study of photoinduced anisotropy in chalcogenide …...summary study of photoinduced anisotropy in...
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Study of Photoinduced Anisotropy in
Chalcogenide Ge-As-S Thin Films
Thèse
Kristine Palanjyan
Doctorat en Physique
Philosophiae Doctor (Ph.D.)
Québec, Canada
© Kristine Palanjyan, 2015
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Résumé Étude de l'anisotropie photo-induite dans les couches minces vitreuses de chalcogénures
Cette thèse porte sur l'étude expérimentale de la photosensibilité de verres de chalcogénures (ChG) sous la forme de couches minces. Plus particulièrement, elle est dédiée à l’étude des modifications photoinduites de leurs propriétés optiques ainsi qu’aux changements structuraux qui y sont liés au niveau atomique. Une étude systématique des propriétés des ChG sélectionnés dans le système vitreux Ge-As-S a été réalisée en fonction de la concentration relative des éléments Ge, As et S, de l’épaisseur des couches minces déposées ainsi que des différentes conditions expérimentales d’exposition au faisceau laser. Tout d’abord nous nous sommes intéressés au band gap optique du matériau, au décalage du bord d'absorption et au changement de sa pente qui sont les résultats d’arrangements atomiques complexes dans le réseau désordonné du ChG.
Ensuite, les résultats expérimentaux ont démontré que la composition vitreuse Ge25As30S45 possède la plus forte photosensibilité et notamment la valeur la plus élevée de biréfringence photo-induite (PIB) parmi les verres des systèmes Ge-As-S et As-S. La conversion de liaisons homopolaires (Ge-Ge, As-As) à hétéropolaires (Ge-S, As-S) a de plus été mise en évidence pour expliquer ce phénomène. En outre, la modélisation théorique simple que nous avons proposée avec une certaine approximation, montre que la valeur locale du PIB peut être d’un ordre de grandeur plus élevée que sa valeur moyenne. Les changements dynamiques d’absorption photo-induite étudiés pour différentes conditions expérimentales sont caractérisés par de forts changements asymétriques et non-monotones durant l'excitation et la relaxation. Ces changements ont été décrits par un modèle phénoménologique unipolaire que nous avons proposé et qui est basé sur certaines conversions séquentielles de liaisons se produisant après le franchissement d’une barrière énergétique donnée (estimée sur la base de nos mesures). Puis cette photosensibilité élevée des couches minces Ge-As-S a été utilisée pour l'enregistrement d’un réseau polarisé et pour la fabrication d’une lentille à gradient d’indice (GRIN) sur la surface, obtenus par irradiation laser à une longueur d’onde correspondante à la valeur de son band gap optique. La variation des efficacités de diffraction maximale obtenues pour les hologrammes scalaires et vectoriels a été discutée en considérant les différentes unités structurales identifiées et le rôle des transitions électroniques directes et indirectes dans ces deux types de réseaux. La stabilité thermique des hologrammes vectoriels a été montrée expérimentalement grâce à l’ajout de l’élément germanium Ge dans la composition de la couche mince. Enfin, les forces optiques des lentilles obtenues ainsi que les distorsions de front d'onde et l’effet de vieillissement ont été caractérisés à l’aide de capteurs Shack Hartmann.
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Summary Study of photoinduced anisotropy in chalcogenide vitreous thin films
This PhD thesis refers to the experimental study of photosensitivity of chalcogenide glassy (ChG) thin films and their induced structural changes at the atomic level. A systematic study of the ChG properties is presented as a function of the elemental composition in the selected Ge-As-S system and the film thickness. More particularly, the goals of this work were to evidence and characterize the photoinduced birefringence and dichroism effects, to investigate the mechanisms involved and to correlate experimental observations with theoretical modeling.
The first part of the work was dedicated to the study of the optical properties, specifically the optical band gap of the prepared composition within the Ge-As-S vitreous system to reveal the most appropriate composition for further photoinduced effects examination. The shift and slope change observed for the absorption edge (associated with the optical band gap) according to the film thickness resulted from complex atomic (re)arrangements in the ChG network. The experiments carried out for the photoinduced effects have permitted to determine the best composition to be Ge25As30S45 among the Ge-As-S and As-S glasses in terms of higher photosensitivity and higher value of photoinduced birefringence (PIB) produced by the conversion from homopolar (Ge-Ge, As-As) to heteropolar (Ge-S, As-S) bonds. Moreover, the simple theoretical model proposed herein showed, with some approximation, that the local value of the PIB in these ChG thin films may be one order of magnitude higher than its average value. Then, the dynamic study of the photoinduced absorption revealed a strong asymmetric and non-monotonic behavior as a function of the irradiation laser power. To account for this specific behavior, a new unipolar phenomenological model is proposed based on sequential bond conversions occurring beyond an estimated energetic barrier.
The photoinduced anisotropy of these ChG Ge-As-S thin films was then used to record polarization gratings and gradient index lenses (GRIN). The maximum diffraction efficiencies achieved between scalar and vector holograms was discussed by means of involved structural units and the role played by indirect and direct electronic transitions. In addition, an improved thermal stability of the recorded vector holograms was experimentally shown after incorporation of germanium Ge into the material composition. The optical performance of the obtained lenses as well as the wave front distortions, aging effect and so on were studied by means of Shack Hartmann sensor.
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Contents Résumé ............................................................................................................................................................. iii
Summary ............................................................................................................................................................ v
Contents ........................................................................................................................................................... vii
List of Tables .................................................................................................................................................... xi
List of Figures ................................................................................................................................................. xiii
Acknowledgments .......................................................................................................................................... xxi
Foreword ....................................................................................................................................................... xxiii
Chapter 1 ............................................................................................................................................................ 1
Genaral Introduction .......................................................................................................................................... 1
1.1 Chalcogenide Glasses and Thin Films ..................................................................................................... 2
1.1.1 Definition and history of Chalcogenide Glasses .............................................................................. 2
1.1.2 Optical Properties of Chalcogenide Glasses..................................................................................... 4
1.1.3 Thermal Properties of Chalcogenide Glasses ................................................................................. 11
1.1.4 The Ge-As-S Glass System ............................................................................................................ 12
1.2 Photoinduced Phenomena in Chalcogenide Glasses and Thin Films .................................................... 17
Brief History ........................................................................................................................................... 18
1.2.1 General Classification of Photoinduced modification in Chalcogenide Glasses ............................ 19
1.2.2 Rheological/Mechanical Photoinduced Effects .............................................................................. 21
1.2.3 Structural Photoinduced Effects ..................................................................................................... 22
1.2.4 Chemical Photoinduced Effects ..................................................................................................... 24
1.2.5 Optical Photoinduced Effects ......................................................................................................... 24
1.2.6 Existing Models Describing the Photoinduced Anisotropy ........................................................... 31
1.3 Objectives and Novelty .............................................................................................................................. 36
Chapter 2 .......................................................................................................................................................... 39
The Absorption Edge Study of Chalcogenide Ge-As-S Thin Films ................................................................ 39
Glass optical quality .................................................................................................................................... 39
Abstract ....................................................................................................................................................... 44
2.1 Introduction ....................................................................................................................................... 45
2.2 Experimental ..................................................................................................................................... 46
2.3 Results and Discussion ...................................................................................................................... 48
2.4 Conclusion and Prospects ................................................................................................................. 51
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Chapter 3 .......................................................................................................................................................... 53
Study of Average Photoinduced Birefringence in Ge-As-S Thin Films .......................................................... 53
Abstract ........................................................................................................................................................ 55
3.1 Introduction ........................................................................................................................................... 56
3.2 Experimental .......................................................................................................................................... 57
3.3 Results ................................................................................................................................................... 58
3.4 Photosensitivity Study in Thin Films ..................................................................................................... 59
3.5 Discussions ............................................................................................................................................ 66
3.6 Summary and Conclusion ...................................................................................................................... 68
Chapter 4 .......................................................................................................................................................... 71
Study of Local Photoinduced Birefringence in Ge-As-S Thin Films ............................................................... 71
4.1 Introduction ................................................................................................................................................ 74
4.2 Experimental Set-Up and Procedure .......................................................................................................... 75
4.3 Results and Discussions ......................................................................................................................... 77
Chapter 5 .......................................................................................................................................................... 83
Study of Photoinduced Dichroism in Ge-As-S Thin Films .............................................................................. 83
Abstract ........................................................................................................................................................ 85
5.1 Introduction ........................................................................................................................................... 86
5.2 Experimental Method ............................................................................................................................ 87
5.2.1 Thin Film Preparation and Characterization Methods .................................................................... 87
5.2.2 Photoinduced Dichroism Investigation Procedure ......................................................................... 88
5.3 Results ................................................................................................................................................... 89
5.3.1 Thin Film Characterization ............................................................................................................. 89
5.3.2 Photoinduced Dichroism Investigation .......................................................................................... 91
5.4 Discussion .............................................................................................................................................. 96
5.4.1 Polarized Raman Spectroscopic Study ........................................................................................... 96
5.4.2 Proposed Model ............................................................................................................................ 100
5.5 Conclusions ......................................................................................................................................... 106
Chapter 6 ........................................................................................................................................................ 109
Polarization Holograms in thin films of Ge25As30S45 Glass ....................................................................... 109
6.1 Introduction ......................................................................................................................................... 109
6.2 Experimental method ........................................................................................................................... 111
6.3 Results ................................................................................................................................................. 113
6.4 Discussion ............................................................................................................................................ 117
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6.5 Conclusion ........................................................................................................................................... 120
Chapter 7 ........................................................................................................................................................ 121
Application of Photoinduced sensitivity in Ge-As-S Chalcogenide Thin Films: GRIN Lens Formation . 121
Photoinduced GRIN Lens Formation in Chalcogenide Ge-As-S Thin Films ............................................ 123
Abstract ..................................................................................................................................................... 123
7.1 Introduction ......................................................................................................................................... 124
7.2 Experimental method........................................................................................................................... 126
7.3 Results and Discussion ........................................................................................................................ 126
7.4 Conclusions and Prospects .................................................................................................................. 131
Chapter 8 ........................................................................................................................................................ 133
General Conclusion........................................................................................................................................ 133
Experimental Method .................................................................................................................................... 139
A.1 Fabrication Method of Chalcogenide Glasses .................................................................................... 139
A.1.1 Bulk Glass Fabrication ................................................................................................................ 139
A.1.2 Glass Substrate Cleaning Procedure ............................................................................................ 142
A.2.1 Thermal Evaporation Technique ................................................................................................. 143
A.2.2 Sputtering Evaporation Technique .............................................................................................. 145
A.2.3 Electron-Beam Evaporation Technique (used in this work) ........................................................ 146
A.3 Thermal analysis ................................................................................................................................. 148
A.4 Elemental Microanalysis by Energy Dispersive X-Ray Spectroscopy Coupled to Scanning Electron Microscope (EDX-SEM) ........................................................................................................................... 150
A.5 Micro-Raman Spectroscopy ............................................................................................................... 153
Bibliography .................................................................................................................................................. 157
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List of Tables
Table 1.1: Classification of the main photoinduced modifications .................................................................. 20
Table 2.1: Elemental analyses of the bulk and thin film of composition Ge25As30S45. The experimental error
of the measurement is estimated to be around 5 At.%. ........................................................................... 47
Table 3.1: Material analyses of the composition Ge25As30S45. The experimental error of this measurement is
estimated to be of the order of 3%-5%. ................................................................................................... 57
Table 5.1: Depolarization ratio calculated from polarized and depolarized Raman spectra before and
after laser exposure at 514.5 nm............................................................................................................ 100
Table 5.2: Bond energies (in kJ/mol) in Ge-As-S ChG, from [180]. ............................................................ 104
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List of Figures
Figure 1.1: Periodic table showing the elements (highlighted in blue) usually combined with chalcogen
elements (highlighted in orange) to fabricate ChG. .................................................................................. 3
Figure 1.2: Photographs of ChG fabricated in the research group of Prof. Younès Messaddeq, COPL, Laval
University (source: http://www.cercp.ca). From left to right: gallium germanium sulfide, arsenic sulfide
and arsenic selenide glasses. ..................................................................................................................... 4
Figure 1.3: Optical transmission of the three families of chalcogenide glass, compared to silica and fluoride
glass, from [9]. The glass thickness is 2mm.............................................................................................. 5
Figure 1.4: Schematic representation of the electronic band structure in amorphous semiconducting materials.
Arrows A and B, C show the optical electronic transitions in the Weak Tail Absorption (WTA)/Urbach
and Tauc regimes, respectively. ................................................................................................................ 6
Figure 1.5: Typical spectral dependence of the optical absorption coefficient in amorphous semiconductors.
In the A and B regions, the optical absorption is controlled by optical transitions between tail and tail,
and tail and extended states, respectively. In the C region, the optical absorption is dominated by
transitions from extended to extended states. In the domain B, the optical coefficient follows Urbach
rule. In the region C, the optical absorption coefficient follows the Tauc’s relation, from[13]. ............... 7
Figure 1.6: Compositional variation of the refractive index and optical band gap of chalcogenide glass thin
films [17]. .................................................................................................................................................. 9
Figure 1.7: Ternary Ge-As-S system diagram presenting: (i) the GeS2-As2S3 and Ge2S3-As2S3 vitreous
stoichiometric tie lines (red dot lines); (ii) the GeS-As4S4 compound tie line (brown dot line); the
vitreous domain reported by Musgraves et al. [23] and the S-rich and S-poor (blue region) vitreous
compositions ranges. ............................................................................................................................... 14
Figure 1.8: Structure models of (a) three-dimensional continuous random network like fused silica SiO2 or
chalcogenide glass GeS2 and (b) two-dimensional distorted layers in As2S3 chalcogenide glass, from
[27]. ......................................................................................................................................................... 15
Figure 1.9: As2Se3 thin film sample with photo-darkened spots indicated by the arrows, from [84]. ............. 25
Figure 1.10: Reversible change in the density of state of the valence band of amorphous As2Se3 thin film,
from [90]. ................................................................................................................................................ 26
Figure 1.11: Photo-darkening E in ChG as a function of where Ti is the irradiation temperature and Tg
is the glass transition temperature. Measurements of E were performed at temperatures below Ti, from
[91]. ......................................................................................................................................................... 26
Figure 1.12: Observation of the photodarkening effect in amorphous As2S3 thin film and its partial recovery
after thermal annealing. The black solid line is the band edge of the as-deposited film, the blue line is
the band edge of the irradiated film and the red one is the band edge of the irradiated film after thermal
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annealing. The numbers 1 to 7 represent the successive cycles of irradiation/annealing applied to the
As2S3 thin film, from [100]. .................................................................................................................... 28
Figure 1.13: Photoinduced anisotropy in a pnictogen–chalcogenide system before (a) and after (b) excitation.
Open circles represent pnictogen atoms and solid circles chalcogens ones, from [49]. .......................... 33
Figure 1.14: The dielectric tensor after irradiation by a linear polarized light (a) and an unpolarized light
(b), from [121]. ........................................................................................................................................ 34
Figure 1.15: The projection of the As4S4 molecule in the parallel plane of the As-As bond, from [122]. ....... 35
Figure 2.1 Images in transmission of: a) Polished slice of home-made Ge25As30S45 glass sample (16 mm
diameter); b) Commercial chalcogenide As2S3 glass window of 25 mm diameter placed between two
polarizers parallel to each other. .............................................................................................................. 40
Figure 2.2 Ge25As30S45 glassy thin film (of 3 µm thickness and 2.5 x 5 cm dimensions) placed between two
parallel polarizers. ................................................................................................................................... 41
Figure 2.3: Photograph of a typical Ge25As30S45 glass rod and polished slice. ................................................. 46
Figure 2.4: Ge25As30S45 glass thin films prepared with different thicknesses. ................................................. 47
Figure 2.5: Ge25As30S45 optical band gap as a function of the film thickness. ................................................. 48
Figure 2.6: Absorption coefficient spectra of Ge25As30S45 for different film thicknesses. ............................... 49
Figure 2.7: Normalized Raman spectra of Ge25As30S45 thin films of different thicknesses. ............................ 50
Figure 3.1: Typical transmission spectra of obtained thin ChG films. ............................................................. 58
Figure 3.2: Absorption coefficients of the Ge25As30S45 as function of probe’s energy obtained for
photoexposition intensity of 8W/cm2 for 60 min. The solid curve corresponds to the unexposed case;
the dashed curve corresponds to the photoexposed case. ........................................................................ 59
Figure 3.3: The experimental setup used for the study of PIB: P-polarizer, M–mirror, - half wave plate, S-
sample; A-analyzer; F1 and F2-filters, d-diaphragm, D-detector. ............................................................ 60
Figure 3.4: Typical cycle of excitation and partial relaxation of the PIB in the Ge25As30S45 film.
The solid curve shows the experimental result and the dashed one (behind the experimental curve)
represents the fitted curve. The thickness of the film was 1.5 µm and the excitation intensity was
8W/cm2. ................................................................................................................................................... 61
Figure 3.5: Dependence of the PIB upon the amount of As in the film of Ge-As-S. ....................................... 62
Figure 3.6: The dependence of the established (saturated) value of PIB upon the excitation intensity for the
composition Ge25As30S45. ........................................................................................................................ 63
Figure 3.7: Normalized Raman spectra of thin Ge-As-S films for different compositions: Ge25As10S65 (dotted
black line), Ge25As20S55 (short dash dotted red line), Ge25As30S45 (dashed green line), Ge25As35S40 (short
dotted blue line), Ge25As40S35 (solid cyan line). ...................................................................................... 65
Figure 3.8: Normalized Raman spectra of Ge25As30S45 bulk glass (dotted black line) and thin films unexposed
(short dash dotted red line) and exposed at 2.14 W/cm2 (dashed green line), 4.24 W/cm2 (short dotted
blue line) and 7.87 W/cm2 (solid cyan line) for 60 min. ......................................................................... 68
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Figure 4.1: Transmission spectra (in non-polarized light) of non-exposed (black, dotted curve) and exposed
(red, solid curve) Ge25As30S45 thin films of 3 µm thickness. Samples were irradiated at 514 nm for 30
min. ......................................................................................................................................................... 75
Figure 4.2: Experimental setup used for the study of PIB: P-polarizer, M–mirror, - half-wave plate, S-
sample; A-analyzer; F1 and F2-filters, d-diaphragm, D-detector. ............................................................ 76
Figure 4.3: Transmitted intensity of the probe beam (3 m thick sample is used versus time for pump
intensity of 8W/cm2. The points 1 and 1’ represent the established values of excitation and 2 and 2’
represent the established values of relaxation corresponding to the probe transmission without (drawn
by triangles in the fig.) and with analyzer (drawn by squares in the fig.), respectively. ......................... 79
Figure 4.4: Dependence of the established values of PIB (under CW excitation) upon the excitation intensity
for the 3 m Ge25As30S45 thin film. The line is used to guide eyes only. ................................................ 80
Figure 4.5: Experimentally measured dependence of the established output probe intensity upon the input
pump intensity for the 3 m Ge25As30S45 thin film. The line is used to guide eyes only. ....................... 81
Figure 4.6: Average (solid curve) and local maximum (at the input front of the ChG film, dotted curve)
values of the established PIB as a function of pump intensity for the 3 m Ge25As30S45 thin film. Solid
and dashed lines are used to guide eyes only. ......................................................................................... 82
Figure 5.1: Experimental setup used for the PID study: M1 and M2 - mirrors; /4 - quarter wave plate (placed
on the path of the probe beam), S – ChG sample; W- Wollaston prism; BS1 and BS2 - polarization
insensitive beam splitters, P1 and P2 – polarizers, D1-4 – photo detectors. Note: BS1 allows using the Ar+
laser (514.5 nm) as probe and pump beams simultaneously. In a different experiment, the BS1 is
removed to use the He-Ne laser (632.8nm) as probe and the Ar+ laser (514.5 nm) as pump. ................. 89
Figure 5.2: (a) Elemental chemical composition measured by SEM-EDAX analyses at 9 distinct points on a
Ge25As30S45 thin film of 7µm thickness. The horizontal (black dashed) lines correspond to the nominal
values (b) Raman spectra recorded at 9 distinct points on the same Ge25As30S45 thin film (numbers 1-9
correspond to their locations, as depicted in the inset) and normalized at 215 cm-1. .............................. 90
Figure 5.3: Transmission spectrum of a 3 µm thick Ge25As30S45 thin film. Vertical arrows show the position
of the band gap and sub band gap lights used in the present work for excitation (at 514.5 nm) and
probing (at 632.8 nm), respectively. ....................................................................................................... 91
Figure 5.4: Transmission (right vertical axis, in red) and reflection (left vertical axis, in black) of horizontal
IHT, IHR (solid lines) and vertical IVT, IVR (dashed lines) components for the sample of 3 µm thickness.
Pump beam (at 514.5 nm) is vertically polarized, Ip= 10 W/cm2; probe beam was obtained from a He-
Ne laser (at 632.8 nm), Ipr = 3.5 mW/cm2. ............................................................................................. 92
Figure 5.5: Transmission (right vertical axis, in red) and reflection (left vertical axis, in black) of horizontal
IHT, IHR (solid lines) and vertical IVT, IVR (dashed lines) components for the 3 µm thick sample. Pump
beam is vertically polarized (at 514.5 nm), Ip = 10 W/cm2; probe beam is also obtained from the same
Ar+ laser (at 514 nm), Ipr = 1mW/cm2. .................................................................................................. 93
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Figure 5.6: Kinetics of the photoinduced dichroism PID (measured at 514.5 nm) in a 3 µm thick Ge25As30S45
thin film annealed at 350°C. .................................................................................................................... 94
Figure 5.7: Kinetics of PID as evidenced by the sum of transmitted and reflected beam powers for two
orthogonal polarization components (normalized to their initial value). (a) probe He-Ne laser, at 632.8
nm; (b) probe Argon-ion laser, at 514.5 nm. The excitation was achieved with a vertically polarized
pump at 514.5 nm. Ip = 10 W/cm2, d = 3 µm. Letters h and v correspond to horizontal and vertical
polarization components of the probe in the established excitation state. Letters h’ and v’ show the
values of same components in the partial relaxation state. ...................................................................... 95
Figure 5.8: Averaged amplitudes of the decrease of the summed (and normalized) transmission and reflection
intensities (T+R) measured from its initial (maximum) value (1-(T+R)) up to the steady state of excitation
(a) and relaxation (b) for two orthogonal components; horizontal (black solid curve) and vertical (dotted
red curve), as a function of pump intensity. The excitation and probing were performed at 514.5 nm. The
polarization of the pump is vertical. The thickness of the film of Ge25As30S45 is 3 µm. ............................ 96
Figure 5.9: Polarized and depolarized Raman spectra recorded at 632.8 nm before (a) and after (b) vertical
polarized laser exposition at 514.5 nm (0.3W/cm2) during 45 min on the Ge25As30S45 thin film. .......... 98
Figure 5.10: Measured temperature dependence of the Ge25As30S45 thin films surface as a function of the
pump intensity. Squares represent the experimental data and circles represent the theoretical estimation
results using the equation (5.3). ............................................................................................................. 102
Figure 5.11: Photoinduced darkening (PD) of the Ge25As30S45 thin film at different oven temperatures for two
orthogonal components (horizontal: black solid curve and, vertical: dot red curve): a- excitation, b-
relaxation. Intensity of the pump was 3 W/cm2 and the thickness of the film was 3 µm. ..................... 103
Figure 5.12: Qualitative reproduction of the dynamics of absorption changes during the relaxation of a 4-
level system with consecutive conversion between bonds. ................................................................... 105
Figure 6.1: Experimental set-up for the vector hologram study: pump – Ar-ion laser; probe - He-Ne laser;
/2- half-wave plate; /4- quarter-wave plate; WP-wollaston prism; S-sample; D1, D2, D3-detectors. 112
Figure 6.2: Normalized diffraction efficiencies of the vector holograms recorded in Ge25As30S45 (black) and
As2S3 (red) thin films as a function of the temperature. Lines are guide to the eye. ............................. 113
Figure 6.3: Diffraction efficiency (%) of vector (solid line) and scalar (dashed line) holograms recorded in
the same Ge25As30S45 thin film as a function of pump intensity. The vector hologram was recorded by
(RCP+LCP) polarizations beams while the scalar hologram was recorded with two linearly s-polarised
beams. Lines are guide to the eye. ......................................................................................................... 114
Figure 6.4: Dynamic diffraction efficiency (%) of vector (sold line) and scalar (dashed line) holograms
recorded in the same Ge25As30S45 thin film. The vector hologram was recorded by (RCP+LCP)
polarizations beams while the scalar hologram was recorded by two linearly s-polarized beams. The
pump intensity was 4W/cm2. ................................................................................................................ 115
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Figure 6.5: Optical microscope images of recorded gratings on the same Ge25As30S45 thin film: (a) scalar
gratings written by (s+s) polarization beams; (b) vector gratings written by (RCP+LCP) polarization
beams; (c) vector gratings written by (s+p) polarization beams. .......................................................... 116
Figure 6.6: Diffraction efficiency of +1 and -1 diffracted orders as a function of the rotation angle of the
quarter-wave plate (the elasticity of the incident probe beam polarization). Ge25As30S45 thin film
thickness is 7 µm. Lines are guide to the eye. ....................................................................................... 117
Figure 7.1: Wave front of the probe beam exiting the GRIN lens measured by the Shack-Hartmann: exposure
time was 30 min and the power was 8W/cm2. The thickness of the thin film was 5 μm. The asymmetry
of the wave front profile is due to the inhomogeneity of the excitation laser beam. ............................. 127
Figure 7.2: (a) Lens optical power dependence on pump intensity for different irradiation durations,
observed with parallel and perpendicular probe polarizations (with respect to pump polarization); (b)
Lens optical power dependence on irradiation time for different pump intensities, observed with parallel
and perpendicular probe polarizations (with respect to pump polarization). ........................................ 128
Figure 7.3: Scanning electron microscope (SEM) images of surfaces of a damaged sample. ....................... 129
Figure 7.4: Modification of the measured optical power over time for different pump intensities examined
with two probe polarizations: parallel to irradiation polarization (a) and perpendicular to irradiation
polarization (b). ..................................................................................................................................... 130
Figure 7.5: Raman spectra (a) and EDAX elemental quantitative analyses (b) of the Ge-As-S films of 5 µm
thickness (freshly evaporated (dashed curve) and 180 days stored in ambient atmosphere after the
irradiation (solid curve)). ...................................................................................................................... 131
Figure 7.6: 2D profiles of the sample before (black curve) and after (red curve) irradiation showing the
absence of surface modification (expansion or contraction). Inset: magnification of the surface profile
of the irradiated zone. ............................................................................................................................ 132
Figure 9.1: (a) Scheme of the experimental set-up for the preparation of chalcogenide synthesis ampoule ; (b)
Tubular rocking furnace and (c) thermal profile used to melt, fine, quench and anneal the chalcogenide
glass within its silica ampoule. .............................................................................................................. 140
Figure 9.2: Thin film deposition techniques, from [219]. .............................................................................. 143
Figure 9.3: Schematic representation of a thermal evaporation chamber: B – heated boat, S – substrate, H –
heating system for the substrate, V – vacuum (from [222]). ................................................................. 144
Figure 9.4: Schematic illustration of thin film deposition dependence on angle of evaporation beam direction
(from [222]). ......................................................................................................................................... 145
Figure 9.5: Schematic representation of the sputtering evaporation apparatus : T – target electrode ; S –
substrate electrode ; P – plasma ; V – vacuum and H – heater (from [222]). ........................................ 146
Figure 9.6: Schematic representation of the e-beam evaporation method. .................................................... 147
Figure 9.7: DSC traces of the Ge25As30S45 thin film and crushed bulk glass pieces (y-axis: unit: 0.5
mW/mg/div.). ........................................................................................................................................ 150
Figure 9.8: Quantification (EDAX analyse) of chemical elements of composition Ge25As30S45. .................. 151
Figure 9.9: Scheme describing the main electron-matter interactions. .......................................................... 152
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Figure 9.10: Schematic energy diagram describing the Rayleigh and Raman scatterings. The line thickness
indicates the signal strength from the various transition state shown by black horizontal lines. ........... 154
Figure 9.11: Schematic representation of Raman Depolarization Ratio. ....................................................... 155
Ծնողներիս…
À Mon Amour…
բոլոր ջանքերի և սիրո համար որ ամեն պահ ինձ հետ է…
pour tous les efforts consentis et ton amour de tous les instants…
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Acknowledgments
I would like to express my gratitude to all those who have supported me throughout
this PhD project since my arrival in Quebec City, 4 years ago. It is always difficult to
express with words the emotions of gratitude, especially in some foreign language, so my
sentences will be shorter than my thoughts.
First, I am deeply indebted to my supervisors Prof. Tigran Galstian and Prof. Réal
Vallée for giving me the opportunity to achieve my doctoral studies at the Center for
Optics, Photonics and Laser (COPL) at Laval University. The completion of this PhD
thesis would certainly not have been possible without their continued support,
encouragement, guidance and dedication to this work. I want to express my sincere
gratitude for all useful comments, advices, valuable remarks they gave me through the
periodic and frequent meetings. Their motivation, enthusiasm, and immense knowledge
that they shared honorably with me, were essential to do this work. I want to mention their
contribution both in professional and personal aspects, which help me from the first day till
now for my integration to the Quebec society.
I would like to thank Prof. Younès Messaddeq to have revised my thesis manuscript
prior to its deposition. His insightful comments and suggestions from the point of view of a
chemist helped me noticeably in improving my work. His contribution was significant
since the beginning of my doctoral studies and I am also sincerely grateful to his research
group, for the support in preparing material and for the fruitful discussions during the
entire project, especially to Sandra Helena Messaddeq, Prof. Igor Skripachev, Yannick
Ledemi and Mohammed El-Amraoui.
Sincere thanks to all the technical and administrative staff of COPL and Laval
University for their help and support, in particular to Patrick Larochelle who was always
available and able to solve rapidly the technical issues inherent to experimental
investigations.
Thanks to the Natural Sciences and Engineering Research Council of Canada
(NSERC) agency for their financial support.
I would like to address my thanks to Renan Cariou to have revised and improved my
English in the first part of this thesis.
xxii
I also express my thanks to all staff of ultrafast laboratory in Yerevan State University,
especially to Prof. Levon Mouradian, Garik Yesayan and Artur Kirakosyan.
Thanks to the Armenian community here in Quebec city, and especially to my friends
Karen, Amalya, Elina, Ani, Vahe , Anush and all who were close to me for cheering me up
when necessary. Special thanks to Karen for all the important discussions about physics
problems that we had.
Finally, I extend special and most important thanks to my family: my mother, my
father, my sister and her family who all have supported me in this adventure so far from
my home in Erevan, and to my husband met here in Quebec, Yannick, for his patience and
unremitting encouragement, for our long scientific (and not) discussions which have made
possible the completion of this PhD project. Իմ բոլոր հաղթանակներն ու
հաջողութունները նաև ձերն են: Շնորհակալ եմ Ձեզ....սիրում եմ Ձեզ անսահման:
Merci également à ma belle-famille: votre amour sincère et votre soutien m’ont
apporté beaucoup plus que vous ne le pensez. Les mots d'encouragement venus du cœur
motivent en profondeur. Je vous aime.
xxiii
Foreword
This thesis mainly aims at detailing the examinations of photoinduced anisotropy
(under irradiation of band gap light) in thermally stable chalcogenide glass (ChG) thin
films belonging to the ternary Ge-As-S system. The investigations reported in this work
lead to important information related to the physical properties of the studied ChG
material, at both macroscopic and microscopic levels, and to the understanding of the
mechanisms involved in the studied photoinduced effects, encompassing thus different
aspects from fundamental physics science to applicative science for the optics and
photonics high-tech industry.
Chapter one proposes a brief review on the subject of ChG glasses and their main
photoinduced properties and modifications.
The major and original part of the thesis is based on three first author articles and two
proceedings published in peer-reviewed journals during my PhD studies. Chapter two
presents a SPIE proceeding published in 2014 (K. Palanjyan, R. Vallée T. Galstian , ‘Band
gap dependence upon thickness of chalcogenide Ge-As-S thin films’, Proc. SPIE 9288,
Photonics North 2014, 92880K (2014)) where the band gap study of the selected ChG
material was presented, and co-authored by my two supervisors, Prs. T. Galstian and R.
Vallée. The subject and methods of realization were at first discussed with them. The
optical experiments of the project were carried out entirely by myself at the COPL
laboratories, at Laval University. After discussing the content and structure of the
manuscript, I provided the original version for further editing and completing.
The third chapter refers to the basic research axis of this thesis: the study of the
photoinduced anisotropy of these ChG thin films. The present chapter presents an article
published in 2013 in Optical Materials Express (K. Palanjyan, S.H. Messaddeq, Y.
Messaddeq, R. Vallée, E. Knystautas, T. Galstian, Photoinduced birefringence in Ge-As-S
thin films, Optical Materials Express, Vol. 3. Issue 6, pp. 671-683 (2013)). The goal of this
study was to investigate the photoinduced birefringence (PIB) and understand the
mechanisms responsible for the observed anisotropic changes. The preparation of bulk
samples, their evaporation to obtain thin films and the experimental set-up for the PIB
xxiv
measurement were realized within the frame of this work. The measurements and
characterizations were performed by using different experimental techniques, as optical
spectroscopic transmission, thermal analysis, elemental microanalysis through energy
dispersive X-ray spectroscopy, micro-Raman spectroscopy, etc., for a better understanding
of the PIB effect through its relationship with the ChG properties and structure. This work
was realized in collaboration with the research groups of Profs. Y. Messaddeq and E.
Knystautas from Laval University. The respective roles of the authors are as follows: first,
all authors participated in the subject discussions. The glass was prepared in the laboratory
of Prof. Y. Messaddeq with the help of Dr. S.H. Messaddeq and Dr. I. Skripachev. Then,
the preparation of the thin films was achieved thanks to the e-beam evaporator available in
the laboratory of Prof. E. Knystautas with the assistance of Dr. S.H. Messaddeq. The thin
film characterizations and main optical experiments were then carried out at COPL
laboratories by me. Following initial discussions with Profs. T. Galstian and R. Vallée,
experiments were performed by S.H. Messaddeq and myself. All results were then
discussed with the authors. The manuscript was essentially written by myself, after
discussing the content and structure with Profs. T. Galstian and R. Vallée. My version of
the manuscript was communicated to all the co-authors who then added some editing and
completing remarks.
In the fourth chapter, additional investigation of the PIB effect is reported thanks to a
thorough measurement of its local value. Such study of the local value of the PIB, besides
the determination of their average values, may be useful for integrated optic and photonic
devices. Some approximations were considered to allow the realization of both the
experiments and modeling (achieved with MatLab program). A giant local anisotropy was
observed from these ChG Ge-As-S thin films. The article was published in 2015 in Optical
Materials Express (K. Palanjyan, R. Vallée, T. Galstian, Observation of giant local
photoinduced birefringence in Ge25As30S45 thin films ,Optical Materials Express, Vol. 5
Issue 5, pp.1122-1128 (2015) ) authored by my two supervisors, Profs. T. Galstian and R.
Vallée, and myself. The subject and methods of realization were first discussed with my
supervisors. The experimental part was entirely realized by myself at COPL laboratories.
After discussing the content and structure of the manuscript, I provided the original version
for further editing and completing.
xxv
The fifth chapter reveals the second major result of the current thesis: the
photoinduced absorption changes (photoinduced dichroism) of the Ge-As-S thin films.
Besides the values of dichroism measured experimentally, a phenomenological model is
proposed to explain the asymmetric and non-monotonic behavior of photoinduced
absorption dynamical changes, and also to account for the mechanisms which cause these
photoinduced modifications. The chapter presents an article published in 2015 in the
Journal of Non-Crystalline Solids (K. Palanjyan, R. Vallée and T. Galstian, Journal of
Non-Crystalline Solids 410 (2015) 65–73)). The authors were my two supervisors and
myself. The subject and methods of realization were at first discussed with them. The
optics experimental part was entirely realized by me at COPL laboratories. After
discussing the manuscript’s content and structure with my supervisors, I provided them my
version of manuscript for further editing and completing.
Finally, the sixth and seventh chapters describe two different applications based on the
obtained results. The sixth chapter corresponds to the study of the polarized holograms
recorded on these ChG thin films with the comparison between the scalar and vector
holograms as well as the thermal stability assessment for the latter. The seventh chapter
deals with the formation of gradual variation of the refractive index (GRIN lenses) on these
ChG Ge-As-S thin films. The studies of the optical performance and the wave front
distortions of the obtained lenses were performed upon different experimental conditions.
Results were presented at an international conference and published in 2014 in the peer-
reviewed SPIE proceedings (K. Palanjyan, R. Vallée T. Galstian, ‘Photoinduced GRIN
lens formation in chalcogenide Ge-As-S thin flms’, Proc. SPIE 9288, Photonics North
2014, 92880L (2014)). In both cases, the authors were my two supervisors and myself. The
subject and methods of realization were at first discussed with them. The optics
experiments were entirely realized by me at COPL laboratories. The preparation of these
articles was realized according to the same way as for the previous ones.
Aside from these main results, the eight chapter consists in the general conclusion of
the thesis while the Appendix brings on one hand some additional descriptions of the
experimental method used for the preparation and characterization of the studied bulk and
thin film and on the other hand, a review of the known methods for thin films fabrication.
xxvi
Co-authors:
Tigran Galstian: Center for Optics, Photonics and Laser, Department of Physics,
Engineering Physics and Optics, Laval University, Pav. d’Optique-Photonique, 2375 Rue
de la Terrasse, Québec, G1V 0A6, Canada.
e-mail: [email protected]
Réal Vallée: Center for Optics, Photonics and Laser, Department of Physics, Engineering
Physics and Optics, Laval University, Pav. d’Optique-Photonique, 2375 Rue de la
Terrasse, Québec, G1V 0A6, Canada.
e-mail: [email protected]
Younès Messaddeq: Center for Optics, Photonics and Laser, Department of Physics,
Engineering Physics and Optics, Laval University, Pav. d’Optique-Photonique, 2375 Rue
de la Terrasse, Québec, G1V 0A6, Canada.
e-mail: [email protected]
Sandra H. Messaddeq: Center for Optics, Photonics and Laser, Department of Physics,
Engineering Physics and Optics, Laval University, Pav. d’Optique-Photonique, 2375 Rue
de la Terrasse, Québec, G1V 0A6, Canada.
e-mail: [email protected]
Emile Knystautas : Center for Optics, Photonics and Laser, Department of Physics,
Engineering Physics and Optics, Laval University, Pav. d’Optique-Photonique, 2375 Rue
de la Terrasse, Québec, G1V 0A6, Canada.
e-mail: [email protected]
Publications (during the PhD thesis):
Reviewed periodicals
K. Palanjyan, S.H. Messaddeq, Y. Messaddeq, R. Vallée, E. Knystautas, T.
Galstian, Photoinduced birefringence in Ge-As-S thin films, Optical Materials
Express, Vol. 3. Issue 6, pp. 671-683 (2013).
xxvii
K. Palanjyan, R. Vallée, T. Galstian, Experimental Observations of Photoinduced
Bond Conversions in Ge-As-S Thin Films, Journal of Non-Crystalline Solids, Vol.
410, pp 65–73, (2015).
K. Palanjyan, R. Vallée, T. Galstian, Observation of giant local photoinduced
birefringence in Ge25As30S45 thin films, Optical Materials Express, Vol. 5 Issue
5, pp.1122-1128 (2015).
Conference Presentations
K. Palanjyan, R. Vallée T. Galstian, ‘Band gap dependence upon thickness of
chalcogenide Ge-As-S thin films’, Proc. SPIE 9288, Photonics North 2014, 92880K
(2014).
K. Palanjyan, R. Vallée T. Galstian, ‘Photoinduced GRIN lens formation in
chalcogenide Ge-As-S thin films’, Proc. SPIE 9288, Photonics North 2014, 92880L
(2014).
1
Chapter 1
General Introduction
Chalcogenide glasses (ChG) based materials have been studied for several decades
and utilized today in a myriad of applications requiring infrared light transparency or
transmission, infrared sensing, phase change memories, optical data storage, etc.
Dedicated for a long time to military restricted or scientific utilizations, ChG materials
are nowadays finding civilian applications. First, the most developed and used
technology to date is the optical data storage with the of CD-RW, DVD-RAM,
DVD±RW and BLU-ray discs which are based on ChG thin film alloys exhibiting a very
fast and reversible transformation from crystal to vitreous state under a specific laser
exposure. Second, their remarkable mid-infrared transparency in the two infrared
atmospheric windows (3-5 µm and 8-12 µm) is now utilized for civilian night vision and
thermal imaging devices, for instance. Indeed, the simultaneous development of vitreous
compositions with properties meeting the standards for practical use and large scale
production have led to a very cost effective alternative to the single crystal germanium
technology for the fabrication of infrared lenses and windows. Such progress, associated
with the development of infrared detectors, has resulted in lower manufacturing costs of
infrared detection and imaging systems.
This chapter is dedicated to a state of the art of photoinduced phenomena in ChG and
ChG thin films. To present an exhaustive review of all the investigations and works
reported in this field to date is not the intent here, however a general overview of the
unique photosensitive character specific to ChG, which is by far one of the most light-
sensitive materials, will be provided.
After a brief historical perspective, the specific properties which make these
materials so unique and their related practical applications will be described in more
details. The photoinduced phenomena reported in these glasses will then be reviewed.
Emphasis will be given to the effects observed under continuous wave (CW) laser
2
illumination. Finally, the different models proposed in the literature to describe these
phenomena at the atomic and molecular short range structural level, including those
related with the photoinduced anisotropy changes, will be discussed.
1.1 Chalcogenide Glasses and Thin Films
1.1.1 Definition and history of Chalcogenide Glasses
Generally, glasses or vitreous materials constitute a unique class of materials. They
were defined by Zarzycki as non-crystalline solids featuring the phenomenon of glass
transition [1]. Among the known glasses, one can distinguish three main categories: the
metallic glasses, the organic/polymeric glasses and the inorganic glasses. The latter
category comprises the well-known silicate glasses but also the phosphate glasses, the
heavy metal oxide glasses, the halide glasses (particularly the fluoride glasses) and the
ChG. All these types of glasses are characterized by specific features owing to their
chemical composition and will therefore be used for specific applications. For instance,
heavy metal oxide glasses possess heavier elements than silicate ones, leading to glasses
with higher refractive indices and extended transmission window in the infrared region.
However, among all of them, only the ChG are transparent so far in the infrared region,
up to 20-25 μm depending on their compositions, making them the most promising
candidates for mid-infrared optical and photonic applications like lenses for infrared
cameras [2], planar waveguides for integrated optics [3] or infrared sensors [4].
In the 1930’s, a research group from University of Hannover in Germany proposed
the term “chalcogen”, which means “ore former” (from “chalcos” old Greek for “ore”) as
the characteristic name of the group VI of elements including oxygen O, sulfur S,
selenium Se, tellurium Te and “chalcogenides” for their compounds [5] (see Figure 1.1).
Twenty years later, in the 1950’s, after the first report on arsenic sulfide glasses, the term
chalcogenide was used to distinguish the oxide glasses (essentially silicate glasses) from
those based on S, Se and Te. Those glasses (ChG) indeed exhibit a unique mid-infrared
transparency compared to the traditional oxide glasses [6]. Following studies on glasses
based on selenium and tellurium showed even further transmission than sulfide ones in
4
Figure 1.2: Photographs of ChG fabricated in the research group of Prof. Younès Messaddeq, COPL, Laval University (source: http://www.cercp.ca). From left to right: gallium germanium sulfide, arsenic sulfide and arsenic selenide glasses.
Nowadays, ChG belonging to the Ge-As-Se, As-S and As-Se systems for instance
are commercialized as passive infrared optics and optical fibers by many companies like
Amorphous Materials Inc., Umicore IR glass, Schott, CorActive, IRflex, IRradiance,
Diafir and others.
1.1.2 Optical Properties of Chalcogenide Glasses
1.1.2.1 Infrared Transmission
The exceptional transparency of ChG in the mid-infrared region directly depends on
the nature of the anion, i.e. S, Se or Te. Indeed, due to their high atomic weight,
increasing from sulfur (MS = 32.06 g/mol) to tellurium (MTe = 127.6 g/mol) associated
with the high atomic weight of the cations (As, Ge, Ga, Sb, etc.) with whom they form
the glass, sulfide, selenide and telluride glasses are characterized by very low maximum
phonon energies (respectively, 350-425 cm-1, 250-300 cm-1 and 150-200 cm-1) as
compared to those of oxide (~1100 cm-1 for silica glass) or fluoride glasses (~540 cm-1).
As a consequence, owing to their very low multiphonon frequency, sulfide, selenide and
telluride glasses exhibit the most extended transmission in the mid-infrared region, up to
20-25 µm, while sulfide glasses, whose anionic weight is lower, use to transmit mid-
infrared light up to 10-12 µm. Between them, selenide glasses are transparent to around
15-16 µm. The transmission spectra of these glasses are represented in Figure 1.3 and
compared with those of oxide and fluoride glasses.
10 mm 10 mm 10 mm
5
Figure 1.3: Optical transmission of the three families of chalcogenide glass, compared to silica and fluoride glass, from [9]. The glass thickness is 2mm.
The mid-infrared optical domain of the electromagnetic spectrum is of major interest
from a technological point of view since it includes the telecommunication wavelengths,
the thermal radiation domain, the spectral signature range of the so-called greenhouse
gases (water vapor, carbon dioxide, methane, nitrous oxide, and ozone), the spectral
footprint range of the biological molecules, etc.
The need for materials capable to operate (emit, detect or conduct light) in this range
is thus continuously growing for many applications as detection of the weak mid-infrared
emission lines of an orbiting plane, remote CO2 detection, monitoring in real time of
pollutants in the environment [10], help in medical diagnostic through efficient optical
biosensors [11], etc., besides the traditional defense & security applications in the mid-
infrared (thermal imaging, laser countermeasures, etc.).
1.1.2.2 Optical Band Gap
The multiphonon absorption defines the long wavelength (infrared) cut-off of
transparency windows of a glassy (dielectric) material, while its short wavelength cut-off
is characterized by the electronic transitions from the valence to the conduction band.
ChG are also semi-conducting materials and present therefore a band gap between their
valence and conduction bands in their energy diagram, as schematized in Figure 1.4 [12].
As glass is a disordered material, localized states can exist within the material band gap.
These localized states form then the band tails which further reduce the minimum energy
required for an electronic transition to occur from the valence band to the conduction
band, defining the optical band gap. The optical band gap of inorganic glasses usually
6
corresponds to wavelengths in the ultra-violet and visible range. In ChG, where the
anions (S, Se or Te) possess high energy lone-pair electrons, the optical band gap is
narrowed. The absorption edge is thus shifted to longer wavelengths: in the green / red
and near mid-infrared regions for the sulfide and selenide / telluride glasses, respectively.
The coloring and opacity of ChG observed in Figure 1.2 are directly correlated to this
optical band gap shift.
Figure 1.4: Schematic representation of the electronic band structure in amorphous semiconducting materials. Arrows A and B, C show the optical electronic transitions in the Weak Tail Absorption (WTA)/Urbach and Tauc regimes, respectively.
In vitreous materials, the absorption can be divided in three regions: the Tauc region,
the Urbach and the weak tail absorption (WTA) regions which can be easily identified in
the short wavelength edge in the transmission or absorption spectrum (see Figure 1.5)
[13]. In the Urbach region (domain B in Figure 1.5), the absorption coefficient α(E) is an
exponential function as:
α(E) exp (E/EU)
Where EU is the Urbach energy (characteristic energy related to the width of the valence
(or conduction) band tail states, and may be used to compare the “widths” of such
localized tail states of different material) and E is the energy of incident photons. In this
region take place the electronic transitions between the tail of a band (corresponding to
the localized states of a band) and the defects state (represented by the arrows B in Figure
1.4). In the Urbach regime, the values of absorption coefficient α range from 1 to 104
Defects states
Energy state density of a perfectly ordered system
Localized tails states
Ener
gy
Energy density of state
B A Forbidden band
Valence band
Conduction band
C
7
cm-1. In the domain A which corresponds to the weak absorption tail of an absorption
spectrum in Figure 1.5 occur the electronic transitions from tail to tail (represented by the
arrows A in Figure 1.4). The absorption coefficient α in this regime is very weak, below 1
cm-1. In this case, α shows a gradual exponential behavior:
α(E) exp (E/EW)
which is referred to as a ‘weak absorption tail’ or ‘residual absorption’. EW is the energy
characteristic to the width of the defect states in the bandgap.
Sometimes, the two latter regimes are joined in a single one, the so-called Urbach tail
absorption. This regime of absorption depends on the temperature.
Figure 1.5: Typical spectral dependence of the optical absorption coefficient in amorphous semiconductors. In the A and B regions, the optical absorption is controlled by optical transitions between tail and tail, and tail and extended states, respectively. In the C region, the optical absorption is dominated by transitions from extended to extended states. In the domain B, the optical coefficient follows Urbach rule. In the region C, the optical absorption coefficient follows the Tauc’s relation, from[13].
In the Tauc region (domain C in Figure 1.5), the localized electronic states do not
contribute to the absorption phenomenon. The transitions between the valence and
conduction bands, shown by the arrows C in Figure 1.4, are similar to those observed in
ideal crystals. The absorption coefficient α(E) is then given by the Tauc equation:
α(E) 2
8
where E0 is the optical gap energy and E is the energy of the incident photons. These
transitions correspond thus to the absorption of higher energy photons (of shorter
wavelengths) in comparison with the Urbach absorption. Typical values of absorption
coefficient α in the Tauc region are . The optical gap energy E0 and Urbach
energy EU can be employed sometimes to characterize the local disorder in a glass
structure [12].The good understanding of the above described notions is important when
the material under study is an amorphous semiconductor like chalcogenide glassy thin
films. Careful studies of the absorption edge may indeed provide useful information not
only about the state of disorder, but also about the glass structure and its defect content.
The existence of weak bonding arrangement in ChG, evidenced by their extended Urbach
absorption tail, can be directly correlated to their extraordinary photosensitivity [14]. The
irradiation of ChG glasses with near band gap or sub band gap laser light may generate
various effects on these localized states and will be discussed later in this chapter.
1.1.2.3 Refractive Index
Another important characteristic that makes the chalcogenide glass materials unique
is their high refractive indices (from 2.0 to 3.6 and above) when compared to those of
oxide, phosphate or fluoride glasses (~1.45-1.6). Such glasses with high refractive indices
may be used in various photonic devices as photonic crystals or omnidirectional
reflectors for instance [15, 16].
The linear refractive index of a material essentially depends on its density and its
polarizability. Their high value in ChG is thus easily explained by the fact that these
glasses are based on heavy and polarizable elements which are covalently bonded within
the network. Depending on the vitreous system and the glass composition within the
latter, the ChG may exhibit wide variations of the refractive index, as shown in Figure 1.6
where the refractive index and optical band gap of different vitreous systems are
depicted. It can be seen that refractive index and optical band gap can respectively vary
from 2 to 3.6 and from 1.5 to 2.8 eV according to the glass composition/system.
9
Figure 1.6: Compositional variation of the refractive index and optical band gap of chalcogenide glass thin films [17].
Several techniques exist to characterize the refractive index of a glass. One of them
is indirect and consists in calculating first the Fresnel losses at normal incidence from a
recorded absorption/transmission spectrum through the following relation:
0
02
11
1n1nR
TT
where R is the reflection losses (light is reflected at both input and output interfaces)
and T0 is the maximum transmission at a given wavelength. However, other direct and
more accurate methods of refractive index measurement are usually preferred: the
ellipsometry and the prism coupling techniques for instance.
1.1.2.4 Optical Non Linearity
Chalcogenide glasses are recognized for their excellent nonlinear optical properties
which overcome those of all the known vitreous materials [18]. When a material is
exposed to low intense light, its response is linear with the electromagnetic field, E
, the
polarization P
of the material is thus expressed by:
P
= (1) E
When exposed to an intense electromagnetic beam, its response becomes nonlinear
and its polarization is then expressed by:
10
P
= (1) E
+ (2) E
. E
+ (3) E
. E
. E
+ …
Where (1) is the linear susceptibility (first order) while
(2) and (3) are the second
and third order susceptibilities, respectively (susceptibilities (n) are tensors of n+1 rank).
The coefficients decrease then rapidly for higher order terms. The importance of the
second and third order terms depend on the field strength E
.
The polarization P
originates from the creation of dipoles due to the displacement of
the negative and positives charges in a system (e.g. molecule), under the action of the
electric field E. When the electric field E
is weak, this charges displacement is weak, the
dipole thus oscillates harmonically, and the polarization of the material is linear. On the
other hand, when the electric field E is strong, this charge displacement is strong, the
dipole oscillation becomes anharmonic, and the material polarization is nonlinear.
The second order nonlinear properties exist only in non-centro-symmetric materials.
When an inversion center exists for the considered material, the components of the tensor
related to the second order susceptibility cancel each other by symmetry. Therefore, in
centro-symmetric materials like glasses, there is no second order nonlinear effect.
However, it is possible to break the glass isotropy by different glass treatments, called
poling. For example, the thermal poling consists in heating the glass while applying an
electric field in order to generate a permanent polarization of the glass, allowing the
observation of second order nonlinear effects such as the second harmonic generation, the
Pockels effect, the frequency addition, or the optical rectification.
The third order nonlinear properties are present in centro-symmetric materials and
more specifically in glasses. Among the different optical nonlinear phenomena, one can
cite the third harmonic generation, the frequency addition, the four-wave mixing or the
optical Kerr effect. The latter is particularly utilized in telecommunications for signal
processing.
As mentioned in the previous section, ChG exhibit high linear and non-linear
refractive indices. Susceptibilities (3) of about two orders of magnitude higher to that of
silica were measured in ChG [18]. Non-linear refractive index measured in silica is about
⁄ at 1.06 µm [19]. In arsenic sulfide As2S3 glass, this value is about 80
times higher and incorporation of Se to As-S composition increases it by about 400 times
11
[20]. This is explained by the presence of covalent, highly polarizable homopolar Se–Se
bonds in the glass structure as identified by Raman spectroscopy. The measurements of
non-linear optical properties of Ge-Se-As ChG glasses reveal that the substitution of
germanium (Ge) for arsenic (As) reinforces the non-linearity while replacing selenium
(Se) by arsenic (As) does not [21].
1.1.3 Thermal Properties of Chalcogenide Glasses
Chalcogenide glasses are defined as soft materials because their characteristic
temperatures are lower compared to those of the traditional silicate based glasses. These
temperatures, i.e. the glass transition temperature Tg, the crystallization temperature Tx
and the melting temperature Tm are defined in the Appendix A3. For instance, the glass
transition temperature Tg of fused silica glass is around 1100˚C. In comparison, the Tg of
arsenic sulfide As2S3 glass is around 180˚C. The network of vitreous As-S is known to be
two-dimensional. Addition of germanium to the As-S matrix results in a network
reticulation, giving rise to a three-dimensional network and thus induced an increase in Tg
to about 300-350˚C, depending on the glass composition. Germanium sulfide based
glasses usually have Tg around 300-400˚C, while GaLaS glasses have the highest
characteristic temperatures among the ChG (Tg around 450-550˚C). Selenium-based
glasses have lower characteristic temperatures than sulfur-based ones, due to lower bond
energies and average bond strength. Tellurium-based glasses have even lower
characteristic temperatures than selenium-based ones, for the same reason. Therefore, the
thermal behavior of a glass is essentially related to its chemical composition and its
network reticulation, through the energy of chemical bonds forming the network.
Assessing thermal characteristics of a glass, especially chalcogenide glass, is thus crucial
not only for its preparation processing but also to understand how its structure evolves
with the chemical composition.
Glasses with low tendency to crystallize, i.e. with a large difference between the
onset crystallization and glass transition temperatures (see in Appendix A.3), are in
general required to produce large bulk samples and optical fibers. However, glasses with
increased crystallization ability may also find application in phase change optical
memory for instance. The critical cooling rate is also an important key factor for glass-
12
forming, particularly for thin film fabrication. If the glass melt is cooled, or quenched, at
an insufficient cooling rate, crystallization will occur. The fabrication of large
chalcogenide glass bulks or preform for optical fiber fabrication is usually restricted to a
certain range of glass compositions because of their critical cooling rate. In thin film
fabrication processing, very high cooling rate can be achieved, extending thus the glass-
formation domain of a vitreous system, as will be discussed in the Appendix A.1-A.2.
Different thermal analysis techniques can be employed to characterize ChG, such as
differential scanning calorimetry (DSC) used in this work (and described in Appendix
A.3), thermogravimetric (TG) analysis, thermomechanical (TMA) analysis, etc. These
techniques permit to probe one property, such as sample heat capacity, weight loss or
expansion as a function of temperature under a controlled heating ramp. As briefly
mentioned previously, they provide important information not only for optical fiber
drawing or precision molding experiments for example, but also for the understanding of
glass structure and its related behavior. In the same way, the viscosity of ChG is of major
interest for glass processing as for any other glass and is assessed through different
thermal analysis techniques depending on the range of viscosity (which is of more than
14 orders of magnitude) of interest.
1.1.4 The Ge-As-S Glass System
Arsenic sulfide As-S glasses have been widely investigated over the past sixty years
and are still the subject of many studies besides their current practical utilization for mid-
infrared optical glasses and fibers. As can be seen in Figure 1.7, the vitreous domain of
the As-S binary system is large around the stoichiometric composition As2S3 (= As40S60,
at.%), allowing the fabrication of many possible compositions with sulfur S content
ranging from about 55 to 100 at.% (and arsenic (As) content ranging from 45 to 0%,
respectively). Glassy arsenic sulfide is also obtained in a second compositional range,
around 40 at.% of S (60 at.% of As), while crystallization and/or phase separation is
observed below 35% and between 45 and 55% of S. Likewise, such extended vitreous
domain can be an advantage to control the glass optical and/or thermal properties by
simple adjustment of its elements concentration.
13
Addition of Germanium (Ge) to the As-S system gives rise to a ternary system whose
domain of glass-forming ability may also be particularly vast with a large range of non-
stoichiometric compositions, as shown in Figure 1.7. Numerous Ge-As-S compositions
different from the stoichiometric GeS2-As2S3 and Ge2S3-As2S3 pseudo-binary vitreous
compounds (red dot lines in Figure 1.7) may form glass. Unlike other glasses like those
based on oxides or halides, the preparation of non-stoichiometric composition of Ge-As-S
system (with an excess or default of S anions) is possible as the preparation can be
realized from each single element separately. This provides a unique flexibility to finely
tune glass properties such as optical band gap, linear or nonlinear refractive indices,
photo-sensitivity, etc.
However, the vitreous domains reported in the literature for the Ge-As-S ternary
system are often inconsistent owing to the use of different conditions for glass synthesis
(e.g. cooling rate) or unconsidered influence of impurities [22]. Figure 1.7 shows the
vitreous domain recently reported by Musgraves and his colleagues [23]. To facilitate the
reading of this diagram, three regions within the vitreous domain can be distinguished: (i)
first, the S-rich region which contains compositions with S concentration higher than
~66.7 at.% (corresponding to left corner of the diagram in Figure 1.7); (ii) second, a S-
poor region, identified in blue and subdivided in three sub regions (labeled A, B, C)
which will be described later and; (iii) third, an intermediate region localized between the
last two, with S content comprised between 66.7 to about 40 at.%.
15
leading to the reticulation of the glass network (3D) [24]. Thus, some related glass
properties (like glass characteristic temperatures or chemical stability) are greatly
improved, making the Ge-As-S glasses promising materials for optical, nonlinear optical,
optoelectronic and photonic applications [25, 26].
Figure 1.8: Structure models of (a) three-dimensional continuous random network like fused silica SiO2 or chalcogenide glass GeS2 and (b) two-dimensional distorted layers in As2S3 chalcogenide glass, from [27].
The intermediate region, comprised between the GeS2-As2S3 tie line and S = 40 at.%
line (left-border of the third region, in blue in Figure 1.7) is then considered as sulfur
deficient (except for the GeS2-As2S3 tie line). A reduction up to a complete disappearance
of the sulfur rings or small chains from the glass structure is observed in this region [23].
Hereby, with the progressive transition from the GeS2-As2S3 tie line to the opposite
arbitrary limit, an increase of the content of Ge-Ge and As-As homopolar bonds can be
observed. This is due to the fact that Ge cation (whose expected coordination number is
four) and As cation (whose expected coordination number is three) do not find enough S
anions (whose expected coordination number is two) to form heteropolar bonds.
Last, the S-poor region can be seen here as a highly sulfur deficient compositional
range. This domain includes glass compositions with less than 40 at.% of S, while 66.7
at.% are theoretically required to satisfy the glass stoichiometry. An increase of content
of the Ge-Ge and As-As homopolar to the detriment of heteropolar Ge-S/As-S ones is
therefore expected and experimentally shown by Raman spectroscopic studies [23]. To
16
further describe the structural modifications which accompany these high sulfur
deficiencies, one has divided this domain into three sub regions:
- in the region A: the As content is relatively low (≤ 20 at.%) while the Ge content is
more than two times higher (≥40 at.%);
- in the region B: the As and Ge contents are relatively close one to each other,
ranging from about 20 to 50 at.%, approximately;
- in the region C: the As content is relatively high (≥40 at.%) while the Ge content is
two times lower (≤ 20 at.%).
According to this sub division, one can reasonably assume that decreasing the S
content further would induce the formation of Ge-As heteropolar bonds at some point,
especially in the sub region B. Whereas in regions A and C, the probability to encounter
Ge-As bonds might be lower due to the cations ratios Ge/As and As/Ge close to two,
respectively. However, despite the fact that the existence of Ge-As heteropolar bonds is
commonly accepted and even considered as glass-forming with regard to the extended
vitreous domain observed for Ge-As-S, there is still no direct and absolute evidence of
this, as discussed in the literature [22, 23].
Although the description given here for the Ge-As-S glass structure seems rather
simple, numerous intricacies were also observed through the different structural
investigations carried out by research groups over the world. And even if this glass
system was first reported in 1966 by Myuller et al. [24], structural studies are still
currently reported in the literature, e.g. in 2014 by Musgraves et al. [23]. Besides the
traditional Raman spectroscopic studies, powerful characterization tools like the
Extended X-ray Absorption Fine Structure (EXAFS) technique at the germanium Ge and
arsenic (As) K-edges were also employed [28] to show that the covalent network
structure of the Ge-As-S glasses with stoichiometric and S-excess compositions are built
up with corner-sharing Ge(S1/2)4 tetrahedra and As(S1/2)3 trigonal pyramids. In the S-
deficient glasses, the Ge and As EXAFS studies carried out by Aitken et al. show that As-
As bonds are preferentially encountered at lower S deficiencies while Ge-Ge bonds take
place at higher levels of S deficiency [29]. Raman spectroscopic studies have also shown
that As-As bonding at intermediate levels of S deficiency leads to the formation of
discrete As4S3 and As4S4 based molecular clusters especially in the As-rich region (close
17
to As4S4 in Figure 1.7) [23, 30]. It is also worth mentioning that in S-excess glasses, Ge–
Ge, As–As or Ge–As bonds are not observed.
Glass networks can be generically classified in terms of their elastic response into
floppy, intermediate and stress-rigid phases, as proposed by the topological model by
Philips and Thorpe [31] which is based on the average coordination number < r > of the
glass. The average coordination number < r > of a network provides a measure of its
constraint. Here, for Gex-Asy-S1-x-y glasses, we assume that the coordination number of
Ge is 4, that of As is 3, and that of S is 2; the average coordination number < r > is thus
given by < r > = 4xGe + 3yAs + 2(1-x-y)S where x, y are the respective relative atomic
fraction for Ge and As. According to Philips and Thorpe, floppy glasses have < r > below
2.4, stressed-rigid glasses (i.e. with over-constrained network) have < r > above 2.4 and,
intermediate glasses have < r > equal to 2.4 (i.e. stress-free).
Hundreds of scientific papers have been published on Ge-As-S glasses and thin films
fabrication, properties and structural characterizations. While a great number of them
have been dedicated to the elucidation of the network structure as a function of the glass
temperature, another large part has been devoted to their unique photo-sensitivity. ChG
can indeed undergo significant modifications of their structure and related properties
under specific light irradiation. These photoinduced phenomena reported in bulk and thin
film ChG constitute the topic of the next section of this chapter.
1.2 Photoinduced Phenomena in Chalcogenide Glasses
and Thin Films
In this section, we will focus our attention on different aspects of metastable
photoinduced changes that can occur in ChG. The photoinduced phenomena in ChG
glasses and thin films were the main topic of many published book chapters and books [8,
32].
18
Brief History
The phenomenon of photoinduced anisotropy was first discovered by Weighert in
1920 [33]. Later, in 1956, were observed the first paramagnetic defects (electron and hole
resonance) generated in silica by laser irradiation [34]. Then, in the 1960’s, the
mechanisms of radiation-induced compaction of vitreous silica were studied, based on the
weakening or rupture of a Si-O bond, making the oxygen volume smaller [35]. About ten
years later, the Russian research group of Zhdanov and Tikhomirov has examined the
quasi-stable optical and electronic anisotropy in chalcogenide amorphous thin films [36]
and bulk glasses [37] by irradiation with polarized light. Meanwhile, photoinduced
refractive-index changes in germanium doped optical waveguides were studied [38].
These modifications were used later in optical fiber telecommunication.
Separately, Kostyshin showed in 1966 the phenomenon of photosensitivity in oxide
glasses when brought into contact with a metal [39]. Shimuzu et al. extended this study to
the metal/chalcogenide systems [40]. They reported light-stimulated metal ions (Ag+,
Cu2+) diffusion in Ge-Se and As-S systems (photo-doping). This phenomenon has been
used to develop some applications such as holography, photo-lithography and photo-
resistivity [41].
The structural photoinduced effects were first observed in the 1970’s with the study
of the photodecomposition (photodissociation) of As2S3/As2Se3 thin films, which
revealed high absorption coefficients, reflectivity change, and oxidation process [42]. At
the same time, the opto-thermal structural changes of amorphous semiconductors have
been examined by the process of high-speed crystallization [43]. The high-speed light-
induced transition and reversibility between amorphous and crystalline phases have been
used later for the CD-RW and DVD±RW technology [44]. From the late 90’s and up to
date, the effects induced by pulsed femtosecond or nanosecond lasers, like micro-
machining or Bragg gratings inscription are attracting a large interest for their large
potential applications in optics and photonics [45].
The interaction of such fs-laser with glassy matter is indeed currently opening a wide
variety of optic and photonic applications [46]. In the following section, we will focus
our discussion on the photoinduced effects obtained through continuous wave (CW) laser
19
beam ChG irradiation with an emphasis to the binary As-S and ternary Ge-As-S systems,
the latter being the system of interest for this thesis.
1.2.1 General Classification of Photoinduced modification in
Chalcogenide Glasses
A classification of the photo-structural modifications may be defined in terms of
reversibility, i.e. reversible and permanent effects, or in terms of the character of the
change, i.e. scalar and vector changes (see Table 1.1 (a)).
The reversibility character of any photoinduced change in a glassy material can be
defined as its ability to recover its initial state, i.e. before irradiation, at temperatures
below the glass transition temperature. On the contrary, when a photoinduced change is
permanent and can only be erased by heating the glassy material above its glass transition
temperature (and even higher than its melting temperature), enabling therefore the atomic
mobility within the vitreous network, one considers the effect as permanent.
Nevertheless, in many cases, the photoinduced changes observed in ChG exhibit a partial
reversibility character, i.e. the glassy material is not able to completely recover its initial
state after an exposition-erasing cycle.
Besides the reversibility/irreversibility character of the photoinduced changes, their
scalar/vector character has also been proposed to help in distinguishing them.
Photoinduced effects are qualified as vector when they depend on the polarization state of
the inducing light, e.g. dichroism or birefringence [47, 48]. On the other hand, scalar
photoinduced changes do not depend on the light polarization, e.g. photo-darkening or
photo-refraction [49, 50]. The scalar modification of a sample does not imply a
reorganization of its structure along with some preferential direction provided by laser
illumination. These scalar and vector modifications can exhibit both reversible and
permanent behaviors depending on the mechanism activated during the photoexcitation.
Some examples are presented in the Table 1.1 (b) to classify the reversibility of
photoinduced modifications.
20
Table 1.1: Classification of the main photoinduced modifications
a- Photoinduced changes classification in terms of recovery and effect of light.
b- Classification of the scalar and vector modifications.
Scalar photoinduced effects
Reversible Permanent
- photo-darkening [49, 50] (e.g. color centers).
-photo-refraction [51] (e.g. transfor- mation of basic structural entities) - photo-crystallization [52] (e.g.
optical memory recording). -photo-expansion/contraction [53] (e.g. structural flexibility).
- photo-darkening [49, 50] (e.g. structural modification). -photo-bleaching [54] (e.g. band conversion, oxidation). - photo-expansion [55] (e.g. incoroporation of oxygen in the glass structure). - photo-refraction [56] (e.g. pulsed laser deposited thin films).
Vector photoinduced effects
Reversible Permanent
- w/o - dichroism [47, 48, 57] (e.g. optical switches, optical memories). - birefringence [47, 48, 58] (e.g. nanogratings).
Photoinduced Changes
Ability to recover
Permanent
Metastable Temporary
Scalar Vector Reversible
Permanent: can’t be recovered without remelting the glass Metastable: recovered on heating to Tg Temporary: recovered on removing the light Scalar: doesn’t depend upon the polarization of the irradiation light Vector: depends upon the polarization of the irradiation light
Effet of light polarization
21
1.2.2 Rheological/Mechanical Photoinduced Effects
Effects of light irradiation on several glass properties such as viscosity, glass
transition temperature, elastic constants, micro-hardness, stress relaxation, surface
morphology etc., have been investigated. Among the rheological changes reported in
ChG materials, one can cite the athermal light-induced plasticity, which consists in a
micro-hardness decrease owing to chemical bonds weakening. The group of Trunov and
Bilanich has widely studied the dynamic changes in ChG film plasticity under
illumination with band gap light [59]. They reported a photoinduced transition from
vitreous to plastic state, known as the photo-plastic effect [60]. Similarly, the photo-
viscous effect corresponds to a decrease of glass viscosity under light illumination, as
discovered by Nemilov et al. [61].
The photoinduced fluidity (PIF) effect consists in a permanent deformation
(macroscopic shape change) of a ChG (in the form of fiber or flake) [62]. The authors
have shown the PIF effect achieved in an As2S3 glass through the combined effect of sub-
band gap light illumination and application of an external mechanical stress. Moreover, it
should be noted that PIF is prevented at higher temperatures. The observed macroscopic
changes, e.g. fiber elongation, imply that PIF is a cumulative effect of structural
rearrangements occurring at the molecular level, among the intramolecular (covalent,
strong) and/or intermolecular (van der Waals, weak) bonds. Yannopoulos et al. have
suggested that such transformation in As2S3 is based on the breakage of As-As bonds,
resulting in suspended As atoms at the ends of As4S4 molecules, which can then bond
with excess S atoms residing in the interstices of the structure. The cage-like As4S4
molecules are therefore transformed into a planar-like configuration that is more
favorable for network polymerization [63]. Hence, in the PIF effect, the presence of
homopolar As-As bonds might be one of the possible factors responsible for the glass
structure destabilization. The simultaneous action of light illumination and material
stretching revealing fiber elongation, can also be called the photoinduced ductility
effect [64].
On the other hand, opposite effects to the PIF were also reported: the so-called
athermal photo-hardening of ChG, where the formation of additional intermolecular
22
bonds takes place, transforming the photo-processed structure to a more rigid
configuration. The glass is thus strengthened upon illumination [65]. Generally, the glass
softening (PIF) is observed for ChG film compositions with germanium (Ge) content
smaller than those of arsenic (As) or sulfur (S), while glass hardening is observed in film
containing more germanium. In both cases, a significant release of sulfur occurs, but in
the first case this will shift the glass structure toward a molecular As4S4 structure, while
in the second case this will induce an increase of 3D connections of the glass network
[66].
Another vector phenomenon called the photoinduced opto-mechanical effect also
reveals an increase or decrease in plasticity of ChG thin films upon light irradiation. This
arises from the atomic structural changes which depend on the light polarization [67]. In
this effect, a reversible anisotropic volume change may be induced, i.e. a contraction
occurring along the direction of the electrical field vector and an expansion perpendicular
to that direction.
Finally, one can also cite the anisotropic surface corrugation effect observed in
ChG As-S thin films upon irradiation by linearly polarized light. Narrow fringes and
streaks on the surface parallel and perpendicular to the electrical field of light were
observed [68].
Later in the text the term ‘wrong’ bond may be used to qualify homopolar bonds like
As-As in ChG materials. The term ‘wrong’ is employed because such type of bond
between two similar chemical species, e.g. As-As, S-S or Ge-Ge, is not energetically
favorable and theoretically not expected.
1.2.3 Structural Photoinduced Effects
Among the most known photoinduced structural modifications in ChG materials
upon band gap or sub band gap light irradiation, one can cite the photo-contraction and
photo-expansion effects, which correspond to a material local volume change. First, an
permanent photo-contraction (also called photo-densification) of ~1% was observed in
As2S3 films upon band gap illumination [69]. Then, the photo-expansion of
chalcogenide As2S3 thin films was first reported by Hamanaka and his colleagues [70].
They reported a photoinduced volume expansion of ~0.5% in ChG bulk and thin films,
23
and at the same time they observed the effect recovery upon annealing near its glass-
transition temperature, ~ 200 °C. The observed increase of thickness was explained by
heteropolar to homopolar bond conversion under light illumination, since the homopolar
bonds (2.57 Å) are longer than the heteropolar bonds (2.24 Å)[71]. These contraction and
expansion effects can produce micro-scale modifications. The macroscopic expansion of
chalcogenide glass was also reported, together with a photo-darkening (PD) effect upon
band gap illumination, in addition the expansion could be recovered with annealing [72].
However, parallel studies on PD and photo-expansion have demonstrated that there is no
direct correlation between both effects. The latter increases earlier than PD under band
gap light irradiation, while under sub band gap light irradiation, photo-expansion
increases more gradually than PD [73]. Another research group explained the effect of
photo-expansion by the presence of free carriers and suggested that photo-structural
changes are caused by Coulomb repulsion of crystal-like fragments of the glass due to
different mobility of electrons and holes [74]. They showed that upon photo-excitation,
the thickness of As2S3 films rapidly increased, reaching a maximum before slowly
decreasing with time. This behavior is similar to the degradation of photocurrent in
amorphous chalcogenides which suggests that the presence of free carriers is essential for
the photo-expansion effect [75]. In general, photo-darkening and photo-expansion were
observed in As-based ChG while photo-bleaching and photo-contraction were observed
in Ge-based ChG [54]. The role of oxygen in the photo-expansion effect of As-free Ge-
Ga-S ChG bulk and thin films has also been demonstrated [55, 76].
Another similar photoinduced structural changes in ChG thin films are the athermal
photoinduced transformation and photoinduced amorphization effects [77]. These
effects reveal the amorphization of films previously crystallized by annealing, similarly
to the Ge2Sb2Te5 phase-change materials used in DVD±RW technology [44]. The photo-
amorphization of As-Se thin films, with As4Se4 quasispherical molecules structure, is
based on the irradiation of strained homopolar (‘wrong’) As-As bonds [77]. Moreover, it
is important to mention that in this As-Se system, a slight structural difference was
observed between the as-evaporated and photo-amorphized films.
In addition, another well-known structural photoinduced effect observed in ChG thin
films is the photo-crystallization effect [52]. In this work, the material was exposed
24
simultaneously by two sources of laser with parallel polarizations and with band gap and
sub band gap photon energy, respectively. The authors also examined the suppression of
the photo-crystallization process, revealing that the process is more efficient by exposing
only one of the above mentioned sources, because while the sub band gap light creates
nuclei with a certain optical axis, the band gap light breaks them.
1.2.4 Chemical Photoinduced Effects
Modifications of the material chemical properties upon light irradiation have also
been reported. Among them, one can cite the photo-amplified oxidation effect,
consisting in a photo-oxidation, as reported in Ge-Se and As-S amorphous ChG thin films
[78]. In the photo-oxidation process of Ge-rich ChG films, the glassy network can be
destroyed, resulting in an permanent photo-bleaching (PB) effect. It is observed that
photo-oxidation effect is usually easier in as-prepared than in annealed thin films.
Photo-dissolution and photo-doping effects which describe the unique properties
of metal dissolution (on the surface) or diffusion (deeper in the material volume) into
ChG layers under light illumination were first reported by Russian scientists in 1966 [79].
A more general consideration of photo-doping is a solid-state reaction, in presence of
light, between the metal layer and the ChG material. This heterogeneous reaction is
characterized by a high complexity of participating electron and ion transfer and transport
processes [80]. Interdiffusion effects between layers in multilayer thin films under light
irradiation have also been reported [81].
Last, permanent photoinduced changes as photo-evaporation [43], photo-
decomposition [82] or photo-polymerization [83] were also reported in the literature on
as-evaporated and even on annealed ChG thin films.
1.2.5 Optical Photoinduced Effects
1.2.5.1.Photo-darkening (PD) and Photo-bleaching (PB)
Among the scalar photoinduced modifications in ChG, the most important are the
photo-darkening (PD) and photo-bleaching (PB) effects. The photo-darkening effect is
an optical darkening of the light-exposed material and is generally permanent. However,
25
the PD effect also usually appears along with reversible modifications of other properties,
such as hardness, density, glass transition temperature, elastic constants or dissolution
rate.
The first observation of PD effect was made over 40 years ago by Berkes et al. who
observed a dark area appearance on a 1 μm thick film of As2Se3 exposed with linearly
polarized light [42]. The Figure 1.9 below shows similar photo-darkened spots (the new
state called darkened state due to the lower transparency of the film).
Figure 1.9: As2Se3 thin film sample with photo-darkened spots indicated by the arrows, from [84].
Later, De Neufville et al. also reported the PD phenomenon by irradiating As2S3 thin
films with band gap light (Eg~2.4 eV). It is assumed that the PD is caused by a decrease
of the optical band gap energy of the material, in other words a red-shift of the glass cut-
off wavelength (absorption edge) [85]. Practically, the PD is observed in amorphous
films thinner than 10-20 μm because this effect increases the absorption coefficient for
the light of wavelength corresponding to the optical band gap and, consequently, the
effective depth of light penetration in the sample is diminished [86].
It is important to mention that the structure disorder plays an important role on this
phenomenon as only amorphous materials and not crystals display this effect.
Nevertheless, the amorphous character of a material is not enough to observe the PD
effect. For example, Kolobov et al. pointed out the essential role of the chalcogen
element, in particular their lone pair electrons [87]. They examined annealed and photo-
darkened states of amorphous As-Se thin films, and by using X-ray Photoemission
Spectroscopy (XPS), showed a shift towards lower energies (by 0.2 eV) in the valence
band (see Figure 1.10) which is mostly formed by the lone pair electrons of the
chalcogenide elements. In addition, they assumed that the conduction band was not
responsible for the PD effect because the valence band edge shift was of the same
26
magnitude as the decrease in the optical band gap. The chalcogens themselves, like
amorphous selenium, show such phenomena with specific features [88]. The
hydrogenated As-S and As-Se films also show PD [89].
Figure 1.10: Reversible change in the density of state of the valence band of amorphous As2Se3 thin film, from [90].
Many compositional studies on PD have been reported so far. As shown in Figure
1.11 for elemental and stoichiometric chalcogenide alloys, the sulfide shows the greatest
change, followed by the selenide, while the telluride shows the smallest one [91]. Hence,
a sulfide glass should be preferred to a selenide or telluride glass to obtain enhanced
optical photoinduced changes.
Figure 1.11: Photo-darkening E in ChG as a function of
⁄ where Ti is the irradiation
temperature and Tg is the glass transition temperature. Measurements of E were performed at temperatures below Ti, from [91].
27
In non-stoichiometric binary As(Ge)-S(Se) glasses, the PD tends to become maximal
for the compositions with average coordination number Z of 2.67 [92]. For ternary
systems, however, it is strongly dependent on the system and glass compositions. For
example, several researchers have demonstrated that PD in Ge-As-S thin films (with Z =
2.3–3.0) induced by band gap illumination shows a maximum at the composition with Z
≈ 2.67 [93]. By contrast, in Ge-As-Se thin films (with Z = 2.2–2.6), it has been shown
that the PD and photo-expansion effects induced by sub band gap illumination become
smaller at Z ≥ 2.45 [94]. The reasons for this contradiction could be the difference of
determination way of PD in these two works, which is defined in the first one by means
of shift of the absorption edges and in the other one as the transmittance decrease at sub-
photon energies.
The opposite effect of PD, the photo-bleaching (PB) effect, can occur in ChG under
illumination of sub band gap light. PB corresponds to a whitening of the material upon
light illumination and is due to an increase of its optical band gap, resulting in a blue-shift
of the absorption edge. Once again, these PB effects are strongly dependent on the
material composition and preparation method. Typically, PB is observed in Ge-based
ChG, specifically in evaporated Ge-S thin films, where the oxidation state of germanium
atoms was shown to play a role [95, 96]. It is also worth mentioning that thermal
enhancement of PB (by annealing) usually requires higher temperatures than those
required to favor PD [97]. In some cases, e.g. in As2Se3 thin films, the PB effect is
characterized by a thermal threshold which means that the photo-bleaching can be
observed only above a given temperature called the optical bleaching threshold, that is
lower than the thermal bleaching temperature [97].
It may be then envisaged that selected compositions can exhibit both PD and PB
effects, erasing the photoinduced optical changes. Moreover, illumination in vacuum
tends to suppress the PB effect, and its thermal reversibility has rarely been
demonstrated. PB in bulk glasses is very exceptional, which implies that almost of all PB
is caused by permanent structural changes such as homopolar to heteropolar bond
conversion, oxidation [98, 99].
Both the PD and PB effects are classified as an permanent effect, which means that
the shift of the absorption edge of as-deposited ChG thin films is not reversible thermally.
28
Actually, these effects are partially reversible, as presented in Figure 1.12 which
illustrates the shift of the absorption edge of a As2S3 thin film upon irradiation of band
gap light [100]. The black solid line corresponds to the absorption edge of the as-
deposited As2S3 thin film while the blue line presents the absorption edge of the same
sample after irradiation, showing a clear shift toward lower energies (longer wavelengths)
or in other words, a PD effect. The red line in Figure 1.12 shows the absorption edge of
the material after thermal annealing close to its glass transition temperature. One can
notice a blue-shift of the edge, corresponding thus to a partial recovery toward the
material as-deposited state after thermal annealing. Nonetheless, one can also notice that
the recovery is not complete. Successive irradiation/annealing cycles were then applied to
the thin film, as identified by the numbers 2 to 7 in Figure 1.12, showing an excellent
reproducibility of the phenomenon, taking into account that it will never completely
recover.
Figure 1.12: Observation of the photodarkening effect in amorphous As2S3 thin film and its partial recovery after thermal annealing. The black solid line is the band edge of the as-deposited film, the blue line is the band edge of the irradiated film and the red one is the band edge of the irradiated film after thermal annealing. The numbers 1 to 7 represent the successive cycles of irradiation/annealing applied to the As2S3 thin film, from [100].
29
During the irradiation or annealing process, it is believed that ‘wrong’ homopolar
bonds (As–As and S-S) are broken and form more energetically favorable heteropolar
bonds (As-S), resulting in a more chemically ordered glass network.
1.2.5.2 Photoinduced Optical Anisotropy
Glassy amorphous materials are, by definition, an isotropic material, implying that
their properties will not depend on any crystal orientation, as can be observed in
anisotropic crystalline materials. Among the known glassy materials, the ChG are unique
in the sense that they can become anisotropic upon light illumination. This effect, the
photoinduced optical anisotropy, constitutes the main photoinduced vector phenomenon
and can be divided in two types, namely the dichroism and birefringence. Other
examples of photoinduced optical anisotropy also exist, such as the induced difference of
photoluminescence intensity or the difference in the fine structure of the X-ray absorption
edge for polarizations of the control light beam parallel and perpendicular to the direction
defined by the polarization of the beam used in PD. Photoinduced anisotropy is a totally
optically reversible effect as it depends only on the polarization orientation of the
inducing light source [58]. The optical axes defining the anisotropy can be rotated by
changing the polarization direction of the irradiation light [101].
Dichroism (or photo-dichroism, PID) is the dependence of the material absorption
with the propagation direction of light [102]. The electric field component of an incident
light wave that is parallel to the transmission axis will pass throughout with relatively
smaller attenuation, whereas light with polarizations in all other directions will be
attenuated.
Birefringence (or photo-birefringence, PIB) refers to the refractive index
dependence on the polarization state and the direction of the propagating beam [102]. A
consequence of birefringence is that any unpolarized light entering in such a material is
broken into two different rays with different polarizations and different propagating
velocities. The result is a phase difference between two orthogonal polarizations.
Contrariwise there is no phase change for the beam propagated along the optical axis.
The effects of photoinduced anisotropy in ChG thin films achieved by light
irradiation, i.e. PID and PIB, were observed by many research groups [36, 50]. Linearly
30
polarized light mainly acts to break or weaken covalent bonds, resulting in the
appearance of a selected direction (optical axis) in the film. These observations were
generally reported in simple binary ChG systems with large photosensitivity, like in
arsenic sulfide (As-S) [103, 104], but also in more complex system and compositions
[105, 106]. Among all these observations on ChG thin films, maximum values for the
PID effect of 10-15% were reported in as-prepared Ge-As-S films (while for the
elementary or binary ChG films, such value varies between 1.5% and 2.5%) [105];
whereas the maximum reported values for PIB were n ≈ 0.002 in As2S3 thin films [107].
Both dichroism and birefringence anisotropies can be induced by either linearly or
circularly polarized light. Fritzsche showed in his theoretical work that even with an
unpolarized beam irradiation, it is possible to establish anisotropy [108]. This theory was
later experimentally tested and validated by Tikhomirov and Elliott [109]. Both effects
(PID and PIB) can occur simultaneously from the same inducing source [36, 110].
However, the obtained experimental results showed the total independence of these two
photoinduced phenomena by means of their characteristics including kinetics,
temperature and spectral excitation dependency.
Furthermore, it is important to mention that these photoinduced vector effects are
completely independent from the scalar effects [111]. In their work, Lyubin and
Tikhomirov demonstrated the simultaneous measurements of the buildup and destruction
or reorientation of PID along with photo-darkening. The PID can be erased by circularly
polarized or unpolarized light [36, 111], likewise by thermal annealing. Besides, to
achieve a complete disappearance of the photoinduced effects, much lower temperatures
are required to erase the vector effects (frequently, they can relax even at room
temperature) than to erase the scalar ones [110]. Then the spectral dependencies are also
different for photoinduced vector and scalar effects. On one hand, vector effects are more
efficiently induced by irradiation of sub band gap light (high energy photons). On the
other hand, scalar effects are more efficiently induced by band gap light (low energy
photons) [112]. Finally, this study by Lyubin and Tikhomirov has shown in some
chalcogenide compounds the presence of great photoinduced dichroism, without any
evidence of scalar effect [37, 111].
31
Several other photoinduced anisotropy phenomena have also been reported in the
literature. For instance, the photoinduced girotropy effect, which consists in
photoinduced optical activity (photoinduced circular birefringence) and photoinduced
ellipticity (photoinduced circular dichroism), was reported by the same authors, Lyubin
and Tikhomirov [58]. The elastically deformed As2S3 glasses show the photo-elastic
birefringence and dichroism effects [68, 113]. These latter anisotropies generated by
uniaxial compression reveal the optical absorption edge shifts to lower photon energies
(red-shift) and some isotropic optical modifications, which can be relaxed by illumination
and annealing. In the same experiments, the authors have revealed another strong
photoinduced effect of ChG glasses, the photoinduced scattering of sub band gap light
irradiation. This appears through the change of the form of the transmitted beam
(diffusion) and also the decrease of transmitted light intensity. This photoinduced
scattering effect was also observed in As-based ChG microlenses which have been used
to focus a laser beam through an optical fiber [114]. Another effect is the anisotropy of
transmittance in which the ChG glass induces a rotation of the polarization of incident
laser beam. The optical activity was explained in the case of As2S3 glass by the natural
consequence of the asymmetric molecular structure, therefore with anisotropy of the
polarizability tensor [115]. In some other ChG compositions, particularly in GeSe2 thin
films, the transmittance oscillation effect was observed, revealing a periodic oscillation
in time of transmittance and reflectance under irradiation with a CW laser [68].
1.2.6 Existing Models Describing the Photoinduced Anisotropy
At least ten models have been proposed in the literature to describe the involved
mechanism(s) in reversible photoinduced changes [90, 116, 117], but none of them has
founded a principal mechanism until now. The first reason is simply because the structure
of the glassy materials is not fully determined and understood up to date. To investigate
the glass structure at the short range order, the radial distribution and Extended X-ray
Absorption Fine Structure (EXAFS) analyses have been carried out, showing that the
structures before and after illumination are similar, although some changes are detected
during illumination [118]. Investigations of the photoinduced modifications can also be
performed at the medium range order but identification of defective structures (such as
32
dangling bonds) remains difficult in disordered matrices. Another reason lies on the
characteristic complexity of the photoinduced effects: many kinds of macroscopic
properties are modified by light illumination, but they are not produced by single atomic
structural changes, making difficult their complete understanding. In addition, it remains
debatable whether an intermolecular change can cause the observed modifications in
macroscopic structural (elastic and thermal) and chemical properties.
The photoinduced anisotropy was first reported in chalcogenide vitreous
semiconducting films more than 25 years ago by Zhdanov et al. [36]. Nevertheless, the
microscopic mechanisms responsible for this phenomenon are still not clearly revealed,
impeding a true and general consensus among the scientists’ community. Here, we will
summarize some of the most known models. All of them are based on microscopic
mechanisms occurring at the atomic level to describe the measureable macroscopic
effect, but there is no method to directly measure these microscopic mechanisms.
The local distortion of the structure induced by light, e.g. the change of the local
bonding, gives rise to a local anisotropy and depends on the polarization of the inducing
photon, along with some asymmetric structural units, which are sensitive to the electric
field of the inducing light polarization. In most of the proposed models for the
mechanism(s) of anisotropy, the notion of intimate valence alternation pairs (IVAPs) is
used as local structural elements. These charged defects exist naturally in ChG. First
proposed in 1990 by Lyubin and Tikhomirov [119], the notion of IVAPs was then
developed in a model based on a pnictogen (e.g. As) and chalcogen (e.g. Se) elements by
Elliot and Tikhomirov [49]. This last model is depicted in Figure 1.13 and consists of a
pyramidal structure formed by a pnictogen and three chalcogenide atoms.
33
Figure 1.13: Photoinduced anisotropy in a pnictogen–chalcogenide system before (a) and after (b) excitation. Open circles represent pnictogen atoms and solid circles chalcogens ones, from [49].
As can be seen Figure 1.13, they proposed a dual recombination where an electron is
excluded from (or a hole excluded from
) and is trapped by a (or by
,
respectively) of the same IVAP. This results in a transformation of the IVAP from
into
. Elliot and Tikhomirov suggested a two-step process: in the first step, one
photon creates a neutral center
and then the neutral centers
thermally return to stable charged centers
(Figure 1.13 (a)) or
(Figure 1.13
(b)). This results in a macroscopic change in the glass structure, where the inducing light
polarization is perpendicular to the IVAP lone pair orientation.
Besides, to explain the photoinduced anisotropies in ChG, Fritzsche proposed
another model [108, 120] which permits to observe the sign of anisotropy and also to
predict that even an unpolarized light beam can induce optical anisotropy in these
materials, as experimentally confirmed later by Tikhomirov and Elliott [109]. These
micro-volumes are based on IVAPs [108], but apart from them, all optical transitions,
interband, Urbach tail and defect transitions are polarization dependent. If the linearly
polarized (at z-axis) light propagates in the y- direction, as shown in Figure 1.14(a), then
only the polarization directions in the x- and z-axis will be absorbed.
34
Figure 1.14: The dielectric tensor after irradiation by a linear polarized light (a) and an unpolarized light (b), from [121].
The absorption coefficients are αz < αx = αy and the refractive indices nz < nx = ny. If
the irradiation made by unpolarized light propagates in the y- direction, the absorption
decreases in micro-volumes for x- and z- directions and increases for y- direction (Figure
1.14 (b)). The dielectric tensor, in this case, is an ellipsoid with the main axis in the
direction of propagation (y-axis), so αz = α x<α y and the refractive indices nz = nx < ny.
Tikhomirov and Elliott confirmed this model experimentally [109]. In their results, the
glass appeared isotropic for the light propagating at the direction of the incident beam (y-
axis of Figure 1.14 (b)), but the anisotropy was observed in the two orthogonal directions
(x- and z-axis of Figure 1.14(b)).
Asatryan et al. proposed another hypothesis based on the analysis of the excited
states in the areas of circular and linear polarizations and transitions between them in
As2S3 thin films [122]. They suggested that As2S3 involves to form As4S4 cage-like
molecules with two homopolar As-As bonds (see Figure 1.15). These bonds have greater
polarizability compared to that of strong covalent As-S bonds. Accordingly, the
polarizability tensor of the As4S4 molecule can come close to the ellipsoid with an
anisotropy axis perpendicular to the homopolar As-As bonds, which is in-plane direction
in Figure 1.15.
35
Figure 1.15: The projection of the As4S4 molecule in the parallel plane of the As-As bond, from [122].
The light excites preferably these As-As homopolar bonds parallel to the polarization
of light, resulting in the As4S4 bonds breaking and replacement of the destroyed
molecules by new As4S4 ones with homopolar As-As bonds perpendicular to the electric
vector of the light, inducing a decrease of the parallel component. However, two
competitive processes of bond breaking and bond reconstruction (with nearest molecule)
can occur simultaneously. This mechanism can explain the existence of different possible
evolution, where some regions may extend due to new neighbor structural formations. It
is also clear that these structural changes can modify the energy band structure near to the
band gap, permitting therefore to determine the shift of the absorption edge, i.e. the PD or
PB effects.
36
1.3 Objectives and Novelty
Being amorphous semiconductors, ChG exhibit a variety of photoinduced
phenomena when irradiated with light whose photon energy is comparable with the band
gap of the material, offering to these materials a wide range of technological applications
(optical data storage, telecommunications, IR optics, etc.).
The current PhD project was directed to study the photoinduced modifications of
ChG thin films, based on their optical and thermo-mechanical properties. A review of the
photoinduced anisotropies reported on these materials in the literature up to date was
presented in the first chapter. The review of the prior art on photoinduced effects on ChG,
particularly on the selected Ge-As-S ternary system, helped us to define and to emphasize
the objectives of this thesis and its originality throughout the obtained results.
To ensure the novelty of this PhD research project, the academic objectives were
defined as follows:
Realize a thorough study of the photoinduced anisotropy phenomena on the
selected ChG materials;
Determine the best composition in terms of photosensitivity (highest as possible);
Reveal the mechanism responsible for both photoinduced birefringent and
dichroitic changes in the ChG materials.
Besides, potential optical and/or photonic applications based on the new
fundamental/academic obtained results were targeted:
By recording thermally stable vector and scalar holograms in the ChG thin films;
By creating a gradual variation of the refractive index on these ChG materials
(GRIN lenses).
The first step of examination was toward the study of the band gap of chosen
chalcogenide materials, as the photoinduced phenomena usually occur under band gap
light illumination. The originality of this study resides in the revealing of some new
feature: the dependence of the ChG band gap on film thickness. The shift of the
absorption edge and the change of its slope were observed as a function of the film
thickness. The explanation of such observed phenomena is tied to localized states
37
existing in the thin films. The obtained results provide the opportunity to choose the
optimal thickness of thin film for our further studies.
The third chapter includes the study of photoinduced birefringence (PIB) of thin
films prepared with different elemental concentrations among arsenic (As), germanium
(Ge) and sulfur (S). The original systematic study carried out allowed to determine the
composition containing 30 at.% of Arsenic as the best one, i.e. exhibiting the highest PIB
value. We believe that the further diminution of the PIB observed with increasing the As
content is related to the occurrence of Ge-As heteropolar bonds owing to the high S-
deficiency in the material, as mentioned in the previous chapter.
In the fourth chapter, the examination of PIB effect by calculating its local value was
pursued, since it could be of interest for integrated optical or photonic devices where very
thin layers and controlled influence of polarization mode dispersion are usually required.
The presented results are based on the approximation that the material optical absorption
is changing non-linearly. The originality of this work resides in the highest local value of
PIB ever reported in the literature, nlocal > 0.112, which is one order of magnitude higher
than its average value.
The fifth chapter describes the photoinduced absorption changes, particularly the
value of PID (about 0.25%), observed in these Ge-As-S ChG thin films. A novel
contribution is given about the investigation of underlying mechanisms responsible for
the photoinduced changes. The implemented experiment has permitted to observe the
homopolar toward heteropolar bond conversion under laser light irradiation. In addition,
a phenomenological unipolar model is proposed to explain the non-monotonic behavior
of dynamic changes of PID. Last, the presence of photoinduced bond changes through an
energetic barrier (estimated at 14kJ/mol) is reported for the first time.
The sixth and seventh chapters explore some potential applications based on the
photoinduced changes investigated in this work. On one hand, the improved thermal
stability of the vector and scalar holograms recorded on the Ge-As-S thin films after
addition of germanium has been shown for the first time. On the other hand, the most
appropriate conditions to obtain a gradual variation of the refractive index on these
materials were defined, therefore forming new GRIN lenses.
39
Chapter 2
The Absorption Edge Study of Chalcogenide Ge-As-S
Thin Films
Glass optical quality
As presented in the previous chapter, the Ge-As-S vitreous system was preferred to
carry out our investigations not only for the large photo-sensitivity of its glasses, but also
for their larger thermal and mechanical stability compared to other ChG (e.g. As2S3),
which is important in view of potential applications. The bulk glass samples were
prepared by melting high purity starting elements (polycrystalline germanium (Ge),
arsenic (As) and sulfur (S)) in a fused silica ampoule sealed under vacuum (up to
10−3 Pa). The detailed procedure used in this work to prepare the glass samples is given
in the Appendix (section A1). Chalcogenide glass rods of about 60 mm in length and
15 mm in diameter were obtained after removal from the synthesis ampoule. Slices of
about 2 mm in thickness were then cut and polished prior to the optical characterizations.
An important quality criterion for the fabrication of optical glasses is their optical
homogeneity, i.e. the absence of any unmolten particles, inclusions, cracks, bubbles, or
even striae (which correspond to density macro-fluctuations in the material volume). The
striae are formed during the solidification (cooling) of the glass and they are related to its
capacity to dissipate efficiently the thermal energy during its cooling. Such issue is well-
known in glass technology since glassy materials usually possess very low thermal
conductivity.
Unfortunately, the bulk samples we prepared were not perfectly uniform in terms of
optical quality and contained some striae, as can be seen in Figure 2.1 which shows the
photographs of two samples, each of about 2 mm thickness, placed between two
polarizers. Figure 2.1 (a) presents a home-made glass slide of composition Ge25As30S45
visualized by means of a CCD camera and an expanded beam from a diode laser
40
operating at 980 nm. Such observation way was required because the Ge25As30S45 bulk
glass is opaque to the visible light. Significant non-uniformities are observed for this
home-made bulk sample if compared to a commercial optical window of As2S3 (from
Amorphous Materials Inc., observed with sun light in Figure 2.1 (b)). The presence of
striae, as evidenced by the transmission image in Figure 2.1 (a) is explained by the
incapacity of the material to evacuate homogeneously its thermal energy during the
quenching, as above mentioned. Such issue is well-known by glass manufacturers who
have developed solutions specifically adapted to the glass nature and its properties, like
its thermal conductivity and critical cooling rate. A further development processing might
certainly have minimized the formation of such striae in the Ge25As30S45 glass to attain
the optical uniformity of the As2S3 optical window (Figure 2.1 (b)). Nevertheless, such
process development is generally time-consuming and expected to be challenging for the
Ge25As30S45 glass if compared to the As2S3 glass. Although the latter is already a
commercially developed glass, it should be also taken into account that its critical cooling
rate is much lower than that of vitreous Ge25As30S45. In other words, the thermal energy
management during As2S3 glass cooling is much easier than that of Ge25As30S45 glass
cooling, facilitating thus the elimination of striae in the former.
Figure 2.1 Images in transmission of: a) Polished slice of home-made Ge25As30S45 glass sample (16 mm diameter); b) Commercial chalcogenide As2S3 glass window of 25 mm diameter placed between two polarizers parallel to each other.
Studying photoinduced phenomena (such as birefringence) in such non uniform bulk
samples would constitute a rather complicated task, particularly if we consider that the
typical spot diameter of laser beams on the glass surface is about 0.7 mm. Instead of
5mm
b) b
5mm
a)
41
developing the fabrication process of Ge25As30S45 bulk glasses, which was not the goal of
this thesis, as previously described, we preferred to study of the photo-sensitivity
properties of the Ge25As30S45 material in the form of optically homogeneous thin films
(with thicknesses of 3-7 μm). Usually, the ChG thin films are fabricated by thermal
evaporation or pulsed laser deposition techniques, as described in the Appendix A.2.3.
However, in the ternary Ge-As-S system, where the elements possess significantly
different evaporation temperatures, it is more advantageous to use the electron beam
evaporation method (under vacuum) to ensure a better control of the thin film
stoichiometry. The evaporation conditions used in this work are detailed in the Appendix
A.2.3.
The Figure 2.2 shows the image of one of the prepared Ge25As30S45 thin films placed
between two parallel polarizers. As we can see from this photograph, the obtained thin
film is uniform and free of striae.
Figure 2.2 Ge25As30S45 glassy thin film (of 3 µm thickness and 2.5 x 5 cm dimensions) placed between two parallel polarizers.
The vitreous character of the prepared thin films, as well as of the bulk samples, was
confirmed by the measurement of the glass transition temperature by thermal analysis
(see DSC traces presented in Appendix A3) and by the observation of halos on the X-ray
diffraction patterns (when the structure is ordered, sharp peaks corresponding to the X-
rays diffracted by the crystals are observed on the X-ray diffraction patterns).
Further characterization of these thin films were carried out by elemental analysis
technique, which was routinely utilized to verify the chemical composition of the
evaporated films vs that of the glass used as evaporation source. The electron dispersive
5mm
42
X-ray analysis technique (coupled to a scanning electron microscope, here a FEI, Quanta
3D FEG equipment) was used for this purpose. The principle of this technique is
presented in the Appendix A.4. The analyses performed on the prepared samples indicate
a very good control of the evaporation process through an excellent conservation of
stoichiometry: chemical compositions experimentally measured on the prepared thin
films and glass samples used as evaporation source are identical within 1 at.%.
The next important step, before starting the examination of the photoinduced
changes, has consisted in determining the band gap and related features of these
materials. This study is presented in the current chapter and is based on the article
published in 2014 in the journal Proceedings of SPIE after being presented in the
Photonic North 2014 Conference. In this work, we studied the band gap and the
absorption edge of the selected Ge25As30S45 glass composition as well as its dependence
on the film thickness. The justification of the relative concentrations chosen for each
element (particularly, the atomic ratio As/S) will be discussed in the third chapter.
The goal of the work was to investigate the effect of the presence of localized states
in the mobility gap inherent to thin films in order to understand the electron transition in
such disordered systems. The shift of absorption edge and change of band gap slope were
observed by varying the film thickness. All the physical parameters may be therefore
dependent on the film thickness.
43
Résumé de l’article inséré
Dépendance du band gap optique de couches minces vitreuses de
chalcogénure Ge-As-S en fonction de leur épaisseur
Nous avons étudié des couches minces du verre Ge25As30S45 évaporées par la
technique de faisceau d'électrons. Nous avons analysé les spectres de transmission de ces
couches minces de même composition nominale, obtenues dans des conditions
identiques, mais avec quatre épaisseurs différentes variant de 1 à 7 micromètres. Toutes
les couches fabriquées ont été recuites pendant 1h à 300°C (en-dessous de la température
de transition vitreuse de ce verre). En conséquence, nous avons observé un déplacement
du bord de transmission d'environ 100 nm vers les plus courtes longueurs d’onde en
fonction de l’épaisseur d’échantillon. Nous avons calculé le band gap optique de ces
couches minces recuites et nous avons observé que la pente du bord d'absorption devient
moins abrupte et que la largeur de bande diminue quand leur épaisseur augmente. En
outre, cette diminution de la bande est accompagnée d'un élargissement des queues et des
états localisés, ce qui indique une augmentation du degré de désordre dans le réseau
vitreux. Cela pourrait s’expliquer par la densité élevée de défauts et des liaisons
pendantes dans les couches plus minces puisqu’il y a moins de matière déposée. Ceci
implique donc une augmentation à la fois du degré de désordre et de la concentration de
défauts, et par conséquent la diminution du band gap optique.
44
Proceedings of SPIE 9288, Photonics North 2014, 92880K (2014)
Band Gap Dependence upon Thickness of Chalcogenide Ge-
As-S Thin Films
K. Palanjyan*, R. Vallée, T. Galstian
Center for Optics, Photonics and Laser,
Department of Physics, Engineering Physics and Optics, Laval University,
2375 rue de la Terrasse, Québec (Qc), G1V 0A6 Canada.
Abstract
We have studied thin films of Ge25As30S45 glass evaporated by electron-beam
technique. We have analyzed the transmission spectra of thin films of the same
nominal composition, obtained under identical conditions, but with four different
thicknesses varying from 1 to 7 micrometers. All fabricated films were annealed for
1h at 300oC (below the glass transition temperature of this glass). As a result, we
observed a thickness dependent blue-shift of about 100 nm of their transmission edge.
We have calculated the optical band gap of those annealed thin films and we have
observed that the slope of absorption edge becomes less abrupt and the band gap
decreases when their thickness increases. Furthermore, this band gap decrease is
accompanied with a broadening of the tails and localized states, which indicates an
increase of the degree of disorder in the vitreous network. This could be explained by
the higher density of defects and dangling bonds in the thinner films since the amount
of deposited material is smaller. This implies therefore an increase of both the degree
of disorder and the concentration of defects, and consequently the decrease of the
optical gap.
Keywords: ChG, thin films, optical band gap, e-beam evaporation
2.20 2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.600
200
400
600
800
1000
1200
1400
1600
1800
Abs
orpt
ion
coef
ficie
nt (c
m-1)
Photon energy (Ev)
1µm 3µm 5µm 7µm
1 2 3 4 5 6 7
2.25
2.30
2.35
2.40
2.45
2.50
2.55
Phot
on e
nerg
y (E
v)
The thickness of the thin film (µm)
200 300 400 5000,0
0,5
1,0
As-
As
Ge-
Ge
Ge-
S; A
s-S
3µm 5µm 7µm
Nor
mal
ized
inte
nsity
(a. u
.)
Raman shift (cm-1)
45
2.1 Introduction
Chalcogenide glasses have attracted attention due to their wide range of infrared
transmission, high non-linear refractive indices, low phonon energy and photosensitivity.
Chalcogenide glasses are non-crystalline semiconductors and, as for the well-known
crystalline semiconducting materials, their charge transport properties are interesting
from the point of view of basic physics as well as of device technology. Thus, the
determination of the band structure in such materials is of first importance. The common
feature of ChG is the presence of localized states in the mobility gap as a result of the
absence of a long-range order as well as various inherent defects. The understanding of
the electron transport in such disordered systems requires the investigation of gap states
[123, 124].
Several works in the literature state the variation of the optical gap as a function of
the thickness of chalcogenide glassy films [123-128]. All these studies refer to very thin
layers (< 400 nm) obtained by thermal evaporation. In general, the gap and the thickness
of the film decrease simultaneously. The presence of unsaturated bonds in deposited
amorphous films has also already been evidenced [12]. These bonds are responsible for
the formation of defects (in the films with small thickness), which produce localized
states in the band gap of the amorphous thin film. The phenomenon of the band gap
increase may be explained by the fact that more defects and dangling bonds are expected
when layers are thinner. As the amount of matter is reduced, the degree of disorder and
concentration of defects are both increased, resulting in a decrease of the thin film optical
band gap [14], while in the case of thicker films, greater deposition builds up more
homogeneous network, thus minimizing the number of defects and localized states,
thereby increasing the optical energy gap.
Incorporation of germanium (Ge) in chalcogenide glassy semiconductors allows
controlling both electrical and optical properties of glasses [125]. Compositions obtained
by adding Ge to As–S glasses are interesting since their electrical conductivity increases
and the density of states at the Fermi level decreases with thickness. Moreover, with
increasing the film thickness, all physical parameters will change accordingly due to the
change of the density of localized states in the mobility gap of thin films. This increase of
46
localized states’ density can be attributed to the increase of defect states and surface
defect states as well.
In the present work, we present a study of the optical and electrical properties of the
Ge25As30S45 amorphous thin films. The effect of annealing and thickness on the optical
energy gap is studied.
2.2 Experimental
The bulk samples of Ge25As30S45 composition have been prepared according to the
traditional synthesis procedure, i.e. by melt-quenching technique in an evacuated fused
silica ampoule sealed under vacuum (10−3 Pa) [129]. The synthesis was carried out by
using high purity Ge, As and S raw materials. Glass melting and fining was performed in
a rocking tubular furnace at 850°C during 8 h and quenched in water at room
temperature.
Glass rods of about 60 mm in length and 15 mm in diameter were obtained after
removing from the synthesis ampoule. Slices of about 2 mm in thickness were cut and
polished (Figure 2.3). The glass samples of nominal composition Ge25As30S45 have dark-
red color and an absorption edge around λvis = 530 nm for thin films of 1.5 µm thickness.
Figure 2.3: Photograph of a typical Ge25As30S45 glass rod and polished slice.
Then, thin films of different thicknesses (from 1.5 μm to 7 µm) were deposited by
electron beam evaporation from the Ge25As30S45 bulk glass onto BK7 glass substrates
held at room temperature. The electron beam voltage was 4 kV (in a vacuum of 10-6 Pa)
and the deposition rate was 10 Å/s. Next, the fabricated films were annealed in ambient
atmosphere for 1h at the glass transition temperature, Tg 350°C, and slowly cooled
47
down to room temperature. The Figure 2.4 shows an image of the prepared thin films
with different thicknesses.
Figure 2.4: Ge25As30S45 glass thin films prepared with different thicknesses.
Both the homogeneity and the elemental compositions of the prepared thin films
have been checked by using the energy dispersive analysis of X-ray technique (EDAX-
SEM) in different points. An excellent accordance was found between the experimental
values measured from the bulk glass and from the evaporated thin films (Table 2.1).
Table 2.1: Elemental analyses of the bulk and thin film of composition Ge25As30S45. The experimental error of the measurement is estimated to be around 5 At.%.
Element Bulk
(At.%)±5%
Thin film
(At.%) ±5%
Ge 24.1 22.2
As 29.8 30.1
S 46.1 47.7
The optical absorption measurements for chalcogenide thin films indicate that the
absorption mechanism is due to an indirect transition [105, 123, 124, 130]. In our
previous work [105], we have reported a blue-shift of about 100 nm of their transmission
edge in the short wavelength region after annealing at a temperature close to that of the
glass transition Tg. The effect of thermal annealing on the optical energy gap is related to
the stability of the chalcogenide film. Annealing at a temperature below the glass
transition temperature increases the optical energy gap. This effect can be explained by
1.5µm 3µm 5µm 7µm
48
the density of states model proposed by Mott and Davis [5]. During thermal annealing,
the unsaturated defects are gradually annealed out [13], producing a large number of
saturated bonds. This, in turn, reduces the density of localized states in the band gap, and
consequently increases the optical energy gap.
The absorption coefficient spectra were calculated from the recorded transmission
spectra by using the Beer-Lambert law:
where I and I0 are the intensity of the transmitted and incident lights, respectively; α
is the linear absorption coefficient (in cm-1) and x is the sample thickness. As the
transitions are indirect, the value of the optical gap for different thin films can be
obtained by extrapolation of the linear part of the curve (by using the Tauc method [9]):
√
2.3 Results and Discussion
We can observe (in Figure 2.5) the decrease of the optical band gap with increase of
film thickness.
Figure 2.5: Ge25As30S45 optical band gap as a function of the film thickness.
Moreover, one can observe (from Figure 2.6) that the slope of the absorption edge is
decreasing with the film thickness increase, indicating the broadening of the tails and
localized states and the increase of the degree of disorder in the system.
1 2 3 4 5 6 7
2.25
2.30
2.35
2.40
2.45
2.50
2.55
Phot
on e
nerg
y (E
v)
The thickness of the thin film (µm)
49
2.20 2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.600
200
400
600
800
1000
1200
1400
1600
1800
7µm 5µm 3µm
Abs
orpt
ion
coef
ficie
nt (c
m-1
)
Photon energy (Ev)
1µm
Figure 2.6: Absorption coefficient spectra of Ge25As30S45 for different film thicknesses.
As mentioned in the introduction, several works in the literature are reporting the
opposite phenomenon, i.e., a decrease of the optical band gap with the thickness decrease
[123, 128]. This is explained by the fact that more defects and dangling bonds are
expected in thinner layers, the material then becoming more uniform with increasing its
thickness. However, it is also important to mention that in these works, the evaporation
technique and even the film compositions were different from those studied in the present
work. Furthermore, the thicknesses of those studied thin films were at the order of
hundreds of nanometers, which is well below the films under examination here.
A similar phenomenon (to that described here) was reported in the literature, but on a
thin film composition without Ge [131]. The authors have considered the columnar
structure of layers to explain the observed effect. It was shown that the increase of the
films’ thickness induces an increase of the free space around the columns and thus a local
decrease of the layer density. We can therefore expect that the thicker, and thus the less
dense, layers have the largest number of defects, resulting thus in a decrease of the
optical band gap. In reference [132], the authors have shown the broadening of tails of
localized states with the thickness increase, suggesting an increase of the degree of
disorder in the system.
Note that Harshavardhan et al. [133] have already evidenced the role played by
columnar structures. By depositing thin films of glassy Ge25Se75 with different angles
between the source of evaporation and the substrate, they have observed a variation of the
50
width of localized states, confirming thus the modification in the columnar structure of
the thin films. On the contrary, when the same experience is reproduced with glassy
As2S3 thin films, which do not have a columnar structure, the width of the localized states
does not vary [133].
This hypothesis is also supported by the results of Kuzukawa et al. [54], Marquez et
al. [134] and Shimakawa et al. [135]. In these studies, the thermal annealing of columnar
Ge-Se layers (obtained by thermal evaporation) leads to a densification of the layers,
resulting in a decrease of the thickness. The optical band gap of the annealed layer is
greater than that of non-annealed layers. This clearly indicates a dependence of the
optical band gap with the density of the columnar layers. In another study, El-Sayed
Samanoudy et al. [128] have associated the increase of the optical band gap of annealed
layers with the decrease of the free space (in other words, the contraction of structure)
and the bonds rearrangement.
In addition we have recorded micro-Raman spectra (equipment-LABRAM 800HR
Raman spectrometer from Horiba Jobin Yvon, probe- CW He-Ne laser at 632.8 nm with
intensity of 30 mW/cm2 and duration of acquisition <10 sec) on the cross-sections of the
thin films with different thicknesses (Figure 2.7).
200 300 400 5000.0
0.5
1.0
As-
As
Ge-
Ge
Ge-
S; A
s-S
3µm 5µm 7µm
Nor
mal
ized
inte
nsity
(a. u
.)
Raman shift (cm-1)
Figure 2.7: Normalized Raman spectra of Ge25As30S45 thin films of different thicknesses.
As we can see from the figure above, there is no significant change of the relative
content of heteropolar Ge-S and As-S bonds, neither on the relative content of homopolar
51
Ge-Ge and As-As bonds, as a function of thin films thickness. However, the micro-
Raman spectroscopy technique is restricted to structural analysis at the short-range order.
We can thus suggest here the role played by the network disorder at the medium- and
long-range order (density, compactness). Moreover, it is worth considering that the
thermal annealing of the thin films was carried out under ambient atmosphere. Indeed,
such material can be sensitive to surface oxidation, particularly at high temperatures.
Another important experimental parameter, which can play a significant role, is the
substrate temperature during the evaporation. In our case, BK7 substrates were held at
room temperature, but its heating during evaporation could probably help to improve the
layer uniformity and more specifically avoid the formation of multilayers.
2.4 Conclusion and Prospects
We have studied the optical band gap dependence upon the film thickness prepared
by e-beam technique. We observed: (i) a blue-shift of their transmission edge after
thermal annealing; (ii) a band gap decrease with increasing the thickness and; (iii) the
change of slope of the absorption edge (which becomes less abrupt) with increasing the
thickness.
On the other hand, the examination by micro Raman spectroscopy of the thin film
cross-sections has shown that no significant change of relative content of both
heteropolar Ge-S, As-S and homopolar Ge-Ge, As-As bonds occurs with increasing the
film thickness, suggesting a role played by the medium - and/or long-range structure
disorder. Further studies are presently ongoing to investigate an eventual role played by
surface oxidation during thermal annealing.
Acknowledgments
We acknowledge the financial support from the Canadian Foundation for
Innovations (CFI), the Fonds Québecois de la Recherche sur la Nature et les
Technologies (FQRNT), and the Natural Sciences and Engineering Research Council of
Canada (NSERC). We would like also to thank Y. Messaddeq for help and Y. Ledemi for
his valuable advices.
53
Chapter 3
Study of Average Photoinduced Birefringence in
Ge-As-S Thin Films The ChG are amorphous semiconductors which exhibit a variety of photoinduced
phenomena upon irradiation with light whose photon energy is comparable with that of
the material optical band gap. The photoinduced effects include changes in the structural,
mechanical, chemical, optical, and/or electrical properties. The study of these
modifications and more generally the study of the photosensitivity of these materials may
be useful for a better understanding of the fundamental aspects prior to any application of
such materials in the high-tech industry. The present chapter has two objectives: first, to
investigate the optical changes produced under different conditions of laser irradiation
and; second, to study the changes at molecular level and describe the involved
mechanisms. The understanding of the latters was the motivation to undertake a detailed
local structural analysis through micro-Raman spectroscopy.
As previously mentioned at the beginning of the previous chapter, efforts were
dedicated in this thesis to study the photo-sensitivity of vitreous Ge-As-S thin films with
high optical homogeneity. Photo-induced phenomena were not investigated in the home-
made bulk Ge-As-S glass samples because of their optical inhomogeneity (presence of
striae).
The present chapter is based on a published article that discusses the measured
average photoinduced birefringence (PIB) in Ge-As-S thin films to determine the
composition exhibiting the highest value and compare its photosensitivity with that of
other binary ChG systems reported in literature or used in the industry. A Raman
spectroscopic study is presented herein to describe and explain the observed
photoinduced changes.
54
Résumé de l’article inséré
Étude de la biréfringence photo-induite dans des
couches minces vitreuses de chalcogénures Ge-As-S
en fonction de la concentration en As
Nous présentons ici l’étude de la biréfringence photo-induite (PIB) observée dans les
couches minces vitreuses Ge-As-S déposées par la technique d'évaporation par faisceau
d'électrons (e-beam). L’étude a été menée en fonction de la concentration en arsenic As,
comprise entre 10% et 40 at.%.
Les études réalisées par spectroscopie Raman sur différentes compositions vitreuses
Ge25AsxS75-x (10 ≤ x ≤ 40 at.%) avant et après irradiation ont permis de décrire les
modifications structurales associées, conduisant ainsi à une meilleure compréhension du
phénomène. Il a été montré que l'augmentation de la concentration d’arsenic favorise la
formation de liaisons homopolaires. La rupture des liaisons homopolaires par l'exposition
à un laser dans le visible à 514 nm est proposée comme mécanisme responsable des
changements structuraux photo-induits menant à la création du PIB.
55
OPTICAL MATERIALS EXPRESS, Vol. 3, No. 6, pp. 671-683, 2013
Copyright © OSA
Study of Photoinduced Birefringence vs As Content in
Thin Ge-As-S Films
K. Palanjyan, S. H. Messaddeq, Y. Messaddeq, R. Vallée, E. Knystautas, T.
Galstian*
Center for Optics, Photonics and Laser,
Department of Physics, Engineering Physics and Optics, Laval University,
Pav. d’Optique-Photonique, 2375 Rue de la Terrasse, Québec, Canada G1V 0A6
* Corresponding author : [email protected]
Abstract
Thin films of Ge-As-S glass are prepared by e-beam evaporation technique.
Photoinduced birefringence (PIB) is studied as function of the As content with
concentrations ranging from 10 at.% to 40 at.%. Raman spectroscopy is used as
additional tool to explain the corresponding changes undergone by the material system.
The breakdown of homopolar bonds is suggested as a possible mechanism of
photoinduced structural changes leading to the creation of the PIB.
OCIS codes: (000.2190) Experimental physics; (160.5335) Photosensitive materials;
(310.6860) Thin films, optical properties
56
3.1 Introduction
Chalcogenide glasses (ChGs) have attracted great deal of attention thanks to their
potential applications in integrated photo programmable infrared photonic circuits [136].
Very often, the research on ChGs was done on material systems, which were relatively
easy to fabricate and had relatively high photosensitivity [76, 90, 107, 137-139]. An
example of such system is the arsenic sulphide (AsS), which is an excellent glass forming
compound with good chemical stability and providing wide transmission range (from
visible to IR), high refraction index, photosensitivity and nonlinear optical properties
[140]. However, this material also has important drawbacks, such as the relatively low
glass transition temperature (Tg ≈ 180C), which may compromise its practical
applications. Thus, for the present study we have selected the glassy system Ge-As-S
since the incorporation of Ge into AsS increases the glass transition temperature (up to
Tg ≈ 350C1) and improves the mechanical and thermal properties of the glass [30].
The structure of those materials consists of interconnected trigonal (As2S3) and
tetrahedral Ge(S1/2)4 units [141]. Optical studies were already reported on annealed films
of such glasses fabricated by thermal vapor deposition. Thus, scalar photosensitivity
properties of a glass composition (As20S60Ge20), containing relatively low content of As,
were described [142]. The dependence of photoinduced band gap changes upon the
content of As was also reported [143]. Photoinduced birefringence (PIB) and
photoinduced dichroism phenomena were studied in a glass composition containing low
amount of As, e.g. (As8S60Ge32) [105]. We think that the understanding of corresponding
mechanisms and structure formation as a function of As concentration is very interesting
and useful task. In the present work, we present such detailed study of the PIB in thin
films based on Ge-As-S. In addition, the e-beam technique used here for thin film
deposition is different (allows better control of composition). Also, we start our
investigations (in the current report) by using fresh (not annealed) films. Finally, Raman
spectroscopy is used as parallel tool for their structural analysis.
1 DSC traces presented in Appendix A.3.
57
3.2 Experimental
Glass samples were prepared by melting high purity starting elements (Ge, As and S)
in a quartz ampoule evacuated up to 10−3 Pa. All procedures including synthesis,
distillation and glass production were carried out in a closed system. After melting in a
rocking furnace at 850°C during 8 h the ampoule was removed, quenched in water and
annealed at the glass transition temperature, Tg, near 400°C. Glass rods about 60 mm in
length and 15 mm in diameter were obtained after cutting the quartz ampoule. Pieces of
around 2 mm in thickness were cut and polished. The obtained samples ranged in color
from yellow to dark-red, corresponding to a shift of the visible absorption edge λvis from
450 nm to 630 nm.
Usually, thin chalcogenide films are fabricated by thermal evaporation or pulsed
laser deposition techniques [131, 144]. However, in the ternary system Ge-As-S, where
the transition temperatures of elements are quite different, it is more advantageous to use
the method of electron beam vacuum evaporation [145]. Thus, films of 1.5 μm thickness
were fabricated (by electron beam evaporation from the crushed ingots with an electron
beam voltage of 4 kV in a vacuum of 10-6 Pa) onto BK7 glass substrates held at room
temperature. The deposition rate was 10 Å/s (measured continuously by a quartz-crystal
monitor, Temescal FTM). It is known that such low deposition rate produces a chemical
composition, which is rather close to that of the bulk (starting) material. Indeed, the EDX
analysis of our samples indicated that those compositions are close (within 4 at.%, see
below). The maximum deviation was observed for Ge content. The study of those films
was carried out in an electron microscope (FEI, Quanta 3D FEG). The corresponding
analysis results are presented in the Table 3.1 for only one composition as example (see
later for detailed analyses).
Table 3.1: Material analyses of the composition Ge25As30S45. The experimental error of this measurement is estimated to be of the order of 3%-5%.
Element Bulk (at %) Thin film (unexposed) (at %) Thin film (exposed) (at %) O 4.6 6.4 12.2 Ge 24.1 20.2 18.8 As 25.2 25.7 24.4 S 46.1 47.7 44.7
58
3.3 Results
The transmission spectra (obtained by means of Varian Cary 500 spectrophotometer)
are shown on Figure 3.1 for different glass compositions of thin films with thicknesses of
1.5 µm. A shift of the band gap position is observed as the As content increases.
400 500 600 700 800 900 10000
20
40
60
80
100
400 500 600 700 800 900 10000
20
40
60
80
100
Ge25As40S35
Ge25As35S40
Ge25As30S45
Ge25As20S55
Tra
nsm
issio
n (%
)
wavelength (nm)
Ge25As10S65
Ge25As40S35
Ge25As35S40
Ge25As30S45
Ge25As20S55
Tra
nsm
issio
n (%
)
wavelength (nm)
Ge25As10S65
Figure 3.1: Typical transmission spectra of obtained thin ChG films.
Namely, the corresponding numerical values of the band gap (calculated from our
experimental data by using the Tauc method, [146]) are as follows: 3.26 eV for
Ge25As10S65, 3.40 eV for Ge25As20S55, 2.42 eV for Ge25As30S45, 2.28 eV for Ge25As35S40,
and 2.08 eV for Ge25As40S35.
We note that there is an important but rather continuous dependence of the band gap
position with respect to the S content x for low concentrations, x < 45 at.%. However,
this dependence become more drastic and we observe a strong jump between the values
45 at.% < x < 55 at.%, while the band gaps of films with x = 55 at.% and x = 65 at.%
are rather close.
59
3.4 Photosensitivity Study in Thin Films
To study the photosensitivity of thin ChG films, a linearly polarized CW Argon ion
laser (operating at 514.5 nm) was used for excitation at normal incidence (from the ChG
side). Samples were exposed during 60 min with different intensities, varying from 2
W/cm2 to 15 W/cm2. We first notice, from the Table 3.1, the significant increase of the
amount of oxygen after photo exposition indicating a photoinduced oxidation of the film.
There are indeed some observations indicating that the photobleaching phenomenon of
Ge-S films is mainly related to the oxidation of germanium atoms [95, 96, 98].
In addition the absorption coefficients were determined from transmittance
measurements using the Lambert-Beer-Bouguer law and those spectral measurements
show a clear shift of the band gap towards the shorter wavelengths (the so-called photo
bleaching process), as presented in the Figure 3.2 for one glass composition (see also
Ref.[85]).
2,25 2,30 2,35 2,40 2,45 2,50 2,55 2,60
after irradiation
Abs
orpt
ion
coef
ficie
nt (a
.u.)
Photon energy (eV)
before irradiation
Figure 3.2: Absorption coefficients of the Ge25As30S45 as function of probe’s energy obtained for
photoexposition intensity of 8W/cm2 for 60 min. The solid curve corresponds to the unexposed case; the dashed curve corresponds to the photoexposed case.
Note that, in the As2S3 composition, usually the opposite process is observed (photo
darkening).
The experimental setup, used to study the PIB, is schematically shown in Figure 3.3.
61
Figure 3.4: Typical cycle of excitation and partial relaxation of the PIB in the Ge25As30S45 film.
The solid curve shows the experimental result and the dashed one (behind the experimental curve)
represents the fitted curve. The thickness of the film was 1.5 µm and the excitation intensity was
8W/cm2.
The theoretical fit of obtained experimental curves was made by a bi-exponential
function ( ⁄ ) (
⁄ ) and the corresponding coefficients for
two excitation contributions were found to be comparable (of the same order of
magnitude) in terms of their amplitudes both during excitation
⁄ and
relaxation ⁄ . However, their characteristic times were drastically different (at
least by an order of magnitude) during the excitation ( and ) and
during the relaxation ( and ). Based on those data one cannot
conclude about the exact nature of the excitation microscopic mechanisms. Yet, the
partial (approximately 30%) relaxation of the PIB (after the removal of the excitation
laser beam) demonstrates one very fast ( ) and important channel of relaxation
and another, relatively smaller (by a factor of 3) and significantly ( ) slower,
channel. This information is used hereafter in our further analyses.
We have studied the behavior of PIB for various As contents. We have chosen the 4
W/cm2 exposition intensity to study the PIB in Ge-As-S films of the same thickness (
1.5 µm), but containing different levels of As. Figure 3.5 shows the obtained dependence
0,2
0,4
0,6
0,8
1
1,2
0 2000 4000 6000 8000 10000 12000 14000
Tran
smis
sion
(u.a
.)
Time (seconds)
Excitation OFF
Excitation ON
62
of the (stabilized under excitation) PIB upon the content of As for concentrations up to
35% (the probe beam was strongly absorbed for glass compositions with higher As
contents).
10 15 20 25 30 350.025
0.030
0.035Ph
otoi
nduc
ed b
irefr
egen
ce
n
xAs
(at.%)
Figure 3.5: Dependence of the PIB upon the amount of As in the film of Ge-As-S.
We can see, from Figure 3.5, that the PIB shows a change in trend (an “inflection
point”), that is, it increases with the increase of the content of As up to 30% (the
composition Ge25As30S45), but then decreases for higher As contents. This also is
important information to be used in our discussion (see hereafter).
Various excitation intensities were used to study the PIB for the “optimal” glass
composition Ge25As30S45. For each intensity, we have studied the PIB behavior at
different parts of the same ChG sample to avoid the errors due to sample to sample
variation. As expected, the growth of the PIB was faster for higher intensities (not shown
here) and the growth process stabilized at different levels (see
Figure 3.6); higher levels of PIB were achieved for higher excitation intensities.
63
2,1 4,2 6,3 8,4 10,5 12,6 14,7
0,015
0,020
0,025
0,030
0,035
0,040
Excitation intensity (W/cm2)
Phot
oind
uced
bir
efre
genc
e
n
Figure 3.6: The dependence of the established (saturated) value of PIB upon the excitation intensity
for the composition Ge25As30S45.
The obtained curves also were fitted by bi-exponential functions to characterize the
dynamics of excitation, the established values of the PIB as well as their partial
relaxation. It is worth to note that the use of higher intensities (>15 W/cm2) in our
samples was limited by the damage threshold of the ChG films (in fact by their photo
oxidation and evaporation of As).
As one can see, from the
Figure 3.6, the PIB first increases almost linearly with the excitation intensity and
then saturates for intensities at the order of 8 W/cm2. It seems that the PIB is created by a
limited number of pre-existing photosensitive units, but not newly photo created ones.
We note that very large values of PIB were achieved (n ≈ 0.03±0.003) at excitation
intensities of the order of 8 W/cm2. Compared to typical ChG compositions, those are
rather high values of PIB, particularly for the ChG composition with such high value of
Tg.
To further study the possible mechanisms involved in the process of described above
PIB, Raman scattering measurements were performed (by using a LABRAM 800HR
Raman spectrometer from Horiba Jobin Yvon) with normal back scattering configuration
in the wavenumber region, ranging from 150 cm-1 to 600 cm-1. The 632.8 nm line of a
64
CW He-Ne laser (30 mW/cm2) was used as light source. Duration of acquisition was kept
very short (<10 sec) to ensure the absence of material modification caused by the
irradiation of the He-Ne laser.
Figure 3.7 shows the measured Raman spectra for the compositions of Ge-As-S films
(versus the ratio As/S) used in the study of PIB (see Figure 3.5). In the Raman spectra of
Ge-As-S films, four bands and one shoulder may be clearly seen at 215, 242, 345, 490
and 430 cm-1, respectively. To facilitate their analyses, the recorded spectra have been
first reduced by subtracting a polynomial baseline linking the first minimum (before the
first band observed near to 200-250 cm-1) and the minimum at the end of the spectrum (at
600 cm-1). Then the spectra were normalized at 242 cm-1 (which corresponds to the
vibrational mode of Ge-Ge bond) in order to observe the dynamic changes of homopolar
As-As bonds (corresponding to 215 cm-1) and heteropolar Ge-S, As-S bonds
(corresponding to 345 cm-1).
The structure of Ge-As-S glasses already was studied by several authors using
Raman spectroscopy [30, 141, 147, 148] and other techniques such as Extended X-Ray
Absorption Fine Structure [28] or Fourier Transform Infrared Spectroscopy [141]. It
consists mainly of a three-dimensional network of Ge(S1/2)4 tetrahedra and As(S1/2)3
pyramids. The main Raman signature of those structural units is centered at 345 cm-1 and
is attributed to overlapped symmetric stretching Ge-S and As-S bonds. In our results, we
note (in Figure 3.7) that the relative intensity of this band is larger for the Ge25As10S65
sample (which is stoichiometric), than for other glass compositions, all being S-deficient.
The lack of sulfur atoms in the network will indeed limit the formation of Ge(S1/2)4 and
AsS3 structural units, which are predominant in stoichiometric compositions and even in
sulfur-excess ones [28]. The attribution of the band at 430 cm-1 is more ambiguous but
was discussed by Tanaka ([148] and references therein) and was associated either to
vibrational modes of edge-sharing bitetrahedra Ge2S2+4/2 (a structural unit known to exist
in both crystalline and glassy GeS2) or to vibrational modes of S-S dimers, which can be
encountered into the network through small sulfur chain and/or sulfur bridging 2
tetrahedra.
A weak band can be observed at 490 cm-1 (Figure 3.7) on the Ge25As10S65 spectrum
whereas it is strongly reduced (invisible) for other compositions. It can be attributed to
65
the S-S stretching vibration in S8 rings or short Sn chains [30]. Such sulfur chains or rings
are observed in S-rich glasses, more seldom in stoichiometric composition, and inexistent
in S-deficient glasses, which is consistent with the spectra presented in Figure 3.7.
The band, centered at 242 cm-1, that exists not only for the stoichiometric
composition, Ge25As10S65, but also for all S-deficient compositions, can be attributed to
the vibration of ethane-like structure S3Ge-GeS3 characterized by homopolar Ge-Ge
bonds [149]. Finally, a shoulder at 215 cm-1 is observed in the spectrum of the
Ge25As20S55 film. The relative intensity of the band at 215 cm-1 is increased with the
increasing As content (As/S ratio) in the film, or in other words, with increasing the lack
of S in the network. This band was attributed to the As-As homopolar bond [150]. The
relative intensity of the As-As band becomes higher than the Ge-Ge band in the spectra
of films where As content exceeds the Ge one. This result is in accordance with previous
studies reported by Aitken et al [28] where it was evidenced that As-As homopolar bonds
are formed prior to Ge-Ge ones in S-deficient Ge-As-S and no Ge-As bond was observed.
200 300 400 500 600
Ge25
As40
S35
Nor
mal
ized
Inte
nsity
(u.a
)
Raman Shift (cm-1)
Ge25
As10
S65
Ge25
As35
S40
Ge25
As20
S55
Ge25
As30
S45
Figure 3.7: Normalized Raman spectra of thin Ge-As-S films for different compositions:
Ge25As10S65 (dotted black line), Ge25As20S55 (short dash dotted red line), Ge25As30S45 (dashed green line), Ge25As35S40 (short dotted blue line), Ge25As40S35 (solid cyan line).
We can also notice (in Figure 3.7) that the bands corresponding to both homopolar
As-As and Ge-Ge bonds are relatively more intense (for all S-deficient films) than the
66
band centered at 340 cm-1 that is attributed to the heteropolar bonds Ge-S and As-S.
Although it is difficult to make some quantitative conclusions from these observations, it
is clear that S-deficient films contain much larger amount of Ge-Ge and As-As
homopolar bonds than the stoichiometric one.
3.5 Discussions
The increase of the PIB with increasing As content, observed in Figure 3.5, can be
thus correlated to the increasing content of homopolar bonds (pre-existing before the
photo exposition) within the glass network evidenced through the Raman spectra. Indeed
the PIB intensity of all S-deficient films is higher than the one obtained for stoichiometric
composition. For the moment, it is difficult to conclude on which of the As-As and Ge-
Ge bonds plays more important role in the PIB. On one hand, the highest value of PIB
was measured for the Ge25As30S45 thin film (see Figure 3.5), i.e. a composition whose
content of As-As bonds is expected to be larger than Ge-Ge ones, Figure 3.7. On the
other hand, a lower value of PIB was measured in both Ge25As20S55 and Ge25As35S40
samples. It is worth noting that the relative intensities of the As-As Raman band of the
latter samples are respectively lower and higher than that of the Ge25As30S45 sample, as
shown in Figure 3.7. This thus suggests the existence of an optimal ratio of homopolar
bonds As-As/Ge-Ge to obtain strong PIB. By considering the PIB results and the Raman
spectra discussed above, this optimal ratio is here close to 1. In other words, a larger PIB
is observed when the amounts of As-As and Ge-Ge homopolar bonds are similar in the
material. Nevertheless, Raman spectroscopy does not bring us to absolute quantitative
results. Indeed, despite the fact that Aitken et al.[28] have shown that As-As homopolar
bonds are formed prior to Ge-Ge ones in S-deficient Ge-As-S bulk glasses, our study was
performed on thin films. The method of preparation of thin films through e-beam
evaporation (used here), is different from the melt-quenching technique used for bulk
samples, and is known to promote formation of homopolar bonds to the detriment of
heteropolar ones [151].
The Raman spectra of bulk for the composition Ge25As30S45 and for the thin film,
obtained from that bulk (presented in Figure 3.8), corroborate this feature: the thin film
67
spectrum exhibits a higher relative intensity of the 215-240 cm-1 band, assigned to the
As-As and Ge-Ge homopolar vibrational modes, than the spectrum recorded on the bulk
glass of same composition. It is worth to remind that the thin films have been prepared
from the bulk glassy samples. The origin of higher degree of structural disorder, i.e.
formation of homopolar bonds, observed in thin films vs bulk glass can be explained by
the higher cooling rate obtained during evaporation process [151].
Therefore, the mere role played by As-As homopolar bonds in the PIB effect cannot
be argued from the results described here and the existence of both As-As and Ge-Ge
homopolar bonds has to be considered in the origin of such effect. In addition, our photo
excitation dynamic study has shown that more than one excitation mechanisms are
involved in the PIB process.
In fact, the role played by homopolar bonds in photoinduced phenomena in ChG
glasses has been well established by several authors in various material systems [66, 152,
153]. Interestingly, it can be noticed that the inflection point of PIB in our studies (see
Figure 3.5) happens for the composition Ge25As30S45, which is placed on the Ge – As2S3
tie line in the Ge-As-S ternary diagram [22]. Further studies, based on composition
belonging to this tie line, are ongoing in order to provide more information about the
origin of the PIB effect.
Finally, to study the evolution of heteropolar vs homopolar bonds before and after
the laser irradiation, we have recorded the Raman spectra on the Ge25As30S45 thin films
exposed with the same Ar ion laser beam at different intensities (the exposition time was
60 minutes). As previously, the obtained Raman spectra were corrected (baseline
subtraction) and normalized at 242 cm-1, presented in Figure 3.8. It can be seen that, by
increasing the laser exposition, the band at 340 cm-1 increases monotonously. It can be
therefore assumed that the laser exposition indeed promotes the breaking of homopolar
bonds and the formation of heteropolar bonds.
68
200 300 400 500
exposed at 7,87W/cm2
exposed at 4,24W/cm2
exposed at 2,14W/cm2
Nor
mal
ized
inte
nsity
(u.a
.)
Raman shift (cm-1)
bulk
unexposed film
Figure 3.8: Normalized Raman spectra of Ge25As30S45 bulk glass (dotted black line) and thin films unexposed (short dash dotted red line) and exposed at 2.14 W/cm2 (dashed green line), 4.24 W/cm2
(short dotted blue line) and 7.87 W/cm2 (solid cyan line) for 60 min.
Such effect may be seen as a structural relaxation of the network with the
replacement of homopolar bonds by heteropolar bonds, tending toward the polymerized
structure encountered in bulk glass. Similar results already were reported in the literature
[151]. We believe that the inflection phenomenon (when the PIB starts to decrease while
the As content continues to increase above the 30 at.%) is related to the phase transition
between the two zones of glass formation, described in Ref. [22].
3.6 Summary and Conclusion
Thin films, produced by e-beam evaporation deposition, were produced with good
control of stoichiometry in Ge-As-S glassy system. PIB study was performed in such
films with different As contents, ranging from 10 at.% to 40 at.% %. High PIB values
were observed which increase as the content of As increases up to 30 at.%. Raman
spectroscopy was performed for different glass compositions before and after photo
exposition for better understanding of this phenomenon. It was shown that increasing the
concentration of the arsenic is favoring the formation of the As-As and Ge-Ge homopolar
69
bonds (because of the decrease of the concentration of sulfide anions in the network). It
was also shown that the exposition with visible light, promotes the breaking of
homopolar bonds, increases the mobility of atoms and favors the formation of heteropolar
bonds, accompanied by structural polymerization.
Additional comments (unpublished):
The original systematic study carried out in this chapter has allowed to determine the
composition containing 30 at.% of Arsenic as the most sensitive in terms of PIB value. A
diminution of the PIB value was then observed when increasing the Arsenic content to 40
at.%. To explain this behavior, one could consider the position of the studied
compositions within the Ge-As-S vitreous diagram presented in the first chapter (Figure
1.7), with its different sub sections related to the sulfur excess or deficiencies. The first
composition investigated in this glass series is Ge25As10S65, which is placed on the
stoichiometric GeS2-As2S3 tie line. Thus, there are no or very few As-As homopolar
bonds expected in this glass and the PIB value is low, as expected. Then, by increasing
the As concentration to 20 and 30 at.%, glass compositions are included in the so-called
intermediate region, as discussed in section 1.1.4. In that case, the presence of homopolar
As-As bonds is confirmed and their content gradually increases with increasing the
arsenic (As) concentration, leading to an expected increase of the PIB value. Finally, a
decrease of the PIB value is observed for the composition containing 40 at.% of As. We
assume here that this unexpected decrease is related to the highly probable existence of
Ge-As heteropolar bonds in this compositional range (sub-region B in the S-poor domain
in Figure 1.7). The formation of the Ge-As bonds, despite the absence of any direct
evidence of their existence in the literature, might then occur to the detriment of As-As
homopolar bond formation, impeding then further increase of the PIB effect.
Acknowledgments
We acknowledge the financial support of Canadian Institute for Photonic Innovations
(CIPI), Fonds Québécois de la Recherche sur la Nature et les Technologies (FRQNT) and
Natural Sciences and Engineering Research Council of Canada (NSERC). We thank Y.
Ledemi for valuable comments on Raman spectra. We also thank TLCL Optical Research
inc for the material help and advice.
71
Chapter 4
Study of Local Photoinduced Birefringence in Ge-As-S
Thin Films
Due to their unique photosensitive properties, the ChG films have found various
applications in optics and photonics, particularly in integrated optics devices, photonic
chips, photonic circuits, etc. However, while the average photosensitivity of these
materials is usually large enough to justify and permit their practical utilization, knowing
the local photosensitivity could also be important for specific application. For instance,
the local value of photoinduced anisotropy can strongly influence the polarization mode
dispersion and spectral broadening of transmitted light.
Here, we continue the study of the photoinduced birefringence initiated in the
previous chapter and focus our investigation on the local value of the PIB. As we have
already shown, spatially average PIB in Ge-As-S films has relatively high value
compared to those reported for other binary or ternary ChG systems. In this chapter,
based on simple polarimetric measurements and theoretical modeling (with some
approximations), we reveal that the local value of PIB in Ge-As-S thin films can be one
order of magnitude higher than the average value measured in the same material.
72
Résumé de l’article inséré
Observation de la biréfringence photo-induite locale
dans des couches minces vitreuses Ge-As-S
La biréfringence photo-induite (PIB) locale est étudiée dans des couches minces de
verre Ge-As-S préparées par la technique d'évaporation par faisceau d'électrons.
L'excitation laser du matériau est effectuée sous atmosphère ambiante à 514.5 nm et la
PIB est mesurée par un laser He-Ne à 632.8 nm (incident du même côté). Sur la base des
résultats expérimentaux obtenus, nous montrons que la valeur locale du PIB dans ce
matériau peut atteindre une valeur de ≈ 0.11, ce qui est, à notre meilleure connaissance, la
plus haute valeur de PIB jamais rapportée dans la littérature.
73
OPTICAL MATERIALS EXPRESS, Vol. 5, No. 5, pp. 1122-1128, 2015
Copyright © OSA
Observation of giant local photoinduced birefringence
in Ge25As30S45 thin films
K. Palanjyan,1* R. Vallée1 and T. Galstian1
1Center for Optics, Photonics and Laser, Department of Physics, Engineering Physics
and Optics, Laval University,
Pav. d’Optique-Photonique, 2375 Rue de la Terrasse, Québec (Qc) G1V 0A6, Canada
* Corresponding author : [email protected]
Abstract
Photoinduced birefringence (PIB) is studied in thin films of Ge25As30S45 glass prepared
by e-beam evaporation technique. Excitation of the material is done in air at 514.5 nm
and the PIB is monitored with a He-Ne laser at 632.8nm (incident from the same side).
Based on the obtained experimental results, we show that the local value of PIB in this
material can reach a value of ≈ 0.11, which is, to the best of our knowledge, the highest
value ever reported in the literature.
OCIS codes: (260.1440) Birefringence; (310.6860) Thin films, optical properties,
(160.5335) Photosensitive materials, (160.2750) Glass and other amorphous materials
74
4.1 Introduction
Chalcogenide glass (ChG) films are the subject of numerous investigations due to their
potential applications in photonics based on their wide transmission range (from visible
to IR) and high photosensitivity [103, 107, 137, 138, 154]. Integrated optics devices that
use ChG thin films are particularly interesting for the fabrication of high-index-contrast
planar waveguide-coupled microdisk resonators, planar waveguides integrated on a
photonic chip, submicrometer-thick low-loss waveguides, photonic circuits, which are
capable to process optical data streams entirely in the optical domain, etc. [11, 155-160].
In many guided optical devices (particularly those manufactured by photoexposition
or photo patterning) the local birefringence is an important parameter, which has to be
carefully considered due to its role played in the polarization mode dispersion. Thus,
spectral broadening was already studied in some photonic chips and attributed to a non-
uniform birefringence [160]. The control of the polarization modes was described in
similar devices by using the photoinduced birefringence (PIB) [161]. The PIB in ChG
materials was studied by many research groups (see for instance [162, 163]). However, to
the best of our knowledge, all previous reports are based on the use of ChG films with
thicknesses L that are noticeably larger than the typical penetration depth l of the
excitation light. The typical values of l are at the micrometer scale, which, by the way, is
of the same order of magnitude as the ChG film thicknesses used in integrated optical
systems. As a result, the obtained values of PIB in research laboratories are spatially
averaged and do not represent the real values that would be generated in photo patterned
thin integrated circuits employing ChG films.
The aim of the present work is to investigate the local value of the PIB. To that
purpose we have chosen the photosensitive Ge-As-S glass family, known for its structural
stability. Indeed, the incorporation of germanium Ge into arsenic sulfide As-S glass
enhances the material properties by increasing its network connectivity, resulting thus in
a three-dimensional structure and a large increase of glass transition temperature (Tg
~350C as compared to ~180˚C for As-S glasses).
Several studies of the above-mentioned photo-induced scalar phenomena in this Ge-
As-S vitreous system have been reported in the literature [99, 164]. Figure 4.1 shows the
75
transmission spectra of annealed thin (of 3µm thickness) films of Ge25As30S45 before
(black dotted curve) and after (red solid curve) irradiation with bandgap light. Depending
on the fact if the samples were as-prepared or annealed, the studies revealed both photo-
induced bleaching and darkening, respectively. Moreover, a unique non-monotonous
phenomenon, the coexistence of fast photodarkening and slow photobleaching was also
observed in Ge-As-S glasses [106].
500 600 700 8000
20
40
60
80
100
Tran
smis
sion
(%)
Wavelength (nm)
non-exposed exposed
Figure 4.1: Transmission spectra (in non-polarized light) of non-exposed (black, dotted curve) and exposed (red, solid curve) Ge25As30S45 thin films of 3 µm thickness. Samples were irradiated at 514 nm for 30 min.
In addition, some preliminary investigations of PIB and photoinduced dichroism
(PID) phenomena have already been conducted in similar compounds [105, 129]. In a
recent work, we have studied the spatially averaged PID in Ge25As30S45 films and
proposed a model of consecutive bond transitions to explain its dynamics [165]. Here,
we show that the local value of PIB in Ge25As30S45 thin films could be an order of
magnitude higher than its previously reported average value [129].
4.2 Experimental Set-Up and Procedure
Bulk glass samples of Ge25As30S45 composition were first synthesized by the traditional
melt-quenching method in silica ampoule sealed under vacuum. The thin films (with
77
(F2) was used to transmit only the wavelengths above 600 nm (to reduce the noise from
the pump beam). A diaphragm (d) was also used in front of the photo detector (D)
to further reduce noise.
It is worth mentioning that we might use multiple optical arms or a rotating polarizer
(instead of a fixed one) to obtain more information. However, the use of a simple
traditional polarimetric setup was preferred here to compare easily the obtained PIB
results with those previously reported.
4.3 Results and Discussions
The transmission spectrum (recorded on a Varian Cary 500 spectrophotometer) of the
unexposed Ge25As30S45 thin film exhibits typical Fabry-Perot oscillations that become
noticeable for larger than 600 nm [19]. Thus, in our study of PIB, we have to consider
not only the transmission, but also the reflection of the probe. In addition, the transmitted
probe beam intensity is relatively low due to the high absorption of the material at 632.8
nm. For this reason, instead of using an analyzer that is strictly perpendicular with respect
to the original probe’s polarization, we aligned them at 50˚ with respect to each other.
This helped to increase the sensitivity of the set-up. Furthermore, we have normalized the
transmission (after the analyzer) by the values of measured transmission without analyzer
(see typical curves on Figure 4.3). Finally, we have used the approximation of small
photo induced dichroism (at the probe wavelength). As we can see from our calculations
(described below), these operations allow the elimination of contributions (in the value of
the PIB) from the Fabry-Perot oscillations as well as from the material absorption [19].
In general, the intensity of the probe beam, after the sample (before the analyzer), may
be calculated (as ) by using the Jones vector of the probe [166], which may
be presented in the first approximation as:
√
[
] (4.1)
where and are the photo induced parallel and perpendicular components (with
respect to the pump beam’s polarization) of absorption and of the phase delay,
respectively, and is the ChG film’s “transmission” matrix related to Fabry-Perot
78
reflection losses. In general, those losses are polarization dependent. However, given
their interferential nature (involving reflections from front and back interfaces of the ChG
film) and the strong absorption of the film, we shall make an approximation of
polarization independence of those loses to simplify the main demonstration of this work
(the high local birefringence). We can thus present those losses by a scalar coefficient T.
In the same way, the intensity of the probe beam, after the analyzer, may be expressed
(as ) by the corresponding Jones vector as:
√
[
] (4.2)
where is the angle between the original probe polarization and the analyzer.
It is easy to verify that we can exclude the influence of the Fabry-Perot losses in the
calculation of the PIB if its polarization dependence is small (neglected) and if we use the
following normalized form of probe intensity:
( )
(4.3)
As the wavelength of He-Ne laser beam is relatively far from the ChG’s band gap, in the
first approximation, the PID (at 632.8 nm) can be neglected also (α ≈ α⊥) to further
simplify the analyses. This hypothesis is confirmed by the experimental results reported
in Ref. [165], which show that the photoinduced absorption (often called “photo
darkening” or PD) in these materials may be rather significant, while the PID is very low
at this probe wavelength
⁄ . Thus, we can see that the normalized transmitted
intensity value depends only upon the birefringence:
(4.4)
) (4.4’)
where , which, in the case of a uniform PIB , is
The corresponding experimental study was performed on ChG thin films of 3 µm
thickness with pump intensity of 8W/cm2. Figure 4.3 shows the typical dynamics of
excitation and partial relaxation process.
79
100 10003
4
5
6
7
2
2'
1'
without Analyser with Analyser
Time (sec)
Inte
nsity
w/o
ana
lyze
r (m
W/c
m2 ) Excitation ON
Excitation OFF
1
1
2
3
4
5
6
7
Inte
nsity
with
ana
lyze
r (m
W/c
m2 )
Figure 4.3: Transmitted intensity of the probe beam (3 m thick sample is used versus time for pump intensity of 8W/cm2. The points 1 and 1’ represent the established values of excitation and 2 and 2’ represent the established values of relaxation corresponding to the probe transmission without (drawn by triangles in the fig.) and with analyzer (drawn by squares in the fig.), respectively.
We can clearly observe (Figure 4.3) that, with switching-on the pump beam, the
probe beam’s transmission first sharply increases and then decreases very slowly. We
observe a partial recovery of the transmitted light when the excitation beam is switched-
off, leading to a significant remnant PIB.
Based on the equation (4.4’), we can calculate the average (along the thickness of the
film) birefringence
by using the measured normalized intensity. This last one is
obtained (from the experimental data, presented in Figure 4.3) by dividing the established
value of transmitted intensity recorded with analyzer (point 2) by the one recorded
without analyzer (point 2’). Furthermore, these values of PIB can be calculated (using our
experimental data) for various pump intensities and thus plotted as a function of pump
intensity, as presented in Figure 4.4. As one can see, the slope of the PIB decreases with
increasing the excitation intensity and becomes negative above 8 W/cm2. This behavior
confirms our previous results, which have pointed out that the PIB is created by a limited
number of pre-existing photosensitive units, but not newly photo created ones [129].
80
2 4 6 8 10 12 14 160,012
0,015
0,018
0,021
0,024
0,027
0,030
Phot
oind
uced
bire
freg
ence
(n)
Excitation Intensity (W/cm2)
Figure 4.4: Dependence of the established values of PIB (under CW excitation) upon the excitation intensity for the 3 m Ge25As30S45 thin film. The line is used to guide eyes only.
It is worth to mention that no change of thickness after exposition was observed by
DekTak profilometer study (within the error limit: ±0.01nm), excluding the possibility of
photo-induced expansion or depression, which could affect our calculations of the PIB
values.
Our additional studies of the PD and PID also show the same linear character of
excitation in the low excitation range of the pump intensities [165]. Indeed, we can see
that, for the 3 m Ge25As30S45 thin film, the maximal average value of the PIB is nav=
0.03±0.003.
In fact, as mentioned in the introduction, we should also consider the exponential
decrease of the pump intensity in the material. Thus, the relative phase delay (due to the
local PiB) will also change in the material with an exponential law and for a given
thickness (L = 3 m) and initial intensity, the differential phase modulation of the probe
beam will be:
∫
(4.5)
where is the value of the local anisotropy at z.
As we can see from Figure 4.4 and Ref.[129, 165], for the range of low pump intensities,
both PID ( ) and PIB ( ) are linearly proportional to the input pump intensity:
81
(4.6)
where is the coefficient of proportionality that we shall use later as a fit parameter.
Therefore, the output probe intensity, corresponding to , will be expressed as:
(
)
(
∫
) (4.7)
In our experiments, we have measured the output probe’s intensity dependence
(measured after the analyzer) upon the input pump intensity values, as presented in
Figure 4.5. Furthermore, we have adjusted the μ parameter (in the eq. 4.6) so that the
calculated value of the normalized output probe intensity (z = 3 µm) becomes equal to
the measured one (see Figure 4.5). The optimal values of the parameter μ were found for
all pump intensities. Then, we have calculated the local value of (eq. 4.5 and eq. 4.7)
by using the obtained values of μ. Figure 4.6 shows the obtained dependences of the
average value of the PIB and its local maximum values calculated at the “input” front of
the ChG film) for different pump intensities (incident from the same “input” side).
2 4 6 8 10 12 14 16
0.004
0.006
0.008
0.010
0.012
0.014
0.016
0.018
Prob
e ou
tput
inte
nsity
(W/c
m2 )
Pump intensity (W/cm2)
Figure 4.5: Experimentally measured dependence of the established output probe intensity upon the input pump intensity for the 3 m Ge25As30S45 thin film. The line is used to guide eyes only.
82
2 4 6 8 10 12 14 16
0.02
0.04
0.06
0.08
0.10
0.12
average value local maximal value
Phot
oind
uced
bir
efre
genc
e (
n)
Exitation Intensity (W/cm2) Figure 4.6: Average (solid curve) and local maximum (at the input front of the ChG film, dotted curve) values of the established PIB as a function of pump intensity for the 3 m Ge25As30S45 thin film. Solid and dashed lines are used to guide eyes only.
As we can see from Figure 4.6, the obtained local value of the PIB is almost one order
of magnitude higher (maximum value achieved here is nloc > 0.112) than the spatially
averaged value that is usually reported in earlier studies.
4.4 Summary and Conclusion
By simple polarimetric experiments and theoretical modeling (with an approximation
of weak PID and weak polarization dependence of Fabry-Perot reflections), we have
shown that the local value of the PIB in Ge25As30S45 chalcogenide thin films may be an
order of magnitude higher than its average value, usually reported in the literature. This
should be taken into account during the design of new photo patterned integrated optics
devices (channel waveguides, gratings, etc.) based on ChG thin films.
Acknowledgments We acknowledge the financial support of Canadian Institute for Photonic Innovations
(CIPI), Fonds Québécois de la Recherche sur la Nature et les Technologies (FRQNT) and
Natural Sciences and Engineering Research Council of Canada (NSERC). We also thank
Lens vector/TLCL Optical Research Inc. for their valuable advices.
83
Chapter 5
Study of Photoinduced Dichroism in Ge-As-S Thin
Films
Among the numerous photoinduced phenomena studied in ChG materials for both
fundamental and practical applications (discussed in detail in the first chapter), the
photoinduced anisotropy under band gap light irradiation has constituted the topic of
interest of the present thesis. In the previous two chapters, we have discussed about the
photoinduced refractive index changes, the involved mechanisms, and the average and
local values of the PIB in Ge-As-S thin films. We have also shown that these ChG thin
films can exhibit a strong dichroic photoinduced modification of the absorption with the
propagation direction of light.
The present chapter presents an article published in the Journal of Non-Crystalline
Solids and reports on the investigation of the photoinduced dichroism (PID) in annealed
Ge-As-S thin films. This study aimed at understanding the underlying mechanisms
responsible for this photoinduced phenomenon and at identifying the nature of the
involved photo-sensitive units. Bond conversion was evidenced by Raman spectroscopy.
The combined use of dynamic PID behavior, temperature and excitation intensity
dependence measurements has permitted to evidence the existence of photoinduced bond
changes beyond an energetic barrier. Furthermore, a phenomenological model was
proposed to explain the non-monotonic behavior of the PID.
84
Résumé de l’article inséré
Observations expérimentales de conversion photo-induite de liaisons
dans les couches minces vitreuses Ge-As-S
Le phénomène d’anisotropie photo-induite a été étudié dans les couches minces
vitreuses de composition Ge25As30S45 préparées par la technique d'évaporation par
faisceau d'électrons (e-beam). Des couches minces d’une épaisseur allant de 3 à 7 µm ont
été préparées puis recuites à une température légèrement inférieure à celle de la transition
vitreuse (Tg). Elles ont ensuite été caractérisées par spectroscopie de transmission UV-
Vis-NIR, analyse thermique, micro-analyse élémentaire par spectrométrie de rayons X à
dispersion d'énergie et par micro-spectroscopie Raman.
Le dichroïsme photo-induit (PID) a été étudié via l’excitation laser à une longueur
d’onde de 514 nm (équivalente à l’énergie du band gap optique du matériau). Nous avons
de plus étudié sa cinétique en enregistrant les spectres de transmission et de réflexion des
couches minces sous différentes intensités d'excitation et différentes températures. La
conversion de liaisons hétéropolaires (Ge-S, As-S) vers des liaisons homopolaires (Ge-
Ge, As-As) a été mise en évidence par des études spectroscopiques de micro-Raman
polarisé.
Nous avons enfin proposé un modèle phénoménologique unipolaire suggérant la
conversion photo-induite de liaisons au-delà d’une barrière énergétique (estimée à 14
kJ/mol) afin d’expliquer le comportement non monotone de PID observé
expérimentalement.
85
Journal of Non-Crystalline Solids 410 (2015) 65–73
Experimental Observations of Photoinduced Bond Conversions in Ge-As-S Thin Films
K. Palanjyan*, R. Vallée, T. Galstian
Center for Optics, Photonics and Lasers,
Department of Physics, Engineering Physics and Optics, Laval University,
Pav. d’Optique-Photonique, 2375 Rue de la Terrasse, Québec, Canada G1V 0A6
* Corresponding author : [email protected]
Abstract
Photoinduced anisotropy phenomena have been investigated in Ge25As30S45
chalcogenide glassy thin films prepared by the electron beam evaporation technique. Thin
films of thicknesses ranging from 3 to 7 µm were prepared, annealed slightly below to
glass transition temperature Tg and characterized by means of optical transmission,
thermal analysis, elemental microanalysis through energy dispersive X-ray spectroscopy
and by micro-Raman spectroscopy. Photoinduced dichroism (PID) was studied using
band gap 514 nm laser excitation. Its kinetics was studied by observing the transmission
and reflection of the film at different excitation intensities and temperature. The
conversion of homopolar (Ge-Ge, As-As) toward heteropolar (Ge-S, As-S) bonds was
confirmed by polarized micro-Raman spectroscopic studies.
A phenomenological unipolar model that suggests the presence of photoinduced
bond changes through an energetic barrier (estimated at 14kJ/mol), is proposed to explain
the experimentally observed non-monotonic behavior of PID.
Keywords: chalcogenide glass, thin film, photoinduced effects, birefringence,
dichroism, polarized Raman spectroscopy
86
5.1 Introduction
Optical properties of chalcogenide glasses (ChG) have been studied for decades
because of the fundamental [110, 167] and practical interests raised by these unique
materials [168-170]. Particularly intensive was the study of photoinduced phenomena,
such as the photoinduced band gap shift (commonly called photo-bleaching, PB, or
photo-darkening, PD, for a band gap shift toward shorter or longer wavelengths,
respectively) and the photoinduced anisotropy (PA), which includes photoinduced
birefringence (PIB) and dichroism (PID). The majority of those studies were performed
on binary glass systems [104, 163, 169, 171-173]. However, more complex material
compositions were also synthesized and studied [76, 106, 164]. Recently, we have
reported the studies of PIB on as-prepared Ge25AsxS(75-x) films, where x was ranging from
10 to 40 at.% [129]. These observations allowed us to find the “optimal” glass
composition (Ge25As30S45), which provided the highest PIB value ( ). The
initial studies of PID in such ternary glassy (GexAs(40-x)S60) films were reported in [164].
It was shown that the PID value in as-prepared Ge-As-S films is of the order of 10-15%,
which is substantially larger than the PID value reported in most of the previously studied
elementary or binary ChG films (1.5% in a-Se films and 2.5% in As-Se films). In
addition, it was shown that the PID value was considerably smaller in thermally annealed
films as compared with the as-prepared ones. However, important aspects of PID remain
to be investigated concerning the underlying mechanism responsible for those
photoinduced phenomena and the nature of the photo-sensitive units involved.
The aim of the current work is to study the origin of the photoinduced absorption
changes based on complementary tools, such as Raman spectroscopy, dynamic PID
behavior, as well as temperature and excitation intensity dependence measurements. We
show hereafter how the chemical bonds are changed by photo-excitation (supported by
the results of a polarized Raman spectroscopic study). We also propose a
phenomenological model suggesting that the photoinduced local heating facilitates some
subsequent heteropolar to homopolar bond conversion (supported by intensity,
temperature and dynamic studies).
87
5.2 Experimental Method
5.2.1 Thin Film Preparation and Characterization Methods
To study the Ge-As-S photo-sensitivity properties, glassy thin films of Ge25As30S45
composition were fabricated with a thickness varying from 3 to 7 μm by the electron
beam (e-beam) evaporation technique (with an electron beam voltage of 4 kV in a
vacuum of 10-6 Pa) from the bulk glass of the same composition. Glassy thin films were
deposited on BK7 glass substrates held at room temperature. More details about the
preparation of the Ge25As30S45 bulk glass and thin films can be found in our previous
work [129]. However, unlike in our earlier work [129], the freshly evaporated thin film
samples were annealed here (prior to any characterization or photoinduced effects study)
at 350˚C during 2 hours and slowly cooled down to room temperature. Such particular
attention was paid to ensure the complete removal of any residual stress and thus to
improve the reproducibility of our experiments, specifically for the further photoinduced
dichroism investigation.
Quantitative elemental analysis was carried out through Energy Dispersive X-
ray spectroscopy coupled to a FEI Quanta 3D FEG Scanning Electron Microscope
(EDAX-SEM). Unpolarized Raman spectroscopy was used to investigate the Ge25As30S45
thin films chemical homogeneity by recording spectra at 9 distinct points on the surface.
For the latter, a LabRam 800HR micro-Raman spectrometer from Horiba Jobin-Yvon
was used with a CW He-Ne laser at 632.8 nm. Laser intensity lower than 30 mW and
short acquisition times (< 10 s) were set to ensure the absence of any photoinduced
modification for each sample, which was furthermore carefully verified after each
measurement.
In addition, differential scanning calorimetry (DSC) analysis was performed by using
a Netzsch DSC Pegasus 404F3 apparatus in aluminum sealed pans at a heating rate of
10˚C/min on scratched and powdered Ge25As30S45 thin films. The optical transmission
spectra of the obtained films were recorded between 500 nm and 800 nm using a Cary
500 (Varian) double beam spectrophotometer. The refractive index of the thin films was
measured by means of the prism coupling technique (Metricon 2010M) at 532 nm while
88
their thickness was measured by using both prism coupling (Metricon 2010M) and
profilometry techniques (DekTak, Veeco 150).
5.2.2 Photoinduced Dichroism Investigation Procedure
It is worth mentioning that the PID measurements (by means of a simple
transmission polarimetric set-up) are noticeably affected by the choice of the initial
transmission and reflection conditions (defined by the initial values of the thickness d and
the refractive index n of the ChG thin film) since the transmission variations (due to
photo excitation) will be affected not only by the birefringence and absorption changes,
but also by the Fabry-Perot modulation. To eliminate this ambiguity, we have measured
simultaneously the intensity of four beams, corresponding to two polarization
components of the reflected (vertical, , and horizontal, ,) and transmitted (vertical,
, and horizontal, ,) probe beams. Figure 5.1 schematically shows the corresponding
experimental set-up. The pump beam was obtained from the CW Argon ion laser,
operating at 514.5 nm. For the probe beam, a He-Ne laser, operating at 632.8 nm, was
used in a first step prior to be replaced (in the second phase of our experiments) by the
Argon ion laser beam, which was then divided by means of the polarization insensitive
beam splitter (BS1) to obtain vertical (s) polarized probe and excitation (or pump) beams
of the same wavelength. The typical intensities of the pump ( ) beam were comprised
between 1.5 W/cm2 and 16 W/cm2 while that of the probe beams ( ) were 5mW/cm2
for both wavelengths. The pump beam (with vertical polarization) was redirected on the
ChG sample (S) by means of the mirror M2 at an incidence angle of 10°. The probe beam
(passed through a quarter wave plate ⁄ to generate circular polarization) was used at a
small incidence angle (≤ 5o).The diameters of the probe and pump beams at the surface of
the ChG thin film werepr ≈ 0.3 mm and p ≈ 2 mm, respectively. The transmitted
(through the sample) probe beam passed through a Wollaston prism (W) and the output
two polarization components (vertical and horizontal) were detected by means of photo-
detectors D1 and D2. The reflected probe beam was split into two components by using
the polarization insensitive beam splitter BS2. The resulting two beams passed through
89
two orthogonal polarizers P1 and P2 and were detected by photo detectors D3 and D4. All
detectors were calibrated prior to the experiment.
Figure 5.1: Experimental setup used for the PID study: M1 and M2 - mirrors; /4 - quarter wave
plate (placed on the path of the probe beam), S – ChG sample; W- Wollaston prism; BS1 and BS2 - polarization insensitive beam splitters, P1 and P2 – polarizers, D1-4 – photo detectors. Note: BS1 allows using the Ar+ laser (514.5 nm) as probe and pump beams simultaneously. In a different
experiment, the BS1 is removed to use the He-Ne laser (632.8nm) as probe and the Ar+ laser (514.5 nm) as pump.
5.3 Results
5.3.1 Thin Film Characterization
The variation of the measured elemental chemical composition is presented in Figure
5.2 (a) while the 9 recorded Raman spectra (normalized at 215 cm-1) are presented in
Figure 5.2 (b). The horizontal (black dashed) lines, in Figure 5.2 (a), show the theoretical
(expected nominal) values for this composition.
/4
D1
M1
M2
BS1
S W
D2 BS2
P1
P2
Pump
Probe He-Ne laser (632.8 nm)
Ar+ ion laser (514.5 nm)
D3 D4
90
Figure 5.2: (a) Elemental chemical composition measured by SEM-EDAX analyses at 9 distinct
points on a Ge25As30S45 thin film of 7µm thickness. The horizontal (black dashed) lines correspond to the nominal values (b) Raman spectra recorded at 9 distinct points on the same Ge25As30S45 thin film
(numbers 1-9 correspond to their locations, as depicted in the inset) and normalized at 215 cm-1.
One can see in Figure 5.2 (a) some deviation of the measured chemical composition
compared to the nominal one. The measured amount of germanium (Ge) is rather uniform
over the surface of the film with a maximal deviation of 1.5 at.%. More significant
fluctuations are observed for arsenic (As) and sulfur (S) with relative deviations of 5.5
at.% and 7 at.%, respectively. Nevertheless, such elemental concentration deviations are
not unusual in ChG thin films and also have to be weighed against the experimental error
of SEM-EDAX analyses. The main bands observed in the normalized Raman spectra of
the Ge25As30S45 thin film (Figure 5.2 (b)) at 215 cm-1, 242 cm-1 and 345 cm-1 remain
almost unchanged with the same relative intensities regardless of where it was recorded
on its surface (describing a square of 3 cm side, with 1.33 cm spacing between successive
points, as depicted in the inset of Figure 5.2 (b)). This indicates the good uniformity of
chemical bonds for the fabricated thin film. A more detailed description of the chemical
bonds, associated with these Raman bands, is presented later in the discussion section.
The obtained characteristic temperatures, i.e. the glass transition temperature
and the onset of crystallization temperature , indicate a
relatively good thermal stability against crystallization i.e. .
91
A typical thin film visible transmission spectrum (recorded here on a film of 3 µm
thickness) is presented in Figure 5.3. The corresponding numerical value of the band gap
was calculated with the Tauc method [174] to be 2.42 eV (corresponding to
). The absorption coefficient of the evaporated films was measured (at
) to be . As one can see from Figure 5.3, there are
significant Fabry-Perot oscillations in the optical transmission spectrum. The two vertical
arrows show the positions (on the thin film transmission spectrum) of the band gap
excitation laser wavelength (514.5 nm, CW Argon ion laser) and of the sub-band gap
laser wavelength (632.8 nm, CW He-Ne laser), which was used here to probe the
photoinduced phenomena. The measured refractive index of the film at 532 nm is
Figure 5.3: Transmission spectrum of a 3 µm thick Ge25As30S45 thin film. Vertical arrows show the position of the band gap and sub band gap lights used in the present work for excitation (at 514.5
nm) and probing (at 632.8 nm), respectively.
5.3.2 Photoinduced Dichroism Investigation
The dynamic variations of the above mentioned four intensity components during a
typical cycle of excitation and relaxation (excitation at 514.5 nm and probing at 632.8
92
nm) are shown in Figure 5.4. The probe and pump intensities (i.e. and ) were
respectively 3.5 mW/cm2 and 10 W/cm2 while the thickness of the film was 3 µm. We
can see that the powers of both polarization components of transmitted (right vertical
axis) probe drastically decrease after switching on the pump beam (at t = 0 s) before
progressively approaching steady-state values. In contrast, the intensities of reflected
beams (left vertical axis) increase in a similar drastic way as photo-excitation switches on
before an overshoot to steady-state. One can also notice that the decrease of transmitted
probe intensity is more pronounced for , i.e. the probe polarization that is parallel to
the pump. Accordingly, we observe a smaller increase of the reflected polarization
component that is parallel to the polarization of the pump. Finally, we observed a
progressive non-monotonic partial “recovery” upon the removal of the pump beam (at t ≈
2500 s).
Figure 5.4: Transmission (right vertical axis, in red) and reflection (left vertical axis, in black) of
horizontal IHT, IHR (solid lines) and vertical IVT, IVR (dashed lines) components for the sample of 3 µm thickness. Pump beam (at 514.5 nm) is vertically polarized, Ip= 10 W/cm2; probe beam was obtained
from a He-Ne laser (at 632.8 nm), Ipr = 3.5 mW/cm2.
Significant differences are observed during the excitation and relaxation processes
when the Argon ion laser beam is used as a probe (Figure 5.5). The probe and pump
intensities (i.e. and ) in this case were respectively 1 mW/cm2 and 10 W/cm2 and
the thickness of the film was 3 µm. In fact, the photoinduced changes are more
93
pronounced for the reflected beams with the 514.5 nm probe as compared to the 632.8
nm probe (Figure 5.4). Larger attenuation is also observed for the polarization
component, which is parallel to the polarization of the pump beam. Moreover, during the
relaxation process, i.e. after blocking the pump beam at t = 3400 s, probe beams exhibit a
drastic decrease followed by a progressive monotonous increase up to a steady-state.
Figure 5.5: Transmission (right vertical axis, in red) and reflection (left vertical axis, in black) of horizontal IHT, IHR (solid lines) and vertical IVT, IVR (dashed lines) components for the 3 µm thick
sample. Pump beam is vertically polarized (at 514.5 nm), Ip = 10 W/cm2; probe beam is also obtained from the same Ar+ laser (at 514 nm), Ipr = 1mW/cm2.
The described changes are different for polarizations which are parallel and
perpendicular with respect to the (vertical) polarization of the pump beam, thus
confirming the presence of PID effect [164]. However, given the specificity of
photoinduced changes observed in transmitted and reflected probe beams of the Argon
ion laser (larger variations in reflection than in transmission), we shall define the PID
(denoted below as D) by the following expression:
(5.1)
where Iv and Ih correspond to the sums of intensities of transmitted and reflected
polarizations:
94
and .
Figure 5.6 shows the typical dynamics of the PID (as defined by Eq. (5.1))
generation and partial relaxation for the 3 µm thick Ge25As30S45 film irradiated by a pump
beam of 10 W/cm2. As one can see, the PID quickly increases when the pump is switched
on (t = 0 s), stabilizes and shows a partial relaxation when the pump is switched off (at t ≈
6000 s).
Figure 5.6: Kinetics of the photoinduced dichroism PID (measured at 514.5 nm) in a 3 µm thick
Ge25As30S45 thin film annealed at 350°C.
We note that the PID reaches the steady-state value of 0.25%. It is also worth
mentioning that the sign of the PID is the same for all pump intensities and all film
thicknesses. Following the same approach (of using the sums of transmission and
reflection beams for each polarization), Figure 5.7 shows the dynamics of the total power
(T+R) for both (vertical and horizontal) polarization components of the probe,
normalized to its initial value (before the excitation). The results in Figure 5.7 (a) and (b)
show the dynamics of the PID process obtained with He-Ne and Argon ion laser probe
beams, respectively. We can see that for both probes, once the pump is switched on (at t
= 0 sec), the total power in both polarization components is decreasing monotonically:
rapidly first and then slowly reaching a steady-state. On the contrary, when the pump
95
beams are switched off (see Figure 5.7 (a) and (b) for both probe wavelengths), a non-
monotonous partial relaxation behavior is observed.
Figure 5.7: Kinetics of PID as evidenced by the sum of transmitted and reflected beam powers
for two orthogonal polarization components (normalized to their initial value). (a) probe He-Ne laser, at 632.8 nm; (b) probe Argon-ion laser, at 514.5 nm. The excitation was achieved with a vertically
polarized pump at 514.5 nm. Ip = 10 W/cm2, d = 3 µm. Letters h and v correspond to horizontal and vertical polarization components of the probe in the established excitation state. Letters h’ and v’
show the values of same components in the partial relaxation state.
In agreement with our expectations, less absorption and less dichroism were
observed when the process was probed at 632.8 nm rather than at 514.5 nm. We note that,
in the case of monitoring at 632.8 nm, after switching off the pump laser, the signal
drastically increases, oscillates and stabilizes (Figure 5.7 (a)). However, besides a
transient spike, no noticeable changes are observed when monitoring the relaxation
process at 514.5 nm (Figure 5.7 (b)).
The above-described experiments were conducted at different pump intensities and
repeated several times in order to better understand the mechanisms involved. For each
pump intensity, the amplitude of the decrease of the summed intensities (T+R) was
measured from its initial (maximum) value up to steady state of excitation (points h and v
in Figure 5.7) and up to steady state of relaxation (points h’ and v’ in Figure 5.7). The
obtained values were then averaged and plotted as a function of pump intensity, as shown
in Figure 5.8 (a) and (b). Error bars correspond to the minimum and maximum
experimental values measured for each pump intensity.
96
Figure 5.8: Averaged amplitudes of the decrease of the summed (and normalized) transmission and reflection intensities (T+R) measured from its initial (maximum) value (1-(T+R)) up to the steady state of excitation (a) and relaxation (b) for two orthogonal components; horizontal (black solid
curve) and vertical (dotted red curve), as a function of pump intensity. The excitation and probing were performed at 514.5 nm. The polarization of the pump is vertical. The thickness of the film of
Ge25As30S45 is 3 µm.
As one can see in Figure 5.8, the averaged steady-state values of amplitudes of
changes (both in excitation and relaxation states) monotonously increase up to a certain
value (corresponding to pump intensity close to 10 W/cm2) before decreasing for higher
pump intensities.
5.4 Discussion
5.4.1 Polarized Raman Spectroscopic Study
Based on the unexpected observation that the intensities of both polarizations of both
probe beams are decreasing, as observed in Figure 5.7, one can conclude that the
mechanism of photoinduced changes cannot be explained by the well-known
photoinduced rotation (reorientation) of the same dipoles (such as valence alternation
pairs [110], which are present in As2S3) [175]. In fact, one could rather hypothesize that
the processes at work here originate from photoinduced chemical changes of species
97
having higher absorption of the probe polarization component, which is parallel to the
pump beam, as shown in Figure 5.4 and Figure 5.7.
To support this assumption, we have carried out polarized Raman spectroscopy
analyses on our samples. For this purpose, the same micro-Raman spectrometer (as
previously mentioned) was used with the same conditions except that here, polarizers
were placed on the laser beam path. This technique, which was recently used by
Musgraves et al. to evidence the structure of Ge-As-S glasses as a function of their
nominal composition [23], is usually utilized to measure and distinguish Raman
symmetrical modes from antisymmetrical stretching, bending and rocking modes. The so-
called polarized Raman spectra relate to the polarization component of the scattered light,
which is parallel to that of the incident beam. Symmetrical excitation modes are generally
dominant in this configuration. The notation HH (horizontal incident light – horizontal
scattered light) or VV (vertical incident light – vertical scattered light) is then employed
[175]. On the other hand, the depolarized Raman spectra refer to the polarization
component of the scattered light that is perpendicular to the polarization of the incident
beam, and the HV (horizontal incident light – vertical scattered light) or VH (vertical
incident light – horizontal scattered light) notation is then used. Vibrational excitation
modes, like antisymmetrical stretching, bending and rocking modes are then dominant.
The implemented procedure was as follows: first, the polarized (HH and VV) and then
depolarized (HV and VH) Raman spectra were successively recorded from 100 cm-1 to
1200 cm-1 with a He-Ne laser beam (operating at 632.8 nm) focused on the surface of the
Ge25As30S45 thin film of 3 μm thickness with a 100x microscope objective. Low laser
power (<30mW) as well as neutral optical filters were used to avoid any photoinduced
material changes during the spectra recording. Then, the He-Ne laser beam was switched
off and, while maintaining the focussing conditions, the film was exposed with the beam
of an Argon-ion laser (operating at 514.5 nm) with an intensity of 3 W/cm2, with a
vertical V polarization (through the same 100x objective) during 45 min. Finally, the
laser exposition at 514.5 nm was switched off, and once again, recordings of polarized
(HH and VV) and depolarized (HV and VH) Raman scattering spectra were performed
(with the He-Ne probe) on the same irradiated spot by using the same acquisition
parameters. All experiments were performed at room temperature. This procedure has
98
been repeated at different places on the Ge25As30S45 film to ensure the accuracy of the
recorded data. The polarized and depolarized Raman spectra, recorded before and after
the laser exposure at 514.5 nm, are presented in Figure 5.9 (a) and (b), respectively. Note
that only the region of interest, i.e. from 100 cm-1 to 600 cm-1, is presented on the Raman
spectra in Figure 5.9.
Figure 5.9: Polarized and depolarized Raman spectra recorded at 632.8 nm before (a) and after (b)
vertical polarized laser exposition at 514.5 nm (0.3W/cm2) during 45 min on the Ge25As30S45 thin film.
The Raman spectra of Ge25As30S45 glasses exhibit two main broad bands consisting
in four overlapping peaks centered at 215 cm-1 (attributed to homopolar As-As bonds),
242 cm-1 (homopolar Ge-Ge bonds) and 345 cm-1 (ascribed to both heteropolar Ge-S and
As-S bonds)[129]. Then, as one can see in Figure 5.9 (a) and Figure 5.9 (b), the polarized
HH spectrum is slightly more intense than the polarized VV spectrum before and after
laser exposure, suggesting the presence of a slight initial anisotropy of the Ge25As30S45
thin film, before the pump laser exposure. Meanwhile, no significant difference is
observed between both depolarized HV and VH spectra. Moreover, the comparison
between the spectra (both polarized HH/VV and depolarized VH/HV) recorded before
and after the 514.5 nm laser exposure clearly reveals an “inversion” of relative intensity
of the 215-242 cm-1 and 345 cm-1 bands. This indicates the formation of heteropolar As-S
and/or Ge-S bonds to the detriment of homopolar As-As and/or Ge-Ge bonds, induced by
the laser exposure at 514.5nm, evidencing the chemical change of the dipoles’ nature.
200 400 6000
100
200
300
400
1 HV 2 VH 3 VV 4 HH
Inte
nsit
y (a
rb. u
n.)
Raman shift (cm-1)
1
2
3
4
After excitation
b
100 200 300 400 500 6000
200
400
600
800
4
3
2
Inte
nsit
y (a
rb. u
n.)
Raman shift (cm-1)
1 HV 2 VH 3 VV 4 HH
a
1
Before excitation
99
In order to gain a better quantitative insight on the nature of the photoinduced
changes in the bonds, we have computed the intensity ratio ρ IHV/IHH (or IVH/IVV) of the
depolarized and polarized Raman spectra, namely the ‘depolarization ratio’, which
provides structural information on the symmetry of the vibrational modes. This ratio was
shown to vary from 0 to 0.75 in amorphous materials [176]. Raman bands with a
depolarization ratio close to 0.75 are not polarized while values of
signify the existence of polarized bonds. The calculated values of depolarization ratio for
homopolar bonds and and for heteropolar bond
are ranging from 0.2 to 0.4 (see Table 5.1), indicating the strongly
polarized character of both types of bonds before and after laser exposure. Although the
depolarization ratios for a given band, calculated by using IHV/IHH or IVH/IVV, slightly
differ, they follow the same trend after laser exposure. Indeed, we can notice from the
depolarization ratios reported in Table 5.1 a decrease of after laser
exposure, while and practically do not change, no matter
how they were calculated (i.e. by using ⁄ or ⁄ ). This indicates that Raman
modes, associated to the homopolar As-As bonds (215 cm-1), are more symmetrical after
laser exposure meanwhile those associated to Ge-Ge homopolar and heteropolar Ge-
S/As-S bonds (345 cm-1) are more asymmetrical.
It is important to notice that the decrease of is more prominent in the
V (vertical) direction (see ⁄ ), which corresponds to the 514.5 nm laser beam
polarization, suggesting a stronger sensitivity of homopolar As-As bond with respect to
the laser beam polarization. In contrast, there is no significant increase of the
and corresponding to either the horizontal or vertical
directions, indicating less sensitivity of homopolar Ge-Ge and heteropolar Ge-S/As-S
bonds to the beam polarization.
100
Table 5.1: Depolarization ratio calculated from polarized and depolarized Raman spectra before and after laser exposure at 514.5 nm.
⁄ ⁄
before after before after
As-As 0.2±10% 0.17±9% 0.3±10% 0.19±9%
Ge-Ge 0.26±10% 0.22±13% 0.26±12% 0.23±13%
As-S
Ge-S 0.14±6% 0.2±12% 0.22±10% 0.26±10%
5.4.2 Proposed Model
The particular character of the absorption process (i.e. rapid decrease followed by a
slower one) is rather intriguing, particularly in the case shown in Figure 5.7 (b). There
were previous reports on excitation in similar compositions exhibiting the same type of
behavior (e.g., describing the process by a model of fast darkening followed by slow
bleaching) [106]. However we are not aware of any report of this particular behavior for
relaxation phenomena. We have tried to reproduce the dynamics of experimentally
observed power changes (Figure 5.7 (b)) by means of bi-exponential theoretical fits:
⁄⁄ (5.2)
Those fits may be done using two different physical models. Namely, the first
(bipolar) model considers (similar to that reported in [106]) the presence of two
excitation channels (into two excited chemical species with different contributions, A1
and A2, and characteristic times, 1 and 2) with opposete signs of absorption changes;
one is rapidly increasing the initial absorption (photo-darkening), while the second one is
slowly decreasing it (photo-bleaching). In this case, after the fit, the amplitudes of
corresponding contributions may be presented as , and characteristic
times as and . However, it must be noted that the obtained
fitting errors were very high by using this bi-exponential function. In contrast to the case
of excitation, the bi-exponential analysis of the relaxation process with the bipolar model
provides a better fit. The corresponding mechanism would suggest that the dynamically
101
excited species quickly relax into species which are responsible for photo-darkening. In
the same time, there is also a direct relaxation into species responsible for photo-
bleaching, which is however much slower. In this case, the corresponding contributions
may be expressed by amplitudes ,
and characteristic times,
and . If we limit ourselves to the above-mentioned considerations
only, it is still not clear why the stationary behavior of excitation should be non-
monotonous.
The second (unipolar) model of excitation considers the presence of two
mechanisms of excitation with the same signs (both increasing the ChG thin film
absorption, but with different contributions, A1 and A2, and characteristic times, 1 and
2). In this case, for the given excitation intensity, those contributions may be described
(after the fit) by the following set of parameters: , and characteristic
times, and . Obviously, both excitation channels here are
increasing the light absorption, while the second one is almost 24 times slower (and more
than twice smaller) than the first one. The fits of the monotonic excitation and relaxation
processes are very good (with low errors) within the unipolar model. However, this
model could still be used if we suggest that a consecutive bond conversion is occurring:
that is, if the relaxation into species that are responsible for the stronger photo-darkening
of the ChG, is much faster (upon the switch-off of excitation) and if then some of those
species may further be converted into species that are responsible for weaker photo-
darkening (e.g., similar to those obtained during the equilibrated excitation regime). This
might happen through an energetic barrier and this is where the above-described
particular non-monotonic behavior (dependence upon the pump intensity, Figure 5.8) of
the photo-darkening (PD) and PID (both during the established excitation and established
relaxation) might support the hypothesis of the existence of such a sequential transition.
Indeed, the initial linear-saturated increase of PID (with the increase of pump intensity)
shows that there is a given (limited) number of species, which may be excited to generate
the PD and PID. However, the further decrease (not just saturation) of the PD and PID
with the further increase of the pump intensity is rather intriguing. In fact, the
corresponding point of decline (or “turning point”) might be related to some changes of
102
the Ge25As30S45 ChG matrix (e.g., due to the resonant bond breaking and/or the increase
of the temperature of the ChG) or to the way how the excitation is distributed.
In fact, the role played by the temperature and, more particularly, by the local
heating (induced by the laser beam exposition) should be taken into account. To estimate
the latter, we can use the following approximate relation [177]:
(5.3)
where is the absorption coefficient, is the sample thickness
(we consider also the substrate), is the pump intensity (power density),
is the thermal diffusivity [178], is the thin
film density (estimated from the bulk density measurement) and
is the specific heat [179]. This theoretical dependence is depicted in Figure 5.10 along
with the actual Ge25As30S45 thin film temperature changes as measured during the photo-
excitation using a thermal camera (Jenoptik, Variocam) with a close-up lens (with spatial
resolution of 50 m and acquisition delay of 5 sec between each measurement). We used
an emissivity value of 0.71 to measure this material (measured on a Ge25As30S45 sample).
Figure 5.10: Measured temperature dependence of the Ge25As30S45 thin films surface as a function of
the pump intensity. Squares represent the experimental data and circles represent the theoretical estimation results using the equation (5.3).
We note a rather good agreement between the experimental and theoretical data. We
can also see that the “turning point”, observed in Figure 5.8, corresponds to the pump
103
intensities of the order of 10W/cm2. At this point, the temperature of the Ge25As30S45 thin
film is approximately . It should be remembered here that this temperature is
well below the measured glass transition temperature of the thin film ( .).
Thus, such intensities (and corresponding temperatures) could not dramatically change
the behavior of the ChG matrix. Furthermore, to clarify the relative role of the
photoinduced resonant bond-breaking (versus the role of the temperature alone), we have
used a relatively weak excitation power (3W/cm2), but we have studied the generation of
the PID for different temperatures of the Ge25As30S45 thin films (by placing it in a heating
oven with optical windows). The averaged steady-state values for excitation and
relaxation, measured as a function of the oven’s temperature, are presented in Figure 5.11
(a) and (b), respectively.
Figure 5.11: Photoinduced darkening (PD) of the Ge25As30S45 thin film at different oven temperatures
for two orthogonal components (horizontal: black solid curve and, vertical: dot red curve): a- excitation, b-relaxation. Intensity of the pump was 3 W/cm2 and the thickness of the film was 3 µm.
As we can see from Figure 5.11, the ‘turning point’ is observed for an oven
temperature of . By assuming that the heating of the sample induced by the
laser beam is approximately the same at room temperature than at 100C, then we can
estimate that the overall temperature of the sample will be the sum of two contributions,
i.e. from the laser exposure and from the oven. As mentioned, the experiment was
performed for a pump intensity of 3W/cm2, which corresponds to a measured heating of
104
about 60°C (see Figure 5.10). Therefore, the ‘turning point’ is observed at approximately
of the local temperature of the thin film.
The obtained results show the important role of the temperature and indicate that
there is a phenomenon that starts to play a significant role when the temperature of the
ChG overcomes some critical temperature Tc, which is well below the glass transition
temperature Tg. The following Table 5.2 presents some of the bond energies reported in
[180] that might appear in the ChG under study:
Table 5.2: Bond energies (in kJ/mol) in Ge-As-S ChG, from [180]. Bond Energy (kJ/mol)
As-As 385±10.5
Ge-Ge 264±6.8
As-S 379±6.3
Ge-S 534±3
S-S 430±0.03
Although the bond energies are generally quite different, the difference between the
As-As and As-S bond energies is of the order of 10 kJ/mol. Considering that, at room
temperature (25 ), the thermal energy is ⁄ , the same energy at Tc would
be of the order of ≈14 kJ/mol, which is of the order of the above-mentioned bond energy
difference. Thus, some of the newly created and strongly absorbing “extra” (or
overpopulated) As-S bonds may be transformed back into less absorbing As-As bonds if
the Ge25As30S45 thin film temperature is close or above Tc. Perhaps there are other
molecular units (e.g., triple or more complex structures), which might have similar
energetic barriers, but we are not aware of them.
The above-mentioned transitions may be modeled by means of an effective four level
excitation system. Indeed, we can suppose that the initial (ground) state (level 1) is
composed of various units (including relatively unstable homopolar bonds), which may
be excited into transient units (level 4), which may form various bonds after their
relaxation. Thus, a dynamic equilibrium may be achieved between continuous excitation
and relaxation processes. Among many possible bonds (and corresponding energetic
levels), two ‘effective’ levels (corresponding to different types of chemical bonds) may
105
be considered specifically. One of those levels (the level 2) is defined by chemical bonds,
which are relatively low absorbing. The next level (level 3) is defined by chemical bonds,
which strongly absorb light. This level is quickly “over-populated” (relaxation from level
4 to level 3). Furthermore, there is an energetic barrier between those two bonds (levels 2
and 3) that is comprised between 10 and 14 kJ/mol. Therefore, when the intensity of
excitation (and thus the local temperature of the sample) is high enough, before the heat
is evacuated (once the excitation is switched off), there is a further relaxation
(consecutive bond conversion) from level 3 to level 2.
A corresponding qualitative simulation, which is based on the above-mentioned
hypothesis of dynamically stabilized excitation of a four level system, reproduces very
easily (Figure 5.12) both the excitation and particularly the non-monotonic relaxation
dynamics observed in our experiment.
0 20 40 60 80
-68,0
-67,5
-67,0
-66,5
-66,0
-65,5
-65,0
Rel
axat
ion
(arb
.un.
)
Time (sec.)
Figure 5.12: Qualitative reproduction of the dynamics of absorption changes during the relaxation of a 4-level system with consecutive conversion between bonds.
Before concluding, we would like to address another issue. We have also observed a
strongly asymmetric variation of the transmitted and reflected beam powers: the
photoinduced reduction of the reflected power being much stronger in the case of the
probe beam at 514.5 nm (Figure 5.5).We think that this phenomenon may be explained
by the combination of two processes: the first one is the presence of strong light
extinction at the interface (air-ChG thin film) caused by the light refraction and
106
reflection, which are different from the case of purely dielectric interfaces [181]. The
absorption of the reflected light here might be considered with some depth of penetration,
which makes the reflection from this interface very sensitive to absorption changes. The
second one is the interferential reflection/transmission from the ChG thin film, despite
the strong absorption of the probe. Very likely, for our experimental conditions, the
starting working point corresponds to the case when the reflected power decreases with
the (photoinduced) decrease of the refractive index n. In this case, the transmitted beam
power is supposed to increase in non-absorbing stratified structures according to the
Fabry-Perot effect. However, in our case, both transmitted and reflected beams suffer
from strong photoinduced absorption. The combination of those two mechanisms may
generate stronger changes for the reflected light compared to the transmitted one. Further
studies are necessary to clarify this point.
In the meantime, given that the pump beam power is decreasing exponentially upon
its propagation inside of the Ge25As30S45 thin film, the observed changes are mainly
occurring within a rather narrow air-ChG thin film interface area (approximately at
). Then the local photoinduced absorption changes (increase) may be
estimated to be , which is extremely large if compared to the initial
material absorption of (the absorption is almost doubled).
5.5 Conclusions
In this work, we have investigated the photoinduced anisotropy effects in e-beam
evaporated Ge25As30S45 ChG thin films through a careful polarized Raman spectroscopic
study. Dynamic absorption studies have been performed on the prepared thin films by
using various wavelengths, polarizations and propagation directions (transmission and
reflection). We have observed strong asymmetric changes of light power both during
excitation (a rapid decrease followed by a progressive one) and relaxation (non-
monotonic). The non-monotonic behavior was also observed in stationary conditions
upon the excitation intensity and environmental temperature. A phenomenological
unipolar model was proposed to account qualitatively for the experimental observations.
According to this model, the photo-excitation induces breakage of the homopolar bonds
107
(As-As and Ge-Ge) and brings the Ge25As30S45 ChG thin film into a dynamically
equilibrated state with bond distribution including originally existing, newly created and
intermediate excited states. The removal of excitation leads to the partial relaxation and
formation of new (mainly heteropolar) bonds including some sequential bond
conversions.
Acknowledgements
This research was supported by the Natural Sciences and Engineering Research
Council of Canada (NSERC). We would like to thank our collaborators, E. Knystautas
(thin film deposition), Y. Messaddeq (bulk glass preparation and DSC analyses), M.
Andrews (initial trials of Raman scattering) and Y. Ledemi for his valuable advice.
109
Chapter 6
Polarization Holograms in thin films of Ge25As30S45 Glass
ABSTRACT: Vector hologram recording is experimentally studied on amorphous
chalcogenide Ge25As30S45 thin films prepared by e-beam evaporation technique. The high
diffraction efficiency of these materials is based on their high photoinduced anisotropy
previously demonstrated [129]. Vector holographic gratings were obtained
under illumination by circularly polarized Ar+ laser beam operating at 514 nm (near the
thin film band gap) and were analyzed by comparison with scalar recording. Moreover,
we have shown a better thermal stability of the vector holograms inscribed in the
Ge25As30S45 thin films compared to those recorded on chalcogenide As2S3 thin films
(fabricated by the same e-beam evaporation method).
6.1 Introduction
The photoinduced modification of refractive index and absorption has been
intensively studied in chalcogenide glass (ChG) systems [111].
The ability to change the optical properties of ChG thin films under illumination with
band gap light [143, 182-184] makes them very suitable materials photo inscribed guide
or holographic recordings, achieved by the spatial modulation of either intensity or
polarization [48, 185, 186]. The polarization optical recording in such optically
anisotropic materials has potential applications for the implementation of new
polarization optical elements such us polarization beam splitters, retardation plates and
optical switches.
The addition of germanium (Ge) into well-known arsenic sulfide ChG system not
only improves the mechanical and thermal properties of the glass but also increases its
photosensitivity and more particularly its photoinduced anisotropy, as shown in chapter
2. In the latter, the optical isotropy which is characteristic to any glassy material, was
indeed broken in ternary Ge-As-S glassy thin films under polarized laser exposition [129,
165]. The measured high values of photoinduced linear dichroism and photoinduced
110
birefringence which are both photoinduced anisotropy effects, make them excellent
candidates for holographic recording investigations. Contrary to scalar holograms,
recorded vector holograms can be employed as diffractive elements for applications in
real time spectral photopolarimetry [187].
There are few reports in the literature about vector hologram recording in ChG
materials. The first vector holograms recorded in ChG have been experimentally and
theoretically studied by Kwak and co-workers in 1988 [188]. They have recorded vector
holographic gratings with a spatial period of 2 µm and a maximal diffraction efficiency
of 0.19% in arsenic sulfide As2S3 films by using two Argon Ar+ laser beams (operating at
514 nm) with orthogonal linear polarizations. The vector holographic gratings recorded in
these thin films have only exhibited the zero and first order diffractions, whereas the
scalar holographic gratings have further displayed the second and third diffraction orders.
This was due to the weak response of the material compared to the scalar one. Later,
Mitkova et al. [189] have investigated in details the properties of vector holograms in
amorphous chalcogenide Se70Ag15I15 films (with thickness 0.5–1.0 µm). They have
shown that the observed high diffraction efficiency with scalar holograms written in the
same material, can be caused by the different involved recording mechanisms. By using
two orthogonally polarized Ar+ laser beams (operating at 488 nm), they have recorded a
maximal diffraction efficiency of 1%. These vector holographic gratings were stable for
six months. Increasing the polarization recording efficiency in ChG thin films can be
achieved by using two opposed circularly polarized recording beams instead of two
orthogonal linearly polarized beams [189]. Also, it is important to mention the works of
Asatryan et al. directed in one hand to the study of polarization holograms inscribed in
As2S3 ChG thin films through different recording configurations and on the other hand to
the investigation of the different dipole interactions responsible for relief modulations
[48, 190]. They have presented interesting results regarding the impurities of recorded
polarization holograms and showed that the modulation period depends on recorded
beams polarization state (linear, circular, etc.), resulting in photoinduced mass transport
phenomenon.
In this paper, we report on the recording of polarization diffraction gratings in Ge-
As-S ChG thin films with two plane waves of equal intensities and orthogonal circular
111
polarizations. Moreover, the behavior of recorded vector holograms were compared with
that of scalar holograms, recorded in thin films of same chemical composition. Besides,
the stability over time of the gratings inscribed in the Ge-As-S films was evaluated and
compared to that of gratings written in As2S3 thin films fabricated by the same e-beam
evaporation technique.
6.2 Experimental method
ChG thin films (of about 7 μm thickness) were prepared by electron beam
evaporation method (with an electron beam voltage of 4 kV in a vacuum of 10-6 Pa) from
the bulk glass of composition Ge25As30S45 onto BK7 glass substrates held at room
temperature. The films were then annealed at 350˚C during 2 hours in an electrical oven
in ambient atmosphere to remove any residual stress induced by the deposition. More
details about the method employed for the fabrication of the Ge25As30S45 bulk glass and
thin films can be found in our previous work [129].
The experimental set-up used to record polarization holograms is shown in Figure
6.1. Hologram recording was achieved with two circular orthogonally polarized (left-
hand and right-hand rotation) continuous wave (CW) Argon laser beams of equal
intensities of about 4W/m2 and operating at 514 nm.
112
Figure 6.1: Experimental set-up for the vector hologram study: pump – Ar-ion laser; probe - He-Ne laser; /2- half-wave plate; /4- quarter-wave plate; WP-wollaston prism; S-sample; D1, D2, D3-detectors.
To record the scalar holograms, we have first split the initial linearly s-polarized
beam of the CW Argon ion laser (at 514 nm) into two beams with the same polarization
and their intensities were equalized through a filter (F), resulting thus only in an intensity
modulation. The inscription of vector holograms was then achieved by two recording
configurations: first, by using orthogonally polarized linear s and p beams, by means of
Wollaston prism (WP). Second, the vector hologram was recorded by two circular
orthogonally polarized (right-hand (RCP) and left-hand (LCP) rotation) beams which
were obtained with the help of two quarter–wave plates placed after mirrors M1 and M2.
A simple polarimeter was used to measure the circularity of these beams. The Stocks
parameters of left and right circular beams were S2= -0.0271, S3= -1.1267 and S2= -
0.0207, S3= 1.0580, respectively. The intensities of these components were equalized
through a filter (F). The angle between these beams was about 5° while the spot size was
about 2 mm. They overlapped at the same point on the sample (S), writing then the
holographic grating with a periodically modulated polarization and almost constant
intensity over the sample surface. A vertically (s-polarization) polarized He–Ne laser
beam, operating at 632.8 nm, was used as a probe to study the inscribed gratings and
diffracted orders recorded by the detectors (D1, D2, D3). All experiments were carried out
at room temperature.
D1
S /4
/2
Ar+-ion
WP
D2
D3
He-Ne
113
6.3 Results
As mentioned in the introduction, the addition of germanium Ge into the binary
arsenic sulfide glass results in larger photoanisotropic sensitivity besides the
improvement of its mechanical and thermal properties thanks to an increased glass
network connectivity. Therefore, a higher thermal stability can be expected as well for
the holographic gratings inscribed in Ge-containing thin films (e.g. like the composition
under study here, Ge25As30S45) than in other ChG thin films, arsenic sulfide As2S3, for
instance. To verify this assumption, thin films of the latter composition were prepared by
using the same by e-beam evaporation method. The same vector holograms (recorded by
RCP+LCP pumps) were inscribed in Ge25As30S45 and As2S3 ChG thin films, both of 7
µm thickness. The samples were then heated at different temperatures during the
relaxation to study dynamically the efficiency change as a function of annealing
temperature. Figure 6.2 shows the normalized diffraction efficiencies obtained from the
Ge25As30S45 (solid line) and As2S3 (dashed line) ChG thin films as a function of
temperature. The intensity of each probe beam was 4W/cm2 in both cases.
Figure 6.2: Normalized diffraction efficiencies of the vector holograms recorded in Ge25As30S45 (black) and As2S3 (red) thin films as a function of the temperature. Lines are guide to the eye.
0 50 100 150 200 250 300 350
0.0
0.2
0.4
0.6
0.8
1.0
Tg(Ge
25As
30S
45)T g(As2S3)
Nor
mal
ized
diff
ract
ion
effic
ienc
ies
Temperature (oC)
As2S
3
Ge25
As30
S45
114
As the refractive index of these ChG thin films is high (≥ 2.4), the diffraction
efficiency ( ) was calculated by:
where is the intensity of the first diffracted order and is the intensity of order zero.
As we can see in the Figure 6.2, the gratings efficiency of the As2S3 thin film is
drastically falling down to 0, i.e. the gratings are completely erased at a temperature of
200˚C. Whereas in the Ge25As30S45 thin film, the gratings still exhibit 40% of relative
efficiency at 300˚C and only vanish at 350˚C.
Then, the Figure 6.3 presents the diffraction efficiencies of scalar and vector
holograms recorded on the same setup as a function of pump intensity. The scalar
hologram was recorded with two linearly s-polarized probe beams, whereas the vector
hologram was recorded with two orthogonally polarized circular probe beams. As we can
see in the Figure 6.3, the efficiency was maximal for the scalar hologram at a pump
intensity of 4W/cm2. Hence, the results presented further in the text were obtained at this
pump intensity.
Figure 6.3: Diffraction efficiency (%) of vector (solid line) and scalar (dashed line) holograms recorded in the same Ge25As30S45 thin film as a function of pump intensity. The vector hologram was recorded by (RCP+LCP) polarizations beams while the scalar hologram was recorded with two linearly s-polarised beams. Lines are guide to the eye.
2 3 4 5 6 7
10
15
20
25
30
35
40
vector scalar
Diff
ract
ion
effic
ienc
ies (
%)
Intensity (W/cm2)
115
Furthermore, the holograms recorded in the Ge25As30S45 thin films were studied to
compare their behavior and involved mechanisms with those well-known of the As2S3
thin films. Figure 6.4 shows the kinetics of the diffraction efficiency of vector and scalar
holograms as a function of exposure time. The scalar holograms were recorded with two
linearly s-polarized beams and the vector holograms with two orthogonal (s+p) or
(RCP+LCP) polarizations. In Figure 6.4, we can observe for the vector hologram a
maximum diffraction efficiency of 12% at a pump intensity Itotal = 4 W/cm2, while a
diffraction efficiency of about 35% is reached for the scalar gratings of the same sample.
Optical microscopy images of the obtained gratings are presented in the Figure 6.5.
In the Figure 6.5 (a), the gratings period observed for the scalar hologram is around 3.1
µm. In the Figure 6.5 (b), the gratings period observed for the vector holograms recorded
by two circularly polarized light (RCP+LCP) is about 2.5 µm while for the vector
gratings obtained with two orthogonal linearly polarized light (s+p), shown in the Figure
6.5 (c), the period is about 1.23µm. Except for the polarization state of exposition light,
all the experimental conditions, i.e. pump intensity, angle between the probe beams, etc.,
were kept identical for this study.
Figure 6.4: Dynamic diffraction efficiency (%) of vector (sold line) and scalar (dashed line) holograms recorded in the same Ge25As30S45 thin film. The vector hologram was recorded by (RCP+LCP) polarizations beams while the scalar hologram was recorded by two linearly s-polarized beams. The pump intensity was 4W/cm2.
0 2000 40000
5
10
15
20
25
30
35
40
Diff
ract
ion
effic
ienc
y (%
)
Time (sec)
vector scalar
Pump OFF
116
Figure 6.5: Optical microscope images of recorded gratings on the same Ge25As30S45 thin film: (a) scalar gratings written by (s+s) polarization beams; (b) vector gratings written by (RCP+LCP) polarization beams; (c) vector gratings written by (s+p) polarization beams.
It is also known that the vector gratings recorded by two orthogonal-circularly
polarized beams can be used for the measurement of the ellipticity (or the Stokes
parameter S3) of any polarized light beam by means of detection of +1 and -1 diffracted
orders [191]. Moreover, one has to mention that DekTak profilometry measurements and
atomic force microscopy (AFM) measurements did not reveal any volume modulation at
the surface of the recorded holograms.
These holograms are polarization selective elements: indeed, as one of the diffraction
efficiencies (the right or left circular polarization of probe) becomes minimal, the other
one becomes maximal when changing the polarization state of the reading beam. Figure
6.6 shows the dependence of the diffraction efficiencies (of holograms recorded with two
beams, left and right circular polarized, of Argon ion laser operating at 514 nm) of
diffraction orders (+1 and -1) versus the angle of rotation of a quarter-wave plate placed
on the path of the reading (probe) beam. The recording beam intensity was 4 W/cm2 and
time of exposure was over 30 min. An He-Ne laser beam at 632.8 nm was used as probe.
As we can see in the Figure 6.6, the intensities of the diffraction orders change
correspondingly with the variation of the polarization of the probe beam.
≈ 3.1µm ≈ 1.3µm ≈ 2.7µm
a b c
117
Figure 6.6: Diffraction efficiency of +1 and -1 diffracted orders as a function of the rotation angle of the quarter-wave plate (the elasticity of the incident probe beam polarization). Ge25As30S45 thin film thickness is 7 µm. Lines are guide to the eye.
6.4 Discussion
As it was expected, the addition of germanium Ge to the glassy matrix has increased its
thermal stability compared to the As-S glass one by increasing the network reticulation.
As we can see from the Figure 6.2, the gratings efficiency of the As2S3 thin film starts to
decrease at 100˚C and at about 200˚C, which is close to its glass transition temperature
(Tg ≈ 180˚C), the grating erased drastically. While at those temperatures, the efficiency
for Ge25As30S45 gratings is still 100%. The continuous heating damages the grating and
decreases its efficiency. However, the slope of this decrease is gradual and even at 300˚C
the Ge25As30S45 gratings exhibit about 40% of relative efficiency. The complete erasure
occurs at 350˚C, which is close to the glass transition temperature of this composition (Tg
≈ 350˚C). Such stability increases therefore the number of potential applications of this
glass composition as polarization selective optical element.
As presented in the previous section, the diffraction efficiency is higher in scalar
gratings than in vector gratings. There is few report in the literature about the involved
photoinduced anisotropic mechanisms in ChG thin films with regard to the recording of
-100 -80 -60 -40 -20 0 20 40 60 80 1000.000
0.005
0.010
0.015
0.020
0.025
+1 -1
Diff
ract
ion
effic
ienc
ies (
%)
Angle of rotation of the quarter wave plate (o)
118
polarization holograms. Ozols et al. [192] assumed that the photoinduced anisotropy
originates from the reorientation and generation of defects (D centers) by both band gap
and sub band gap lights [58, 193]. This hypothesis was supported by the EXAFS
measurements performed in As-Se thin films. An increase of the next-nearest-neighbour
distance around the Se atoms was found and its magnitude was related with the direction
of light polarization [194]. The main reason why the vector hologram recording is less
efficient than the scalar hologram recording in ChG films is the much smaller
concentration of D centers. Indeed, vector hologram recording involves indirect
electronic transitions, whereas scalar recording involves direct ones. The authors have
also shown an easy erasure of vector holographic gratings by one of the recording beams
due to the fact that a periodical D centers orientation distribution can be easily destroyed
by light (the He–Ne laser photon energy is about twice the activation energy of D
centers). This conclusion is also confirmed by the easier inscription of vector gratings at
higher pump intensities.
Furthermore, to understand the dynamic changes of the first order diffraction
efficiency observed for the scalar and vector holograms (see Figure 6.4), we should
understand which type among thin or thick these recorded holograms are: that depends on
two factors, i.e. geometrical
and physical
, where d is the
thickness of the sample, n is the refractive index, is the gratings period, and is the
dielectric constant of the material given by:
For thin holograms, and .
In our case,
Thus, and
Therefore, the holograms studied here cannot be considered nor thin neither thick. This
also confirmed the recorded value of diffraction efficiency, which was 35%, while for the
thin holograms the maximum theoretical value could be 33.9%. The oscillations we
observe in the vector holograms (Figure 6.4) can be related to the fact that they are not
completely pure and to the presence of some intensity modulation which decreases the
119
diffraction efficiency. The non-purity of the recorded vector gratings is also notable in the
Figure 6.6, where we can see some deformation of the curve and even peaks shifting.
Such kind of deformation was already reported in the similar study in As2S3 ChG thin
films by Asatryan et al. [48]. Such behavior was explained by the creation of
superimposed undesirable scalar gratings during the vector hologram recording. This
hypothesis was confirmed by studying the kinetics of recording and optical erasure of
vector holograms. The last was not possible to carry out in the case of recording
(RCP+LCP) polarization configuration, which confirms the non-purity of recorded vector
holograms.
Another interesting aspect of the present study is the period difference among the
recorded gratings and particularly, the period doubling for the vector hologram recorded
by (RCP) and (LCP) beams compared to (s) and (p) polarization recording ones. The
hologram recording is based on the phenomenological model of anisotropy proposed by
Fritzsche who suggested the presence of different polarizable anisotropic units (native or
photoinduced) [120]. Asatryan et al. have postulated in their study the existence in As2S3
ChG thin films of these uniaxial ellipsoidal microvolumes which can have different
polarizability tensor (disk-like and cigar-like) [190]. Based on this assumption, they
examined the polarization states of interference gratings for all configurations and
calculated the period of corresponding relief modulation. Their analysis demonstrated
that in the case of the cigar-like microvolumes, the period of relief modulation recorded
with (s+p) polarizations will be two times smaller than the (RCP+LCP) recorded case.
The peaks of relief grating obtained by (s+p) recording configuration occur, when the
recorded polarization is circular and do not depend on the direction of circularity (right or
left). On the contrary, in the (RCP+LCP) configuration case, the effective recorded light
polarization is always linear but directed differently. The peaks of grating occur when the
orientation of defects, i.e. the polarization is strictly the same. Thus, in the (s+p)
configuration, these peaks of relief modulation are double during the same period
compared to (RCP+LCP) configuration. Therefore, the period of the grating was two
times smaller in (s+p) recording configuration case. As we can see from the Figure 6.5,
these cigar-like microvolumes can be also present in the Ge25As30S45 composition, as the
120
same phenomenon of period doubling occurs, depending on the method used for
recording the vector holograms.
6.5 Conclusion
In summary, the recording of vector and scalar holographic gratings in Ge25As30S45
ChG thin films was experimentally studied and discussed with regard to anterior works
conducted on As2S3 thin films. The choice of the material is motivated by the significant
improvement of the thermal stability of the polarization holograms recorded in these thin
films compared to those recorded in AsS thin films. This is achieved thanks to the
addition of germanium Ge to the As2S3 vitreous network, which results in an increase of
its connectivity and thus provides a better mechanical and thermal stability.
Then, experimental results have shown different behaviors for the vector and scalar
holographic recordings. The diffraction efficiency and the gratings period for scalar
holograms recorded in Ge-As-S films at 514 nm are higher than those of the vector
holograms recorded, as already reported in As2S3 thin films [192]. This may be due to
the lower concentration of charged D centers compared to the concentration of sites of
photoinduced structural changes (in our case homopolar As-As and Ge-Ge bonds), and/or
to the indirect electronic transitions in the vector recording case which are less efficient
than direct transitions in the scalar recording case. In our experiments, maximal recorded
diffraction efficiencies of 35% and 12% were measured for scalar recording and vector
recording, respectively. In addition, the obtained results confirm, that the mechanism
previously proposed to explain the gratings period doubling in As2S3 thin films for some
type of recording configuration, as well as the non-uniform polarization selectivity of
recorded holograms, also works for the Ge-As-S ChG composition. Recorded
polarization gratings can be used to measure the polarization state (ellipticity) of
polarized light beam.
121
Chapter 7
Application of Photoinduced sensitivity in Ge-As-S
Chalcogenide Thin Films: GRIN Lens Formation
The present chapter proposes a second application taking advantage of the photo-
sensibility of the Ge-As-S thin films: the inscription of gradual variation of refractive
index, the so-called GRIN lenses. Unlike the other known methods to fabricate such
lenses (by neutron irradiation, chemical vapor deposition, ion exchange, ion stuffing and
polymerization, thermal treatment or UV irradiation), we report here the formation of
GRIN lenses by a simple CW Argon laser irradiation of the Ge25As30S45 ChG thin film.
The studies of the optical performance and the wave front distortions of the obtained
lenses were performed by using a Shack Hartmann wave front sensor as a function of the
different conditions (pump and probe intensity, polarization, etc.) used to inscribe these
GRIN lenses.
122
Résumé de l’article inséré
Seconde application basée sur la photo-sensibilité de couches minces
vitreuses de chalcogénures
Nous décrivons ici la formation photo-induite de lentille à gradient d'indice (GRIN)
dans des couches minces vitreuses de chalcogénures de composition Ge25As30S45. Nous
avons examiné les changements d'épaisseur de ces échantillons par profilométrie 3D à
pointe (Dektak) et la performance optique ainsi que les distorsions de front d’onde des
lentilles obtenues en utilisant un capteur Shack Hartmann. La formation de lentilles
GRIN est liée au déplacement du bord de transmission vers des longueurs d'onde plus
courtes (augmentation du band gap optique, effet de photo-blanchiment ou photo-
éclaircissement) induit par l’exposition à un faisceau laser continu (Argon). La
biréfringence photo-induite de ce matériau est à l'origine de la formation de lentilles
GRIN anisotropes [254].
123
SPIE proceedings 9288, Photonics North 2014, 92880L (2014).
Photoinduced GRIN Lens Formation in Chalcogenide Ge-As-S
Thin Films
K. Palanjyan, R. Vallée, T. Galstian*
Center for Optics, Photonics and Laser,
Department of Physics, Engineering Physics and Optics, Laval University
Abstract
We describe the photoinduced formation of gradient index (GRIN) lenses in thin
films of chalcogenide glass (ChG) of Ge25As30S45 composition. We examine the changes
of thickness of these samples by DekTak profilometry, as well as the optical performance
and wave front distortions of the obtained lenses by using a Shack Hartmann sensor. The
GRIN formation is related to the photoinduced shift of the band gap towards shorter
wavelengths (so-called photo-bleaching effect). The corresponding photoinduced
birefringence of this material is in the origin of anisotropic GRIN lenses formed [129].
Keywords: chalcogenide glasses, GRIN lenses
124
7.1 Introduction
The gradient index (GRIN) method is based on the gradual variation of the refractive
index of an optical material to obtain lenses, prisms or more complicated refractive index
patterns while maintaining flat external surfaces. This kind of gradient-index component
can be manufactured from different materials (including glasses [195-197], plastics [198-
200], germanium single crystals [201], and polycrystalline zinc selenide or sodium
chloride [198, 199, 202]) in different forms, such as rod lenses, fiber couplers and optical
fibers for applications in telecommunications and image-transmission systems. Interest in
GRIN lenses has greatly increased due to their ability to reduce spherical and chromatic
aberrations [198, 199]. There are three types of refractive index gradients: the axial, the
radial or the spherical gradient. Numerous techniques have been used to produce
refractive index variations in glasses and plastics. Those techniques include neutron
irradiation [203], chemical vapor deposition [202], ion exchange [204], ion stuffing and
polymerization [205], as well as thermal treatment [206] or UV irradiation [207].
Different mechanisms are involved. For example, the bombarding of a boron-rich glass
(such as BK7) with neutrons creates a change in the boron concentration and thus a
change in the index of refraction [208]. The main difficulty of such a technique is the
large number of neutrons required to create an index change and the fact that this gradient
is not permanent.
Another technique, widely used in telecommunications, is the chemical vapor
deposition. This method is based on the deposition of chemicals on the surface of a
substrate resulting from a chemical gaseous phase reaction, producing thus a solid layer.
The chemical composition of the deposited solid can be then tailored by a fine tuning of
the reaction parameters, like the concentration of gaseous reactants [202, 209].
Glasses and polymers are two well-known materials employed for GRIN lenses
manufacturing, obtained by irradiation or polymerization techniques [206]. For polymeric
materials, the gradient can be generated by their irradiation with UV light via an energy-
controlled process or simply by laser beam. By this technique, lenses with large
geometries can be obtained with a relative ease and arbitrary gradient profiles can be
manufactured [210, 211].
125
One can also notice the method based on ion-exchange process where ions from a
molten bath of a salt material (such as lithium bromide) diffuse into a glass and exchange
for ions in the glass, usually sodium ions [204]. A one-for-one exchange occurs, and
therefore a gradient in composition is created. The profiles are nonetheless limited to
Gaussian, Lorentzian, and linear shapes [212]. Besides, another chemical method, based
on the phase separation, exists [205, 213]. This technique usually implies a special glass,
which can support a controlled phase separation by heat-treatment and a chemical etching
(e.g. with acid) to remove one of the separated phases. When immersed into the chemical
bath, the ions or the molecules can also diffuse into the glass material, creating a gradient.
Nevertheless, if the phase separation is not uniform, then the gradient is not uniform.
The crystal growing method should also be mentioned [203] [214]. In this technique,
the crystal (single or polycrystalline) is pulled from a seed, e.g. a sodium chloride seed.
As a function of time, more sodium chloride is pulled out from a molten NaCl/AgCl bath,
making thus the silver chloride concentration increase in the bath. As the crystal
continues to be pulled, the silver chloride concentration continues to increase to such a
point that the crystal is forced to take a small amount of silver chloride. As the growth
continues, the composition of silver chloride in the bath continues to increase, and more
silver chloride is forced into the crystal, creating therefore a gradient within the grown
crystal.
In the present work, we describe the formation of GRIN lenses in Ge25As30S45 ChG
by a simple irradiation with the CW Argon laser. In our previous work [129] , we have
shown a shift of the optical band gap of this material towards shorter wavelengths after
irradiation (so-called photo-bleaching effect) and a relatively high photoinduced
birefringence. Here, we examine the changes of thicknesses of the Ge25As30S45 glass
samples by DekTak profilometry. We also study the optical performance and wave front
distortions of obtained lenses by using a Shack Hartmann wavefront sensor. Lenses
obtained under different conditions are also examined for different probe polarizations.
126
7.2 Experimental method
The Ge25As30S45 glass samples were prepared by melting high purity starting
elements (germanium (Ge), arsenic (As) and sulfur (S)) in a fused silica ampoule
evacuated to 10−3 Pa. More details about the procedure used to prepare the glass samples
can be found in our previous work [129]. Then, the thin film deposition was carried out
by electron beam evaporation technique. Once again, more details about the conditions of
evaporation are presented in the reference [129]. The fabricated films were annealed for
1h at the glass transition temperature, and slowly cooled down to room
temperature. Next, the prepared ChG films samples of 3 to 5 μm thickness were
irradiated by a linearly (S) polarized CW Argon ion laser (operating at 514.5 nm) at
normal incidence (from the ChG side). Samples were exposed in air
during 30, 60 and 120 min with different laser intensities, varying from 0.5 W/cm2 to 5
W/cm2. Then, the thickness changes of these samples were studied by surface
profilometry (Dektak Veeco 150) while the optical performance and wavefront
distortions (of obtained GRIN lenses) were examined by a Shack Hartmann sensor
(Imagine Optics, Shack Hartmann HASO 3-42). Lenses obtained under different
conditions were finally examined for different probe (from a He-Ne laser, operating at
632.8 nm) polarizations (parallel and perpendicular to the direction of the polarization of
the excitation).
7.3 Results and Discussion
The modifications of the planar wave front (before irradiation) of the probe beam,
crossing these thin films after the excitation laser exposition were observed by using the
Shack Hartmann sensor. Figure 7.1 shows the typical image of the obtained wave front of
the probe beam that traversed the obtained lenses, measured for the sample of 5 µm
thickness. For the illustrated case, the irradiation time was 30 minutes and the intensity of
irradiation was ⁄ . The intensity profile of the excitation laser beam was
Gaussian with a diameter of 3 mm (and consequently the GRIN lens dimension was
considered as the same).
127
Figure 7.1: Wave front of the probe beam exiting the GRIN lens measured by the Shack-Hartmann: exposure time was 30 min and the power was 8W/cm2. The thickness of the thin film was 5 μm. The
asymmetry of the wave front profile is due to the inhomogeneity of the excitation laser beam.
The surface profilometry measurements did not reveal any noticeable change of the
film’s surface after its irradiation2. Thus, the observed changes in the probe wave front
are related to the refractive index modifications.
Furthermore, the dependences of optical power (in diopters) upon pump intensity and
irradiation time of these lenses were studied. The Figure 7.2 (a) shows the dependence of
the calculated (using the experimental data obtained with the Shack-Hartmann sensor)
optical powers of the lenses as a function of the intensity of the excitation laser for probe
polarizations parallel and perpendicular to the direction of the polarization of excitation
beam (dashed and solid curves, respectively) for different irradiation times (different
colors).
2 See the corresponding 2D profiles in the end of chapter.
128
Figure 7.2: (a) Lens optical power dependence on pump intensity for different irradiation durations,
observed with parallel and perpendicular probe polarizations (with respect to pump polarization); (b) Lens optical power dependence on irradiation time for different pump intensities, observed with
parallel and perpendicular probe polarizations (with respect to pump polarization).
The Figure 7.2 (b) shows the dependence of the measured optical power of the lenses
as a function of the irradiation time for probe polarizations parallel and perpendicular to
the direction of the polarization of excitation (dashed and solid curves respectively) for
different intensities of the pump laser (different colors).
As we can see in Figure 7.2 (a) and (b), the obtained optical powers depend on the
irradiation time and laser intensity. The obtained lenses are divergent and the optical
power is greater for higher pump intensities, but the longer irradiations deform the
created wave fronts. In all cases, the absolute value of the induced optical power is
greater when observed with perpendicular polarized probe (with respect to pump
polarization). The highest optical power (-0.43 diopters) is achieved by a 30 min
irradiation with the pump intensity of 8W/cm2.
We can measure the optical anisotropy based on the difference of optical powers in
perpendicular and parallel directions.
The optical power D (in diopters) of a GRIN lens may be calculated as the invers of
its focal distance F (in meters) for both polarizations of the probe
,
where r is the radius of the spot ( ) and d is the thickness of the sample
a b
30 45 60 75 90 105 120-0,45
-0,40
-0,35
-0,30
-0,25
-0,20
-0,15
-0,10
-0,05
Opt
ical
pow
er (d
ptr)
Temps (min)
4 W/cm2_parallel 4 W/cm2_perpendicular 6 W/cm2_parallel 6 W/cm2_perpendicular 8 W/cm2_parallel 8 W/cm2_perpendicular
4 5 6 7 8-0,45
-0,40
-0,35
-0,30
-0,25
-0,20
-0,15
-0,10
-0,05
0,00
Opt
ical
pow
er (d
ptr)
Pump intensity (W/cm2)
parallel_30min perpendicular_30min parallel_60min perpendicular_60min parallel_120min perpendicular_120min
129
( ). The photoinduced birefringence can thus be calculated as
. Thus, we can conclude that and the value for the intensity
8W/cm2 is which is close to the value that we reported in our previous work
[129].
We have examined the same samples at different times to assess their stability and
ageing behavior. After one month of storing in lab conditions under ambient atmosphere
and without any particular precautions, some damages were observed on the surface of
those samples (such as cracks or traces of moisture). The Figure 7.3 shows scanning
electron microscope (SEM) images of these damaged surfaces after 1 month.
Figure 7.3: Scanning electron microscope (SEM) images of surfaces of a damaged sample.
The Figure 7.4 shows the evolution of optical power in perpendicular (a) and parallel
(b) probe polarization directions for different pump intensities after six months.
130
Figure 7.4: Modification of the measured optical power over time for different pump intensities
examined with two probe polarizations: parallel to irradiation polarization (a) and perpendicular to
irradiation polarization (b).
The further analyses of these samples (transmission spectra, Raman spectra and
elemental EDAX analyses) have revealed some compositional changes for the As and S
elements after six months of storing in ambient atmosphere. Surface oxidation is
occurring with time, resulting in the diffusion of oxygen atoms within the films volume.
In all cases, we noticed a progressive decrease of the lens power until its complete
vanishing 180 days after the irradiation (Figure 7.4).
The preliminary study of this aging phenomenon shows that it is not related to a
chemical relaxation effect of the Ge25As30S45 thin film (conversion of homopolar bonds
towards heteropolar ones) given the absence of any change in the corresponding Raman
spectra (Figure 7.5 (a)). On the other hand, the elemental chemical analyses, performed
by EDAX technique on the sample surface (Figure 7.5 (b)), reveal a surface oxidation,
which is a well-known problem of ChG materials if no special precaution is taken for
their storing.
a b
0 20 40 60 80 100 120 140 160 180
-0,4
-0,3
-0,2
-0,1
0,0
parallel polarization
Opt
ical
pow
er (d
ptr)
Time (days)
4 W/cm2
6 W/cm2
8 W/cm2
0 20 40 60 80 100 120 140 160 180-0,4
-0,3
-0,2
-0,1
0,0
4 W/cm2
6 W/cm2
8 W/cm2
perpendicular polarization
Opt
ical
pow
er (d
ptr)
Time (days)
131
Figure 7.5: Raman spectra (a) and EDAX elemental quantitative analyses (b) of the Ge-As-S films of 5 µm thickness (freshly evaporated (dashed curve) and 180 days stored in ambient atmosphere after
the irradiation (solid curve)).
7.4 Conclusions and Prospects
We have used the irradiation of a photosensitive chalcogenide glass thin film of
Ge25As30S45 composition with a CW Argon laser (near the glass optical band gap; at
514.5 nm) to build GRIN lenses. The measurements of the probes wave fronts, passing
through these lenses, were performed for different exposure times and intensities by
using a Shack-Hartmann wave front sensor. The obtained lenses have shown negative
optical powers, ranging from -0.05 Diopters to -0.45 Diopters. The lenses obtained by
this technique have Gaussian profiles with a diameter comparable to the excitation laser
spot ~ 3 mm. The original thickness of examined films was 5 micrometers. The surface
profile studies (performed by means of surface profilometry) did not reveal noticeable
changes in the thin film surface after irradiation, indicating that the observed changes are
only related to the refractive index of the material. The wave fronts were examined for
different polarizations (parallel and perpendicular to the direction of the polarization of
the irradiation) and we obtained different optical powers of GRIN lenses for those
polarizations. The difference of optical powers for those polarizations allows us to
roughly estimate the photoinduced birefringence value which is .
150 200 250 300 350 400 450 5000,0
0,2
0,4
0,6
0,8
1,0
5 µmN
orm
aliz
ed In
tens
ity
Ranan shift (cm-1)Raman shift (cm
-1)
Nor
mal
ized
inte
nsity
0
10
20
30
40
50
OSAsGe
At.
%
Elements
freshly evaporated 180 days after the irradiation
a b a b
132
The examination of the aging process shows that these lenses are not permanent,
with obtained optical powers decreasing and even disappearing after long time (6
months). We think that external factors are changing the chemical nature of the thin films
(surface oxidation) and, therefore, the obtained optic powers are decreasing and even
disappearing with time. Further investigations are required to understand the origin of
this long term instability.
Acknowledgments
We acknowledge the financial support from the Canadian Foundation for
Innovations (CFI), the Fonds Québecois de la Recherche sur la Nature et les
Technologies (FQRNT), and the Natural Sciences and Engineering Research Council of
Canada (NSERC).
Additional comments (unpublished)
The figure below presents the 2D profiles recorded by DekTak profilometer of the
sample before (black curve) and after (red curve) irradiation. As can be seen, the
thickness of the examined sample was 5 µm and no significant changing of the surface
profile (expansion or contraction) was observed after the exposition. The inset of the
figure magnifies the surface profile of the irradiated zone.
0 1 2 3 4 50
1
2
3
4
5
1 2 3 45.00
5.05
5.10
5.15
5.20
5.25
5.30
irradiation zone
substrate surface
Thic
knes
s (µm
)
Distance (mm)
not exposed exposed
Thic
knes
s (µm
)
Distance (mm)
Figure 7.6: 2D profiles of the sample before (black curve) and after (red curve) irradiation showing the absence of surface modification (expansion or contraction). Inset: magnification of the surface
profile of the irradiated zone.
133
Chapter 8
General Conclusion
The aim of this PhD thesis was to explore in details the high photo-sensitivity of
ChG which make them attractive materials for optics and photonics. In the first chapter,
the properties and applications of ChG were reviewed as well as the known methods for
their preparation and characterization in the bulk and thin film forms. Emphasis was
given to the techniques employed in this work and the study of the Ge-As-S ternary
system. The obtained results were reported and discussed throughout six chapters (from
chapter 2 to chapter 7), each one based on an article published in a peer-reviewed
scientific journal. The whole work presented in these chapters was directed to evidence,
study, understand and discuss with regards to the prior art the photoinduced effects in Ge-
As-S ChG under band gap light irradiation.
In a first step, as described in the second chapter, we studied the optical band gap
of the ChG Ge-As-S thin films prepared by e-beam technique, and particularly its
dependence upon the film thickness. We have analyzed the transmission spectra recorded
from annealed thin films of the same Ge25As30S45 composition and obtained under
identical conditions, but with four different thicknesses. A slope change and a blue-shift
of the absorption edge were observed with decreasing film thickness and related with a
higher density of defects in thinner films. The micro-Raman spectroscopic study
evidenced the important role played by the medium- and/or long-range structural disorder
of the network.
The third chapter referred to the study of the photoinduced birefringence (PIB) in
vitreous bulk and thin films of different Ge25AsxS75-x compositions with x (arsenic
concentration) ranging from 10% to 40 at.% to reveal the most photo-sensitive
composition. The Raman spectroscopic study permitted to show the formation of
homopolar Ge-Ge and As-As bonds to the detriment of Ge-S and As-S heteropolar ones
with increasing arsenic concentration. Moreover, we demonstrated that the exposition
134
with visible light at 514 nm increased the atomic mobility by breaking some homopolar
bonds and favoring the formation of new heteropolar ones.
The fourth chapter was a direct continuation of this study with a focus on the
observation and characterization of the local value of PIB obtained through polarimetric
experiments and estimated by simple theoretical modeling. This additional study allowed
us to clarify the comprehension of the PIB phenomenon, which is of first interest to
design new photo-patterned integrated optics devices (channel waveguides, gratings, etc.)
based on ChG thin films.
Then, the fifth chapter was dedicated to the dynamic study of the photoinduced
absorption changes of the ChG thin films under 514 nm band gap laser excitation. These
studies were performed on the Ge25As30S45 composition, selected for its higher PIB value.
The dynamic changes of absorption were investigated for various light wavelengths,
polarizations and propagation directions (transmission and reflection). Based on the
strong asymmetric changes and non-monotonic behavior of both transmission and
relaxation, we proposed a phenomenological model accounting qualitatively for the
experimental observations. In this model, the photo-excitation induces the breakage of
homopolar bonds (As-As and Ge-Ge) and brings the ChG Ge25As30S45 thin film into a
dynamically equilibrated state with a bond distribution including those originally
existing, the newly created and some intermediate-excited states. When the photo-
excitation is stopped, the system partially relaxes and new (mainly heteropolar) bonds are
formed, conducting to some sequential bond conversions which were confirmed by
polarized micro-Raman spectroscopic studies. This model also proposes that the bond
conversion occurs beyond an energetic barrier to explain the non-monotonic behavior of
PID observed experimentally.
In the last two chapters (sixth and seventh chapters, respectively), two different
applications based on the high photo-sensitivity were presented, particularly the high PIB
and PID values, of Ge25As30S45 thin films. The first one was the recording of polarization
hologram in the ChG thin film. The vector holographic gratings recorded in Ge25As30S45
thin films at 514 nm were experimentally studied and analyzed in comparison with the
scalar ones. A better diffraction efficiency and higher grating period were then evidenced
in the scalar holograms. This may be due to: (i) first, a lower concentration of the charged
135
defect centers in the material compared to that of the sites where photoinduced structural
changes take place (here, the As-As and Ge-Ge homopolar bonds) and; (ii) second, the
direct transitions in the scalar recording case which are more efficient than the indirect
electronic transitions in the vector recording one. The results obtained can then be used to
optimize the polarization hologram recording in ChG thin films, and to measure the
ellipticity of any polarized light beam. Moreover, higher thermal stability of the recorded
polarization holograms was observed in Ge-As-S thin films compared to As2S3 films,
illustrating the role played by germanium in the improvement of the glass properties
(resistance, durability, etc.). The second application was the formation of gradient index
(GRIN) lenses in the ChG Ge25As30S45 thin films based on their high photoinduced
birefringence PIB under band gap light irradiation (by CW Argon laser). The samples
were examined by means of DekTak profilometry and Shack Hartmann sensor to
determine their surface modifications, their optical performance and the wave front
distortions of the obtained lenses. The study of the surface profile did not reveal
noticeable changes in the thin film surface after irradiation, indicating that the observed
changes were only related to the refractive index of the material. The measurements of
the probe wave fronts passing through these lenses were performed for different exposure
times and intensities by using a Shack-Hartmann wave front sensor. The GRIN formation
is related to the photoinduced shift of the band gap towards shorter wavelengths (so-
called photo-bleaching effect).
In summary, the main results and achievements of the present thesis are as follows:
Experiments showed a band gap decrease and a slope change of the
absorption edge (which becomes less abrupt) with increasing the Ge-As-S film
thickness;
Experiments showed an average Photoinduced Birefringence increase in
the Ge-As-S ChG with increasing As content up to 30 at.%: the Ge25As30S45 glass
composition was determined as the most sensitive in terms of PIB effect;
The maximum PIB value measured was about 0.02 (one order higher than
that of the ChG compositions studied previously);
136
The determined local value of PIB was almost one order of magnitude
higher (nloc > 0.112) than the highest average value ever reported in the
literature;
The photoinduced absorption changes were demonstrated to originate from
the homopolar to heteropolar bond conversions, which occur beyond an energetic
barrier calculated to be approximately 14kJ/mol;
The hypothesis that pump beam intensity decreases exponentially in the
material has led to local absorption changes ( ) close to the
initial material absorption ( );
Experiments showed the recording of scalar and vector holograms with
maximum diffraction efficiencies of about 35% and 12%, respectively;
The grating period of the scalar holograms recorded in the thin films at
514 nm was approximately 2.4 times larger than that of the vector holograms
recorded;
Experiments showed a better thermal stability of the vector holograms
recorded in Ge-As-S thin films compared to As-S ones, evidencing the role
played by germanium (Ge) in the improvement of the mechanical and thermal
properties of the glass;
The irradiation with band gap light allowed the formation of lenses with
negative optical powers ranging from -0.05 Diopters to -0.45 Diopters. The lenses
obtained by this technique possess Gaussian profiles with a diameter of about 3
mm. A decrease of the obtained optical powers was observed by means of aging
process (total vanishing of the effect after about 6 months);
The difference of optical powers obtained for parallel and perpendicular
polarizations versus the direction of the polarization of the irradiation leads to an
estimation of the photoinduced birefringence of .
The work achieved in this thesis has provided new insights on a particular
composition of the Ge-As-S ternary system. During the past 40 years, the Ge-As-S
ternary system was the topic of numerous investigations around the world owing to the
richness of its structural features and its spectacular photo-sensitivity. Nevertheless, the
137
complex structure and observed photo-structural modifications of Ge-As-S glass are not
completely elucidated throughout its vast compositional range to allow for application in
optics and photonics devices in the future.
First, additional investigations should be performed under controlled atmosphere to
show whether oxygen plays significant role during laser exposition. In addition, aging
effects should also be explored further, to characterize the lifetime of induced effects and
the long-term stability of these materials. Such information is crucial in view of
integrating these materials in devices. Different strategies to improve the material
durability, such as the deposition of protective coatings, might be considered.
Furthermore, the two potential applications demonstrated in this thesis for the
Ge25As30As45 composition should be repeated over a wider range of compositions, given
the vast vitreous domain of this system. One has also to consider that the latter vitreous
domain might also be extended by evaporating polycrystalline Ge-As-S compounds; it is
indeed well-established that thin film evaporation leads to larger glass-forming domain
than glass bulk fabrication.
Finally, a complete comprehension of the photoinduced effects underlying
mechanisms, including their theoretical modeling, will probably require similar thorough
investigations as those reported in this thesis over a wide range of compositions,
following the recent structural study carried out on more than 150 glass compositions.
Such systematic and careful work does indeed seem to be inevitable to attain one day a
clear and complete comprehension of the Ge-As-S vitreous system.
139
Appendix
Experimental Method
The appendix covers the main experimental methods used throughout this thesis
work. We will successively describe: (i) the fabrication of the chalcogenide glass samples
in silica ampoule sealed under vacuum (alternative techniques will be also mentioned);
(ii) the thin film preparation by e-beam evaporation technique (we will also briefly
describe some other well-known evaporation techniques as thermal evaporation,
sputtering evaporation and chemical vapor deposition, for instance); (iii) the thermal
analysis (DSC) and the elemental microanalysis techniques used to characterize the
prepared samples and; (iv) the (polarized) micro-Raman spectroscopy which was used to
probe the photoinduced phenomena.
A.1 Fabrication Method of Chalcogenide Glasses
A.1.1 Bulk Glass Fabrication
The traditional way to synthesize chalcogenide glass (ChG) is by melt-quenching of
the starting materials into a fused silica ampoule sealed under vacuum. Such specific
attention is required for two main reasons:
- First, to prevent any contamination of the starting elements with oxygen
and water during the melting,
- Second, to maintain the desired stoichiometry by avoiding any material
losses due to high vapor pressure of chalcogenides at their melting temperature.
The starting materials (in this thesis: germanium (Ge), arsenic (As) and sulfur (S))
are precisely weighed (±2 mg) to prepare a glass of desired weight (here, 30 g) and
loaded into the fused silica ampoule as represented in Figure 9.1 (a). The internal
diameter of silica tube used in this work is 15 mm. This tube is then evacuated under
primary vacuum (around 10-3 mbar). Once the ampoule is sealed under vacuum, it is
141
- Prior to removing the synthesis ampoule from the tubular furnace to achieve the
glass quenching (step e), the rocking is stopped and the ampoule is maintained in
vertical position for about 10-20 min, to allow the glass melt to form an
homogeneous glass rod, free of bubbles;
- Rapid quenching by immersing the synthesis ampoule in water at room
temperature until complete solidification of the glass melt (step e);
- Glass annealing at temperature close to the glass transition temperature (around
300˚C) for 6h to remove any residual stress induced by the quenching (step f);
- Slow cooling down to room temperature at around 60˚C/h.
Then, the obtained chalcogenide glass rods of about 60 mm in length and 15 mm
diameter are removed from their synthesis ampoule, cut into slices with a diamond saw
and polished with silicon carbide papers whose grain size is progressively decreased to
obtain sample surface with a good optical quality for further characterizations.
The principle of the method above described remains the same to prepare essentially
all the known ChG based on As, Ge, Ga, S, Se and/or Te by adapting the temperature(s)
and thermal profile(s) to control the reaction, melting and homogenization of the used
starting materials, as well as the quenching rate and consecutive glass annealing.
Apart the melt-quenching technique to prepare bulk chalcogenide glass, one can also
mention some marginal preparation techniques: the chemical vapor deposition (CVD)
method which was implemented by the group of D. Hewak in United Kingdom [215] and
the sol-gel technique [216]. The CVD process, closed to the MCVD one (M for modified)
which is a mature technology widely used to produce ultra-high purity silica preform for
telecom optical fibers for instance, is based on the reaction between chemicals precursors
in the gaseous state, leading to a suit which is further densified (vitrified) at high
temperature. The main advantage of this technique lies on the ultra-high chemical purity
of the used precursors, e.g. gaseous SiCl4 and GeCl4, which permits to manufacture the
obtain glasses free of impurities like transition metals, etc. This technique was the key
factor for the success of silica optical fiber production at the end of last century.
However, it is not easily implementable for all the types of glasses.
Except the work by D. Hewak’s group, there are very few works reporting the
fabrication of high purity ChG by using the CVD process. To the best of our knowledge,
142
the first was reported for Ge-Se binary glasses, obtained from the gaseous reaction
between GeCl4 and SeCl2 by Katsuyama et al. in 1986 [217]. High purity germanium
sulfide glasses were also prepared [218]. Nevertheless, this technique usually leads to the
formation of amorphous and/or crystalline particles/powders which have to be further
melted and quenched in order to obtain bulk glasses. But on the other hand, the CVD
technique is by definition also appropriate for the fabrication of thin films and
particularly chalcogenide thin films [218].
A.1.2 Glass Substrate Cleaning Procedure
The fabrication of ChG thin films consists in the vacuum evaporation of crushed
ingots of Ge-As-S glass on commercial glass substrates of 1 mm thick BK7 glass (or
BK7 with an ITO layer). Prior to the film deposition, the glass substrates were cleaned to
remove any undesired particles and impurities from their surface according to the
following procedure:
- First, the substrates are cleaned and rinsed in an ultrasonic cleaner using
Extran MN-1 powder detergent dissolved in deionized water,
- Then, the glass slides are placed in a rack inside a beaker washed/rinsed
for 4 cycles in the ultrasonic cleaning bath using the Extran MN-1 powder
detergent in deionized water,
- After removing the substrates from the bath, the slides are blown dry with
high purity filtered air,
- Finally, complete drying is achieved by baking the substrates overnight in
an oven at 105 °C.
It has been determined from the above procedure that 4 wash and rinse cycles
produce optimum cleaning results.
A.2 Chalcogenide Glass Thin Film Fabrication
The different techniques for thin film deposition can be divided in two sections: the
physical vapor deposition (PVD) and chemical vapor deposition (CVD, as introduced
earlier in this chapter). The former can be then divided in two sub-sections, namely
143
vacuum and sputtering, whereas the latter can be divided in gas-phase and liquid-phase
sub-sections, as summarized in the Figure 9.2 which was adapted from [219].
Figure 9.2: Thin film deposition techniques, from [219].
We will not describe here all the above cited techniques [219] but we will focus on
those which are commonly used to prepare ChG thin films: (i) the thermal evaporation
(not reported in Figure 9.2); (ii) the sputter deposition and; (iii) the electron-beam
evaporation. It is worth mentioning that these techniques can be combined together, when
co-evaporation is required (e.g. to evaporate simultaneously materials with strongly
different vapor pressures) [220]. The most important points in the preparation of thin
films are the substrate cleaning by chemical and physical methods (especially for thinner
films), as previously specified, the control of the chemical composition variation along
the film thickness (which may vary from ∼10 nm to ∼50 μm, depending on the targeted
application) and finally, the stabilization and homogenization of thin films by means of
annealing.
A.2.1 Thermal Evaporation Technique
The simplest way to prepare a film is by thermal evaporation: material to be
evaporated is placed into a boat (B) and heated under vacuum, as represented in Figure
144
9.3. Chalcogenide glasses are particularly suitable for this technique due to their
relatively low melting points and high vapor pressure [221]. The produced glass melt
then evaporates and the vapor is condensed onto a substrate (S), forming a thin
amorphous film.
Figure 9.3: Schematic representation of a thermal evaporation chamber: B – heated boat, S –
substrate, H – heating system for the substrate, V – vacuum (from [222]).
The temperature control of the substrate (through the heater H) is extremely
important as it conducts the atomic structure, including the ratio of ring and chain
molecules, their size and length as well as the deposition rate, especially for Ge-based
chalcogenide thin films [223]. However, this evaporation technique is not very suitable
for the deposition of multicomponent materials whose compounds/elements melting
temperatures are very different. Indeed, this can lead to different evaporation rates, partial
sublimation, etc, resulting in thin films inhomogeneity, local variation of their properties
like its optical band gap [98] and overall a lack of repeatability. To avoid such issues, the
flash evaporation may be employed. This alternative technique consists in filling a very
hot boat with a powdered sample to evaporate it almost instantaneously [224].
Moreover, it is worth mentioning that in this thin film evaporation technique, special
attention has to be paid to the angle of evaporation. Indeed, structural inhomogeneity and
porosity of deposited films can be avoided by changing the angle of substrate vs the
incident evaporation beam, as represented in Figure 9.4. Oblique angles of evaporation
may lead to the formation of columnar-growth morphology, where the direction of the
145
columns in the thin films (at an angle β with the normal to the film) and the direction of
the evaporation beam (at an angle α) do not lie parallel to each other [95, 225]. This
effect is particularly prevalent in Ge-based chalcogenide materials but curiously seems to
be much less pronounced for the case of As-based ones. Besides, such obliquely-
evaporated chalcogenide thin films possessing a columnar microstructure exhibit
enhanced permanent photostructural effects.
Figure 9.4: Schematic illustration of thin film deposition dependence on angle of evaporation beam
direction (from [222]).
A.2.2 Sputtering Evaporation Technique
The other widely used amorphous thin films preparation technique is the sputtering,
in which the target material (bulk) is ablated by bombardment with energetic ions from
an electrical plasma struck in a vacuum chamber with presence of an inert gas, e.g. Ar (or
reactive gas, e.g. H2, which can react chemically with the bulk and the resulting films, in
the case of chemical sputtering). Here, the ejected material is simply physically
transferred to the substrate in the form of ionized atoms or clusters of atoms (Figure 9.5).
The depositions rates are generally in the order of 1–10 Å/s.
146
Figure 9.5: Schematic representation of the sputtering evaporation apparatus : T – target electrode ;
S – substrate electrode ; P – plasma ; V – vacuum and H – heater (from [222]).
The main advantage of the sputtering technique lies in the fact that most elements
have rather similar sputtering rates, which permits to obtain multicomponent thin films of
same or close composition with that of the bulk source (target). In general, the sputtering
technique tends to produce denser films than those prepared by other evaporation
methods. Sputtering can be used to deposit STAG (Si-Te-As-Ge alloys), Ge2Sb2Te5 thin
films for DVD applications [226], Si3N4 films (using a N2-containing gas), etc. However,
when this technique implies to use the target as an electrode, it can only be used to sputter
(evaporate) metals or other electrically conductive materials. In the case of poorly
conductive materials, like the chalcogenide glass systems Ge–As–S(Se), radiofrequency
sputtering has to be used to fabricate the thin films [227].
A.2.3 Electron-Beam Evaporation Technique (used in this work)
The electron beam evaporation method, also called e-beam method, was preferred to
the thermal one in the present thesis because of the large mismatch between Arsenic and
Germanium evaporating temperatures, which are respectively 613°C and 2830°C at
atmospheric pressure (we can consider that this mismatch still exists at low pressure).
The sputtering method, which could also be envisaged to prepare the ChG films of this
study, was not chosen for two main reasons: the absence of equipment dedicated to ChG
evaporation at Université Laval and the necessity to prepare large glass sample targets,
147
usually about 40-50 mm diameter, which is more challenging from the preparation
method for ChG described in section A.2.
The e-beam evaporation method allows to evaporate and deposit a wide variety of
materials including refractory metals (such as tungsten), low vapor pressure metals (such
as platinum), and alloys. Since in this method a large amount of heat is concentrated on a
very small area, high rates of deposition are possible. The process begins under a vacuum
of 10-5 Torr or less. A tungsten filament inside the electron gun is heated. The gun
assembly is located outside the evaporation zone to avoid any contamination with the
evaporated material. When the filament becomes hot enough, electrons are emitted.
These electrons then form a beam which is deflected, accelerated and focused on the
material to evaporate in the crucible by means of electro-magnetic fields. When the
electron beam strikes the target surface (usually material chunks, bulk), the kinetic energy
of the electrons is transformed at the impact into thermal energy (heat), inducing its
evaporation. The experimental set-up is schematized in Figure 9.6.
Figure 9.6: Schematic representation of the e-beam evaporation method.
It is important to remind one that the energy provided by a single electron is quite
small and that the heating is accomplished simply by virtue of the vast number of
electrons, hitting the surface of the material to evaporate. This energy then vaporizes the
target material. The energy level achieved in this manner is more than several million
148
watts per square inch. Due to the intensity of the heat generated by the electron beam, the
material holder (the crucible) must be water cooled and refractory to prevent its melting
(e.g. tungsten crucible).
The substrate is fastened to a horizontal rotating support, which permits to obtain a
uniform film thickness during the evaporation. The distance between the target and the
substrate is 20 cm. The material to evaporate (crushed ingots of Ge-As-S glass) is placed
within a tungsten crucible at room temperature in the evaporation chamber. First of all, a
primary vacuum is achieved in the vacuum chamber to a pressure of 10-3 Torr. Then, the
diffusion pump is started and when the pressure attained 10-6 Torr, which generally
corresponds to the vacuum required for ChG evaporation, the electrons bombardment of
the glass source is initiated. The electron beam is accelerated to a high kinetic energy and
is focused onto the source. The acceleration voltage is over 4 kV. The used deposition
rate of the films is about 10 Å/s, which was measured continuously by a piezoelectric
quartz-crystal (Temescal FTM) placed parallel to the substrate in the vacuum chamber. It
is known that such low deposition rate ensures a very close chemical composition
compared to that of the bulk (starting) material. The evaporation time is typically 1 hour
to obtain thin films of about 1.5 micrometer thicknesses. The film thickness is controlled
by using the piezoelectric quartz-crystal used to control the deposition rate.
Besides the techniques above described, ChG thin films can also be prepared by:
- Spin coating [228], which is a inexpensive method by using appropriate solutions;
- Chemical vapor deposition (CVD) [217], which is also used to obtain hydrogenated
films, generally in the semiconductor industry;
- Laser ablation method for As2S3 and other materials [229] by using pulsed and
continuous-wave laser sources;
-Other marginal methods based on various chemical reactions, for example sol–gel
production of Ge–S films [230], etc.
A.3 Thermal analysis
The thermal analysis technique permits to explore the material behavior as a function
of temperature while submitted to a heating ramp. It allows to evidence the physical and
149
chemical changes of a material upon its heating at a constant rate and thus to determine
the temperature at which occur these changes. Indeed, these transformations undergone
by the material are mostly accompanied with a release or a consumption of heat (thermal
energy), leading to exothermic or endothermic phenomenon, respectively, and detection
by the equipment as a variation of heat flow in the sample.
For these measurements, the Differential Scanning Calorimetry (DSC) technique was
used, through a DSC Netzsch 404F3 Pegasus apparatus. The analyses were performed as
follows: small glass pieces are weighed (5 to 15 mg) and placed into an aluminum pan
(crucible) which is then mechanically sealed off. The pan is then placed into the chamber,
close to a reference pan (without sample). Both are heated up to 600˚C at a constant rate
of 10˚C/min. The heat flow absorbed or released by the sample is then recorded. The
accuracy of DSC measurements is about ± 2˚C.
The characteristic temperatures of a glassy material are its glass transition
temperature, Tg, its onset of crystallization temperature Tx, its peak crystallization
temperature Tp and its melting temperature TM. They can be determined through a DSC
thermogram, as shown in Figure 9.7 where DSC traces recorded on the material studied
in this thesis, i.e. the Ge25As30S45 ChG. The traces of finely grinded Ge25As30S45 bulk
(solid line) and scratched and powdered thin films (dashed line) are presented in Figure
9.7. It is worth noting that the DSC technique rarely permits to assess the melting
temperature TM of specialty optical glasses like chalcogenide ones.
150
100 200 300 400 500 600
0.0
0.5
1.0
1.5
2.0
Tx = 485oC
Tg = 470oC
Tg = 396oC
Hea
t flo
w (a
rb.u
n.)
Temperature (oC)
bulk thin film
Tg = 357oC
Figure 9.7: DSC traces of the Ge25As30S45 thin film and crushed bulk glass pieces (y-axis: unit: 0.5
mW/mg/div.).
In the Figure 9.7, one can observe a decrease of about 40˚C of the glass transition
temperature Tg of the thin film if compared to that of the bulk. Moreover, a higher ΔT
value (Tx-Tg) is observed for the thin film, indicating its better thermal stability against
crystallization compared to the bulk glass. Such feature is expected and illustrates the
better glass-forming ability of ChG thin films vs bulk (owing to higher glass cooling
rates).
A.4 Elemental Microanalysis by Energy Dispersive X-Ray
Spectroscopy Coupled to Scanning Electron Microscope
(EDX-SEM)
To qualify, quantify and thus verify the elemental chemical compositions of the
fabricated samples and compare them with the nominal ones, we utilized the Energy-
dispersive X-ray spectroscopy (EDX) technique coupled to a FEI, Quanta 3D FEG
153
During the relaxation process, an electron from the outer shells will move to the hole
created by the electron initially ejected from the inner shell. During this transition, a
certain amount of energy is lost. This energy can be released in the form of an X-ray or
transferred to an outer electron which will is then ejected and easily detectable. The
Auger electrons are of low energy and characteristic to the atom from where they were
emitted. They can thus provide information about the chemical composition and more
particularly of the sample surface (Auger spectroscopy). The emitted X-rays are also
characteristic to the electron shell and the atomic element from where they were emitted
and can be quantitatively measured through the Energy Dispersive X-ray spectroscopy.
A.5 Micro-Raman Spectroscopy
The Raman spectroscopy is a widely used vibrational spectroscopic technique to
characterize material from a structural point of view. This technique is based on the
inelastic scattering of a monochromatic radiation focused on the material and permits to
identify different stretching, vibrational, rotational or other low-frequency modes of
chemical bonds [231].
The Raman effect was discovered in 1928 by Chandrasekhara Venkata Raman, who
proposed two types of light scattering from a material during its exposure with a
monochromatic light [232]:
the elastic Rayleigh scattering, which does not involve energy transfer
between molecules and the incident photon, thus, the scattered photon has the
same energy as the incident light and the photons reflect at the same angle as the
incident angle;
the inelastic Raman scattering, which involves the exchange of energy and
a slight reflection angle variation. The scattered photon energy differs from the
incident photon energy. Those differences carry information about molecule
vibrations and can be used for characterization.
Raman spectra consist of Stokes and anti-Stokes lines which are symmetrical about
the Rayleigh line. Stokes Raman scattering occurs as a result of photon absorption to a
virtual state, followed by relaxation to a higher order phonon level (the change in energy
154
between the original state and the new state causes an up-shift), as schematically
presented in Figure 9.10. In anti-Stokes Raman scattering, a photon is absorbed from a
higher order phonon level to a virtual state followed by depopulation to the ground state
(the change in energy between the original state and the new state causes a downshift), as
schematically presented in Figure 9.10.
Figure 9.10: Schematic energy diagram describing the Rayleigh and Raman scatterings. The line
thickness indicates the signal strength from the various transition state shown by black horizontal lines.
The Raman scattering intensity is very low compared to the Rayleigh one (about10-6
to 10-9 times lower) [233]. Therefore, high-power light sources and adapted acquisition
durations are required to produce a sufficient number of detectable Raman-scattering
photons. Generally, the monochromatic light used in Raman spectroscopy is a laser with
a wavelength that varies from 532 to 785 nm.
Micro-Raman spectroscopy has evolved in 1966 to make this technique independent
of sample sizes down to the dimension determined by the diffraction limit [234].
Nowadays, micro-Raman spectroscopy is a major structural characterization technique in
both industry and academic research.
During the Raman spectrum acquisition, the intensity of inelastic scattering is
recorded by a photo-detector and measured in wavenumber (cm−1) corresponding to the
Raman shift relative to the Rayleigh line. The obtained spectrum can be then normalized
155
by the strongest or more defined peak and provides some qualitative information to
describe the structure.
Polarized Raman spectroscopy gives information about the molecular orientation and
symmetry of the bond vibrations in a molecule [235]. For this purpose, the same micro-
Raman spectrometer can be used with the same conditions except that here, polarizers are
placed on the laser beam path (see Figure 9.11). This technique is usually utilized to
measure and distinguish Raman symmetrical modes from antisymmetrical stretching,
bending and rocking modes.
Figure 9.11: Schematic representation of Raman Depolarization Ratio.
The obtained polarized Raman spectra relate to the polarization component of the
scattered light, which is parallel to that of the incident beam. On the other hand, the
depolarized Raman spectra refer to the polarization component of the scattered light that
is perpendicular to the polarization of the incident beam. Symmetrical excitation modes
are generally dominant in this configuration.
The characterization of the symmetry of bond vibrations is done by calculating the
depolarization ratio, ρ, for a particular peak:
ρ= I⊥ / III
Where I⊥ is the intensity of the Raman band with polarization perpendicular to the
laser beam, and III is the intensity with polarization parallel to the laser beam. This ratio
was shown to vary from 0 to 0.75 in amorphous materials [176]. If ρ < 0.75, the band can
156
be considered polarized, while the bands with a depolarization ratio ρ close to 0.75 are
not polarized. The ratio ρ > 0.75 corresponds to an anomalously polarized band.
In this thesis work, the unpolarized and polarized micro-Raman spectroscopic
techniques were used first to verify the structure of the fabricated ChG thin films and
then to understand the underlying photo-structural modification and chemical
mechanisms of the bond modification responsible for the observed photoinduced
phenomena upon laser irradiation.
157
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