on the growth of rare earth doped liyf4 thin film by pulsed laser

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Dr. M.Anwar-ul-Haq

On the growth of rare earth doped LiYF4thin films by pulsed laser deposition

Prof. Paola BicchiDr. Stefano Barsanti

Department of Physics,University of Siena, Italy

Department of Physics, University of Sargodha,

Pakistan

International Scientific Spring at NCP from March 01-06, 2010

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Layout of the presentationIntroduction

Experimental setup

Conclusions

Thin filmsPulsed Laser Deposition (PLD)

ResultsNd3+:LiYF4 thin films

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Development of rare earth (RE) ions-doped LiYF4

(YLF) fluoride thin films with characteristics suitable for their use as active medium in micro-lasers sources in the 1-2 μm, via Pulsed Laser Deposition (PLD)

Materials used

Aim of the research workIntroduction

SubstratesSubstrates Pure mono-crystalline YLF

TargetsTargets RE ions- doped mono-crystalline YLF

Nd3+ or Tm3+

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Why thin film in micro-laser area? [1,2]Introduction

[2] C. L. Bonner, A. A. Anderson, R. W. Eason, D. P. Shepherd, D. S. Gill, C. Grivas and N. Vainos, Opt. Lett. 22 (1997) 988[1] D. B. Chrisey and G. K. Hubler, (Eds.), Pulsed laser deposition of thin films, John Wiley, New York (1994)

The removal of the heat in excess from the active media

Maximizes the interaction zone between pump and active media

Enhancement of the confinement of the radiations

Reduction of the lasing threshold

The realization of active optical devices in film shape would magnifies all advantages of the micro-lasers systems by favoring;

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Thin Films [3]

Growth Mechanism [4]

on a suitable substrate or on previously deposited layers

Dimensions from fractions of a nanometer to several micrometers in thickness

Realization

[3] K. Wasa, M. Kitabatake and H. Adachi, Thin film materials technology: sputtering of compound materials, Springer, Heidelberg (2004)[4] A. Rockett, The materials science of semiconductors, Springer, New York (2007)

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Major ways of thin films growth [5]Thin films

Volmer-Weber growth(Island growth)

Stranski-Krastinov growth(mixed growth)

Frank-van de Merwe growth(layer by layer growth)

[5] J. A. Venables, G. D. T. Spiller and M. Hanbucken, Rep. Prog. Sci. 47 (1984) 399

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Liquid Phase EpitaxyThermal EvaporationSputtering

Molecular Beam EpitaxyChemical Vapour DepositionIon Implantation

Pulsed laser deposition

Ejection of materials from the target by highly energetic laser pulses, with subsequent deposition/condensation on a suitable substrate

[1] D. B. Chrisey and G. K. Hubler, (Eds.), Pulsed laser deposition of thin films, John Wiley, New York (1994)

Thin film growth techniques [1]Thin films

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Typical PLD experimental setup

Interaction of the photons with the target causes material ejection via a thermal and / or electronic process

The ablated plume is a mixture of energetic particles such as atoms, molecules, electrons, ions, sub-microns or micron-sized solid particles and molten globules

[6] P. R. Willmott and J. R. Huber, Rev. Mod. Phys. 72 (2000) 315

Pulsed Laser Deposition (PLD) [1,6]Thin films

[1] D. B. Chrisey and G. K. Hubler, (Eds.), Pulsed laser deposition of thin films, John Wiley, New York (1994)

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laser external to the ablation/deposition chamber,

flexibility of the experimental set-up,

possibility of getting thin films of almost any kind of material,

deposition can be performed either in vacuum or in presence

of a controlled background atmosphere

stoichiometry in the film can be maintained,

films growth rates can be controlled,

multiple layer films can be grown,

deposition on substrates kept at any temperature, is possible

[7] J. Schou, Appl. Surf. Sci. 255 (2009) 5191

[6] P. R. Willmott and J. R. Huber, Rev. Mod. Phys. 72 (2000) 315

Advantages of PLD [6,7]Thin filmsPLD

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Crystals doped with rare earth Laser sources in IR

Disadvantages Sensitive to thermal shocks even for slow thermal gradient and to OH¯ radical contamination during growth even a few ppm

OxidesFluorides

Lower phonon energy

Stronger emission cross sectionsAdvantages of fluorides over oxides [8, 9]

Films of RE ions-doped fluorides

Excellent optical properties + Benefits of thin film[8] A. A. Kaminskii, Laser Crystals, Springer-Verlag, New York (1981)

[9] C. Garapon, S. Guy, S. Skasasian, A. Bensalah, C. Champeaux and R. Brenier, Appl. Phys. A 91 (2008) 493

Why fluorides?Thin filmsPLD

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Weight

Chemical Formula

Properties of YLF crystal [11]

Monoaxis (a, c)Crystallographic axis

no = 1.4485ne = 1.4708Refractive Indexes

3.99 g/cm3Density

0.12- 7.3 µmTransparency

5.07 MohsHardness

819°CMelting Point

TetragonalCrystal Structure

171.8 amu

LiYF4

Unit cell of YLF [10]

Substrate mono-crystalline YLF

Radius ~ 4.7 – 7.2 mm

Thickness ~ 2 mm

[11] R. L. Aggarwal, D. J. Ripin, J. R. Ochoa and T. Y. Fan, J. Appl. Phys. 98 (2005) 103514

[10] E. Garcia and R. R. Ryan, Acta Crystallogr., Sect C 49 (1993) 2053

Substrate materialExperimental

setup

YLF Crystal

Experimental setup

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2.1 x10201.5

1.4 x10201

Nd3+

N (cm-3)Concentration (at. %)Dopant

Target crystals

NdNd3+3+:LiYF:LiYF44 (Nd:YLF)(Nd:YLF)

Radius ~ 4.7 – 7.2 mm, Thickness ~ 3 mm

Target materialsExperimental

setup

The YLF and Nd:YLF mono-crystals used during our PLD experiments were either grown by the NEST growth facility in Pisa or provided by a commercial supplier (VLOC, USA).

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Ablation setup Deposition setup

Film growth systemExperimental

setup

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LaserLaser

PlumePlume

ShutterShutter

Rotating Rotating target holdertarget holder

Substrate heaterSubstrate heater//holderholder

Heater Heater ConnectionsConnections

Photos of the UHV chamberExperimental

setup

Auxiliary Auxiliary viewportsviewportsVacuum gaugesVacuum gaugesGas valveGas valve

Laser entrance Laser entrance windowwindow

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Knudsen layerAdiabatic expansion

Heated substrate

PLD processExperimental setup

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Film in-situ analysisExperimental setup

Realization of the film growth

To verify the presence of the rare earth ions in the film

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CCD camera photo of the Nd:YLF films Experimental

setup

The film was deposited with laser fluency of 10 J/cm2 in 1 Pa of He atmosphere at a substrate temperature of 650 °C.

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Determining the kind of film deposited

To determine if the orientation from the substrate to the film has been transferred

Surface quality

Concentration of Nd³+ ions present

Film ex-situ characterisationsExperimental setup

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Simplified level scheme Absorption curve

Simplified level scheme and absorption curve for Nd:YLF [12,13]Experimental

setup

[13] J. R. Ryan and R. Beach, J. Opt. Soc. Am. B:Opt. Phys. 9 (1992) 1883

[12] A. A. S. da Gama, G. F. de Sa, P. Porcher and P. Caro, J. Chem. Phys. 75 (1981) 2583

• The fluorescence profiles of Nd3+-doped YLF crystals depend on they being recorded with E || or E ⊥ to the crystal c-axis [13].

• The Nd3+ 4F3/2 manifold lifetime is concentration dependent in such a way that lower concentrated samples manifest a higher lifetime and vice versa [13].

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Setup for the ex-situ characterizationExperimental

setup

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Results

Ablation in vacuum

Ablation in 1 Pa of He

Plume analysisResults

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Plume analysisResults

• The expansion velocities of the plume species were found reduced in presence of 1 Pa of He.

• Both in vacuum and in 1 Pa of He, the expansion velocity of most of the plume components saturates beyond 8 J/cm2.

• In vacuum, all the plume species were focused along the target normal except lightest neutral Li, which was found to point preferentially at 16° to the target normal.

• In presence of 1 Pa of He, Li and all other plume species were focused along the target normal.

• The FWHM of the angular distribution curves of the plume components was found reduced in 1 Pa of He compared to the similar measurements done in vacuum and gave indication of the confinement of the plume in the presence of 1 Pa of He.

• Ablation threshold in vacuum for all the species was found to be 1.7 ± 0.3 J/cm2 with exception of Li for which 0.7 ± 0.3 J/cm2.

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Plume shape

(a) Vacuum (b) 1 Pa of He

Results

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Nd:YLF thin filmsResults

PLD in vacuum

PLD in 1 Pa of He

Low (4 J/cm2) and high (10 J/cm2) laser fluency

Deposition at Ts = 650°C

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High fluency (10 J/cm2)

Deposition conditionsLaser wavelength 355 nm

Laser fluency 10 J/cm²Ablation time 20’

Repetition rate 10 Hz

Laser Pulse duration 13 ns

Target Nd:YLF 1.5% at.

Substrate YLF

Vacuum 1 1 ××1010--44 PaPa

Target-substrate distance 35 mm

Temperature of the substrate 650°C

Results• Vacuum - Ts=650°C

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Heating/cooling cycle of the substrate for film depositionResults• Vacuum - Ts=650°C

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Films in-situ analysis

Realization of the film

Presence of Nd3+ ions in the film

Portion of the LIF spectrum following 355 nm excitation, recorded in the Nd:YLF bulk crystal and from a film grown in vacuum with 10J/cm2 laser fluency.

Example of interference pattern producedby a film deposited in vacuum


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Films ex-situ characterizations

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4F3/2 → 4I11/2

Unpolarized, normalized, fluorescence spectra

λλexcexc = 806.6 nm= 806.6 nm

4F3/2 → 4I9/2

Crystalline Nd:YF film


[14] S. Barsanti, F. Comacchia, A. Di Lieto, A. Toncelli, M. Tonelli and P. Bicchi, Thin Solid Films 516 (2008) 2009

From Ref. [14]

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Life time measurement of the 4F3/2 manifold

Film deposited at Ts=650°C in vacuum with Fl = 4 J/cm2 [14]

Film deposited at Ts=650°Cin vacuum with Fl = 10 J/cm2

τFilm = 242 ± 5 μs

τTarget = 464 ± 2 μs

[14] S. Barsanti, F. Comacchia, A. Di Lieto, A. Toncelli, M. Tonelli and P. Bicchi, Thin Solid Films 516 (2008) 2009


Nd:YF film

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Deposition in presence of 1 Pa of He

Deposition parameters

85’Ablation time4 J/cm²Laser fluency

355 nmLaser wavelength

10 HzRepetition rate

13 nsLaser Pulse duration

35 mmTarget-substrate distance

650°CTemperature of the substrate

1 Pa of He1 Pa of HeBack ground atmosphere

YLFSubstrate

Nd:YLF 1.5% at.Target

Results• 1 Pa of He - Ts=650°C

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4F3/2 → 4I11/24F3/2 → 4I9/2

Crystalline Nd:YLF film

Inhomogeneous


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Average Nd3+ ion concentration in the film greater than in the target

Concentration of Nd3+ ions in the film

Single exponential decay

τFilm average = 437 μs


4F3/2 manifold lifetime

λλexc exc = 806.6 nm

Variation from point to point ~ ± 10%


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Volmer-Weber growth

Morphological analysisResults• 1 Pa of He - Ts=650°C

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Deposition conditionsLaser wavelength 355 nm

Laser fluency 10 J/cm²Ablation time 20’

Repetition rate 10 Hz

Laser Pulse duration 13 ns

Target Nd:YLF 1.5% at.

Substrate YLF

Back ground atmosphere 1 Pa of He1 Pa of He

Target-substrate distance 35 mm

Temperature of the substrate 650°C

High fluency (10 J/cm2)Results• 1 Pa of He - Ts=650°C

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Crystalline Nd:YLF film

4F3/2 → 4I9/24F3/2 → 4I11/2

Homogeneous


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Optical analysis Optical analysis ⎪⎪⎪⎪ and and ⊥⊥ to to cc--axisaxis

4F3/2 → 4I9/24F3/2 → 4I11/2

The transition of interest for the possible lasing action was favored in the film as much as in the bulk.


4F3/2 → 4I9/24F3/2 → 4I11/2

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The global spectral intensity, in both the emissions is higher when E || to c-axis with only exception of

2//

867863

867||863|| ≈⎟⎟⎠

⎞⎜⎜⎝

⎛

⊥⊥ FilmIIII

3//

arg867863

867||863|| ≈⎟⎟⎠

⎞⎜⎜⎝

⎛

⊥⊥ etTIIII

Transition 4F3/2→ 4I9/2

The spectral profile in the film and in the bulk changes in the same way in shifting the polarization from E || to E ⊥to the c-axis.


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The only mismatch

( ) 2.1arg10471053 ≈etTII

Transition 4F3/2→ 4I11/2

The spectral profile in the film and in the bulk remains the same in shifting the polarization from E || to E ⊥ to the c-axis.

( ) 2.110531047 ≈FilmII

Some features compatible with a considerable if not complete transfer of orientation from the substrate

to the film


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Concentration of Nd3+ ions in the film

Single exponential decay

τFilm = 468 ± 5 μs


4F3/2 manifold lifetime

τTarget = τFilm

λλexc exc = 806.6 nm

Same Nd3+ ion concentration in the film and the target


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Morphological analysis

Mixed growth


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Conclusions

In fact in this case:

• The grown film was a crystalline Nd:YLF film.

• It was homogeneous.

• It showed some spectral features compatible with a consistent, even if not complete, transfer of the substrate orientation to the film.

• The concentration of the dopant ions was transferred from the bulk to the film.

• It had a rather good surface quality

The best film produced which showed promising optical qualities to reach the goal of this project was the Nd:YLF one obtained in 1 Pa of He with a laser fluency of 10 J/cm2, when Ts was 650°C.

We succeeded to deposit crystalline YLF films doped with Nd3+.

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Publications1. P. Bicchi, M. Anwar-ul-Haq and S. Barsanti In: A. N. Camilleri (Ed.),

Radiation Physics Research Progress, ISBN: 978-1-60021-988-8, Nova Science Publishers, Inc., Hauppauge, NY (2008), pg. 193-217.

2. S. Barsanti, M. Anwar-Ul-Haq and R. Bicchi, Thin Solid Films 517 (2009) 2029-2034.

3. M. Anwar-ul-Haq, S. Barsanti, A. Bogi and P. Bicchi, Opt. Mat. 31 (2009) 1860-1863.

4. M. Anwar-ul-Haq, S. Barsanti and P. Bicchi, IEEE NANO 2009, ISBN:978-981-08-3694-8, (2009) 373-376.

5. M. Anwar-ul-Haq, S. Barsanti and P. Bicchi, DGaO Proceedings 2009-Http://www.dgao-proceedings.de, ISSN:1614-8436, (2009) P35.

6. A. Bogi, S. Barsanti, M. Anwar-ul-Haq, P. Bicchi, Appl. Phys. A 98 (2010) 153-159

on the growth of rare earth doped liyf4 thin film by pulsed laser

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