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Chapter-I
1
General Introduction to Mixed Metal Chalcogenide Semiconductor
1.1 Introduction to metal chalcogenide semiconduct ors (MCS)
Chalcogenide is a chemical compound consisting of at least one
chalcogen ion and at least one more electropositive element. It means Binary
compounds of the chalcogens are called chalcogenides. Although all group
16A group elements of the periodic table are defined as chalcogens. This
group is also known as the oxygen family. It consists of the elements oxygen
(O), sulfur (S), selenium (Se), tellurium (Te), the radioactive element polonium
(Po), and the synthetic element ununhexium (Uuh). Group 16 A elements
namely oxygen (O), sulfur (S), selenium (Se) and tellurium (Te) are shown in
Figure 1.1 in their natural state. The term ‘Chalcogen’ was proposed by
scientist Werner Fisher in 1930. It comes from the Greek words (chalkos,
literally ‘copper’), and (genes, born). Thus the chalcogens give birth to,
produce copper. Although the literal meanings of the Greek words imply that
chalcogen means copper-former, this is misleading because the chalcogens
have nothing to do with copper in particular. "Ore-former" has been suggested
as a better translation, both because the vast majority of metal ores are
chalcogenides, and because the word Chalcogen in ancient Greek was
associated with metals and metal-bearing rocks.
The chalcogenides are an ore forming elements more commonly they
are referred as Sulphide, Selenide, and Telluride, rather than for oxides [1-3].
Oxygen and sulfur are nonmetals, and selenium, tellurium, and polonium are
metalloid or semiconductors. Nevertheless, tellurium as well as selenium is
acts as a metal when it in elemental form.
Chalcogenide glasses contain S, Se, or Te alloyed with Group IV, V
and VI elements. The group IV elements have four covalent bonds with their
nearest, neighbors and group VI elements have two covalent bonds.
Chalcogenides have strong potential for applications in photonics [4-6]. Photo
darkening has been used to fabricate waveguides, and some chalcogenides
show extremely high fast optical nonlinearities which could be used for all-
Chapter-I
2
optical switching, and band gap that can be matched to the mid-IR
telecommunications wavelengths.
Figure 1.1 The elements like oxygen (O), sulfur (S) , selenium (Se) and
tellurium (Te) in their natural state. (Source: Chalkogene.jpg)
1.2 Literature survey on VA-VIA group chalcogenides thin films
The V-VI group (V = As, Bi, Sb; VI = S, Se, Te) elements forms binary,
ternary, quaternary or even pentanary chalcogenides by direct chemical
reactions. All the V-VI group chalcogenides are coloured hence shows high
optical absorption coefficient and are studied intensively because of their
semiconducting and thermo cooling behavior. And find wide variety of
applications in various fields like thermoelectric, photoconductive etc [7-8].
In V-VI chalcogenide crystals shows formation of antisite defects and
influenced by the bond polarity. The presence of antisite defects results in
excess of Bi/Sb ratio in V-VI type chalcogenide crystals. Decrease in bond
polarity increases the antisite defects [9]. Antisite defects in Bi2Te3, Sb2Te3,
Bi2Se3 and Sb2Se3 crystals are due to low bond polarity of Bi-Te, Sb-Te, Bi-Se
and Sb-Se bond respectively.
Chapter-I
3
1.2.1 Antimony selenide (Sb2Se3) thin films
Orange red coloured selenides of antimony are potential solar light
absorber in devices for photovoltaic conversion of solar energy. Electronic
structure of single crystal and amorphous Sb2Se3 were studied by J. C.
Shaffer et al. They measured normal incidence reflectivity of Sb2Se3 upon
both crystalline and amorphous samples [10]. Structure of antimony selenide
is shown in the Fiture 1.2. Koichi Shimakawa et al. were studied
compositional dependence of the optical gap in amorphous semiconducting
alloys. They classify it into three types (classes A, B and C), is consistent with
the calculation based on effective medium percolation theory which interprets
the compositional dependence of the conductivity of chalcogenide glasses
[11].
Figure 1.2 Structure of Antimony Selenide (Sb 2Se3)
(Source:Sb2Se3 structure.jpg)
The crystal structure of Sb2Se3 has been redetermined with 610
independent reflections, using three-dimensional intensities measured on a
computer-controlled Philips PW 1100 single-crystal diffractometer and it
shows that Sb2Se3 is isostructural with Sb2S3 and Bi2Se3, by G. P. Voutsa et
al. [12]. Composition dependence of electrical properties of simultaneously
Chapter-I
4
evaporated Sb-Se thin films was explained by P. S. Nikam and R .R. Pawar
[13]. The composition dependence of the optical constants in amorphous
SbxSe1−x thin films has been discussed by H. A. Zayed et al. The analysis of
the absorption coefficient data revealed that existence of two optical transition
mechanisms, depending on the value of x, indirect transitions for SbxSe1−x thin
films (x = 0.1, 0.4, 0.5, 0.7, and 0.9) and a forbidden direct transition for x =
0.3. They reported the optical energy gap Eoptg was found to vary from
0.24eV for (x = 0.9) to 1.92eV for (x = 0) [14]. K. Y. Rajpure et al. reported
effect of the substrate temperature on the properties of spray deposited Sb–
Se thin films from non-aqueous medium. The analysis of the absorption
coefficient data reveals that as the substrate temperature increases, the
optical bandgap value of the material increases. It has also been found, for
the film deposited at 2000C and annealed in N atmosphere, that the
polycrystalline material follows the direct optical transition with energy gap (Eg
opt) equal to 2.14eV [15]. P. Arun et al. were studied potential of Sb2Se3 films
for photo-thermal phase change optical storage [16].
Electrodeposition of Sb2Se3 thin films from alkaline bath were done by
J. D. Desai et al. The optical absorption studies of the Sb2Se3 thin films give
direct band gap of 1.14 eV [17]. A comparative study of the properties of
spray-deposited Sb2Se3 thin films prepared from aqueous and nonaqueous
media by K. Y. Rajpure et al. Thermoelectric power (TEP) measurement
studies revealed that the films prepared from both media showed p-type
conductivity, with Seebeck coefficients of 46.2 and 18.3 µV/°C for the
polycrystalline and amorphous Sb2Se3 thin films, respectively [18]. A. M.
Fernandez et al. were carrying out preparation and characterization of
Sb2Se3 thin films prepared by electrodeposition for photovoltaic applications.
However, the analysis also shows the presence of excess Sb. The optical
energy gap of the annealed samples is 2.00 eV. The morphology changes
from amorphous to polycrystalline, when the samples are annealed [19].
Studies on deposition of antimony triselenide thin films by chemical method
like SILAR by B. R. Sankapal et al. [20].
A. P. Torane and Bhosale were carried out preparation and
characterization of electrodeposited Sb2Se3 thin films from non-aqueous
media. The XRD patterns of the films obtained by varying compositions and
Chapter-I
5
concentrations showed that the as-deposited films are polycrystalline with
relatively higher grain size for 1:1 composition and 0.05 M concentration. The
optical band gap energy for indirect transition in Sb2Se3 thin films is found to
be 1.195 eV [21]. Theoretical and experimental study of the conduction
mechanism in Sb2Se3 alloy was done by E. Abd. El-Wahabb et al. The
theoretical result is also consistent with the experimental ones, where they
attributed the high values of dielectric constant and increasing frequency by
assuming a decrease in the bond energies [22]. Synthesis of Sb2Se3 nanorod
using β-cyclodextrin was done by K. Sudip et al. He prepared nano rod
obtained by the addition of β-cyclodextrin (5 mM) in a reaction mixture of
potassium antimony oxide tartrate and sodium selenosulfate in alkaline pH
(∼10.80), while chain like structures of antimony selenide are formed at a
lower concentration of β-cyclodextrin (2mM) [23].The preparation of antimony
chalcogenide and oxide nanomaterials was prepared by P. Christian and P.
O’Brien. They synthesize antimony chalcogenides Sb2X3 (X = O, S, Se, Te) by
a colloid route. They found that several materials with regular nano
morphology exhibit a change in structure depending on the reaction
conditions. A range of morphologies are found including rods, wires, tubes
and wafers [24]. Polycrystalline thin films of antimony selenide via chemical
bath deposition and post deposition treatments was done by Y. Rodriguez et
al. Evaluation of band gap from optical spectra of such films shows absorption
due to indirect transition occurring in the range of 1–1.2 eV [25].
E. R. Shaaban et al. were carry out compositional dependence of the
optical properties of amorphous antimony selenide thin films using
transmission measurements. The results indicate that the value
of Egopt decrease with the increase in the amount of Sb at expense of Se. The
chemical bond approach has been applied successfully to interpret the
decrease of the glass optical gap with increasing Sb content [26]. M. S. Iovu
et al. proposed photoconductivity of amorphous Sb2Se3 and Sb2Se3:Sn thin
films. The proposed interpretation of the observed phenomena involves the
examination of the processes of deep capture and recombination on the
charged defects and is in accordance with the general tendency of a change
in concentration of U− centers during the introduction into the chalcogenide
film of the positively charged impurity such as tin [27]. Microwave-assisted
Chapter-I
6
synthesis of Sb2Se3 submicron rods, compared with those of Bi2Te3 and
Sb2Te3 were done by Bo Zhou and Jun-Jie Zhu. The synthesis of Sb2Se3 was
based on the polyol reducing process. The morphologies of the compounds
were mainly determined by their inherent anisotropic crystal structures. The
optical properties of as-prepared Sb2Se3 were also characterized by UV–vis
diffuse reflectance spectroscopy and the band gap (Eg) can be derived to be
1.16 eV [28]. Synthesis and optical properties of Sb2Se3 nanorods, which are
of diameter around 40–100 nm and length could be of several micrometers.
The band gap of the nanorods is found to be 1.78 eV. Photoluminescence of
the Sb2Se3 nanorods excited at 450 nm showed emission peak at 587 nm
were reported by J. Ota and S. K. Srivastava [29].
Electronic structures of antimony selenide (Sb2Se3) from GW
calculations were carried out by R. Vadapoo et al. They studied the electronic
band structure of antimony selenide using density functional theory (DFT)
within the generalized gradient approximation (GGA) with GW corrections.
Their result shows that Sb2Se3 has an indirect energy band gap of 1.21 eV;
however, a direct transition only 0.01 eV higher than the band gap (1.22 eV)
[30]. One-dimensional Sb2Se3 nanostructures and their solvothermal
synthesis, growth mechanism, optical and electrochemical properties were
discussed by J. Ma et al. They reported that the synthesized Sb2Se3 nanowires
could be expected to be potentially used in lithium ion batteries as well as
solar energy and photoelectronics [31].
1.2.2 Antimony telluride (Sb2Te3) thin films
Antimony telluride is a grey, crystalline solid. It has been investigated
for its semiconductor properties. It can be transformed into both n-type and p-
type semiconductors by doping with an appropriate dopant. Sb2Te3 forms
the pseudobinary intermetallic system germanium - antimony - tellurium with
germanium telluride, GeTe. Like bismuth telluride, Bi2Te3, antimony telluride
has a large thermoelectric effect and is therefore used in solid state
refrigerators. Study on antimony telluride has been carried out by many
researchers. MOCVD of Bi2Te3, Sb2Te3 and their superlattice structures for
thin-film thermoelectric applications was reported by R. Venkatasubramanian
et al. [32]. Optical constants of Bi2Te3 and Sb2Te3 measured using
spectroscopic ellipsometry by Cui et al.
bismuth telluride (Bi
spectroscopic ellipsometry (SE).
Sb2Te3 samples after being etched in diluted NH
characterize the over layer and
Growth of Bi2Te3 and Sb
Giani et al. [34]. Thermoelectric properties of p
alloys manufactured by rapid solidification and hot pressing was r
H. C. Kim et al. These alloys were fabricated by mechanical alloying and hot
pressing to characterize the thermoelectric properties [35].
Sb2Te3 thin films were carried out by
some physical properties of tetradymite
with CdS [37]. Further
Sb2Te3 polycrystalline films were studied by
found that the resistance of the polycrystalline films strongly depends on the
grain size and inter-
the temperature dependence of the forbidden band in Bi
explained by V. Ku
shown in Figure 1.3.
Figure 1.3 Structure of antimony telluride (Sb
T. Thonhauser et al.
thermoelectric properties of Sb
anisotropic nanocrystalline sb
7
spectroscopic ellipsometry by Cui et al. They present the optical constants of
bismuth telluride (Bi2Te3) and antimony telluride (Sb
spectroscopic ellipsometry (SE). Analysis was performed on two
samples after being etched in diluted NH4OH solution in order to
characterize the over layer and confirm the reliability of the results [33].
and Sb2Te3 thin films with MOCVD were reported by
Thermoelectric properties of p-type 25%Bi
alloys manufactured by rapid solidification and hot pressing was r
These alloys were fabricated by mechanical alloying and hot
pressing to characterize the thermoelectric properties [35].
thin films were carried out by P. Arun et al. [36]. P. Lost'ak
some physical properties of tetradymite-type Sb2Te3 single crystals doped
Further Influence of grain size on the electrical properties of
polycrystalline films were studied by P. Arun and Vedeshwar
found that the resistance of the polycrystalline films strongly depends on the
-granular voids [38]. A study of tunneling spectroscopy of
the temperature dependence of the forbidden band in Bi2Te3
V. Kulbachinskii et al. [39]. Structrue of antimoy telluride is
shown in Figure 1.3.
Figure 1.3 Structure of antimony telluride (Sb
T. Thonhauser et al. discovered influence of negative pressure on
thermoelectric properties of Sb2Te3 thin films [40]. Chemical synthesis of
anisotropic nanocrystalline sb2Te3 and low thermal conductivity of the
Chapter-I
optical constants of
) and antimony telluride (Sb2Te3) by using
Analysis was performed on two
OH solution in order to
confirm the reliability of the results [33].
thin films with MOCVD were reported by A.
type 25%Bi2Te3+75%Sb2Te3
alloys manufactured by rapid solidification and hot pressing was reported by
These alloys were fabricated by mechanical alloying and hot
pressing to characterize the thermoelectric properties [35]. Ageing effect of
P. Lost'ak reported
single crystals doped
Influence of grain size on the electrical properties of
Arun and Vedeshwar. They
found that the resistance of the polycrystalline films strongly depends on the
A study of tunneling spectroscopy of
3 and Sb2Te3 wad
antimoy telluride is
Figure 1.3 Structure of antimony telluride (Sb 2Te3)
influence of negative pressure on
Chemical synthesis of
conductivity of the
Chapter-I
8
compacted dense bulk were reported by W. Wang and et al. Results obtained
indicated that a very low thermal conductivity of about 1 W/mK at 300 K,
comparing to 4.7 W/mK of the polycrystalline bulk, was achieved and nano
structured Sb2 Te3 is potentially a good candidate for engineered nano
composites that can lead to high thermoelectric figure-of-merit [41].
Vibrational properties of crystalline Sb2Te3, phonon dispersion relations and
infrared and Raman spectra of crystalline Sb2Te3 were computed within
density functional perturbation theory. Overall good agreements with
experiments were obtained by G. C. Sosso et al. [42].
Physical properties of Bi2Te3 and Sb2Te3 were discussed by O. Vigil-
Gal et al. deposited by close space vapor transport. The dependence of the
film properties on the substrate temperature was explored over a wide range
by keeping the source-to-substrate thermal gradient [43]. S. Shanmugan et al.
has been synthesized nano structured Sb2Te3 thin films by stacking the
elemental layer which is allowed to isochronal annealing at various
temperatures in the presence of Argon atmosphere [44]. The optimization of
ion beam sputtering deposition process for Sb2Te3 thin films deposited on
BK7 glass substrates was reported by Ping et al. [45]. Semiconductor
nanocrystals functionalized with antimony telluride ions for nano structured
thermoelectric material. They showed that the possibility of designing
nanostructured thermoelectric materials using colloidal inorganic nano
crystals functionalized with molecular antimony telluride complexes [46].
1.2.3 Bismuth selenide (Bi2Se3) thin films
Bismuth selenide (Bi2Se3) is a compound of bismuth and selenium. It is
used as a semiconductor and a thermoelectric material. Naturally occurring
selenium vacancies act as electron donors and it often acts as a semimetal. It
plays number of applications in thermoelectric materials thermoelectric effect,
topological insulators. Chalcogenide thin films were prepared by solution
growth technique on polymer surfaces were deposited by P. Pramanik et al.
[47]. Electrical properties of bismuth selenide (Bi2Se3) thin films prepared by
K. L. John with reactive evaporation. Hall effect measurements show that the
films have a carrier concentration of ≈ 1.02 × 1019 cm−3 with n-type
conductivity [48]. Chemical deposition of bismuth selenide thin films using N,
N-dimethylselenourea were carried out by
that deposited films exhibit strong optical absorption corresponding to a band
gap of about 1.7 - 1.41 eV. These values decrease to about 1.57
upon annealing the films at
Bismuth selenide is shown in Figure 1.4
Figure 1.4 Structure of
Thin films of Bi2Se
room temperature using selenium dioxide as a selenium ion source by an
electro deposition technique
gap of the film was 0.55 eV
determined by D. Nataraj
three kinetic parameters (activation energy (E
the frequency factor (v)) is derived from the Johnson
theory to describe the constant heating rate DSC spectra. Using the activation
energy from Kissinger plot, theoretical constant heating rate DSC traces are
generated for arbitrary values of 'v' and 'n' using the analytical equation. Then
theoretically generated DSC
one and the unknown values 'v' and 'n' are determined [51].
deposition and characterization of glassy bismuth (III) selenide thin films has
been studied by B. Pejova
Microwave-assi
Harpeness et al. In this reaction nano sized Bi particles were obtained as an
9
dimethylselenourea were carried out by V. M. Garcia et.al.
that deposited films exhibit strong optical absorption corresponding to a band
1.41 eV. These values decrease to about 1.57
upon annealing the films at for 1 h in nitrogen [49].
is shown in Figure 1.4
Figure 1.4 Structure of bismuth selenide (Bi 2
Se3 have been prepared from an aqueous acidic bath at
room temperature using selenium dioxide as a selenium ion source by an
electro deposition technique by A. P. Torane et al. They found optical band
gap of the film was 0.55 eV [50]. Kinetic parameters of Bi
D. Nataraj et al. According to him an analytical equation with
three kinetic parameters (activation energy (Ea), transformation order (n) and
the frequency factor (v)) is derived from the Johnson-Mehl
describe the constant heating rate DSC spectra. Using the activation
energy from Kissinger plot, theoretical constant heating rate DSC traces are
generated for arbitrary values of 'v' and 'n' using the analytical equation. Then
theoretically generated DSC spectra is compared with that of the experimental
one and the unknown values 'v' and 'n' are determined [51].
deposition and characterization of glassy bismuth (III) selenide thin films has
B. Pejova and et al. [52].
assisted synthesis of nanosized Bi2Se3 was reported by
In this reaction nano sized Bi particles were obtained as an
Chapter-I
V. M. Garcia et.al. They reported
that deposited films exhibit strong optical absorption corresponding to a band
1.41 eV. These values decrease to about 1.57 - 1.06 eV
for 1 h in nitrogen [49]. Structure of
2Se3)
have been prepared from an aqueous acidic bath at
room temperature using selenium dioxide as a selenium ion source by an
They found optical band
Kinetic parameters of Bi2Se3 thin films
n analytical equation with
), transformation order (n) and
Mehl-Avrami (JMA)
describe the constant heating rate DSC spectra. Using the activation
energy from Kissinger plot, theoretical constant heating rate DSC traces are
generated for arbitrary values of 'v' and 'n' using the analytical equation. Then
spectra is compared with that of the experimental
one and the unknown values 'v' and 'n' are determined [51].Chemical
deposition and characterization of glassy bismuth (III) selenide thin films has
was reported by R.
In this reaction nano sized Bi particles were obtained as an
Chapter-I
10
intermediate [53]. X. Qiu et al. introduced a new chemical method to produce
nano structured films for thermoelectric studies and applications. In this
method variation in synthetic parameters was studied to control the scale,
morphology, and composition of the films and enables the thermoelectric
transport properties to be optimized reliably and less expensively. With the
help of this method large-scale production of functional thin films for
applications such as catalysis and energy conversion was prepared [54].
Effect of structural, electrical and optical properties of electrodeposited
bismuth selenide thin films in polyaniline aqueous medium were studied by S.
Subramanian et al.He obtained the optical band gap energy 2.35 eV for as-
deposited Bi2Se3 thin film [55]. Bismuth selenide thin films were successfully
prepared by thermal evaporation by T. E. Manjulavalli et al. [56].
A new nanostructure of double-layer thin films (DLTFs) has been
introduced to Bi2Se3 as thermoelectric films through a facile one-step and low-
temperature solution route by Z. Sun and et al. [57]. R.H. Bari et al. were
synthesized bismuth selenide by chemical bath deposition method. They
reported band gap increase as Bi/Se ratio increases [58]. Electrodeposition
and characterization of thermoelectric Bi2Se3 thin films were discovered by
Xiao-long et al.[59]. Synthesis and thermoelectric properties of Bi2Se3
nanostructures Bi2Se3 nano flakes were synthesized via solvothermal route at
different synthesis conditions using DMF as a solvent by K. Kedal et al. The
surface morphology and crystal structure of the nanoflakes were analyzed,
and the results showed that the as-prepared samples were rhombohedral
phase of Bi2Se3. The size of the Bi2Se3 nano flakes increases with the
synthesis temperature. From the thermoelectric property measurement, the
maximum ZT value of 0.096 was obtained at 523 K and a ZT value of 0.011
was obtained at room temperature. The as-prepared Bi2Se3 nano flakes
exhibit a higher Seebeck coefficient and a low thermal conductivity compared
with the bulk counterpart at room temperature, which can be attributed to their
nano scale size. The improvement on the thermoelectric property indicates
the promising aspect of the as-prepared Bi2Se3 nanoflakes as a good
thermoelectric material at room temperature [60].
H. Peng et al. carried out the electrodeposition of Bi2Se3 nanowires on
an anodic aluminum oxide template. They reported that the composition and
Chapter-I
11
surface morphology of Bi2Se3 nanowires can also be improved by adding
surfactant [61]. Growth and characterization of Bi2Se3 thin films by pulsed
laser deposition using alloy target were reported by Lijian Meng et al.
Bi2Se3 films have been deposited on Si substrates by pulsed laser deposition
technique using a Bi2Se3 alloy target. All the deposited Bi2Se3 films were non-
stoichiometric and Bi-rich [62].
1.2.4 Bismuth telluride (Bi2Te3) thin films
Bismuth telluride (Bi2Te3) is a gray powder that is a compound of
bismuth and tellurium. It is a semiconductor which, when alloyed with
antimony or selenium is an efficient thermoelectric material for refrigeration or
portable power generation. T.C. Harman et al. prepared and studied some
physical properties of Bi2Te3, Sb2Te3, and As2Te3 from purified elements by
several techniques. They enumerated advantages and disadvantages of the
various techniques. Also electrical and thermal properties are presented as
functions of temperature and impurity concentration [63]. Pawlewicz et al.
reported resistivity of Bi2Te3 from 1.3 K to 300 [64]. K. J. George and B.
Pradeep reported preparation and properties of co-evaporated bismuth
telluride [Bi2Te3] thin films [65]. The effect of film thickness and deposition
temperature on the thermoelectric power and resistivity of Bi2Te3 films
prepared by vaccum evaporation on glass substrate was reported by
Damodara Das and Soundarajan [66]. Witold Brostow et al. [67] reported
electric and thermoelectric properties of electrodeposited bismuth telluride
(Bi2Te3) thin films. Xiaochuan Xu et al. [68] studied template synthesis of
heterostructured polyaniline/Bi2Te3 nanowires. Hydrothermal synthesis of
single-crystalline Bi2Te3 nanoplates was given by Yongbin Xu [69].
Kwon, Sung-Do et al. [70] fabricates bismuth telluride-based alloy thin film
thermoelectric devices grown by Metal Organic Chemical Vapour Deposition
(MOCVD) technique. Zhanli Chai et al. [71] synthesize polycrystalline
nanotubular Bi2Te3.
E. Koukharenko et al. were fabricated bismuth telluride materials by
ultrarapid quenching. Foils are obtained with a thickness varying from 10 to 60
µm. The thermoelectric properties were determined by measuring electrical
resistivity, Seebeck coefficient and Hall coefficient. The influences of
Chapter-I
12
quenching temperature and heat treatment on the Seebeck coefficients were
studied along with the variation of thermoelectric properties with temperature.
N-type degenerated materials were obtained with a carrier concentration of
1027 m−3[72]. S. M. Souza et al. reported structural, thermal, optical, and
photoacoustic of nanocrystalline properties of Bi2Te3 produced by mechanical
alloying. The PAS results suggest that the contribution of the interfacial
component to the thermal diffusivity of nanostructured Bi2Te3 is very
significant [73].
L. Plucinski et al. reported robust surface electronic properties of
topological insulators: Bi2Te3 films grown by molecular beam epitaxy. The
surface electronic properties of the important topological insulator Bi2Te3 are
shown to be robust under an extended surface preparation procedure, which
includes exposure to atmosphere and subsequent cleaning and
recrystallization by an optimized in situ sputter-anneal procedure under
ultrahigh vacuum conditions. Clear Dirac-cone features are displayed in high-
resolution angle-resolved photoemission spectra from the resulting samples,
indicating remarkable insensitivity of the topological surface state to cleaning-
induced surface roughness [74]. Wei et al. studied bismuth telluride thin films
with layered nanostructure have been fabricated by radio frequency (RF)
magnetron sputtering. They reported that substrate temperature is a key
factor on the microstructure and transport property of bismuth telluride thin
films. High temperature was beneficial for the formation of layered
nanostructure and the enhancement of power factor. The highest power factor
was obtained on the thin film deposited at 400℃. However, Te deficiency was
observed in these thin films. Thus thermoelectric property would be further
enhanced by optimizing composition of these thin films [75].
Zhu et al discovered that, being a best known thermoelectric material
and a topological insulator at ambient condition, magic bismuth telluride
(Bi2Te3) under pressure transforms into several superconducting phases,
whose structures remain unsolved for decades. They also solved the two
long-puzzling low high-pressure phases as seven- and eightfold monoclinic
structure, respectively, through particle-swarm optimization technique on
crystal structure prediction. They experimentally discovered that above
14.4 GPa Bi2Te3 unexpectedly develops into a Bi-Te substitutional alloy by
Chapter-I
13
adopting a body-centered cubic disordered structure stable at least up to
52.1 GPa. The continuously monoclinic distortion leads to the ultimate
formation of the Bi-Te alloy due to the Bi→Te charge transfer under pressure.
Our research provides a route to find alloys made of nonmetallic elements for
a variety of applications [76].
The anisotropic thermoelectric transport properties of Bi2Te3 and
Sb2Te3 under strain were investigated by N. F. Hinsche et al. It was found that
due to compensation effects of the strain-dependent thermopower and
electrical conductivity, the related power factor will decrease under applied in-
plane strain for Bi2Te3, while being stable for Sb2Te3. A clear preference for
thermoelectric transport under hole doping, as well as for the in-plane
transport direction was found for both tellurides. [77].
Figure 1.5 Structure of Bismuth Telluride (Bi 2Te3)
H. B. Zhang has carried out experimental evidence of the nanoscaled
topological metallic surface state of Bi2Te3 and Sb2Te3 films. They were
designed an experiment to clarify that topological insulators that their
theoretically predicted nanoscaled metallic surface state (3–5 nm) was never
Chapter-I
14
been demonstrated substantially by experiments by measuring the surface-
state and bulk-state resistances of topological insulators of Bi2Te3 and Sb2Te3
thin films. They found that the measured surface-state resistivity was lower
than that of the bulk-state by 5 orders of magnitude, indicated that the
nanoscaled surface state (3–5 nm) is metallic. These results definitely showed
that the bulk state exhibits a typical temperature dependence of insulators
[78].
G. Wang et al. reported topological insulator thin films of Bi2Te3 with
controlled electronic structure, using a joint ARPES/STM. They studied
theoretical experimental and first-principles. They demonstrate that the
electronic properties of the Bi2Te3 thin films can be regulated by altering the
MBE growth conditions without extrinsic dopants. A conversion from n- to p-
type conduction in the Bi2Te3 thin films is observed when the Si substrate
temperature is increased above a critical temperature [79].
A simple and new synthesis method of quality single crystalline Bi2Te3
nanowires combining the OFF-ON method with post-sputtering and annealing
was demonstrated by J. Kang et al. In step one; Bi nanowires were grown by
the conventional OFF-ON method. In step two, a Bi 2Te 3 thin films were in
situ deposited onto the Bi nanowire-including substrate by RF sputtering,
followed by the post-annealing at a high temperature well above the melting
point of Bi. Bi2Te3 nanowires are synthesized during the high temperature
annealing by the atomic inter-diffusion between the Bi core and the Bi2Te3
shell. Growth of this method yielded homogeneous, stoichiometric Bi2Te3
nanowires with high single-crystallinity and no observable defects, which were
hard to achieve using the conventional OFF-ON growth from a single
compound source. These results are used studies on high-efficiency
thermoelectric devices and topological insulators taking advantage of Bi2Te3
nanowires [80]. Figure 1.5 shows atomic layers in the Bi2Te3 crystal structure.
Dashed lines indicate Vander Waals gaps. The octahedral coordination is
highlighted for a Te atom.
1.2. 5 Ternary Bi2(Te1-xSex)3 chalcogenide thin films
Ternary chalcogenide thin films of V-VI compounds are the most
studied materials in thermoelectric research. Structural and electrical
Chapter-I
15
properties of the chalcogenide glasses such as Bi30 Se (70-x) Tex system were
studied by Z. Abdel- Khalek Al et al. [81]. Damodara Das et al. [82] reported
structural and electrical resistivity data of (Bi0.75Sb0.25)2Te3 thin films and are
analyzed using the effective mean free path model. Aboulfarah et al. [83]
discussed effects of VI/V ratio on electrical and thermoelectric properties of p-
type (Bi1-xSbx)2Te3 elaborated by metal-organic vapour deposition. N.
Keawprak et al. [84] investigates the thermoelectric properties of p- type
(Bi0.24Sb0.76)2Te3 material after pulse discharge sintering process. Pawar and
Bhosale [85] reported preparation and properties of Bi2-xAsxS3 thin films by
solution-gas interface technique. J. H. Keily and Dong-Hi Lee [86]
demonstrated that Bi2Te2.4Se0.6/Bi0.5Sb1.5Te3 thermoelectric generators can be
fabricated using the flash-evaporation technique and output characteristics of
the thermoelectric device can be significantly altered by variation of thermo
element dimensions. H. E. Atyia [87] reported structural properties and
thermoelectric power of thermally evaporated InSbTe3 thin films.
Damodara Das and Mallik [88] studied principal scattering processes
occurring in thin films of (Bi0.25Sb0.75)2Te3 alloy. L. D. Ivanova and Yu V.
Granatkina [89] were finds the optimal compositions of Sb2Te3-Bi2Te3 solid-
solution system for use in the p-legs of the low-temperature stages of
magneto-thermoelectric coolers. The pulsed magnetron sputtering technique
was applied for the preparation of layered Bi2Te3 and Sb2Te3 thin films [90].
Volklein et al. [91] have reported on the transport properties of Bi0.5Sb1.5Te3
films deposited on SiO2 substrates and their dependence on various
annealing conditions. Murali and Andavana [92] studied characteristics of
slurry coated CdSeTe films. Yang et al. [93] reported p-type
(Bi2Te3)0.25(Sb2Te3)0.75 via BMA and subsequent hot pressing, and study the
effects of BMA cycles and hot pressing temperature on its thermoelectric
properties. Bhatnagar and Bhatia [94] interpreted the results of their
measurements of the ac conductivity in Ge20S80-xBix on the basis of the
correlated barrier-hopping model. B. M. G. Marisol et al. [95] reported the
fabrication of high-density Sb-rich Bi2-xSbxTe3 nanowire arrays by means of
the electro deposition technique. Desai et al. [96] studied micro hardness of
Sb and Se doped Bi2Te3 single crystals. Shahab Derakhshan et al. [97]
Chapter-I
16
introduced the synthesis, crystal structure, electronic band structure, as well
as temperature dependent electrical conductivity of the ternary Pb4Sb6Se13.
Adam A. et al. [98] studied effect of laser irradiation on the optical
properties of amorphous Se96-xTe4Gax thin films. The electrical resistivity and
Hall effect were studied for the vacuum evaporated and annealed 1% Sb
doped bismuth alloy films of various thickness by V. Damodara Das and M. S.
Jagadeesh [99]. The morphology, structure, composition and the relation
between the Seebeck coefficient and the electro deposition parameters of the
(Bi1-xSbx)2Te3 film were reported by Huang et al. [100]. Doriane Del Frari et al.
[101] work concerns with pulse electro deposition of (Bi1-xSbx) 2Te3, through a
comparison of the different pulse parameter influences. M. Nedelcu et. al.
[102] deposited thick thermoelectric films by electro deposition. Comparative
study of the electrochemically prepared Bi2Te3, Sb2Te3 and (BixSb1-x) 2Te3
films was carried out by Doriane Del Frari et al. [103]. H. W. Jeon et al. [104]
have studied the effect of Sb2Se3 addition to Bi2Te3-Sb2Te3 pseudo-binary
alloy grown by the Bridgman method. P. Sharma et al. [105] examined the
structure of the glassy Ge20Se80-xBix alloys theoretically and found to be in
good agreement with experimentally observed values. D. I. Bletskan and
Uzhgorod [106] reported glass formation in binary and ternary chalcogenide
system. Svechnikova and Zemskov [107] studied growth of graded n-Type
Bi2Te2.85 Se 0.15 Crystals.
1.2.6 Quaternary Bi Sb (Te1-xSex)3 chalcogenide thin films
Effect of selenium doping on corrosion and electrochemical
performance of Pb-Sb-As-Se was studied by Z. Ghasemi A. Tizpar [108]. C.
M. Lee et al. [109] develops new compounds based on the Te-Ge-Bi-Sb
quaternary system by adding bismuth into Te5Ge4Sb. Abouel and et al. [110]
reported crystal structure and optical properties of quaternary system of Bi-
Sb-Te-Se. Arne Olsen et al. [111] determined the space group symmetry and
crystal structure of Tl3SbS3-xSex by a combination of powder X-ray diffraction,
electron diffraction, and high resolution electron microscopy. The effect of
replacement of antimony atoms by bismuth atoms on the electrical properties
of compounds of the melt-quenched thermally evaporated Ge25Sb15-xBixS60
chalcogenide system are reported by M. M. El-Samanoudy [112]. In our
Chapter-I
17
material research laboratory R. M. Mane et al. [113] worked on synthesis and
characterization of new quaternary MoBiInSe5 mixed metal chalcogenide thin
films.
1.3 Aim and objectives of present research work
The compounds of V-VI group elements forms binary, ternary and
quaternary chalcogenides. These metal chalcogenides are of great
importance due to their optical absorption, thermal energy gaps, and direct
type of mode of optical transitions. Hence these materials can be applicable in
photovoltaic devices and high frequency power sensors.
The development and performance of the Electrochemical Photovoltaic
(ECPV) cells depends on a large degree of the material employed for their
fabrication and construction. Thin film ECPV cells are being developed in
order to reduce the cost of photovoltaic systems. Thin film photo electrodes
are expected to be cheaper to prepare owing to their reduced material costs,
energy costs, handling costs and capital costs. However, thin films have to be
developed using new semiconductor materials which will give better
conversion efficiency. In these context nanocrystalline metal oxides, metal
chalcogenides and dye sensitized thin film electrodes such as TiO2, InO,
Fe2O3 and II-VI, II-III-VI Transition Metal Dichalcogenide (TMDC) etc. have
received much popularity as a thin film photoelectrode materials. They are
popular because of their better optoelectronic & electrical performance, long
term stability, simple chemosynthesis methods and cost effectiveness.
The optical absorption coefficient of semiconductor electrode may be
increased by loading organic dye called as dye sensitized ECPV cell. In dye
sensitized ECPV cell process of light absorption and charge separation is
differentiated. Their simple construction offers the hope of a significant
reduction in cost. The light absorption is performed by the monolayer of dye
on the surface of semiconductor photoelectrode. The dye containing
transition metal complex absorbs light photons and transfer an electron to
semiconductor photoelectrode, the process is known as injection [114].
Chapter-I
18
• The research work to be undertaken and its significance
For several decades, thermoelectric devices have attracted extensive
interest because of their excellent features such as no moving parts, quiet
operation, environmentally friendly, high reliability, and so forth. The
thermoelectric device can convert thermal energy from a temperature gradient
into electric energy. Bi2Te3, Sb2Te3 , their solid solutions Bi2(Te Se)3 and
[Bi,Sb]2 (Te Se)3 are of great interest for near-room temperature
applications in thermoelectric cooling and thermoelectric generator devices.
Now a days there has been exponentially increasing interest in developing
the thin film solar cells as one of the alternative energy source since it is
clean, non-polluted way of energy generation which need comparatively little
maintenance and abundantly available solar energy. Photovoltaic energy
conversion through semiconductor / liquid junction route is developing fastly
and becoming one of the popular alternatives to present energy crisis. The
photoconductivity properties of several chalcogenide have been investigated
by many workers [115,116]. If chalcogenide used in pure state then they have
difficulties with optical memory devices. So they are mixed with some impurity
atoms (Bi, Te, Ge, Sb, As etc.), which give higher sensitivity, higher
crystallization temperature and smaller ageing effects [117].
The addition of antimony to chalcogenide glasses is generally
accompanied by a marked change in their electrical and photoelectrical
properties. Though, it is possible to find some papers in the literature dealing
with the effect of the addition of Sb on electrical and photoelectrical properties
of chalcogenide glasses [118,119]. The addition of another element in binary
systems has been quite useful in improving some of the properties of glassy
semiconductors. Through addition of third element stabilize the structure,
which makes ternary system more stable thermally, the density of defect
states is increased, which affects the photoconductive properties [120,121].
Taking in to account the above facts, systematic studies were planned to
synthesize and characterize ternary Bi2(Se 1-xTex)3 thin films and antimony
doped Bi2(Se 1-xTex)3 ,where (x = 0.02, 0.04, 0.06, 0.08 and 0.10) thin films
are also investigated by simple arrested precipitation technique. The
preparative conditions such as bath temperature, pH, deposition time, speed
of the substrate rotation were finalized using following characterization
Chapter-I
19
techniques to check suitability of these films as a thermoelectric properties
and photo electrode in energy conversion device. Bi2(Se1-xTex)3 and
antimony doped Bi2(Se1-xTex)3 thin films will be obtained on to the glass and
conducting substrate supports in an alkaline medium using simple arrested
precipitation technique (APT) . The deposition conditions and preparative
parameters growth mechanism in film formation will be finalized at the initial
stages of research work. After deposition in thin films post treatments to the
as deposited semiconductor thin films will be given to influence the
optoelectronic characteristics using light sensitive dyes (transition metal
complexes). It is proposed to obtain monolayer of xylenol orange.
Ru(II)(dcbpy)2(NCS)2 on the surface of semiconductor thin film electrode.
• Characterization of Bi2(Se1-xTex)3 and Sb (III) doped Bi2(Se 1-xTex)3
thin films
As deposited and post-treated thin films will be then characterized to
check its suitability for the fabrication of injection ECPV cells. The
compositional structural microscopic, optical analyses will be done by means
of spectrophoto-metric, EDAX, XRD, SEM, AFM etc. techniques. The
electrical transport properties of the films such as, electrical conductivity and
thermoelectric power, thermal conductivity will be examined as a function of
thin film composition and temperature. Temperature dependence of electrical
conductivity, type of electrical conduction etc., will be investigated. The optical
absorption, band gap and the type of optical transition will be determined for
series of thin film electrode materials.
• Fabrication of dye-sensitized injection ECPV Cell
ECPV cells will be fabricated using the sensitized semiconductor
photoelectrode and suitable electrolyte systems. Construction of ECPV cell is
as below.
Dye sensitized
semiconductor
photoelectrode
Electrolyte system
aqueous and/or
non-aqueous
Conducting
glass/inert counter
electrode
Chapter-I
20
Fabricated ECPV cell will be characterized through current-voltage,
capacitance-voltage and spectral response. An attempt will be made to
correlate opto-electronic properties with film composition and preparative
parameters. The available data will be then interpreted and the results will be
reported in thesis.
Chapter-I
21
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