general introduction to mixed metal chalcogenide...

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

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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

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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

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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

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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

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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


-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


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

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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.


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


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



theoretically generated DSC spectra is compared with that of the experimental

one and the unknown values 'v' and 'n' are determined [51].


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


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


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



spectra is compared with that of the experimental

one and the unknown values 'v' and 'n' are determined [51].Chemical


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

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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

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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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general introduction to mixed metal chalcogenide...

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