2.1 Introduction - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/22827/2/chapter ii.pdf ·...

GROWTH AND STUDY OF OPTICAL AND ELECTRICAL PROPERTIES OF CHEMICALLY DEPOSITED CdS1-xSex:Ag NANOCOMPOSITE THIN FILMS FOR SENSOR APPLICATION

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2.1 Introduction

The formation of thin films is one of the aspects of such development where the

restriction as dimensional growth rules out the existence of two dimensional thin films.

This has vast range of applications in different areas. Thin films can be prepared from

variety of materials such as metals, semiconductors, insulators or dielectric and many

more materials with different kind of preparation techniques. For the deposition of thin

films various preparation methods have been developed [1, 2]. New methods are also

being evolved to improve the quality of thin films deposit with maximum reproducible

properties and minimum variation in their compositions. The required thin film

properties and versatility can be obtained by using proper thin film preparation method.

In the thin film deposition process, following basic steps are to be considered,

• The initial materials used for the deposition is prepared in the atomic, molecular

or finest particulate form before the deposition.

• The finest form of materials is transported on the substrate in the form of vapor

stream or the solid or the spray.

• Deposition of the material on the substrate and the film growth by nucleation-

growth process.

The thin film preparation methods can be distinguished from each other by the

way in which the above mentioned basic steps are affected. In principle, one can get the

thin films of required properties by modifying above steps. The choice of preparative

method is however, guided by several factors particularly the melting point of the

materials, it’s stability, purity, dissociation constants in solutions and other

characteristics of deposits, etc. The control over all these preparative parameter can be

achieved by several methods [3].

----------------------------------------------------------------------------- Ph. D. Thesis submitted by Mr. Jagannath Babu Chaudhari

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2.2 Deposition Techniques

The process of thin film deposition occurs by multiple techniques with its

requirement of being single or multicomponent, alloy/oxide/compound coating on

substrate of different shapes and size. Depending on the nature of way used for the thin

films deposition the technique can be broadly classified as:

• Physical deposition techniques and

• Chemical deposition techniques.

2.2.1 Physical deposition techniques

The physical deposition techniques are those in which the material required for

deposition is made available in the atomic, molecular or particulate form before

deposition. This is usually done at sufficiently high temperature. The condensation of

vapor on substrate material kept at relatively low temperature yields thin film.

Physical deposition can be further subdivided in:

• Thermal evaporation,

• Molecular beam epitaxy,

• Sputtering,

• Electron beam evaporation,

• Ion plating and

• Activated reactive evaporation.

The detailed description of all the physical vapor deposition techniques is beyond

the aim of this dissertation.


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2.2.2 Chemical deposition techniques

Chemical deposition techniques are widely used for the growth of thin films due

to their versatility for depositing a very large number of elements and compounds, easily

controllable synthesis parameters at relatively low deposition temperature, etc. [4-6]. The

films can be deposited with required stoichiometric in the form of vitreous and

crystalline layers with high degree of preparation and purity. The various chemical

deposition processes are as follows,

Chemical bath deposition (CBD),

Successive ionic layer adsorption and reaction (SILAR),

• Electrodeposition,

• Anodization,

• Electroless deposition,

• Screen printing,

• Spray pyrolysis and

• Chemical vapor deposition (CVD).

A brief introduction of chemical deposition techniques is given below:

2.2.2a Chemical bath deposition (CBD)

Chemical bath deposition, which is also known as controlled precipitation or

solution growth method, or simply chemical deposition, is recently emerged as a

versatile method for the deposition of metal chalcogenide thin films. In CBD metal

chalcogenide thin films occurs due to substrate maintained in contact with dilute

chemical baths containing the metal and chalcogenide ions. The film formation takes


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place when ionic product exceeds solubility products. Lokhande et al. [12] has published

a review article with an emphasis on describing the deposition of various semi

conductors. A review by Mane and Lokhande [5] reported the effect of number of

deposition parameters on structural, optical, electrical and morphological properties of

chemically deposited films such as CdS, CdSe, CdSSe and number of binary and ternary

thin films. Recently, Hodes [6] wrote a book entitled “Chemical Solution Deposition of

Semiconductor Films”. In his book, he had explained fundamental mechanisms of thin

film formation and different aspect related to CBD process, ECPV properties of different

semiconductors, quantum size effect, etc. The chemical bath deposition has numerous

advantages such as:

• The method does not require sophisticated instruments.

• It is ideally suited for large area deposition and substrate of accessible as well as

non-accessible nature.

• Electrical conductivity of the substrate material is not an essential criterion.

• The deposition is possible even at low temperature and avoids the oxidation or

corrosion of the metallic substrate.

• An intimate contact between the reacting solution and the substrate material

permits for pinhole free and uniform deposits on the substrate of complex shape

and sizes.

• Stoichiometry of the deposit can easily be maintained since the basic building

blocks are ions instead of atoms.

• Mixed and doped film structures could be obtained by merely adding the mixent /

dopant solution directly in to the reaction bath.


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• The process of film growth is slow, that facilitate better orientation of the

crystallite with improved grain structures.

• The method can be used to deposit a large number of metal chalcogenides.

2.2.2b Successive ionic layer adsorption and reaction (SILAR)

SILAR is relatively new method firstly introduced by Nicolau et al. [7, 8] for the

deposition of metal chalcogenide thin films. It is based on immersion of glass substrate

in to separately place anionic and cationic precursors and rinsing between every

immersion in double distilled water to avoid homogeneous precipitation. In this method,

when substrate is immersed in cationic precursor, cations are adsorbed on the substrate

surface. Rinsing the substrate in double distilled water can separate the unadsorbed or

excess ions out and avoid homogeneous precipitation. When this substrate is immersed

in anionic precursor solution, anions react with preadsorbed cations. The unreacted

powder material can be removed by rinsing the substrate in double distilled water. This

step may be termed as one complete cycle. After repeating such appropriate cycles,

multilayer film formation of appropriate thickness takes place. Quality and thickness of

the film mainly depend upon preparative parameters. A review by Pathan and Lokhande

[9] reports the advantages of SILAR over CBD. Pathan [10] reported some calculations

based on cost effectiveness of SILAR over CBD.

2.2.2c Electrodeposition

It is the process of depositing a material on a substrate; substrate is used as a

electrode by electrolysis, the chemical changes being brought about by the passage of a

current through an electrolyte. The Faraday’s laws govern the phenomenon of

electrolysis. When a metal electrode is dipped in a solution containing ions of that metal,


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a dynamic equilibrium is set up. The electrode gains a certain charge on itself which

attract oppositely charged ions and molecules holding them at the electrode / electrolyte

interface. A double layer consisting of a layer of water molecules interposed

preferentially adsorbed ions and outer layer of the charge opposite to that of the electrode

is formed. During deposition, ions reach the electrode surface, stabilize on it, and release

their ligands (water molecules or complexing agent), release their charges and undergo

electrochemical reaction. The rapid depletion of the depositing ions from the double

layer is compensated by a continuous supply of fresh ions from the bulk of the

electrolyte. The transport of the ions to the depletion region occurs due to the diffusion

owing to concentration gradient and migration because of applied electric field and

conventional current. The factors influencing the electrodeposition process are:

• pH of the solution,

• Current density,

• Bath composition,

• Temperature of the bath,

• Electrode shape and

• Agitation.

2.2.2d Anodization

-----------------------------------------------------------------------------

It is an electrolyte process in which the metal substrate is used as a anode in a

suitable electrolyte. When an electric current is passed the surface of the metal is

converted in to its oxide having decorative, protective or other properties. The cathode is

graphite or also a metal where H2 evolves. The required oxygen originates from the

electrolyte used. The pH of the electrolyte plays an important role in obtaining the

coherent films. Thickness of the oxide layer depends on the type of metal, voltage

Ph. D. Thesis submitted by Mr. Jagannath Babu Chaudhari

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applied, temperature of bath and time of deposition. “These may be specially used for

depositing some metal oxides.”

2.2.2e Electroless deposition

Films can be grown on either metallic or nonmetallic substrate by dipping them

in appropriate solution of metals salt without the application of any external electrical

field. Deposition may occur by homogeneous chemical reactions usually reduction of

metal ions in a solution by a reducing agent. If this occurs at a catalytic surface it is

called an electro less deposition. Silvering is most widely used of these techniques. The

growth rate and degree of crystallinity depend upon the temperature of the solution. One

of the main advantages of such a method is to deposit the films on non accessible

surface, i.e., on the inner side of glass tubes etc.

2.2.2f Screen printing

Screen printing is essentially a thick film process in which a paste, containing

film material, is screen printed by a conventional method on a suitable substrate.

Subsequently, the substrate is fixed under appropriate conditions of the film and

temperature, to yield rugged components bounded to the substrate. The substrate that

have smooth surface, capability of withstanding for higher temperature, mechanical

strength and high thermal conductivity and are compatible with film material pastes are

used (alumina, beryllia, magnesia, thoria and zirconia).

2.2.2g Spray pyrolysis

Spray pyrolysis is essentially a thermally stimulated reaction between the clusters

of liquid/vapour atoms of different chemical species. The spray pyrolysis technique


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involves spraying of solution (usually aqueous) containing soluble salts of the constituent

atom of the desired compound on a substrate maintained at elevated temperature. The

sprayed droplets on reaching the hot substrate under go pyrolytic decomposition and

form a single crystal or cluster of crystallites of the product. The volatile by-products and

excess solvent escape in the vapour phase. The thermal energy decomposition and

subsequent recombination of the species and sintering and recrystallization of the

crystallites is provided by the hot substrates. The atomization of chemical solution in to a

spray of fine droplets is affected by the spray nozzle with the help of carrier gas, which

may or may not be involved in the pyrolysis reaction. The chemicals used for this

method have to satisfy the following conditions.

• The desired thin film material must be obtained as a result of thermally activated

reaction between the various species/complexes dissolved in the spray solution.

• The remainder of the constituents of the chemical including the carrier liquid

should be volatile at the pyrolysis temperature.

The nature of the substrate, chemical nature and concentration of spray solution,

and chemical spray parameter determine the growth of the film by spray pyrolysis. The

film deposited by this method are generally strong and adherent, mechanically hard,

pinholes free and stable with time and temperature. The topography of the film is

generally rough and depends on spray conditions. Substrate surfaces get affected in the

spray process and the choice is limited to glass, quartz, ceramic or oxides, nitrate or

carbide coated surfaces.


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2.2.2h Chemical vapor deposition (CVD)

A simple definition of chemical vapor deposition is the condensation of reactants

from the gas phase on to the substrate to form solid deposition, where reaction occurs to

produce a solid deposit. A liquid or solid compound to be deposited is made gaseous by

volatilization and is caused to flow either by a pressure difference or by the carrier gas to

the substrate. The chemical reaction is initiated near the substrate. In some processes the

chemical reaction may be motivated through an external agency such as heat, RF field,

light, X-rays, electric field or gas flow discharge, electron bombardment, etc. The

morphology, microstructure and adhesion of deposit are strong functions of the nature of

the reaction and the activation process. The possible reactions that are involved in CVD

are: thermal decomposition, hydrogen reduction, nitridation, oxidation, chemical

transport reaction and combined reactions. In most of the reactions, the deposition is

heterogeneous in character. Homogeneous reactions may occur in gas phase resulting in

powder form or flaky deposits.

The feasibility of CVD process can be predicted by studying thermodynamics of

the reactions. The reaction kinetics and mechanism of film growth are so different in

individual processes that generalized account is not possible. However, certain important

features are common to all those methods are: i) CVD set-ups are simple and fast recycle

times are possible, ii) high deposition rates are achieved, iii) depositions of the

compounds and multi-component alloys and control over their stoichiometry is possible,

iv) epitaxial layer of high perfection and low impurity content can be grown, v) objects

of complex shape and geometries can be coated and vi) in-situ chemical vapour etching

of the substrates prior to the deposition is possible.


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2.3 Theoretical Background of Chemical Bath Deposition (CBD)Technique

Chemical Bath Deposition technique (CBD) is a low cost technique used for the

growth of large area thin films of metal chalcogenide semiconductors for variety of

applications in different fields. In the chemical bath deposition technique, thin films of

semiconducting materials are deposited on the substrate immersed in dilute solution

containing metal ions and a source of sulfide, selenide or hydroxide ions [11-21]. The

basic principle used in this method is, the controlled precipitation through the use of

suitable complexing agents and amount by controlling the anions in the reaction bath

through the setting of appropriate chemical equilibrium, thin film deposition can take

place. The deposition of thin films can take place through the condensation of metal and

sulphide/selenide ions over the substrate surface. The early research in this area has been

presented in a 1982 review article [11], which attracted many researchers to work in this

area. The progress in this area is given in a 1991 review article [12], which contains the

collection of literature of more than 40 binary and ternary compounds (CdS, CdSe, CuS,

ZnS, InSe, Bi2S3, Bi2Se3, CdS-Bi2S3, CdS-Bi2Se3, SnS, Cd1-xZnxS, Cd1-xZnxSe, CuInS2,

CuInSe2, CdSSe etc.) grown by this technique on different types of substrates. The

considerable progress in this deposition technique makes it feasible for producing

multilayer thin films of metal chalcogenides as well as production of new material alloys

with improved surface and thermal stabilities. Thus, chemical bath deposition technique

is used for the deposition of thin films for number of electronic devices [11], such as

cathode ray tube, color TVs, electroluminescent devices, radiation detectors and laser

colors teleprojectors [12, 13], photovoltaic devices [14-16], window layer material in

tandem solar cells [18, 19], absorber layer material in solar cells[15] and photo

detectors[17]. Hence, looking towards the advantages lead down by chemical bath


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deposition technique, it has been used for thin film preparation in the present research

work.

The above-mentioned variety of prospective applications particularly in the field

of photovoltaic devices [21-24], photosensitive detectors [25] and various electronic and

optoelectronics devices [26-28], solar cells [29] has promoted renewed interest in

chemically deposited thin films.

The thin film growth by this method on the substrate takes place by ion-by-ion

condensation instead of cluster-by-cluster condensation or by adsorption of particles

from colloidal solution. The deposition parameters are easily controllable and better

crystalline orientation and improved grain structure can be obtained. Deposition

conditions like the bath temperature, pH of the solution, speed of rotation of substrate,

concentration of solution, reaction time, deposition time and reaction rate, etc. can be

easily controlled to obtained uniform and adherent film deposition at low temperature

[30]. Literature survey reveals the formation of number of binary chalcogenides such

CdS, CdSe, and ternary chalcogenides CdSSe using this simple technique.

It is already been proved that, solid solution system of two or more

semiconductor phases are of great importance in view of their significant role in

technological advancements. They are weakly disordered systems in which disorder is

generated by random distribution of the substitutional atoms at the sites of corresponding

sublattice. So they present unusual combination of semiconducting properties. CdSSe are

such extensively studied solid solution systems.


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2.3.1 Solubility and ionic product

When a sparingly soluble salt AB is placed in water, a saturated solution

containing A+ and B- ions in contact with the undissolved solid AB is obtained and

equilibrium is established between the solid phase and the ions in the solutions as,

AB(s) = A+ + B– (2.1)

Applying law of mass action to this equilibrium,

(2.2)

where, CA+, CB

– and CAB are the concentrations of A+, B–, and AB in the solution,

respectively.

)(SCCC

K =AB

BA −+

The concentration of a pure solid phase is a constant number.

+

−+=K

CCK BA (2.3)

i.e., CAB(s) = a constant = K`

or KK` = CA+CB

- (2.4)

As K and K` are constants, the product KK` is also constant, say Ks, therefore equation

(2.4) becomes

Ks = CA+CB

- (2.5)

The constant Ks is called the ‘solubility product (SP)’ and the expression (CA+CB

-) is

called as the ‘ionic product (IP)’.

When the solution is saturated, the ionic product (IP) is equal to the solubility

product (SP). It follows that when the IP exceeds the SP, i.e., IP/SP>1, the solution is

supersaturated and precipitation occurs and ions combine on the substrate and in the

solution to form nuclei. When the IP < SP, the solution will be unsaturated. The

temperature, solvent and particle size are three main factors, which affect the SP [31-33].


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2.3.2 Growth of thin films

Thin films of metal chalcognides are obtained by the bulk precipitation of

solution. The reaction of a metal solution takes place with an aqueous solution of a

compound capable of giving chalcogenide ions under suitable conditions. Thiourea,

thioacetamide or sodium selenosulphate are the commonly used compounds, which

furnish sulphur or selenium ions respectively by hydrolysis in alkaline media [34-38]. In

case of alkaline bath chemical deposition of film take place by the process of ion-by-ion

and cluster-by-cluster deposition. The ion-by-ion growth results in thin, hard, adherent

and specularly reflecting films whereas cluster-by-cluster growth gives thick, powder

diffusively reflecting films.

Once nucleation occurs, the deposition rate rises rapidly until the rate of

deposition becomes equal to zero i.e. IP = SP or S = 1 (S = degree of supersaturation).

Consequently, film attains a terminal thickness, which is maximum attainable thickness

(under given set of experimental conditions) therefore it is also called ‘terminal

thickness’. For nucleation to start, certain time period requires, it is referred to as

‘incubation period’. When substrates are suspended in container before forming the

complex in the solution, film thickness increases linearly and hence, show that the nuclei

for the formation of film are provided by the solution itself.

2.3.3 Precipitate formation in the solution

-----------------------------------------------------------------------------

The formation of a solid phase from a solution involves two steps one is the

nucleation growth and other is particle growth. The size of the particles of a solid phase

is dependent upon the relative rates at which these two competing processes take place. It

also depends on deposition temperature, rate of mixing reagents, concentration of

reagents and the solubility of the precipitate during precipitation. All of these can be

related to the relative supersaturation of the system. A state of supersaturation may be

Ph. D. Thesis submitted by Mr. Jagannath Babu Chaudhari

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achieved by lowering the temperature of an unsaturated solution and formation of the

solute in the solutions at a fixed temperature.

For any precipitate, there is some minimum number of ions or molecules required

to produce a stable second phase in contact with a solution called a nucleus. The rate at

which nuclei formed in a solution is dependent on the degree of supersaturation. In

highly supersaturated solution, the rate of nucleation increases exponentially.

Rate of nucleation = K0 (Q – S)X (2.6)

where, K0 and X are constants and X>1, Q the excess concentration above saturation s

the concentration at saturation. The second step is the growth of particles already present

(AB)

in the solution. This begins when nuclei or other seed particles are present. In case of

ionic solid, the process involves deposition of cations and anions on appropriate sites.

(AB)n + A+ + B– (AB)n+1 (2.7)

+ –

where ‘ st com ine in

le particle (AB) te of gro th is directly p

supersaturation.

surface area of exposed solid and K0` is a constant that is characteristic

the precipitation, the relatively few nuclei formed will grow to give a small number of

large particles. With high supersaturation, many more nuclei are formed initially and

nucleation may occur throughout the entire precipitation process. As a result, there are

many more centers upon which the growth process can take place, none of the particles

can become very large and colloidal suspension is formed.

n+1 + A + B (AB)n+2 (2.8)

n’ is the minimum number of A+ and B– ions that mu b order to yield

the stab n. The ra w roportional to the

Rate of growth = K0`A(Q – S) (2.9)

where ‘A’ is the

of the particular precipitate. If the supersaturation is maintained at a low level throughout


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The colloidal suspension consists of finely divided solid particles in a liquid

phase with diameters of 0.01 to 0.1 µm. Under some circumstances, colloidal particles

can com

drop wise addition of

of both the ions to be precipitated and the

reagent

following equation.

H4+ +

ea

s negl ro erties e

control

e together and adhere to one another and the resulting solid is called a ‘colloidal

precipitate’ and the process by which it is formed is termed as ‘coagulation’ or

‘agglomeration’. Colloidal particles when agglomerated have quite different properties

from a crystalline solid since the particles are arranged irregularly.

2.3.4 Controlled precipitation in the solution

The degree of supersaturation can be reduced by the slow,

the reagent and by the use of dilute solutions

. In precipitation, from homogeneous solution, the precipitating agent is

chemically generated in the solution. Local reagent excess does not occur because the

precipitating agent appears slowly and homogeneously throughout the entire solution;

the relative supersaturation is thus kept low. The necessity for careful control of the pH

has long been recognized. This is accomplished by making use of the hydrolysis of urea,

which decomposes into ammonia and carbon dioxide as follows:

CS(NH2)2 + H2O 2NH3 +CO2 (2.10)

Ammonia hydrolysis in water provides OH– ions according to the

NH3+H2O N OH- (2.11)

Thus urea is often used for the homogeneous generation of hydroxide ion. Ur

possesse igible basic p p soluble in wat r and its hydrolysis rate can be easily

led. It hydrolyses rapidly at 363-373 K and hydrolysis can be quickly terminated

at a desired pH by cooling the reaction mixture to room temperature. The use of a

hydrolytic reagent alone does not result in the formation of a compact precipitate. The

physical character of the precipitate will be very much affected by the presence of certain


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anions. The main function of ‘suitable anion’ is the formation of a basic salt, which

seems responsible for the production of a compact precipitate. The pH of the initial

solution must be appropriately adjusted. Moreover, by varying the rate of the chemical

reaction producing the precipitant in homogenous solution, it is possible to alter further

the physical appearance of the precipitate, the slower the reaction, the larger are the

crystals formed. Homogeneous precipitation of crystalline precipitates also results in

larger crystallites as well as improvement in the purity.

2.4 Effect of Preparative Parameters

The terminal thickness and the rate of deposition depend upon the number of

nucleation centers, supersaturation of the solution (given by ratio of IP/SP), and stirring.

The gro

tration leads to an increase in ‘X’ ion

larger thickness is obtained. However, above a

certain

n concentration decreases with increase in concentration of

uently, the rate of reaction and hence precipitation is reduced

leading

wth kinetics depends on the concentration of ions, their velocities and nucleation

and growth processes on immersed substrates. The effect of various deposition

conditions on these parameters is discussed below.

2.4.1 Anion compound concentration

The increase in anion (X) compound concen

concentration and a film of MX with

concentration, when rate of reaction becomes high, precipitation also becomes

important which leads to lesser amount of MX on the substrates and hence lower

thickness is obtained.

2.4.2 Complexing agent

The metal io

complexing ions. Conseq

to a larger terminal thickness of the film. Stability constants indicate affinity of

the complexant to the metal ion and their tendency to keep the metal ions in solution

even at a pH range where metal hydroxide precipitation is possible. If complex formed is


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too stable, the reduction of metal ion by weak reluctant may be completely inhibited. If

the complexant is not sufficiently stable, spontaneous reduction in the bulk of solution

will take place if strong reducing agents are used. The complexant selected should not

lead insoluble complexes that are complex with no charge on them. Negatively charged

complexes greatly influence the properties of the metal ion and hence the mechanism of

deposition. Complexing agents used are acetates, hydroxyacetates, succinates, hydroxy

propionates, triethanol amine, malonates, glycollates, lactates, pyrophosphates, citrates,

tartarates, ammonia, disodium salt of ethylenediamine tetra-acetic acid (Na2 EDTA)

[39], etc.

2.4.3 pH

The reaction rate and the rate of deposition depend on supersaturation: the lower

aturation, the slower the formation of MX (where M is metal ion concentration

and X

e anion of the compound (x-compound)

erature. At higher temperatures, the dissociation is greater and gives

higher

the supers

is anion ion concentration). If the concentration of OH– ion in the solution is

higher, the M ion concentration will be lower and the reaction rate will be slow. With an

increase in the pH as the M ion concentration decreases, the rate of deposition of MX is

decreased and increases the terminal thickness. At a certain pH, the concentration of M

ions decreases to a level such that the ionic product of M and X becomes less than the

solubility product of MX and a film is formed.

2.4.4 Temperature

The dissociation of the complex and th

depend on the temp

concentrations of M and X ions (number of metal and chalcogenide ions), which

result in higher rates of depositions. The thickness increases or decreases with increase in

the bath temperature depending on the conditions under which films are prepared. At low

pH values, supersaturation is high even at low temperature and increases further with


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increasing temperature. This results in formation of precipitation and consequently lower

thickness is obtained. At high pH values, precipitation is limited due to the low

supersaturation and most of the product is formed on the substrate surface. Further the

thermal dissociation of complex and anion compound is increased at high temperatures

so that more M and X ions are available for MX formation and thus higher thickness is

obtained.

2.4.5 Substrate

Film formation can take place only under certain conditions, i.e., either under

ions for MX formation or when the substrate has special properties

facilita

he

m insoluble chalcogenide under the same conditions of deposition and

provide

ctivity is

lled photoconductivity. This effect occurs in organic or inorganic

materia

of photon of sufficiently high energy by a

optimum condit

ting for formation of single crystal films. The second condition favours the

formation because when the lattice of the deposited material matches well with that of

the substrate the free energy change is smaller, which facilitates nucleation formation.

2.4.6 Doping

Impurities in the starting materials can be incorporated into the films only if t

impurities for

d their corresponding ionic product is greater than the solubility product. Doping

affects thickness, physical and chemical properties of the host film [40].

2.5 Photodetection

When a semiconductor is illuminated by light often increase in its condu

observed and it is ca

ls. With the combination of different parameters such as spectral response, dark

resistance, rise and decay time of the photoconductivity, etc., only few materials are

useful for practical uses in photodetection.

The photoconductivity process can be understood with the help of band picture of

solid. In intrinsic materials, the absorption----------------------------------------------------------------------------- Ph. D. Thesis submitted by Mr. Jagannath Babu Chaudhari

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solid ra

σph = e(nµn – pµp) (2.12)

where s and holes

ely. µn µp

photo conducting

produ ing ‘f’

p h e n n p p) (2.15)

The process involved in photoconductivity is quite complex [41] due to presence

of defect in the film materials such as vacancies, dislocation, grain boundaries, etc. in

materia

ises electron from valance band to conduction band, creating holes in valence

band. In extrinsic material free carriers are also produced by exciting the donor or

acceptor atoms.

Photoconductivity of material at constant light intensity is given by

‘e’ is electronic charge, ‘n’ and ‘p’ be the number of electron

respectiv and are the corresponding mobility. Photoconductivity is induced by

absorption of light wave. The condition for such phenomenon is hν ≥ Eg. The threshold

frequency ν = e.Eg/h with the incident light energy greater than eEg there will be change

in carrier density with out changing there mobility, thus affecting the conductivity. The

change in conductivity of material due to light irradiation is given by,

∆σph= e (∆n µn+ ∆pµp) (2.13)

This when expressed in terms of light intensity of excitation (L) on a

material c electron hole pairs per unit volume per unit photon absorbed and

free carrier life time ‘τ’ through the relation,

∆n = f. τn and ∆p = f. τp (2.14)

∆σ = f = (µ τ + µ τ

l which acts as trapping or recombination centers of the carriers, these

imperfections present in the solid plays an important role in heterojunction. The traps are

localized as positive potential centers for electrons and as negative potentials for holes.

So there will be localized discrete energy levels, i.e., traps in the band gap in the vicinity


46


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of the conduction band and valance band, respectively. When the traps are filled, the net

charge then vanishes.

When electron or hole falls in the trap, it spends some finite time there and will

no long

Consider a shallow trapping level ‘Et’ placed between the bottom of conduction

band ‘

(2.16)

here c carrie

tocurrent Iph is proportional to number of carriers available for

conduc

(2.17)

en ity L, but in

(2.18)

er be free to move and then it is raised to conduction or valance band. The traps

can capture electrons and holes and the probabilities of the opposite charges recombine is

greatly increased thereby terminating there free lifetime. Such traps are called

‘recombination centers’. These centers affect the density of free carriers and there free

lifetime. Hence photoconductivity of the material is affected.

Ec’ and fermi level ‘Ef’. Then number of carriers ‘n’ available to conduction

current will be less due to trapping of carriers than the steady state number ‘n0’ generated

by incident light. The relation between n and ‘n0’ is given by

( )KTE

NN

nn

nnn c −==−

expTt0

w N umber of trap rs, NT – is the density of state in conduction band, nt – n

density of traps.

The pho

tion.

( )KTEI Ph

−∝ exp

In trap free cases, photoconductivity is proportional to the light int s

presence of traps Iph will not be a linear function of L; it is modified as

Iph α Lx


47


------------------------------------------------------------------------------------------------------------------------

where the magnitude of ‘x’ varies from 0.5 to 1.0, the Iph versus ‘L’ relation varies from

linear to sub linear [42]. The current will mainly be carried by the carriers of one sign,

i.e. majority carriers [43]. The recombination process is of two types, i.e. radioactive and

non radioactive. The spectral response of photocurrent gives important information like

thermally stimulated current. The photocurrent spectra shows peak near absorption edge

which is related to energy band gap of solid [44]. The peaks with lower magnitude are

also observed and these are due to impurity levels in the forbidden gap. It is thus possible

to probe inside the energy structure especially of trap and recombination centers by

photoconductivity measurements.

Fig. 2.1: Schematic diagram illustrating different types of electronic transitions due to photon absorption.[41]

Electrons, holes, or- traps, excited states

There are few other parameters, which are important in assessing the efficiency

of photoconductivity. These parameters are life time (t) of carriers; collision cross

section (s) of traps, concentration of traps (Nt), time required by electron (hole) between

two electrodes, voltage applied in between two electrodes (V), etc.

When photon is absorbed by semiconductor various electronic transitions takes

place leading to photoconductivity of materials. These are shown in Fig. 2.1. Here ‘1’

shows the production of electron hole pair in conduction and valance band, respectively


48


------------------------------------------------------------------------------------------------------------------------

due to absorption of high-energy photon. ‘2’ indicates absorption of photon at a localized

imperfection site producing free electron and also a hole.

Hole being bound to a neighboring defect center and free electron moves to the

conduction band contributing to photoconductivity under an impressed voltage. ‘3’

shows, absorption of photon as a result of which electron from valance band is raised to

unoccupied impurity level, which (i.e., electron) remain bound to defect site leaving a

free hole in valance band for conduction. ‘5’ and ‘5`’ represent the capture of electron in

trap center and its thermal excitation to conduction band, respectively. ‘4’ and ‘4`’ are

similar process for holes. ‘6’ and ‘7’ represent the capture of hole and electron

respectively in the two-recombination centers. ‘8’ represent the combination of free

electron directly with free hole; this radioactive emission is ‘edge emission’. ‘9’

represents capture of electron by excited centers containing a hole. ‘10’ represents hole

capture by excited center containing electron. The transition ‘9’ and ‘10’ may be

radioactive.

Any transition that creates additional free carriers, it effectively increases the free

life time as well as the photosensitivity of the material. The ratio of increase in

conductivity of the material in presence of light to the conductivity in darkness gives the

value of photosensitivity and is given by equation

(2.19) tivityPhotosensiD

Dph

D

Dph

VVV

I=

II −−=

∆=

σσ

(2.20) 100tivityPhotosensi % XV ⎬⎨=

VV

D

Dph

⎭

⎫

⎩

⎧ −

where, ‘Iph’ and ‘ID’ are current under illumination and in dark, respectively. ‘Vph’ and

‘VD’ are the corresponding voltages, respectively. Assuming, however, that the specimen

resistance ‘Rph’ and ‘RD’ under illumination and dark is much larger than standard

resistance used across which voltages are measured.


49


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