introduction to thin films chapter -i...introduction to thin films chapter -i 4 1.2...

Introduction to thin films Chapter -I

1

1.1 Introduction

“A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to

several micrometers in thickness”. Many of the electronic semiconductor devices are the

main applications benefiting from thin film construction. The semiconducting material, in

thin film form are of particular interest because it has a various number of applications viz.

transparent electrodes, photovoltaic devices, solar front panel display, surface acoustic

wave devices, low emissitivity coating for architectural glass, various gas sensors and heat

reflectors for advanced gazing in solar cells. Due to surface and interface effects; properties

of thin film differ considerably from those of bulk and this will dominates overall behavior

of the thin films.

Thin film plays an important role in the nanotechnology and nanoscience development.

Solar cell is an important application of thin film technology from the point in view of

global energy crunch, which converts the energy of the solar radiation into useful and

constructive electrical energy. Window material is the main condition for thin film solar

cells, which allows passing through; the visible region of solar spectrum but reflects the IR

Radiation. A large number of different deposition techniques are used for the construction

of thin films for structural, morphological and optical applications, as outlined in chapter

'Thin Film Deposition Techniques' by H. K. Pulker. The two most important categories are

physical vapour deposition (PVD), namely thermal vaporization and sputtering, and

chemical vapour deposition (CVD). It is clear, that for each deposition technique

appropriate coating materials are required. The PVD process normally use of inorganic

elements or compounds and gases, where as the CVD process use of dip coating and

spinning, liquid inorganic and organic compounds and gases. Liquid compounds and gases


2

are normally purchased directly from the producer, because it needs no special

preparation. Solid materials have to be compact and in the suitable form or shape, free of

gas inclusions or even are prepared according to a special recipe. Targets must also fulfill

structure requirements i.e. grain size, texture, precipitation. These operations are the task

of companies specialised in the production of coating materials and targets. This chapter

focuses on solid coating. The requirements on these materials are discussed, their

properties are listed and their production is described.

1.1 Deposition techniques

The wide classification of thin film deposition techniques is showed in Chart 1.1.

Among all these thin film deposition techniques, electrochemical deposition offers a wide

range of advantages over more expensive and vacuum based other methods of thin film

deposition. Along with being a simple, inexpensive and economic method, it has its own

advantage of no wastage of material, no any production of gases, it does not require very

pure starting material etc. The art and science of electrodepositing metal and metallic

alloys and anodization have been developed for more than a century [1-2].


3

Chart. 1.1: Classification of thin film deposition techniques

Resistive heating Electron beam

evaporation Flash evaporation Laser evaporation Arc evaporation Radio frequency (RF)

heating

Chemical vapor deposition Laser chemical vapor

deposition Photochemical vapor

deposition Plasma-enhanced chemical

vapor deposition Metal organochemical vapor

deposition

Glow discharge DC sputtering Getter sputtering Triode sputtering Radio frequency (RF) sputtering Magnetron sputtering Face target sputtering AC sputtering Ion beam sputtering

Electrodeposition Chemical bath deposition Successive ionic layer adsorption

and reaction (SILAR) Anodization Electroless deposition Spray pyrolysis Liquid phase epitaxy Sol gel process

Thin Film Deposition Techniques

Physical Chemical

Vacuum evaporation Gas Phase

Sputtering Liquid Phase


4

1.2 Electrodeposition technique

Electrodeposition is the process in which by the action of electric current, usually a

metallic coating is produced on a surface of electrode. By putting a negative charge on the

object to be coated the deposition of a metallic coating onto an object is achieved, this

object is immersing into a solution which contains a salt of the metal to be deposited (in

other words, the object to be plated is made the cathode of an electrolytic cell). The

metallic ions of the salt carry a positive charge and are thus attracted towards the object.

When positively charged metallic ions reaches the negatively charged object (that is to be

electroplated), it provides electrons to reduce the positively charged ions to metallic form.

Electrodeposition is often also called “Electroplating” [3].

Preparation of thin films using electrodeposition technique has several attractive

features [2].

1. It is an isothermal (temperature remains constant) process in which, morphology

and the thickness of the thin films can be easily controlled by parameters such as

electrode potential and current.

2. Relatively uniform films can be obtained on substrates of complex shape.

3. The deposition rate is higher than all other physical and chemical methods.

4. The equipments required are cheaper and does not require sophisticated

instrumentation.

5. Electrodeposition generally has low operating temperature. Apart from the obvious

advantages in terms of energy saving, the low deposition temperature avoids high

temperature effects such as contamination, inter-diffusion and dopant

redistribution etc.


5

1.3.1 Basics of Electrodeposition

Electrodeposition is based on principal of electrolysis in which it is due to passage

of electric current through an electrolyte, chemical reaction occur at electrode electrolyte

interface. Before commencing to study basics of electrodeposition, following "electrical"

terms are essential to know [1, 2, 4, 5].

i) Electrolyte – It is the bath of a conducting medium in which the flow of electric current

takes place by migration of ions. This bath may be aqueous, non-aqueous or molten, and it

must contain suitable metal salts. Sometimes, a stabilizer is included to improve the

quality of the electrodeposit. An ideal additive should not become incorporated in the film

but should lead to improvement of its surface finish, adhesion, uniformity, etc.

ii) Electrode - It is a conductor through which an electric current enters or leaves an

electrolyte. An electrode connected to negative terminal is referred as a cathode where as

another is referred as anode. At cathode, positive ions are discharged or negative ions are

formed or other reducing reactions occur. At anode, negative ions are discharged or

positive ions are formed or oxidizing reactions occur.

iii) Electrode potential - The difference in potential between an electrode and the

immediately adjacent electrolyte, measured against or referred to, an arbitrary zero of

potential is called an electrode potential. Static and dynamic electrode potentials are the

two electrode potentials those exist, when current is not flowing and passing between the

electrode and the electrolyte.

iv) Equilibrium electrode potential - An equilibrium electrode potential is defined as a

static electrode potential in which the electrode and electrolyte are in equilibrium with

respect to a specified electrochemical reaction.


6

v) Standard electrode potential - A standard electrode potential is one of the equilibrium

potential, for an electrode in contact with an electrolyte, in which all the components of a

specified electrochemical reaction are in their standard state. The standard state for a gas

is at one atmospheric pressure, and it is constant for a solid.

vi) Reference electrode - A reference electrode is as an electrode on which the state of

equilibrium of a given reversible electrochemical reaction is permanently secured under

constant physical and chemical conditions. Equilibrium potential of standard hydrogen

electrode (VHF) is 0 V, whereas it is +0.2415 V for saturated calomel electrode (SCE).

For the passing of an electric current two electrodes are immersed in an electrolyte, the

potential is applied across them. The electric current through the electrolyte is due to

cations and anions (positively and negatively charged ions). The function of applied

potential is to direct the cations towards positively charged electrode and anions towards

the negatively charged electrode i.e. towards the appropriate electrodes, where their

charges are neutralized and they are set free as atoms or molecules. The net result is that

metal (cation) is deposited on the cathode from the solution of metal ions according to

following process.

Mn+ + ne- M (1.1)

On the other hand, if the electrolyte contains more than one ionic species that can be

simultaneously deposited, then the electrodeposition process for these two types of ionic

species can be written as

M+ + e- M (1.2)

N+ + e- N (1.3)


7

or M+ + N+ + 2e- MN (1.4)

Accordingly, one can deposit a compound or an alloy of a multicomponent system.

When a electrolysis is carried out in the electrolyte, metal is deposited on cathode at the

same time anode is dissolved in the solution. The amount of dissolution and deposition is

determined by the quantity of electricity passed.

Faraday’s laws of electrolysis give the relationship between them, as follows,

i) The weight of the metal deposited at cathode (or dissolved from anode) is

proportional to the quantity of electricity (charge).

ii) When the same quantity of electricity is passed through several solutions in series,

the amounts of the metals deposited (or dissolved) are proportional to the chemical

equivalent weights of the respective metals. The chemical equivalent weight of a metal

is its atomic weight A divided by the valance Z of the ion from which discharge is taking

place.

The quantity of electricity that requires to deposits one equivalent weight of the metal is

called the Faraday F, which is equal to 9.65x104 C. Thus for I x t Coulombs (where I is

current in amperes and t is time in second) is passed, the quantity of metal deposited, W is

given by

ZAtI

ZA

FtIW

500,96 (1.5)

During electrodeposition, the actual mass of the electrodeposited material in general, does

not correspond to the electrolysis current. Part of the electrolysis current is consumed in

producing chemical changes other than one desired. For example, the current can also be


8

used up in decomposing the electrolyte or in evolving some gases. These chemical changes

are undesirable and waste energy.

The current efficiency (): It is defined as the ratio of the desired chemical change to the

total chemical change. This is calculated from the relationship.

% =

Nernst equation is a fundamental equation of electrochemistry and is particularly

significant for the electrode processes and is follows.

CFZTREE ln..

0 (1.7)

Where,

E - Electrode potential,

E0 - Standard electrode potential,

R - Gas constant,

T - The absolute temperature of the electrolyte,

Z - Charge on the ion,

F - Faraday's constant

C - Concentration of ions in gram ions per liter.

Nernst equation clearly suggests that electrode potential is proportional to activity

(concentration) of ions.

Weight of metal actually deposited

Weight of metal calculated using Faraday’s law

× 100 (1.6)


9

1.3.2 Steps involved in electrodeposition process

When current flows through the electrolyte the cations and anions move towards the

cathode and anode, respectively and deposit on the electrode after undergoing a charge

transfer reaction. In general the electrodeposition reaction occurs in the following

successive steps [1].

i] Ionic transport

ii] Discharge

iii] Breaking of ion-legand bond

iv] Incorporation of adatoms on to the substrate followed by nucleation and growth,

these steps can be schematically represented by the following Fig 1.1

Fig. 1.1: Approximate regions in which various stages of ion transport occur leading to

electrodeposition


10

Within 1-1000 Å from the substrate four steps occurs with each having its own

operation region. Ions that are sufficiently away from the electrode surface i.e. greater than

1000 Å (distance) can move towards electrode surface under the influence of current

density, concentration gradient and potential gradient. In the electrolyte ionic species are

normally surrounded by a hydration sheath or by some other complex forming ion or

legand present in the electrolyte. They move together (with hydration sheath or legand) as

one entity and arrive near the electrode surface where the ion-legand system either donate

electron to the anode or accepts electron from the cathode. This ionic discharge reaction

occurs in the electrolyte between 10 to 1000 Å from the electrode. The discharged ions

arrive near the electrode, where step-by-step they lead to the formation of a new solid

phase or growth of an electrodeposited film. The atoms thus deposited have a tendency to

form either an ordered conglomerate of crystalline phase or disordered amorphous phase.

The electrodeposit formation steps of transport, discharge, ion-ligand breaking, nucleation

and growth are interlinked.

1.3.3 Factors governing electrodeposition

Several factors affect the growth during deposition from electrolyte bath and hence

grain size and thickness. These parameters and their effect on grain size and thickness are

described below.

1. Current density

It is always desirable to an adequately high current density and thereby to increase

the rate of deposition. Due to low current discharge of ions at the cathode occurs at a low

rate causing slow deposition. Within a certain limit, as current density is raised the growth

rate of nuclei enhances and the deposits will be fine grained. The deposits obtained under


11

these conditions will be obviously crystalline. High current density increases the rate of

deposition and with further increase in current density beyond limit give foggy and spongy

deposits. The concentration of metal in the cathode decreases and the polarization

increases with excess increase in the limiting current density. The resultant films are

amorphous in nature.

2. Temperature

The rate of diffusion enhances and the ionic conductivity of the bath increases with

rise in temperature. Increase in temperature of deposition bath cause an increase in

crystallite size. This increase in crystallite size corresponds to a decrease in polarization at

higher temperatures. Higher current densities are possible at high temperatures and hence

it is possible to obtain fine grained and smooth deposition by heating the bath solution.

3. Metal ion concentration

Normally the plating bath is always an aqueous solution-containing compound of

metal to be deposited. It is advantageous to use higher concentration of metal components

in the bath solution. A high current density can be employed in high metal bearing bath. AS

the cathode polarization decreases which increases the crystallite size due to increase in

metal concentration under given condition.

4. Nature of anions and cations

The cathodic deposition of metals from their simple salts is affected by a nature of the

salt anions. For many metals the effect of anions on the over potential and on the nature of

deposits formed is observed. Generally over potentials decrease from anion to anions in

the following order,

PO33- >, NO3- > SO42- > ClO4- > NH4SO3- > Cl- > Br-> I-


12

Rougher and porous grained deposits tendency increases in the order as shown above. The

presences of indifferent cations in the solution are usually NH4+, Na+, K+, H+ ions. As general

rule, metal over potentials rise with increase in concentration of hydrogen ion.

5. Hydrogen ion concentration (pH)

Control of pH of plating bath is necessary in order to operate a bath with optimum

efficiency and maintain the desired physical properties of the deposit. Besides very low pH

may be lead to accumulation of hydroxide ions in the vicinity of the cathode and

consequent precipitation of basic salts, which may get included in electrodeposition,

thereby altering deposit properties. The efficiency of metal deposition is lowered when all

aqueous solutions contains H+, in fact in every deposit from an aqueous there is a

possibility of the hydrogen gas formation at the cathode due to H+ ions. This efficiency and

hydrogen discharge potential partly depends upon hydrogen ion concentration. At very

low pH, the bath permits the use of higher current density to produce a sound deposit

relatively high efficiency.

6. Addition agents

In almost all cases of electrodeposition of metals, it is observed that, addition of small

quantities of certain substances often result in production of fine grained, smooth and

nanocrystalline deposits. Such substances are known as addition agents and depending

upon the specific effect they produce, they are called levelers, stress reducers, and

brighteners etc. These addition agents are usually organic substances, which are colloidal

in nature. Minute quantities of these organic substances are sufficient to cause a

remarkable change in the form of deposits. Among the substances used as addition agents,

the following may be mentioned. Gelatine, glue albumin, agar sugars, aldehydes etc.


13

7. Complexing agents

To form stable complex ion the unstable metal ions are capable of combining

chemically with neutral molecules and with ions of opposite sign. The combination is

through the covalent bond, when neutral molecules interact with positively charged metal

ions to yield negatively charged complex ions. Complex compounds in a plating bath serve

two purposes. Firstly they make possible to maintain a high metal concentration but low

metal ion concentration. The complex ions continuously supply the simple ions necessary

for the discharge at the cathode because the complex compound serve as a resever. A low

metal ion concentration enables the production of deposits with small grains and improves

the throwing power. Secondly, complex formation enables us to enhance significantly the

solubility of slightly soluble salts.

1.3.4 Experimental setup of electrodeposition technique

Fig. 1.2 shows the simple electrodeposition experimental setup. Electrodeposition is

defined as the process in which the deposition takes place in the form of thin layer on a

substrate by electric current. The bath is specially designed chemical solution that contains

the desired metal dissolved in a form of submicroscopic positively charged metallic

particles. The object (metal ions) that is to be plated is submerged into the electrolyte,

placed generally at the center of the bath, which acts as a negatively charged cathode. The

positively charged anode completes the electric circuit; those may be at opposite edges of

the plating tank, thus causing film deposit on both sides of the cathode. Battery is used as

power source; providing the necessary current required for chemical change. This type of

circuit arrangement directs electrons (negative charge carriers) into a path from the power

supply to the cathode (usually substrate). Now, in the bath the electric current is carried


14

mostly by the positively charged ions from the anode towards the negatively charged

cathode. This movement makes the metal ions in the bath to migrate towards extra

electrons that are located at the cathode's surface or near the cathode’s surface outer layer.

Finally, by means of electrolysis the metal ions are removed from the solution and are

deposited on the surface of the object as a thin layer [2].

Fig. 1.2 Experimental setup of a simple electrodeposition technique

1.4 Mechanism of nucleation and growth

The growth of an electrodeposit from an electrolyte involves a phase transfor-

mation from ionic species in the solution to a solid phase on the electrode. This phase


15

transformation is the cumulative effect of ionic transport, discharge, nucleation, and

growth. Practically all the literature and text on the nucleation and growth of

electrodeposits deals with metals. The nucleation processes during the electrodeposition

of semiconductors are expected to exhibit similar behavior as the metals.

1.4.1 Pathways for the growth of an electrodeposition

The entire pathway for the growth can be divided into four following steps shown

in Fig. 1.3.

Step 1 Transport of the ions in the electrolyte bulk towards the interface.

Step 2 Discharge of the ions reaching the electrode surface, giving rise to adatoms.

Step 3 Nucleation and growth, where two alternative routes are possible, Route A growth

assisted by surface diffusion. Route B growth assisted by formation of clusters and critical

nuclei.

Step 4 Formation of monolayer and final growth of electrodeposit

Surface defects such as steps, kinks and dislocations generally control the growth

kinetics. The kinks sites and screw dislocations together sustain the growth of the

electrodeposit.

Ion by ion mechanism Ion and cluster mechanism


16

Fig. 1.3 Schematic representation of steps involved in the electrodeposition process.

1.5 Characterization techniques

1.5.1 Structural and morphological properties

Characterization is an important step in the development of better-quality

materials. The complete characterization of any material consists of phase analysis,

structural elucidation, compositional characterization, surface characterization, and micro-

structural analysis, which have strong bearing on the properties of materials. This has led

to the emergence of variety of advanced techniques in the field of materials science. In this

section different analytical instrumental techniques are used to characterize our thin films

described with relevant principles of their working and operation.

1.5.1.1 X-ray diffraction (XRD) technique

X-ray diffraction is a tool for the investigation of the crystal structure and phase of

material. This technique had its beginnings in von Laue's discovery in 1912 that crystals


17

diffract x-rays, the manner of the diffraction revealing the structure of the crystal. Fig. 1.4

shows the schematics of X-ray diffractometer. The phenomenon of X-ray diffraction

consists of reflection of X-rays from the different crystallographic planes of material and

Bragg’s law governs it,

2d sin = n (1.8)

Where,

d - Interplaner spacing,

- Bragg’s angle or diffraction angle,

- Wavelength of x-ray used, and

n - Order of diffraction.

In powder diffraction method the crystal to be examined is reduced to a fine

powder and placed in a beam of a monochromatic x-rays. Each particle of the powder is the

tiny crystal, or assemblage of smaller crystals, oriented at random with respect to incident

beam of x-rays. For example some of the crystals will be correctly oriented so that their

(101) planes, can reflect the incident beam. Other crystals will be correctly oriented for

(111) reflections and so on.

The principle of x-ray powder diffractometer is that every set of lattice planes will

be capable of reflection. Ideally, according to Bragg’s law, for the particular d value, the

constructive interference of x-rays should occur only at particular value i.e. Bragg’s angle

and for all other angles there should be destructive interference and intensity of diffracted

beam will be minimum for destructive interference.


18

Fig. 1.4 Schematics of X-ray diffractometer

Identification of phases

Phases of different materials can identified by the following way: The observed d-

values (interplaner spacing) are compared with standard d-values from ASTM

(international American Standard for Testing of Materials) standard data file or JCPDS

(Joint Committee for Powder Diffraction Standards) data file, for the same material

synthesized by standard chemical method. This analysis reveals the different phases

present in the sample and miller indices (h, k, l) of the atomic planes. The lattice

parameters for the unit cell of the phase present are calculated using equations given by

Cullity [6]. Absence of reflection peaks indicates amorphous nature of the sample. A single

reflection peak indicates an epitaxial growth while many reflection peaks indicates

polycrystalline (heteroepitaxial) growth. However, if the size of the diffracting tiny crystal

2

X-raySource

Sample

Detector

Receiving SlitDiverging Slit


19

is small, there is no more complete destructive interference at d, which broadens the

peak corresponding to diffracted beam in proportion to the size of the tiny crystal. This can

be used to calculate the crystal size. The crystallite size was calculated using the relation

given by Scherrer and formulated [7] as,

B

D

cos9.0

(1.9)

Where,

D - Crystal size,

B - Diffraction angle,

- Wavelength of X-rays and

- line broadening at Full Width at Half Maxima (FWHM).

1.5.1.2 Scanning Electron Microscopy (SEM)

Scanning electron microscope (SEM) is an instrument that is used to observe the

surface morphology of the sample at higher magnification. As compared to an optical

microscope it has higher resolution and depth of focus [8]. The ray diagram of scanning

electron microscope is shown in Fig. 1.5. Basically SEM is used for topographical and

compositional observations of surfaces, internal structure observation, elemental analysis

of specimen, crystalline structure, internal characteristics observation and magnetic

domain observation.

Interaction of electron with elements has been extensively used for the

characterization of materials. Scattering of electron from the electrons of the atom results

into secondary electrons, backscattered electrons, auger electron, characteristics x-rays,


20

continuous x rays and photons of varies energies. These scattered electrons give

information about the nano and microstructure of sample in the form of image. These

images are classified as (a) secondary electron image and (b) backscattered electron

image. A secondary electron image is most generally used to study surface topography. In

this case, detector is sensitive to electrons that emerge from the specimen with energy

greater than 50 eV. Scintillator is used as a detector which is held at a high positive

potential of several KV and the secondary electrons are accelerated into scintillator to give

visible light which is then detected by photomultiplier [9].

In SEM, since electrons being charged particles to avoid change in their density they

require vacuum environment, an electron beam is directed towards the sample in a

vacuum of 10-4 to 10-10 torr using electromagnets. The resolution and the depth of field of

the image are determined by the spot size and beam current. This beam current and spot

size are adjusted by objective lenses and condenser lenses. The detector detects the back

scattered and secondary electrons emitted by sample. The secondary electrons are

produced due to interaction of beam electrons and weakly bound electrons in the

conduction band of sample and are mostly used for investigation of surface morphology.

The gain of electron beam may be amplified and utilized to control the brightness of the

spot. The sample preparation is relatively easy.

The sample used for the scanning must be electrically conducting in order to avoid

charge build up caused by the impinging electrons and resulting in jumping of beam. Non

conducting samples can be scanned by coating conducting layer of Pt-Au alloy.


21

Fig. 1.5 Ray diagram of scanning electron microscopy

1.5.1.3 Field Emission Scanning Electron Microscopy (FE-SEM)

Field Emission Scanning Electron Microscope is abbreviated as FE-SEM. The FESEM

can be classified as a high vacuum instrument with pressure less than 110−7 Pa in the ions

pumps. The vacuum allows electron movement along the column without scattering and

helps prevent discharges inside the instrument. The vacuum design is a function of the

electron source due to its influence on the cathode emitter lifetime. There are two classes

of emission source: field emitter and thermionic emitter. Emitter type is the main

difference between the Scanning Electron Microscope and the Field Emission Scanning

Electron Microscope. Now a day, three-dimensional features can be observed due to the

large depth of field available in the FE-SEM.


22

Principle

Electrons generated by a field emission source are accelerated in a field gradient,

under vacuum. The beam passes through electromagnetic lenses, focusing onto the

specimen. As result of this beam bombardment, different types of electrons are emitted

from the specimen. A detector catches these secondary electrons and an image of the

sample surface is constructed by comparing the intensity of these secondary electrons to

the primary scanning electron beam. Finally the image is displayed on a monitor.

1.5.2 Energy-dispersive X-ray spectroscopy (EDS or EDX) Analysis

Chemical characterization or elemental analysis of a sample is does with the help of

an analytical technique called as Energy-dispersive X-ray spectroscopy (EDS or EDX). It

relies on the investigation of an interaction of some source of X-ray excitation with sample.

Its characterization capabilities are due in large part to the fundamental principle that each

element has a unique atomic structure allowing unique set of peaks on its X-ray spectrum

[10]. High-energy beam of charged particles such as electrons or protons, or a beam of X-

rays, is focused into the sample in order to stimulate the emission of characteristic X-rays

from a specimen being studied. At rest, an atom within the sample contains ground state or

unexcited electrons in discrete energy levels or electron shells bound to the nucleus. The

incident beam may excite (knocks) an electron in an inner shell, ejecting it from the shell

while creating an electron hole where the electron was. An electron from an outer, higher-

energy shell then fills the hole of inner shell, and the difference in energy between the

higher-energy shell and the lower energy shell may be released in the form of an X-ray. The

number and energy of the X-rays emitted from a specimen can be measured by an energy-

dispersive spectrometer. As the energy of the X-rays is characteristic of the difference in


23

energy between the inner and outer shell (two shells), and of the atomic structure of the

element from which they were emitted, this allows the elemental composition of the

specimen to be measured [10].

Fig. 1.6 Principle of EDS

EDS is often contrasted with its spectroscopic counterpart, WDS (wavelength dispersive X-

ray spectroscopy). WDS differs from EDS in that it uses the X-rays diffraction on special

crystals as its raw data. WDS has a much more finer spectral resolution than Energy-

dispersive X-ray spectroscopy (EDS). WDS also avoids the problems associated with

artifacts in EDS e.g. noise from the amplifiers and microphonics, also false peaks. In WDS,

only one element can be analyzed at a time, while EDS gathers a spectrum of all elements,

within limits, of a sample.

1.5.3 Optical properties

1.5.3.1 Absorption

Due to optical photon absorption the equilibrium situation in semiconductor can be

disturbed by generation of carriers. Optical photon incident on any material may be

transmitted, reflected, or absorbed. The phenomena of radiation absorption in a material is


24

altogether considered to be due to (a) valence band electrons (b) inner shell electrons (c)

free carriers including holes as well as electrons and (d) electrons bound to localized

impurity centers or defects of some type. In study of the fundamental properties of

semiconductors, the absorption by the first type of electrons is of great importance. At

absolute zero temperature, for an ideal semiconductor, the valence band would be

completely full of electrons so that electron could not be excited to a higher energy state

from the valence band. Absorption of quanta of adequate energy tends to transfer the

electrons from valence band to conduction band. The optical absorption spectra of

semiconductors normally exhibit a sharp rise at a certain value of the incident photon

energy which can be attributed to the excitation of electrons from valence band to

conduction band i.e. it may also involve acceptor or donor impurity levels, traps etc. Energy

and momentum conservation must be satisfied in optical absorption process.

Fig. 1.7 E-k diagrams showing (a) direct and (b) indirect inter-band transition.

k´

k

Eg

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

k

E

0

Conduction Band

Valence Band

----

----

----

---

k k k´

Conduction Band

Valence Band

(a) (b)


25

Direct and indirect transitions, mainly these are two types of optical transitions that

can occur at the fundamental edge of the crystalline semiconductor. Both involve the

interaction of an electromagnetic wave with an electron in the valence band, which is rose

across the fundamental gap in the conduction band. On the other hand indirect transition

also involves simultaneous interaction with lattice vibration. Thus the wave vector of the

electron can change in the optical transition. The momentum change being taken or given

up by phonon. The direct interband optical transition involves a vertical transition of

electrons from the valence to the conduction band such that there is no change in the

momentum of the electrons and energy is conserved which is as shown in Fig. 1.7 (a).

Hence a wave vector k for electron remains unchanged in E-K space. Absorption coefficient

α for simple parabolic bands and for direct transition is given by the relation (1.10) [11].

h

nEhA g

(1.10)

Where,

Eg - Separation between bottom of the conduction and top of the valence band,

h - Photon energy,

n - Constant (1/2 or 3/2 depending on whether transition is allowed or forbidden)

A - Constant depending upon the transition probability for direct transition.

For allowed direct transitions n =1/2 and for allowed indirect transition n = 2. Thus

if the plot of (hν)2 against hν is linear then the transition is direct allowed. The band gap

energy Eg is determined by extrapolating the linear portion of the curve to the energy axis


26

at =0. Let’s visualize a situation in Fig. 1.7(b), where interband transition takes place

between different k-states. Since these must satisfy the momentum conservation laws. The

only way such a transition can take place is through the emission or absorption of a

phonon with wave vector q as;

K’ ± q = k + K (1.11)

The transition defined by equation (1.12) is termed indirect transition. For indirect

transitions

h

nEhA g (1.12)

Where, Eg = Eg’ ± Ep, Eg is indirect band gap energy and Ep is the phonon energy. For

allowed transition n=2 and for forbidden transition n=3. The band gap energy is

determined by extrapolating the linear portion of the plot (h)n versus h to the energy

axis at =0.

1.5.4 Fourier Transform Raman (FT-Raman) Spectroscopy

The advent of FT-Raman makes Raman spectroscopy more useful in chemical

analysis because, the fluorescence is reduced (or eliminated) by measuring spectra in the

near-IR region. The FT technique involves measuring the Raman effect using an ND:YAG

laser emitting at 1064 nm and interfacing the Raman sampling module with an FT-IR

instrument [12]. The technique was first suggested in 1964 by Chantry and Gebbie [13]. At

present, most FT-IR instruments can be coupled with a Raman accessory to obtain Raman

spectra and use the FT-IR capabilities with computer manipulations and software


27

programs developed for IR spectroscopy. The interest in FT-Raman has increased

significantly since it reduced or eliminated fluorescence, has high resolution, good

frequency accuracy, and collects stokes and anti-stokes. It measures the intensities of light

of many frequencies simultaneously. The spectrum is then converted into a conventional

spectrum by means of Fourier transformation using a computer algorithm. FT-Raman

spectroscopy is often referred to as a time-domain spectroscopy. The distinctive feature of

the FT technique is that, information for all the wavelengths falls on the detector at all

times. This provides improved resolution, spectral acquisition times, and signal to noise

ratio over conventional dispersive Raman spectroscopy. FT-Raman spectroscopy is a

viable tool for studying thin films, especially those that contain absorbing or fluorescent

moieties.

It is also apparent that FT-Raman spectroscopy can provide information about

chemical moieties not associated with electronic chromospheres and can be carried out

with molecules which give rise to resonance enhancement when visible or UV excitation is

used. This is an important consequence or near-IR excitation Raman spectroscopy and will

lead to a greater understanding of the structure and chemistry of thin films of polymers

and highly colored materials. FT-Raman spectra plays an important role in medical field, to

study various dynamic processes of interest in the paint industry, food industry, to

characterize dyes in dye industry, in metal corrosion study, petroleum industry, forensic

analysis and mining industry. It is much more useful to analyze pesticide contamination of

fruits and vegetables. Due to these vast applications the as deposited thin films are to be

characterized by FT-Raman spectra.


28

1.6 Literature Survey of copper sulphide thin films

M. Nagarathinam and et al. reported self-Assembled Cu (II) coordination polymer to

Shape controlled CuS nanocrystals using solvothermal reactions , the precursor as well as a

sacrificial template to synthesize covellite CuS nanomaterials, a hydrogen-bonded 1D

coordination polymer, {[Cu(HSglu)(H2O)].H2O}n, 1, has been used. He explained the role of

the chelating ligand and solvents in the self-assembly process [13]. The multiple roles of L-

cysteine in the formation of metal sulfide nanostructures were elucidated by Benxia et al.

three typical precursors with flakes, spherical and solid microspheres have formed in the

three initial reaction solutions with different ratios of L-cysteine to CuCl2.2H2O was

observed [14]. By a precipitation reaction in the water phase of a Pickering emulsion Janus

CuO/CuS colloids are prepared by Dong et al. he found that Janus CuO/CuS colloids did not

rotate at the emulsion interface. His findings are important to understand the

fundamentals of Pickering emulsion [15].

Xue and et al explained synthesis of CuS nanofibers with different helical pitches by

gelator 1 as templates; by using H2S as the sulfur source, straight and bending helical CuS

nanofibers with a pitch of 100-200 nm could be fabricated in butyl acetate and benzene-

butanol gel systems [16]. Formation Mechanism and synthesis of CuS Hollow Spheres

under mild conditions with spherical aggregates of Cu2O nanoparticles as sacrificial

templates is reported by H.Zhu. The results show that the fast synthesis of hollow spheres

at low temperature by loose aggregates of Cu2O nanoparticles is the key factor for

synthesis. By adjusting the aggregation degree of the Cu2O nanoparticles the thickness of

the shell can be controlled easily [18]. Recently, Shinkai et al. reported silica nanomaterials

with different morphologies by changing concentration of the gelator [19] or transcription


29

temperature, [20] which suggested that the mineralization conditions played an important

role on the controlling of the morphologies of the inorganic nanostructures. Formation of

homogeneous core-shell CuS microspheres via thermolysis is explained by Y. Chen and et

al. as deposited cupric sulfide CuS microspheres undergo a reduction to tetragonal cuprous

sulfide Cu2S microspheres without change in morphology via solid-state annealing at

400°C under N2 atmosphere [21]. Cu2O and CuS uniform hollow spheres were successfully

synthesized by chemical transformation of in situ formed sacrificial templates containing

Cu (I) in aqueous solutions bromide source used for the formation of intermediate

templates can adjust shell thickness of these hollow spheres, Cu2O hollow spheres were

directly obtained at room temperature while well crystallized CuS hollow spheres are

obtained after a hydrothermal treatment at 160°C [22].

Kumar and et al. verified that precursor anions play a significant and important role

on the formation of the Cu1.8S and CuS when dissociated in ethylene glycol. Monophasic

rhomboheral Cu1.8S was obtained from the [Cu(tu)3]Cl, and pure hexagonal CuS was the

product of dissociation from [Cu4(tu)9](NO3)4 . 4H2O. A mixture of Cu1.8S and CuS was

obtained from the precursor containing the sulfate anion, [Cu2(tu)6]SO4.3H2O [23]. By

simple wet chemical method CuS nanorods are synthesized P. Roy and et al. he investigated

the possible mechanism for the growth of CuS nanorods and nanoparticles. He also

concluded that the presence of twins in nanorods also results in the poor

photoluminescence property showing low intensity PL emission [24]. Template free

nanowires are synthesized by A. Ghahremaninezhad and et al. used dilute copper and TU

solution for pulsed electrodeposition. These nanowires are prepared by pulse

electrodeposition with reverse (anodic) potential of 0.07 V (20 ms) and forward (cathodic)


30

potential of -0.85 V (10 ms). XRD analysis confirmed that the metal-deficient copper sulfide

of Cu1.94S is present in the structure of the nanowires and XPS analysis showed formation

of copper sulfides (CuS and Cu2-xS) [25]. Kriegel and et al. reported the solutions consisting

of Cu1.97S NCs spontaneously self assembled in micrometer-sized supercrystals have been

studied from the structural and morphological point of view; he concluded that the light

absorption properties of Cu1.97S NCs change upon arrangement into supercrystals [26].

1.7 Literature survey of copper selenide thin films

R.S. Mane and et al. reported preparation of copper (I) selenide crystalline thin films

for hetero-junction solar cells by chemical bath deposition method. The optical band gap of

thin film was found to be 1.73 eV from optical absorption studies. Thermally induced

voltage shows P-type behavior of thin film, which may find interesting applications in

hetero-junction solar cells as an absorber layer [27]. Through a simple reaction of sodium

selenosulfate with metal copper at room temperature in alkaline Na2SeSO3 aqueous

solution spherical copper selenide nanoparticles (NPs) were prepared by Y. Yang and et al.

A galvanic approach to the preparation of different phases of copper selenide is reported,

the effect of VOC has been investigated in the formation of different phases of copper

selenide (Cu2−xSe and CuSe) [28].

Thin films of copper selenide obtained from chemical baths gives rise to the optical

absorption in the visible-ultraviolet region may be interpreted in terms of direct allowed

transitions with band gap in the 2.1-2.3 eV range and indirect allowed transitions with

band gap 1.2-1.4 eV, with p-type conductivity, in the range of (1-5) x 103 ohm-1 cm-1 shows

cubic structure as in berzelianite, Cu2-xSe is reported by Garcia and et al. [29]. Phosphine-

Free P-type copper (I) selenide nanocrystals in hot coordinating solvents are synthesized


31

by S. Deka and et al. In the current-voltage measurements he found film resistivities of ∼6

× 10-3 Ω cm these films show a distinct optical absorption shoulder in the UV and a peak in

the NIR region with a high absorption coefficient [30]. By the brush electrodeposition

technique on conducting tin oxide substrates Cu2Se thin films were prepared by Murali and

et al. The films were found to be exhibit optical direct band gap value of 2.18 eV with

polycrystalline and cubic structure. With deposition temperature the value of the

resistivity decreases and also mobility and carrier concentration of the films increased

with deposition temperature. The films exhibited reasonable photo activity from

photoelectrochemical studies [31]. Nonequilibrium carrier dynamics in copper selenide

nanowires and nanocrystallites in femto and picosecond time domains by the means of a

transient dynamic grating technique were investigated by Statkute and et al. Bulk and

quantum confinement approaches were used to fit the experimental results using

nonequilibrium carrier fast relaxation, recombination, and trapping mechanisms [32].

Z. Deng demonstrate the synthesis of two-dimensional single-crystal berzelianite

(Cu2-xSe) nanosheets and nanoplates via a simple, green and environmentally begin

method of injecting Cu(I)-complex precursor into Se-solution in paraffin liquid he does not

use expensive and toxic Phosphine ligands. He got the products of the cubic phase and

high-quality single-crystal two-dimensional nanostructure [33]. Choi et al. explained

synthesis of copper selenide nanodiscs by Colloidal Synthesis of Cubic-Phase and their

optoelectronic properties are studied. The optical band gap of nanodiscs was estimated to

be 1.55 eV and compared with the band gap of bulk Cu2-XSe, the larger value resulted from

the nano size effect of materials [34]. On glass substrates Cu2−xSe thin films were prepared

by chemical bath deposition technique and its structural, electrical and optical properties


32

were studied by Mamun et al. transmittance and reflectance of copper selenide thin films

were found in the range of 5–50% and 2–20% respectively, with absorption coefficient of

∼108m−1. The band gap for direct transition varies in the range of 2.0–2.3 eV and that for

indirect transition is in the range of 1.25–1.5 eV [35].

1.8 Literature survey of copper telluride thin films

By wet chemical method homogeneous green luminescent copper telluride (Cu2Te)

nanoparticles are synthesized in a single reaction at 70 °C within 9 h. With a very high

productive yield the method ensures almost complete utilization of the precursors; it is

found that the diameters of the particles are in the range of 25-30 nm. Pushpendra Kumar

et al. also reported that the direct and indirect band gaps of the Cu2Te nanoparticles were

2.04 and 3.05 eV respectively [36]. Suresh Kumar reported synthesis and characterization

of copper telluride nanowires through electrochemically template-assisted dc

electrodeposition route, X-ray diffraction pattern showed the structure of copper telluride

as hexagonal structure and single crystalline. The optical band gap of copper telluride

nanowires was found to be 3.092 eV for 100 nm and 3.230 eV for 50 nm diameters [37].

L. Zhang demonstrated the hydrothermal synthesis nanostructured films of copper

and silver tellurides, nanowires, nanorods, nanobelts, nanosheets, and hierarchical

dendrites were obtained. He explained a low-temperature solution-phase method to grow

low-dimensional nanostructured metal tellurides with controllable morphologies [38].

Fulari et al developed the method of holography to determine the stress, strain, fringe

width and thickness of copper telluride thin films. It is observed that stress to substrate

decreases with increase in deposition time; thickness and mass of thin film. The films are

found to be polycrystalline with orthorhombic structure with optical band gap energy


33

ranging from 1.1 eV to 1.3 eV [39]. Patan et al. reported the preparation of copper telluride

thin film by modified chemical bath deposition it based on the immersion of the substrate

into separately placed cationic and anionic precursors, the film consists of different phases

of copper telluride. Electrical resistivity of film at room temperature was found to be of the

order of 101 ohm cm [40].

Boron-doped diamond electrodes were prepared by hot filament chemical vapor

deposition (CVD) technique using the standard mixture of H2/CH4 by Fernandes and et al.

He investigated electrodeposition of Cu, Te and Cu-Te thin films on BDD electrodes. The

electrochemical behavior of Cu-Te system changed slightly, during the process of

deposition occurred at different potentials when compared with the electrochemical

process of Cu or Te [41]. Using a thermal evaporation technique thin films of cupric

telluride (CuTe) of thickness from 50 to 200 nm have been prepared by K. Neyvasagam et

al. The temperature variation of the resistivity indicates that CuTe in thin-film form is

semiconducting. The resistivity of CuTe bulk material increases monotonically with

temperature with no evidence for semiconducting behaviour. Activation energies of the

films have been evaluated and are in the range 0.05 and 0.15 eV. CuTe thin films and the

bulk material are found to be of relatively uniform grain distribution and polycrystalline

[42]. Simple galvanostatic electrochemical deposition method was used to synthesize

polycrystalline Cu7Te4 dendritic microstructures at room temperature the phase and

morphology of the as prepared product were characterized and are hexagonal.

Experimental results showed that high deposition current promoted the formation of

dendritic Cu7Te4 microstructures reported by Y. Zhang and et al. [43]. Dong and et al.

explained synthesis of Cu(2−X)Te nanowires by a microwave-assisted solvothermal method


34

using a self-sacrificial template, single-crystalline nanowires with diameters of 100–200

nm and lengths of several micrometers were obtained. The electrical conductivities of

Cu(2−X)Te nanowires was found to be (5.99–7.19)×103 Ω−1 m−1 at temperatures ranging

from 300 to 450 K [44].

1.9 Scope of the present work

From literature it is observed that, in the area of nanomaterials, semiconductor

transition metal chalcogenides, such as CuS, CuSe and CuTe have attracted increasing

interest due to their excellent physical and chemical properties. In particular, shape-

controllable synthesis of low dimension nanocrystals has become a new and interesting

research focus because of the strong relationship between the physical properties and the

shape of nanocrystals. Copper sulfides and selenides in different stoichiometries are

widely used as p-type semiconductors in solar cells, as optical filters, and as superionic

materials. Due to their unique optical and electrical properties, they are also widely

applied in thin films and composite materials. Copper sulfide nanocrystals, such as

nanoparticles, nanorods and tubular crystals, have been synthesized by solid-state

reactions, irradiation, sonochemical, hydrothermal, solvothermal, and CVD methods.

Microemulsions have been employed as an ideal media for the synthesis of nanocrystals.

Reports are also available on the application of hydrothermal microemulsions in the

synthesis of CuS nanorods. Silver nanodisks and nanoprisms have been prepared and the

synthesis of CuSe flakes has also been reported. However, copper sulfide nanodisks flakes

and hollow spheres by electrodeposition have not been reported yet.

Recently, nanocrystalline semiconductors have been of great research interest due

to their unusual physical properties, which are quite different from those of their


35

corresponding bulk materials and their isolated atoms or molecules. The fabrication of

low-cost, efficient solar cells made of green materials is a main goal of energy-related

research. Several copper based materials, such as Cu2S, CuInS2, CuInSe2, CuInxGa1-xSe2, and

Cu2ZnSnS4, have been explored to date in photovoltaics, mainly as thin films, but recently,

colloidal nanocrystals of these materials have also been developed and used to make solar

cells. Today “all-nanocrystal” or organic-inorganic nanocomposite films can be prepared

over large areas using various deposition techniques. Also, copper selenide, a superionic

conductor, has been studied in thin-film applications in photovoltaics, optical filters, and

dry galvanic cells (as a solid electrolyte). It can form in many stoichiometries (Cu2Se, Cu2-

xSe, CuSe, Cu2Se3) and phases. Copper (I) selenide (Cu2Se, Cu2-xSe) crystallizes generally in

the face centered cubic berzelianite phase. Cu2-xSe has both a direct band gap of 2.2 eV and

an indirect band gap of 1.4 eV (at the limit for solar cell applications) and shows p-type

conductivity. Nanocrystals of Cu2-xSe have been prepared via various routes, including

colloidal synthesis methods in hot surfactants.

In contrast, CuSe has a hexagonal phase at room temperature, and it is often found

as impurity in copper (I) selenide. The interest in copper selenides CuxSey is motivated by

their application in photovoltaic technology. Moreover, in recent years there has been a

growing interest in CuxSey as nonlinear optical elements. Fast carrier separation and slow

independent recombination warranting sufficient voltage at the contacts is essential for

the solar cell application, whereas for nonlinear optical elements a fast nonlinear response

to the light intensity. The ordered alumina matrix has shown photonic features acting as a

two-dimensional photonic crystal with grating period 200−600 nm. Doping alumina

photonic crystals by copper selenide nanostructures is a future challenge that could lead to


36

the development of tunable photonic crystals in the visible and near infrared. The

development and implementation of the mentioned technologies requires a deep

understanding of processes involving carrier response to the light.

Therefore, the preparation technique plays a vital role. Various deposition

techniques such as vacuum evaporation, electron beam evaporation, R.F. sputtering, ion

beam sputtering, magnetron sputtering, CVD and sol-gel have been employed. Among the

others, Electrodeposition method is the advantageous because of its simplicity, cheapness, low

cost of the starting materials also electrodeposition results in pin hole free and uniform

deposits are easily obtained since the basic building blocks are ions instead of atoms.

Preparative parameters such as deposition time, and concentration of precursor were

optimized, which are desirable for industrial, solar cell and gas sensor applications.

The detailed investigation of the effect of preparative parameters (like substrate

cleanness, concentration of solution, deposition time) on structural, morphological,

compositional, and optical properties will be carried out. The X-ray diffraction (XRD)

technique will be used for the structure identification and crystallite size determination.

The surface morphology of the films will be studied using field emission scanning electron

microscopy (FE-SEM). The compositional analysis will be carried out using X-ray

photoelectron spectroscopy EDAX. The optical properties will be studied in the visible

range of spectrum using spectrophotometer.


37

References

[1] A. Brenner, "Electrodeposition of alloys”. Vol. I and II", Academic Press, New York

(1963) 284-288

[2] R. Pandey, S. Sahu, S. Chandra "Handbook of Semiconductor Electrodeposition", Marcel

Dekker, Inc., New York (1996) 64-87

[3] J. George, “Preparation of Thin Films”, Marcel Dekker. New York (1992) 13-19

[4] C. Lokhande, S. Pawar, Phys. Status Solidi-A, 111 (1989) 17

[5] S. Glasstone, "An Introduction to Electrochemistry", Litton Educational Publishing, Inc.,

New York (1942) 226-305

[6] B. Cullity, “Elements of X-rays Diffraction”, 2nd Edition, Addison-Wesley, London,

(1978) 324-344

[7] A. Taylor, “X-ray Metallography”, Wiley, New York, (1942) 709

[8] S. Banerjee, “Proc. of National Workshop on Advanced Methods for Mater.

Characterization” (NWMMC-2004) Mat. Res. Soc. India (2004)

[9] O. Wells, “Scanning electron microscopy” McGraw Hill Inc., (1974) 208-210

[10] J. Goldstein, “Scanning Electron Microscopy and X-Ray Microanalysis”, (2003) 297-353

[11] F. Micheltti, P. Mark, Appl. Phys. Lett., 10 (1967) 136

[12] T. Hirschfeld, E. Schildkraut, “Laser Raman Gas Diagnostics”, Plenum Press, New York

(1974) 379-388

[13] G. Chantry, H. Gebbie, C. Hilsum, Nature, 203 (1964) 1052

[14] M. Nagarathinam, J. Chen, J. Vittal, Crystal Growth Design., 9 (2009) 2457

[15] L. Benxia, Y. Xie, Yi Xue, J. Phys. Chem. C, 111 (2007) 12181

[16] L. Dong, H. Yongjun, S. Wang, J. Phys. Chem. C, 113 (2009) 12927


38

[17] P. Xue, R. Lu, D. Li, M. Jin, C. Tan, C. Bao, Z. Wang, Y. Zhao, Langmuir, 20 (2004) 11234

[18] H. Zhu, J. Wang, D. Wu, Inorg. Chem., 48 (2009) 7099

[19] J. H. Jung, K. Nakashima, S. Shinkai, Nano Lett. 1 (2001) 145

[20] J. Jung, S. Lee, J. Yoo, K. Yoshida, T. Shimizu, S. Shinkai, Chem. -Eur. J., 9 (2003) 5307

[21] Y. Chen, L. Chen, L. Wu, Crystal Growth Design, 8 (2008) 2736

[22] J. Gao, Q. Li, H. Zhao, L. Li, C. Liu, Q. Gong, L. Qi , Chem. Mater., 20 (2008) 6263

[23] P. Kumar, M. Gusain, R. Nagarajan, Inorg. Chem., 50 (2011) 3065

[24] P. Roy, K. Mondal, S. Srivastava, Crystal Growth Design, 8 (2008) 1530

[25] A. Ghahremaninezhad, E. Asselin, D. G. Dixon, J. Phys. Chem. C, 115 (2011) 9320

[26] I. Kriegel, R. Jessica, E. Como, A. Lutich, J. Szeifert, J. Feldmann, Chem. Mater., 23 (2011)

1830

[27] R. Mane, S. Kajve, C. Lokhande, S. Han, Vacuum, 80 (2006) 631

[28] Y. Yang, S. Hu, J. Solid State Electrochem., 13 (2009) 477

[29] V. Garcia, P. Nair, M. Nair, J. Cryst. Growth, 203 (1999) 113

[30] S.Deka, A. Genovese, Y. Zhang, K. Miszta, G.Bertoni, R. Krahne, C. Giannini, L. Manna, J.

Am. Chem. Soc., 132 (2010) 8912

[31] K. Murali, R. Xavier, Chalco. Lett., 6 (2009) 683

[32] G. Statkute, R. Tomasiunas, A. Jagminas, J. Appl. Phys., 101 (2007) 113715

[33] Z. Deng, M. Mansuripur, A. Muscat, J. Mater. Chem., 19 (2009) 6201

[34] J. Choi, N. Kang, H. Yang, H. Kim, S. Son, Chem. Mater., 22 (2010) 3586

[35] A. Mamun, A. Islam, A. Bhuiyan, J. Mate. Sci., 16 (2005) 263

[36] P. Kumar, K. Singh, Crystal Growth Design, 9 (2009) 3089

[37] S. Kumar, V. Kundu, A. Vohra, S. Chakarvarti, J. Mater. Sci., 22 (2011) 995


39

[38] L. Zhang, Z. Ai, F. Jia, L. Liu, X. Hu, J. Yu, Chem. Eur. J., 12 (2006) 4185

[39] V. Fulari, V. Malekar, S. Gangawane, Prog. In Electro. Research C, 12 (2010) 53

[40] H. Pathan, C. Lokhande, D. Amalnerkar, T. Seth, Appl. Surf. Sci., 218 (2003) 290

[41] V. Fernandes, J. Matsushima, M. Baldan, A. Azevedo, M. Linardi, N.Ferreira, ECS

Transactions, 25 (2010) 209

[42] K. Neyvasagam, N. Soundararajan, Ajaysoni, G. Okram, V. Ganesan, Phys. Status. Solidi-

B, 245 (2008) 77

[43] Y. Zhang, Y. Ni, X. Wang, J. Xia, J. Hong, Crystal Growth Design, 11(2011) 4368

[44] G. Dong, Y.Zhu, G.Cheng, Y.Ruan, Mater. Lett., 76 (2012) 69

introduction to thin films chapter -i...introduction to thin films chapter -i 4 1.2...

Documents