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1 Sol-gel processing…………………………….............................. 3 1.1 Introduction………………………………………............ 3 1.2 Chemistry of Precursor Solutions………………………... 6 1.2.1 Metal Salt Solutions……………….......................6 1.2.2 Ions Solvations…………………….......................6 1.2.3 Hydrolysis………………………..........................7 1.3 Colloidal Particles and Sols…………………………….... 8 1.4 Sintering…………………………………………..............8 1.5 Nucleation & Growth of Particles in Liquid Medium……9 1.6 Other Synthesis Processes………………………..............10 1.7 Application of Sol-Gel Processing…………………….....11 1.8 Functions of Sol-Gel Coatings…………………………...12

2 Physics of semiconductor……………………………………......13 2.1 Introduction……………………………………………… 13 2.2 Band Structure and Effective Mass……………………… 14 2.3 The Periodic Table of Semiconductors…………...............15 2.4 Small Size, Big Change…………………………............. 16

3 Nanotechnology………………………………………….............17 3.1 Introduction……………………………………………....17 3.2 Products and Applications……………………………......17 3.3 Donors, Acceptors and Deep Traps……………………... 18 3.4 Particle Size Determination………………………………20 3.5 Properties of Individual Nanoparticles…………...............21 3.6 Bulk to Nanotransition…………………………................22 3.7 Semiconducting Nanoparticles…………………...............23 3.8 Photoluminescence…………………………………….....23

4 Thin film-I Physics and Chemistry of thin films………………………….25

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CONTENTS

4.1 Introduction……………………………………………..... 5 4.2 Special Properties of Thin Films……………………….... 25 4.3 Typical Steps in Making Thin Films…………………….. 25 4.3.1 Thermal Accommodation………………............... 26 4.3.2 Binding…………………………………………… 26 4.3.3 Surface Diffusion……………................................ 27 4.3.4 Nucleation………………………………............... 27 4.4 Schematic Diagram to Show How Nuclei Growth Initially…………………………………………...27 4.4.1 Island Growth……………………………………. 28 4.4.2 Island Coalescence………………………………. 28

5 Thin film-II Deposition and Application……………………………........30 5.1 Various Deposition Techniques……………………..30 5.1.1 Chemical Vapour Deposition……………….30 5.1.2 Spray Pyrolysis……………………………...31 5.1.3 Vacuum Evaporation………………………..32 5.1.4 Sputtering……………………………………33 5.1.5 Ion-Associated Deposition Techniques……..35 5.1.6 Other Methods…………………………........37 5.2 Thin Film Application……………………………….38

6 ZnO and Composite Experimental, Results and Discussion…………………..............41 6.1 Introduction……………………………………………….41 6.2 Zinc Oxide (ZnO)………………………………...............41 6.3 ZnO-Al2O3 Nanocomposites……………………...............42 6.4 Experimental……………………………………...............43 6.4.1 Instruments Used for Characterization…………..44 6.6 Results and Discussion…………………………...............48 CONCLUSION………………………………………….........................53

REFERENCES…………………………………………………………54

2

Chapter 1Sol-gel processing

1.1. Introduction

A sol is a dispersion of the solid particles (~ 0.1-1 mm) in a liquid where only the Brownian

motions suspend the particles. A gel is a state where both liquid and solid are dispersed in each

other, which presents a solid network containing liquid components. Sol-gel processing of new

organic-inorganic, inorganic-inorganic, metal-inorganic etc. hybrid materials has gained

increased interest in the last decade. These materials are synthesized by chemically incorporating

organic polymers into inorganic networks, resulting in excellent and even unique properties.

Sol: peptization

It is defined as an action of dispersing colloidal particles in a liquid medium, so that this

dispersion remains stable. This stable powder dispersion can be termed a colloidal suspension,

or a sol. The stability, which is involved, is kinetic. As a matter of fact, the surface to volume

ration of spherical particles of radius r is:

This ratio goes to infinity as r goes towards zero. So, colloidal particles have a high

surface to volume ratio and a high total surface energy.

The kinetic stability of a sol is due to the fact that the aggregation of these colloidal

particles can be very slow. Practically, the dispersion must remain stable for a sufficient time.

This depends on interaction force between particles. Most often, the dilution in the solvent is

sufficient to consider that only interaction between near neighbor particle pairs is sufficient.

These interactions can be divided into:

1. Vander Waals interactions, which mostly introduce attractive force

between the particles.

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2. Electrostatic interaction, which introduce repulsive forces between the particles.

3. Steric interaction, which occur between the solvent and organic macromolecules

adsorbed on the surface of the particles.

4. In some systems magnetic interaction.

Gels: structure and classification

The solid volume proportion can be extremely low in a gel, even lower than in some gases near

their critical temperature.

Most often, the inorganic gels comprise two phased in the thermodynamic sense: the

solid network, and the liquid matrix. It is even possible for a solid network to be a polymer of

the monomeric liquid component, in which case the gel is named an isogel.

Flory’s Classification

Flory proposed a general classification of both organic and inorganic gels. It comprises

a) Lameller gels such as mesophase and clays.

b) Covalent gels largely represented in organic chemistry

c) Gels constituted by local crystallization of polymeric chains.

d) Particulate gels in which macroscopic particles of various shapes are linked to each

other to form a porous network.

Ceramists` Classification

In the present state of knowledge, ceramists have consequently adopted a more simple

classification of gels comprising:

A: polymeric gels

B: colloidal gels

A: Polymeric Gel

(i) Organic Gels: The element most prone to build extended polymeric molecules, which are

not hydrolyzed by water, is carbon organic gels which comprise a polymeric carbon backbone,

constitute a large class.

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(ii) Silica Gels: The second element most prone to polymerization is silicon. Polymerization

can build unidimensional, bi dimensional, or three-dimensional polymers depending on the

conditions. The correlation between the polymerization connectivity and the oxide content in the

final gel have allowed to show that an, increasing water/alkoxide hydrolysis ratio, as well as

increasing dilution, an increasing connectivity of the network. Silica gels, which can be

classified as polymeric, are obtained in rather acidic conditions (below pH 2.5).

B: Colloidal gelsGels which can be classified as colloidal, according to the size of the solid particles which makes

the tri-dimensional network, can differ from each other by:

1. The size, shape and crystallographic structure of the colloidal solid particles, which

macroscopically appears as quite fibrous, may be composed of spherical particles

linked in a very linear fashion.

2. The type of linkage between the particles.

Silica Colloidal gels

Colloidal gels are formed with silica in a pH range from ≈6. They are composed of spherical

particles linked by siloxane bonds. Preferential growth of the necks between two particles

produces a cylindrical fibrillar structure during aging.

Example of silica gel made in excess water, termed xerogel because they are dried by

evaporation.

The gel can also be dried in supercritical conditions, in which causes a dry gel is termed

an aerogel. On a scale of 1nm, the structure consists of dense primary silica particles (Pa=2200

kg. m-3). These primary particles are aggregated into roughly spherical secondary particles with

a radius of order of a few nm. The secondary aggregates are themselves arranged in chains, on

length from 50 to 100 nm, which give its macroscopic apparent density to the aerogel.

Transport Properties in the liquid of a gel

The liquid matrix that impregnates the gel network is important for all transport properties

through the liquid medium. However the gel network has a significant influence, it can for

instance be utilized as a barrier to the diffusion of particles or of big species. For this reason,

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gels are often used as media to grow all kinds of organic or inorganic mono-crystals, because

polycrystalline aggregation is made difficult by the gel network, while the diffusion of small

molecular species is as fast as in a pure liquid and it is not distributed by convection.

Advantages

The main advantages of Sol-gel powders are:

1. An easier control of aggregation.

2. A good stability in time, after for years.

3. In the case of Multi-components oxides, a good dispersion of the cations on a fine

scale;

4. A good attenuation of the communication problems due to confinement of the

powders in a liquid, which is often water.

5. Reversibility of the Sol-Gel transition for some systems, which makes it possible to

make corrections.

So, this Sol-Gel processes are often more convenient than other techniques.

1.2. Chemistry of precursor solutions:

1.2.1. Metal Salt Solutions

In sol-gel processing, when metal salts are used they are often dissolved in an aqueous medium.

The metal salt dissociates into ions, which disperses in the solution, and the anions negative

charge balances the positive charge of the metal atom. The cation and anion then have the same

absolute formal charge Z. These anions are sometimes considered as impurities; they must, for

example, be eliminated in order to produce pure oxide ceramic. However, they can also be

invaluable in channeling the chemical transformations within the solution. The ions firsts

dissolves in the water owing to its polar nature.

1.2.2 Ions salvations:

Since, water has dipolar moment, the positive charge Z+ of a cation attracts the partial negative

charge, that is the oxygen atom, of H2O molecules (∂(0)<0). As a consequence, the cation is

entrapped by a number N of water molecules, which constitutes, since they are the first

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neighbours, the first solvatation shell. This shell is tightly bonded to the metal cation M z+ so that

the chemical formula of the complex formed by the solvated ion is [M(H 2O)N] z+. The value of N

is fixed for a given type of metal; its value often ranges from 4 to 8 and is frequently equal to 6,

such as in [A1(H2O)6] 3+ Water also solvated the proton and the most frequent value of N for H+

is 4. Hence, the complex [H(H2O)4]+ also written [H9O4].

1.2.3 Hydrolysis:

Hydrolysis is the deprotonation of a solvated metal cation it corrists the metal M in the first

solvantation shell. As a consequence, the aquo ligand meleculus H2O that is bonded to the metal

is either transformed into a hydroxo ligand, if only one proton leaves, or into an oxo ligand, O2-,

it two protons detaches.

● Hydrolysis of cations as Function of their Nature:

Hydrolysis is a complex technique that depending on the conditions, gives rise to a great variety

of colloidal structure ranging from metals to hydroxide and including oxides and oxi-hydroxides.

Of those, the colloidal oxides are often so strongly solvated that the water molecules are tightly

bonded to the complex and it is difficult to know the exact chemical formula of the particle. The

behavior of the cations in aqueous solutions can be summarized according to their nature and

with respect their final type of ligands.

● Cations with valence I

These cations have a low charge, a low electronegativity, and an oxidation number of I. They

are not hydrolyzed in solution but remain as solvated cations of the form [M(H2O)N]Z+. They

form basic oxides, which liberate OH- anions when reacting with water. An example is Na.

●Cations with valence II

These cations have an oxidation number of II and a slightly higher charge and electronegativity

than those with valence I. They therefore undergo condensation more extensively and precipitate

in the form of hydroxides with formula M(OH)2. In solution, most of those cations (Mn,Co,Ni)

form compact tetrameters that later transform into solid hydroxides with lamellar structure such

7

as the one of Brueite, CdI2. Cu creates liner polymers that crystallize as Cu(OH)2 in a lamellar

structure similar to that of boehmite AlO(OH).

●Cations with valence III

Those cations with an oxidation number of III, such as Al, Fe, Cr, Sc, Y and the rage earth

elements have a very rich aqueous chemistry. They easily polymerize in a variety of different

polycations and constitute various solid phases, each more or use polymeric. These phases are

produced by a succession of olation and oxolation reactions, which lead to the formation of

several polynuclear complexes. Those complexes later form oxyhydroxide solids.

1.3 Colloidal particles and sols

Fine ceramic particles are in great demand to make part which sinter well at a temperature as low

as possible. In some cases, all particles made in a given liquid medium process have the same

shape, as well as very narrow size dispersion; they are termed “monodies persed” or

“monosized”.

The synthesis of non-dispersed microspheres presents a large theoretical interest to

understand the optical, magnetic, electokinetic, corrosion and catalytic properties of colloidal

matter.

1.4 Sintering

Sol-gel ceramics just after during and even heat treatments at intermediate temperatures often

have a very high specific surface area and an extremely small grain size. Hence, both sintering

and grain growth tend to be rigorous. Sintering behavior of a two dimensional random loose

packing of equal-size spheres after interparicle distance shrinkage of (a) 0% (b) 4% (c) 8%.

To sinter compounds which comprise several cations, such as mullite 3Al2O3. 2SiO2, the

inter-diffusion of cations is necessary. Hence, as for the simple oxides, crystallization often

occurs simultaneously with sintering. Sol-gel processing makes it possible to directly obtain

powders which easily sinter, while in conventional ceramic processing, length grinding times are

necessary to make small size particles so as to lower the temperature where diffusion can

efficiently.

8

All the hydrolysis-condensation condition of sol-gel synthesis has an effect on the

sintering, in the same way, as they are responsible for a large variability in the nature of the first

phase, which crystallize.

1.5 Nucleation and Growth of particles in a Liquid Medium

● Relationship between hydrolysis, condensation, and the formation of solid

particles

The formation of solid particles can be described as a process of nucleation and growth,

according to the

Hydrolysis => condensation => nucleation => growth

Solid particles can form as the result of heterogeneous nuecleation on foreign inclusions, such as

dusts or the products from uncontrolled hydrolysis. This does not make possible to obtain a

well-defined quality of powder, for instance easy to sinter. Rather, it is preferable to aim at

a) A fine size (between 01 and 1 um)

b) A narrow size distribution

c) An equiaxed shape (e.g. spheres)

d) A non-agglomerated state

To achieve these results it is necessary to avoid any heterogeneous nucleation.

● Nucleation rate

Nucleation is due to statistical thermodynamic fluctuations, in agreement with the Bottzmann

statistics. Bimolecular collisions responsible for nucleation are enhanced by aging, heating and

participation of chemical ligands. Aging gives more time for bigger fluctuation. Heating

accelerates all chemical steps. The ligands modify the types of complexes from which

nucleation can occur. For ionic precursors, the nucleation rate often depends on the metal cations

concentration with a reaction order of 4 to 10, although the detailed kinetics depends largely on

the nature of each cation.

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1.6 Other synthesis processes

● Physical and Chemical Vapor Deposition

The P.V.D. and C.V.D. Techniques have reached a high level of efficiency, especially when the

nucleation and growth in vapor phase are monitored with a pulsed laser. They can also be used

with alkoxides as the chemical precursors.

They produce extremely fine and pure particles. Immediately after growth the particles

are non-agglomerated. The main advantage of these techniques is to allow the production of

non-oxide and metal powders, which are not feasible in liquid medium. The final powders are

smaller than by precipitation.

● Pyrolysis of Precursors

The various pyrolysis methods have in common with the sol-gel processes the use of the same

precursors, and to require a thermal treatment. They make it possible to produce ZrO 2, Y2O3, rare

earth oxides powders of Gd2O3, Dy2O3 and Yb2O3. The mechanism of decomposition is different

for each precursor.

High purity oxides (upto 99.95%) are formed according to the reaction;

2MO(OR)4=> MO2 + M(OR)4

In the case of Zr, the decomposition of an alkoxide is somewhat different and proceeds according

to the two following steps:

ZrO(OR)4 ROH => Zr(OR)4 + ROH at 1250C

Then:

ZrO(OR)4 => ZrO2 + 2ROH + olefin at 3000C

The particles priority is better than 99.99% the size of 80% of them is less than 10mm. They are

cubic and transform to the monoclinic stable phase at 400°C .

● Spraying techniques

A variation of the co-decomposition technique consists in maintaining the various components in

a solution and to decompose them by spraying onto a hot surface. It is also possible to melt a

precursor in a flame or plasma and to spray it onto a cold surface. Plasma can be realized in an

10

induction coupling. Compounds which were synthesized by such techniques include (Ni, Zn)

Fe2O4 and PbCrO4.

● Freeze Drying

In the technique of Freeze-drying to constituent of small droplets are immobilized by freezing at

low temperature. Freezing can be done at -800C in hexane, where water transforms to ice

without segregation and is sublimated without passing through the liquid state. This technique

was first used to prepare reactive alumina, Al2O3-Cr2O3, Spinel MgAl2O4, and Sm2O3.

● Liquid Drying

The method of drying in a liquid medium has been initially synthesized radioactive pellets, such

as UO2 and ThO2. Then it has been extended to other materials such as ZrO2 conducting powders

of SnO2-In2O3, Al2O3 and (MgMn) Fe2O4. In this technique, a solution is atomized in the vortex

of a rapidly stirred hygroscopic liquid such as acetone, methanol or isofluoroprapanol, with the

help of emulsifying agents.

● Aerosols Hydrolysis

Monodispersed powders can be made by the technique of aerosols where the chemical reactions

are carried out on droplets in a vapor phase. For instance, powders of TiO2, Al2O3 and various

other compounds have been obtained. The aerosols make it possible to aim at a given size and

purity and they provide excellent atomic homogeneity in compounds.

1.7 Application of sol-gel processing

Sol: The current applications of ceramics as finished products in the sol state concern essentially

the Ferrofluids. These sols comprise magnetic colloidal particles such as Fe3O4, which can more

when an appropriated magnetic field is applied to the sol. As a sol is kinetically stable, the fluid

in which the magnetic colloidal particles are dispersed more all together with these particles,

hence they can be used as transmission fluids in non-gravity.

Gel: This gel can be deposited as paint in thin layers on large areas, with a thickness of 50 to

1000 nm. The semi-conducting properties of the V2O5 gels are due to the fact that vanadium can

11

have two different valence states; +4 and +5. Hence, the electrons can be transferred between

the corresponding energetic levels by optical or by thermal activation. These gels have been

patented for antistatic coatings on the dorsals of photographic films. They are appreciated for

their lower sensitivity to humidity. They can also be applied in electrical switching device.

Xerogels and Aerogels: Aerogels often have a very high specific surface area up to 800

m2/g. This property makes them outstanding thermal and acoustic insulation materials. They

can be applied in materials. They can be applied in acoustic delay lines and in piezoactive

antireflective acoustic coatings of thickness ¼.

1.8 Functions of sol-gel coatings

Sol-gel coating can have many functions. The most frequent ones are optical functions, because

oxide is transparent to visible light wavelengths. They can transmit, absorb, and reflect,

radiations with a given wavelength. They can also be used to protect a substrate against

corrosion, abrasion, or scratch, be chemically and thermally stable, or even stable against some

radiation.

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Chapter 2Physics of semiconductor

2.1 Introduction

A substance for which the width of the forbidden gap between the Valence Band (VB) and the

Conduction Band (CB) is relatively small is called a semiconductor. As the forbidden gap, also

known, as the Band Gap is narrow, at room temperature, some of he valence electrons acquire

sufficient thermal energy to move into the conduction band.

Semiconductors are a group of materials having electrical conductivity intermediate

between metals and insulator. It is significant that the conductivity of these materials can be

varied over orders of magnitude by changes in temperature, optical excitation and impurity

content. This variability of the electrical properties makes the semiconductor materials natural

choices for the electronic device investigation.

There are numerous semiconductor materials. Among these, Si is used for the

majority of semiconductor devices; rectifiers, transistors and integrated circuits are usually made

of Si. The compounds are used most widely in devices requiring the emission or absorption of

light. For example, light emitting diodes (LEDs) are commonly made of such compounds as

GaAs, Gap and mixed compounds such as GaAsP. Fluorescent materials such as those used in

Television screens usually are II-VI compound semiconductors such as ZnS, CdS etc. light

detectors are commonly made with InSb, CdSe, or other compounds such as the lead salts PbTe

and PbSe. Si and Ge are also widely used as infrared and nuclear radiation detectors.

An important microwave device, the Gunn diode, is usually made of GaAS, as are

semiconductors lasers. Thus the wide range of semiconductor materials offers considerable

variety in properties and provides device and circuit engineers with much flexibility in the

design of electronic functions.

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The electronic and optical properties of semiconductor materials are strongly affected by

impurities, which may be added in precisely controlled amounts. Negative charge carriers to

positive charge carriers use such impurities to vary the conductivities of semiconductors over

wide ranges and even to alter the nature of the conduction processes. For example an impurity

concentration of one ppm can change a sample of Si from a poor conductor to a good conductor

of electric current. This process of addition of impurities is called doping.

2.2 Band Structure and Effective Mass

The basic description of a semiconductor is its band structure, i.e. the variation of energy E with

wave-vector k. The most important bands are: Valence band - the last filled energy level at T=0

K Conduction band - the First unfilled energy level at T=0 K. The valence band maximum is at k

=0, is known as the gamma point. Where the conduction-band minimum also occurs at k =0, the

semiconductor is said to be a direct band semiconductor. At non-zero k =0, the semiconductor is

an indirect-band semiconductor. In addition to these two main conduction bands other bands may

also be present. In III-V semiconductors, Ge and Si there are 3 valence bands with maxima

at k =0. These are the light-hole , heavy-hole and spin-orbit split-off band . The bands in a

semiconductor material are approximated by parabolic functions of k close to the band edges.

Conduction-band:

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Ea

EdDonor levels

Acceptor levels

Conduction Band

Valence Band

Valence-band:

2.3 The Periodic Table of Semiconductors

A semiconductor is a crystalline solid that in its pure form exhibits a conductivity midway

between that of metals and insulators. The three semiconductor materials that Livermore is

studying for possible use as sensors and detectors are silicon, germanium, and diamond. Silicon

accounts for almost 99 percent of all commercial semiconductor products. Germanium became

famous when the transistor was invented but has since been replaced largely by silicon.

Diamond, a monocrystal of carbon, has the physical properties of a wide-optical gap

semiconductor, but current technologies do not allow its use as a semiconductor.

These three materials comprise some of the Group IV elements on the periodic

table, as shown below. Tin, the fourth potential semiconductor material in this group, has the

physical properties of a semiconductor at low temperatures but at room temperature behaves like

a metal. These four materials are elemental semiconductors.

Elements in Groups II and VI and in Groups III and V are often combined to form

compound semiconductors. Gallium–arsenide is a typical Group III/V compound semiconductor

often used in microwave devices and optoelectonics. Most experiments designed to explore the

optical properties of semiconductor nanoclusters have focused on such Group II/VI compound

semiconductors as cadmium–selenium.

I II III IV V VI VII VIII

Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon

Magnesium Aluminum Silicon Phosphorous Sulfur

Zinc Gallium Germanium Arsenide Selenium

Cadmium Indium Tin Antimony Tellurium

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2.4 Small Size, Big Change

Reducing any piece of material from a chunk that we might recognize to the nanometer scale

changes virtually all of its most basic properties in a fundamental way. Its shape and crystalline

structure change, as do its melting and boiling temperatures. Its magnetic properties may be

different at the nanoscale. Its optical and electronic properties also change.

In a nanosemiconductor, an effect known as quantum confinement occurs when

electrons and “holes” in the material are confined. (A hole is the absence of an electron; the hole

behaves as though it were a positively charged particle.) Typically, quantum confinement causes

the material’s optical gap—the energy difference between filled states and empty states—to

widen. A larger optical gap prompts dramatic changes in electronic and optical properties. Bulk

silicon when stimulated does not emit visible light, but in 1990, researchers found that

nanoparticles of silicon do.

Livermore researchers and others have since determined that silicon nanoparticles

emit different colors of light depending on their diameter. In 1997, germanium nanoparticles

were found to emit light. In the last two years, other Livermore scientists have discovered that

the optical gap of nanodiamond does not change until its size is reduced to less than

2 nanometers. Nanoparticles are also different from the bulk form of the material in that the

percentage of atoms at or near the surface of the particle is far greater. The surface of

nanoparticles thus plays a large role in determining the particle’s electronic and optical

properties.

16

Chapter 3

Nanotechnology

3.1 Introduction

A nanometer is one billionth of a meter, and the term “nanotechnology” refers to engineered

structures, devices and systems that have a length scale of 1-100 nanometers. At these length

scales, materials begin to exhibit distinct properties that affect their physical, chemical and

biological behavior.

  Particles at the nanoscale, or nanoparticles, have very high surface areas, and their

behavior and mobility can be changed. This creates unlimited possibilities for products and

applications.

Many properties of solids depend on the size range over which they are measured.

When measurements are made in the micrometer or nanometer range, many properties of

material change, such as mechanical ferroelectric and ferromagnetic properties. The solids at the

next lower level of size namely, the nanoscale level, perhaps from 1 to 100 nm.

Many important nanostructures are composed of the group IV elements Si or Ge,

type iii-iv semiconducting compounds such as GaAS, or type ii-iv semi conducting materials

such as Cds, so these semiconductors materials will be used to illustrate some of the bulk

properties that become modified with incorporation into nanostructures.

 3.2 Products and Applications

Nanotechnology is divided into the following three approaches, which in turn give way to

specific products and applications

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Top-Down—A given bulk material is reduced in size to produce nanometer-scale particles,

which are then either systematically inserted into larger structures or used as an admixture to

other materials.

Bottom-Up—Larger structures are built up atom-by-atom or molecule-by-molecule or are

allowed to grow through self-assembly.

Self-Assembly—Components spontaneously assemble, usually by moving in a solution or gas

phase, until a stable structure of minimum energy is reached. A living cell is the most successful

“nanofactory” known to humankind.

  Nanoparticles are currently used in the electronic, magnetic, optoeletronic,

biomedical, pharmaceutical, cosmetic, energy, catalytic and materials industries. Areas that

produce the greatest revenue for nanoparticles include chemical-mechanical polishing, magnetic

recording tapes, sunscreens, automotive catalyst supports, biolabeling, electroconductive

coatings and optical fibers.

  Nanoparticles are also used in the medical field to aid in drug delivery and medical

imaging, and it is predicted that nanotechnology will contribute to new cancer therapies, new

treatments for infections and brain diseases and new drugs with fewer side effects.

  Advanced nanotechnology, or that which works with artificial intelligence,

nanorobots and self-assembly, is expected to increase significantly.

  Nanotechnology is also expected to play a major role in environmental protection.

Nanoparticles may be used in contaminant neutralization, magnetic techniques, special filtering

and cleaning methods, environmental decontamination, energy conservation and in the

production of energy-efficient devices.

3.3 Donors, Acceptors and Deep Traps

When a type V atom such as P, As, or Sb which has five electrons in its outer or valence electron

shell, is a substitutional impurity in Si it uses four of these electrons to satisfy the valence

requirements of the four nearest neighbour silicons, and the one remaining electron remains

weakly bound. The atom easily donates or passes on this electron to the conduction band so it is

called a donor, and the electron is called a donor electron.

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A type III atom such as Al or Ga, called an acceptor atom, which has three electrons

in its valence shell, can serve as a substitutional defect in Si, and in this role it requires four

valence electrons to bond with the tetrahedron of nearest neighbor Si atoms. To accomplish this,

it draws or accepts an electron from the valence band, leaving behind a hole at the top of this

band. This occurs easily because the energy levels of the acceptor atoms are in the forbidden gap

slightly above the valence band edge by the amount ∆Ea relative to KBT, as indicated in fig1.

Th

e

donor and acceptor atoms are known as shallow centers, that is shallow taps of electrons or

holes, because their excitation energies are much less than that of the band gap (∆Ed, ∆Ea<<Eg).

There are other centers with energy levels that lie deep within the forbidden gap, after closer to

its center than to the top or bottom, in contrast to the case with shallow donors and acceptors.

Since generally Eg>>KBT, these traps are not in exciting or ionizing them are not small.

Example of deep centers is defects associated with broken bonds, or strain involving

displacements of atoms.

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Fig.1. Sketch of the forbidden energy gap showing acceptor levels the typical distance ∆Ea above the top of the valence band, donor levels the typical distance ∆Ed below the bottom of the conduction band, and deep trap travels nearer to the centre of the gap. The value of the thermal energy KBT indicated on the right

KBT

Deep trap levels

Conduction Band

Donor levels

Acceptor levels

Valence Band

▲ED

▲EA

Excitons

An ordinary negative electron, called a positron, situated a distance r apart in free space

experience an attractive force called the Coulomb force, which has the value –e2/4пЄ0r2 where e

is their charge and Є0 is the dielectric constant of free space. A quantum mechanical calculation

shows that the electron and positron interact to form an atom called positronium which has

bound-state energies given by the Rydberg formula introduced by Niels Bohr in 1913 to explain

the hydrogen atom

E=-(e2)/(8пЄ0αo n2) = -6.8/n2 eV

Where a0 is the Bohr radius given by a0=4пЄoћ2/moe2=0.0529nm, mo is the free

electron (and positron) mass, and the quantum number n takes on the values n=1,2,3,……. . For

the lowest energy or ground state, which has n=1, the energy is 6.8 eV, which is exactly half the

ground state energy of a hydrogen atom, since the effective mass of the bound electron proton

pair in the hydrogen atom. Fig.2 shows the energy levels of a positronium as a function of the

quantum number n. The analog of a positronium in a solid such as a semiconductor is the bound

state of an electron-hole pair, called an exciton.

An exciton has the properties of a particle; it is mobile and abele to move around

the lattice. It also exhibits characteristic optical spectra.

20

e

hCoulomb force

n=3

n=2

n=1

E

k

Eb

Fig.2. First few energy levels in the rydberg series of a hydrogen atom (left) and the states below the band edge

3.4 Particle size determination

The most significant way to determine the size of a micrometer-sized grain is to look at it in a

microscope, and for nanosized particles a transimission electron microscope (TEM) serves this

purpose.

Another method for determining the sizes of particles is by measuring how they

scatter light. The extent of the scattering depends on the relationship between the particle sized

and the wavelength λ of the light, and it also depends on the polarization of the incident light

beam.

The sizes of particles <2nm can be conveniently determined by the method of mass

spectrometry. The mass spectrometer made use of the typical magnetic field mass analyzer.

3.5 Properties of individual nanoparticles

Because nanoparticles have 106 atoms or less their properties differ from those of the same

atoms bounded together to form bulk materials. Nanoparticles are generally considered to be a

number of atoms or molecules bounded together with a radius of <100 nm. A nanometer is

10-9m or 10Å, so particles having a radius of about ≤1000Å can be considered to be

nanoparticles. Figure 3 gives a somewhat arbitrary classification of atomic clusters according

their size showing the relationship between the number of atoms in the cluster and its radius.

21

106

105

104

103

102

10

1 пЄ0

1

10

Molecules

Nanoparticles

Bulk

The

size of the nanoparticles is smaller than critical lengths that characterized many physical

phenomena. Generally, some critical length, a thermal diffusion length, or a scattering length

can characterize physical properties of materials, for example. The electrical conductivity of a

metal is strongly determined by the distance that the electrons travel between collisions with the

vibrating atoms or impurities of the solid. This distance is called the mean tree path or he

scattering scattering length. If the sizes of particles are less than this characteristic length, it is

possible that new physics or chemistry may occur.

3.6 Bulk to nanotransition

In a cluster with less than 100 atoms, the amount of energy needed to ionize it i.e. to remove an

electron from the cluster, differs from the work function. The work function is the amount of

energy needed to remove an electron from the bulk solid. Clusters of gold have been found to

have the same melting point of bulk gold only when they contain 1000 atoms or more. Fig.4 is a

plot of the melting temperature of gold nanoparticles vs. the diameter of the particle. In general,

bulk appears that different physical properties of clusters reach the characteristic value of the

solid at different cluster sizes. The size of the cluster where the transition to bulk behavior

occurs appears to depend on the property being measured.

22800

850

900

950

1000

1050

1100

1150

1200

1250

1300

0 50 100 150 200 250 300

Temperature (K)

Diameter (Å)

Fig.3.Distinction between molecules, nanoparticles and bulk according to the number of atoms in cluster.

3.7 Semiconducting nanoparticles

The most striking property of nanoparticles made of semi-conducting elements is the pronounced

changed in their optical properties compared to those of the bulk material. There is a significant

shift in the optical absorption spectra toward the blue (shorter wavelength) as the particle size is

reduced.

The existence of the exciton has a strong influence on the electronic properties of

the semiconductor and its atom and has energy levels with relative spacing analogous to the

energy levels of the hydrogen atom but with lower actual energies. Fig.5 represents the optical

absorption spectra of cuprous oxide (CU2O), showing the absorption spectra due to the exciton,

when the size of nanoparticles becomes smaller than or comparable to the radius of the orbit of

the electron-hole pair, then there are two situations, called the weak-confinement and strong-

confinement regimes.

23

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2.12 2.13 2.14 2.15 2.16

Photon energy (eV)

Log (transmission)

Fig.4.Melting temperature of gold nanoparticles vs. particle diameter (10Å=1nm).

Fig.5. Optical absorption spectrum of Hydrogen like transitions of excitons in Cu2O.

3.8 Photoluminescence

The technique of photoluminescence excitation (PLE) has a standard one for obtaining

information on the nature of nanostructures such as quantum dots. In case of nanoparticles the

efficiency of luminescence decreases at high incoming photon energies.

The photoluminescence excitation involves scanning the frequency of the excitation

single, and recording the emission within a very narrow spectral range. Fig.6 illustrates the

technique for the case of ~5.6nm CdSe quantum dot nanoparticles. The solid line in fig.6a plots

the absorption spectrum in the range from 2.0 to 3.1 ev, and the superimposed dashed line shows

the photoluminescence response that appears near 2.05eV. The sample was then irradiated with

a range of photon energies of 2.13-3.5eV, and the luminescence spectrum emitted at the photon

energy of 2.13 eV is shown plotted in fig6b, as a function of the excitation energy. The

downward-pointing arrow on fig 6a indicates the position of the detected luminescence. It is

clear from a comparison of the absorption and luminescence spectra of this figure that the

photoluminescence is much better resolved. This is because each particle size emits light at a

characteristic frequency so the spectrum reflects the emission from only a small fraction of the

overall particle size distribution.

24

Energy (eV)

Optical densityIntensity (arbitrary units)

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

650 600 550 500 450 400

(b)

(a)

0

0 0

10K

10K

Fig.6 – Spectra taken at 10K for 5.6 nm diameter CdSe quantum dots. (a) Absorption spectrum (solid line) and photoluminescence spectrum (dashed line) obtained with excitation at 2.655 eV

.

Chapter 4Thin Film-IPhysics & chemistry of thin films

4.1 Introduction

When a bulk semiconductor material is converted to a thin film (i.e. 2-dimensional structure) the

physical and chemical properties will change to some extent (e.g. stability, electrical properties

etc.). A vast change in physical properties such as thermal properties, mechanical properties,

optical properties etc is observed when we go for size reduction from bulk to nanometer range.

We can define a thin film as a coating layer that is so thin (normally <1m in thickness) that its

surface properties are important compared to its bulk counterparts.

4.2 Special Properties of Thin Films

Thin films are different from bulk materials. Thin films may be:

1. Not fully dense

2. Under stress

3. Different defect structures from bulk

4. Quasi - two dimensional (very thin films)

5. Strongly influenced by surface and interface effects

6. This will change electrical, magnetic, optical, thermal, and mechanical

properties.

4.3 Typical steps in making thin films

Emission of particles from source (heat, high voltage)

25

Transport of particles to substrate (free vs. directed)

Condensation of particles on substrate.

 

4.3.1 Thermal accommodation

Impinging atoms must lose enough energy thermally to stay on surface. Assume that E = kT so

we can talk about energy or temperature equivalently

Thermal accommodation coefficient (aT)

4.3.2 Binding

Two broad types of surface bonds:

●Physisorption (physical adsorption)

○Van der Waals type

26

○Weak bonds

○0.01 eV

●Chemisorptions (chemical adsorption)

○Chemical bonds

○Strong bonds

○1 - 10 eV

The atoms stay on the surface by the competition between the impinging atoms (deposition) and

desorption of atoms

4.3.3 Surface diffusion

Allows clusters of adsorbed atoms to form .If clusters are stable then film forms. The distances

through which the atoms move are given by random walk analysis [Ref: F. Reif "Fundamentals

of Statistical and Thermal Physics" p. 486]. Diffusion distance (X) is given by

4.3.4 Nucleation

How do clusters form? => Nucleation

Two competing processes in cluster formation:

1. Clusters have a condensation energy per unit volume (ДG V) which lowers

the desorption rate (higher barrier)

2. Clusters have a higher surface energy than individual atoms

Clusters want to break up to minimize energy

Capillarity Model (= heterogeneous nucleation)

4.4 Schematic Diagram to show how nuclei grow initially

27

4.4.1 Island Growth

1. Island growth (Volmer - Weber)

●Form three-dimensional islands

●Source:

○Film atoms more strongly bound to each other than to substrate

○And/or slow diffusion

2. Layer by layer

growth (Frank - van der Merwe)

●Generally highest crystalline quality

●Source:

○Film atoms more strongly bound to substrate than to each other

○And/or fast diffusion

3. Mixed growth (Stranski - Krastanov)

●Initially layer by layer

●Then forms three dimensional islands

●Change in energetics

4.4.2 Island Coalescence

There are three common mechanisms:

●Ostwald Ripening

28

●Sintering

●Cluster Migration

● Ostwald ripening

□Atoms leave small islands more readily than large islands

□More convex curvature => higher activity => more atoms escape

● Sintering

□Reduction of surface energy

● Cluster migration

□Small clusters (<100 Å across) move randomly

□Some absorbed by larger clusters (increasing radius and height)

Chapter 5

29

Thin Film-IIDeposition & Application

5.1 Various deposition techniques

The deposition techniques that can be employed to grow transparent conducting oxide films are:

1. Chemical vapour Deposition (CVD)

2. Spray Pyrolysis

3. Vaccum evaporation

4. Sputtering

5. Ion-associated deposition Technique

a) Ion-beam deposition

b) Ion ploting

6. Other method

a) Electroless chemical growth technique

b) Sol-gel technique

c) Laser-associated deposition technique

d) Anodization

5.1.1 Chemical Vapour Deposition

Chemical vapour deposition (CVD) is one of the most important techniques for producing thin

films of semiconductor materials. This technique involves a reaction of one or more gaseous

reacting species on a solid surface (substrate). In this process, metallic oxides are generally

grown by the vapourization of a suitable organometallic compound. A vapour containing the

condensate material is transported to a substrate surface, where it is decomposed, usually by a

heterogeneous process. The nature of the decomposition process varies according to the

composition of the volatile transporting species. The decomposition condition should be such

that the reaction occurs only at or near the substrate surface and not in the gaseous phase, to

avoid formation of powdery deposits which may result in haziness in the films.

30

The vapours of a volatile compound are carried by a carrier gas, e.g. O2, N2, Ar from

a hot bubbler through a heated line to a reaction chamber to which oxygen or water vapour is

introduced. In this growth chamber the vapour decompose and the homogeneous oxide films

form at the preheated substrate surface. The quality of the films depends on various parameters

such as substrate temperature, gas how rate and system geometry. In order to obtain the best

quality films all these parameters should be optimized are controlled.

The main advantages of using the CVD process are simplicity, reproducibility and

the ease with which it can be adopted for large-scale production without requiring vaccum as an

essential requirement for deposition. Moreover, due to the low cost of equipment used, the cost

of production of thin films by this technique is reasonably low. Using this technique, films of

high purity, stoichiometry, and structural perfection can be obtained.

5.1.2 Spray Pyrolysis

Spray pyrolysis is based on the pyrolytic decomposition of a metallic compound dissolved in a

liquid mixture when it is sprayes onto a preheated substrate. The method depends on the surface

hydrolysis of metal chloride on a heated substrate surface in accordance with the reaction.

MClx+YH2O MOy + xHCl

In which M is any host metal such as Sn, Tn, Zn etc. of the oxide films. The

conventional spray pyrolysis technique is shown in figure below. The spray nozzle with the help

of a filtered carrier gas accomplishes the atomization of the chemical solution into a spray of

fine droplets. The carrier gas and solution are fed into the spray nozzle at predetermined

pressures and how rates. The geometry of the gas and the liquid nozzles strongly determine the

spray pattern, size distribution of droplets and spray rate, which in turn determine the growth

kinetics and hence the quality of the films obtained. The other important control parameters that

affect the quality of the films are the nature and temperature of the substrate, the solution

31

composition, the gas and solution flow rates, the deposition time and the nozzle-to-substrate

distance.

In general, the films grown at the substrate temperatures less than 300 0c are

amorphous in nature, whereas at higher substrate temperatures, polycrystalline films are formed.

In order to obtain films with good conductivity, it is normally essential that complete oxidation

of the metal be avoided. This is generally achieved by adding an appropriate reducing agent

such as propanol, ethyl alcohol or pyrogallol.

5.1.3 Vaccum evaporation

The basic setup is shown in the figure below. A resistively heated tungsten or tantalum source, or

an electron beam heated source, can be used to evaporate the charge. The substrate heater is

placed above the substrate to heat it to the required temperature. Oxygen or an argon-oxygen

mixture can be admitted through the calibrated leak value. The important control parameters are

the substrate temperature, evaporation rate, source to substance distance and oxygen partial

pressure. The transport conducting oxide films can be evaporated in three ways-

i) By directly evaporating metal oxides, e.g SnO2, In2O3, Cd2SnO4

ii) By reactive evaporation of the metal presence of oxygen.

iii) Post oxidation of metal films.

When oxide materials are evaporated there is always some deficiency of oxygen in

the films. Either the films must be evaporated in the partial pressure of oxygen or some post

deposition heat treatment in air is essential to achieve good quality films. In reactive evaporation,

the corresponding metal or alloy is evaporated at rates of 100-300 Ǻ min-1 in oxygen atmosphere

onto substrates heated to about 4000C. In the case of post-oxidation of metal films, mainly the

32

Fig. Spray Pyrolysis System

oxidation temperature controls the conductivity and transparency of the films, which is usually in

the range 350-5000C.

5.1.4 Sputtering

Compared with other deposition techniques the sputtering process produces films with higher

purity and better-controlled composition provides films with greater adhesive strength and

homogeneity and permits better control of film thickness. The sputtering process involves the

creation of a gas plasma (usually an inert gas such as argon) by applying voltage between a

cathode and anode the cathode is used as target holder and the anode is used as substrate holder.

Source material is subjected to intense bombardment by ions. By momentum transfer, particles

are ejected forms the surface of the cathode and they diffuse away from it, depositing a thin film

onto a substrate. Sputtering a normally performed at a pressure of 10-2-10-3 Torr.

Normally, there are two modes of powering the sputtering system. In a DC

sputtering system, a direct voltage is applied between the cathode and the anode. This process is

restricted to conducting targets, such as tin or indium.

33

Fig. The basic set up for vacuum evaporation system.

In RF sputtering, which is suitable for both conducting and insulating targets, a high

frequency generator (13.56 MHZ) is connected between the electrodes.

Magnetron sputtering is particularly useful where high deposition rates and low substrate

temperatures are required.

34

The quality of the films depends on various deposition parameters such as sputtering

rate, substrate temperature, sputtering gas mixture etc.

Experimental sputtering systems usually have small targets and low production

rates, whereas commercial production systems have large targets and rapid substrate transport to

maximize production rates. Typical conditions for DC sputtering of these materials are :

Cathode voltage 1-5 KV

Cathode current density 1-10 mA cm-2

Cathode substrate distance 2-4 cm

Working gas pressure 10-2 Torr.

For RF sputtering, power 200-1000W is delivered to the target from an RF

generator of 13.56 MHz via an impedance matching network. These conventional sputtering

processes offering simply controlled and reproducible film growth rates, suffer from two main

disadvantages:-

i) Low deposition rates

ii) Substantial heating of the substrate due to its bombardment by secondary

electrons from the target.

By employing a magnetic field to confine these electrons to a region close to the

target surface, the heating effect is substantially reduced and the plasma is intensified, leading to

greatly increased deposition rates. The source target is usually a water-cooled magnetron

cathode, and a sufficiently uniform magnetic field of 0.02-0.1 tesla is required to confine the

electrons near the target surface. The magnetic field is parallel to the target surface and

orthogonal to the electric field. In conventional magnetron sputtering systems, there is always

resputtering of the film due to bombardment of high energy O-ions.

5.1.5 Ion-associated deposition techniques

Where deposition of abherent coatings is required at low temperatures, ion-assisted deposition

techniques are promising alternatives. The ion-associated deposition techniques can be broadly

divided into two categories: -

35

i) Ion-beam deposition and

ii) Ion-plating

i) Ion-beam deposition

Ion-beam deposition can be used in two different configurations, namely sputtering and

evaporation. In the case of ion-beam sputtering are used to bombard the target, whereas in the

case of evaporation, the ions of suitable reactive gas are allowed to impinge on the substrate.

Unlike conventional sputtering systems, ion-beam sputtering involves minimal

intrinsic heating and electron bombardment. Moreover, in this process the arrival rate of

depositing species can be controlled precisely by a typical ion-beam sputtering system used for

deposition of ITO films. An argon ion beam from a Kaufman ion source is directed onto the hot

pressed ITO target to supply the depositing species. The argon ion-beam source is usually

capable of giving beams within maximum ion energies upto 100-1500 ev and beam currents upto

30-50 mA.

Recently a dual beam ion sputtering technique has been used for the deposition of a

variety of films. Two different ion sources are used; one for sputtering the metallic species and

the other for providing the reactive gas ions. In the case of ion-beam evaporation, a thermal

evaporator i.e. electron beam or a tungsten boat, is used for the evaporation of metallic atoms.

An ion beam gun of reactive gas (O2) with energies upto 500 eV is used.

ii) Ion-platting

In the ion-plating method, continuously bombarding the substrate and growing film with

energetic particles significantly enhance deposition energies. Ion plating can be used in an

36

evaporative or sputter mode. In both case the deposition species are ionized after leaving the

source.

The system used in the evaporative mode comprises a vacuum chamber containing

an evaporative source and a high voltage cathode. The deposition chamber is initially evacuated

at ~10-6 Torr and subsequently filled with argon, raising the pressure to ~10 -2 to 10-3 Torr. A DC

voltage of -500 to -3000 v is applied to the cathode.

In the case of sputter mode system, the reactive gas is activated by ionization and

driven towards the substrate by coupling an RF field to the substrate. The power of the applied

ion-plating RF field is normally in the range of 100-500 W. In general, the partial pressure of

oxygen in the system is ~1.5x10-3 Torr and that of argon 3.5x10-3 Torr for reactive ion plating of

transparent conducting oxide films.

5.1.6 Other methods

● Electroless chemical growth Techniques

In this process, the substrate is immersed in an aqueous solution of metal chloride. Solid phased

of metal hydroxide or metal hydrous oxide is formed, which on heating yield to metal oxide. The

important parameters, which control the deposition process, are the composition of the initial

solution and its pH value. Film thickness depends on deposition time with pH as a parameter.

● Laser-associated deposition techniques

The laser evaporation deposition technique has been successfully used to grown high Tc

superconductors and semiconductor films of good quality. The main advantage of this method is

that it can be used to grow highly oriented films at low substrate temperatures. This is due to the

fact that the particles ejected using lasers have sufficient kinetic energy to arrange the structure

of the films on the substrate.

● Anodization

Anodization has been used as simple and efficient method for converting metals into their

oxides. However, attempts to anodically oxidize tin or indium have been only partially

37

successful. The metal to be oxidized is employed as an anode and is dipped in an electrolyte

from which it attracts oxygen ions.

The oxygen ions combine with the metallic atoms to form oxide molecules. The

rate or film growth depends on the temperature and the kind of electrolyte used. Anodization

can be carried out either at constant current or at constant voltage conditions. It should be

mentioned that it is not possible to grow films of large thickness using this techniques.

5.2 Thin film application

● Gas sensor

The gas chemisorption onto the surface of the semiconductor material influences its electrical

conductivity significantly. It is this large and reversible variation in conductance with active gas

pressure that has made semiconductor materials attractive for the fabrication of gas sensing

electronic devices.

In case of gas sensor, the most promising semiconductor transparent coatings are tin

oxide, zinc oxide, and indium tin oxide. The most commonly used semiconductor material for

gas sensing is tin oxide. The most important use of SNO2 sensors in the home is in gas leak

alarms.

Semiconductor-based gas sensors are being presently used for detecting H2, CH4,

LPG to prevent leakage, for detecting so for pollution control, and as alcohol sensors for

controlling drunken driving.

● Corrosion protection:

Aluminums have a high and fairly constant reflectivity in the visible region and a reflectivity

≈99% in the thermal infrared region. Because of this and its light weight and low cost,

aluminium is being used in a varity of optical application, i.e. mirrors of high optical quality and

large decorative panels on buildings. It corrodes easily in a humid atmosphere and its advantages

optical properties are lost.

To overcome this problem a protective layer is deposited on the aluminium. This is

achieved either by an anodization technique where an aluminium oxide layer is electrotytically

38

grown on the aluminium surface, or by depositing a tin oxide on it. But the anodized process is

not suitable for thin film surface for its drawbacks.

Tin oxide coatings exhibit protective properties, which are superior to those of

anodic layers. There is no effect of sulfuric acid and sodium hydroxide solution on tin oxide

coated aluminium surfaces.

It can be seen that reflectivity of the tin oxide coated samples has not changed at all.

In addition to providing better chemical stability, a tin oxide coating also enhances the

mechanical strength of the aluminium surface.

● Antireflection coatings

The highest damage threshold antireflection coatings are single layers deposited by room

temperature, non-vacuum technique.

The sol-gel, Teflon and NSP coatings are all single layers produced using non-

vacuum techniques. The sol gel coatings are the most widely used for large aperture, high damage

threshold applications. Electron beam deposited multi-layers are used in most other applications

and are the coatings most widely available commercially.

At 1064 nm the damage thresholds of sol-gel coatings generally do not change,

whereas e-beam coatings can show increases remaining form nothing to a factor of 2 or better,

depending on the wavelength and the materials. Natural solution process coatings show increases

on the order of a factor of 2. Teflon coating show a laser-conditioning effect at both 1064 and

355 nm. Therefore have also been reports that laser or sputter cleaning of the substrate surfaces

can also increase the threshold of e-beam-deposited AR coatings.

● Wear-resistance coating:

Wear may be defined as undesirable and unintentional deterioration of objects due to use by the

removal of material form one or both of the rubbing surfaces.

1) Adhesive Wear

When two metal surfaces are rubbing against each other, intense local heating and high pressures

are produced at the interface by colliding asperities. This results in the formation of metallic

39

junctions, which are of appreciable size by molecular standards. This is only the first stage of

the adhesive wear mechanism.

2) Abrasive Wear

Abrasive wear occurs when the two rubbing surfaces differ widely in their hardness and the

harder surface has a certain degree of roughness, so that its surface asperities dig into the softer

surface, literally ploughing out furrow which break away forming loose wear particles.

3) Chemical Wear

This type of wear occurs when a surface prone to corrosion is being rubbed against another

surface. This happens when the reaction-product layer formed by corrosion is not adherent and

wear resistant and can be easing removed by rubbing, thus exposing the surface of the material to

the environment and allowing the chemical attack to continue farther.

4) Surface fatigue Wear

Fatigue wear occurs at surfaces in rolling contact and is characterized by sudden pitting or

flacking-off of the surfaces on a large scale due to repeated cyclic stress, usually terminating the

life of the mechanical system.

Wear resistant coatings are coatings of hard materials such as cr, co, si, Mo, W, C

and their alloys and carbides and nitrides of refractory metals and their mixture along with their

composite with metals. The properties or the coatings may depends on the technique of

depositing and the depositing parameters.

40

Chapter 6ZnO and CompositeExperiment, Results & Discussion

6.1 Introduction

In the field of nanoscience the tailoring of the properties of the materials in nanodimension

involves different nanostructures and nanocomposite thin films. Most of the technological,

electronic and optoelectronical applications utilize materials in thin film forms. The films

themselves could be amorphous, single-crystalline or nanocrystalline and are used for electronic

thin film devices, for wear, chemical or oxidation protection, as well as for their optical

properties (e.g., anti-reflection) and transparent conducting films. Transparent conducting oxides

play a key role in optoelectronic devices such as flat panel devices, photovolatic and

electrochromic devices.2 Wide band gap semiconductors such as ZnSe, GaN and ZnO have

attracted much attention owing to their usefulness in the field of optoelectronic.

6.2 Zinc Oxide (ZnO)

Zinc oxide occurs in the nature as the mineral zincite. Zinc oxide crystallizes in the hexagonal

wurtzite (B-4 type) position of hexagonal close packing. Every oxygen atom lies within a

tetrahedral group of four zinc atoms, and all these tetrahedral point in the same direction along

the hexagonal axis giving the crystal its polar symmetry. The lattice constants are a= 3.24 Å and

C=5.19Å.

41

The bulk electronic properties of ZnO are described by a tight bonding Hamiltonial

obtained be fitting self-consistent pseudopotential bulk band structure. The surface band

structures have been calculated using the scattering theoretical method. The grain size of the thin

films of ZnO lies in the range 50-300A. The donor levels in ZnO lay in the range 0.02-0.05 eV,

depending on carrier concentration. These hydrogen-like donor levels can be produced either

due to oxygen vacancies (V0) or due to incorporation of hydrogen, indium, lithium or zinc. In

this case of gallium-doped ZnO films, there are three donor levels at 30,60 and 150 meV and an

acceptor level at ~0.72 eV below the conduction band. The deep lying acceptor level is probably

due to chemisorbed oxygen. A similar type of acceptor level has also been obtained at 0.80 eV

below the conduction band in undoped ZnO films. Among these materials thin films of ZnO hold

better prospect because of its good optical properties in combination with its large excitonic

binding energies of around 60 meV. This large excitonic binding energy ensures excitonic

features in the optical spectra even at the room temperature. Transparent conducting metal oxide,

ZnO is a II-VI semiconductor and crystallizes in the hexagonal wurtzite structure having large

band gap Eg of 3.37 eV at room temperature. This tuning is achieved in two ways: (i) confining

the nanoparticles in proper matrix thus arresting the growth of the nanoparticles and (ii) by

incorporating various dopant into the lattice site of ZnO thus expanding the valence level

(Burstein-Moss shift). The explicit relationship between the intrinsic band gaps of ZnO with the

particle size is exploited in order to seek its possible use in the optoelectronic devices. On the

other hand, a decrease in the size of the nanocrystals means higher surface to volume ratio that

can incorporate significant surface related defects, disorder and randomness in the system. These

defects on its part decrease the excitonic emission efficiency necessitating minimization of these

defects. Capping or embedding the materials in an appropriate matrix can minimize these surface

defects. High band gap materials such as SiO2, MgO and Al2O3 are used to cap the ZnO

nanoparticles, thus confining the growth of the nanoparticles. The tailoring of the optical band

gap to the ultra-violet (UV) edge of the spectra and a large excitonic binding energy enshrines

ZnO as a good candidate for optoelectronic applications in the visible and ultraviolet regions.

6.3 ZnO-Al2O3 nanocomposites

ZnO and Al2O3 films, both are transparent in the visible region. ZnO-Al2O3 nanocomposite films

may provide advantages in the applications in optoelectronic by allowing the control of particle

size, refractive index and surface roughness. The crystal structure of ZnO (hexagonal wurtzite,

42

a=3.249 Å, b=5.206 Å) and Al2O3 are entirely different, however the ionic radii of Zn+2 (72 pm)

and Al+3 (53 pm) may replace each other in the matrix depending on the increasing ionic

interaction among the atoms of the lattice. Nanocrystalline ZnO exhibits strong n-type

conductivity with the electrons to move in the conduction band as charge carriers thus exhibiting

improved electrical properties in comparison to its bulk counterpart. On the other hand The

frequency dependent electrical properties of the thin film greatly depends on the structural

homogeneity and stability of the composite based devices and can highlight the relative

contribution of the grain, grain boundary and the defect states to the total ac response under a

given set of experimental condition.

6.4 Experimental

For the preparation of the ZnO-Al2O3 nanocomposites, the sol was prepared in two parts, one

was the ZnO part and the other was the Al2O3 part. All reagent grade chemicals were purchased

from Merck (India) Ltd. For the preparation of the ZnO part, zinc acetate, 2-propanol and

diethanolamine (DEA) were used. The molar ratio of zinc acetate and 2-propanol was

approximately 1:55. For total dissolution of the zinc acetate, 0.006 mole of DEA was required.

The zinc acetate was first mixed with 2-propanol under stirring and then DEA was added to the

above solution drop wise under constant stirring. Then this part was stirred for one hour to get a

transparent sol.

In the preparation of Al2O3 part, aluminum nitrate (Al(NO3)3, 6H2O) was used as the

precursor. The aqueous solution of the precursor was refluxed for one hour and then treated with

NH3 solution (25%). The white precipitate (boehmite) thus obtained was dissolved in (12:1)

volume ratio of ethanol and water. 3 cc of acetic acid was added to it drop wise under constant

stirring and was stirred for half hour. During the process the shape and the size of the sol particle

changed. At the end of three hours of stirring a completely transparent and viscous sol was

obtained.

The ZnO and Al2O3 sol were mixed and stirred for two hours to get the desired

transparent sol for coating. ZnOx-(Al2O3)1-x nanocomposites films (x=0.20 and 0.50) were

deposited on properly cleaned quartz substrates by dip coating technique (5cm/min). The as

coated samples were annealed in a preheated chamber at 573K, 673 K, 773 K, 873 K and 973K

in air for 25 minutes. The nanocomposite thin film thus obtained was a random assembly of ZnO

43

nanoparticles embedded in Al2O3 matrix. In the latter section, the nanocomposite thin film with

x=0.20 and 0.50 will be denoted by Sample A and Sample B, respectively.

6.4.1 Instruments used for characterization

The morphology, crystalline size and phase of the nanocomposite samples were determined by

transmission electron microscope (TEM) (JEOL 2010 electron microscope), atomic force

microscopy (AFM) (Nanoscope IV scanning probe microscope controller) and using powder X-

ray diffraction (XRD) (Seifert 3000 diffractometer) method. Optical absorption spectrum of the

products was recorded by a spectrophotometer (Schimadzu 2401).

● X ray diffractometer

Introduction:

X- ray powder diffraction technique is one of the most powerful techniques for qualitative and

quantitative analysis of crystalline compounds. The technique provides information that cannot

be obtained in any other way. The information obtained includes types and nature of crystalline

phases present, structural make up of phases, degree of crystallinity, and amount of amorphous

content, micro strain and orientation of crystal.

Principles

When a material is irradiated with a parallel beam of monochromatic X rays, the atomic lattice of

the sample acts as a 3-dimensional diffraction grating, causing the X ray beam to be diffracted to

specific angles related to the inter atomic spacing. This X ray pattern is recorded by film

methods or angular measuring X ray detector method. By measuring the angles of diffraction, the

inter atomic spacing of the material can be determined and used to identify the crystallographic

structure of the material.

X ray Diffraction is essentially valuable to the study of epitaxial layers and other

thin film materials. Using precision lattice parameter measurement methods, the lattice mismatch

(delta-a) of the epitaxial layer and the substrate can be determined to within 0.0005. Lattice

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parameters plotted Vs temperature, as in a high temperature Diffractometer, can be used to

obtain the co-efficient of thermal expansion.

● Transmission electron microscope

A TEM works much like a slide projector. A projector shines a beam of light through the slide,

as the structures and objects on the slide affect the light passes through it. These effects result

only in certain part of the light being transmitted through certain parts of the slide. This

transmitted beam us then projected onto the viewing screen, forming an enlarged image of the

slide.

TEMs work in the same way except that they shine a beam of electrons (like the light) through

the specimen (like the slide). Whatever part is transmitted is projected onto a phosphor screen for

the user to see. A more technical explanation of a typical TEMs working is as follows:

●The electron beam is produced described in below

1) The “virtual source” at the top represents the electron gun , producing a

stream of monochromatic electrons.

2) The stream is condensed first by the condenser lens. This lens is used both

to form the beam and limit the amount of current in the beam. It works in conjunction with the

condenser aperture to eliminate the high-angle electrons to form the beam.

3) The beam is then constricted by the condenser aperture (usually not user

selectable), eliminating some high-angle electrons.

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4) The second condenser lens forms the electrons into a thin, tight, coherent

beam.

●The beam is restricted by the condenser aperture (usually user selectable),

knocking out high angle electrons (those far from the optic axis, the dotted line down the center).

●The beam strikes the specimen and parts of it are transmitted.

●This transmitted portion is focused by the objective lens into an image.

●Optical Objective and Selective Area metal apertures can restrict the beam; the

Objective aperture enhancing contrast by blocking out high-angle diffracted electrons, the

Selected Area aperture enabling the user to examine the periodic diffraction of the electrons by

order arrangements of atoms in the sample.

●The image is passed down the column through the intermediate and projector

lenses, being enlarged all the way.

●The image strikes the phosphor image screen and light is generated, allowing the

user to see the image. The darker areas of the image represent those areas of the sample that

fewer electrons were transmitted through (they are thicker or denser). The lighter areas of the

image represent those areas of the sample that more electrons were transmitted through (they are

thinner or less dense)

HITACHI H-600 Transmission Electron Microscope was used to study the growth

and microstructure of the sample.

● UV-VIS spectrometer

UV/VIS spectrophotometry is used to determine the absorption or transmission of UV/VIS light

(180 to 820 nm) by a sample. It can also be used to measure concentrations of absorbing

materials based on developed calibration curves of the material.

A sample is placed in the UV/VIS beam and a graph of he transmittance or

absorbance versus the wavelength is obtained. Alternatively, the samples are prepared in known

concentrations and the UV/VIS Spectrophotometer. Results are then graphed to make a

calibration curve from which the unknown concentration can be determined by its absorbance.

HITACHI U-3410 spectrometer is used to measure the extent to which the samples

under consideration absorb or transmit light of different wavelengths. The instrument

46

automatically records a graph of absorbance or transmittance or reflectance vs. wavelength within

a spectral range of 185 to 2600 nm.

● Atomic force microscopy

AFM is one of many techniques, which fall under the Scanned Probe Microscopy (SPM) family

of instruments. In all of these SPM techniques a small probe (10-100 nm radius of curvature) is

raster scanned by a piezoelectric device over a sample to produce an image of the sample

surface, or near surface region.

A simple schematic of an AFM instrument is given in Figure below. In this

instrument the probe tip is mounted on the end of a triangular cantilever arm, similar to a

diamond stylus mounted on the end of a record player arm. A piezoelectric device raster scans

the sample beneath the probe tip. As the probe tip undergoes attractive or repulsive forces, the

cantilever will bend. This bending of the cantilever can be monitored by bouncing a laser beam

(a simple diode laser, like that found in a CD player works well) off of the cantilever onto a 2-

element photodiode (Position Sensitive Photo Diode). In normal operation the tip-sample force is

held constant by a computer controlled feedback loop that examines the force (bend of the

cantilever) and tells the piezoelectric device whether to move the sample closer or farther away

in order to maintain the set force value. The AFM image produced by taking the feedback signals

at each pixel of the raster scan is a measure of the topography of the sample.

The AFM uses the attractive or repulsive forces encountered by a probe tip when it

is in close proximity to a sample surface (<200 nm). There are three main modes of AFM

operation that are currently in use: Contact, Non-Contact and Intermittent Contact (Tapping).

Contact AFM is done by bringing the tip to a distance at which repulsive forces dominate the tip-

47

Fig. Schematic Diagram of AFM

sample interaction. Non-Contact AFM is done such that the tip-sample interaction is in the

attractive or Van der Waals regime. In order to perform measurements in this attractive force

region the cantilever is oscillated with a low amplitude (<5 nm), near its resonant frequency. For

Non-Contact AFM the force is measured by comparing the frequency and/or amplitude of the

cantilever oscillation relative to the driving signal. Tapping or Intermittent Contact mode is also

done by oscillating the cantilever near its resonant frequency, but the amplitude is significantly

higher (~10-50 nm). This Intermittant contact mode operates in the repulsive force region, but

touches the surface only for short periods of time, in order to reduce damage to potentially

fragile samples (i.e. biological molecules).

6.6 Results and Discussion

● X-Ray diffraction

In Figure 1 the X-ray diffraction (XRD) spectra for the ZnO-Al2O3 nanocomposite thin films are

shown. As is observed from Figure 1c that Samples A (ZnO-Al2O3 20: 80) does not show

48

Fig. 1

any distinguishable characteristic peak of ZnO even though it was annealed at temperature as

high as 973 K. This absence of peak may be attributed to the lower molar percentage of ZnO in

the Al2O3 matrix. Detection of the characteristic peaks of Al2O3 at this temperature is highly

improbable as (i) Al2O3 crystallizes at very high temperature (our previous experience in this

regard is that below 1073K alumina powder remains amorphous in nature and the first of peak

occurs for γ-Al2O3 when the sample is annealed at 1073 K for as minimum as 1 hour) and (ii)

alumina thin film drawn on quartz substrate crystallizes only after rapid thermal processing

(RTP) at high temperature. This because of the fact that during regular annealing the slow

diffusion of Si+4 ion in the alumina lattice site breaks the periodicity of the crystal and XRD

spectra for this thin film is similar to that coming from amorphous samples. Recognition of the

ZnO peaks is completed for Sample B (ZnO-Al2O3 50:50) annealed at 873 K and 973 K, as

shown in Figure 1a-b. Figure 1b shows indication of low intensity peaks of ZnO at 2θ = 31.621°

and 36.851°. The peaks are assigned as (100) and (101) planes of wurtzite ZnO [JCPDS File No.

35-0664]. Figure 1a shows the XRD pattern for Sample B annealed at 973 K which gives an

additional peak at 2θ =56.266° for (110) plane of the same wurtzite phase of ZnO. The

appearance of these peaks with notable intensity at higher molar concentration of ZnO is due to

the easy detection of the material in the nanocomposites and also owing to enhanced particle size

in the matrix. The noticeable feature of the XRD pattern presented here is the appearance of a

very small peak at 2θ =59.368° (marked with "`asterisk"). At this value of 2θ no known peak of

wurtzite or cubic ZnO matches well, closer examination reveals that this 2θ corresponds to (511)

plane of ZnAl2O4 [JCPDS File No. 05-0669].

● Transmission electron microscope

49

High magnification TEM micrographs of ZnO-Al2O3 nanocomposites annealed at 573 K are

shown in the Figure 2a-b. As is revealed form the images exact determination of the size of the

nanoparticles is not possible as the micrographs show highly agglomerated grains of the

nanocomposites with sizes vary from 10 nm (Sample A, Figure 2a) to 18 nm (Sample B, Figure

2b).

● Atomic force microscope

The atomic force microscopy (AFM) images of the surface of 1.0 × 1.0 µm2 for the samples A

and B annealed at 573 K are shown in the Figure 3a-b. The homogeneity of the nanocomposite

thin films is evident from the AFM images. The grain sizes vary for the two

samples and are largest for the Sample B (Figure 3b). The smoothness of the top surface of the

thin film is measured in terms of the roughness, which is a basic parameter indicating the

deviation of a surface with respect to a perfect plane. The root mean square roughness R (rms) is

defined as

50

Fig. 2

Fig.3

where, Zi is the Z value of each point, Zav is the average of the Z values and N is the number of

points. For Sample A and B annealed at 573 K the roughness (rms) is measured as 1.18 nm and

0.77 nm, respectively, indicating smoother surface for the latter film.

● Optical Absorption Study

Figure 4a-b shows normalized absorbance spectra of single layered ZnOx (Al2O3)1-x

51

Fig.4

nanocomposite films with thickness (t) of 119 nm annealed in the temperature range of 573-973

K. The spectra are normalized in order to compare the intensity of the absorbance peaks. The

spectra indicate (i) a long tail with varying absorbance at lower energies, (ii) sharpening of the

absorption peak with increasing annealing temperature, (iii) broadening of the absorption peak

with higher concentration of ZnO in the nanocomposite. The appearance of a long tail at lower

energy region may be due to various defect related states, and on the other hand broadening

arises due to the disorder and randomness in the thin film. The variation of the transmittance (T)

with wavelength () can be expressed as T= Aexp(-4πκtλ), where κ is the extinction coefficient

and is given by κ=λα/4π. A is nearly equal to unity and α is the absorption coefficient. Then α

can be expressed as α =(1/t)ln(1/T).

For Sample A (Figure 4a) the band edge shifts from 5.35 eV to 5.58 eV for an

increase in annealing temperature of 573K to 973 K while the absorption peaks shifts from 4.85

eV to 5.15 eV. On the contrary, In Figure 4b (Sample B) it is seen the the high intensity

absorption peaks do not appear except for the one annealed at 973 K, while at the same instant

the absorption edge shifts from 5.30 eV (573 K) to 5.44 eV (973K). The reasons for this

broadening of the spectra are that (i) a relative lower concentration of Al2O3 in the composite

basically loosen the extent of capping of the ZnO nanoparticles, thus incorporating surface

defects to creep in the grain boundary and (ii) an inhomogeneous distribution of the

nanoparticles, which for having an individual transitional frequency of their own creates an

envelope of broad absorption at the absorption edge.

Jeong and Park reported such blue shifting of the absorption edges with increasing

concentration Al dopant in the ZnO owing to the ionic substitution of Al+3 into the Zn+2 site in

the lattice. The reason for this is that the density of electron decreased after the Al +3 ion is

substituted into the Zn+2 ion sites in the films thus lowering the valence level. In our sample as

the case may be that with increasing annealing temperatures some of the Al+3 ions may diffuse

into the ZnO lattice site in the film resulting in the Burstaein-Moss shift in the absorption

spectra. Oppositely, it can also be concluded that with the diffusion of the Zn+2 ions in the Al2O3

lattice site the band gap of the latter is lowered considerably. But the fact that the band gap

widens with the increase in annealing temperatures may safely lead us to conclude that the ionic

diffusion occurs in the other way round, otherwise, more and more diffusion of the Zn+2 ion

52

would have lead to a decrease in band gap of Al2O3 at higher annealing temperatures, contrary to

the observed results. Such ionic diffusion in nanocrystalline materials is very dominant owing to

(i) relaxation of the grain boundary structures, which occurs through relative grain displacements

and thus reducing the free volume of grain boundary structures and (ii) the grain boundary

dislocations whose transformations greatly effect the diffusion process.

ConclusionZnO and ZnO-Al2O3 thin films were prepared by dc magnetron sputtering on Si and quartz glass

substrates. ZnO-Al2O3 thin films remain transparent in the short wavelength range (<350nm)

because of the increased bandgap Eg in the film. By Al atoms doping in ZnO grains, ZnO-Al2O3

films have excellent electrical conductivity.

In the experiment when we prepared alumina we cann’t use any other acid except

acitic acid for peptization because if we used any other acid like HCl or HNO3 there are some

amount of Cl or N in the solution which we could not washed out. Also we are very careful about

the cleanness of the beaker where alumina is prepared because if there is some ammonia in the

beaker then it is opposed the peptization. Also when we prepared ZnO from Zn-acetate, Zn-

acetate can’t resolve in 2-propanol. So Diethanolamine mixed in the solution. It separates Zn and

acetate and then Zn resolve in 2-propanol.

Zinc oxide based coating have recently received much attention because they have

advantages over the more commonly used indium and tin-based oxide films are usually more

expensive than zinc oxide films Pure zinc oxide films, although transparent, are usually high

resistive. Non-stoichiometric and doped zinc oxide films, however, have high conductivities, but

non-stoichiometric films are not very stable at high temperatures. For practical applications,

therefore, doped ZnO films are more suitable.

ZnO films are used in various pressure transducers, acoustic wave and acoustic

optical devices with their piezoelectric properties. They are useful as transparent panel display

and solar cells while doped with Al, In etc, and as catalytic combustion sensors while doped with

Pt and Pd

Another important application of ZnO films is the use of a buffer layer for the

growth of GaN films. Recently, improvement of the GaN blue light emitting diode has made it

53

possible to display full color by semiconductor. GaN film is lack of high quality, closely lattice

matched substrate materials. ZnO is closely lattice matched to GaN and has the same crystal

structure. Therefore, the ZnO films offer the potential to be a substrate on which high-quality

GaN films may be grown. So, ZnO-Al2O3 films are used as a buffer layer and transparent

electrodes of blue emitter diode. ZnO:Al films have the advantages that they can be produced at

low substrate temperature and etched easily.

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