28

CHAPTER 3

EXPERIMENTAL TECHNIQUES

3.1 INTRODUCTION

There are so many deposition methods available for preparing thin

films. However, every method has its own specific limitations and involves

compromises with respect to process specifications, expected film properties,

substrate material limitations, cost, etc; these limitations redistrict us to select

the deposition technique. A detailed study has been made about the various

methods available for thin film deposition in this chapter.

3.2 PHYSICAL VAPOUR DEPOSITION

The physical vapour deposition (PVD) process are atomistic

deposition process in which material is vaporized from a solid or liquid source

in the form of atom or molecules and transported in the form of a vapour

through a vaccum or low pressure to the substrate where it condenses.

Typically PVD processes are used to deposit films with thickness in range of

few nanometres to thousand nanometers. Deposition rates are 10-100 A° (1-10

nanometres per second). The important physical vapour deposition processes

are vaccum evaporation, sputtering, arc evaporation and ion planting. In

vaccum evaporation, atoms are removed from the source by thermal means,

where as in sputtering, they are dislodged from target surface through impact

of gaseous ions. If the evaporated material is transported through a reactive

gas, the technique is called reactive evaporation. Flash evaporation technique

29

is used when we have to deposit a multi- component material which cannot be

heated to the evaporation point together. Ion plating refers to a process in

which the substrate and film are exposed to a flux of high-energy ions during

deposition this can provide dense coatings at relatively high gas pressures

where gas scattering can enhance surface coverage. Though these methods

appear simple they require advance sophisticated arrangements to control the

film properties. Maissel & Glang (1970).

3.3 CHEMICAL METHODS

There are verities of other important methods available for

preparation of thin films. Methods are chemical vapour deposition, chemical

bath deposition, electro less deposition, electrolytic deposition and

anodization.

3.3.1 Chemical Vapour Deposition (CVD)

Chemical vapour deposition is the deposition of atoms or molecules

by the high temperature reduction or decomposition of chemical vapour

precursor species which contain the material to be deposited between a carrier

gas and an Organo-metallic precursor. It includes hydride chemical vapor

deposition method, trichloride chemical vapor deposition method and metal-

organic chemical vapor deposition method. Chemical vapour deposition is the

condensation of compound or compounds from the gas phase on to a substrate

where reaction occurs to produce a solid deposit .The deposited material may

react with other gaseous species in the system to give compounds. Chemical

vapour deposition processing is generally accompanied by volatile reaction by

products and unused precursor species.

The chemical reaction is initiated at or near the substrate surface,

which produces the desired material in the form of a deposit on the substrate.

30

In some process, the chemical reaction may be activated through an external

agency, such as, application of heat, RF field, light or X-rays, an electric or

glow discharge and electron bombardment. The microstructure and adhesion

of the deposit is strongly influenced by the nature of the chemical reaction and

the activation process.

3.3.2 Chemical bath Deposition (CBD)

The Chemical bath deposition (CBD) method is one of the cheapest

methods to deposit thin films and nanomaterials, as it does not depend on

expensive equipment and is a scalable technique that can be employed for

large area batch processing or continuous. Chemical deposition techniques are

relatively low cost processes and can be easily scaled up for industrial

applications.

Most of the chemical bath consists of one or more metal salts Mn+,

a source for the chalcogenide X (X = S, Se, Te) and typically a complexing

agent, in an aqueous solution. The deposition of metal chalcogenide occurs via

the following four steps.

1. Equilibrium between the complexing agent and water,

2. Formation/dissociation of ionic metal-ligand complexes[ M(L)i]n-ik

3. Hydrolysis of the chalcogenide source; and

4. Formation of the solid.

During the step 3, the metal cations are pulled out of the solution by

the desired non-metal species provided through the hydrolysis of the

chalcogenide source, to form the solid film. The kinetics of the step 3 is highly

sensitive to the solution pH and temperature, as well as to the catalytic effects

of certain solid species that may be present, which in turn decides the rate of

31

formation of thin film on the surface of the substrate or bulk precipitation. The

basic principle involved behind the formation of desired solid film/bulk MmXn

(step 4) is the rising concentration of Xm- from step 3 causes the ionic product

[Mn+]m [Xm-] n to exceed the solubility product. During step 2, the formation of

complexed metal ions allows control over the rate of formation of solid metal

hydroxides, which competes with step 4 and which would otherwise occur

immediately in the normal alkaline solutions. These steps together determine

the composition, growth rate, microstructure and surface topography of the

resulting thin films.

3.3.3 Sol- Gel Method

Out of the different methods available for the preparation of

nanoparticles and nanocrystalline TiO2 thin films, the sol gel method is

simple, inexpensive, non-vacuum and low temperature technique. This sol-gel

process offers many advantages like, excellent control of the stoichiometry of

precursor solutions, eases of compositional modifications, customizable

microstructure, and eases of introducing various functional groups, requires

relatively low annealing temperature and has the possibility of coating over

large area substrates.

The unique property of sol-gel process is the ability to go all the

way from the molecular precursor level to the product level, allowing a better

control of the whole process and the synthesis of tailor made materials for

different applications. Sol-gel method is more suitable to prepare materials

because it permits molecular-level mixing and processing of the raw materials

and precursors at relatively lower temperature and produces nano-structured

bulk, powders and thin films. Hence the sol-gel technique is the very attractive

32

method to produce films for photovoltaic applications for which large-area

films are required at low cost, Pierre et al (2002).

In a typical sol-gel process, independent solid colloidal particles

ranging from 1 nm to 1 m are formed from the hydrolysis and condensation

of the precursors, which are usually inorganic metal salts or metal organic

compounds such as metal alkoxides. It is usually easy to maintain such

particles in a dispersed state in the solvent.

In the next stage, these colloidal particles can be made to link with

each other by further sol condensation, while they are still in the solvent, so as

to build a three-dimensional open grid, termed gel Pierre et al (2002) The

transformation of a sol to a gel constitutes the gelation process (figure 3.1).

Figure 3.1 Sol-Gel formations

33

3.3.4 Sol-Gel Chemistry

The sol and gel formation is based on the hydrolysis and

condensation of precursors. Most work in the sol-gel field has been performed

by the use of alkoxides as precursors. Alkoxides provide a convenient source

solvents, especially alcohol. Alcohols enable a convenient addition of water to

start the reaction. Another advantage of the alkoxide route is the possibility to

control rates by controlling hydrolysis and condensation by chemical means

(Schmidt 1988) with an alkoxides as a precursor, sol-gel chemistry can be

simplified in terms of the following reaction equations (3.1) Livage et al

(1992).

(1) Hydrolysis (hydroxylation) of metal alkoxides:

-M-OR + H2O -M-OH + ROH (3.1)

(3.2)

As shown in the equation 3.2, the mechanism involves

nucleophilic attack of a negatively charged HO - group onto a positively

charged metal M + and transfer of a proton from the water to a negatively

charged OR group of the metal and release of the resulting ROH molecule. As

soon as reactive hydroxyl groups are obtained, the formation of branched

34

oligomers and polymers with a metal oxo based skeleton and reactive residual

hydroxo and alkoxy groups occurs through a polycondensation process.

2) Condensation:

(3.3)

In this case X means either an H or R (an alkyl group). Oxolation is

also a three step nucleophilic substitution reaction which occurs through the

elimination of H2O or ROH. Generally, under a stoichiometric hydrolysis ratio

(H2O/M < 2), the alcohol producing condensation is favored, whereas the

water forming condensation is favoured for large hydrolysis ratios (H2O/M

>>2). The hydrolysis and condensation are responsible in the transformation

of metal alkoxide precursors to a metal oxo macromolecular network. The

recombination of these metal oxo polymers leads to the production of well

dispersed structures which occupy the whole volume. When these oxo

polymers reach macroscopic sizes, the reaction bath becomes a gel, inside

which, the solvent, reaction by products and free polymer are trapped. If the

polymerized structures do not reach macroscopic sizes, sols are produced.

Precipitates are formed if the reactions produce dense rather well dispersed

structures (equation 3.3) (Brinker et al 1990), (Sanchez et al 1994).

Applications for sol-gel process derive from the various special

shapes obtained directly from the gel state (monoliths, films, fibers, and mono

sized powders) combined with compositional and microstructural control and

low processing temperatures. Compared with other methods, such as the solid-

state method, the advantages of using sol-gel process include

35

(1) The use of synthetic chemicals rather than minerals enables high purity

materials to be synthesized.

(2) It involves the use of liquid solutions as mixtures of raw materials. Since

the mixing is with low viscosity liquids, homogenization can be achieved

at a molecular level in a short time.

(3) Since the precursors are well mixed in the solutions, they are likely to be

equally well-mixed at the molecular level when the gel is formed thus on

heating the gel, chemical reaction will be easy and at a low temperature.

3.3.5 Successive Ionic Layer Adsorption (SILAR)

Among the di

simplicity of the successive ionic layer adsorption and reaction (SILAR)

method and its potential application for large area deposition make it very

substrates are immersed into separately placed cationic and anionic precursors

and precipitate formation in the solution, i.e. wastage of the material was thus

avoided. Also, SILAR can be used to deposit compound materials on a variety

of substrates such as insulators, semiconductors, metals.

Adsorption is the basic building block of the successive Ionic layer

adsorption and reaction method. Adsorption is the collection of a substance on

the surface of another one, and is possible due to attractive force between ions

in the solution and surface of the substrate. It is also a surface phenomenon

between ions and surface of the substrate. The forces may be cohesive force or

Van der Waals force or chemical attractive force. Atoms or molecules of

substrate surface possess unbalanced or residual force and that holds the

substrate particles. The successive ionic layer adsorption and reaction is based

36

on sequential reaction at the substrate surface. The substrate is rinsed in

solvent after each reaction, which enables heterogeneous reaction between the

solid phase and the solvated ions in the solution. The successive ionic layer

adsorption and reaction process is intended to grow thin films of water

insoluble ionic or ion covalent compounds by heterogeneous chemical

reaction at the solid solution interface between adsorbed cations and anions.

Figure 3.2 represents the deposition process of thin films using

successive ionic layer adsorption and reaction method. It consists of four

different steps such as adsorption, rinsing, reaction and rinsing. 1) Adsorption:

In the first Step, the cations present in the precursor solution are adsorbed on

the surface of the substrate and form the Helmholtz electric double layer. This

layer is composed of two layers: the inner (positively charged) and outer

(negatively charged) layers.

Figure 3.2 Schematic representation of SILAR method

The positive layer consists of the cations and the negative forms the

counter ions. 2) Rinsing: In this step, excess adsorbed ions are rinsed away

from the diffusion layer. 3) Reaction: In this reaction step, the anions from the

37

anionic precursor solutions are introduced to the system. Due to the low

stability of the material a solid substance is formed on the interface. This

process involves the reaction of cation surface species with the anionic

precursor. 4) Rinsing: In the last step, the excess and un-reacted species and

the reaction by product from the diffusion layer are removed.

After five minutes of drying these steps are repeated again and this

results in the formation of a thin layer of the material. During one complete

cycle the maximum increase in film thickness is theoretically one monolayer.

If the measured growth rate exceeds the lattice constant of the material, a

homogeneous precipitation in the solution should take place. The factors

affecting the growth phenomena are the quality of the precursor solution, pH

value, counter ions, complexing agent, pre-treatment of the substrate,

individual rinsing and dipping times.

3.3.6 Dip coating method

Dip coating technique can be described as a process in which the

substrate is immersed in the solution for a desired period. After that the

substrate is withdrawn with a uniform withdrawal speed. The dip coating is

done under controlled temperature and atmospheric conditions. During sol-gel

thin film formation via dipping, polymeric or particulate inorganic precursors

are concentrated on the substrate surface by a complex process involving

gravitational draining with concurrent drying and continued condensation

reactions. The structure of films deposited from polymeric precursors depends

on such factors as size and structure of the precursors, relative rates of

condensation and evaporation, capillary pressure, and substrate withdrawal

speed. In our present work TiO2 nanocrystalline this films were prepared by

Sol- gel dip drive method.

38

3.4 CHARACTERIZATION TECHNIQUES

3.4.1 X-ray Diffraction (XRD)

The X-ray diffractometer consists of three parts, a basic diffraction

unit, a counter goniometer and an electronic circuit panel with an automatic

recorder as shown in figure 3.3. The diffraction angles and intensity of lines

which describes the conditions for constructive interference of X-rays

scattered from atomic planes of a crystal. The condition for constructive

-rays, d is the

-

rays. The factor d is the related to the (h k l) indices of the planes and the

dimension of the unit cells. It is therefore seen that

Figure 3.3 Schematic of XRD measurement

the diffraction direction is solely determined by the structure and size of the

unit cell.

The cr

cos0.94D .

39

relation tancosD

as the length of

the dislocation lines per unit volume of the crystal can be evaluated from the

relation 2D1 . The intensities of the diffracted beams depend on the possible

diffraction directions and the lattice parameters.

3.4.2 Optical properties

3.4.2.1 Introduction

Optical properties of thin films generally deals with absorption,

transmission, reflection, refractive index (n), extinction coefficient (k) and

absorption coefficient . Total energy incident on a thin film is conserved

through various processes namely, reflection, transmission and absorption. In

some cases scattering of light also occurs.

Optical properties of a solid emanate from its interaction with

electromagnetic waves and are manifested in optical frequencies. These

interactions may cause several transitions in its band structure such as band to

band, between sub bands or impurity levels and a band, transitions of free

carrier within a band and also resonance due to lattice vibrations. Hence a

detailed study of the absorption band spectra is likely to provide very good

information about the electronic band structure of thin films (Goswami 1996).

Thus the knowledge of optical properties of solid films has widely contributed

to the phenomenal growth of their applications in scientific technological and

industrial applications. Thin films are being used in optical devices, such as,

mirror coatings, interference filters, antireflection coatings, absorption filters,

optical and thermal detectors etc., Nath & Chopra (1973). Moreover, optical

characteristics of films are strongly influenced by the process parameters and

40

the deposition method. Optical films are primarily characterized by

absorption/transmittance and refractive index. The absorption/transmittance

versus wavelength graph can be divided basically into three regions (1) UV,

(2) Visible and NIR and (3) IR and far-IR. The desired region of high

absorption/transmittance is located in the second region and it strongly

depends on the material purity and stoichiometry. In region 1 the absorption

depends on electronic structure of the material and in region 3 it depends on

lattice vibrations or in the case of semiconductors, on free carrier absorption.

In general, mixtures of various compounds evaporate non

uniformly. Films with perfect optical homogeneity are rare in homogenities

with gradual increase or decreases of refractive index with film thickness are

more compared to in homogenities with abrupt changes and it is due to

accidental changes in the deposition parameters, like pressure rate,

temperature etc.,

3.4.2.2 Experimental details

The electromagnetic wave fields vary periodically with an angular

-direction with a complex

= j ( )1/2 = + i (3.1)

where is the magnetic permeability, the permittivity the

attenuation factor and the phase factor. The phase velocity of the wave is

given by

)1/2 (3.2)

41

In vacuum this wave travels with a phase velocity equal to that of

light (c) while it moves with a lesser velocity in any other material medium.

The refractive index of the medium is given by the ratio of these two velocities

as

)1/2 (3.3)

For a no 3.3) becomes

n = ( ) 1/2 or n2 = (3.4)

being a complex quantity it can be replaced by *,

where * = '- j " and by similar reasoning should be replaced by

*. Then the refractive index n can also be expressed as complex quantity n*

that is

n* = (n+jk) = ( *)1/2 = ( '- j ")1/2 (3.5)

where n and k are respectively the real and imaginary components

of n* and are known as the refractive and absorption indices.

From equation (3.5)

' = n2 k2 and " = 2nk (3.6)

where ' and " are termed as the optical dielectric constants of the

material. These equations reveal that both the dielectric constants ( ' and ")

and optical constants (n and k) are interrelated.

42

Further n and k can be evaluated from

21

21

1]tan21[21n (3.7)

21

21

21

1tan21

1tan21k ( 3.8)

Another important parameter, the absorption coefficient is

calculated using the transmittance (T) value measured for a particular

wavelength and the film thickness (t) using the relation

tTln

(3.9)

0eII (3.10)

where x is the distance through which the electromagnetic wave

travels to change its intensity from I0 to I. The absorption index or the

extinction coefficient k which is the attenuation per unit radian may be written

as

4

k (3.11)

The absorption of radiation that gives rise to transition of electrons between

the valence band and conduction band is of two types.

a) Direct Transition: The necessary condition for a direct transition to take

43

place is that in the excitation process no change in the k-values of the

electron should occur. The following dependencies are observed during

this transition

(Ev - Ei) ½ for allowed transitions

(Ev - Ei) 3/2 for forbidden transitions

v is the energy of the top of

the valence band and Ei is the energy of the initial state from which the

transition is made.

a) Indirect Transition: The transition involving a change in the crystal

momentum is termed as indirect transition. In this case absorption of

both a photon and a phonon or the absorption of a photon and the

emission of a phonon take place. The following dependencies are

observed during this indirect excitation.

(Ev - Ei)2 for allowed transitions

(Ev - Ei)3 for forbidden transitions

The refractive index of thin films often differs from that of bulk

material. The refractive index of a material at optical frequencies is mainly

determined by polarizability of the valence electrons. In compounds the type

of bonding also influences the index. The analysis of the wavelength

dependence of the optical constants n and k is of considerable interest due its

optoelectronic applications. The optical parameters can be estimated by means

of the equations corresponding to the propagation of electromagnetic waves

through the layers of plane-parallel faces consisting of an absorbing thin

semiconductor film and a transparent glass substrate. The value of k can be

evaluated from the formula.

44

tk4 (3.12)

and the refractive index.

cos22)(1n)(1)(1nT

1122

210

22

212 (3.13)

Where 210

21

20

1 )n(nnn , 2

21

22

21

2 )n(nnn and tn 2 1 .

0, n1 and n2 are

the refractive index of air, film and glass respectively. Substituting the

experimental values for T, A, t n0 and n2 in the above equations can lead to

solving the values of n1 and k1 using the method with different iteration till the

desired convergence can be achieved.

3.4.3 Thickness Measurement

Thickness is one of the most important parameter, which influences

various properties of the films. Hence a very accurate measurement of

thickness is vital. Various techniques are adopted for the measurement of

thickness up to a fair accuracy. In the present study the following two methods

are adopted to determine the thickness and they are cross-verified for

accuracy. The multiple beam interferometry is adopted in the present work

because of its simplicity and accuracy. When a wedge of small angle is formed

between un-silvered glass plate illuminated by monochromatic light as shown

in figure 3.4, broad fringes are seen arising from the glass on the two sides of

45

are measured and the thickness (t) is deduced from the equation 2

t ,

matic light.

Figure 3.4 Multiple beam interferometer and Fizeau fringes

Films are deposited on to glass substrate and a silvered thread

having fine edge (flat type) is tied to the substrate to form a channel. An over

coating of silver is given to produce a lower reflecting plane, which is forming

the step profile in multiple beam interferometer. A parallel beam of

monochromatic light is used at normal incidence to form Fizeau fringes. The

fringe spacing and the displacement measurement are found out and the

thickness of the film is calculated by using traveling microscope of least count

0.001 cm. The average of the calculated thickness values at different points is

taken to be the thickness of the film. It is checked with gravimetric method.

46

3.4.4 Spectrophotometer

The essential components of a spectrophotometer (as shown in

Figure 3.5) include: (1) a stable source of radiant energy, (2) a monochromator

to resolve the radiation into component wavelengths or bands of wavelengths,

(3) sample compartment and (4) a radiation detector.

3.4.4.1 Sources of radiation

A tungsten filament lamp is the most satisfactory and inexpensive

source of visible and infrared radiation. The filament is heated by a stabilized

D.C. power supply, or by a storage battery. The tungsten filament emits

continuous radiation in the region between 350 nm and 2500 nm. For source

of ultraviolet radiation, the hydrogen lamp and deuterium lamp are used. They

consist of a pair of electrodes, which are enclosed, in a glass tube provided

with a quartz window and filled with hydrogen or deuterium gas at low

pressure. When a stabilized high voltage is applied to the electrodes, an

electron discharge occurs which excites other electrons in the gas molecules to

high-energy states. As the electrons return to their ground state they emit

radiation, which is continuous in the region roughly between 190 nm and 350

nm. Similarly, a xenon discharge lamp is used as a source of ultraviolet ration.

The xenon lamp produces higher intensity radiation, but it is not as stable as

the hydrogen lamp. It also emits visible radiation, which may appear as stray

radiation in ultraviolet applications.

3.4.4.2 Monochromator

The sources of radiation commonly used to emit continues radiation

over wide ranges of wavelengths. However, narrow band widths have many

advantages: (1) narrow band radiation will allow the resolution of absorption

47

bands which are quite close to each other, (2) with narrow band radiation a

peak may be measured at its absorption maximum, thus increasing the

sensitivity and (3) the absorption of narrow band radiation will tend to show

absorbed is measured. Monochromators resolve wide band polychromatic

radiation from, the source into narrow bands or monochromatic radiation with

monochromator include: (1) an entrance slit which admits polychromatic

radiation from source; (2) a collimating device, either a lens or a mirror; (3) a

dispersion device, either a prism or grating which resolves the radiation into

component wavelengths; (4) a focusing lens or mirror and (5) an exit slit. The

effective bandwidth of radiation emerging from the monochromator depends

on several factors, including the dispersing element and the slit widths of both

the entrance and exits slits. Narrow slit widths isolate narrow bands; however,

the slit width also limits the radiant power, which reaches the detector.

Therefore, the minimum bandwidth may be determined by the sensitivity of

the detector.

3.4.4.3 Detection devices

Any detector absorbs the energy of the photons, which strike it and

converts this energy to a measurable quantity such as the darkening of a

photographic plate and electric current or thermal changes. Most modern

detectors generate an electric signal. Any detector must generate a signal

which is quantitatively related to the radiant power striking it. The noise of a

detector refers to the background signal generated when no radiant power

from the sample reaches the detector. This noise may be caused either by

random changes within the detector itself or by electrical pick up of other

48

signals in the vicinity of the detector unit. Important requirements for detectors

include: (1) high sensitivity with a low noise level in order to allow the

detection of low levels of radiant power, (2) short response time, (3) long term

stability to insure quantitative response and (4) an electronic signal which is

easily amplified for typical readout apparatus and exits slits. Narrow slit

widths isolate narrow bands; however, the slit width also limits the radiant

power, which reaches the detector. Therefore, the minimum bandwidth may be

determined by the sensitivity. Ultraviolet and visible photons possess enough

energy to cause photo-ejection of electron when they strike surfaces, which

have been treated with specific types of compound. Their absorption may also

cause bound, non-conduction electrons to move into conduction bands in

certain semiconductors. Both processes generate an electric current, which is

directly proportional to the radiant power of the absorbed photons. Device,

which employs these systems, are called photoelectric detectors and are sub-

classified as phototubes and photovoltaic cells.

Figure 3.5 Block diagram of Spectrophotometer.

49

3.4.4.4 Refractive Index

The refractive index of thin films often differs from that of bulk material.

The refractive index of a material at optical frequencies is mainly determined by

polarizability of the valence electrons. In compounds the type of bonding also

influences the refractive index. The analysis of the wavelength dependence of the

optical constant refractive index (n) is of considerable interest due to its opto-

electronic applications.

The optical parameters can be estimated by means of the equations

corresponding to the propagation of electromagnetic waves through the layers of

plane-parallel faces consisting of an absorbing thin semiconductor film and a

transparent glass substrate. The refractive index can be calculated from the

transmission spectra using the interference fringe region by the method of Manifacier

et al. In this method the maximum transmittance (Tmax) and minimum transmittance

(Tmin) are considered to be continuous functions of wave length through the refractive

envelopes of the maxima Tmax minima Tmin

refractive indices of the films are calculated using the relations by (Manifacier et al

1976)

21

21

20

22 )nn(NNn (3.14)

minmax

minmax1021

20

TT)T(Tn2n

2nnN (3.15)

where no and n1 are the refractive indices of air and glass respectively.

50

3.4.5 MORPHOLOGY

Morphology gives the form and structure of a surface. Surface

roughness is one of the important parameters; it affects the light absorption

capacity. Substrate roughness affects the morphology and growth kinetics in

three primary ways namely, nucleation, impurity diffusion and sometimes by

acting as a metallurgical shunt. The following paragraphs discusses in details

about the techniques to study the morphology of deposited films.

3.4.5.1 Scanning Electron Microscopy (SEM)

SEM is a useful technique for the direct observation of surfaces,

employed to predict the growth mechanisms leading to reminiscent

structures. In a SEM analysis, the areas or micro-volumes to be examined

are irradiated with a fine electron beam produced by the electron gun and

focussed by electron lenses. Scanning coils deflect this beam and sweeps it

over the film surface. A cathode ray tube is scanned synchronously with the

electron beam Brightness of display tube is modulated by the signal which

arises from interactions of the beam with film surface. The strength of this

signal is thus translated into image contrast.

The types of signals produced when electron beam impinges on

specimen surface include secondary electrons, Auger electrons,

characteristic X-rays and photons of various energies. These signals are

obtained from specific emission volumes within the samples, which

ultimately determine surface topography, crystallography, composition, etc.

In the present work surface morphology of the deposited TiO2 films were

analyzed by scanning electron microscopy Hitachi S-3400 N SEM.

51

3.4.5.2 Energy Dispersive Analysis of X-rays (EDAX)

EDAX helps to determine the elemental contents in sample. In

this technique, an energetic beam of electrons is allowed to be incident on the

film. These incident electrons interact in elastically with both the inner shell

electrons and outer shell electrons of the atoms of thin sample material

generating X-rays. Outer shell electrons generate soft X-rays due to this

interaction whereas innermost shells generate characteristic X-rays, which

depend on energies of these shells and hence are characteristic of atoms

radiating these X-rays. Hence by analyzing the energy of these characteristic x-

rays, typical of which are K , K , L , L etc., information about the type of

atoms present in the sample and their concentration can be determined. In the

present study JEOL 840 SEM-EDX has been used to determine the

elemental contents present in the prepared, TiO2 thin films.

3.4.6 Fourier Transform Infra-Red Spectroscopy (FTIR)

Fourier transform infrared spectroscopy has become the standard

technique for chemical characterisation. Infrared spectroscopy is one of the

most powerful tools available for identifying organic and inorganic

compounds. Indeed most molecular species absorb infrared radiation. It is

based on the fact that the absorbed radiation stimulates molecular vibration.

These vibrations are characteristics for each organic functionality, such as

methyl or aldehyde groups for example. Each molecular species has a unique

infrared absorption spectrum. Unlike a dispersive instrument (grating

monochromator or spectrograph) an FTIR spectrometer collects all

wavelengths simultaneously. This feature is called the Multiplex or Felgett

advantage. Fourier transform infra-red spectrometer collects and digitizes the

interferogram, performs the FT function and displays the spectrum.

52

Fourier transform infrared spectrometer is typically based on a

Michelson interferometer. It consists of a beam splitter, a fixed mirror and a

mirror that transmits back and forth, very precisely. The beam splitter is made

of a special material that transmits half of the radiation striking it and reflects

the other half. Radiation from the source strikes the beam splitter and

separates into two beams. One beam is transmitted through the beam splitter to

the fixed mirror and the second is reflected off the beam splitter to the moving

mirror. The fixed and moving mirrors reflect the radiation back to the beam

splitter. Again, half of this reflected radiation is transmitted and half is

reflected back at the beam splitter, resulting in one beam passing to the

detector and the second back to the source. The quality of FTIR spectrum is

dependent on sample contact rather than sample thickness. In the present work

the samples were analysed using the instrument ABB Bommn, MB 3000,

Canada.

3.4.7 Atomic Force Microscopy (AFM)

The Atomic Force Microscope consists of a cantilever with a sharp

tip at its end and it is used to scan the specimen surface. The cantilever is

typically silicon or silicon nitride with a tip radius of curvature in the order of

nanometre. When the tip is brought into proximity of a sample surface, forces

between the tip and the sample lead to a deflection of the cantilever according

include mechanical contact force, vander waals forces, capillary forces, etc.

Along with force, additional quantities may simultaneously be measured

through the use of specialized types of probe. Typically, the deflection is

measured using a laser spot reflected from the top surface of the cantilever

into an array of photodiodes. Using a Wheatstone bridge, strain in the AFM

cantilever due to deflection can be measured.

53

If the tip was scanned at a constant height, a risk would exist so as

the tip collides with the surface, causing damage. Hence, in most cases a

feedback mechanism is employed to adjust the tip-to-sample distance to

maintain a constant force between the tip and the sample. Traditionally, the

sample is mounted on a piezoelectric tube, which can move the sample in the

direction for maintaining a constant force, and the and y directions for

crystals may be employed, with each responsible for scanning in the x, y and z

directions. This eliminates some of the distortion effects seen with a tube

scanner. In newer designs, the tip is mounted on a vertical piezo scanner while

the sample is being scanned in X and Y using another piezo block. The

resulting map of the area z = f(x, y) represents the topography of the sample.

The AFM can be operated in a number of modes, depending on the

application. In general, called contact mode and a variety of dynamic (or non-

contact) modes where the cantilever is vibrated.