3.1 Introduction - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/22827/1...dark lines against a white background with a fringe width which can be made as small as λ /100. By

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

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3.1 Introduction

The properties of the thin films and aspects of growth mechanism can be well

understood by its characterization of the films. The characterization techniques used for

the thin films are thickness measurement, X-ray diffraction (XRD), scanning electron

microscopy (SEM), energy dispersive analysis of X-ray (EDAX), optical absorption and

electrical resistivity measurement techniques were utilized in the present study. Table 3.1

summarizes different characterization techniques used in the present study with their

utility to find out various properties.

Characterization technique Properties studied

Fizeau Interferometer Thickness measurement

X-ray Diffractometer (XRD) Crystal structure, Crystallite size

and lattice parameters.

Surface Electron Microscope (SEM) Surface morphology

Energy Dispersive Analysis of X-ray

(EDAX)

Quantitative analysis / Elemental

analysis

Ultra violet-Vis spectrometer Optical absorption coefficient,

transmission and energy band gap,

etc.

Two-probe electrical resistivity Resistivity/ activation energy

Table 3.1: Summary of different characterization techniques.

----------------------------------------------------------------------------- Ph. D. Thesis submitted by Mr. Jagannath Babu Chaudhari 53


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3.2 Thickness Measurements

The film thickness being one of the important parameter, which affects the

properties of the thin film, is measured by various methods viz., weight difference,

interference fringes and ellipsometry method.

3.2.1 Weight difference method

Name of the measurement system itself defines its aspect of calculation of

thickness where the weight of the substrate is measured before and after deposition of

material hence the weight of material is calculated. But only the weight of material is not

significant to be calculated hence it requires knowing the dimensions of deposition also,

i.e., area of the substrate. Suppose that ‘m’ is a mass of the film deposited on the

substrate which covers area ‘A’ cm2, the thickness ‘t’ is calculated by using mass of

deposit ‘m’ and density ‘ρ’ of the material. The value pertaining to the bulk material is

usually taken for ‘ρ’ even though the actual density of thin film is low. The thickness is

calculated using the equation [1].

(3.1) ⎥⎦

⎢⎣

=ρA

t ⎤⎡ m

The mass ‘m’ of the film deposited was measured by using a single pan sensitive

electronic microbalance.

3.2.2 Interference fringes method

When two reflecting surfaces are brought into close proximity, interference

fringes are produced, the measurement of which makes possible a direct determination of

the film thickness and surface topography with high accuracy [1].

Two types of fringes are utilized for thickness measurements, (a) Fizeau method

of fringes of constant thickness using a multiple beam and (b) Fringes of equal chromatic

order method (FECO). The Fizeau fringes of equal thickness are obtained in an optical

apparatus of the type shown in Fig. 3.1. The interferometer consists of two slightly



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inclined optical flats, one of them supporting the film, which forms a step on the

substrate. When the second optical flat is brought in contact with the film surface, and

the interferometer is illuminated with a parallel monochromatic beam at a normal

incidence and viewed with a low-power microscope, dark fringes can be observed which

trace out the points of equal air-gap thickness. The two adjacent fringes are separated by

λ/2 (where λ is the wavelength of light used). If the surfaces of the optical flats are highly

reflecting and very close to each other, the reflected fringe system consists of very fine

dark lines against a white background with a fringe width which can be made as small as

λ /100. By adjusting the relative positions of the flats to form a wedge-shaped air gap, the

fringes can be made to run in straight lines perpendicular to the steps on the opaque film.

The fringes show a displacement expressed as a fraction of the λ/2 fringe spacing gives

the film thickness and it can be measured to about tenth of a fringe.

In second method, instead of the air wedge, two parallel plates illuminated with

white light are used. Fringes occur at wavelength for which t/λ is constant so that

resultant spectrum is banded. This method is called as “Fringes of Equal Chromatic

Order (FECO)”. In reflection, fringes appear dark on bright background. If one of the

plates is covered by a film, a displacement ∆λ is seen in the nth order fringe, from which

film thickness can be calculated as [1], Ο

Α∆

=λ2

mt (3.2)

3.2.3 Ellipsometry method

Ellipsometry provides a non-destructive method for measuring thickness of films.

This technique has also been called polarimetry. A plane-polarized light reflected from

an absorbing substrate at non-normal incidence assumes elliptical polarization. The

ellipticity (the ratio of minor to major axis) of the reflected beam is determined by the



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relative phase difference ‘∆’ and the azimuth, is the ratio of the reflection amplitudes of

the parallel and perpendicular components of a reflected beam.

The optical constant (n0, k0) of the reflecting surface can be determined from the

values of ∆ and ψ, as given by the equation below,

(3.3) ( )( ) ⎥

⎦

⎤⎢⎣

⎡∆+

∆−+=− 2

2221

2

122

122

cos2sin1sin2sin2costan1sin

ψψψφϕnkn oo

(3.4)

The experimental arrangement utilized to measure ∆ and ψ is called an

“ellipsometer”. A schematic arrangement of a typical ellipsometer is shown in Fig. 3.2.

Analysis of the ellipticity of the reflected beam is carried out by any of the standard

methods. A commonly used arrangement [2] employs a plane-polarized incident light

with its plane of polarization at 450 to the plane of incidence. The reflected light passes

through a compensator and an analyzer, which are rotated to give the smallest intensity

of the reflected light. The vibration direction of the compensator determines the angle

‘X’ between the great semi-axis of the ellipse and the plane of incidence. The difference

between the position of the compensator and the analyzer gives the ellipticity angle γ.

The values of ψ and ∆ can be determined from the relations

( )21

21

221

cos2sin1sinϕϕn

(3.5)

and (3.6)

2cos2sintansin∆+

∆=

ψψψkn oo

One can also set the ana

until the detector gives the min

this case is called the ‘principa

plane of polarization and the pla

--------------------------------------------------Ph. D. Thesis submitted by Mr. Jaganna

sin2 γ =sin2ψ.sin∆

tan2x = tan2ψ.cos∆

lyzer at 450 by rotating the polarizer and the compensator

imum of the reflected light. The angle of incidence for

l angle of incidence φ1’ and angle between the restored

ne of incidence is the ‘principal azimuth ψ’.

--------------------------- th Babu Chaudhari 56


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(a) Fizeau

(b) FECO

Fig. 3.1: Diagram of optical system of interferometer (a) Fizeau meth

f fringes of constant thickness using a multiple beam, (b) Fringes ofequal chromatic order method (FECO) [1].

od o

Under this condition the major and minor axes of the polarization ellipse of the

reflected light lie in and perpendicular to the plane of incidence. Equation (3.3) and (3.4)

then simplify to

ψφφ )4costan1(sin 22

12

02

02 nkn +=−

T

direct ca

--------------Ph. D. The

ψψφφ 2cos2sintansin 12

12

12

00

11

nkn =

he general Drude equation [Equations (3.3) and (3.4)] as such do

lculations of the refractive indices and thickness from the values of

--------------------------------------------------------------- sis submitted by Mr. Jagannath Babu Chaudhari

(3.7)

(3.8)

not allow

∆ and ψ.

57


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Vasicek [2] developed a solution for a transparent film on a glass and obtained a table of

values of ψ and ∆ for the films of thickness up to 1 to 1.5 µm and for refractive indices

of 1.2 to 2.75 for a substrate of index 1.5163 and λ = 5.890 Å. Computations of the

optical constants are conveniently handled by an electronic computer Archer [3] and

prepared a chart of ∆ and ψ values of a range of thicknesses and indices of films on a

silicon substrate. Thus a given point in the (∆, ψ) chart uniquely defines a film thickness

and index. In the present study, film thicknesses were measured by the laser

ellipsometry, with an incident angle of 700 and wavelength 632.8 nm.

Fig. 3.2 Schematic representation of an ellipsometer (polarization spectrometer) along with related components [1]

3.3 X –Ray Diffraction (XRD)

X-ray diffraction (XRD) technique is most important analytical tool for the thin

film structural analysis. Diffraction of the coherent radiation by three dimensional arrays

of atoms in the crystal was first observed by Friedrich, Knipping and Laue [3] while

experimenting with X-rays. At the same time Bragg [4] observed the reflection of X-rays



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by crystal and stated that the incident beam was reflected by a set of lattice planes (hkl)

if,

(3.9) λθ nd =sin2

where ‘d’ is lattice spacing, ‘λ’ is wavelength of monochromatic X-rays, ‘θ’ is the angle

between incident beam and the planes (hkl), ‘n’ is the order of reflection (n = 1, 2,

3,…..). By using the X-rays of known wavelength and measuring the angle of diffraction

of the most intense peak, it is possible to determine the interplanar spacing‘d’ in the

crystal [5-7]. These values of ‘d’ can then be compared with known values. The closest

matching of the value directly indicates the predominant phase present, with its

magnitude. The powder technique in conjunction with diffractometer is most commonly

used for thin films. In this instrument, the diffracted radiations are detected by counter

tube moving along with angular range of the reflection and intensities are recorded by

computer system in synchronously advancing strip charts. In Braggs Brentano

diffractometer, the specimen is mounted at the center of diffractometer. It is rotated by an

angle around the axis in thin film plane and counter is rotated about the same axis by an

angle of ‘2θ’.

The ‘d’ value calculated using the Bragg’s equation for known value of θ, λ and

n. The x-ray diffraction data thus obtained is printed in tubular form on paper and is

compared with Joint Committee on Powder Diffraction Standard (JCPDS) or American

Standard Testing for Materials (ASTM) data cards. This x-ray diffraction data can also

be used to determine the particle size, structural factors, residual stresses, miller indices,

etc. [8].

3.3.1 Indexing for polycrystalline films

A given material always produce unique diffraction pattern of its own, whether

that material is present in pure form or as one constituent of mixture of materials. The



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powder pattern of material is characteristic of that material and forms a sort of

fingerprints by using those patterns the material can be identified. Every powder pattern

is characterized by the set of lines positions ‘2θ’ and set of relative line intensities ‘I’.

But the angular positions of these lines depend upon the wavelength used, and on a more

fundamental quantity is interplanar spacing ‘d’ of the lattice planes forming each line.

Hanawalt and co-workers [9,10] found that it was possible to code all crystalline solids

on the basis of inter-planer spacing or d-spacing as calculated from Bragg’s equation

and the relative intensity of the peaks that appears in the diffraction pattern. The

thousands of the substances are covered by the index that supplied by American Society

for Testing of Material (ASTM) or Joint Committee on Powder Diffraction Standard

(JCPDS) [11]. Sometimes a compound with same chemical composition can be

crystalline in different states. If a crystalline sample is chemically pure, it can be

identified in its present state by matching its diffraction pattern with JCPDS or ASTM

cards. A given substance in its pure state or in mixture of substances always produces a

characteristic diffraction pattern and can be used in diffraction method of chemical

analysis. Qualitative and quantitative analysis are also possible, because the intensities of

diffraction lines due to one phase of the mixture depend upon proportion of that phase in

mixture. Detail treatments of chemical analysis by x-ray diffraction were given by Klug

and Alexander [12] and Zwell and Danko [13].

Fig. 3.3: Schematic diagram of x-ray diffraction

Incident X-rays Scattered beam Detector

Sample2Θ



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3.3.2 Crystallite size determination from the broadening

Scherrer [8] first showed that the mean dimension ‘D’, of the crystallite

composing a powder is related to pure x-ray diffraction broadening ‘β’, by the equation,

(3.10)

where ‘k’ is constant approximately equal to unity and related both to crystalline

shape and to the way in which ‘β’ and ‘D’ are defined. The quantity ‘β’ is to be carefully

distinguished from the breadth of diffraction line as actually observed under given

experimental condition. ‘β’ is the pure breadth of powder reflection free of all

broadening due to the experimental method employed in observing it. Some investigators

defined ‘β’ as angular width at half maxima intensity.

The quantity ‘k’ in the Scherrer formula (here ‘β’ is in radians)

(3.11)

assumes various numerical values ranging from as small as 0.70 to as much as 1.70

depending on number of factors,

• The crystallite shape,

• The indices (h k l) of the reflecting plane,

• The particular definition of β that is adopted. (Whether the half maxima or the

integral breadth of peak) and

• The particular definition of D, the crystallite dimensions that is adopted.

The four factors listed above govern the value of ‘k’. The crystallite shapes is in

most cases initially unknown and further more the shape probably varies considerably

from one crystallite to other crystallite for number of substances. It is not possible to

obtain the approximate value of the absolute dimension.

An useful approach to the subject of crystalline size determination is based upon

the determination of ‘d’ as the cube root of value of crystallite, V1/3, a concept induced by

θβλ

cosD =

k

0.cos

Dθβ

Akλ=



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Murdock. This is very satisfactory approach when the crystallite dimensions are of the

same order of size, when the linear dimension ‘d’ is taken as the diameter.

For half maxima breadth β1/2, k = 1.107.

Since the diameter of the sphere is related to its volume by,

(3.12) ddV 0.3 =⎟⎞

⎜⎛ Π= 806.

631

1

⎠⎝

The corresponding ‘k’ is smaller by the factor 0.806, when the ‘D’ is defined as cube

root of the volume.

In fact, in very few cases there would be an error exceeding 10 percent be incurred. If a

constant value of k = 0.9 were employed,

(3.13) Ο

Α=θβλ

cos.9.0

.D

3.4 Electron Microscope (EM)

3.4.1 Scanning electron microscope (SEM)

This is one of the most useful and versatile instrument for investigation of surface

topography, micro structural features, etc. of material in thin film form. [15–17]. The

basic function of scanning electron microscope is to produce an image of three

dimensional appearance derived from the action of an electron scanning beam across the

surface of the specimen. The resolution is better than 6 nm. Under suitable condition, the

depth of focus that is at least 300 times greater than an optical microscope can be

achieved. The SEM can have magnification ranging from few times to several hundred

thousands times. The upper magnification is limited only by the resolution available.

SEM is primarily used to study the surface or near surface structure of the bulk

specimens.

The SEM technique is used primarily for the examination of thick (electron

opaque) samples. Electron which are emitted or back scattered from the specimen are

collected to provide,



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• Topological information (i.e. detail shape of specimen surface) if the low energy

secondary electrons (< 50 eV) are collected and

• Atomic number and orientation information if higher energy back scattered

electrons are used, or if the leakage current to earth is used.

The sharpness and contrast of SEM micrographs and depth of field in images are

depending upon three major electron beam parameters,

• Electron probe size,

• Electron probe current,

• Electron probe convergence angle.

For highest resolution image, electron probe size must be as small as possible.

For the best image quality and x-ray microanalysis, electron probe current must be as

large as possible. For the best depth of field where the large range of heights on the

specimen appears in focus, convergence angle must be as small as possible however the

probe current will be inescapably reduced when the probe size and angle of convergence

are made very small. Small beam angle can be achieved by using small objective lens

aperture and large working distance or both. Therefore, these parameters must be

selected properly to obtain the appreciable imaging condition in each situation [18, 19].

Since the image is formed by secondary electron emission, the sample must be

conducting. However, the surface of nonconducting material can be studied by coating

the material surface with gold, palladium film. The thickness of gold film is usually 500

to 1000 Å [20]. The samples were then loaded in the sample holder of scanning electron

microscope unit.

SEM micrographs were also used to calculate the grain size of the film by using

Cottrell’s method [21]. This method relates the number of intercept of grain boundaries

per unit length ‘PL’ and is given by,



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(3.14) Mr

nP ⎥⎤

⎢⎡=L .2 ⎦⎣ π

where ‘n’ is the total number of intercepts and ‘M’ is the magnification and ‘r’ is the

radius of the circle drawn. Using PL grain size ‘L’ can be calculated by using the formula

as,

(3.15)

A schematic diagram of scanning electron microscope is shown in Fig. 3.4.

11−

=LP

L

Fig. 3.4 Schematic diagram of scanning electron microscope [22]

3.4.2 Energy dispersive x-ray analysis (EDAX)

Energy dispersive x-ray analysis is the powerful tool for the compositional

analysis of thin film materials in the sample. As the electron beam scans the surface of

sample, the emerging x-ray is supplied to the x-ray analyzer, which has been set for

element of interest. Whenever the x-ray of that energy is detected, a dot is brightened at



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the corresponding point on the display tube of the SEM. By positioning the SEM

electron beam on the spot of interest, a simultaneous semi qualitative analysis of all the

elements can be performed by plotting the count versus energy (keV). The heights of the

peak are not exact measure of concentration because not all elements are excited with

same efficiency.

For a more qualitative measurement, known element standard must be employed

[23]. However, the modern equipments having the computer interface are capable to

determine the sample composition very accurately using the software stored element

standards.

3.5 Optical Properties

Optical properties of solid emanate from its interactions with electromagnetic

waves and are manifested in optical frequencies. The effect of such interaction in optical

frequencies results in optical behavior. Optical properties of film have been studied

extensively primarily because of there application in various optical and optoelectronic

devices. It has been found that there is considerable deviation of optical parameters from

that of the bulk material. The optical study of a solid, concern not only with physical

phenomenon like reflection, refraction, absorption, transmission and interference of light

but also on interaction of photon energy with matter and consequent change in there

electronic states. The study of optical properties of solids helps in understanding of

electronic and atomic structure of these materials [24]. Absorption studies provide

simple means for the evaluation of absorption edge, optical energy band, optical

transition that may be direct or indirect, allowed or forbidden and also of the nature of

the solid material.



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A simple way to determine the optical properties of solid is by illuminating the

sample with light and then measure the reflection, transmittance or absorbance as a

function of photon energy [25]. This was experimented by spectroscopic methods.

3.5.1 Ultraviolet-visible (UV-Vis) spectroscopy

The ultraviolet region of spectrum is generally considered to the range from 200

to 400 nm and visible range from 400 to 800 nm. The corresponding energies from these

regions are about 150 to 72 and 72 to 36 K cal moles-1, respectively. The energy of these

magnitudes corresponds to electronic state of many molecules.

In UV-VIS spectroscopic method, to measure absorption spectra, which are

primarily due to light absorption resulting from excitation of electrons in atoms or

molecules is important study to the optical properties of the material [26]. The physical

measurement of molecular absorption of ultra violet light involves passing of light

through the material. The radiant power of incident beam of the light can be denoted by

I0 and the power of transmitted beam is I. Using these two terms it is possible to define

the transmittance ‘T’ as,

(3.16) 0I

IT =

But the radiation in molecules of the material is logarithmically related to transmittance,

so absorbance ‘A’ and transmittance ‘T’ are related by,

(3.17) ( )TA log−=

Lambert-Beer law basically gives qualitative absorbance measurement. According to

which the transmitted light intensity through non-reflecting medium is given by [26],

(3.18) cmTln −=α

e

t

T t

1

−

−

= α



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where ‘t’ and ‘α’ are thickness and absorption coefficient of medium, respectively.

Absorption coefficient is a measure of energy attenuation or losses, when it

travels through the material. The main source of absorption in solid is electrons. The

spectrophotometer is an instrument capable of producing transmittance (T) or absorbance

(αt) as a function of wavelength (λ). In the present study, optical absorbance of the films

were recorded using PerkinElmer UV-Vis spectrophotometer Lambda 25, which is like

most of the spectrophotometers, measures ‘I0’ and ‘I’ simultaneously. The computer

interface then calculate the absorbance ‘A’ which is given by log10(I0/I) and is plotted as

a function of wavelength. Spectrophotometer provides an option to plot the transmission

spectra (i.e., percentage transmission T as a function of wavelength). Film were grown

on glass substrate, the substrate absorption was corrected by the instrument’s computer

interface, taking measurements with reference to another similar glass slide.

3.5.2 Optical band gap

Due to the optical (photon) absorption, the equilibrium situation in semiconductor

material can be disturbed by generation of carriers [27]. Optical photon incident on any

material may get either reflected or absorbed or transmitted. The phenomenon of optical

absorption in the material is altogether considered to be due to (i) inner cell electrons, (ii)

valence band electrons, (iii) free carrier including holes as well as electrons and (iv)

electron bound to localized impurity center or defects of same type.

An ideal semiconductor, at the absolute zero temperature, has valence band

completely filled with electrons, so electrons from the valence band could not be excited

to higher energy state. The absorption of the quanta of sufficient energy tends to transfer

electrons from valence band to conduction band. This band-to-band transition is the main

contributor to absorption process in solids [28]. In crystalline materials, the transition of



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electrons from valence band to conduction band can be divided in two types, i.e., direct

and indirect process, depending upon the position of conduction band minima (CBM)

respective to valence band maxima (VBM) in K-space, (i) in direct inter band optical

transition; the wave vector ‘k’ for element remains unchanged in E-k space. The direct

transition is possible when CBM and VBM are at the same k and involves only photons.

Here momentum also does not change and conservation of energy is given by,

(3.19) νhEE PhotongapE ===∆

In indirect inter band optical transition the wave vector k of element is changed in

E–k space. The indirect transition is possible when CBM and VBM are at different point

in k-space. The energy and momentum conservation in such transition can be explained

by the emission or the absorption of phonons (i.e. process involves a simultaneous

interaction with lattice vibration [29–31].

The conservation of energy is given by,

(3.20)

where ‘+’ for phonon absorbed and ‘–’ for phonon emitted simultaneously with

absorption of photon.

The conservation of momentum is,

(3.21) ∆P = ħ∆k = ħk photon ± ħk phonon ≈ ħkphonon

where kphonon is phonon wave vector absorbed or emitted, hence, the light incident on

solid losses energy in excitation of electrons to such transitions. The energy band gap

(crystalline sample) or optical band gaps (amorphous sample) can be calculated using

absorption coefficient in crystalline or polycrystalline materials. The nature of optical

transition (direct or indirect) near the absorption edge can be determined by the relation

between ‘α’ and optical energy gap Eg. Assuming the band to be parabolic in nature, the

absorption coefficient for the direct absorption is related to the band gap.

(3.22) ( )gEAh −= ννα nh

∆E = hνphoton ± hνphonon



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For indirect absorption, Macforlane and Robert showed that,

(3.23)

where ‘θD’ is the Debye temperature ‘Ep’ is the phonon energy. For small phonon

energies only second term contributes. The possible values of ‘n’ are [32–35],

n = 1/2, direct allowed transition,

n = 1/3, direct forbidden transition,

n = 2, indirect allowed transition,

n= 3, indirect forbidden transition.

Band gap is calculated by plotting the graph between (αhν)1/n versus hν and look for the

values of ‘n’ which gives best linear graph. The value of ‘n’ decides the nature of the

graph or transition involved.

For materials having direct band gap, the graph shows the single linear portion

(equation 3.22), which is extrapolated to determine Eg (the x – axis intercept). For the

material with indirect energy gap, the graph plotted between (αhν)1/n versus hν

( ) ( )

⎥⎥

⎦

⎢

⎠⎞

⎝⎞

= Aθθ

να ⎥⎥

⎢

⎢⎢

⎣⎟⎜

⎛−

−−+

−⎟⎠

⎜⎝⎛

+−

T

EEgh

T

EEghh

D

nP

D

nP νν

exp11exp

⎤⎡

Fig. 3.5 Schematic diagram of UV-VIS spectrophotometer [36].



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(equation 3.23) shows two linear portions of different slopes giving two intercept on x –

axis. (Eg – Ep) and (Eg + Ep) from which Eg can be calculated. The transmission spectrum

was recorded with a PerkinElmer UV-Vis spectrophotometer Lambda 25. A schematic

diagram of UV-Vis spectrophotometer is shown in Fig 3.5.

3.6 Electrical Characterization

The use of thin films as resistors, contacts and interconnections has lead to

extensive study of conductivity, temperature dependence, the effect of thermal

processing stability and so on. Investigation of the critical resistivity as a highly

structure sensitive properties make it possible to gain insight into the structural and

electrical properties of the metal films which is important from both the theoretical and

practical point of view.

The contact methods are most widely used for the measurement of resistivity.

These methods include two-point probe, four-point probe and the spreading resistance.

The two-point probe method is simple, easy to use and useful for high resistive thin

films. In this method constant voltage ‘V’ is applied between two fixed position probes

separation ‘d’ in cm and current passing through a sample of known dimension (cross

sectional area ‘A’ in cm2) is measured with an appropriate current meter. For uniform

sample, resistivity is given by,

Ω-cm (3.24) ⎟⎠⎞

⎜⎝⎛=

IdAVρ

In case of semiconducting thin films, the resistivity decreases with increase in

temperature. The thermal activation energies ‘Ea’ are calculated by using equation

⎟⎠⎞

⎜⎝⎛ −=

KTEaexp0ρρ (3.25)

where symbols have their usual meanings. Hence we plot Log (ρ) versus 1/T and its

slope leads to the estimation of activation energy.



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The experimental setup used to study the variation of resistivity with temperature

is schematically shown in Fig. 3.6. The brass plate of size 10 cm × 0.5 cm is grooved at

the bottom side so as to fit the heating element (Toni, 60 watt) parallel to the length of

the plate, in order to achieve the uniform temperature. The sample is mounted on the top

of the plate. The thin film of size 1 × 1 cm2 on the glass substrate is used for the

resistivity measurement. Silver paste was applied for making the good ohmic contacts to

the film. A mica sheet was used between the film and the brass plate to provide the

insulation. The temperature is measured with chromel-alumel thermocouple, which is

fixed at the center of the sample. The temperature was recorded on digital panel meter (0

to 199.9 mV range) (DPM) supplied by Omega Electronics, Jaipur. Aplab power supply

is used for passing current through the film and the current was measured with MECO

made (MIC-60E) digital current meter.

1- Specimen holder (Brass),

2- Mica sheet,

3- Sample (thin film),

4- Thermocouple,

5- Heater,

6- Copper wire.

Fig. 3.6 Schematic circuit diagram for the measurement of electrical resistivity of thin film [Lab. setup]



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3.7 I-V Measurements

The current voltage (I-V) measurements of ohmic contact device were carried out

by using Lab-Equip instrument. It consists of a metal cold finger through which the

sample can be connected. The sample fits flat with its back fixed on the cold finger with

a mica sheet below it. Silver paste was used to make contacts. I-V measurement was

made between the top and the back contacts, using PC based I-V measurement system.

This equipment is also used for studying photosensor properties of the as-deposited films

by irradiating the samples with known intensity of light. Fig. 3.6 shows the photograph

of Lab-Equip I-V measurement system with PC.

Fig. 3.7 Photograph of I-V characteristic measurement [Lab setup].



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References

1. K. L. Chopra; in: Thin Film Phenomenon, Mc-Graw Hill Book Co.; New York

(1969).

2. A. Vasicek; in: J. Opt. Soc. Am. 37 (1947) 145, 979.

3. R. J. Archer, J. Opt. Soc. Am; 52 (1962) 970.

4. Sir Lawarence Bragg, in: ‘The Crystalline State’ vol.-1, A General Survey, G.

Bell and Sons Ltd. London, (1949).

5. W. L. Jolly; in: ‘The Synthesis and Characterization of Inorganic Compounds’;

Prentice Inc. (1970), 263.

6. F. A. Settle, ‘Handbook of Instrumental Techniques for Analytical Chemistry’

(Paerson Educational Publisher) (2004).

7. W. M. D. Settle, ‘Instrumental Method of Analysis’ (CBS) (1986).

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3.1 Introduction - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/22827/1...dark lines against a white background with a fringe width which can be made as small as λ /100. By