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Chapter-II
Thin Film Deposition and Characterization Techniques
Part A:
Thin Film Deposition Techniques
2. A. 1 Introduction
Thin films are thin material layers ranging from fractions of a
nanometer to several micrometers in thickness. Electronic semiconductor
devices and optical coatings are the main applications benefiting from thin film
construction. A familiar application of thin films is the household mirror which
typically has a thin metal coating on the back of a sheet of glass to form a
reflective interface. The process of silvering was once commonly used to
produce mirrors. A very thin film coating (less than a nanometer) is used to
produce two-way mirrors. Thin-films are used to produce thin-film batteries.
Thin film technology is widely used in photoelectrochemical and dyesensitized
solar cells [1]. In case of ceramic thin films, they are in widely used
in coating technology. This is due to its relatively high hardness and inertness
of ceramic materials to make this type of thin coating of interest for protection
of substrate materials against corrosion, oxidation and wear. In particular, the
use of such coatings on cutting tools can extend the life of these items by
several orders of magnitude. Research is being done on a new class of thin
film inorganic oxide materials, called amorphous heavy-metal cation multicomponent
oxide, which could be used to make transparent transistors that
are inexpensive, stable and environmentally benign [2-3].
Definition and basics of thin films
A definition of thin film is “low dimensional material created by
condensing, one by one, atomic/molecular/ionic species of matter”. The
thickness is typically less than several microns. Whatever be the film
thickness limit, an ideal film can mathematically be defined as a
homogeneous solid material contained between two parallel planes and
extended infinitely in two directions (say x, y) but restricted along the third
direction (z), which is perpendicular to the (x, y) plane. The dimension along
the z-direction is known as film thickness (t). Its magnitude may vary from a
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Chapter-II
limit t=o to any arbitrary value say to 10 µm or more but always remaining
much less than those along the other two directions i.e. x and y.
A basic step involved in thin film deposition is, it needs substrate
because characteristics of thin films depend on the surface condition of the
substrate. It formed after depositing the materials into atomic/molecular scale
by physical and/or chemical measures. Properties of thin films are mainly
depending on the cleanliness and nature of the substrate, the deposition
conditions like temperature, pH, time concentration etc. Post deposition heat
treatment and passivation like parameters are important process in thin film
fabrication. Material properties are basically depending on nucleation and
kinetics of thin film formation. The nucleation involves adsorption, deposition,
condensation and migration of atoms. This process takes place under super
saturation, rapid heat treatment and nonequilibrium thermodynamics.
Therefore, it is planning to prior to this improvement in our best knowledge of
thin films, implement in to real devices [4].
In ancient years, thin solid films are used in antireflection coatings for
lenses, automobile headlights and multilayer interface filters. Now a day’s
thin-film technologies are being developed as a means of substantially
reducing the cost of photovoltaic systems. The reason for this is that thin-film
modules are expected to be cheaper to manufacture owing to their reduced
material costs, energy costs, handling costs and capital costs. The needs of
new and improved optical and electronic devices have stimulated the study of
thin solid films of elements, as well as binary, ternary and quaternary systems.
Earlier on the basis of the physical dimension namely thickness the difference
between thin and thick was made. But now days such difference is made on
the basis of its applications. If coating is used for its bulk properties such as
corrosion resistance then it is a thick film. If coating is used for surface
properties such as electron emission, it is a thin film. A large number of
materials are being used today for coating applications [5].
2. A. 2 Deposition techniques for thin films
Deposition methods of thin films can be divided into two major
categories: i) physical deposition processes and ii) chemical deposition
processes.
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Chapter-II
Several techniques have been developed (depending on the desired
film properties) for the deposition of the thin films of the metals, alloys,
semiconductors, ceramic, polymer and superconductors on a variety of the
substrate materials. The properties of thin films are extremely sensitive to the
method of preparation. Underlying the performance and economics of thin film
components are the manufacturing techniques that are used to produce the
devices. Each method has its own merits and demerits and of course no one
technique can deposit the thin films covering all the desired aspects such as
cost of equipments, deposition conditions and nature of the substrate
material. The vast varieties of thin film materials, their deposition, processing,
fabrication techniques, spectroscopic characterization, optical characterization
probes, physical properties and structure-property relationships are the key
features of such devices and basis of thin film technologies.
Physical methods cover the deposition techniques which depend on
the evaporation or ejection of the material from a source that is evaporated or
sputtered. Where as chemical methods depend on specific reaction conditions
like temperature, pH, concentration of precursors etc. [6]. The chemical
reactions may also depend on thermal effects as in vapour phase deposition
and thermal growth. However, in all these cases a definite chemical reaction
is required to obtain the final film. When one seeks to classify deposition of
films by chemical methods, one finds that are available, into two more
classes. The first of these classes is concerned with the chemical formation of
the film from medium e.g. electroplating, chemical reduction plating and
vapour phase deposition. A second class is that of formation of this film from
the precursor ingredients e.g. anodization, gaseous anodization, thermal
growth, sputtering ion beam implantation, chemical vapor deposition (CVD),
metal organo-chemical vapor deposition (MOCVD) and vacuum evaporation.
The methods summarized under the classifications given are often
capable of producing films defined as thin films i.e. 1 µm or less and films
defined as thick films i.e. 1 µm or more. However, there are certain techniques
which are only capable of producing thick films and these include screen
printing, glazing, electrophoretic deposition, flame spraying and painting. Thin
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Chapter-II
film deposition techniques are broadly classified under two heading as listed
in Table 2. A.1.
Table 2. A. 1 Broad classification of thin film deposition technique
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Chapter-II
2. A. 3 Classification of thin film deposition techniques
Thin film deposition techniques are classified as follows.
2. A. 3. 1 Physical techniques
a) Sputtering
Bombardment of a surface of target material with energetic particles, it
is possible to cause ejection of the surface atom then this process is known
as sputtering. The ejected atoms can be condensed on to a substrate to form
a thin film. Various theories have been put forward to account the mechanism
of sputtering [7]. This method has various advantages over normal
evaporation techniques in which no container contamination will occur. It is
also possible to deposit alloy films which retain the composition of the parent
target material. DC sputtering, radio frequency sputtering and magnetron
sputtering methods are the oldest types of sputtering used. High pressure
oxygen sputtering and facing target sputtering are the two new methods
introduced for deposition of thin films for applications in superconducting and
magnetic films.
b) Ion plating
The ion plating technique is developed by Mattox. In this atomistic,
essentially sputter-deposition process the substrate is subjected to a flux of
high energy ions, sufficient to cause appreciable sputtering before and during
film deposition. The benefits of ion plating process are clean surface, high
purity and dryness, compatibility with semiconductor integrated circuit
processing and epitaxial film growth. However, there are certain
disadvantages such as slow deposition rates, difficult stoichiometric control,
high temperature post deposition annealing often required for crystallization
and high capital expenditure. Ion plating using magnetron sputtering also has
been reported.
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Chapter-II
c) Physical vapour deposition
Physical vapour deposition (PVD) is fundamentally a vaporization
coating technique, involving transfer of material on an atomic level. It is an
alternative process to electroplating.
Physical vapour deposition processes takes place through following
steps:
i) Evaporation - The solid material to be deposited is physically converted
to vapour phase.
ii) Transportation - The vapour phase is transported across a region of
reduced pressure from the source to the substrate.
iii) Reaction - In some cases coatings will consist of metal oxides, nitrides,
carbides and other such materials. In these cases, the target will
consist of the metal. The atoms of metal will then react with the
appropriate gas during the transport stage. For the above examples,
the reactive gases may be oxygen, nitrogen and methane. In instances
where the coating consists of the target material alone, this step would
not be part of the process.
iv) Deposition - The vapour condenses on the substrate to form the thin
film. The conversion from solid to vapour phase is done through
physical dislodgement of surface atoms by addition of heat in
evaporation deposition or by momentum transfer in sputter deposition.
The third category of PVD technique is the group of so called augmented
energy techniques including ion, plasma or laser assisted depositions [8, 9].
d) Evaporation
There are two popular evaporation technologies, which are e-beam
evaporation and resistive evaporation each referring to the heating method. In
electron-beam evaporation, an electron beam is aimed at the source material
causing local heating and evaporation. In resistive evaporation, a tungsten
boat containing the source material is heated electrically with a high current to
make the material evaporate. Many materials are restrictive in terms of what
evaporation method can be used. Evaporation or sublimation techniques are
widely used for the preparation of thin layers. A very large number of
materials can be evaporated and if the evaporation is undertaken in vacuum
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Chapter-II
system, the evaporation temperature will be considerably reduced. The
amount of impurities in the growing layer will be minimized. In order to
evaporate materials in a vacuum, a vapor source is required that will support
the evaporant and supply the heat of vaporization. While allowing the charge
of evaporant to reach a temperature sufficiently high to produce the desire
vapour pressure, rate of evaporation is maintained in such a way that vapor
source does not react chemically with the evaporant. To avoid contamination
of the evaporant and hence of growing film, the support material itself must
have a negligible vapour pressure and dissociation temperature of the
operating temperature [10,11].
2. A. 3. 2 Chemical techniques
a) Metal organo chemical vapor deposition (MOCVD)
Metal organo chemical vapor deposition (MOCVD) is the chemical
vapour deposition method of epitaxial growth of materials.These materials are
metal organics and metal hydrides containing the required chemical
elements. It consists of heating an organometallic solution, which evaporates
and is deposited on a heated substrate. The films grown by this method which
generally requires expensive, sophisticated apparatus are usually
homogeneous. This is a crucial attribute for the study of optical properties.
MOCVD is a chemical vapor deposition process that uses metalo-organic
source gases. For example indium phosphide could be grown in a reactor on
a substrate by introducing Trimethylindium ((CH 3 ) 3 In) and phosphine (PH 3 ).
This technique is preferred for the formation of devices incorporating
thermodynamically metastable alloys and it has become a major process in
the manufacture of optoelectronics.
b) Cathodic deposition
This is a standard method of electroplating. Two metal electrodes are
dipped into an electrolyte solution and on application of an external field
across the electrodes metal ions from the solution are deposited on cathode
as a film. Deposition of the films is mainly controlled by the electrical
parameters such as, electrode potential and current density.
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Chapter-II
c) Electro deposition
Electro deposition is the process of depositing a substance by the
passage of electric current through the conducting medium producing a
chemical change. It is also known as electroplating. Electro deposition is
divided in to two technologies namely Electroplating and Electroless plating.
The phenomenon of electrolysis is governed by the Faraday’s laws, when a
metal electrode is immersed in a solution containing ions of that metal, a
dynamic equilibrium M + x (M- Metal atom and X = S, Se and Te) is set up.
The factors influencing the electro deposition process are i] current density ,ii]
bath composition, iii] pH of the electrolyte, iv] temperature of the bath, v]
agitation and vi] electrode shape.
It consists of an anode and a cathode immersed in a suitable
electrolyte. By using electrical current to reduce cations of a desired material
from a solution and coat that material as a thin film onto a conductive
substrate surface and usually some gas generation at the counter electrode.
Now days there have been considerable interests in the electro deposition
[12].
In the electro less plating deposition of films can be done directly by
chemical reaction, without the application of any electrode potential. This
process is desirable since it does not require any external electrical potential
and contact to the substrate during processing. Unfortunately, it is also more
difficult to control with regards to film thickness and uniformity. This is simple
technique that does not require high temperature.
d) Spray pyrolysis
Spray pyrolysis is a process in which a thin film is deposited by
spraying a solution on a heated surface, where a constituent react to form a
chemical compound. The sprayed droplets on reaching the hot substrate
undergo pyrolytic decomposition and form a single crystal or cluster of
crystallites of the product. The other volatile byproducts and excess solvents
escape in the vapour phase. The thermal energy for decomposition,
subsequent recombination of the species, sintering and re-crystallization of
the crystallites is proved by hot substrate. The nature of the fine spray
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Chapter-II
droplets, with the help of a carrier gas depends upon spray nozzle. The
growth of a film by a spray pyrolysis is determined by nature of the substrate,
solution as well as spray parameters. The films are generally strong and
adherent, mechanically hard, pinhole free and stable with temperature and
time. The morphology of the films is general rough and that will depend upon
the spray conditions. The surface of the substrate gets affected in the spray
process and the choice is limited to glass, quartz, ceramics or oxides, nitride
or carbide coated substrates.
e) Chemical bath deposition
Films can be grown on either metallic or nonmetallic substrates by
dipping them in appropriate solutions of metal salts without the application of
any electric field. Deposition may occur by homogeneous chemical reactions
usually reduction of metal ions in solution by a reducing agent. If this occurs
on a catalytic surface it is called as electro less deposition.
Among the methods mentioned in the Table 2.1, the chemical methods
are economical and easier than that of the physical methods. But there is no
ideal method to prepare thin films, which will satisfy all possible requirements.
Among the chemical methods, the Chemical Bath Deposition (CBD) method is
the most popular today because large number of conducting and
semiconducting thin films can be prepared by this technique. It is also popular
due to its simplicity and low cost. In this technique, the thin films can be
deposited on different substrates like glass, ceramic, metallic etc. Many
studies have been conducted over about three decades on chemical bath
deposition method for the preparation of thin films. There after due to good
productivity of this technique on a large scale and simplicity of the apparatus,
it offered most attractive way for the formation of thin films of metal oxides,
metallic spinal type oxides, binary chalcogenides, ternary chalcogenides,
superconducting oxides etc. It is simple and low cost technique and is formed
in a two step fashion, first is sensitizing and second is deposition. It has
capability to produce large area of high quality adherent films of uniform
thickness [13-19].
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Chapter-II
f) Anodic oxidation
This is an electrolytic method for producing oxide films on the surface
of metal. These metals form the anode that dips into a liquid electrolyte such
as a salt and acid solution. Oxide ions are attracted to the anode to form a
thin layer of metal oxide. On increasing the field strength, more oxide ions
diffuse through the oxide layer to the metal surface and hence the oxide layer
grows thicker.
g) Deposition by chemical reactions
Chemical reaction either takes place on the surface of the dipped
substrate or in the solution itself, where a mixing of components on the
surface to be coated is required. Most of the coatings
The most widely used deposition methods are listed below;
Homogeneous chemical reduction of a metal ion solution by a reducing
agent regardless the substrates.
Electroless plating for the deposition of metallic coating by controlled
chemical reduction that is catalyzed by the metal or alloy being
deposited.
Conversion coatings forming a sacrificial layer containing compound of
the metal substrate.
Displacement deposition or galvanic deposition makes use of the
electro negativity differences of metals.
Arrested Precipitation Technique (APT) means a metal ion is arrested
by organic complexing agent which is then made available by slow
dissociation of the organometallic complex at specific pH value.
Among the various chemical deposition systems, chemical bath
deposition has attracted a great deal of attention because of its overriding
advantages over the other conventional thin film deposition methods.
The chemical bath deposition method for the preparation of thin films
has recently been shown to be an attractive technique because of its
simplicity, convenience, low cost, low temperature and it has been
successfully used for depositing ternary metal chalcogenide thin films [20].
Understanding of the chemistry and physics of the various process
involved in a deposition processes has now made possible to obtain undoped/
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Chapter-II
doped, multi-component semiconductor thin films of usual/unusual and
metastable structure.
h) Laser Chemical Vapour Deposition (LCVD)
Laser Chemical Vapour Deposition (LCVD) is a recently developed
technique using a laser source. It is a modification of conventional CVD. This
process holds great potential for the production of small and complex metal
and ceramic parts. Recent research significant to LCVD is reviewed,
summarizing the general state of knowledge in the field, and discussing
important challenges that remain. The basics of the LCVD process and the
various deposition techniques, including: photolytic versus pyrolytic deposition
and fiber growth versus direct writing methods are considered. The
application of heat transfer and chemical kinetic models to the LCVD process
as a means for predicting deposit properties is described. The deposition
process is considered with respect to efforts to increase deposition rates and
to control deposit shapes. Modern process control techniques for measuring
deposition temperature and growth rate are also discussed.
i) Arrested Precipitation Technique (APT)
APT is modified chemical bath deposition method. APT is based on
Ostwald ripening law is simple and inexpensive method used for deposition of
wide variety of metal chalcogenide thin films [21-23]. It can be distinguished
from other conventional techniques as follows:
It is simple, inexpensive and does not require sophisticated
Instrumentation.
It is ideally suited for large area thin film depositions. Substrate
surfaces of both accessible and non-accessible nature could easily be
deposited.
Stoichiometry of the deposits can be maintained since the basic
building blocks are ions instead of atoms.
The deposition is usually at low temperature and avoids oxidation or
corrosion of the metallic substrates.
Electrical conductivity of the substrate material is not an important
criterion.
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Chapter-II
Wide varieties of conducting/non-conducting substrate materials can
be used.
Slow film formation process facilitates better orientation of the
crystallites with improved grain structures over the substrate surface.
Doped and mixed films could be obtained by merely adding the dopant
solution directly into the reaction bath.
Dissociation rate of organo metallic complex to release free metal ions
for reaction is well control by maintaining the pH of reacting solution.
An intimate contact between reacting species and the substrate
material permit pinhole free and uniform deposits on the substrate.
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Chapter-II
Section B: Thin Film Characterization Techniques
2. B. 1 Introduction
In this section different analytical techniques used to characterize the
thin films are described with relevant principles of their operation and working.
Nowadays, there has been increasing demand for deposition of thin films for
their wide range of potential applicability in the various fields of science and
technological progress in modern society. The complete characterization of
any material consists of compositional characterization, phase analysis,
structural elucidation, surface characterization and micro-structural analysis
which have strong bearing on the properties of materials. A lot of efforts have
been taken to obtain thin films of required properties such as thickness,
texture, uniformity, adhesivity, orientation etc. for particular applications. Thin
film properties such as structural, morphological, electrical, optical and
electrochemical of a given material are strongly dependent on the method of
deposition, the substrate material, the substrate temperature, rate of
deposition, and background pressure. In this section operation and working of
different analytical instruments used to characterize our thin films are
described with relevant principles. Here we use the APT deposition method
for the deposition of our thin films. This method is economical and easier than
other physical methods. Number of conducting and semiconducting thin films
can be prepared by using this technique [24-26].
2. B. 2 Surface profilometry
Thickness measurement can be done by two methods. One is weight
difference density method and another is surface profilometer. Weight
difference method is time consuming. So as compared to weight difference
method surface profilometer is superior to measure thickness of the film. The
method chosen should be convenient, reliable and simple. A typical
profilometer is a basic tool that allows quick measurement of the physical
thickness of films, providing a sharp edge of the film. The thickness of film is
the most significant parameter that affects the properties of the thin films. Any
known physical quantity related to film thickness can be used to measure the
thickness. In Figure 2.1 schematic ray diagram of typical profilometer is
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Chapter-II
shown. It mainly measures film thickness (step height) and roughness. Film
thickness includes a) changes of 200 Å to 65µm and b) vertical resolution of
about 10 Å 0 . Horizontal resolution depends on tip radius. In our experiment we
used the XP-1 Ambios technology surface profilometer (contact mode) having
1 A o resolution as shown in Figure 2.2. Electro-magnetic sensors detect the
vertical motion of the stylus as it is moved horizontally across the sample. But
in case of soft film stylus penetrates in to the film.
Figure 2. 1 Schematic ray diagram of typical profilometer
(http://micromachine.stanford.edu/~hopcroft/Research/mattest.html)
Figure 2. 2 Photograph showing the XP-1 Ambios Technology surface
profilometer
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Chapter-II
2. B. 3 UV-Visible spectrophotometry
Figure 2. 3 Schematic of UV-Vis spectrophotometer
(Source: UVVis_fig1.jpgcp.chem.agilent.com)
Spectrophotometer is optical instrument that measures the intensity of
light transmitted or reflected by objects as a function of wavelength. Figure 2.3
shows schematic of UV-Vis spectrophotometer. Semiconductors absorb light
with energy larger than their band gap. Absorption measurement can
therefore be used to estimate their band gap energy. Measurements at low
temperature are more meaningful due to the reduced amount of band tailing.
To find dependence of the absorption coefficient on frequency for
direct/indirect gap semiconductors, the material is made thin and its surfaces
are polished to reduce scattering. Light incident on material is absorbed if it
can cause an electronic transition. In semiconductors this process can occur
by means of several mechanisms including,
• Direct interband (band-to-band) transition
• Indirect interband transitions
• Impurity-to-band and impurity-impurity transitions
• Excitonic transitions
• Interband transitions
• Phonon transitions
In an ideal semiconductor, at absolute zero temperature the valence
band would be completely full of electrons so that electron could not be
excited to a higher energy state from the valence band. Absorption of quanta
of sufficient energy tends to transfer the electrons from valence band to
conduction band. The optical absorption spectra of semiconductors generally
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Chapter-II
exhibit a sharp rise at a certain value of the incident photon energy, which can
be attributed to the excitation of electrons from valence to conduction band
(may also involve acceptor or donor impurity levels, and traps). The
conservation of energy and momentum must be satisfied in optical absorption
process. Basically there are two types of optical transitions that can occur at
the fundamental edge of the crystalline semiconductor, direct and indirect.
Both involve the interaction of an electromagnetic wave with an electron in the
valence band, which gives rise across the fundamental gap in the conduction
band. However, indirect transition also involves simultaneous interaction with
lattice vibration. Thus the wave vector of the electron can change in the
optical transition. The momentum change being taken or given up by phonon.
The direct inter band optical transition involves a vertical transition of
electrons from the valence band to the conduction band such that there is no
change in the momentum of the electrons and energy is conserved as shown
in Figure 2. 4 and Figure 2. 5.
Figure 2 .4 A simple representation of band structure difference between
metals, semiconductors, semimetals and insulators
(Source: http://3.bp.blogspot.com/_Lm7Zn1DZFbc/TFJPKekhUgI/AAAA
AAAAAHE/ruEruqrAHf4/s1600/figu1.png)
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Chapter-II
Figure 2.5 Direct band and indirect band semiconductors
In semiconductors, the optical absorption spectrum has been found to
have three distinct regions:
i) High absorption region (α ≥ 104 cm -1 )
ii) Exponential edge region (1≤ α ≥ 104 cm -1 )
iii) Weak absorption tail (1≤ α cm -1 )
The optical transition is denoted by a vertical upward arrow as shown
in Figure 2.5. For simple parabolic bands and for direct transition, absorption
coefficient (α) is given by the equation [27, 28].
A
α = (hν
−Eg) n
(2.1)
hν
where α is the absorption coefficient, Eg is the separation between
bottom of the conduction and top of the valence band,hν photon energy , A is
constant parameter that depends on the transition probability, h’ is Planks
constant, ‘ν’ is frequency and n assumes values of 1/2, 3/2 and 3 for allowed
direct, allowed indirect, forbidden direct and forbidden indirect transitions
respectively. For allowed direct type of transitions:
(αhν) 2 = A (hν-Eg) (2.2)
where, Eg is the separation between bottom of the conduction and top
of the valence band, hν is the photon energy, n is constant and is equal to 1/2
or 3/2 depending on whether transition is allowed or forbidden and A is a
constant depending upon the transition probability for direct transition. If the
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Chapter-II
plot of (αhν) 2 against hν is linear then the transition is direct allowed. The
band gap energy Eg is determined by extrapolating the linear portion of the
curve to the energy axis at α =0.
In spectrophotometer light from the lamp enters in to the
monochromator, which disperses the light and selects the particular
wavelength chosen by the operator for the measurement. The light beam of
selected wavelength is passed alternately through the sample and along the
reference path. The reference and sample light beams pass through the cell
compartment, consisting of a reference space and a sample space. The two
light beams converge on the detector. Quantitative measurements in chemical
analysis are done by comparison of the absorption with the absorbance of
known concentration of the element.
In present study UV-Visible spectrophotometer (Shimadzu model 3600,
Japan) was used to determine absorption spectra of the sample in the
wavelength range 400 to 900 nm. A glass slide of same thickness and size
was used as reference throughout all the measurements. One side of the film
was removed with the help of cotton swab moist in dilute HCl. The layer
thickness of the as deposited samples was measured by surface profilometer.
Absorption spectra were analyzed to determine absorption coefficient, optical
band gap E g and mode of optical transition for all the compositions.
2. B. 4 X-ray diffraction (XRD)
X-ray Diffraction study (XRD) is essential in characrisation technique
for determination of crystal structure and lattice parameters. Diffraction in
general occurs only when the wavelength of the wave motion is of the same
order of magnitude as the repeat distance between scattering centers. Figure
2.6 shows the schematics of X-ray diffractometer.
Diffraction is nothing but Bragg’s law and is given as,
2 d sin θ = n λ (2.3)
where,
d = interplaner spacing
θ = diffraction angle
λ = wavelength of x-ray
n = order of diffraction
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Chapter-II
Figure 2. 6 Schematics of X-ray diffractometer.
The basic principles of X-ray diffraction are found in textbooks [29-33].
X-rays are ideally suited for probing the structural arrangement of atoms and
molecules in a wide range of materials. The energetic x-rays can penetrate
deep into the materials and provide information about the bulk structure.
For thin films, the powder technique in conjunction with diffractometer
is most commonly used. In this technique the diffracted radiation is detected
by the counter tube, which moves along the angular range of reflections. The
intensities are recorded on a computer system. The d values are calculated
using equation 2.4 for known values of θ, λ and n. The X- ray diffraction data
thus obtained is printed in tabular form on paper and is compared with Joint
Committee Power Diffraction Standards (JCPDS) data to identify the unknown
material. The sample used may be powder, single crystal or thin film. The
crystallite size of the deposits is estimated from the full width at half maximum
(FWHM) of the most intense diffraction line by Scherrer’s formula as follows
[34].
0.9λ
D =
βcosθ
(2.4)
where, D is crystallite size, λ is wavelength of X-ray used, β is full width
at half maxima of the peak (FWHM) in radians, θ is Bragg's angle. The X- ray
diffraction data can also be used to determine the dimension of the unit cell.
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Chapter-II
This technique is not useful for identification of individuals of multilayer or
percentage of doping material.
2. B. 5 Scanning electron microscopy (SEM)
Scanning electron microscope is an instrument that is used to observe
the morphology of the solid sample at higher magnification, higher resolution
and depth of focus as compared to an optical microscope [35,36]. It uses a
focused beam of high kinetic energy electrons to generate a variety of signals
at the surface of solid specimens. When an electron strikes the atom, variety
of interaction products are evolved as shown in Figure 2.7. It illustrates these
various products and their use to obtain the various kinds of information about
the sample. The signals that derive from reveal information about the sample
including external morphology (texture), chemical composition and crystalline
structure and orientation of materials making up the sample. Scattering of
electron from the electrons of the atom results into production of
backscattered electrons and secondary electrons. Electron may get
transmitted through the sample if it is thin. Primary electrons with sufficient
energy may knock out the electron from the inner shells of atom and the
excited atom may relax with the liberation of Auger electrons or X-ray
photons. All these interactions carry information about the sample. Auger
electron, ejected electrons and X-rays are energies specific to the element
from which they are coming. These characteristic signals give information
about the chemical identification and composition of the sample. Area ranging
from approximately 1 cm to 5 microns in width can be imaged in a scanning
mode using conventional SEM techniques (magnification ranging from 20X to
approximately 30,000X, spatial resolution of 5 to 10 nm).
Variety of interaction products evolved due to interaction of electron
beam and sample in scanning electron microscope. A well-focused monoenergetic
(~25 KeV) beam is decelerated in the solid sample giving various
signals as mentioned above. These signals include secondary electrons,
backscattered electrons, diffracted backscattered electrons etc.
Backscattered electrons and secondary electrons are particularly pertinent for
SEM application, their intensity being dependent on the atomic number of the
host atoms. Each may be collected, amplified and utilized to control the
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Chapter-II
brightness of the spot on a cathode ray tube. To obtain signals from an area
the electron beam is scanned over the specimen surface by two pairs of
electro-magnetic deflection coils.
Figure 2. 7 Variety of interaction products evolved due to interaction of
electron beam and sample in scanning electron microscope
(Source: http://www.physics.utu.fi/tutkimus/materiaalitiede/SEM.html)
So, cathode ray tube (C. R. T.) is the beam in synchronization with this
the signals are transferred from point to point and signal map of the scanned
area is displayed on a long persistent phosphor C. R. T. screen. Change in
brightness represents change of a particular property within the scanned area
of the specimen [37]. The ray diagram of scanning electron microscope is
shown in Figure 2.8. The scattering cross section for back-scattered electrons
is given by equation
49

Chapter-II
Figure 2. 8 The ray diagram of scanning electron microscope.
(Source: Scanning-Electron-Microscope sem.jpg microscopy.ethz.ch)
Q
= 16.2 ∗ 1 0 −
30
2
cot
where, Z is atomic number and E is electric field.
Here the cross-section section is proportional to Z 2 . Hence the back-scattered
electrons are used for the Z contrast or for compositional mapping. SEM
analysis is considered to be non-destructive that is x-rays generated by
electron interactions do not lead to volume loss of the sample, so it is possible
to analyze the same materials repeatedly.
2. B. 6 Energy dispersive X-ray analysis (EDAX)
⎡
⎢
⎣
Z
E
⎤
⎥
⎦
(2.5)
Energy-dispersive
X-ray spectroscopy (EDAX) is an analytical
technique used for the elemental analysis or chemical characterization of a
sample. In EDAX technique a sample is made the target in an X- ray tube and
is bombarded with electrons of suitable energy, it emits characteristics X-rays.
This is the basis of a method of chemical analysis. The emitted X-rays are
analyzed in an X-ray spectrometer and the elements present in the sample
⎛
⎜
⎝
φ
2
⎞
⎟
⎠
50

Chapter-II
are qualitatively identified by their characteristics wavelengths. For
compositions greater than or about 1% and elements separated by few atomic
numbers, energy dispersion analysis is very useful because the intensities are
increased about 100-Fold. The resolution however, of an energy dispersion
instruments is as much as 50 times less than the wavelength dispersion
spectrometer using a crystal. Thus overlapping of lines from nearby elements
may occur. If a sample is irradiated with X-rays of sufficiently high energy, it
will emit fluorescent radiation. This radiation may be analysized in an X-ray
spectrometer and the elements present in the sample identified by their
characteristics wavelengths. Study of thin films, ferrites, composites,
biological samples and pharmaceutical samples are the common application
areas [38]. During EDAX analysis, the specimen is bombarded with an
electron beam inside the scanning electron microscope. The bombarding
electrons collide with the specimen atoms own electrons, knocking some of
them off in the process as shown in Figure 2.9. A position vacated by an
ejected inner shell electron is eventually occupied by a higher-energy electron
from an outer shell. However, the transferring outer electron must give
up some of its energy by emitting an X-ray.
The amount of energy released by the transferring electron depends on
which shell it is transferring from, as well as which shell it is transferring to.
Furthermore, the atom of every element releases X-rays with unique amounts
of energy during the transferring process. Thus, by measuring the amounts
of energy present in the X-rays being released by a specimen during electron
beam bombardment, the identity of the atom from which the X-ray was
emitted can be established.
51

Chapter-II
Figure 2. 9 Energy-dispersive X-ray microanalysis technique
EDAX has been widely used to determine the compositions of
particles, contamination and thin film layers in wafer fabrication. EDAX
analysis is able to provide a good and reliable compositional analysis for
particles and contamination, but for some cases, the EDAX results may not be
accurate, especially for wafer thin film layer identification.
2. B. 7 Atomic force microscopy (AFM)
The atomic force microscopy (AFM) was first developed in 1982 at IBM
in Zurich by Binnig, et al [39,40] .The atomic force microscopy (AFM) probes
the surface of a sample with a sharp tip, a couple of microns long often less
than 100A in diameter [41]. The tip is located at the free end of a cantilever,
which is 100m long. The forces between the tip and sample surface cause the
tip to 200 cantilevers to bend or deflect. A detector measures the cantilever
deflection as tip is scanned over the sample or the sample is scanned under
the tip. The measured cantilever deflection allows a computer to generate a
map or surface topography [42].
Several forces typically contribute to the deflection of an AFM
cantilever. AFM operates by measuring the attractive or repulsive forces
between a tip and the sample. The forces most commonly associated with
52

Chapter-II
atomic force microscopy are inter-atomic force called the Van der Waals
force. The dependence of the Van der Waals force upon the distance
between the tip and the sample is shown in Figure 2.10. The two distance
regimes are labeled in the figure are (a) the contact regime and (b) noncontact
regime.
In the contact regime, the cantilever is held at a distance less than few
angstroms from the sample surface and the inter-atomic force between the
cantilever and the sample is repulsive. In the non-contact regime the
cantilever is held at a distance of the order of tens to hundred of angstroms
from the sample surface and the inter-atomic force between the cantilever and
sample is attractive. In principle, AFM resembles the record player as well as
the surface profilometer. However, AFM incorporates a number of
refinements; these are sensitive detection, flexible cantilever, sharp tips, highresolution
tip-sample positioning and Force feedback.
Figure 2 .10 Schematic diagram of Atomic Force Microscope (AFM)
(Source: AFM_schematic.gif home.agilent.com)
53

Chapter-II
The AFM tip is first brought (manually) close to the sample surface and
then the scanner makes a final adjustment in tip–sample distance based on a
set point determined by the user. The tip, now in contact with the sample
surface through any adsorbed gas layer, is then scanned across the sample
under the action of a piezoelectric actuator, either by moving the sample or
the tip relative to the other. A laser beam aimed at the back of the cantilever–
tip assembly reflects off the cantilever surface to a split photodiode, which
detects the small cantilever deflections.
By maintaining a constant tip-sample separation and using Hooke’s
Law (F = -kx, where F is force, k is the spring constant and x is the cantilever
deflection), the force between the tip and the sample is calculated. Finally, the
distance the scanner moves in the z direction is stored in the computer
relative to spatial variation in the x-y plane to generate the topographic image
of the sample surface as shown in Figure 2.10. A special table to isolate
mechanical and acoustical vibrations is also usually necessary to perform
high resolution (atomic scale) work.
2. B. 8 X-ray photoelectron spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS) is also known as ESCA an
abbreviation for Electron Spectroscopy for Chemical Analysis. XPS detects
all elements with an atomic number (Z) of 3 (lithium) and above. It cannot
detect hydrogen (Z = 1) or helium (Z = 2) because the diameter of these
orbitals is so small, reducing the catch probability to almost zero. The XPS
technique is used to investigate the chemistry at the surface of a sample. It is
a quantitative spectroscopic technique that measures the elemental
composition, empirical formula, chemical state and electronic state of the
elements that exist within a material. XPS spectra are obtained by irradiating
a material with a beam of X-rays while simultaneously measuring the kinetic
energy and number of electrons that escape from the top 1 to 10 nm of the
material being analyzed. XPS requires ultra high vacuum (UHV) conditions.
XPS is used to measure
i) Elemental composition of the surface (top 1–10 nm usually).
ii) Empirical formula of pure materials.
iii) Elements that contaminate a surface.
54

Chapter-II
iv)
Chemical or electronic state of each element in the surface.
v) Uniformity of elemental composition across the top surface or line
vi)
profiling or mapping.
Uniformity of elemental composition as a function of ion beam etching
or depth profiling.
Also XPS routinely used to analyze
inorganic compounds, metal
alloys, semiconductors, polymers, elements, catalysts, glasses, ceramics,
paints, papers,
inks, woods, plant parts, make-up, teeth, bones, medical
implants, bio-materials, viscous oils, glues, ion modified materials and many
others. XPS can be performed using either a commercially built XPS system,
a privately built XPS system or a synchrotron-based light source combined
with a custom designed electron analyzer. Binding energy of each of the
emitted electrons can be determined by using an equation
E =
binding
E
where, E binding is the binding energy of the electron, E photon is the energy
of the X-ray photons being used, E kinetic is the kinetic energy of the electron
as measured by the instrument and φ is the work function of the
spectrometer.
photon
−
(E
kinetic
+
φ)
(2.6)
Figure 2.11 Principle of X-ray photoemission.
(Source: http://www.ifw-dresden.de/institutes/ikm/organisation/dep-
31/methods/x-ray-photoelectron-spectroscopy-xps) xps)
55

Chapter-II
Figure 2.12 Basic components of monochromatic XPS system
An electron near the Fermi level is far from the nucleus, moving in
different directions all over the place and will not carry information about any
single atom. Fermi level is the highest energy level occupied by an electron in
a neutral solid at absolute zero temperature. Electron binding energy is
calculated with respect to the fermi level as shown in Figure 2.11. The core
electron is close to the nucleus and have binding energies characteristic of
their particular element. Detection limits typically ~ 0.1 at % and smallest
analytical area ~10 µm [43]. Figure 2.12 shows basic components of
monochromatic XPS system.
2. B. 9 Conductivity measurement techniques
a) Electrical conductivity (EC) measurements
Thermoelectric materials are utilized to convert temperature gradient
into electricity or to convert electricity into temperature gradient based on the
Seebeck and Peltier effects is currently receiving renewed interest due to
requirement of environmental protection and military applications. The
electrical conductivity of the films was studied by using two point D. C. probe
method. As the contact resistance of the films is very low (10-30 ohm)
compared to film resistance, the two probe method is accurate and hence
used for electrical conductance measurements. Figure 2.13 and Figure 2.14
shows photograph and schematic diagram of the electrical conductivity
measurement unit. The two brass plates of the size 10 cm x 5 cm x 0.5 cm
56

Chapter-II
are grooved at the centre to fix the heating elements. Two strip heaters (65
Watts) were kept parallel in between these two brass plates to achieve
uniform temperature. The two brass plates were then screwed to each other.
The sample was mounted on the upper brass plate at the centre. To avoid the
contact between the film and the brass plate, a mica sheet was placed
between the film and brass plate. The area of the film was defined and silver
emulsion (paste) was applied to ensure good electrical contact to the films.
The working temperature erature was recorded using a Chromel-Alumel
thermocouple (24 gauge) fixed at the centre of the brass plates. Testronix
model - 34C (power supply unit) was used to pass the current through the
sample. The potential drop across the film was measured with the help of
Meco 801 digital multimeter and the current passed through the sample was
noted with a sensitive 4 digit picoammeter (Scientific equipment, Roorkee
DPM 111).The measurements were carried out by keeping the film system in
a light tight box, which was kept at room temperature.
Figure 2.13 The electrical conductivity measurement assembly
57

Chapter-II
Figure 2.14 Design and schematic arrangement for measurements of the
conductivity.
b) Thermoelectric power (TEP) measurements
Fabrication of micro-thermoelectric cooling device using thin film
techniques is important to enhance thermoelectric performance because films
have lower thermal conductivity than those of bulk materials due to strong
phonons scattering at surfaces and film-surface interface. And enhancement
of density of states near the Fermi energy, giving rise to increased Seebeck
coefficient. Thermoelectric power (TEP) measurement is carried out under the
condition of maximum temperature difference and minimum contact
resistance. Care was taken to fulfill this condition while fabricating the
thermoelectric power unit Figure 2.15 and Figure 2.16. Shows photograph
and schematic diagrams of the TEP measurement unit. Thermoelectric power
measurement apparatus consists of two brass blocks. One brass block was
used as a sample holder-cum-heater and the other brass block was kept at
room temperature. The hot and cold junction was kept thermally isolated by
inserting an insulated barrier between the junctions. The size of the film used
in this study was 40 mm x 12.5 mm x 1.35 mm on amorphous glass
substrates, were fixed on two brass blocks. Chromel– Alumel thermocouples
(24 gauze) were used to sense the working temperature. A 65 watt strip
heater was used for heating the sample. The temperature of the hot junction
was raised slowly from room temperature, with a regular interval of 10 K and
58

Chapter-II
the thermoemf was noted up to the highest temperature of 500 K. Silver paste
contacts were made to deposited films with copper wire.
Figure 2.15 The thermoelectric power measurement assembly.
Figure 2.16 Design and schematic arrangement for measurements of the
thermoelectric power
A backellite box was used for proper shielding of the TEP unit, which
also minimises thermal radiation losses to some extent. The mean
temperature was measured with a Meco 801 digital multimeter while the
59

Chapter-II
differential thermal gradient and thermoelectric voltage were measured with
digital Testronix - 8 microvoltmeter [44-47].
c) Thermal conductivity measurement
Thermal conduction is the spontaneous transfer of thermal energy
through matter, from higher temperature to lower temperature region. The
most studied thermoelectric materials are bismuth and antimony
chalcogenides and this family of materials has been the backbone of the
thermoelectric industry over the past four decades. Bi 2 Te 3 with Bi 2 Se 3 alloys
have been an attractive thermoelectric material widely used in cooling devices
and proposed for energy conversion applications at room temperature [48,49].
The performance of thermoelectric materials depends on the
thermoelectric figure of merit (ZT) of the material. Recently progress has been
made in improving the figure of merit of thin film. It is approximately one for
bismuth telluride at room temperature. Bismuth telluride represents the parent
compound of a family of technologically important semiconductor alloys that
are used extensively in modern thermoelectric coolers [50-53]
The efficiency of thermoelectric material is given by a quantity called
the figure of merit, defined as Z= S 2 σ/к, where S is the thermoelectric power,
σ is the electrical conductivity and к is thermal conductivity. To maximize
figure of merit Z, the challenge lies in achieving simultaneously high
thermoelectric power, high electrical conductivity, and low thermal
conductivity. Many research groups are looking for new materials with better
figure of merit and compatible with microelectronic devices. In the last few
years a significant improvement in material properties was reported [54-59].
• Thermal conductivity meter (CT meter)
Thermal conductivity was measured by different types of probes,
namely cylindrical probe, two wire probe, Iron probe and ring probe. In our
experiment ring probe method is used to measure commercial thermal
conductivity [CT meter (Teleph, France)] at room temperature. The CT meter
uses the general concept of the method and the model for the determination
of thermal conductivity. General scheme of CT meter is represented in
60

Chapter-II
Figure 2.17. Thermal conductivity is automatically determined by means of the
control unit computer as shown in Figure 2. 18.
Figure 2.17 General scheme of CT meter
Figure 2.18 Thermal conductivity measurement unit CT-meter
61

Chapter-II
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