Chapter 2 Thin Film Deposition and Characterization...

Synthesis and Characterization of Thin Films of Conducting Polymers for Gas Sensing Applications

Mr. Ravindrakumar G. Bavane, SOPS, NMU, Jalgaon (2014) 2. 1

Chapter 2

Thin Film Deposition and Characterization Techniques

2.1 Introduction:

The development of modern society purely depends on the advancement of

technology, which in turn is not possible with technological progress in the field of thin

film science. Thin films are deposited onto bulk materials (substrate) so as to achieve

required properties. Additional functionality in thin films can be achieved by depositing

multiple layers of different materials. The multilayer thin films can behave as completely

new engineered materials, unknown in bulk form. When multiple layer in is combined

with lithographic pattern in the plane of the films, then variety of microstructure can be

constructed (e.g. Basic technology of IC technology industry, optical waveguide,

micromechanical devices etc.) [1-2]. The difference between thin film and thick film

technologies is that the former involves deposition of individual molecules, while the

later involves deposition of particles. ‘Temperature’ is a key variable in the process of

altering film properties.

Thin films process contains 4 sequential steps viz. source material, transport,

deposition, analysis. The source of film forming material may be a solid, liquid, gas or

vapor. In the transport step, the major issue is uniformity of arrival rate of material over

the substrate area. The third step deposition is the actual thin film process formation onto

the surface of substrate. Deposition behavior is determined by source and transport

factors and also by condition at the deposition surface. The last step in the deposition is

the analysis of the films. It is essential to optimize the deposition process parameters

during the formation of thin films.

Thin film studies have directly or indirectly advanced and enhanced many new areas

of research in solid-state physics and chemistry, which are based on the phenomena

exclusively characteristic of the thickness, geometry, and structure of the film [3].

Various techniques are available for the deposition of thin films with several materials.

Basically, thin- film deposition technologies are either purely physical (evaporative



method), or purely chemical (gas / liquid- phase chemical method). Physical method

covers the deposition techniques which depend on the evaporation or ejection of the

material from a source, i.e. evaporation or sputtering, whereas chemical methods depend

on physical properties. Structure-property relationships are the key features of such

devices and basis of thin film technologies.

Majority of sensor devices based on the material structures are produced by thin film

deposition. Earlier in 1817, Fraunhofer (German Scientist) accidently found an ‘optical

thin film layer’ was generated on the surface of glass during the experimentation. It may

be considered as the beginning of thin film technology. Thereafter, in 1850’s Faraday,

Grove and Edison developed the deposition techniques such as Electrodeposition,

chemical reduction deposition and Evaporation of metallic wires by current respectively

[4]. Recently, Xie et al. [5] reported the fabrication and characterization of a PANI-based

gas sensor by ultra-thin film technology.

2.2 Conducting polymer films -deposition techniques:

There are variety of deposition techniques have been developed to prepare conducting

polymer films, in order to adapt to different sensing materials and different types of

sensor configurations. Thus it is essential firstly to discuss about the deposition of

conducting polymer films.

2.2.1 Sol-Gel technique:

The sol-gel technique has been widely used in the preparation of thin films either

by dip-coating or a spin-coating procedure through a chemical reaction in the form of

solution at low temperature. Sol-gel processing is now accepted technology for forming

thin films and coating. As proved to be technically sounded alternative in some cases as

well as they have also been shown to be commercially viable alternative. The technology

of sol-gel thin film has been around for over 35 years. In 1939, Jenaer Glaswer Schott &

Gen, was first registered the patent based on sol - gel processing for silicate sol - gel films

formed by dip coating. By utilizing the sol - gel process, it is possible to fabricate

advanced materials in a wide variety such as ultrafine or spherical shaped powders,

Synthesis and Characterization of Thin Films of Conducting Polymers for Gas Sensing Applicat

Mr. Ravindrakumar G. Bavane, SOPS, NMU, Jalgaon (2014)

thin film coatings, fibers, porous or dense materials, e

containing the desired oxide or non

dipping. A spinning process involves depositing a small puddle of a solution resin onto

the centre part of substrate and then spi

acceleration will cause the resin to spread over and eventually off, the edge of substrate

leaving thin film. Final film thickness and other properties will depend on the nature of

resin (viscosity, drying rat

Figure 2.1:

The sol - gel process involves transition of

solid “gel” phase. In general , the term sol

nanoparticles dispersed in a liquid solution so as to agglomerate together resulted in a

continuous three- dimensional network exten

- gel reaction mainly consists

subsequent hydrolysis and

formation of a ‘sol’ (colloidal solution). This ‘sol’ is further coated on substrate by

using spin - coating technique to form

of sol-gel process.



thin film coatings, fibers, porous or dense materials, etc [6]. In this process a solution

containing the desired oxide or non-oxide precursor is applied to a substrate by spinning,


the centre part of substrate and then spinning the substrate at high speed. Centripetal



resin (viscosity, drying rate, percent solid, surface tension etc.).

2.1: Schematic representation of sol-gel process

gel process involves transition of a solution from a liquid "sol”

. In general , the term sol - gel refers to a process in which solid


dimensional network extending throughout the liquid (gel form).Sol

gel reaction mainly consists of two steps viz hydrolysis and condensation. The

subsequent hydrolysis and polycondensation reaction of the precursor resulted in

(colloidal solution). This ‘sol’ is further coated on substrate by

coating technique to form a thin film. Fig. 2.1: Schematic representation


2. 3

tc [6]. In this process a solution

oxide precursor is applied to a substrate by spinning,


nning the substrate at high speed. Centripetal



[7]

from a liquid "sol” into a

a process in which solid


ding throughout the liquid (gel form).Sol

hydrolysis and condensation. The

the precursor resulted in the

(colloidal solution). This ‘sol’ is further coated on substrate by

Schematic representation



The following are the steps involved under sol-gel technique:

Step 1: ‘Sol’ preparation:

The starting materials used in preparation of “sol" is usually a precursor solution

such as inorganic metal salts or metal organic compounds such as metal alkoxides [8].

The deionized (DI) water then added in precursor solution with solvent such as ethanol.

The prepared solution must have a proper molar ratio. The continuous phase in a sol is a

liquid and the dispersed phase is a solid. The difference between a sol and a noncolloidal

liquid is that solid nanoparticles are dispersed throughout the liquid in a sol. The final

mixing of solution undergoes hydrolysis and polycondensation so as to form wet gel.

Step 2: Spin- On Deposition:

In this process, a small amount of prepared solution ( just before its gel point) is

placed onto the center of a substrate as shown in Figure 2.1,afterwards the substrate is

spin at high speed in order to spread the fluid uniformly. The thickness of the film can

be controlled by varying the speed of spinning.

Step 3: Drying:

The water along with other liquids (solvent) entrapped within the pores of the gel

structure, which is removed during this stage. Drying is performed at a temperature of

about 400ºF (~204ºC). After drying, the gel gets converted into a monolithic icroporous

structure called ‘Xerogel’. Whereas drying at super- critical condition preventing

collapsing of the gel network and it gives a macro- porous low density structure

called ‘Aerogel’.

Process parameters:

The microstructure of the films can be easily modified by controlling the following

parameters:

• The ratio between alkoxide and water

• Type of catalyst used

• Temperature

• Type of solvent

• pH



• Relative and absolute concentrations of the reactants

Applications [9-10]:

1. Protective and decorative coatings and electro

glass, metal and other types of substrates in

2. Solid- state components to high surface area resulted thin films, coatings and fibers.

3. The sol – gel route opens new ways for the powerless processing of shaped

materials such as films, micro

4. Bioreactors and biosensors can also be easily made.

2.2.2 Dip-coating:

‘Dip coating’ is a simple technique widely used for the deposition of thin films.

coating technique, a substrate to be coated is immersed in a liquid. Further, it withdrawn

with a well-defined withdrawal speed under controlled temperature and atmospheric

conditions. The thickness is primarily affected by fluid density, fluid viscosity, and

surface tension of the liquid. A faster withdrawal speed pulls more fluid up onto the

surface of the substrate before it has time to flow back down into the solution. The

coating thickness can be calculated by the Landau

Where, (h) is coating thickness

(v) is viscosity

(γLV) is liquid-va

( r) is density

(g ) is gravity

Dip coating- process flow:

The dip coating process can be separated into five stages:

The base material to which it is desired to produce a film is dissolved; the concentrat

is controlled to obtain the desired film thickness.



• Relative and absolute concentrations of the reactants

Protective and decorative coatings and electro- optic components can be applied to

glass, metal and other types of substrates in this method.

state components to high surface area resulted thin films, coatings and fibers.

gel route opens new ways for the powerless processing of shaped

materials such as films, micro- spheres or fibers.

biosensors can also be easily made.

‘Dip coating’ is a simple technique widely used for the deposition of thin films.


withdrawal speed under controlled temperature and atmospheric



ate before it has time to flow back down into the solution. The

coating thickness can be calculated by the Landau-Levich equation 2.1 [11].

…………. (2.1)

Where, (h) is coating thickness

vapour surface tension,

process flow:

The dip coating process can be separated into five stages:

The base material to which it is desired to produce a film is dissolved; the concentrat

is controlled to obtain the desired film thickness.


2. 5

optic components can be applied to

state components to high surface area resulted thin films, coatings and fibers.

gel route opens new ways for the powerless processing of shaped

‘Dip coating’ is a simple technique widely used for the deposition of thin films. In Dip


withdrawal speed under controlled temperature and atmospheric



ate before it has time to flow back down into the solution. The

Levich equation 2.1 [11].

The base material to which it is desired to produce a film is dissolved; the concentration



1. Immersion: Insert the substrate in the solution phase as shown in the solution of the

coating material at a constant speed as shown in Fig. 2.2 and keep it for particular time.

2. Start-up: The substrate has remained inside the solution for a while and is starting to

be pulled up.

3. Deposition: The thin layer deposits itself on the substrate while it is pulled up. The

withdrawing is carried out at a constant speed. The speed determines the thickness of the

coating (faster withdrawal gives thicker coating material).

4. Drainage: If in case the excess liquid will drain from the surface.

5. Evaporation: In last stage the solvent evaporates from the liquid, forming the thin

layer.

In the continuous process, the steps are carried out consecutively.

Figure. 2.2: Different stages of dip coating process [12]

The applied coating may remain wet for several minutes till the solvent evaporates. This

process can be accelerated by heated drying. In addition, the coating may be cured by a

variety of means including conventional thermal, UV, or IR techniques depending on the

coating solution formulation. Once a layer is cured, another layer may be applied on top

of it with another dip-coating / curing process. Thus the multi-layer stack can be

constructed using this method. When dipping a substrate into a chemical polymerization

solution, part of the polymer will be deposited onto its surface. The thickness of the film

is usually controlled by dipping time. Another similar process involves alternatively

immersing a substrate into the monomer and oxidant solutions. The adsorbed monomer

will be polymerized on the surface of substrate. More recently, an angle-dependent dip

coating process has been developed.



Advantages [13-14]:

Homogeneous film.

Both sides can be coated in a single process.

Amount of coating liquid required is less than that for spin coaters.

Cost-effective technique.

Applications:

Dip-coating is widely used in industry for coil coating, roll coating [15]

2.2.3 Spin-coating:

‘Spin coating’ (spin-on) has been used for several decades for the deposition of

thin films. In spin-coating method a small puddle of a fluid resin is placed onto the center

of a substrate and then spinning the substrate at constant speed. Centripetal acceleration

will cause the resin to spread over, and eventually off, the edge of the substrate thereby

leaving a thin film of resin on the surface. Final film thickness and other properties will

depend on the nature of the resin (viscosity, drying rate, percent solids, surface tension,

etc.) and the parameters chosen for the spin process [16]. Factors such as final rotational

speed, acceleration, and fume exhaust; contribute to define the properties of coated films.

Spin coating is an extensive practice in modern science and engineering, where it is used

to deposit uniform coatings of organic materials to uniformly distribute on a flat surface.

Figure 2.3: The different "stages" of spin coating- a) Dispensation (not modeled)

b) Acceleration (not modeled). c) Flow dominated. d) Evaporation dominated



Fig. 2.3 shows the different "stages" of spin coating.

method for preparing films from soluble conducting polymers. In this process, the

conducting polymer solution is spread on a rotating substrate .

solvent, a thin film was formed. Repeating above process is feasible, which can control

the thickness of the film. Concentration of the solution and rotating rate of the substrate

are also plays important roles in adjusting the thickness of the film produced [17]. This

method can coat conducting polymers on both conducting and insulating substrates.

In order to produce a solid film via spin coating, the solution must evapora

the starting point, the concentration of the solute is uniform. The solvent evaporates

uniformly over the entire surface area causing solids concentration (t) to increase,

independently of (r) {since h is independent of (r) for uniform films}. Makin

additional approximations: i) that (c) is independent of (z), ii) that the volume of the

liquid solution is equal to the volume of the solvent plus the volume of the solute, and iii)

that the evaporation rate is only a function of experimental parame

one may define a volume per unit area (L) of liquid and (S) of solid, so that[c(t)]=S/(S+L)

and [h(t)]=S+L. The initial film thickness will reduce uniformly as before, but the

thinning rate is different. The rate of change of S and

rate (e) is [18.]

Advantages:

The spin coating technique has many advantages in coating operations as

1. Spin coating is a fast (only few seconds per coating) and easy method to generate thin

and homogeneous organic films out of solute ions.

2. Spin coating is a procedure used to apply uniform thin films to flat substrates

on large surface area too.



he different "stages" of spin coating. Spin-coating is a simple


lymer solution is spread on a rotating substrate . After evaporation of



also plays important roles in adjusting the thickness of the film produced [17]. This


In order to produce a solid film via spin coating, the solution must evapora



independently of (r) {since h is independent of (r) for uniform films}. Makin



that the evaporation rate is only a function of experimental parameters such as spin speed,



thinning rate is different. The rate of change of S and L due to outflow and evaporation at

……………………………… (2.2

The spin coating technique has many advantages in coating operations as


and homogeneous organic films out of solute ions.

2. Spin coating is a procedure used to apply uniform thin films to flat substrates

.


2. 8

coating is a simple


After evaporation of



also plays important roles in adjusting the thickness of the film produced [17]. This


In order to produce a solid film via spin coating, the solution must evaporate. At



independently of (r) {since h is independent of (r) for uniform films}. Making the



ters such as spin speed,



L due to outflow and evaporation at

(2.2)

The spin coating technique has many advantages in coating operations as-


2. Spin coating is a procedure used to apply uniform thin films to flat substrates, if in case



3. In the spin coating process the interactions between substrate and solution layer are

stronger than the interactions between solution surface layer and air.

4. Spin coating gives high uniformity over the surface of curved surface curved paths.

5. Spin coating can only be performed on one part at a time, in contrast to dip coating in

which many parts may be processed simultaneously.

6. Its biggest advantage being its lack of coupled process variables

7. The spin speed and the fluid viscosity are the only degrees of freedom, making the

spin coating process very robust.

8. The film thicknesses are easily changed by changing the spin speed, or switching to a

different viscosity photoresist.

9. This is a particularly advantageous method when the fluid or substrate itself has poor

wetting abilities and can eliminate voids that may otherwise form.

10. The slower rate of drying offers the advantage of increased film thickness uniformity

across the substrates.

Applications:

Spin coating is widely used in microfabrication, where it is being used to create

thin films with thicknesses below 10 nm. It is used intensively in photolithography, to

deposit layers of photoresist about 1 micrometer thick.

2.2.4 Spray pyrolysis technique:

‘Spray pyrolysis’ is a simple and low-cost technique that has been used to prepare

the thin film on a variety of substrates like glass, ceramic or metallic. These films were

used in various devices such as solar cells, sensors, and solid oxide fuel cells. Due to the

simplicity of the apparatus and the good productivity of this technique on a large scale, it

offered a most attractive way for the formation of thin films of metal oxides. This

technique is the most popular today because of its applicability to produce variety of

conducting and semiconducting materials and devices [19].

The basic principle involved in spray pyrolysis technique is pyrolytic

decomposition of salts of a desired compound to be de deposited. Every droplet of spray



reaching the surface of the hot substrate undergoes pyrolytic (endothermic)

decomposition and forms a single crystalline or cluster of crystallites as a product. The

other volatile by-products and solvents escape in the form of vapour phase. The

substrates provide thermal energy for the thermal decomposition and subsequent

recombination of the constituent species, followed by sintering and crystallization of the

clusters of crystallites and thereby resulting in coherent film. The required thermal energy

is different for the different materials and also for the different solvents used in the spray

process. The automization of the spray solution into a spray of fine droplets also depends

on the geometry of the spraying nozzle and the pressure of a carrier gas.

Figure 2.4: Schematic diagram of the spray pyrolysis technique

The spray pyrolysis mainly consists of spray nozzle, rotor for spray nozzle, liquid

level monitor, hot plate, gas regulator value and airtight fiber chamber.

a. Spray nozzle:

‘Spray nozzle’ is used to carry the gas whose film is to be deposited. It is made up of

glass and consists of the solution tube surrounded by the glass bulb. At the tip of the

nozzle the vacuum is created, and the solution is automatically sucked in the solution

tube and the spray process starts. The Rotor controls the linear simple harmonic motion

of the spray nozzle over the required length of the hot plate.



b. Liquid level monitor:

The spray rate at a fixed air pressure depends upon the height of the solution

measured with respect to the tip of the nozzle. The arrangement for the change in height

of the solution forms the liquid level monitor system.

c. Hot plate:

The iron disc (with diameter 16 cm and thickness 0.7 cm along with 2000 Watt

heating coil is fixed) served as a hot plate. Maximum temperature of 600 ± 5oC can be

achieved with this arrangement. The chromel-alumel thermocouple is used to measure the

temperature of the substrates. It is fixed at the center of the front side of the iron plate.

The temperature of the hot plate is monitored with the help of temperature controller.

d. Gas regulator valve:

The gas regulator valve is used to control the pressure of the carrier gas flowing

through the gas tube of the spray nozzle. A glass tube of length 25 cm and of diameter 1.5

cm is converted into gas flow meter. Since air pressure depends upon the size of the air

flow meter, the air flow meter should be calibrated from nozzle to nozzle.

e. Air tight fiber chamber:

Since the number of toxic gases is evolved during the thermal decomposition of

sprayed solution, hence it is necessary to fix the spraying system inside with airtight fiber

chamber. This chamber of the size (60 cm x 60 cm x 60 cm) was fabricated. The fiber

avoids the corrosion of the chamber. The outlet of chamber is fitted to exhaust fan to

remove the gases evolved during thermal decomposition. In spray pyrolysis, the process

parameters like precursor solution, automization of precursor solution, aerosol transport

and decomposition of precursor are important factors while studying the structural,

electrical & optical properties, morphology and crystallinity of the thin films. Various

steps during pyrolysis of aerosols are shows in figure 2.5.



Fig. 2.5: Various steps during pyrolysis of aerosols

A) In the first step, an aqueous precursor solution is converted into aerosols (droplets) by

spray nozzle and the evaporation of solvent takes place.

B) In this step, vaporization of the solvent leads to the formation of precipitate as the

droplets approaches towards the substrate.

C) Pyrolysis of the precipitate occurs in succession before the precipitate reaches the

substrate. When the precipitate reaches the substrate, nucleation and the growth of metal

oxide thin films on the substrate take place.

E) Finally, the growth of the nuclei leads to the formation of continuous thin layer of

metal oxide.

Advantages:

It has capability to produce large area, high quality adherent films of uniform

thickness.

Does not require vacuum at any stage.

Spray pyrolysis does not require high quality targets or substrates.

The deposition rate and the thickness of the films can be easily controlled over a

wide range thereby changing the spray parameters.

It operates at low temperature.



It offers an extremely easy way to dope films with virtually any elements in any

proportion, just by adding it in some form to the spray solution.

2.2.5 Electrochemical deposition:

Electrochemical deposition technique in recent decades, evolved from an art to an

exact science. This development is seen as responsible for the ever-increasing number

and widening types of applications in the field of practical science and engineering.

Electrodeposition is a low cost and efficient method to produce thin films. The

most interesting feature in electrodeposition is that, the composition and crystalline

structure of the thermoelectric material can be controlled by adjusting the

electrodeposition parameters. Some of the technological areas in which methods of

electrochemical deposition constitute an essential component are all aspects. It includes

electronics—macro and micro, optics, optoelectronics, and sensors of most types. In

addition, a number of key industries, such as the automobile industry, adopt this method

even when other methods, such as evaporation, sputtering, chemical vapor deposition

(CVD), are available as an option.

Electrochemical reaction is a chemical reaction which happens at the

interface between an electron conductor (electrode) and an ionic conductor (electrolyte)

of an electrochemical cell, because of the transfer of electrons in between the

electrolyte and the electrode. This is basically a ‘REDOX’ (Reduction-Oxidation) type

of reaction. Oxidation reaction happens on the anode and the reduction reaction on the

cathode. In addition electrochemical deposition is the process that uses reduction

reactions to deposit an element or a compound which is dissolved in the electrolyte as

ions on the top of an electrode.



Figure: 2.6 (a) Electrochemical deposition setup (b) Reactions in the

electrodeposition process [RE: reference electrode, WE: working electrode, CE:

counter electrode]

Generally an electrochemical setup consists of a Cathode (negative polarity), anode

(positive polarity), a power supply unit and an electrolyte. The three electrodes are

named as working, counter and reference electrodes, respectively, as shown in the

Figure 2.6. ‘WE’ is the one that we apply a desired potential to supply electrons to

the electrolyte during reduction reaction or transfer electrons from the electrolyte

during oxidation reaction. The ‘RE’ is a half cell which has a well defined potential used

to measure the potential of the ‘WE’. Therefore it should not pass any current through

it. ‘CE’ is used to maintain that current flow.

Normally the surface area of the ‘CE’ is larger than that of the ‘WE’ in order to keep a

uniform current flow through the ‘WE’. Electrodeposition is a well-known technology to

deposit metals and alloys in the industry. It has wide applications from copper deposition

in electronic industry for integrated circuit to zinc or chromium coating for surface

protection.



Kinetics and mechanism of the deposition process [20]:

The rate of the deposition reaction [Eq. 2.3)] is defined as the number of moles of

Mz depositing per second and per unit area of the electrode surface:

ʋ = k [Mz+]…………….( 2.3)

Where, k = rate constant of the reduction reaction and

[Mz+] = represent the activity of Mz+

The rate constant ‘k’ of electrochemical process is interpreted on the basis of the

statistical mechanics and is given by the following expression (2.4)

K= kB *T / h*(-Δ Ge‡/R*T) --------(2.4)

Where, ( kB )is the Boltzmann constant,

(T )is the absolute temperature,

( h) is the Planck constant,

(Ge‡) is the electrochemical activation energy, and R is the gas constant.

The electrochemical activation energy is a function of the electrode potential (E):

ΔGe‡ = ƒ( E) …………… (2.5)

During the electrodeposition, reaction at the electrode, it includes various steps: [21]:

(1) Ionic transfer

(2) Discharge

(3) Breaking up of ion-ligand bond

(4) Incorporation of atoms onto the electrode followed by nucleation and growth

Advantages:

It is simple and low operating temperature technique.

Economical technique because of its low cost apparatus, and negligible waste

materials.

It is not required to have very pure starting materials.



The deposition is mainly controlled by electrical parameters such as electrode

voltage and current density, which are easily adjusted to control the film thickness,

composition, morphology, etc.

An electrochemical synthesis is an oxidation or a reduction reaction. By fine-tuning

the applied cell potential, the oxidizing or reducing power can be continuously varied

and suitably selected.

2.3 Introduction to characterization techniques:

Once synthesized conducting polymers, it is essential to characterized them in

order to check the practical feasibility, material confirmation by a wide array of test.

Advancement in characterization techniques made possible to study the material

properties at atomic scale. The characterization typically has as a goal to improve the

performance of the material. As such, many characterization techniques should ideally be

linked to the desirable properties of the material such as physiochemical, electrical,

surface related properties, optical properties etc. Characterization techniques are typically

used to determine molecular structure, morphology, crystal size, particle size, film

thickness etc. Since there are several advanced characterization methods available to

characterize the PANI for the appropriate analysis.

The following some characterization techniques are used for the characterization

PANI thin films.

2.3.1 Ellipsometry:

‘Ellipsometry’ is a non-invasive, non-destructive measurement technique that has

been used for obtaining the thickness, refractive index, stress and topography of thin

films. It has applications in many different fields from basic research to industrial

applications. Ellipsometry is also becoming more interesting to researchers in other

disciplines such as biology and medicine. Ellipsometry is a very sensitive measurement

technique and provides unequalled capabilities for thin film metrology. The name



"ellipsometry" stems from the fact that light reflected at angle from a sample

has Elliptical polarization

Working Principle:

The basic components of an

generator, sample, polarization analyzer and detector.

diagram of ellipsometer.

Figure 2.7:

The polarization generator and analyzer are constructed of optical components that

manipulate the polarization: polarizer, compensators and phase modulator. A light source

produces unpolarized light which is then sent through a polarizer. The polarizer all

light to perfect electric field orientation to pass. The polarizer axis is oriented between p

and s- planes such that both arrive at the sample surface. The linearly polarized light

reflect from the sample surface, becomes elliptically polarized, and

continuously rotating polarizer orientation relative to the electric field “ellipse” coming

from the sample. The information compared to the known input polarization to determine

the polarization change caused by sample reflection. The in

p- and s-components. The reflected light has undergone amplitude and phase changes for




rization.

The basic components of an ellipsometer are a light source, polarization

generator, sample, polarization analyzer and detector. Figure 2.7 shows the block

Figure 2.7: Basic block diagram of ellipsometer [22]



produces unpolarized light which is then sent through a polarizer. The polarizer all

light to perfect electric field orientation to pass. The polarizer axis is oriented between p

planes such that both arrive at the sample surface. The linearly polarized light

reflect from the sample surface, becomes elliptically polarized, and travels through a



the polarization change caused by sample reflection. The incident light is linear with both

The reflected light has undergone amplitude and phase changes for


2. 17


are a light source, polarization

shows the block

[22]



produces unpolarized light which is then sent through a polarizer. The polarizer allows

light to perfect electric field orientation to pass. The polarizer axis is oriented between p-

planes such that both arrive at the sample surface. The linearly polarized light

travels through a



cident light is linear with both

The reflected light has undergone amplitude and phase changes for



both p- and s-polarized light, and ellipsometry measures their changes. The ellipticity of

the reflected light depends on the optical constants of the thin film and its thickness.

Ellipsometer utilizes two polarizer’s, one is placed before the sample and the

other often called as analyzer placed just prior to the detector. The incident

monochromatic beam is collimated and transmitted through a polarizer as well as

compensator. The reflected beam is then appeared at the analyzer and detector. Here, the

change in phase and amplitude of the beam is observed.

From these orientations and the direction of polarization of the incident light, the

relative phase change and the relative amplitude change are calculated by reflection

from the surface. The measure of these and is given by,

�� (�) =��

�� ……………………. (2.6)

)()( SPSP aa ……………… (2.7)

Thus, the obtained values of Ψ and Δ are used to determine the refractive index,

extinction coefficient and thickness of the films.

Advantages

Non-destructive and non-contact technique

No sample preparation

Solid and liquid samples

Single and multi-layer samples

Accurate measurement of ultra-thin films of thickness < 10nm

2.3.2 Fourier Transform Infrared (FTIR) spectroscopy:

The FTIR spectroscopy is the most powerful tool used for the

identification of chemical bonding (functional groups) of material. The FTIR can be

applied to the analysis of solids, liquids, and gasses. The term FTIR is a method of

infrared spectra by first collecting an interferogram of a sample signal using an

interferometer, and performing a Fourier Transform (FT) on the interferogram to obtain

the spectrum. Chemical IR spectroscopy was emerged as a science in 1800 by Sir



William Herschel. IR spectroscopy is the measurement of the wavelength and intensity of

the absorption of infrared light by a sample. Firstly most IR instrumentation was based on

prism or grating monochromators. Michelson invented interferometer in 1881. In 1949

Peter Fellgett obtained the first IR spectrum by using FTIR spectrometer. In 1966

Cooley-Tukey developed an algorithm, which quickly does a Fourier transform [23].

Principle of IR spectroscopy:

The principle of FTIR is based on the fact that molecular bonds vibrate at various

frequencies depending on the elements and the type of bonds. For any given bond, there

are several specific frequencies at which it can vibrate. According to quantum mechanics,

these frequencies correspond to the ground state (lowest frequency) and several excited

states (higher frequencies). One way to cause the frequency of a molecular vibration to

increase is to excite the bond by absorbing light energy. Infrared light is energetic enough

to excite molecular vibrations to higher energy levels. A molecule that is exposed to

infrared rays absorbs infrared energy at frequencies which are characteristic to that

molecule. For any given transition between two states the light energy (determined by the

wavelength) must exactly equal the difference in the energy between the two states

usually ground state (E0) and the first excited state (E1). In a molecule, the differences of

charges in the electric fields of its atoms produce the dipole moment of the molecule.

Molecules with a dipole moment allow infrared photons to interact with the molecule

causing excitation to higher vibration states. Diatomic molecules do not have a dipole

moment since the electric fields of their atoms are equal. During FTIR analysis, a spot on

the specimen is subjected to a modulated IR beam. The specimen's transmittance and

reflectance of the infrared rays at different frequencies is translated into an IR absorption

plot consisting of reverse peaks. Once an interferogram is collected, it needs to be

translated into a spectrum (emission, absorption, transmission, etc.). The process of

conversion is through the Fast Fourier Transform algorithm. The resulting FTIR spectral

pattern is then analyzed and matched with known signatures of identified materials in

the FTIR library.



The FTIR spectrometer operated on a principle called Fourier transform. The

mathematical expression of Fourier transform can be expressed as

F (ω) = ∫ �(�)��

�� ……………(2.8)

And reverse Fourier transform is

F (x) =�

��∫ �(�)��

�

�� ……………(2.9)

Where, ω is angular frequency and x is the optical path difference in our case.

F(ω) is the spectrum and f(x) is called the interferogram. It is clear that if the

interferogram f(x), is determined experimentally, the spectrum F(ω) can be obtained by

using Fourier transform.

Working of FTIR Spectroscopy:

From Fig. 2.8 the basic components of an FTIR are Infrared source, Interferogram,

sample, detector and signal & data processing. The unique part of an FTIR spectrometer

is the interferometer. Infrared radiation from the source is collected and collimated (made

parallel) before it strikes the beam splitter. The beam splitter ideally transmits one half of

the radiation, and reflects the other half. Both transmitted and reflected beams strike

mirrors, which reflect the two beams back to the beamsplitter. Thus, one half of the

infrared radiation that finally goes to the sample has first been reflected from the

beamsplitter to the moving mirror, and then back to the beamsplitter. The other half of

the infrared radiation going to the sample has first gone through the beamsplitter and then

reflected from the fixed mirror back to the beamsplitter. When these two optical paths are

reunited, interference occurs at the beam splitter because of the optical path difference

caused by the scanning of the moving mirror. On leaving the sample compartment the

light is refocused on to the detector. The difference in optical path length between the two

arms to the interferometer is known as the retardation. An interferogram is obtained by

varying the retardation and recording the signal from the detector for various values of

the retardation.



Figure 2.8: Block diagram of FTIR

The form of the interferogram when no sample is present depends on factors such as the

variation of source intensity and splitter efficiency with wavelength. This results in a

maximum at zero retardation, when there is constructive interference at all wavelength

followed by series of "wiggles". The position of zero retardation is determined accurately

by finding the point of maximum intensity in the interferogram. When a sample is present

the background interferogram is modulated by the presence of absorption bands in the

sample.

As an interferogram is measured with a sample and Fourier transformed, a sample

single beam spectrum is obtained. It looks similar to the background spectrum except that

the sample peaks are superimposed upon the instrumental and atmospheric contributions

to the spectrum. To eliminate these contributions, the sample single beam spectrum must

be normalized against the background spectrum. Consequently, a transmittance spectrum

is obtained as follows.

%T = I/Io ……………( 2.10)

Where, %T - is percentage transmittance;



I - is the intensity measured with a sample in the beam (from the sample single

beam spectrum);

Io - is the intensity measured from the back ground spectrum.

The absorbance spectrum can be calculated from the transmittance spectrum using the

following equation.

A = -log10 T……………… (2.11)

Where, (A) is the absorbance.

The final transmittance/absorbance spectrum should be devoid of all instrumental

and environmental contributions, and only present the features of the sample. If the

concentrations of gases such as water vapor and carbon dioxide in the instrument are the

same when the background and sample spectra are obtained, their contributions to the

spectrum will ratio out exactly and their bands will not occur. If the concentrations of

these gases are different when the background and sample spectra are obtained, their

bands will appear in the sample spectrum.

2.3.3. UV-visible spectroscopy:

UV-visible measurements were applied to obtain information about electronic

structure and transitions of various redox state of compounds. Ultraviolet and visible

(UV-Vis) absorption spectroscopy is the measurement of the attenuation of a beam of

light after it passes through a sample or after reflection from a sample surface.

Absorption spectroscopy, in general, refers to characterization techniques that

measure the absorption of radiation by a material, as a function of the wavelength.

Depending on the source of light used, absorption spectroscopy can be broadly

divided into infrared and UV-visible spectroscopy.



Basic Principle:

Ultraviolet and visible light are energetic enough to promote outer electrons to

higher energy levels, and UV-Vis spectroscopy is usually applied to molecules or

inorganic complexes in solution. The UV-Vis spectra have broad features that are of

limited use for sample identification but are very useful for quantitative measurements.

The concentration of an analyte in solution can be determined by measuring the

absorbance at some wavelength and applying the Beer-Lambert Law. Since the UV-Vis

range spans the range of human visual acuity of approximately 400 - 750 nm, UV-Vis

spectroscopy is useful to characterize the absorption, transmission, and reflectivity of a

variety of technologically important materials, such as pigments, coatings, windows, and

filters. This more qualitative application usually requires recording at least a portion of

the UV-Vis spectrum for characterization of the optical or electronic properties of

materials. Absorption measurements can be at a single wavelength or over an extended

spectral range. When sample molecules are exposed to light having energy that matches a

possible electronic transition within the molecule, some of the light energy will be

absorbed as the electron is promoted to a higher energy orbital. An optical spectrometer

records the wavelengths at which absorption occurs, together with the degree of

absorption at each wavelength. The resulting spectrum is presented as a graph of

absorbance versus wavelength. The peaks in a UV-Vis spectrum are commonly due to n

→ π* and /or π→ π* transitions. Both the shape of the peak(s) and the wavelength of

maximum absorbance (λmax) in spectrum give information about the structure of the

compounds. The combination of electronic absorption spectroscopy and electrochemistry

provides valuable information regarding electrochromic properties of materials.



Figure 2.9: Schematic of a wavelength-selectable, single-beam UV-Vis spectrophotometer [25]

The figure 2.9 shows the Schematic of a wavelength-selectable, single-beam UV-Vis

spectrophotometer. This light is passed through a monochromator to select a single

wavelength (a monochromator uses some dispersive element such as a grating or a prism

to specially separate the colors of light and then a slit to select one of the colors). One of

these beams passes through a reference sample cell which usually contains everything

that is in the sample to be measured except for the molecules you want to know the

spectrum of.

2.3.4. X-ray diffraction (XRD):

X-ray diffraction (XRD) is one of the fundamental experimental techniques used

to analyze the atomic and molecular structure of a crystal, in which the

crystalline atoms cause a beam of incident X-rays to diffract into many specific directions.

By measuring the angles and intensities of these diffracted beams, a crystallographer can

produce a three-dimensional picture of the density of electrons within the crystal. From

this electron density, the mean positions of the atoms in the crystal can be determined, as

well as their chemical bonds, their disorder and various other information.Wilhelm Conrad



Röntgen discovered X-rays in 1895. In 1901 he was honored by the Noble prize for

physics.

Basic Principle:

Crystals are regular arrays of atoms, and X-rays can be considered waves of

electromagnetic radiation. Atoms scatter X-ray waves, primarily through the atoms'

electrons. Just as an ocean wave striking a lighthouse produces secondary circular waves

emanating from the lighthouse, so an X-ray striking an electron produces secondary

spherical waves emanating from the electron. This phenomenon is known as elastic

scattering, and the electron (or lighthouse) is known as the scatterer. A regular array of

scatterers produces a regular array of spherical waves. Although these waves cancel one

another out in most directions through destructive interference, they add constructively in

a few specific directions, determined by Bragg's law:

2*d*sinθ = nλ …………………… (2.12)

Where, θ is the angle between incident X-ray beam and scattering plane, and λ is

the wavelength of incident X-ray.

A powder X-ray diffractometer consists of an X-ray source (usually an X-ray

tube), a sample stage, a detector and a way to vary angle θ. The X-ray is focused on the

sample at some angle θ, while the detector opposite the source reads the intensity of the

X-ray it receives at 2θ away from the source path. The incident angle is than increased

over time while the detector angle always remains 2θ above the source path. Figure 2.10

shows the schematic ray diagram of XRD



Figure 2.10: Schematic ray diagram of XRD

The Scherrer equation, in X-ray diffraction and crystallography, is a formula that

relates the size of sub-micrometre particles, or crystallites, in a solid to the broadening of

a peak in a diffraction pattern [26]. It is named after Paul Scherrer It is used in the

determination of size of particles of crystals in the form of powder.

The Scherrer equation can be written as:

----------------------- (2.13)

Where, (D) is mean size of the ordered (crystalline) domains

(λ) is wavelength of X ray,

(β) is full width and half maxima,

(θ) is Bragg’s angle

2.3.5. Scanning Electron Microscope (SEM):

A Scanning Electron Microscope (SEM) is a tool used for seeing the object at

nanolevel by magnifies it from about 10 times up to 300,000 times. A ‘SEM’ is a type

of electron microscope that produces images of a sample by scanning it with a focused

beam of electrons. In SEM uses electrons instead of light to form an image. In a scanning



electron microscope, a tiny electron beam is focused onto the sample. Simultaneous to

scanning the beam across a selected sample area, generated signals are being recorded

and thereby an image is formed pixel by pixel. The electrons interact with atoms in the

sample, producing various signals that can be detected and that contain information about

the sample's surface topography and composition. The electron beam is generally

scanned in a raster scan pattern, and the beam's position is combined with the detected

signal to produce an image. With SEM one can achieve resolution better than 1

nanometer scale. Specimens can be observed in high vacuum, in low vacuum, and (in

environmental SEM) in wet conditions.The most common mode of detection is by

secondary electrons emitted by atoms excited by the electron beam. The number of

secondary electrons is a function of the angle between the surface and the beam. On a flat

surface, the plume of secondary electrons is mostly contained by the sample, but on a

tilted surface, the plume is partially exposed and more electrons are emitted. By scanning

the sample and detecting the secondary electrons, an image displaying the tilt of the

surface is created.

Since their development in the early 1950's, scanning electron microscopes have

developed new areas of study in the medical and physical science communities. The SEM

has allowed researchers to examine a much bigger variety of specimens. Because the

SEM utilizes vacuum conditions and uses electrons to form an image, special

preparations must be done to the sample. All water must be removed from the samples

because the water would vaporize in the vacuum. All metals are conductive and require

no preparation before being used. All non-metals need to be made conductive by

covering the sample with a thin layer of conductive material.

In a typical SEM, an electron beam is thermionically emitted from an electron

gun fitted with a tungsten filament cathode. Tungsten is normally used in thermionic

electron guns because it has the highest melting point and lowest vapor pressure of all

metals, thereby allowing it to be heated for electron emission, and because of its low cost.

Other types of electron emitters include lanthanumhexaboride cathodes, which can be

used in a standard tungsten filament SEM if the vacuum system is upgraded and FEG,



which may be of the cold

thermally assisted Schottky

The electron beam, which typically has an

is focused by one or two c

The beam passes through pairs of scanning coils or pairs of deflector plates in the

electron column, typically in the final lens

beam in the x and y axes

sample surface.

Figure 2.1

When the primary electron beam interacts with the sample, the electrons lose energy by

repeated random scattering and absorption within a teardrop

specimen known as the interaction volume, which extends from less than 100

approximately 5 µm into the surface. The size of the interaction volume depends on the

electron's landing energy, the atomic number of the specimen and the specimen's density.



cold-cathode type using tungsten single crystal emitters or the

Schottky type, using emitters of zirconium oxide.

The electron beam, which typically has an energy ranging from 0.2

is focused by one or two condenser lenses to a spot about 0.4 nm to 5


electron column, typically in the final lens as shown in figure 2.11, which deflect the

so that it scans in a raster fashion over a rectang

Figure 2.11: Schematic of scanning electron microscopy


repeated random scattering and absorption within a teardrop-shaped volume of the

specimen known as the interaction volume, which extends from less than 100

µm into the surface. The size of the interaction volume depends on the



2. 28

gle crystal emitters or the

KeV to 40 KeV,

nm to 5 nm in diameter.


, which deflect the

fashion over a rectangular area of the

: Schematic of scanning electron microscopy [27]


shaped volume of the

specimen known as the interaction volume, which extends from less than 100 nm to

µm into the surface. The size of the interaction volume depends on the




The energy exchange between the electron beam and the sample results in the reflection

of high-energy electrons by elastic scattering, emission of secondary electrons

by inelastic scattering and the emission of electromagnetic radiation, each of which can

be detected by specialized detectors. The beam current absorbed by the specimen can also

be detected and used to create images of the distribution of specimen current. Electronic

amplifiers of various types are used to amplify the signals, which are displayed as

variations in brightness on a computer monitor (or, for vintage models, on a cathode ray

tube). Each pixel of computer video memory is synchronized with the position of the

beam on the specimen in the microscope, and the resulting image is therefore a

distribution map of the intensity of the signal being emitted from the scanned area of the

specimen. In older microscopes image may be captured by photography from a high-

resolution cathode ray tube, but in modern machines image is saved to a computer data

storage. Backscattered electrons can also be used to form an electron backscatter

diffraction (EBSD) image that can be used to determine the crystallographic structure of

the specimen.

2.3.6. Transmission Electron Microscope (TEM):

Transmission electron microscopy (TEM) is a microscopy technique in which a

high energy electron beam transmitted through a very thin sample, and analyzes the

microstructure of materials with atomic scale resolution. The transmission electron

microscope is used to characterize the microstructure of materials with very high spatial

resolution. It use to examine fine detail—even as small as a single column of atoms,

which is thousands of times smaller than the smallest resolvable object in a light

microscope. In 1927, Hans Bush showed that a magnetic coil can focus an electron beam

in the same way that a glass lens for light. Five years later, a first image with a TEM was

obtained by Ernst Ruska and Max Knoll [28]

In a TEM, the electrons are accelerated at highvoltage (100-1000 kV) to a

velocity approaching the speed of light (0.6-0.9 c); they must therefore be considered as

relativistic particles. The associated wavelength is five orders of magnitude smaller than

the light wavelength (0.04-0.008 Å). Nevertheless, the magnetic lens aberrations limit the



convergence angle of the electron beam to 0.5° (instead of 70° for the glass lens used in

optics), and reduce the TEM resolution to the Å order. This resolution enables material

imaging (section 3.5) and structure determination at the atomic level (section 3.6 and

3.7). The first TEM was built by Max Knoll and Ernst Ruska in 1931, with this group

developing the first TEM with resolution greater than that of light in 1933 and the first

commercial TEM in 1939 [29]. The electrons are focused with electromagnetic lenses

and the image is observed on a fluorescent screen, or recorded on film or digital camera.

An image is formed from the interaction of the electrons transmitted through the

specimen; the image is magnified and focused onto an imaging device, such as

a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as

a CCD camera.

The resolution ρ of a microscope is defined as the distance between two details

just separable from one another. It can be calculated using the Abb theory of images

formation for optic systems [30]. For incoherent light or electron beam:

ρ = 0.61λ/ sinα (Rayleigh criterion) ………….(2.14)

where, (λ) is the wavelength of the light,

(α) the maximum angle between incident and deflected beam in the limit of the

lens aberrations.

For optical microscopy, the resolution is therefore limited by the wavelength of light

(410-660 nm). The X or γ rays have lower wavelength, but unfortunately, high-

performance lenses necessary to focus the beam to form an image.

2.3.7. Nuclear Magnetic Resonance (NMR):

Nuclear magnetic resonance spectroscopy, most commonly known as NMR

spectroscopy, is a research technique that exploits the magneticproperties of

certain atomic nuclei. It determines the physical and chemical properties of atoms or

the molecules in which they are contained. It relies on the phenomenon of nuclear

magnetic resonance and can provide detailed information about the structure, dynamics,

reaction state, and chemical environment of molecules. The intramolecular magnetic field

around an atom in a molecule changes the resonance frequency, thus giving access to



details of the electronic structure of a molecule. Nuclear magnetic resonance was first

described and measured in molecular beams by Isidor Rabi in 1938. The Purcell group

at Harvard University and the Bloch group at Stanford University independently

developed NMR in the late 1940s and early 1950s. Dr. Edward Mills Purcell and Dr.

Felix Blochshared the 1952 Nobel Prize in Physics for their discoveries [31].

Nuclear Magnetic Spectroscopy is based on the fact that when a population of

magnetic nuclei is placed in an external magnetic field, the nuclei become aligned in a

predictable and finite number of orientations. This circulation is called a local

diamagnetic current, generates a counter magnetic field which opposes the applied

magnetic field, the figure illustrates this effect which is called diamagnetic shielding or

diamagnetic anisotropy. The circulation of electrons around a nucleus can be viewed as

being similar to the flow of an electric current in an electric wire. In atom, the local

diamagnetic current generates a secondary, induced magnetic field which has a direction

opposite that of the applied magnetic field. As a result of diamagnetic anisotropy, each

proton in a molecule is shielded from the applied magnetic field to an extent that depends

on the electron density surrounding it. The greater the electron density around a nucleus,

the greater the induced counter field that opposes the applied field. Hence, the magnetic

field strength must be increased for a shielded proton to flip at the same frequency.

Moreover, depending on their chemical environment, protons in a molecules are shielded

by different amounts. As the molecule is attached to more electronegative, it becomes

less shielded.

The potential energy of the precessing nucleus is given by;

E = - μ B cos θ

Where, (θ) is the angle between the direction of the applied field and the axis of

nuclear rotation.

If energy is absorbed by the nucleus, then the angle of precession, q, will change. It is

important to realise that only a small proportion of "target" nuclei are in the lower energy

state (and can absorb radiation). There is the possibility that by exciting these nuclei, the

populations of the higher and lower energy levels will become equal. If this occurs, then



there will be no further absorption of radiation. The spin system is

possibility of saturation means that we must be aware of the relaxation processes which

return nuclei to the lower energy state

The basic arrangement of an NMR spectrometer is shown

positioned in the magnetic field

circuit. The realigned magnetic fields induce a radio signal in the output circuit which is

used to generate the output signal.

actual spectrum. The pulse is repeated as many times as necessary to allow the signals to

be identified from the background noise.

Figure 2.1

Ideally, the NMR spectroscopist would like relaxation rates to be fast

too fast. If the relaxation rate is fast, then saturation is reduced. If the relaxation

rate is too fast, line-broadening in the resultant NMR spectrum is observed.

There are two major relaxation processes;

Spin - lattice (longitudinal) relaxation

Spin - spin (transverse) relaxation

2.3.8 Current-Voltage (I

Electrical characterization of dielectric thin films includes current

capacitance-voltage (C-V) characterization that is useful to study properties and interface



further absorption of radiation. The spin system is


return nuclei to the lower energy state[32].

The basic arrangement of an NMR spectrometer is shown in figure 2.12

positioned in the magnetic field and excited via pulsations in the radio frequency input


used to generate the output signal. Fourier analysis of the complex output produces the

he pulse is repeated as many times as necessary to allow the signals to

be identified from the background noise.

Figure 2.12: Basic arrangement of an NMR spectrometer

Ideally, the NMR spectroscopist would like relaxation rates to be fast

fast. If the relaxation rate is fast, then saturation is reduced. If the relaxation

broadening in the resultant NMR spectrum is observed.

There are two major relaxation processes;

lattice (longitudinal) relaxation

(transverse) relaxation

Voltage (I-V) analyzer:

Electrical characterization of dielectric thin films includes current-voltage (I

V) characterization that is useful to study properties and interface


2. 32

further absorption of radiation. The spin system is saturated. The


in figure 2.12. The sample is

and excited via pulsations in the radio frequency input


Fourier analysis of the complex output produces the

he pulse is repeated as many times as necessary to allow the signals to

asic arrangement of an NMR spectrometer

Ideally, the NMR spectroscopist would like relaxation rates to be fast - but not

fast. If the relaxation rate is fast, then saturation is reduced. If the relaxation

broadening in the resultant NMR spectrum is observed.

voltage (I-V) and

V) characterization that is useful to study properties and interface



of thin films. The primary measurement resource for parametric test is the source/monitor

unit or SMU. SMUs have three basic modes of operation: voltage source, current source

and common. The basic function of an SMU is to perform one of the following source-

measure operation (1) Source Voltage, Measure Current/ or Voltage (2) Source Current,

measure Voltage and /or Current. The test structure is connecting to this CV/IV system

by means Signatones S-probe station. The keithly 4200 semiconductor characterization

system is used for electrical characterization of fabricated MIS structures. It is an

automated system that provides IV characteristic of MIS diode and CV analyzer gives the

capacitance-voltage characteristics of semiconductor devices and test structure. These

tests are easily and quickly configured and run from the Keithly Interactive Test

Environment (KITE).It is as application program designed and developed specially for

characterizing semiconductor devices and materials.

The simplest of two-terminal network elements is the linear resistor. Ohm's law states

that voltage across a linear resistor is directly proportional to the current flowing through

the resistor. The constant of proportionality R is called resistance.

V = R*I or I = V/R ……………….(2.16)

Therefore, on an I-V plane a linear resistor is characterized by a straight line ( Fig. 2.13 )

passing through the origin. The I-V characteristics may be either in the form of a

mathematical expression relating voltage across the element to the current flowing

through it or in form of a graph on the I-V plane. For example, the I-V characteristics of a

linear resistor may be represented either by Equation 2.16 or by a graph of Figure 2.13.



Figure 2.13: Current-Voltage (I-V) characteristics of chemiresistor [33]

In case of a linear resistor it is more convenient to work with the mathematical model.

On the other hand, it may be easier to use the graphical representation of the I-V

characteristics when dealing with non-linear elements.



References:

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Sciences Vol.5: Nanostructured Thin Films and Surfaces., WILEY-VCH Verlag

GmbH & Co. KGaA, Weinheim,(2010).

2. http://www.ist.fraunhofer.de/en.html

3. http://shodhganga.inflibnet.ac.in/bitstream/10603/4025/8/08_chapter%202.pdf

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18. D. Meyerhofer, "Characteristics of Resist Films Produced by Spinning," J. Appl.

Phys., 49, 3993 (1978).

19. S. Kozhukharov, S. Tchaoushev, Journal of Chemical Technology and

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20. Fundamentals Of Electrochemical Deposition, Milan Paunovic, Mordechay

Schlesinger, A John Wiley & Sons, Inc., Publication, (2006).

21. Wilman Septina, Studies on Electrochemical Syntheses of Semiconductor Thin

Films for Photovoltaic Applications, thesis submitted to Osaka University, (2013).

22. http://en.wikipedia.org/wiki/File:Ellipsometry_setup.svg

23. A. Rogalski, Opto−Electronics Review, 20(3), 279–308, (2012).

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26. http://en.wikipedia.org/wiki/Scherrer_equation

27. http://www.purdue.edu/rem/rs/sem.htm

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