Chapter-II

Applications of Thin Films and

Characterization Techniques

Chapter – II

Applications of Thin Films and Characterization Techniques

Sr. No.

Title

Page No.

Part - A

Applications of Thin Films

2. A.1. Introduction 29

2. A.2 Photoelectrochemical (PEC) solar cells 35 Part -B

Thin Film Characterization Techniques 2. B.1 Introduction 53

2. B.2 Optical absorption study 53

2. B.3 Raman spectral study 55

2. B.4 Scanning Electron Microscopy (SEM) 61

2. B.5 Atomic Force Microscopy (AFM) 64

2. B.6 Energy Dispersive Analysis by X-Rays Measurement (EDS)

66

2. B.7 X- ray Diffraction (XRD) 66

2. B.8 Thickness Measurement 68

2. B.9 Electrical Conduction Studies 69

REFERENCES 75

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CHAPTER– II

Applications of Thin Films and Characterization Techniques

Part - A

Applications of Thin Films

2. A.1. Introduction

There are number of applications in various fields. Few of them are

A.R.coatings, solar energy converters, transistors, coating technology,

interference filters, polarisers, narrow band filters, solar cells,

photoconductors, IR detectors, waveguide coatings, temperature. Controlled

aerospace devices, photo thermal solar coatings (such as black chrome,

Nickel, cobalt etc.) Magnetic films in recording device, superconducting films,

microelectronic devices, diamond films, and high coatings are used for

engineering applications, corrosion resistive thin film coatings and decorative

thin film coatings etc.

2. A.1.1. Applications of thin films:

1] Semiconductor:

Historically, the semiconductor industry has relied on flat, two-dimensional

chips upon which to grow and etch the thin films of material that become

electronic circuits for computers and other electronic devices. This thin layer

(only a couple of hundred nanometers thick) can be transferred to glass,

plastic or other flexible materials, opening a wide range of possibilities for

flexible electronics. In addition, the semiconductor film can be flipped as it is

transferred to its new substrate, making its other side available for more

components. This doubles the possible number of devices that can be placed

on the film. By repeating the process, layers of double-sided, thin-film

semiconductors can be stacked together, creating powerful, low-power, three-

dimensional electronic devices. "It's important to note that these are single-

crystal films of strained silicon or silicon germanium. The strain is introduced

in the way we form the membrane. Introducing strain changes the

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arrangement of atoms in the crystal such that we can achieve much faster

device speed while consuming less power."

For non-computer applications, flexible electronics are beginning to

have significant impact. Solar cells, smart cards, radio frequency identification

(RFID) tags, medical applications, and active-matrix flat panel displays could

all benefit from the development. The techniques could allow flexible

semiconductors to be embedded in fabric to create wearable electronics or

computer monitors that roll up like a window shade. "This is potentially a

paradigm shift. The ability to create fast, low-power, multilayer electronics has

many exciting applications. Silicon germanium membranes are particularly

interesting. Germanium has a much higher adsorption for light than silicon. By

including the germanium without destroying the quality of the material, we can

achieve devices with two to three orders of magnitude more sensitivity." That

increased sensitivity could be applied to create superior low-light cameras, or

smaller cameras with greater resolution.

2] Flat Panel Displays:

Developed from the Mykrolis contamination control technologies,

Entegris provides a broad portfolio of liquid and gas contamination control

technologies for the flat panel display fabrication. The Flat Panel Display

(FPD) fabrication environment is among the worlds most competitive and

technologically complex. Device designers and manufacturers continually

strive to satisfy the worldwide consumer’s appetite for larger displays, greater

pixel resolution and feature-rich performance all at a lower cost than the

previous generation of technology. The need to control contamination in air,

gas and liquid process streams is now a paramount focus of process

engineers and designers. Entegris provides the solutions to succeed under

these extreme conditions.

3] Photovoltaic:

In the familiar rigid solar panel, the energy of incoming photons is

converted to electricity in cells containing two thin layers of crystalline silicon.

31

What makes roll-to-roll production of flexible film solar products possible is

replacement of the crystalline silicon with amorphous silicon supplied in high-

solids slurries that can be deposited onto substrates by web-converting

processes like slot die coating. Microlayer film. EDI can outfit its Contour cast

film dies with a new system, which makes it possible to produce film of

standard thickness yet with dozens of exceedingly thin ‘micro-layers’. The

multiple layer-to-layer interfaces create a torturous path for gas molecules and

thus substantially increase the barrier properties of the film. This is critical for

photovoltaic applications, which require barrier layers to prevent performance

losses caused by infiltration of oxygen or moisture vapour.

"Though as yet little known in the solar industry, the continuous-web

production methods familiar to EDI and its converter customers are a key to

developing high-volume, low-cost production of solar electric systems," said

Miller. "In working with solar manufacturers, EDI draws on extensive

experience in other applications that require thin-gauge, optically clear, close-

tolerance films and coatings with critical functionalities, including films and

coatings for flat panel displays and flexible batteries—particularly relevant,

since solar cells are a kind of battery. In comparison with conventional coating

methods, such as spray, roll, and spin coating, slot dies provide greater

control over coating weight and distribution because they are closed systems

into which coating material is pumped at closely pre-determined rates; in turn

this greater control makes possible thinner coatings,"

Many in the solar power industry and the investment community believe

the arrival of the grid parity point when cost of electricity generated by a

rooftop photovoltaic (PV) cell system is equivalent to that purchased from an

electrical utility will mark a major inflection point for the market that will deliver

a huge increase in growth. However, even when true grid parity arrives, it’s

unlikely to generate an abrupt rise in solar system installations due to the high

upfront costs and the long-term return of investing in a rooftop photovoltaic

system In fact, growth is set to moderate during the years when grid parity

arrives for various regions of the world as the industry enters a more mature

phase.

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4] Optical Coatings:

An optical coating is one or more thin layers of material deposited on

an optical component such as a lens or mirror, which alters the way in which

the optic reflects and transmits light. One type of optical coating is an

antireflection coating, which reduces unwanted reflections from surfaces, and

is commonly used on spectacle and photographic lenses. Another type is the

high-reflector coating which can be used to produce mirrors which reflect

greater than 99.99% of the light which falls on them. More complex optical

coatings exhibit high reflection over some range of wavelengths, and anti-

reflection over another range, allowing the production of dichroic thin-film

optical filters.

5] Data Storage:

As the data storage density in cutting edge microelectronic devices

continues to increase, the superparamagnetic effect poses a problem for

magnetic data storage media. One strategy for overcoming this obstacle is the

use of thermomechanical data storage technology. In this approach, data is

written by a nanoscale mechanical probe as an indentation on a surface, read

by a transducer built into the probe, and then erased by the application of

heat. An example of such a device is the IBM millipede, which uses a polymer

thin film as the data storage medium. It is also possible, however, to use other

kinds of media for thermomechanical data storage, and in the following work,

we explore the possibility of using thin film Ni-Ti shape memory alloy (SMA).

Previous work has shown that nanometer-scale indentations made in

martensite phase Ni-Ti SMA thin films recover substantially upon heating.

Issues such as repeated thermomechanical cycling of indentations, indent

proximity, and film thickness impact the practicability of this technique. While

there are still problems to be solved, the experimental evidence and

theoretical predictions show SMA thin films as an appropriate medium for

thermomechanical data storage.

33

6] Optoelectronic:

An optoelectronic thin-film chip, comprising at least one radiation-

emitting region in an active zone of a thin-film layer and a lens disposed

downstream of the radiation-emitting region, said lens being formed by at

least one partial region of the thin-film layer, the lateral extent of the lens

being greater than the lateral extent of the radiation-emitting region. The thin-

film layer is provided for example by a layer sequence which is deposited

epitaxially on a growth substrate and from which the growth substrate is at

least partly removed. That is to say that the thickness of the substrate is

reduced. In other words, the substrate is thinned. It is furthermore possible for

the entire growth substrate to be removed from the thin-film layer. The thin-

film layer has at least one active zone suitable for generating electromagnetic

radiation. The active zone may be provided for example by a layer or layer

sequence which has a pn junction, a double heterostructure, a single quantum

well structure or a multiple quantum well structure. Particularly preferably, the

active zone has at least one radiation-emitting region. In this case, the

radiation-emitting region is formed for example by a partial region of the active

zone. Electromagnetic radiation is generated in said partial region of the

active zone during operation of the optoelectronic thin-film chip.

7] Super capacitor

Since first patents were in the 1950, the idea of storing charge in the

electric double layer that founds at the interface between a solid and an

electrolyte has presented itself as an attractive notation while its considerable

development was carried out by the slander oil company, Cleveland, Ohio

(SOHIO) in the 1960 a lack of sales lead them to license the technology to

Nippon electric company. The first high power double layer capacitor were

developed for military applications by the pinnacle research institute in 1982.

Conventional capacitor store energy electrostatically on two electrode

separated by a dielectric, the capacitance of which is C= €A/d, where € being

the dielectric constant, A surface area and d the dielectric thickness. In a

double layer capacitor, charges accumulate at the boundary between

electrode and electrolyte to form two charge layers with separation of several

34

Angstroms. Super capacitors are electrochemical energy storage device. The

way that super capacitor store energy is based on two principles type’s

capacitive behaviour the electrical double layer capacitance from the pure

electrostatic charge accumulation at the electrode interface and the pseudo-

capacitance developed from fast and reversible surface redox process at

characteristic potential. Super capacitor integrated in to hybrid vehicle, power

source, power control, multi source supply, fuel save etc.

8] Gas sensors

Advancements in micro technology and the evolution of new

nonmaterial and devices have been playing a key role in the development of

very accurate and reliable sensors. The technology of sensors has developed

tremendously in the last few years owning many scientific achievements from

various experiments, offering newer challenges and opportunity to the quest

for every smaller devices capable of molecular level imaging and monitoring

of pathological samples and the macromolecules has lately gained the focus

of attention of the scientific community, particularly for remote monitoring due

to the increasing need for environmental safety and health monitoring.

Gas sensors generally operate different principles and various gas

sensing elements have been developed in past years, out of which resistive

metal oxide sensors comprise a significant part. However these sensing

elements typically operate at an elevated temperature for maximum

performance. This makes higher power consumption which is not suitable for

the inflation. Over the last decayed carbon nanotube has attracted

considerable attention, and great efforts have been made to exploit their

unusual electronic and mechanical properties. These properties make them

potential candidates for building blocks of active materials in nanoelectronics,

field emission devices, gas storage and gas sensors. Among these the room

temperature gas sensing property is very attractive for many applications.

9] Photoelectrochemical (PEC):

In photoelectrochemical experiments, irradiation of an electrode with light

that is absorbed by the electrode material causes the production of a current

35

(a photocurrent). The dependence of the photocurrent on wavelength,

electrode potential, and solution composition provides information about the

nature of the photoprocess, its energetics, and its kinetics. Photocurrents at

electrodes can also arise because of photolytic processes occurring in the

solution near the electrode surface, Photoelectrochemical studies are

frequently carried out to obtain a better understanding of the nature of the

electrode-solution interface. Photoelectrochemistry and Electrogenerated

Chemiluminescence photocurrent can represent the conversion of light

energy to electrical and chemical energy; such processes are also

investigated for their potential practical applications. Since most of the studied

photoelectrochemical reactions occur at semiconductor electrodes, the brief

review of the nature of semiconductors and their interfaces with solutions is

necessary. Consideration of semiconductor electrodes also helps in gaining a

microscopic understanding of electron-transfer processes at solid-solution

interfaces.

2. A.2 Photoelectrochemical (PEC) solar cells

Though the number of applications of thin films in the large

optoelectronics field, its application in PEC is most important as the need of

energy source is increasing day by day hence the conversion of solar energy

to electrical energy is an important task in this era.

2. A.2.1.Basic theory of the Photoelectrochemical (PEC) solar cells

In1950's, much attention has been given to the possible use of

photoelectrochemical effects to convert solar energy directly into electric

power and, as a result of the energy crisis; the study of photoelectrochemical

phenomena has become increasingly more popular and important. Major part

of the present global energy production is now depends on use of fossil fuels.

However, the inherent problems associated with the use of fossil fuels such

as their dwindling sources and the environmental issues force the mankind to

look for new, renewable more sustainable long-term energy solutions to

provide the future energy demand [1, 2].

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One of the most powerful alternatives for future large scale electricity

production is photovoltaic’s, i.e., the conversion of sunlight directly into

electricity. Sunlight is available in most locations, and it provides such an

enormous supply of renewable energy that if the whole global electricity

demand would be covered exclusively by photovoltaic’s, the total land area

needed for light collection would be only a few percent of the world’s desert

area . Solar cells are easy to install and use, and their operational lifetimes

are long, which eliminates the need for continuous maintenance. Since

photovoltaic systems are modular, they are equally well suited for both

centralized and non-centralized electricity production. Therefore their

potential uses range from consumer electronics portable instruments like

calculators, wristwatches etc. to large power plants. Due to its reliability and

stability, solar energy is a good choice in applications where power is very

important and its shortages or outages cannot be tolerated, for example in

hospitals and certain production plants. Photovoltaic systems can be installed

on rooftops and facades of buildings, and they can be combined with solar

water heating systems. The power generated by rooftop solar cells can be

used locally, and the surplus can be exported to the commercial grid if there

is one in the region. The possibility for local electricity production offers

consumers more freedom by reducing their dependence on the availability

and price of commercial electricity. This is a crucial feature especially in

remote areas that lack the infrastructure of electrification. It is actually more

cost effective to install a photovoltaic system than to extend the grid if the

power requirement lies more than about half a kilometer away from the

electrical line. Rooftop photovoltaic installations, both by public institutions

and by individual citizens, are becoming more and more common worldwide

[3, 4]. In the end of the 20th century dominant energy sources switched to

petroleum, natural gas, hydropower and nuclear energy. Since the reserves

of fossil fuel are very limited and are being depleted very fast, search for

alternate sources of energy are being very much important. The world is

looking towards natural resources such as solar energy, wind energy, bio-

energy, ocean energy and geothermal energy etc., out of this solar energy

has greatest potential. In the last few decades’ lot of efforts has been already

gone in utilizing and converting solar energy in different forms.

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Why Go Solar?

A solar cell is a device that uses the photoelectric effect to generate electricity

from light. Solar cells are used to power many kinds of equipment, including

satellites, calculators, remote radiotelephones, and advertising signs.

The simplest type of solar cell is a silicon diode, but research is

continuing into more exotic materials with greater efficiencies. Modern solar

cells are encapsulated in glass-fronted plastic sheets. They have design

lifetimes that exceed forty years. Sunlight provides about 1.36 kilowatts per

square meter, and most solar cells are between 8 and 12 percent efficient. In

desert areas, they can operate for an average of 6 hours per day when

mounted in nonrotating brackets.

Why solar energy?

Solar power is one of the most promising sources of energy for the

21st Century. It's a technology that has truly come of age. After more than 50

years of refinement and use in space as well as large-scale "solar farms" right

here on Earth, solar power is ready for mainstream use like never before.

Going solar is something you should seriously consider for your residence or

commercial facility. It's good for your monthly electric bills, and it's good for

the planet.

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Characteristics of solar energy

� Clean: Solar power systems generate electricity with zero emission of

Nox, CO2, SOx or other gases associated with global warming and acid

rain.

� Renewable: Solar power systems can convert nature's sunlight into an

unlimited supply of energy.

� Abundant: The amount of sunlight striking earth in any given hour

contains light energy equivalent to the total world energy consumption

for one year.

Present Status of the Earth:

The various environmental problems we presently face on earth, such

as global warming and acid rain, can be attributed to the excessive

consumption of fossil fuels. Over dependence on fossil fuels for energy also

affects the health of everyone on Earth.

With global energy consumption continually increasing, we have

rethink seriously over our approach to the supply of energy. Natural energy

sources that do not release CO2 are critical for the creation of a healthier

planet.

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The clock is ticking:

The remaining oil reserves on the earth are equivalent to about 40 years

of consumption at the current rate of consumption. Actions must be taken now

to avert a crisis of depleting energy sources. Adoption of new natural energy

sources is a pressing need for the whole of society.

World energy resources, provide operable reserves

Solar is the remedy:

Utilization of solar energy, which is clean and inexhaustible, is a

promising substitute for fossil fuels. Photovoltaic (PV) modules are already

well established for residential use in many parts of the world. Large-scale

home systems can lead to energy self-sufficiency, while smaller-scale

systems can at minimum help reduce the consumption of fossil fuels, reduce

monthly electric utility bills, and decrease negative impact on the planet.

How Solar Energy Works:

Typical residential solar power systems use roof-mounted photovoltaic

(PV) modules to generate the electricity needed for day-to-day living. By

integrating these systems with the local power grid, homeowners can

automatically sell surplus electricity through the local grid or top up their solar

generated supply by purchasing additional electricity from their local provider

40

as necessary. This highly efficient system delivers a stable supply of

electricity while ensuring that none of the power generated goes to waste.

Conversion of Sunlight to Electricity

what does the word "photovoltaic" mean, and how do photovoltaics

generate electricity from sunlight? "Photo" means light, and "voltaic" means

electricity, so putting the two together simply means electricity from light.

The individual photovoltaic cells that comprise PV modules are made

up of silicon semiconductors. When sunlight strikes the cell, some of it gets

absorbed into the silicon material. This solar energy frees electrons within the

semiconductor. Electric fields within the PV cell force the electrons to flow in

41

specific direction, which creates a current. N-type silicon carries a negative

charge and P-type silicon carries a positive charge. Metal contacts on the top

and bottom of the PV cell allow the current to be drawn off for external use.

Binary and multinary compound thin films of chalcogenide of II-VI, IV-

VI, III-V, VI-VI groups have already emerged as proven and potential

candidates with low production cost and good performance in the field of solar

energy conversion.

� Classification of the Photoelectrochemical (PEC) Solar Cells

A wide range of power technologies exist which can make use of solar

energy reaching earth and converting them into different useful forms of

energy. Depending on built in potential, due to difference in Fermi energy

level solar energy trapping can be classified of two types-direct solar power

and indirect solar power as shown in Table 2.1 [5].

� Direct solar power:-

Direct solar power involves only one step transformation into a usable

form. Eg. Sunlight hits a photovoltaic cell generating electricity. Sunlight hits a

dark absorber surface of a solar thermal collector. The heat energy may be

carried away by fluid circuit.

� Indirect solar power: -

Indirect solar power involves more than one transformation to reach a

usable form. Eg. Vegetation utilize solar energy and covert into chemical

energy by photosynthesis which can later be burned as fuel to generate

42

electricity (Bio-fuel ) methane (natural gas ) may be derived from the bio-fuel.

Wind turbines, hydroelectric dams are powered by solar energy through its

interaction with the earth’s atmosphere and the resulting weather

phenomena. Energy obtained from oil, coal etc., originated as solar energy

captured by vegetation in the remote geological past and fossilized. Oceans

thermal energy production uses the thermal gradient at different depths to

generate powers these temperature differences are due to the energy of the

sun.

In semiconductor – semiconductor (S-S) junction cells two types of

junction are occurred

a) Homojunction:

A homojunction cell consists of shallow p-n junction formed either by

diffusion of dopant into a monocrystalline semiconductor substrate or by

growth of an epitaxial layer onto the substrate.

b) Heterojunction:

A heterojunction is the interference between two dissimilar materials.

The semiconductor –metal (S-M) junction cells is known as

schottaky barrier cell. It is fabricated by deposing a

semitransparent metal film on the semiconductor surface. The

transparent metal film is normally evaporated on a carefully

processed semiconductor surface.

The MIS and SIS junction cells have a thin interfacial layer of an

insulator between a top inducing contact (metal or semiconductor) and a base

semiconductor. The interfacial layer is generally an oxide or the compound

which is always an insulator.

The semiconductor- liquid (SL) junction cells consist of a

semiconductor photoelectrode dipped into an electrolyte solution along with

counter electrode. The counter electrode is a suitable metal or graphite. The

charge transfer process at the semiconductor/ electrolyte interface results in

band bending establishing a potential barrier at the interface. These cells are

further classified into following groups:

� Photogalvonic cells

� Rechargable cells

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� Photoelectrolytic cell

� Photoelectrocatalytic cells

���� Photoelectrochemical cells

Amongst these groups of cells the photoelectrochemical system is

convinient. A PEC effect is defined as one in which irradiation of electrode /

electrolyte system produce a change in electric potential (on open circuit) or in

the current flowing in external circuit (under short circuit conditions) [6].Early

semiconductor electrochemistry studies have shown that the distribution of

potential at the semiconductor – electrolyte interface is almost similar to that

at a simple p-n junction [7]. A direct conversion of solar energy into electrical

energy using a semiconductor electrolyte interface was demonstrated by

Gerisher these are generally known as photoelectrochemical solar cells [8].

As compare to the traditional solid state cells photoelectrochemical (PEC)

solar cells can be much cheaper. This is particularly important because of the

comparatively low solar radiation power density requiring the use of large area

converters. The future prospects of photoelectrochemical solar energy

conversion method depend on how completely its potential advantage can be

realized in practice. Secondly, the photoelectrochemical method is convenient

in that one of its versions - photoelectrolysis, enables light energy to be

directly converted into chemical energy of the photoelectrochemical reaction

products, and thus permits the energy storage problem to be solved along

with proper energy conversion.

During the last two-three decade much work has been done on

photoelectrochemical (PEC) system in search of suitable liquid junction

photovoltaic solar cells [9].

44

Table 2.1 Classification of solar energy utilization.

2. A.2.1.1.Oxidation and Reduction Process

The charge transfer across the semiconductor/ electrolyte interface in dark

as well as in light results in flow of current through the junction. This is the key

concepts for working in photoelectrochemical solar cells and reports are

available which predict the analogy between the semiconductor and

electrolyte [5].

A process in which substance gains an electron is called a reduction

reaction [10].

OX + e- ↔ Red Eo 2. 1

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Where OX and Red are oxidized and reduced species and Eo is the

standard electrochemical potential. The reverse process of losing an electron

is called an oxidation reaction. In a system, where one species loses an

electron and other species gains an electron is called redox system. If Eo is

positive, the reaction proceeds from left to right and when it is negative, the

reaction proceeds from right to left.

� The oxidized and reduced species in an electrolyte are analogous to

the conduction, valance bands respectively.

� The energy necessary to transfer an electron from the reduced

species to the oxidized species is analogous to the band gap; Eg, of

a semiconductor

� The term Efredox can also be explained in a similar manner as that of

the semiconductor Fermi level, Ef

� The redox potentials is potential required to transfer an electron

from redox species to the vacuum level or vice-versa [11].

2. A.2.1.2.Semiconductor-electrolyte Interface

The semiconductor/electrolyte interface was firstly studied and reported by

Brattain and Garret [12]. When semiconductor is dipped in redox solution, as

its chemical potential is different from the redox potential (Eredox), a new

equilibrium is established between the semiconductor and electrolyte solution

by rearrangement of charges. This results in a strong field near the junction.

When the semiconductor-electrolyte junction is illuminated with light having

energy greater than the band gap energy of semiconductor, electron-hole

pairs are produced in the depletion layer. Charge separation takes place due

to the local field present at the interface. The probability of annihilation of a

hole with an electron is reduced by this field. This condition will be optimum

when the light penetration depth is equal to the depletion layer width. So that

all the light is absorbed in the depletion layer and maximum number of

electron-hole pairs are produced in it. These separated charges produce a

counter charge and under open circuit conduction this counter field is

maximum. This is the open circuit voltage. The conduction band and valance

band get shifted due to the counter voltage. The photovoltage is given by the

46

change in the Fermi level as shown in Fig. 2.1. When a counter electrode is

immersed in the electrolyte and connected externally to the semiconductor,

the photo generated electron moves into the bulk of semiconductor through

the external circuit; it reaches the counter electrode to reduce an oxidized

species in the electrolyte. The hole is pushed to the electrode surface where it

oxidizes a species in the electrolyte. The photocurrent depends on the

absorption coefficient of the semiconductor, width of the space charge region,

hole diffusion length, area of illuminated electrode, photon energy and

radiation intensity. Under short circuit conditions, the Fermi levels of the

semiconductor and the potential of the redox couple of the solution are

equalized and a net charge flows during the illumination.

Semiconductor Electrolyte

E

Ec

Ef

Ev

Ec*

Ef , redox

Ev *

(In dark)

(In dark)Ef

* WOx

WRed

nEf *

pEf *

hν

+

−−−−

h+

e –

Fig. 2.1 The position of bands under illumination responsible for

photo induced charge transfer

2. A.2.2. Classification of the Photoelectrochemical (PEC) solar cells

The PEC cells are very similar the Schottky type solid state solar cells.

The principle of charge transfer reaction at the semiconductor – electrolyte

interface forms the basis of various types, of photoelectrochemical solar cells.

Depending on the net free energy change (∆G) in the overall system, PEC

cells can be divided into three categories.

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2. A.2.2.1. Electrochemical photovoltaic cells (∆∆∆∆G = 0)

These cells consists of such a redox couple that the total cathodic and

anodic reactions do not lead to net chemical change i. e. a change in the net

free energy, ∆G = 0. The electrodes do not participate in the chemical

reaction, they only serve as a “shuttle” for the charge transfer mechanism. At

the semiconductor electrode,

(Red)solv. + h+ ↔ (OX) solv. 2. 2

At the metal counter electrode,

(OX) solv. + e- ↔ (Red)solv 2.3

The above cell is the regenerative type PEC cell used for direct production of

electricity.

2. A.2.2.2. Photoelectrolysis cells (∆G > 0)

Effectively two redox couples are present and a net chemical change

takes place in the system by converting the optical energy in to chemical

energy. The photoelectrolysis cells and some electrochemical storage

cells belong to this category. Some examples of reaction for the above

type are

H2O hν H2 + ½ O2. 2.4

CO2 + H2O hν CH2O + O2 2.5

Chemical energy

2. A.2.2.3. Photocatalytic cells (∆G < 0)

In these cells, similar to above two redox couples are present such that

a net chemical changes take place. therefore ∆G < 0 and the optical energy

provide the activation energy for the chemical reaction. Example for

photocatalytic cell is

N2 + 3H2 hν 2NH3 2.6

2.A.2.3. Working of Photoelectrochemical (PEC) solar cells

The fabrication of Photoelectrochemical cell (PEC) for the conversion of solar

energy (sunlight) into electricity or chemical energy, the most important fact is

the long-term stability of cells that show high conversion efficiencies.

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Maximum work is carried out on the stability of semiconductor photoelectrode,

an electrolyte and a counter electrode as shown in Fig. 2.2. All parts play an

important role in the PEC cell. The distance between photoelectrode and

counter electrode is 0.5 cm. When both the electrodes are immersed in the

electrolyte, the band bending of the semiconductor photoelectrode may occur.

The transfer of electrons to or from the electrolyte can takes place only in the

energy region of the conduction band, while the hole transfer can takes place

in the region of the valance band. Such a transfer can occurs between two

states of the same energy, one empty and other filled. When the interface of

the semiconductor photoelectrode-electrolyte is illuminated, the electron-hole

pairs are generated in the depletion layer and are separated by the electric

field present at the interface. The incident energy of photon should be higher

than the bandgap energy of the semiconductor photoelectrode. The electron-

hole pairs generated in the bulk of the semiconductor are essentially lost

through recombination. If a positive potential is applied to the n-type

semiconductor photoelectrode and illuminated, the electron-hole pairs are

generated and the separated electrons rise to the top of the conduction band

and holes in the valence band. This process set up a counter field under

open circuit conductions. The counter field is at its maximum and is the open

circuit voltage; Voc is given by the equation-

Voc = (nKT/e) log [Isc+1/Io] 2.7

On the other hand, the counter electrode is being in the same

electrolyte, the photo voltage acts as a driving force for the electrons to move

under the short- circuit conditions from the semiconductor electrode to the

counter electrode and a regenerative cell is formed and is shown in Fig. 2.3.

The short- circuit current is given by the equation-

Isc = Io [exp (eVoc /nKT)-1] 2.8

The electrons promoted to the conduction band drift towards the

interior, while the holes, the minority carriers, come to the surface of the

semiconductor. Here they encounter the reduced form of the redox couple in

the solution. The component is oxidized by the holes, transported to the

49

counter electrode and therefore gets reduced. This reduction is driven by the

external connection from the semiconductor.

Fig. 2.2. A typical electrochemical photovoltaic cell

h+

e −−−−

Ec

Ef

Ev

Ef Ef,redox

Red. + h+ →→→→ Ox. Ox. + e- →→→→ Red.

WOx

WRed

A+ + e- →→→→ A

A + h+ →→→→ A+

WOx

WRed

n-type Semiconductor Electrolyte Metal

Fig. 2.3. Current flow and energy level diagram for n- semiconductor PEC cell.

50

2.A.2.4. Requirements of Photoelectrochemical (PEC) cells

The performance of photoelectrochemical cells depends upon the

following components.

2.A.2.4.1. Semiconductor photoelectrode

Semiconductor photoelectrode used in PEC cell for the achievement of

good performance should satisfy the following requirements:

� It should remain stable in the dark as well as under illumination.

� Charge carriers in the material should have high mobility and life time.

� The band gap should be such that, maximum span of the solar

spectrum should be utilized.

� The electrode must be stable against corrosion when placed in the

specific redox electrolyte. Low band gap semiconductors generally get

easily corroded. Therefore, the choice of semiconductor materials is a

very important for PEC solar cells.

� Thickness should be large enough to absorb all the incident

radiation.

� It should be of the direct band gap type with high optical

absorption coefficient (104 to 105 cm-1).

� Series resistance Rs should be as small as possible and shunt

resistance Rsh should be large enough, ideally Rs = 0 and Rsh = ∞.

2.A.2.4.2. Electrolyte/redox couple

Another important parameter in the PEC cell is the electrolyte.

Electrolytes consist of the oxidized species and the reduced species.

These species are ionic species which helps to transfer the photo

generated holes from photo electrode to counter electrode. The energy

levels in the electrolyte are similar to the concept of energy states in

the solid and Ef-redox is equivalent to the Fermi energy level of the

semiconductor. When the semiconductor electrode is immersed into

the electrolyte, the equilibrium situation is achieved by the electron

exchange at the surface; the fermi level of the semiconductor adjusts

with each other which produces a barrier height depends on the nature

of the solution species and particular semiconductor. Electrolyte

properties require for the PEC cells are:

51

� Reduction-oxidation reactions should occur appropriate to the

semiconductor band edges.

� Charge transfer rates of oxidized and reduced species at both

semiconductor and counter electrode should be high or

effective.

� Oxidized, reduced species and solvent components should have

photo and thermal stability through useable solar spectrum and

operational temperature range.

� The electrolyte should have minimum optical absorption.

� It should be non-corrosive to electrodes.

� Ionic conductance of electrolyte should permit negligible ohmic

losses.

� Toxicity, reactivity and cost should preferably below.

2.A.2.4.3. The Counter electrode

The counter electrode is important, it must satisfy regenerative

processes, the electrolyte species are oxidized at the counter electrodes

giving no net chemical change in the composition of electrolyte. The

requirements for the counter electrode for better performance in the PEC

cell are:

� When a counter electrode is immersed into the electrolyte, the half

cell potential of the electrode should match with that of the half cell

potential of the semiconductor electrode.

� The counter electrode should not react with electrolyte. i.e. it should

be chemically inert.

� The counter electrode should have a low potential for the reduction

reaction.

� It must be electronically active i.e. the charge transfer between the

counter electrode and redox species in the electrolyte must be fast.

52

2.A.2.5. Efficiency of Photoelectrochemical (PEC) cells

A solar cell's energy conversion efficiency (η) is the percentage of

power converted (from absorbed light to electrical energy) and collected,

when a solar cell is connected to an electrical circuit. This term is calculated

using the ratio of the maximum power point, Pm, divided by the input light

irradiance (E, in W/m²) under standard test conditions and the surface area of

the solar cell (Ac in m²).

m

c

Pη=

E×A 2.9

22

2.A.2.6. Fill Factor of Photoelectrochemical (PEC) cells

This is the ratio of the maximum power point (Imax maximum short

circuit current, Voc Maximum open circuit voltage) divided by the open circuit

voltage (Voc) and the short circuit current (Isc):

max max

sc oc

I VFF=

I V 2.10

2.A.2.7. Merits of Photoelectrochemical (PEC) solar cell

� Since the junction formation is a spontaneous process randomly

oriented crystallites can be used.

� Many complicated processing steps in the fabrication of a p-n-

junction are eliminated or simplified.

� The depletion layer width (W) and the minority carrier diffusion

length (LD) should be large.

� Growth of single crystal with large size need not be required.

� The fabrication of PEC cell should be easy and simple.

� The most important advantage of a PEC cell is it has ability for

chemical storage [13, 14].

53

Part-B

Thin Films Characterization Techniques

2. B.1 Introduction

In the past decade the advancement in science has taken place mainly

with the discovery of new materials. Characterization is an important stage in

the development of exotic materials. The whole characterization of any

material consists of phase analysis, compositional characterization, structural

and surface characterization, which have strong bearing on the properties of

materials. In this section different analytical techniques are used to

characterize thin films are described with related principles of their operation

and working.

2. B.2. Optical absorption Study

The equilibrium situation in semiconductor can be disturbed by

generation of carriers due to optical photon absorption. Optical photon

incident on any material may be reflected, transmitted or absorbed. The

phenomena of radiation absorption in a material is all together considered to

be due to inner shell electrons, valence band electrons, free carriers including

electrons and electrons bound to localized impurity centers or defects of some

type. In the study of fundamental properties of the semiconductors, the

absorption by the second type of electrons is of great importance. 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 higher

energy state from the valence band. Absorption of quanta of sufficient energy

tends to transfer of electrons from valence band to conduction band.

The optical absorption spectra of semiconductors generally exhibits a

sharp rise at a certain value of the incident photon energy which can be

attributed to the excitation of electrons from the valence band to conduction

band may also involve acceptor or donor impurity levels, traps, excitons

etc.The conservation of energy and momentum must be satisfied in optical

absorption process.

54

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 is rose across the fundamental gap to the conduction

band. However, indirect transition 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.

Direct interband 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 Fig. 2.4

(a). The optical transition is denoted by a vertical upward arrow. The forms of

the absorption coefficient ‘α’ as a function of photon energy hν depend on

energy of N(E) for the bands containing the initial and final states. For simple

parabolic bands and for direct transitions [15].

αααα = A (hνννν −−−− Eg)n / hνννν 2.11

where A is a constant depending upon the transition probability for direct

transition, n = 1/2 or 3/2 depending on whether the transition is allowed or

forbidden in the quantum mechanical sense. Eg is the optical gap.

Let’s visualize a situation given in Fig. 2.4 (b) where interband

transition takes place between different k−states. As they satisfy the

momentum conservation laws, the only way such transition can take place is

through the emission or absorption of a phonon with wave vector q i.e.

k' ±±±± q = k + K 2.12

The transitions defined by equation (2.12) are termed as indirect

transitions. For indirect transitions [16]

αααα = A (hνννν −−−− Eg)n / hνννν 2.13

For allowed transition n = 2 and for forbidden transitions n = 3.

The band gap energy ‘Eg’ is determined by extrapolating the linear

portion of the plot of (αhν)n against hν to the energy axis at α = 0.

While discussing the optical absorption edges observed in amorphous

semiconductors the following assumptions are made the matrix elements for

55

(a)

Valance Band

Conduction

Band

'kr

kr k

r

Valance Band

Conduction

Band

hν = Eg

O

E

kr

'kr

kr

(b)

the electronic transitions are constant over the range of photon energies of

interest and K−conservation selection rule is relaxed. This assumption is

made in amorphous semiconductors because near the band edges at least,

∆k ∼ k and thus k is not a good quantum number. On E − k diagram such

transitions would be non−vertical. However, no phonon absorption or

emission processes are invoked to conserve momentum and all the energy

required is provided by the incident photons. Such transitions are termed

non−direct as opposed to indirect. Without knowledge of the form of N(E) at

the band edges, and under the assumption of parabolic bands, the absorption

in many amorphous material is observed to obey the relation (A) with n = 2.

Thus absorption edge of many amorphous semiconductors can be described

by a simple power law, at least over a limited range of the absorption

coefficients, which enables an optical gap ‘Eg’ to be defined.

Fig. 2.4 “Direct interband optical transitions” for a) direct band and b) indirect

band semiconductors. The transitions are represented by vertical arrow.

2. B.3.Raman Spectral Study

Raman spectroscopy is different from the rotational and vibrational

spectroscopy considered above in that it is concern with the scattering of

radiation by the simple, rather than an absorption process. It is named after

the Indian physicists who first observed it in 1928, C. V. Raman. Both

rotational and vibrational spectroscopies are possible. The energy of the

56

exciting radiation will determine which type of the transition occurs-rotational

transitions are lower in energy than vibrational transitions.

In addition to this, rotational transitions are around three orders of

magnitude slower than vibrational transitions. Therefore, collisions with other

molecule may occur in the time in which the transition is occurring. A collision

is likely to change the rotational state of the molecule, and so the definition of

the spectrum obtained will destroyed. Rotational spectroscopy is therefore

carried out on gases at low pressure to ensure that the time between the

collision is greater than the time for transition.

The basic set up of Raman spectrometer is shown above.

Note that the detector is orthogonal to the direction of the incident

radiation, so as to observe only scattered light. The source needs to provide

intense monochromatic radiation, and usually laser. The criteria for a

molecule to be Raman active are also different to other types of spectroscopy,

which required permanent magnetic dipole moment, at least for diatomic

molecules. A molecule will only be Raman if the following gross selection rule

is fulfilled.

� Gross selection rule:

For spectroscopic techniques such as infrared spectroscopy, as seen

in the first section it is necessary for the molecule being analyzed to have a

permanent electric dipole. This is not the case for Raman spectroscopy;

rather it is the polarizability (α) of the molecule which is important.

The oscillating electric field of a photon causes charged particles

57

(electrons and, to a lesser extent, nuclei) in the molecule to oscillate. This

leads to an induced electric dipole moment, µind,

Where µind = (α) E

This induced dipole moment then emits a photon, leading to either

Raman or Raleigh scattering. The energy of this interaction is also dependent

on the polarizability:

Energy of interaction = -1/2α E 2

In comparison with the equation for the interaction energy for other

forms of spectroscopy, it can be seen that the energies of Raman transitions

are relatively weak. To counter this, a higher intensity of the exciting radiation

is used.

For Raman scattering to occur, the polarizability of the molecule must

vary with its orientation. One of the strengths of Raman spectroscopy is that

this will be true for both heteronuclear and homonuclear diatomic molecules.

Homonuclear diatomic molecules do not possess a permanent electric dipole,

and so are undetectable by other methods such as infrared.

� Selection rules arising from quantum mechanics:

A detailed discussion of the quantum mechanical basis for

spectroscopy is beyond the scope of this brief introduction. Here, the quantum

numbers and the allowed transitions involved are simply stated.

� Rotational

To find allowed rotational transitions, the total angular momentum

quantum number for rotational energy states, J, must be considered. Raman

spectroscopy involves a 2 photon process, each of which obeys:

∆J=±1

Therefore, for the overall transition

∆J=±2

Where, ∆J=0 for corresponds to Raleigh scattering,

∆J=0 corresponds to a Stokes transition,

and ∆J= -2 corresponds to an anti-Stokes transition.

� Vibrational

To find allowed transitions the vibrational quantum number, ν must be

58

Source Molecule

hν1

hν1

Source Molecule

hν1

hνr1 hνr2

considered.

For the overall transition,

∆ν = ±1

Transitions where ∆ν = ±2 are also possible, but these are of weaker

intensity.

� Types of scattering

Consider a light wave as a stream of photons each with energy hν.

When each photon collides with a molecule, many things may happen, 2 of

which are considered below:

� Elastic, or Raleigh scattering

This is when the photon simply 'bounces' off the molecule, with no

exchange in energy. The vast majority of photons will be scattered in this way.

� Inelastic, or Raman scattering

This is when there is an exchange of energy between the photon and

the molecule, leading to the emission of another photon with a different

frequency to the incident photon:

59

This scattering phenomenon as shown follows.

� Raman scattering

The molecule may either gain energy from, or lose energy to, the

photon. In the diagram below, 3 energy states of the molecule are shown (E1,

E2, E3). The molecule is originally at the E2 energy state. The photon interacts

with the molecule, exciting it with an energy hν1. However, there is no

stationary state of the molecule corresponding to this energy, and so the

molecule relaxes down to one of the energy levels shown. In doing this, it

emits a photon. If the molecule relaxes to energy state E1, it will have lost

energy, and so the photon emitted will have energy hνr1, where hνr1 > hν1.

60

These transitions are known as anti-Stokes transitions. If the molecule relaxes

to energy state E3, it will have gained energy, and so the photon emitted will

have energy hνr2, where hνr2 < hν1. These transitions are known as Stokes

transitions.

For rotational spectroscopy, both Stokes and anti-Stokes transitions

are seen. For vibrational spectroscopy, Stokes transitions are far more

common, and so anti-Stokes transitions can effectively be ignored [17-22].

Fig. 2.5 Photograph of Multi RAM (Brucker) made Germany.

61

2. B.4 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 [23]. When an

electron strikes the atom, variety of interaction products are evolved. Figure

2.6 illustrates these various products and their use to obtain the various kinds

of information about 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.

A) Uses

a) Topographic images

b) Micro structural analysis

c) Elemental analysis if equipped with appropriate detector (EDAX, WDX)

d) Magnification of 10 - 50,000

B) Samples

e) Minimum size: 0.1 mm; maximum size depends on machine

f) Samples must be conductive or coated with thin conductive layer

g) Compatible with vacuum environment

Principle of Scanning Electron Microscope

A well-focused mono-energetic (~25KeV) beam is incident on a solid

surface giving various signals as mentioned above. Backscattered electrons

and secondary electrons are particularly pertinent for SEM application, their

62

intensity being dependent on the atomic number of the host atoms. Each may

be collected, amplified and utilized to control the 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 and so is the C.R.T. beam in synchronization with this.

Fig. 2.6. Variety of interaction products evolved due to interaction of electron

beam and sample

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. The ray diagram of scanning electron

microscope is shown in Fig. 2.7. The scattering cross section for back-

scattered electrons is given as [24],

Secondary Electrons

Back Scattered Electrons

Auger Electrons

X-rays

Cathodolumenescence

Electromotive force

Electron Beam

Transmitted Beam

63

∗= −

2cot102.16

2

30 φE

ZQ 2.14

where, Z is atomic number and E is electric field.

Here the cross-section is proportional to Z2. Hence, the back-scattered

electrons are used for the Z contrast or for compositional mapping.

Fig. 2.7. The schematic ray diagram of scanning electron microscope.

64

2. B.5 Atomic Force Microscopy (AFM)

The atomic force microscopy (AFM) probes the surface of a sample

with a sharp tip, a couple of microns long often less than 100 Å in diameter.

The tip is located at the free end of a cantilever, which is 100 to 200 µm long.

The forces between the sample surface and tip cause the cantilever to deflect

or bend. 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.

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 atomic force microscopy

are interatomic 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. The two distance regimes are labeled in the figure are the

contact regime and non-contact 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. Figure shows schematic diagram of AFM.In principle,

AFM resembles the record player as well as the surface profilometer.

However, AFM incorporates a number of refinements that enable it to achieve

atomic–scale resolution: Sensitive detection, flexible cantilever, sharp tips,

high-resolution tip-sample positioning and Force feedback [25, 26].

65

Fig. 2.8.a. Inter-atomic force versus distance curve for the operation of AFM

Fig. 2.8.b. Schematic diagram of Atomic Force Microscope (AFM)

Repulsive force Force

contact

Attractive force

Intermitted contact

Non- contact

Distance

(Tip –to-sample separation)

(a)

(b)

66

2. B.6 Energy Dispersive Analysis by X-Rays Measurement (EDS)

In EDS 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

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 [27].

2. B.7 X- ray Diffraction (XRD) Technique

X-ray diffraction (XRD) is a powerful technique for determination of

crystal structure and lattice parameters. The basic principles of X-ray

diffraction are found in textbooks e.g. by Buerger [28], Klug and Alexander

[29], Cullity [30], Tayler [31], Guinier [32], Barrett and Massalski [33].

Figure 2.9 shows the schematics of X-ray diffractometer. 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. This

condition of diffraction is nothing but Bragg’s law and is given as,

2d sinθθθθ = nλλλλ 2.15

Where,

d = interplaner spacing

θ = diffraction angle

n = order of diffraction

67

λ = wavelength of x-ray

2θ

θ

X-ray

Source

Sample

Detector

Receiving SlitDiverging Slit

Fig. 2.9. Schematics ray diagram of X-ray diffractometer.

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 relation (E) 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].

θβ

λ=

cos

9.0D 2.16

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.

68

This technique is not useful for identification of individuals of multilayer’s or

percentage of doping material.

2. B.8. Thickness Measurement

The thickness of film is the most significant parameter that affects the

properties of the thin films. It may be measured either by in-situ monitoring of

the rate of the deposition or after the film is taken out form deposition

chamber. Technique of the first type often referred to as monitor methods

generally allow both monitoring and controlling of deposition rate of film

thickness. Any known physical quantity related to film thickness can be used

to measure the thickness. The method chosen should be convenient, reliable

and simple. One of the most convenient surfaceprofilometer and reliable

method for determining film thickness in this method, area and weight of the

film are measured.

Fig. 2.10 Schematics ray Diagram of Typical Profilometer

Electro-magnetic sensors detect the vertical motion of the stylus as it is

moved horizontally across the sample.

Measure

A) Film thickness (step height)

a) changes of 200 Å to 65 µm

b) vertical resolution of about 10 Å

B) Roughness

69

a) horizontal resolution depends on tip radius problem: stylus

penetrates soft films

In our experiment we used the XP-1 Ambios Technology surface profilometer

(contact mode) having 1 Ao resolution.

Fig.2.11 Photograph showing the XP-1 Ambios Technology surface

profilometer

2. B.9 Electrical Conduction Studies

Surface Electrical Conduction phenomena are well known to have a

strong influence on the electronic properties of bulk semiconductors. When

Electrical Conduction takes place through thin specimens, the carriers are

being subjected to considerable scattering by the boundary surface in addition

to normal bulk scattering. This additional scattering will reduce the effective

carrier mobility below the bulk value and will thus give rise to quantum size

effects. A study of these size effects can yield information on the electronic

structure of a surface and is therefore of considerable fundamental and

practical importance. These phenomena play an important role in the

transport properties of semiconducting film of about 1µm thickness and having

carrier concentration up to 1018 cm-3. Surface transport phenomena in bulk

70

semiconductor have received much attention in recent years. This subject is

well explained by Pulliam et al. [35]. The important Electrical Conduction

Studies i.e. electrical resistively, thermoelectric power (TEP) are discussed

below.

a) Electrical Conductivity Measurement

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

lead to extensive study of conductivity, its temperature dependence, the effect

of thermal processing stability and so on. Investigations of the electrical

resistivity as a highly structure sensitive properties make it possible to gain

insight into the structural and electrical properties of the metal film which is

important from both the theoretical and practical point of view.

The contact techniques are widely used for the measurement of

resistivity. These techniques include two-point probe, four point probe and the

spreading resistance methods. The two-point method is simple and easy to

use. In this technique a constant current I is passed through a sample of

known dimensions (cross-sectional area ‘A’). And the d.c. voltage ‘V’ between

two fixed position probes (separation‘d’) measured either with impedance

voltmeter or potentiometrically. For uniform sample resistively is given by

ρρρρ = (A/I) (V/d) 2.17

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

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

following relation:

σσσσ = σσσσοοοο exp (-∆∆∆∆E /κκκκT) 2.18

Where ∆E is the activation energy for the conduction, κ is Boltzmann constant

and σ0 is the pre exponential constant depending on the material.

Fig. 2.12a. and 2.12b. shows photograph and schematic diagram of the

electrical conductivity measurement unit. The two brass plates of the size 10

x 5 x 0.5 cm 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

71

achieve uniform temperature. The two brass plates are 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 was recorded using a Chromel-Alumel

thermocouple (24 gauge) fixed at the centre of the brass plates. Testronix

model 34 C (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 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.

Fig 2.12a. Photograph showing the electrical conductivity

measurement assembly.

72

Fig. 2.12b. Cross sectional view of electrical conductivity measurement

unit.

b) Thermoelectric Power Measurement (TEP)

If some metal contacts are applied to the two ends of a semiconductor

and if one junction is maintained at higher temperature than the other, a

potential difference is developed between the two electrodes. This

thermoelectric or Seebeck voltage is produced partly because

i) The majority carriers in the semiconductor diffuse from hot to cold junction,

thus giving a potential difference between the ends of the specimen. This

voltage builds upto a value such that the return current just balances the

diffusion current when a steady state is reached.

ii) Other part which contributes to the thermoelectric voltage is the contact

potential difference between metal and semiconductor, which occurs at two

junctions.

73

In the semiconductor, if the charge carriers are predominantly

electrons, the cold junction becomes negatively charged and if the charge

carriers are positive holes, the cold junction becomes positively charged. The

magnitude of the developed voltage is proportional to the difference in

temperature between the hot and cold junction, if the temperature difference

is small. From the sign of the thermoelectric voltage it is thus possible to

deduce whether a given specimen exhibits n-or p-type conductivity.

The thermoelectric power (TEP), which is defined as the ratio of

thermally generated voltage to the temperature difference across the piece of

semiconductor, gives the information about the type of carriers in the

semiconductor.

Fig. 2.13a) and 2.13b) shows photograph and schematic diagram of

the TEP measurement unit. Thermoelectric power measurement apparatus

consist of two brass blocks. One brass block was used as a sample holder-

cum-heater. 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. the thermo emf was noted up to the highest

temperature of 500 K. Silver contacts were made to films with copper wire. A

backellite box was used for proper shielding of the TEP unit, which also

minimises to some extent, thermal radiation losses. The mean temperature

was measured with a Meco 801 digital multimeter while the differential

thermal gradient and thermoelectric voltage were measured with digital

Tektronix microvoltmeter.

74

Fig.2.13a.Photograph showing the thermoelectric power measurement

assembly.

Fig. 2.13b. Cross sectional view of thermoelectric power measurement

unit.

75

REFERENCES

[1] L.D. Partain, “Solar Cells and Their Applications”, John Wiley & Sons,

Inc, N.Y., (1995) 600.

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