CHAPTER-2 GROWTH AND CHARACTERIZATION TECHNIQUESshodhganga.inflibnet.ac.in/bitstream/10603/26504/11/11_chapter 2.p… · MOCVD [6 – 10] and the likes. Similarly, the ultra-thin

CHAPTER-2

GROWTH AND CHARACTERIZATION

TECHNIQUES

Chapter 2: Growth and Characterization Techniques

31

CHAPTER – 2

GROWTH AND CHARACTERIZATION TECHNIQUES

Growth and characterization techniques are the backbone of research work. They play

a crucial role in research. They provide us with the opportunity to explore materials

even up to atomic level. Today technology has reached to new heights, providing us

with sophisticated instruments for precisely controlled growths of thin films. In this

work, vacuum thermal evaporation technique has been used to prepare various

nanostructured materials. Brief descriptions of the growth and characterization

techniques used in this work are presented here. This chapter is divided into two parts:

first part deals with growth technique and the second part deals with characterization

techniques.

Following techniques have been used for growth and characterization of the samples:

2.1 Growth Techniques

2.1.1 Physical Vapor Deposition – Vacuum Thermal Evaporation

Technique

2.2 Characterization Techniques

2.2.1 Structure and Morphology

2.2.1.1 X – Ray Diffraction (XRD)

2.2.1.2 Transmission Electron Microscope (TEM)

2.2.2 Optical Properties

2.2.2.1 UV – Visible - NIR Spectroscopy

2.2.2.2 FTIR Spectroscopy

2.2.2.3 Raman Spectroscopy


32

2.1 Growth Techniques

Thin films are thin material layers ranging from fractions of a nanometer to

several micrometers in thickness. Electronic semiconductor devices and optical

coatings are the main applications benefiting from thin film construction. Some

work is being done with ferromagnetic thin films as well for use as computer

memory. Ceramic thin films are also in wide use. Thin film materials are the key

elements of continued technological advances made in the fields of

optoelectronic, photonic, and magnetic devices. The processing of materials into

thin films allows easy integration into various types of devices. The properties of

material significantly differ when analyzed in the form of thin films. Most of the

functional materials are rather applied in thin film form due to their specific

electrical, magnetic, optical properties or wear resistance. Thin film technologies

make use of the fact that the properties can particularly be controlled by the

thickness parameter. Thin films are formed mostly by deposition, either physical

or chemical methods. Thin films, both crystalline and amorphous, have immense

importance in the age of high technology. Few of them are: microelectronic

devices, magnetic thin films in recording devices, magnetic sensors, gas sensor,

A. R. coating, photoconductors, IR detectors, interference filters, solar cells,

temperature controller in satellite, super conducting films, anticorrosive and

decorative coatings.

Film deposition technology plays a key role in the fabrication of planar devices

such as VLSI circuits used in computers, since microelectronic solid-state planar

devices are all based on material structures grown by thin-film deposition.

Scientist and electronic engineers have continuously demanded films of improved

quality and sophistication for solid-state devices, requiring a rapid evolution of

deposition technology. Equipment manufacturers have made successful efforts to

meet the requirements for improved and more economical deposition systems,


33

and for in situ process monitors and controls for measuring film parameters.

Another important reason for the rapid growth of deposition technology is the

improved understanding of the physics and chemistry of films, surfaces,

interfaces and microstructures made possible by the remarkable advances in

analytical instrumentation. A fundamental understanding of materials leads to

expanded applications and new designs of devices that incorporate these

materials.

The importance of deposition technology is the fabrication of semiconductor

devices, an industry that is totally dependent on the formation of thin solid films

of a variety of materials by deposition from the gas, vapor, liquid or solid phase.

The starting materials, epitaxial films of semiconductors, are usually grown from

the gas phase. Further, these methods can also be realized for the synthesis of

nano sized structures and materials using appropriate modifications. The good –

old and well known synthesis of self-assembled semiconductor quantum dot

structures is due to any of the epitaxial film growth techniques like MBE [1 – 5],

MOCVD [6 – 10] and the likes. Similarly, the ultra-thin discontinuous films

grown by a simple physical vapor deposition like vacuum evaporation [11 – 12]

could lead to size variable quantum dot structures. Thermal evaporation has

advantages of being simple, adoptable for any material, clean, economical, and

capable of growing on any type of substrate. Each method has its own advantages

and disadvantages in view of the final application of the grown structures

depending on the constraints. Different thin film deposition techniques are

classified in Table 2.1


34

Table 2.1 Broad classification of thin film deposition techniques [13]

Further, only one relevant growth methods used in the present work will be discussed

briefly:

Physical Vapor Deposition – Vacuum Thermal Evaporation Technique


35

2.1.1 Physical Vapor Deposition – Vacuum Thermal Evaporation

Technique

PVD processes proceed along the following sequence of steps:

a) The solid material to be deposited is physically converted to vapor phase;

b) The vapor phase is transported across a region of reduced pressure from the source

to the substrate;

c) The vapor condenses on the substrate to form the thin film.

The physical vapor deposition is the production of a condensable vapor by physical

means and subsequent deposition of a thin film from this vapor. The foremost

‘physical’ means of producing a vapor is simple heating of a source material as with

the hot filament source or the molybdenum or tungsten boat depending upon the

nature and melting point of the starting material. These are often called “thermal”

sources. The vapor deposition is performed under high vacuum (≈ 10-6 Torr or 10-4

Pa) in order to attain a desired level of purity of the thin film. The mean free path of

the evaporant particle with respect to collisions with the residual gas in the deposition

chamber depends on the level of vacuum attained by the vacuum pump and is

inversely proportional to the pressure in the chamber. With evaporation, the mean free

path is typically much greater than the distance to the substrate at high vacuum [14].

The advantages of vacuum deposition are its simplicity, economy and efficiency [15 –

17]. A film will condense on a substrate when there exists a supersaturated vapor near

the substrate. And for a given substrate temperature, there is a critical incident flux

above which a film will grow, but below which no deposit is obtained. The greater the

substrate temperature, the greater the critical incident flux. We prepared thin films of

all the materials for our study by vacuum evaporation process using “Hind-Hivac

vacuum coating unit (Model 12A4)”.

The substrates we have used in the present study were glass micro-slides and the films

were grown either at room temperature or at various substrate temperatures as per

requirement for various measurements. The distance between the source and

substrates was kept about 10 cm. The vacuum of the order of 10-6 Torr is achieved


36

with the cascaded effort of a diffusion pump and rotary pump. The starting materials

for thin film preparation were high purity (99.99% pure) grade chemicals. The

evaporated material condenses on the glass substrates, forming a very fine/thin coat.

The chemical and physical condition of the film depends on the vacuum and

cleanliness of the substrate. If the glass substrate is not cleaned properly the films end

up with pinholes or even peel off on heating due to poor film-substrate adhesion. Dust

particles and grease are the main culprits for this. The glass substrates were hence

washed in soap solutions prior to its cleaning in dilute chromic acid for over 8 hours.

They were washed in distilled water followed by acetone rinsing and dried with a hot

air blower before placing in vacuum chamber. A good vacuum ensures low amount of

oxygen in chamber and hence the chances of oxidation would be reduced. A

stoichiometric film same as that of the starting material can be grown if the

dissociation temperature of the molecules of the compound is larger than its melting

point. Usually the materials having low melting point give better stoichiometric films.

Figure 2.1 Hind High Vacuum Thermal Evaporation Unit used for the growth of QDs

in this work.


37

Figure 2.2 Schematic of thermal evaporation thin film coating unit.

Film Thickness Measurements

Film thickness is a very important parameter as it is one of the variables in our thin

film studies. Various quantitative calculations and analyses regarding the film

properties also require the knowledge of the thickness of the film. Thickness as well

as deposition rate of the films were measured using HIND HIVAC Digital Thickness

Monitor Model: DTM - 101 during evaporation and confirmed subsequently by

DEKTEK IIA surface profiler.

DTM - 101 allows improved manual control of the film deposition process by

providing a direct display of film thickness and deposition rate during deposition. In it

the quartz crystal oscillator monitor senses the amount of material accumulated on it.

To relate the film thickness on the sensor to that on the substrate, one makes use of a


38

tooling factor, which is their ratio. The monitor uses a quartz crystal as the basic

transducing element. The quartz crystal is incorporated into an oscillator circuit,

which produces oscillations by piezoelectric effect. The crystal has a resonant

frequency of 6 MHz and static thickness resolution of 1Å [14].

Dektek IIA Surface Profiler is essentially a mechanical probe in which stylus moves

over the film surface. The vertical displacement of this probe influences the output

from a piezoelectric transducer attached to the probe showing the roughness of the

film surface. Thus, the uniformity of the film can also be confirmed by scanning the

probe at different areas of the film. The film thickness can be determined quite

accurately by scanning across the film edge on the substrate.

2.2 Characterization Techniques

2.2.1 Structure and Morphology

2.2.1.1 X – Ray Diffraction (XRD)

X-ray diffraction (XRD) is one of the primary techniques used by mineralogists and

solid state chemists for the characterization of crystalline solids and determination of

their structure. About 95% of all solid materials can be described as crystalline and

when X-rays interact with a crystalline phase, a diffraction pattern is generated as a

result of the interaction between the incident X-rays and the atomic architecture of the

solid. Each crystalline solid has unique atomic architecture and consequently has a

unique characteristic X-ray powder pattern. These patterns can be used as

‘fingerprints’ for identification of solid phases. Once the material has been identified,

X-ray crystallography may be used to determine its structure, i.e. how the atoms pack

together in the crystalline state and the size and the shape of the unit cell, etc.

X – Ray diffraction is a tool for the investigation of the fine structure of matter. At

first, X – Ray diffraction was used only for the determination of crystal structure.

However, now a days the method is applied not only to structure determination, but to

such diverse problems as chemical analysis and stress measurement, to the study of

phase equilibria and the measurement of particle size, to the determination of the


39

orientation of one crystal or the ensemble of orientations in a polycrystalline

aggregate [18]. The XRD technique gives a whole range of information about the

crystallographic aspects of a thin film. It gives the information about the lattice

constants, crystal structure (i.e. epitaxial, polycrystalline, amorphous etc.), orientation,

crystalline size, composition (with the help of standards), ordering in amorphous

films, defects, and stresses in thin films.

Diffraction is essentially a scattering phenomenon in which a large number of atoms

cooperate. Since the atoms are arranged periodically in a lattice, the rays scattered by

them have definite phase relations between them; these phase relations are such that

destructive interference occurs in most directions of scattering, but in a few directions

constructive interference takes place and diffracted beams are formed.

When a monochromatic X – Ray beam is incident on an atom in a material, it scatters

the beam in all directions. Since crystalline materials have atoms arranged in a regular

periodic fashion in three dimensions, the scattered X – Rays can undergo constructive

interference in certain specific directions and thus produce strong X – Ray beams in

those directions which are referred to as diffracted beams.

If incident X – Rays 1 and 2 of wavelength λ (0.7 to 2 Ǻ) strike the atoms A and B in

the two atomic planes with interplanar spacing ‘d’ with an incident angle θ, the

constructive interference between the resulting two scattered rays 1′ and 2′ ( figure

2.3) takes place only

When the angle of incidence is equal to the angle of scattering or diffraction.

When the path difference 2d sin θ between the two scattered rays is an integral

multiple of λ i.e., 2d sin θ = nλ (Bragg’s law).

The intensity of the diffracted X-rays is then measured as a function of the diffraction

angle 2θ and the specimen’s orientation. This diffraction pattern is used to identify the

specimen’s crystalline phases and to measure its structural properties [18]. This

technique does not require elaborate sample preparation and is widely being used for

materials characterization. The crystallite size, s, can be estimated using Scherer’s


40

formula:

(2.1)

where λ is the X – Ray wavelength, B is the full width at half maximum of a

diffraction peak, θ is the diffraction angle, and K is the Scherer’s constant ≈ 0.9 for

usual crystal.

Figure 2.3 X-Ray diffraction from crystal planes.

X – Ray diffraction is a very important nondestructive experimental technique that

has been used for:

Determination of crystal structure and lattice parameters

Qualitative and quantitative analysis of unknown substances

Phase diagram determination

Order-disorder transformation analysis

Residual stress measurement

Determination of the nature of polycrystalline aggregates like particle size,

perfection and texture.


41

Figure 2.4 X – Ray Diffraction system used in this study.

2.2.1.2 Transmission Electron Microscopy (TEM) [19]

TEM is one of most powerful techniques in materials science, which has been widely

used in the characterization of nanocomposites. It has ability to examine the


42

constitutional characteristics of these nanocomposites such as grain shape and size,

crystallinity and chemical variations at a resolution down to the nanometer scale.

With advanced design, modern TEM enables lattice defects, atoms and even their

movements to be seen. The first practical TEM was built by Albert Prebus and James

Hillier at the University of Toronto in 1938 using concepts developed earlier by Max

Knoll and Ernest Ruska. TEMs are capable of imaging at a significantly higher

resolution than light microscopes, owing to much smaller wavelength of electrons (de

Broglie wavelength) than that of light [20]. This enables the instrument's user to see

objects of the order of a few angstrom (10-10 m) which is tens of thousands times

smaller than the smallest resolvable object in a light microscope. The possibility for

high magnifications has made the TEM a valuable analysis method in both biological

and materials research. The greatest advantages that TEM offers are the high

magnification ranging from 50 to 106 and its ability to provide both image and

diffraction information from a single sample. In terms of its construction, a general

TEM usually consists of six basic components, as follows:

1) Source providing illumination: An electron source, commonly used in all TEM,

comprises a filament, which emits electrons either by thermal heating (a so-called

thermionic filament) or through application of high electric field to a metal filament

tip generating field emission electrons (so-called field-emission filament). The field-

emission filament is a lot more expensive and requires much higher vacuum than the

thermionic filament, but offers a very stable source with a greater resolution and

longer life-time.

2) Electrodes: These include a cathode, which accelerates the electrons generated

from the filament to a high energy, ranging from a few hundreds to over million volts.

Although a higher voltage can produce a higher resolution, in fact, most TEM

instruments are operated at energies between 100 kV and 400 kV. This is to reduce

sample damage and the cost of the instrument while still achieving an electron

wavelength as short as possible.


43

3) An optical system: This consists of a series of electromagnetic lenses, such as

condenser lens, objective lens, projective lens as well as intermediate lens. These

lenses help to focus the electrons to produce a small probe beam and form images of

samples. The objective lens is the heart of the microscope. The spherical and

chromatic aberrations inherent in the objective lens are the major limitations to the

resolution of the TEM instrument.

4) A sample chamber: This is where the sample is positioned, and is directly above

the objective lens. It is important that the chamber is spacious enough to allow the

samples to be viewed with a wide range of tilting necessary for the crystal orientation

examination as well as for chemical analysis.

5) Camera(s): Images of the samples can be acquired using a video/scanned camera

which is located beneath a phosphor screen where the images are seen. The

photographs are taken by lifting up the screen and exposing the film in the camera.

However, this recording method has been gradually replaced by using a charge-

coupled device (CCD) camera, which collects a digital image which can be saved

onto a computer.

6) Vacuum system: The TEM runs at a very high vacuum, which is maintained by a

vacuum system. In most cases, such a system comprises a combination of two types

of pumps, i.e. mechanical and diffusion pumps.

The analysis capacity of TEM has been significantly enhanced by integration of

several advanced techniques into the instrument. These techniques include

spectrometers, such as energy-dispersive X-ray analysis (EDX) and electron energy

loss spectroscopy (EELS). Nowadays, there is increasing demand to produce an

image at an atomic scale so that the lattice arrangements within crystalline materials

can be visualized. One well-known technique for this is high resolution TEM

(HRTEM). A very high magnification is necessary in order to obtain a high-resolution

atomic image. However, such an atomic image is not turned out by simply zooming

into an image to a sufficient magnification through an imaging system (comprising of


44

both intermediate and projection lenses) in TEM. As a result, quite a few things must

be taken into account in order to acquire a good quality HRTEM image. First, TEM

column alignment needs to be carried out as accurately as possible, which includes

electron gun and condenser lens alignment, plus astigmatism correction of condenser

lenses and objective lens. Secondly, in a HRTEM, an atomic image of a crystal

structure is only possible if certain conditions are satisfied, one of which - choosing

the optimum defocus - is crucial.

Transmission electron microscopy (TEM) is an imaging technique whereby a beam of

electrons is focused onto a specimen causing an enlarged version to appear on a

fluorescent screen or layer of photographic film, or to be detected by a CCD camera.

TEM can achieve a magnification upto million times. Hence it is the most suitable

method to study the microstructure of thin films. With such a good resolution one can

focus on an individual grain and take the Laue diffraction pattern based on electron’s

dual nature.

Selected Area Electron Diffraction (SAED)

Selected area electron diffraction (SAED) is a crystallographic experimental

technique that can be performed inside a TEM to identify crystal structures and

examine crystal defects [21]. It is similar to X-ray diffraction, but unique in those

areas as small as several hundred nm in size can be examined, whereas X-ray

diffraction typically samples areas several cm in size. SAED is used primarily in

material science and solid state physics, and is one of the most commonly used

experimental techniques in those fields.

In a TEM, a thin crystalline specimen is subjected to a parallel beam of high energy

electrons. As TEM specimens are typically ~100 nm thick, and the electrons typically

have energy of 100-400 kV, the electrons pass through the sample easily. Some

fraction of them will be scattered to particular angles, determined by the crystal


45

structure of the sample, while others continue to pass through the sample without

deflection. By inserting a selected area aperture strip, located below the sample holder

on the TEM column, all of the electron beam will be blocked except for the small

fraction passing through to contribute to a diffraction pattern on the screen. The

resulting diffraction pattern is then recorded on photographic film or using a CCD

camera.

In relation to diffraction patterns, there are three types of solid matter: single crystals,

polycrystals and amorphous materials.

(i) Single crystals consist of atoms arranged in an ordered lattice. An electron beam

passing through a single crystal will produce a pattern of spots. From the diffraction

spots one can determine the type of crystal structure (f.c.c., b.c.c.) and the ‘lattice

parameter’ (i.e., the distance between adjacent (100) planes). Also, the orientation of

the single crystal can be determined: if the single crystal is turned or flipped, the spot

diffraction pattern will rotate around the center beam spot in a predictable way.

(ii) Polycrystalline materials are made up of many tiny single crystals. Any small

single crystal in a polycrystal will have a random distribution of all the possible

orientations. A polycrystal, therefore, will produce a diffraction pattern equivalent to

that produced by a beam passing through series of single crystals of various

orientations. A series of concentric rings are formed, resulting from many spots very

close together at various rotations around the center beam spot. Each circle

corresponds to a different set of Miller indices. From the diffraction rings one can also

determine the type of crystal structure and the ‘lattice parameter’. However, the

determination of the orientation of a polycrystal is not possible since there is no

change for the ring pattern when flipping or turning the polycrystal.

(iii) Amorphous materials do not consist of atoms arranged in ordered lattices, but in

random sites. Therefore, amorphous materials are completely disordered. The electron

diffraction pattern will consist of fuzzy rings of light on the fluorescent screen.


46

Figure 2.5 Schematic diagram of a typical TEM.

Sample preparation

Sample preparation in TEM can be a complex procedure. TEM specimens are

required to be at most hundreds of nanometers thick. Materials that have dimensions

small enough to be electron transparent, such as powders or nanotubes, can be quickly

prepared by the deposition of a dilute sample containing the specimen onto support

grids or films. In our case, we have directly deposited QDs on grid. Grid was kept

inside deposition chamber during the growth. Standard TEM grid size is a 3.05 mm

diameter ring, with a thickness and mesh size ranging from a few to 100 µm (figure

2.6) The sample is placed onto the inner meshed area having diameter of


47

approximately 2.5 mm. Usually grid materials are copper, molybdenum, gold or

platinum. This grid is placed into the sample holder which is paired with the specimen

stage.

Figure 2.6 Copper grid (carbon coated) used in the sample preparation process of

TEM.

A standard TEM is first evacuated to low pressures, typically on the order of 10−4 Pa.

An electron source consisting of a cathode (generally tungsten filament) and an anode

at the top of the microscope emits the electrons which are accelerated to 100 keV or

higher (up to 1MeV). These electrons are projected onto a thin specimen by means of

the condenser lens system and penetrate the sample thickness. Depending on the

density of the material present, some of the electrons are scattered and disappear from

the beam. At the bottom of the microscope the unscattered electrons hit a fluorescent

screen, which gives rise to a "shadow image" of the specimen with its different parts

displayed in varied darkness according to their density. The image can be studied

directly by the operator or photographed with a camera. The scattering processes

experienced by electrons during their passage through the specimen determine the

kind of information obtained. Elastic scattering involves no energy loss and gives rise

to diffraction patterns. Inelastic interactions between primary electrons and sample

electrons at heterogeneities such as grain boundaries, dislocations, second phase

particles, defects, density variations, etc., cause complex absorption and scattering

effects, leading to a spatial variation in the intensity of the transmitted electrons. In


48

TEM one can switch between imaging the sample and viewing its diffraction pattern

by changing the strength of the intermediate lens as shown in the ray diagram in

figure 2.7. A pattern of dots in the diffraction pattern suggests the single crystal nature

while presence of rings in general implies polycrystalline nature of the sample.

Figure 2.7 Ray diagrams for obtaining image and diffraction pattern of sample using

TEM.


49

Figure 2.8 Transmission Electron Microscope used in this study.

2.2.2. Optical Properties

Optical techniques are very useful for the characterization of solids because they

require little sample preparation. The sample is generally unaltered, and the

measurement itself cause no damage (unless a probing laser beam is too intense,

which is usually avoidable). Because an optical beam is easily manipulated, these


50

methods can examine different parts of a structure, at spatial resolutions determined

by the wavelength of the light. Visible to near-infrared light can probe the finest

details of a semiconductor nanostructure or device. This means that optical

measurements can create two-dimensional maps of properties in the plane of the

sample, such as impurity distribution or layer thickness. It is also possible to

differentiate properties along the third dimension, as the light propagates into the

sample with a component perpendicular to its surface. The penetration depth of the

light depends on its wavelength and on the sample properties, so that the region

examined can range from nanometers to micrometers deep. Widely used techniques

for optical characterization are Infrared, Raman, and Photoluminescence spectroscopy

because they are spectroscopic in nature, means that intensity is measured versus

wavelength, which provides the capacity for quantitative analysis.

There are various ways in which light interacts with matter, e.g. absorption, reflection,

scattering, emission etc. The study of optical properties of solids proved to be a

powerful tool in our understanding of the electronic and atomic structure of these

materials [22]. The optical properties of semiconductors and insulators such as

absorption, reflection and dispersion result from the electronic excitations in crystals.

The main emphasis is on the evaluation of the optical constants in a wide spectral

range in order to correlate them to the electronic band structure. The usual way to

determine the optical properties of a solid is to shine monochromatic light onto the

sample and then to measure the reflection, transmittance or absorbance as a function

of photon energy [23]. These are known as spectroscopic methods.

2.2.2.1 UltraViolet-Visible (UV-VIS) Spectroscopy

Spectrophotometers are optical instruments that measure the intensity of light

transmitted or reflected by objects as a function of wavelength. Light from the lamp

enters the monochromator, which disperses the light and selects the particular

wavelength chosen by the operator for the measurement. The light beam of selected

wavelength is passed alternately through the sample and along the reference path. The


51

‘reference’ and `sample' light beams pass through the cell compartment, consisting of

a `reference space' and a sample space. The two light beams converge on the detector.

Quantitative measurements in chemical analysis are done by comparison of the

absorption with the absorbance of known concentration of the element.

Extensively used for determination of trace impurities in semiconductors, alloying

elements in steel, non-ferrous alloys, trace impurities in ceramic materials, trace

impurities in liquids like high purity water, solvents, acids, dyestuffs in food etc.

Transmittance or absorbance of solid or liquid and total diffuse

reflectance/transmittance of solids like large disks, silicon wafers, plastics, glass etc.

can be measured. Band gap determination, electron transition and enzyme activity

studies can also be made.

This is the spectroscopic method to measure absorption spectra which are primarily

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

molecules [24]. The basis of quantitative absorption measurement is provided by the

Bouguer-Lambert-Beer law according to which the transmitted light intensity through

a non-reflecting medium is given as [25]

toeII (2.2)

where Io is the incident intensity, t and α are the thickness and absorption coefficient

of the medium respectively. Absorption coefficient is a measure of the energy

attenuation or loss as it travels through the medium. However, if the medium is also

partly reflecting then [26 – 27]

t

t

o eR

eRII

2

2

1

)1( (2.3)

The velocity of light in a medium of refractive index 'n' can be determined by

n

cv (2.4)


52

where c is the velocity of light in vacuum. However, if the medium is an absorbing

one, the velocity of light in the medium becomes complex and the corresponding

complex refractive index n* is defined as

n* = n + ik (2.5)

where k is the absorption index. The absorption coefficient α is related to the

absorption index by

c

k 2 (2.6)

The major sources of absorption in solids are its electrons. The optical absorption of

the films was recorded using Hitachi U-3900H Spectrophotometer, which like most of

the spectrometers, measures Io and I simultaneously. The computer interface then

calculates the absorbance, A (given by )(log10 II o ), and plots it as a function of

wavelength. For all figures in this thesis A is in arbitrary units. Spectrophotometers

provide an option to plot the measurements as transmission spectra (percentage

transmission, T, as a function of wavelength) as well. Since films were grown on glass

substrates, the substrate absorption was corrected by the instrument’s computer

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

absorbance is related to absorption coefficient for a non reflecting medium (from Eq.

2.2) as

t

A

I

I

t

303.2log

303.2 010 (2.7)

For a reflecting film in the region of strong absorption (small R and large ) near the

fundamental absorption edge, can be calculated by

t

R

t

A )1(log606.4303.2 10 (2.8)


53

neglecting )2exp(2 tR term in equation (2.3). Normally, R is very small near the

absorption edge and therefore the second term in Eq (2.8) is neglected in

determination of optical energy gap. All the important optical constants like refractive

index, transmission coefficient and optical energy gap can be computed using the

values of α or the absorbance.

Optical Band Gap

The major sources of absorption in solids are (1) band-to-band transition (2) excitons

(3) imperfactions and (4) free carriers. However band to band transition is the main

contributor to absorption processes in solid [28]. Basically, there are two types of

optical transitions that can occur at the fundamental absorption edge of crystalline

semiconductors, direct and indirect, depending upon the position of conduction band

minima (CBM) with respect to the valence band maxima (VBM) in k-space. Both

involve the interaction of an electromagnetic wave with an electron in the valence

band, which is raised across the fundamental gap to the conduction band. The direct

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

photons, where the conservation of energy is given as

hEEE photongap (2.9)

It is possible, however, for the two extrema (CBM and VBM) to occur at different

points in k-space. An electron making a transition from such a VBM to CBM in such

a situation is said to be making an indirect transition. The energy and momentum

conservation in such a transition can be explained by the emission or absorption of a

phonon (i.e. process involves a simultaneous interaction with lattice vibration [29 -

31] ). The conservation of energy is given by

phononphoton hhE (2.10)

where phononh is the energy of a phonon that is absorbed (plus sign) or emitted

(minus sign) simultaneously with the absorption of the photon. The conservation of

momentum is satisfied by


54

phononphononphoton kkkk (2.11)

where phononk is the wave vector of the phonon that is absorbed or emitted. Since the

light incident on solid loses energy via exciting electrons to such transitions, the

energy band gap (for crystalline samples) or the optical absorption edge (for

amorphous samples) can be calculated using the absorption coefficient.

If an electron moves from the ith energy level to the jth level by absorbing a photon,

the absorption coefficient is defined in quantum mechanics as proportional to the

transition rate per unit volume of the solid Wji, i.e.

nc

W jiiji

(2.12)

where n is the refractive index of the solid.

The transition rate is defined as the rate of change of the probability for the transition

to take place, where this probability is computed from first order time-dependant

perturbation theory. The probability is related to the perturbation Hamiltonian as [32]

)(2 jiij EHa (2.13)

where the use of Dirac Delta function specifies the conservation of energy

requirement. Since we are dealing with electrons in solids the wave function is

defined by Bloch function. The expression for the perturbation Hamiltonian will

depend on whether phonons are created/absorbed or not, i.e. whether the transition is

indirect or direct. In a crystalline or polycrystalline material the nature of optical

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

relation between and the optical energy gap Eg. Assuming the bands to be parabolic

in nature the absorption coefficient in direct transition is related to the band gap by

ngEhconstth )( (2.14)


55

and for indirect transition by

)exp(1

)(

1)exp(

)(

T

EEhB

T

EEhAh

D

npg

D

npg

(2.15)

where Ep is the phonon energy and D is the Debye temperature. For small phonon

energies only second term contributes. Values that n can take are [29, 33 – 34]:

n = 1/2, for direct allowed transition.

n = 3/2, for direct forbidden (in quantum mechanical sense) transition.

n = 2, for indirect allowed transition.

n = 3, for indirect forbidden transition.

The usual method of calculating band gap is to plot graph between nh 1)( and

h and look for the value of n which gives best linear graph. The value of n will

decide the nature of the energy gap or transition involved as mentioned above. In case

of direct band gap materials the graph shows a single linear portion (equation 2.14)

which is extrapolated to determine Eg (the x-intercept). Indirect band gap materials

show two linear portions of different slopes in nh 1)( versus h plot (equation

2.15) giving two intercepts, viz (Eg - Ep) and (Eg+ Ep) from which Eg can be

determined.

Amorphous Materials

The above equations were derived with the assumption of bands to be parabolic.

However, this assumption is restricted only to crystalline samples. In amorphous

materials the momentum vector k – conservation selection rule is relaxed because of

the lack of the long range order and hence sharp energy bands. Hence, the only type

of transition taking place in amorphous materials is non – direct transition [35]. In a

non – direct transition no phonon absorption or emission process is involved to

conserve momentum and all energy required is provided by the incident photons as


56

opposed to indirect transition in crystalline materials. The absorption in many

amorphous materials is observed to obey the relation

αhν = constt(hν – Eg)2

(2.16)

above the exponential tails, where Eg is the absorption edge or optical gap.

Figure 2.9 UV – Visible setup used in this study.


57

2.2.2.2 Fourier Transform Infrared (FTIR) Spectroscopy

Fourier Transform Infrared spectroscopy (FTIR) is a technique based on the

vibrations of the atoms within a molecule. An infrared (IR) spectrum is obtained by

passing IR radiation through a sample and determining what fraction of the incident

radiation is absorbed at a particular energy. The energy at which any peak in an

absorption spectrum appears corresponds to the frequency of a vibration of a part of a

sample molecule. Moreover, chemical bonds in different environments will absorb

varying intensities and at varying frequencies. Thus IR spectroscopy involves

collecting absorption information and analyzing it in the form of a spectrum - the

frequencies at which there are absorptions of IR radiation (‘peaks’ or ‘signals’) can be

correlated directly to bonds within the compound in question. Because each

interatomic bond may vibrate in several different motions (stretching or bending),

individual bonds may absorb at more than one IR frequency. Stretching absorptions

usually produce stronger peaks than bending, however the weaker bending

absorptions can be useful in differentiating similar types of bonds (e.g. aromatic

substitution).

Infrared (IR) spectroscopy is a chemical analytical technique, which measures the

infrared intensity versus wavelength (wavenumber) of light. Based upon the

wavenumber, infrared light can be categorized as far infrared (4 ~ 400cm-1), mid

infrared (400 ~ 4,000cm-1) and near infrared (4,000 ~ 14,000cm-1) [36, 37].

Infrared spectroscopy detects the vibration characteristics of chemical functional

groups in a sample [36, 37]. When an infrared light interacts with the matter, chemical

bonds will stretch, contract and bend. As a result, a chemical functional group tends to

adsorb infrared radiation in a specific wavenumber range regardless of the structure of

the rest of the molecule e.g., the C=O stretch of a carbonyl group appears at around

1700 cm-1 in a variety of molecules. Hence, the correlation of wavenumber position of

a band in the IR spectrum with the chemical structure is used to identify a functional

group in a sample. The wavenumber positions where functional groups adsorb are

consistent, despite the effect of temperature, pressure, sampling, or change in the


58

molecule structure in other parts of the molecules. Thus the presence of specific

functional groups can be monitored by these types of infrared bands, which are called

group wavenumbers.

Figure 2.10 shows the schematic diagram of a typical FTIR spectrometer. An FTIR

spectrometer obtains infrared spectra by first collecting an interferogram of a sample

signal with an interferometer, which measures all of infrared frequencies

simultaneously. An FTIR spectrometer acquires and digitizes the interferogram,

performs the FT function, and outputs the spectrum.

Figure 2.10 Schematic diagram of a typical FTIR spectrometer

Fig. 2.11 depicts the ray diagram for generation of interferogram by an FTIR

interferometer. An interferometer utilizes a beam splitter to split the incoming infrared

beam into two optical beams. One beam reflects off of a flat mirror which is fixed in

place. Another beam reflects off of a flat mirror which travels a very short distance

(typically a few millimeters) away from the beam splitter. The two beams reflect off

of their respective mirrors and are recombined when they meet together at the beam

splitter. The re-combined signal results from the “interfering” with each other.


59

Consequently, the resulting signal is called interferogram, which has every infrared

frequency “encoded” into it. When the interferogram signal is transmitted through or

reflected off of the sample surface, the specific frequencies of energy are absorbed by

the sample due to the excited vibration of function groups in molecules. The infrared

signal after interaction with the sample is uniquely characteristic of the sample. The

beam finally arrives at the detector and is detected by the detector. The detected

interferogram cannot be directly interpreted. It has to be “decoded” with a well-

known mathematical technique in term of Fourier Transformation. The computer can

perform the Fourier transformation calculation and present an infrared spectrum,

which plots absorbance (or transmittance) versus wavenumber.

Figure 2.11 Ray diagram depicting process of generating interferogram using

interferometer


60

When an interferogram is Fourier transformed, a single beam spectrum is generated.

A single beam spectrum is a plot of raw detector response versus wavenumber. A

single beam spectrum obtained without a sample is called a background spectrum,

which is induced by the instrument and the environments. Characteristic bands around

3500 cm-1 and 1630 cm-1 are ascribed to atmospheric water vapor, and the bands at

2350 cm-1 and 667 cm-1 are attributed to carbon dioxide. A background spectrum must

always be run when analyzing samples by FTIR. When 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

where %T is 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

where A is the absorbance.

In the present work FTIR studies have been carried out in transmittance mode on the

pellets containing dry KBr powder mixed with small amount of powder samples. The

IR spectra consist of the transmittance bands due to functional groups present in the

powder samples only because KBr is transparent to infrared radiations.


61

Figure 2.12 FTIR instrument used in this study

2.2.2.3 Raman Spectroscopy

Raman spectroscopy is a spectroscopic technique based on inelastic scattering of

monochromatic light, usually from a laser source. Inelastic scattering means that the

frequency of photons in monochromatic light changes upon interaction with a sample


62

[38]. Photons of the laser light are absorbed by the sample and then reemitted.

Frequency of the reemitted photons is shifted up or down in comparison with original

monochromatic frequency, which is called the Raman Effect. This shift provides

information about vibrational, rotational and other low frequency transitions in

molecules. Raman spectroscopy can be used to study solid, liquid and gaseous

samples. The Raman effect is based on molecular deformations in electric field E

determined by molecular polarizability α. The laser beam can be considered as an

oscillating electromagnetic wave with electrical vector E. After interaction with the

sample it induces electric dipole moment P = αE which deforms molecules. Because

of periodical deformation, molecules start vibrating with characteristic frequency υm.

Amplitude of vibration is called a nuclear displacement. In other words,

monochromatic laser light with frequency υ0 excites molecules and transforms them

into oscillating dipoles. Such oscillating dipoles emit light of three different

frequencies (figure 2.13) when:

(a) A molecule with no Raman-active modes absorbs a photon with the frequency υ0.

The excited molecule returns back to the same basic vibrational state and emits

light with the same frequency υ0 as an excitation source. This type of interaction is

called an elastic Rayleigh scattering.

(b) A photon with frequency υ0 is absorbed by Raman-active molecule which at the

time of interaction is in the basic vibrational state. Part of the photon’s energy is

transferred to the Raman-active mode with frequency υm and the resulting

frequency of scattered light is reduced to υ0 - υm. This Raman frequency is called

Stokes frequency, or just “Stokes”.

(c) A photon with frequency υ0 is absorbed by a Raman-active molecule, which, at

the time of interaction, is already in the excited vibrational state. Excessive energy

of excited Raman active mode is released, molecule returns to the basic

vibrational state and the resulting frequency of scattered light goes up to υ0 + υm.

This Raman frequency is called Anti- Stokes frequency, or just “Anti-Stokes”.


63

Figure 2.13 Schematic diagram to show the transition states.

About 99.999% of all incident photons in spontaneous Raman scattering undergo

elastic Rayleigh scattering. This type of signal is useless for practical purposes of

molecular characterization. Only about 0.001% of the incident light produces inelastic

Raman signal with frequencies υ0 ± υm. Spontaneous Raman scattering is very weak

and special measures should be taken to distinguish it from the predominant Rayleigh

scattering. Instruments such as notch filters, tunable filters, laser stop apertures,

double and triple spectrometric systems are used to reduce Rayleigh scattering and

obtain high-quality Raman spectra. A Raman system typically consists of four major

components:

i. Excitation source (Laser).

ii. Sample illumination system and light collection optics.

iii. Wavelength selector (Filter or Spectrophotometer).

iv. Detector (Photodiode array, CCD or PMT).


64

A sample is normally illuminated with a laser beam in the ultraviolet (UV), visible

(Vis) or near infrared (NIR) range. Scattered light is collected with a lens and is sent

through interference filter or spectrophotometer to obtain Raman spectrum of a

sample. Since spontaneous Raman scattering is very weak the main difficulty of

Raman spectroscopy is separating it from the intense Rayleigh scattering. More

precisely, the major problem here is not the Rayleigh scattering itself, but the fact that

the intensity of stray light from the Rayleigh scattering may greatly exceed the

intensity of the useful Raman signal in the close proximity to the laser wavelength. In

many cases the problem is resolved by simply cutting off the spectral range close to

the laser line where the stray light has the most prominent effect. Very strong laser

pulse with electric field strength > 109 V·cm-1 transforms up to 50% of all laser pulse

energy into coherent beam at Stokes frequency υ0 - υm (figure 2.13). The Stokes beam

is unidirectional with the incident laser beam. Only the mode which is the strongest in

the regular Raman spectrum is greatly amplified. All other, weaker Raman active

modes are not present. The Stokes frequency is so strong that it acts as a secondary

excitation source and generates the second Stokes line with frequency υ0 - 2υm. The

second Stokes line generates the third one with the frequency υ0 - 3υm and so on.

Stimulated Raman technique enjoys 4 – 5 orders of magnitude enhancement of

Raman signal as compared to the spontaneous Raman scattering. Notch filters which

cut-off spectral range of ± 80 - 120 cm-1 from the laser line. This method is efficient

in stray light elimination but it does not allow detection of low-frequency Raman

modes in the range below 100 cm-1. Stray light is generated in the spectrometer

mainly upon light dispersion on gratings and strongly depends on grating quality.

Raman spectrometers typically use holographic gratings which normally have much

less manufacturing defects in their structure than the ruled ones. Stray light produced

by holographic gratings is about an order of magnitude less intense then from ruled

gratings of the same groove density. Using multiple dispersion stages is another way

of stray light reduction. Double and triple spectrometers allow taking Raman spectra

without use of notch filters. In such systems Raman-active modes with frequencies as

low as 3 – 5 cm-1 can be efficiently detected.


65

In earlier times, people primarily used single-point detectors such as photon-counting

Photomultiplier Tubes (PMT). However, a single Raman spectrum obtained with a

PMT detector in wavenumber scanning mode was taking substantial period of time,

slowing down any research or industrial activity based on Raman analytical

technique. Nowadays, more and more often researchers use multi-channel detectors

like Photodiode Arrays (PDA) or, more commonly, a Charge-Coupled Devices (CCD)

to detect the Raman scattered light. Sensitivity and performance of modern CCD

detectors are rapidly improving. In many cases CCD is becoming the detector of

choice for Raman spectroscopy.

Figure 2.14 Raman Microscope used in the present study.

Raman measurements on solids are carried out mainly for exploring the vibrational

structure or Vibrational Density Of States (VDOS) of the material. Since VDOS

depends on the molecular structure of the basis molecule, it becomes useful for the

analysis of short range order in disordered, glassy and amorphous materials. In a

perfect crystal the first order Raman scattering occurs with either absorption or

emission of an optical phonon of k~0. However, increasing disorder could relax this

selection rule for making first order scattering by phonons of larger k possible. The


66

maximum k of the participating phonons should be of the order of 1/, with their

coherence length. In an amorphous material, with of the order of the nearest

neighbor distance, the degree of disorder is so high that all phonons should participate

in a first order Raman process up to the edge of the Brillouin zone [39]. The Raman

spectrum of amorphous Si reported by Smith et al. [40] illustrates this assumption. An

important, non-trivial conclusion reached by these authors is the fact that, apparently,

the remaining short range order in the amorphous material is sufficient to preserve the

main features of the density of phonon states of the crystalline material. Thus Raman

scattering in amorphous solids can be used to obtain a semi quantitative picture of the

density of phonon states in the amorphous solid and in crystalline solids of similar

coordination and nearest neighbor arrangement. The density of phonon states can be

obtained from the measured Stokes scattered intensity mainly by multiplying the

Bose-Einstein occupation number, assuming the scattering probability is the same for

all phonon modes. However, in this spirit, the measured Stokes scattered intensity, for

all practical purposes, can be treated as the indication of the approximate spectral

dependence of the density of phonon states.

2.3 References

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S. B. Krupanidhi, Jounal of Nanoparticle Research 13, 3 (2011).

[3] Y. C Song, P. J. Simmonds, and M. L. Lee, Applied Physics Letters 97, 22

(2010).

[4] I. Kamiya, T. Shirasaka, K. Shimomura, and D.M. Tex, Journal of Crystal

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Smith, Physica Status Solidi A – Applications and Materials Science 205, 11,

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67

[7] L. Li, G. Liu, Z. Li, M. Li, X. Wang, Y. Qu, and B. Bo, Chinese Optics Letters

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[12] V. Gulia, and A. G. Vedeshwar, Physical Review B 75, 4, 045409 (2007).

[13] Handbook of thin film deposition process and techniques – Principals,

Methods, Equipment and Applications, edited by Krishna Seshan, William

Andrew Publishing, New York, USA (2002).

[14] John E. Mahan, Physical Vapor Deposition of Thin Films, John Wiely & Sons,

Inc. (2000).

[15] L. I. Maissel, and R. Gland (Ed), Hand book of Thin Film Technology,

McGraw-Hill, NY (1970).

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(1970).

[17] Joy George, Preparation of Thin Films, Marcel Decker, NY (1992).

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California, USA (1978).

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Laser Ablation, PhD Thesis, School of Chemistry - University of Bristol (2007)

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[24] Encyclopedia of Spectroscopy by Heinz-Helmut Perkampus, VCH

Verlagsgesellschaft mbh. D-69451 Weinheim (Federal Republic of Germany)

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Semiconductors: Materials and Fundamental Principles, Kluwer Academic

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[36] B. H. Stuart, Infrared spectroscopy: Fundamentals and applications , John Wiley

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[37] B. C. Smith, Fundamentals of Fourier Transform Infrared spectroscopy, CRC

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