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CHAPTER-III

THIN FILM CHARACTERIZATION TECHNIQUES

3.1 CHARACTERIZATION TECHNIQUES

The magnesium indium oxide films deposited on glass substrates were

characterized and their properties were studied using various techniques. X-ray

diffraction system is used for identifying the structural properties of the films.

Scanning electron microscopy and atomic force microscopy are used to study the

surface morphology of the prepared films. The optical characterization was done

using UV-Vis-NIR spectrophotometer. The electrical characterization was analyzed

by linear four point probe technique. The general theory and experimental procedures

of the various techniques used are discussed in detail in this chapter.

3.2 THICKNESS MEASUREMENT

Film thickness is one of the very important attributes of the films to be

determined. The reason is that many properties of the films are dependent on the film

thickness. Thickness is the most important film parameter, which controls the film

properties. Hence precise knowledge of the film thickness is necessary for the

intensive study of the properties of thin films. Wide varieties of methods are available

for measuring thin film thickness. Some of the thickness measurement methods

available are Stylus profilometry, multiple beam interferometry, ellipsometry,

spectrometry, x-ray microanalysis, microgravimetry and cross-sectional scanning

electron microscopy. The type of deposit, deposition technique and the nature of the

substrate dictate the choice of the method. There are various methods to measure the

magnesium indium oxide film thickness. In this work, talystepping techniques have

been used.

Stylus Method-Profilometer

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The surface thickness was determined at a film edge using a stylus type

profilometer (Mitutoyo Surftest SJ-301) in this work. The surftest SJ-301 is a stylus

type surface roughness measuring instrument developed for shop floor use. The SJ-

301 is capable of evaluating surface textures with a variety of parameters according to

various standards and international standard. The measurement results are displayed

digitally/graphically on the touch panel, and output to the built-in printer. The stylus

of the SJ-301 detector unit traces the minute irregularities of the film surface. Surface

rouhness is determined from the vertical stylus displacement produced during the

detecter traversing over the surface irregularities. This type profilometer is a

computerized, highly sensitive surface profiler that measures roughness, step height,

and other surface characteristics in a variety of applications. During measuring, a

diamond-tipped stylus directly contacts the surface and follows height variations as

the sample is moved. The height variations are converted into electrical signals,

producing a profile. The resulting trace represents a cross-sectional view with high

vertical and spatial resolution. Tracing speed, stylus tip diameter, and tip angle were

10 mm/min, 4 mm and 90º, respectively. Fifteen millimeter tracing length (Lt) with

2.5 mm cut-off was used for the measurements. The measuring force of the scanning

arm on the surfaces was 4 mN (0.4 g) which did not put any significant damage on the

surface. Three roughness parameters, mean arithmetic deviation of profile (Ra), mean

peak-to-valley height (Rz), and maximum roughness (Rmax) were commonly used in

studies to evaluate surface characteristics. Ra is the average distance from the profile

to the mean line over the length of assessment. Rz can be calculated from the peak-to-

valley values of five equal lengths within the profile while maximum roughness

(Rmax) is the distance between peak and valley points of the profile which can be

78

used as an indicator of the maximum defect height within the assessed profile [1].

Several factors limiting the accuracy of stylus measurements are:

1. Stylus penetration and scratching of films. This is sometimes a problem in

very soft films.

2. Substrate roughness.

3. Vibration of the equipment.

In order to produce a step on the films, a line was marked on the substrate with

a felt pen before coating and removed later together with the material deposited on the

top of it. Film thickness is thus directly read out. The mark line is made to about 5

mm long, located at the edge of the substrate. The measurements are always carried

out on the inner side of the mark. In order to reduce the measuring error, a mean value

of five measurements at different positions was taken. Consequently the effective film

thickness is derived.

3.3 STRUCTURAL CHARACTERIZATION BY X-RAY DIFFRACTION

(XRD)

The structural characterization of thin films could be obtained from X-ray

diffraction analysis. Generally speaking thin film diffraction refers not to a specific

technique but rather a collection of XRD techniques used to characterize thin film

samples grown on substrates. These materials have important technological

applications in microelectronic and optoelectronic devices, where high quality

epitaxial films are critical for device performance.

X-rays are electromagnetic waves. When an X-ray beam is directed onto a

specimen, many additional non-parallel beams emerge from the material. The angles

at which these beams emerge and their relative intensities, provide information about

the lattice geometry, orientation and arrangement of atoms [2]. Thus much

79

information related to the crystal structure of the films, including grain size, preferred

orientation of polycrystals, defects, stress etc., can be determined from the x-ray

diffraction measurement.

Diffraction of X-rays by a crystal is shown in Fig.3.1.The crystallinity of the

deposited film and the effect of substrate temperature are studied with powder X-ray

diffraction (XRD). Powder XRD is a non-destructive identification technique for

powder or polycrystalline samples. The phenomenon of the X-ray diffraction by

crystals from a scattering process is in which X-rays are scattered by the electrons of

the atoms without change in wavelength. A diffracted beam is produced by such

scattering only when certain geometrical conditions are satisfied, which may be

expressed in either of the two forms, the Bragg’s law or the Laue equations.

Bragg’s law is an equation that relates the wavelength of the incident and

diffracted beams to the crystalline spacing and the angle between the incident beam

and diffracting plane, scattered in a specular fashion by the crystals in the system and

undergo constructive interference in accordance to the law given as,

nλ = 2d sinθ ………………. 3.1

where n is an integer determined by the order, λ is the wavelength of the X-ray, d is

the inter-planar spacing generating the diffraction, and θ is the diffraction angle.

Consider atom A in plane I, and atom B in the adjacent plane II. Geometric

construction shows that the difference in total path length is equal to 2dsinθ. The

exiting beams, A and B, constructively interfere with one another when they are 360

degrees out of phase with one another; or when λ, the wavelength of the incident and

diffracted beams, is equal to the path-length difference. The result is a strong

diffracted beam. Beams also interfere constructively if beams A and B are 720

80

degrees or any multiple of 360 degrees out of phase; hence, the general condition for

constructive interference is Bragg’s law [3].

The characteristic properties of the coated thin film samples were determined

using powder X-ray diffractometer. The term “Powder” really means that the

crystalline domains are randomly oriented in the sample. Therefore when the 2-D

diffraction pattern is recorded, it shows concentric rings of scattering peaks

corresponding to the various d spacings in the crystal lattice. The positions and the

intensities of the peaks are used for identifying the underlying structure (or phase) of

the material. The instrument consists of X-ray tube, sample stage and a detector. In

this equipment the sample stage remains constant whereas the source and the detector

rotate with an angle 2θ. The metal target used for the radiation is copper, whose

characteristic wavelength for the K radiation is 1.54056Å. The diffraction pattern is

obtained by measuring the intensity of scattered waves as a function of scattering

angle when the incident beam strikes the sample. Reflection geometry is used for

these measurements as the substrates are generally too thick for transmission.

The X-ray diffraction patterns of all films were recorded using the following

configuration based X-ray diffractometer.

Model : X’Pert PRO PANalytical

X-ray source : 1.8 kW ceramic Copper tube

Operation potential : 40 kV, 30 mA

Filter : Nickel

Radiation used : CuKα - 1.54056 Å

Detector : X’celerator

From the XRD pattern one can determine the interplanar spacing (d), lattice

parameters and hence the structure of the films. Usually the diffraction peak positions

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are compared with the data from JCPDS cards. Diffraction peak from a lattice plane is

labeled as Miller indices (hkl) and these indices are related to inter-atomic spacing

or‘d’ spacing. For an orthogonal system (α = β = γ = 90°), the‘d’ spacing for any set

of planes is given by the formula:

2

2

2

2

2

2

2hkl c

lbk

ah

d1

++= .......................... 3.2

where, ‘a’, ‘b’ ‘c’ are the cell edges.

For cubic crystals, a = b = c;

2

222

2hkl a

lkhd1 ++

= .............................. 3.3

For hexagonal crystals,

2

2

2

22

2hkl c

la

khkh34

d1

+⎟⎟⎠

⎞⎜⎜⎝

⎛ ++= .......................... 3.4

Using these relations, the Miller indices are assigned for each diffraction peak

obtained in a diffractogram.

The commonly accepted formula for particle size broadening is the Scherrer

formula [4]:

D= θβλ

cos94.0 ............................ 3.5

where, ‘D’ is the grain size, ‘ β ’ is the full width at half maximum (FWHM) of the

respective peak, and ‘θ ’ is the corresponding Bragg’s angle.

The average internal stress developed in the film is determined by the relation [5]:

S = ⎟⎟⎠

⎞⎜⎜⎝

⎛ −δ o

o

aaa

2E .................................... 3.6

where, ‘E’ is the Young’s modulus of the film, ''δ is the Poisson’s ratio of the film,

‘ao’ is the bulk lattice constant, and ‘a’ is the lattice constant of the film.

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The origin of the strain is also related to the lattice mismatch and may be

calculated from the slope of β cosθ vs. sinθ plot using the relation [6]:

θ

β−

θλ

=εtansinD

................................ 3.7

where ‘D’ is the grain size, ‘ β ’ is the full width at half maximum of the peak, and

‘θ ’ is the corresponding Bragg’s peak.

The dislocation density ''δ ie, the dislocation lines per unit area of the crystal can

also be evaluated from the grain size ‘D’ using the formula [7]:

22 m/lines

D1

=δ .................................... 3.8

Also using grain size ‘D’ and film thickness ‘t’, number of crystallites ‘N’ has

been estimated using the relation [7]:

N= areaunit/D1

3 ............................……….3.9

3.4 TRANSMISSION ELECTRON MICROSCOPY (TEM)

Transmission electron microscopy (TEM) is a microscopic technique whereby

a beam of electrons is transmitted through an ultra thin specimen, interacting with the

specimen as it passes through it. An image is formed from the electrons transmitted

through the specimen, magnified and focused by an objective lens and appears on an

imaging screen, a fluorescent screen in most TEMs, plus a monitor, or on a layer of

photographic film, or to be detected by a sensor such as a CCD camera. It is shown in

Fig.3.2. TEM image and selected area electron diffraction (SAED) pattern were

recorded using a JEOL 2010 TEM Instrument.

The electron beam interacts with the sample due to differences in density or

chemistry. The beam that is transmitted through the sample contains information

about these differences, and this information in the beam of electrons is used to form

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an image of the sample. 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 electrons that remain in the beam can be detected

using a photographic film, or fluorescent screen. Areas where electrons have been

scattered in the sample can appear dark on the screen, or on a positive image due to

this scattering.

In material science/metallurgy the specimens tend to be naturally resistant to

vacuum, but must be prepared as a thin foil, or etched so that some portion of the

specimen is thin enough for the beam to penetrate. Preparation techniques to obtain an

electron transparent region include ion beam milling and wedge polishing. Materials

that have dimensions small enough to be electron transparent, such as powders or

nanotubes, can be quickly produced by the deposition of a dilute sample containing

the specimen onto support grids. The suspension is normally a volatile solvent, such

as ethanol, ensuring that the solvent rapidly evaporates allowing a sample that can be

rapidly analyzed. In the present study, wedge polishing was used to reduce the

thickness of glass substrate.

The image can be studied directly and the following information can be

obtained.

Morphology

The size, shape and arrangement of the particles which make up the specimen

as well as their relationship to each other on the scale of atomic diameters.

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Crystallographic Information

The arrangement of atoms in the specimen and their degree of order, detection

of atomic-scale defects in areas a few nanometers in diameter

3.5 SURFACE MORPHOLOGICAL STUDIES

The scanning electron microscopy as well as atomic force microscopy has

been used in the present work for the surface morphological studies. Scanning probe

microscopes are very different from optical microscopes in the sense that they operate

with an extremely small probe tip, barely touching the surface, sensing different

properties at close to atomic resolution in all three dimensions.

3.5.1 Scanning Electron Microscopy (SEM)

Scanning Electron Microscope (SEM) is used primarily for the study of

surface topography of solid materials. The types of signals produced by an SEM

include secondary electrons, back-scattered electrons (BSE), characteristic X-rays,

light (cathodoluminescence), specimen current and transmitted electrons and is shown

in Fig. 3.3. In SEM an electron beam passing through an evacuated column is focused

by electromagnetic lenses on to the specimen surface. The general SEM set-up is

shown in the Fig. 3.4. Essential component of SEM include: electron source “Gun”,

electron lenses, sample stage and detector. The beam is then rastered over the

specimen in synchronism with the beam of a cathode ray tube display screen. In-

elastically scattered secondary electrons are emitted front the sample surface and

collected by a scintillator, the signal from which is used to modulate the brightness of

the cathode ray tube .In this way the secondary electron emission from the sample is

used to form an image on the CRT display screen.

Difference in secondary emission results from charges in surface topography.

If (elastically) back-scattered electrons are collected to form the image, contrast

85

results from compositional differences. Cameras are provided to record the images on

the display screen.

The SEM uses electrons instead of light to form an image. A beam of

electrons is produced at the top of the microscope by heating of a metallic filament.

The electron beam follows a vertical path through the column of the microscope. It

makes its way through electromagnetic lenses which focus and direct the beam down

towards the sample. Once it hits the sample, other electrons (back-scattered or

secondary) are ejected from the sample. Detectors collect the secondary or back

scattered electrons, and convert them to a signal that is sent to a viewing screen

similar to the one in an ordinary television, producing an image.

In the present study, JSM 35CF JEOL is used, which is capable of taking

magnified pictures of solid, dry, conducting and non-conducting specimen.

3.5.2 Atomic Force Microscopy (AFM)

The atomic force microscopy (AFM) is one of a family of scanning probe

microscopes which has grown steadily since the invention of the scanning tunneling

microscope by Binning and Rohrer in the early eighties for which they received the

Nobel Prize for Physics in 1986. AFMs probe the sample and make measurements in

three dimensions, x, y, and z (normal to the sample surface), thus enabling the

presentation of three-dimensional images of a sample surface. This provides a great

advantage over any microscope available previously. With good samples (clean, with

no excessively large surface features), resolution in the x-y plane ranges from 0.1 to

1.0 nm and in the z direction is 0.01 nm (atomic resolution). AFMs require neither a

vacuum environment nor any special sample preparation, and they can be used in

either an ambient or liquid environment.

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Thus the AFM measures the forces acting between a fine tip and a sample.

The tip is attached to the free end of a cantilever and is brought very close to a

surface. Attractive or repulsive forces resulting from interactions between the tip and

the surface will cause a positive or negative bending of the cantilever. The bending is

detected by means of a laser beam, which is reflected from the back side of the

cantilever [8]. The basic concept of AFM is shown in the Fig. 3.5.

AFM can generally measure the vertical and horizontal deflection of the

cantilever with picometer resolution. To achieve this, most AFMs today, use the

optical lever, a device that achieves resolution comparable to an interferometer while

remaining inexpensive and easy to use. The optical lever operates by reflecting a laser

beam on the back of the cantilever. Angular deflection of the cantilever causes a

twofold larger angular deflection of the laser beam. The reflected laser beam strikes a

position-sensitive photodetector consisting of four side-by-side photodiodes. The

difference between the four photodiode signals indicates the position of the laser spot

on the detector and thus the angular deflection of the cantilever. If the tip is scanned

over the sample surface then the deflection of the cantilever can be recorded as an

image which represents the three dimensional shape of the sample surface (deflection

image).

Contact mode

Contact mode AFM is one of the most widely used scanning probe modes, and

operates by rastering a sharp tip (made either of silicon or Si3N4 attached to a low

spring constant cantilever) across the sample [9]. In contact mode, also known as

repulsive mode, an AFM tip makes soft “physical contact” with the surface. The tip is

attached to the end of a cantilever with a low spring constant, lower than the effective

spring constant holding the atoms of the sample together. As the scanner gently traces

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the tip across the sample (or the sample under the tip), the contact force causes the

cantilever to bend to accommodate changes in topography.

As the atoms are gradually brought together, they first weakly attract each

other. This attraction increases until the atoms are so close together that their electron

clouds begin to repel each other electro-statically. This electrostatic repulsion

progressively weakens the attractive force as the interatomic separation continues to

decrease. The force goes to zero when the distance between the atoms reaches a

couple of angstroms, about the length of a chemical bound. When the total van der

Waals force becomes positive (repulsive), the atoms are in contact. The slope of the

van der Waals curve is very steep in the repulsive or contact regime. As a result, the

repulsive van der Waals force balances almost any force that attempts to push the

atoms closer together. In AFM this means that when the cantilever pushes the tip

against the sample, the cantilever bends rather than forcing the tip atoms closer to the

sample atoms. Even if you design a very stiff cantilever, the interatomic separation

between the tip and sample atoms is unlikely to decrease much, the sample surface is

likely to deform.

Thus an extremely low force (~10-9 N, interatomic force range) is maintained

on the cantilever, thereby pushing the tip against the sample as it rasters. Either the

repulsive force between the tip and sample or the actual tip deflection is recorded

relative to spatial variation and then converted into an analogue image of the sample

surface.

The AFM tip is first brought (manually) close to the sample surface, and then

the scanner makes a final adjustment in tip–sample distance based on a set point

determined by the user. The tip, now in contact with the sample surface through any

adsorbed gas layer, is then scanned across the sample under the action of a

88

piezoelectric actuator, either by moving the sample or the tip relative to the other. A

laser beam aimed at the back of the cantilever–tip assembly reflects off the cantilever

surface to a split photodiode, which detects the small cantilever deflections.

But the AFM, not only measures the force on the sample but also regulates it,

allowing acquisition of images at very low forces. The feedback loop consists of the

tube scanner that controls the height of the entire sample; the cantilever, which

measures the local height of the sample; and a feedback circuit that attempts to keep

the cantilever deflection constant by adjusting the voltage applied to the scanner and

is shown in Fig. 3.6. Faster the feedback loop can correct deviations of the cantilever

deflection, the faster the AFM can acquire images; therefore, a well-constructed

feedback loop is essential to microscope performance. A feedback loop, maintains

constant tip–sample separation by moving the scanner in the z direction to maintain

the set point deflection. Without this feedback loop, the tip would “crash” into a

sample with even small topographic features (although this phenomenon can happen

even with careful AFM operation). By maintaining a constant tip-sample separation

and using Hooke’s Law (F = -kx where F is force, k is the spring constant, and x is the

cantilever deflection), the force between the tip and the sample is calculated. Finally,

the distance the scanner moves in the z direction is stored in the computer relative to

spatial variation in the x-y plane to generate the topographic image of the sample

surface.

In contact mode, the sample is often destroyed or even pushed out of the field

of view by the rastering tip. These complications have been addressed through the

development of Tapping Mode AFM. In the Tapping Mode, the AFM tip–cantilever

assembly oscillates at the sample surface while the tip is scanned; thus, the tip lightly

taps the sample surface while rastering and only touches the sample at the bottom of

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each oscillation. This prevents damage to soft specimens and avoids the “pushing” of

specimens around on the substrate. By using constant oscillation amplitude, a constant

tip–sample distance is maintained until the scan is completed. Tapping Mode AFM

can be performed on both wet and dry sample surfaces.

Non-contact mode:

When lifting the probe by at least one nanometer from the sample surface,

only long-range interactions remain. The relevant forces result in general from van

der Waals interactions, electro- and magnetostatic interactions, and, under ambient

conditions, often from the formation of liquid capillaries. Information of the atomic or

nanoscale surface structure gets completely lost.

While van der Waals forces are relatively small and capillary forces can be

avoided by either choosing a sufficiently large working distance or by working on

clean surfaces, electro- and magnetostatic interactions can yield relatively strong

forces. This provides important information about the electrical or magnetic charge

distribution in the near-surface regime of the sample. Since these charge distributions

can be manifold, the lateral variations as well as the range of the resulting interactions

are very different on different samples. In this context near-field operation means that

only charges in probe and sample within a certain volume around the probe apex

contribute to contrast formation. In other words, if the static interaction is modeled in

terms of a multipole expansion of the charge distribution, one usually finds monopole,

dipole and higher contributions which all have to be taken into account up to a certain

degrees. Thus, for the magnetostatic interaction it is very frequently found that the

resulting forces are not simply dipole forces but that the monopole term dominates

contrast formation.

90

AFM uses a position-sensitive photodetector (PSPD) to detect the deflection

of the cantilever and the cantilever’s twist. The reflected laser beam is detected by a

PSPD which is segmented into four quadrants. Thus here AFM measurements were

taken using the instrument: Nanoscope E – 3138 J.

3.6 PHOTOACOUSTIC SPECTROSCOPY

(Thermal property & Optical band gap)

Though much work has been reported on the electrical and optical properties

of the material Magnesium Indium Oxide to the best of our knowledge, no attempts

have been made to understand its thermal properties in addition to the optical

properties [10]. Thermal properties like thermal diffusivity, thermal conductivity and

the thermal effusivity of Transparent Conducting Oxides used in various applications

must be determined very precisely and frequently. The thermal diffusivity is an

important physical parameter that determines the heat transport through the material.

This may be the essential condition of the proper choice of material and subsequent of

the reliability and durability of the material. Since the thermal diffusivity is the

parameter, which depends closely on the microstructural variations, composition and

the processing condition of the samples [11] it should be studied precisely.

Photoacoustic spectroscopy was used to study the thermal and also the optical

properties of the Transparent Conducting MgIn2O4 films. Though the PA technique

has emerged as an important tool for the accurate evaluation of the thermal and

optical properties of a large variety of materials, especially semiconductors [12, 13],

the present report is an attempt to study the thermal and optical properties of the

TCO’s through PAS. According to Rosencwaig – Gersho theory [14] the expression

for the photoacoustic (PA) signal from 1-D heat flow model for a solid sample can be

obtained as

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δ p = --------------- 3.10

where Po is the ambient pressure, To the ambient temperature, Io the absorbed light

intensity and ω= 2 f, where f is the modulation frequency and li, ki and s are the

length (thickness), thermal conductivity and thermal diffusivity of the sample

respectively. Here i = s subscript denotes the sample and i = g denotes the gas medius.

Also σ s = (1 + j) as

where as = ( ω / 2 s ) ½

is the complex thermal diffusion coefficient of the sample.

If the sample is thermally thick, such as

ls as >> 1, eqn (3.10) reduces to

δ p ≈ -3.11

Eqn. 3.11 mean that, for a thermally thick sample, the amplitude of the PA

signal decreases exponentially with the modulation frequency as (1/f) exp (- / )

and the phase decreases as (- a / ), where a = ( ) 2/1

Hence, the thermal diffusivity sα can be obtained either from the amplitude

data or from the phase data (PA depth profile analysis) by exponential fitting it to the

amplitude data or linearly fitting it to the phase data respectively. Knowing the

coefficient a from the fitting procedure, sα is readily obtained from

sα = 2

s

a⎟⎠⎞

⎜⎝⎛π

l --------------- 3.12

If the temperature gradient is generated within the sample along its thickness, there is

a chance of thermo elastic bending (drum effect) [14] where the phase of the signal

has the dependence of modulation frequency as

92

⎥⎥⎦

⎤

⎢⎢⎣

⎡−⎟⎟

⎠

⎞⎜⎜⎝

⎛απ

+φ=φ − 1f21

ss

10

/

ll

tan -----------------3.13

In this case, the thermo elastic bending is also to be considered. The thermo

elastic bending effect could be dominant when the thickness of the sample is large and

also when the thermal diffusivity of the samples is very low.

The depth profile analysis by PA is carried out for two cases (i) glass plate

deposited with the film and (ii) glass plate alone (without the film). Firstly the glass

plate with thin film deposited is taken inside the PA cell and the PA signal is observed

for different chopping frequencies. Thermal diffusivity is then calculated for the

sample from the slope a of the curve for the thermally thick region, using Eqn. 3.12.

Since the film deposited on the glass substrate is used as the sample, the thermal

diffusivity measured here will be the effective thermal diffusivity of glass plate and

the film. Now for the glass plate alone, the PA signal was observed and with these

data of thermal diffusivity of the glass plate ) and the effective thermal diffusivity

( ), thermal diffusivity for the film alone ( s) is computed from eqn [15],

-------------------------3.14

where lg, ls are the thickness of glass and sample and and are there thermal

diffusivities, respectively

The optical band gap of MIO thin films were estimated with the PA

wavelength scanning experiment (PA spectrum) which is a measure of the PA

intensity to the wavelength of the incident light keeping the chopping frequency as

constant say 30 Hz in the present experiment. The PA spectrum measured for the

sample (MgIn2O4) with the substrate was normalized with the PA spectrum of the

glass to obtain the PA spectrum of the films only. At photon energies above the band

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gap, dips will be there in the PA amplitude which is an evidence of resonant

absorptions via interband transitions [16]. From the dips the optical band gap can be

directly found.

Schematic diagram of the photoacoustic spectrometer is shown in Fig. 3.7. In

the experiment, 450W Xe-lamp (Horiba Jobin Yvon, USA) is used as a source. The

sample (film + substrate) is placed in the photoacoustics (PA) cell and the mike is

placed very close to the sample. To get the modulated light, a mechanical chopper (C-

995, Tetrahertz technologies Inc.,USA) is used with the source. The PA signal from

the microphone is fed to a lock-in amplifier (SR-830 DSP Stanford Research, USA).

The light is allowed to fall on the sample through a monochromator (Triax 180,

Horiba Jobin Yvon, USA). The whole set up, the photoacoustic spectrometer is

indigenously integrated and automated with the PC for the fastest data acquisition and

accuracy and calibration.

3.7 OPTICAL CHARACTERIZATION

In early days, the study of the interactions of light with matter laid the

foundations for quantum theory. Today, optical methods are among the most

important tools for elucidating the electron structure of matter. Among the many

available tools, spectrophotometer, photoluminescence spectrometer and Raman

spectrometer are used to explain all the possible transitions such as band-to-band,

excitons, between sub bands, between impurities and bands. In addition, the

transitions by free carrier within a band and the resonances due to vibrational states of

the lattice and of the impurities can be understood.

3.7.1 UV-Vis-NIR spectrophotometer

This is a very effective and common method to characterize the thin films by

means of analyzing their optical spectra e.g., the transmittance and absorbance.

94

Optical properties of films have been studied extensively primarily because of their

applications in various optical and electro-optical devices.

When a beam of UV-Vis-NIR radiation strikes any object it can be absorbed

(A), transmitted (T), scattered (S) and reflected (R). Since some of the energy of the

incident photon is retained in the molecule (or lost by a non-radiative process such as

collision with another molecule) the emitted photon has less energy and hence a

longer wavelength than the absorbed photon [17]. Thus a film may reflect, absorb,

transmit or scatter light and these properties are functions of wavelength.

No material is fully transparent in all optical frequencies and hence there will

always be some absorption in some region of the spectra. Absorption studies provide

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

optical transitions.

Absorption of light by different materials can induce various types of

transitions such as band to band, between sub-bands, between impurity levels and

bands, interactions with free carriers within a band, resonance due to vibrational state

of lattice and impurities. Thus the spectral position of bands determines the types of

transitions occurring during the process. The electronic transition between the valence

and the conduction band can be direct or indirect [18]. If the band energy is plotted

against the wave vector k and the conduction band minimum and the valence band

maximum occur at the same value of wave vector k, a direct transition results. The

absorption edge may occur at hυ= Eg where Eg is the minimum width of the forbidden

energy band of the material. However, if the minimum of the conduction band and the

maximum of the valence band does not coincide i.e., these occurring at different

regions of the k-space, an intense absorption ceases at a wavelength corresponding to

the minimum vertical energy gap. Then a non-vertical or indirect transition may occur

95

resulting in a reduced absorption at frequencies down to hυ= Eg which is the minimum

energy band gap.

Thus the electronic transitions between the valence and the conduction bands

can be direct or indirect. In both cases it can be allowed as permitted by the transition

probability or forbidden where no such probability exists. The transition probability is

related by the following equation:

α = A (hυ – Eg) n ………………………………..3.15

where A is a constant, Eg is optical band gap and n has discrete values like 1/2, 3/2, 2

or more depending on whether the transition is direct or indirect and allowed or

forbidden. In the indirect and allowed cases the index n=1/2 whereas for the direct but

forbidden cases it is 3/2. But for the indirect and allowed cases n=2 and for the

forbidden cases it will be 3 or more. Apparently the plot of (αhυ) 2 or (αhυ) 1/2 against

(hυ) provides the nature and Eg value of a particular film. The magnitude of ‘n’ can be

estimated from the slope of the graph of log (α) vs. (hν) and hence can suggest the

type of transition. The direct optical absorption is illustrated in Fig.3.8. Direct optical

absorption is a first order process, involving only the absorbed photon and can be

represented approximately as a vertical line on the energy versus wavevector (k) plot.

The absorption transition conserves energy so that ΔE = hν and there is no change in

‘k’ between initial and final states except for the small momentum of the photon. An

indirect optical transition is a second order process involving both the absorbed

photon and a simultaneously absorbed or emitted phonon, which occurs when the

minimum of the conduction band and the maximum of the valence band occur at

different values of the wave vector ‘k’ as illustrated in Fig 3.9.

The type of spectrophotometer features a continuous change in wavelength

and an automatic comparison of light intensities of sample and reference material.

96

The ratio of the latter is the transmittance of the sample, which is plotted as a function

of wavelength. The automatic operation eliminates many time-consuming adjustments

and provides a rapid spectrogram.

Here the instrument used to determine the absorption and transmission of the

coated sample is “Hitachi-3400 (double beam)” where the halogen lamp is used as a

source and the wavelength range was about 200-1100 nm. An uncoated corning glass

plate was used as reference. For the measurement, the first procedure is to measure

the transmittance of a “blank material” (usually in air), which is regarded as the

background value with a transmittance of 100%. And then the coated sample is put

into the sample cell and measured. Consequently the recorded transmittance of the

sample is a percent of that of the blank material (air). Since the transmittance of the

air is a constant (100%) in the whole spectral range, the transmittance measuring is

quite simple and accurate. In the same manner absorption value is determined and the

optical band gap value is calculated. The other optical constants which are referred to

as refractive index (n) and extinction coefficient (k) values can also be evaluated.

3.8 FOURIER TRANSFORM INFRARED SPECTROSCOPY (FTIR)

Infrared spectroscopy is one of the most powerful analytical tools for

elucidation of the molecular structures of inorganic and organic compounds. In the

present study, the following FTIR spectrometer had been used to identify the

functional groups and molecular structures of the metal oxide films.

Two types of instrumentation are widely used to obtain infrared spectra:

Dispersive speotrophotometers use a monochromator to produce an infrared

spectrum one resolution element at a time.

97

Michelson interferometers use a moving mirror to create an interference

pattern or interferogram, from which all resolution elements are determined

simultaneously.

The optical system in FTIR spectrometer is very simple: the interferometer

requires two mirrors, an infrared light source, an infrared detector, and a beamsplitter

as shown in Fig.3.10.

The beamsplitter is the heart of the interferometer. Essentially a half-silvered

mirror, the beamsplitter reflects about half of an incident light beam while

simultaneously transmitting the remaining half. One half of this split light beam

travels to the interferometer’s moving mirror while the other half travels to the

interferometers stationary mirror. The two mirrors reflect both beams back to the

beamsplitter where each of the two beams is again half reflected and half transmitted.

Two output beams result: one travels to the detector as the other travels to the source.

Note that each output beam is composed half of light that traveled from the

moving mirror and half of light that traveled from the stationary mirror. Also, the two

output beams contain equivalent, but not necessarily identical information. This fact

results from reflection-induced phase shifts at the beamsplitter and subsequent

interference.

When the two beams return to the beamsplitter, an interference pattern or

interferogram is generated. This interference pattern varies with the displacement of

the moving mirror, that is, with the difference in pathlength in the two arms of the

interferometer. The interference pattern detected by the infrared detector as variations

in the infrared energy level is what ultimately yields spectral information.

98

3.9 ELEMENTAL COMPOSITIONAL ANALYSIS

The composition of the surface layer of a material is a property-related

parameter that provides information about the elements present and the amount of

elements present. Many spectroscopic methods using standard instruments are multi-

elemental analyzers, in that, signals from a number of elements can be registered

simultaneously. Scanning electron spectroscopy with EDS, x-ray photoelectron

spectrometry (XPS) are the techniques used in this study to identify the elemental

content present in the prepared films.

3.9.1 Energy Dispersive Spectroscopy (EDS)

In energy dispersive spectroscopy, an x-ray spectrum is usually displayed as

function of energy. It consists of a continuous signal with superimposed characteristic

elemental line. The positions of the elemental lines are related to the atomic number

of the elements generating them. Elements with increasing atomic number yield lines

at successively higher energy. Beyond certain energy, no x-rays are generated and

therefore the acceleration voltage has to be high enough to be able to excite the

elemental lines. Lighter elements can be analyzed by their K-lines and heavier

elements by their L or M lines.

As x-rays are generated at some depth, they undergo absorption on their way

out of the sample. If the sample contains elements that are close to each other in

atomic number, x-rays from the heavier elements undergo strong absorption by the

lighter elements on their way out. At the same time, this causes fluorescence on the

lighter elements. As a result, the observed intensity from the heavier elements is

reduced, whilst the intensity from the lighter elements is enhanced. To correct these

effects, observed x-ray intensity ratios between sample and standard have to be

multiplied by a correction factor K. For that, the data from the spectometer have to be

99

passed through computer programs that apply ZAF (atomic number, absorption,

fluorescence) corrections.

In the present study, X-ray spectrometer attached to the SEM instrument (JSM

35CF JEOL) was used to perform elemental analysis of all elements down to atomic

number 5 with better geometrical resolution.

3.9.2 X-Ray Photoelectron Spectroscopy (XPS)

The XPS technique is used in many research fields ranging from fundamental

studies of the electron structure of atoms and molecules to technological applications.

The technique has become a very useful tool for the investigation of surface

composition and chemical structure. The principle of the XPS technique is the

emission of electrons from atoms by absorption of x-ray photons. X-ray excitation is

used to induce emission of electrons from the core levels of the atoms [19]. The

schematic illustration of photoelectron emission is shown in Fig. 3.11.

For every element in a compound, there will be a characteristic binding energy

associated with each core atomic orbital. Therefore, each element will give rise to a

characteristic set of emission peaks in the photoelectron spectrum. The position of

peaks is determined by the photon energy and the respective binding energies. The

binding energy of the electron in turn depends on the atomic charge distributions. The

binding energy (BE) of an electron level can be determined by measurement of the

kinetic energy ‘Ekin’ of the photoelectron.

BE = hυ – Ekin - ϕ ……………………..3.16

where, ‘ϕ’ is the work function of the analyzed sample.

The binding energy of an electron level is the energy difference between the

total energy of the final state after electron emission and the total energy of the initial

100

state. If photoelectron emission occurs from the Kth level of an ‘N’ electron atom, the

binding energy can be written as [20]:

BE(k) = Eftot (N-1, k) – Ei

tot(N, k) ……………………. 3.17

Binding energy is the difference between the total energies of the initial state

of an atom and the final state of an ion. It is the Hartee-Fock energy of the electron

orbital, so peaks in the photoelectron spectrum can be identified with specific atoms

and thus surface compositional analysis could be possible. A peak at a particular

energy indicates the presence of an element in the surface under study and the

intensity of the peaks is related to the concentration of the element within the region

of interest. Thus, XPS is a quantitative technique for the compositional analysis, if

two or more elements are present. The basic requirements for a photoemission

experiment are,

(i) Fixed energy radiation source (MgKα X-ray source)

(ii) An electron energy analyzer (which can disperse the emitted

electrons according to their kinetic energy and thereby measure

the flux of emitted electrons of a particular energy).

(iii) A high vacuum environment (to enable the emitted

photoelectrons to be analyzed with out interference from gas

phase collisions).

A schematic drawing of the main components of a modern XPS instrument is

shown in Fig. 3.12. The main components of the system are: an x-ray source, the

sample stage, the lens, the analyzer and the detector, which are enclosed in an ultra-

high vacuum chamber. An electron optical system is present in most cases of a

hemispherical analyzer and an electrostatic lens system is used to focus the electrons

to the analyzer. The voltage between the hemispherical segments is kept constant and

101

the energy discrimination of the photoelectrons is obtained by sweeping the potential

in the lens or by using a grid system in front of the analyzer.

The sensitivity of the instrument depends on the X-ray source used, the

analyzed area, tilt angle of the sample, solid angle over which electrons from sample

are accepted by the lens, efficiency of the lens, the analyzer and the detector. The

energy resolution mainly depends on the inherent width of the electron level, the

inherent width of the X-ray radiation and the resolving power of the spectrometer.

X-ray photoelectron spectroscopy (XPS) was performed using a Multilab-

2000 (Thermo-scientific UK) spectrometer using a monochromic MgKα X-ray source

(1256 eV) with an analyzer pass energy of 10 eV Samples were mounted on SS

sample holder with silver paint.

3.10 ELECTRICAL CHARACTERIZATION

Electrical conduction phenomenon is mostly related to the transport of

electrons (or other charge carriers) under electrical and magnetic fields and also under

thermal gradient. The band structure and electron transport properties of a

semiconducting film are similar to those of the bulk though some additional effects

like electrical conductivity, Hall effect, Hall mobility, thermoelectric power, magneto

resistance, scattering modes of charge carriers etc. have to be taken into consideration.

Usually, the resistivity is measured using four probe technique. The carrier

concentration is calculated from the measured Hall mobility values using Hall effect

measurement set up.

102

3.10.1 Four Probe Method

The most generally used technique in the semiconductor industry for the

measurement of resistivity is the four-point probe. Normally this method is non-

destructive; however, the probe points may damage certain film regions when

excessive probe pressure is applied. The usual geometry is to place the probes in a

line and use equal probe spacing (Fig. 3.13). Current is passed through the outer two

probes and the potential developed across the inner two probes is measured.

Generally, for probes resting on a semi-infinite medium, the resistivity is expressed

as:

1 3 1 2 2 3

2

1 1 1 1o

VI

S S S S S S

πρ

⎛ ⎞⎜ ⎟⎝ ⎠=

+ − −+ +

……………………..3.18

where, ‘S1’, ‘S2’ and ‘S3’ are probe spacings in centimeters. When the probes are

equally spaced then:

ρo = 2πS VI

……………………….. 3.19

However, for thin film samples with thickness t < 0.1 S and boundaries > 20 S

from probes, the resistivity ‘ρ’ is expressed as:

( / )

o

G t sρρ = …………………….. 3.20

The correction factor G (t/s) for infinity thin slice (t/s < 0.1) is 2St

ln 2 and therefore:

ln 2

4.53

t VI

VtI

πρ

ρ

=

≈ ………….. 3.21

and the sheet resistance Rsh is expressed as:

103

4.53shVRI

≈ ………………………3.22

For the present study, the four probe resistivity measurements were carried out

on Magnesium Indium Oxide thin films using an OSAW AC-DC Four point probe

unit, in conjunction with a UNI-INSTA DC power supply. The voltage and current

were measured using a HIL 2161 Digital multimeter and a Keithley 2000 multimeter

respectively.

3.10.2 Hall Effect Measurements

The Hall Effect can be used to measure the mobility, carrier density and sign

of the charge carriers. The Hall Coefficient is the important one which also depends

on the specimen thickness and the sign of the potential developed at the two ends of a

specimen with respect to the electrical and magnetic field directions determines

whether the specimen is n-or p- type. The typical sample geometry used for Hall

effect measurements is shown in Fig.3.14.

Here, the Hall effect measurements are taken using the Ecopia HMS-3000

which is a complete system for measuring the resisivity, carrier concentration, P/N

type and mobility. In order to measure all the above values, the ohmic contacts are

made at four sides of the films using silver paste.

When a specimen is placed in a magnetic field, perpendicular to the direction

of current flow, a field is developed across the specimen in the direction perpendicular

to both the current and the magnetic field and the geometry. The field is called as the

hall field and is given by,

Ey = Jx Bz/ ne ...…………………………3.23

where, Jx is the current density and n is the number of carriers per unit volume. The

ratio,

RH = Ey / Jx Bz = 1/ne .................................. 3.24

104

is called the “Hall coefficient”. A measurement of RH thus determines n. Since ρ = 1/

ne µ for a free electron,

RH = µρ ……………………………..3.25

The sign of the Hall coefficient RH indicates whether the specimen is a current carrier

or charge carrier and thus the Hall voltage is given as,

VH = RHIB / w ……………………….. 3.26

where RH is the hall coefficient, I the current through the sample, w the thickness, B

the magnetic induction [21].

3.11 GAS SENSORS

The most widely studied area of the solid-state gas sensors is based on

semiconductor oxides. It has been known that the absorption of foreign species on

semiconducting surface induces change in the electrical properties of the surface. The

measurands for electrical sensors are emf, resistance and capacitance. The solid-state

sensors are being fabricated in the form of bulk, thin and thick films.

In the present study, gas sensors were fabricated with the prepared oxide thin

film as the sensing element. A static measuring system is employed, which includes

an airtight chamber, the sensor element and the temperature sensor. The test gas is

injected inside the chamber through a needle value. The electrical characteristics of

the sensor are observed by its temperature in air ambient and this response is

considered as a reference response for the calculation of sensitivity. After admittance

of the test gas in predefined amount, the resistance of the sensor is measured once

again for various sensor temperatures.

The gas concentration in ppm can be estimated from the gas law.

PNkTV = …………………….. 3.27

105

where, ‘P’ is the pressure, ‘V’ is the volume of the chamber, ‘N’ is the number of

particles, ‘k’ Boltzmann’s constant, and ‘T’ is the temperature of the chamber.

The sensitivity factor of the film sensor is calculated using the formula:

gas

gasairgas R

RRRR

)(/

−=Δ …………………… 3.28

where, ‘Rgas’ is the value of the sample resistance in the presence of the test

gas and Rair is the sample resistance in dry air.

3.12 CONCLUSION

In this chapter, the author has reported briefly the characterization techniques,

which were used to characterize the magnesium indium oxide films. The instrumental

specifications and analysis methodology are explained in detail. The results are used

to interpret the materials properties of thin films prepared using the RF sputtering

technique developed in the present work and are presented in the following chapters.

106

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