CHAPTER 2

APPARATUS AND EXPERIMENTAL TECHNIQUES USED IN THE PRESENT STUDY

2.1 Introduction

Ternary chalcopyrites are deposited by a number of techniques

such as spray pyrolisis (1, Z), chemical bath deposition (3), flash

evaporation (4), sputtering (5) and three source evaporation (6 ) . Each

of the above method has its own advantages and disadvantages. In

this work, we have used the single source flash evaporation method

for the preparation of thin films of CuInSez, AgInSez and CuAISe2.

It is a cost effective and a convenient method of deposition of

ternary compounds.

2.2. Deposition methods for preparation of films

Different methods are widely used for the deposition of thin

films of desired properties depending upon the source, mdterial

transportation and growth process. Deposition techniques can be

broadly classified as (i) Physical Vapour Deposition (PVD), (ii) Chemical

Vapour Deposition (CVD), (iii) Solution Growth Deposition (SGD) and

(iv) Electro Chemical Deposition (ECD). PVD involves (i) thermal

evaporation, (ii) electron beam gun evaporation, (iii) molecular beam

epitaxy, (iv) activated reactive evaporation and (v) ion plating. CVD,

SGD and ECD are also used for the preparation of thin films. We

shall restrict our discussion to the method used in the present

study. We have employed flash evaporation method for the preparation

of thin films in the present investigation and is dilscussed in detail

below.

2.3 Production and measurement of vacuum

Various degrees of vacuum are classified according to the

pressure ranges as follows (7):

1. low vacuum - 760 - 25 torr

2. medium vacuum - 25 - torr

3. high vacuum - - torr

4. very high vacuum - lo4 - torr

5. ultra high vacuum - below torr

Vacuum deposition method has the following advantages:

(i) minimum impurity concentration in the film (ii) boiling of material

at low temperature in vacuum (iii) controlled growth rate of the

film (iv) reduced oxide formation in the film and (TI) wide selection

of the substrate. To reduce the pressure in a vacuum enclosure, two

different principles employed are (i) physical removal of .gases

from the vessel by exhausting the gas load to outside and (ii)

the condensation or trapping of gas molecules on some part of

the inner surface of the enclosure without discharging the gas.

Mechanical and vapour stream pumps employ the former' and

Cryogenic, Cryosorption, Sublimation and Getter Ion pumps rely on

the latter principle. Mechanical pumps move gases by the cyclic

motion of a system of mechanical parts. Oil seeded rotary pumps

used in the present study come under this category. These are the

, most widely used method for establishing the necessary fore-vacuum

for high vacuum pumps. An eccentrically positioned rotor fits tightly

against a cylindrical seat machined into the stator. Two spring loaded

vanes sliding in diametrically opposed slots in the rotor press against

the inner surface of the stator. Friction and wear is minimized by

a thin oil film which lubricates all the parts of the pump and also

seals the minute gap at the seat. When turning the rotor, air is

drawn at the inlet side into the pump. The crescent-shaped air

volume is then compressed, thus forcing the outlet valve open and

permitting the gas to be discharged. The double stage rotary pump

used in the present work (Hind Hivac ED - 12) has an induction

motor (Crompton Greaves) and a vented outlet valve admitting a

small air flow into the compression section (gas ballast) which reduces

the condensation of water vapour by lowering the compression ratio.

The pumping efficiency is maintained down to torr and ultimate

pressures of about lo4 torr can be obtained. The fluids in rotary

oil pumps are either mineral oils or diphenyl ether:; with relatively

high vapour pressures of to torr at 50°C. The problem

of backstreaming of oil vapours is solved by installing foreline traps

operated either by condensation on liquid nitrogen cooled surfaces

or by adsorption on surface active materials. Schematic diagram of

a vane-type rotary oil pump is shown in figure 2.1.

Fig. 2.1. Cross-section of a rotarv pump 0 - outlet, V - vanes, S - seal, R - rotor ST - stator, I - ~nlet

The idea of evacuating a vessel by momentum transfer from

streaming to diffusing molecules was first described by Gaede (8).

Diffusion pumps are employed in the pressure range of lo-' to

torr. Schematic diagram of a high speed diffusion pump is shown

in figure 2.2. Oil in the boiler is heated using a heater and conGerted

into vapour. ,The vapour raises in the concentric columns and is

limited by the jets due to the comparative high pressure existing

above the boiler in the jet system. The vapour is forced through

the jet aperture where it is directed downwards by the jet deflectors,

while the tubular side jet discharges vapour into the backing system.

The molecules issuing from the jet engulf gas molecules, diffuse into

the vapour streams since not being able to diffuse back due to the

downward deflected vapour. The vapour molecules are carried

downwards successively compressed by such stage and finally removed

to the atmosphere by the backing pump. The oil vapour impinging

on the water cooled pump wall condenses and drains to the boiler

where it is re-evaporated. In order to minimize the emission of

contaminating vapours, oil should be stable in regard to the thermal

decomposition and oxidation at operating temperatures; the other is

a low vapour pressure near room temperature. In our coating plant,

the diffusion pump (Hind Hivac 114D) has an eval-uating speed of

500~s-' with DC 704 (silicone) diffusion oil. The general properties

of silicone fluids are (i) silicone fluids are not oxidized by air at

Fig.2.2. Schematic of a high speed diffusion pump

C - to chamber, J - jet cap, T - throat, W - \cater cooled casing, V - vapour jet, S - stack, N - nozzle, H - hot vapour, F - fore vacuum

operating temperatures (ii) silicone fluids do not crack (iii) it will

not decompose and (iv) tar and carbon fouling are less on the boiler

surface. The Pirani gauge model Hind Hivac - A6 STM is used for

measuring vacuum in the range 0.5 - torr with two heads.

Change of pressure in vacuum system brings about a rise or fall

in number of gas molecules present and hence a , rise or fall in the

thermal conductivity of the gas. Thus the heat loss of constant

voltage electrically heated filament in the system varies with the

pressure. The Pirani gauge head element has high temperature

coefficient of resistance. So a slight change in the system pressure

brings about useful change in filament resistance resulting in an out

of balance current which can be read as pressure on a meter (Fig.2.3).

The filament is often reconditioned if the gauge behaves erratically

when it is filled with any contaminants. The gauge head is first

flushed with acetone and thoroughly dried. 10V AC or DC is applied 1 across the filament to volatize the deposits on the filament. The

Penning gauge (model STM4) is used to measure vacuum in the

range 10" to torr in two ranges with instant range-changing

provided by a toggle switch. This is a cold cathode ionization gauge

consisting of two electrodes; anode and cathode (Fig.2.4). A potential

difference of about 2.3kV is applied between the anode and the

cathode through current limiting resistors. A magnetic field is introduced

at right angles to the plane of the electrodes by a permanent magnet

Fig.2.3. Pirani gauge H - h ~ g h vacuum, V - vacuum svstem. R - reference wire,

S - sensing wire, R1 and R2 - resistances

Fig.2.4. Schematic of a Penning ionization gauge C - cathode (-), A - ring anode (+), M - magnetic field,

V - 2kV, B - ballast resistor (1MR)

having nearly 800 gauss magnetic field which will increase the

ionization current. The electrons emitted from the cathode of the

gauge head are deflected by means of magnetic field applied at

right angles to the plane of the electrodes and are made to take

helical path before reaching the anode loop. Thus following very

long path, the chance of collision with gas molecules is high even

at low pressures. The secondary electrons produced by ionization

themselves perform similar oscillations and the rate of ionization

increases rapidly. Eventually the electrons are captured by the anode

and equilibrium is reached when the number of electrons produced

per second by ionization is the sum of positive ion current to the

cathode and electron current to the anode and is used to measure

the pressure of the gas. If the gauge shows unstable pressure reading

due to the contamination of gauge head by forming a thin layer

of deposits on the anode loop and cathode liner, it is cleaned

chemically by heating for 20 minutes in a solution of 20-30% HN03

and 2-3% HF acids.

The coating piant consists of pumping section, coating chamber

and electrical feed through. Various reviews on vacuum systems and

their ultimate vacuum attained can be obtained from Holland (9), Dushman

(lo), Caswell (11) and Joy (12). A schematic representation of the

vacuum coating plant used is shown in figure 25. Our coating unit

(Hind Hivac 12A 4) consists of O.lm diffusion pump in conjunction

with a rotary pump in the backing stage. The vacuum achieved in

the system with a 0.3m diameter stainless steel bell jar is of the

order of 1odmbar. The coating plant has set-ups for electron beam

evaporation, flash evaporation and ionic bombardment. Photograph of

the unit is shown in figure 2.6.

2.4 Substrate

For deposition of thin films, highly polished and optically flat

micro glass slides are required. Glass slides of 75 x 25 x 1.3mm.

size are used. First, the substrates are cleaned using a liquid detergent.

Then it is immersed in nitric acid for 15 minutes. Substrates are

then cleaned using distilled water. Then they are agitated ultrasonically

in acetone for few minutes. After drying, they are subjected to ionic

bombardment for few minutes before deposition. Substrate heating is

done using a nichrome wire wound over a mica sheet. Coil is

covered using mica sheets. Power supply to the heater coils is given

using a variac. Temperature is measured using a chromel-alumel

thermocouple and can be uniformly controlled. Maximum

temperature attainable is 550°C using the heater.

I I Fig.2.5. Schematic diagram of a vacuum coating plant

1. bell jar 8. high vacuum valve 2. crystal thickness monitor 9. diffusion pump 3. substrate holder with heater 10. backing valve 4. source shutter 11. Pirani gauge 5. filament (source) with LT. supply 12. fore line trap 6. Penning gauge 13. isolation valve 7. roughing valve 14. rotary pump

Fig.2.6. Photograph of the thin film unit and the other instruments used in the laboratory

2.5 Sample preparation

Stoichiometric amounts of spectrographically pure constituent

elements of the compound weighed in a digital microbalance (Sartorius

model BP 210 S) are mixed using a mortar and pestle and sealed

in an evacuated (vacuum - lo4 torr) quartz ampoule. Ampoule

containing the mixture is then fired in a two zone rocking furnace

(3.5kW, max.temp: 1200~C) having a digital temperature controller

within an accuracy of f 5OC. The ampoule is heated slowly (to avoid

explosion) at the rate of 3 ' ~ per minute to a temperature of 1 0 5 0 ~ ~

and is kept at that temperature for 8-12 hours and subsequently

quenched to retain the chalcopyrite phase in the material. The ampoule

is then broken and the sample is powdered well which is used for

the deposition.

2.6 Deposition of films

Flash evaporation is a powerful PVD technique for preparation

of thin films. It is a simple method in which the material is created

in vapour form by means of resistive heating. On heating a material

in vacuum, sublimation takes place and the atoms are transported

and get deposited onto cleaned substrates held at suitable distance

at desired temperatures. The material for deposition is supported on

a source which is heated to produce desired vapour pressure. The

requirements for the source are that it should have a low vapour

pressure at the deposition temperature and should not react with

the evaporant. The shape of the source is designed and fabricated

in such a way as to hold the evaporant material. We have used

both tungsten baskets and tungsten helix as sources for giving leads.

The evaporant material in the powder form is deposited using a

molybdenum boat. Flash evaporation method consists of sprinkling of

the fine powder of the polycrystalline charge using an electromagnetically

vibrating feeder which serves to drop the powdered material onto

a heated molybdenum boat and evaporates instantaneously without

variation in the stoichiometry. Schematic diagram of the flash

evaporation set up is shown in the figure 2.7. The low tension

(L.T.) supply for evaporation sources is obtained from a 230V input

transformer by means of parallel or series connections in the secondary

side of the transformer. The L.T. output from the transformer is fed

through a current meter and a selector switch to L.T. feed through

and filament holders. The unit is connected for 10V/100A ratings

from the transformer.

2.7 Thickness measurement

As the film thicknesses are in the order of wavelength of

light, different optical interference phenomena have been found useful

for the measurement of film thickness (13). In addition to the

optical interference techniques, ellipsometry and absorption spectroscopy

are useful in the thickness measurement of the films. In laboratory,

Fizeau and FECO (Fringes of Equal Chromatic Order) fringes are used

Fig.7.7. Schematic of the flash e ~ ~ a p o r a t i o n se t up

F - feeder. M - materral for deposition, T - transformer, V - a.c. (0-12V), B - boat, 5 - substrate, C - chamber

for producing interference patterns. FECO fringe technique employs

white light at an incident angle of zero degree and the reflected

or transmitted white light is dispersed by a spectrograph, thus offering

a means of varying wavelength (1). Fringes will form for certain

t values of - where 't' is the thickness between the glass plates. h'

FECO can be obtained with 60 silvered surfaces parallel to each

other. Fizeau fringes are generated by monochromatic light and

represent contours of equal thickness arising in an area of varying

thickness 't' between two glass plates such that they form a slight

wedge at an angle Q so that 't' varies between the two plates. The

angle O is made very small so that consecutive fringes are spaced

as far apart as possible. The angle of incidence O is kept near zero

degree and the medium is air (nl = 1.0). Hence the spacing between

A fringes corresponds to a thickness difference of - where h is the 2'

wavelength of the monochromatic radiation being used. Fizeau fringe

pattern and experimental set up is shown in figure 2.8. Film thickness

is given by,

t = h step height - 2 band width

For well defined and sharp fringes, the optical flat (Fizeau

plate) should have high reflectivity and low absorptivity. For accurate

measurement, film surface should be well collimated and monochromatic.

Thickness measurements from 30 to 20,000A can be made to an

accuracy of f l0A.

Fig.2.S. Fizeau fringe pattern and the experimental set up E - eye piece, S - light source, R - partial reflector,

G - substrate with film, s - step height, b - band width

Quartz crystal type thickness monitor (Hind Hivac CFM-1) is

used for measuring the thickness of deposited films while the process

is going on. It consists of an oscillating quartz crystal mounted in

the chamber of vacuum coating plant exposed to the deposition path.

As the material evaporation proceeds, deposits on the chamber crystal

damp the oscillations changing its frequency. The thickness monitoring

unit has facility to indicate both the film thickness and rate of

deposition on separate meters. A controller output is provided at

the back of the control panel to terminate the deposition automatically.

Necessary wiring connections are made to the evaporation source for

the purpose.

The quartz crystal (6.0MHz, monitor crystal) is positioned in

the vacuum chamber so that vapour is deposited both on the substrate

and on a defined area of the crystal surface. A second crystal

(6.5MHz, reference crystal) is mounted in the control unit outside

the vacuum chamber. The difference in the crystal frequency is

amplified and fed into another circuit where it is mixed with the

frequency of the variable oscillator to produce a final difference in

frequency between 0 and 100kHz. Mass of the deposited material

(thin film) causes a reduction in the natural resonant frequency of

the monitor crystal causing an increase in the final difference in

frequency. This change is converted to a DC signal which activates

both the frequency shift meter and the rate meter. Thus both the

thickness of the film and its rate of deposition on the crystal face

are displayed on conventional meters. Rate of deposition is indicated

in HZS-' by a diode pumping type frequency to voltage converter

and through a differentiating circuit for extreme accuracy. Calibration

of frequency shift against film thickness is done using optical

method.

For the monitor crystal (AT cut), frequency of the fundamental

resonance is given by,

where 'd' is the thickness, 'PQ' is the density, 'c' is the shear

elastic constant of the crystal and 'N' is a constant given by;

If a mass 'm' is added to the exposed area A, produces a

change in frequency Af given by,

'k' is a constant and negative sign indicates

frequency (14).

Combining equations (1) and (2) we get,

f2k where Cf = - and is a constant.

NPQ

Assuming uniform thickness and constant density (pf) of the film,

and

Block diagram of the thickness monit'oring unit is shown in

figure 2.9.

2.8 Conductivity cell

Conductivity cell consists of a cylindrical chamber with a

bottom flange and four side tubes made of stainless steel. Three

side tubes are air-tight with glass windows for the provision of

optical characterization. The remaining side tube is used for evacuation

of the chamber using a rotary pump. The top cover is p ro~ided

with an inner tube having liquid nitrogen cavity and a copper finger.

The chamber is made leak proof by using a neoprene '0' ring which

rests inside the groove on the flanges. Sample holder fixed at the

end of the copper finger is covered with mica sheets and the heater

coil is wound over it. The electrical leads to the sample and the

heater are taken out of the chamber through BNC connector.

Temperature of the sample is sensed using a copper-onstantan

thermocouple in contact with the sample. Temperature of the sample

can be varied in the range 7;1< - 800K. A schematic representation

of the conductivity cell is shown in figure 2.10.

Fig.2.9. Block diagram of the quartz crystal thickness monitor COv - crystal oscillator (6.OMllz) itlsidc tllr chamber, CO/M - oscillator (6.5M11z) and mixer,

FC - frequency counlcr. I<O - rcference oscillator, LO - local oscillator FVC - frequency 11) voltage converter, RM - rate meter

Fig.2.10. Cross-section of the conductivity cell C - chamber. I - inner tube, L - liqzid nitrogen cavity, H - heater coils.

W - g l z s window, F - copper finger, \I - mica sheet as insulztor, .A - connection =,ire, G - substrate \\.ith film,

T - thermocou?i~. 6 - bottonl flanjit.

2.9 Electrometer (Keithley model 617)

Keithley model 617 programmable electrometer is a highly

sensitive instrument designed to measure voltage, current, charge and

resistance. Two ways of resistance measurements included in the

standard configuration are constant current method and constant

voltage method (using a built in voltage for greater sensitivity). The

measuring range is between 10pV and 200V for voltage measurements,

O.lfA and 20mA in the current mode, 0.1 R and 200 R (upto 1016 R

with built in voltage source) and lOfC and 20nC in the coulombs

mode. The very high input impedance and extremely low input offset

current allow accurate measurement in situations where many other

instruments would have detrimental effects on the circuit being

measured. A 4; digit display and IEEE-488 interface make easy access

to the instrument data. Autoranging is provided for all functions

and ranges. Zero correct, baseline suppression, one shot triggering,

isolated lOOV voltage source in 50 steps, selectable guarding and

100- point data store are the main provisions of the instrument. A

schematic diagram of the resistance measurements using the .ohms

function as a current source and V/I resistance measurement connections

are shown in figure 2.11(a, b).

2.10 Conductivity and Hall effect measurements

It has been found that van der Pauw technique (15,16) is

the best method for resistivity and Hall effect measurements. In this

method, four small ohmic contacts are made to the periphery of the

Fig.2.11 (b). Schematic diagram of measuring resistance on Keithley using V/I function

1. input 5. black wire 2. voltage source (a - low, b - hgh) 6. resistance 3. 6011 cable 7. shield 4. red wire

F1g.2.11 (a) Schemanc diagram of measurmg resistance on Keithley usmg ohms funchon 3

i / \

3 9 3 0 0

-1 1

I e

5 ,

3 I

2 I I a 0 05 I I

- 0 1 \ 1

MODEL 61 7

specimen having uniform thickness. Let us consider a sample with

successive contacts A, B, C and D fixed at arbitrary places along

the circumference as shown in figure 2.12. The resistance RAB,CD is

defined as the potential difference (VD - VC) between the contacts

D and C per unit current through the contacts A and B. The current

enters the sample through A and leaves it through contact B. Similarly

the resistance RBC,DA can be defined. From this, the resistivity (p)

is uniquely determined by the resistances RAB,CD and RBC,DA according

to the relation (16),

-x RAB, C D ~ exp [ ) + exp rx Rg:DAt) = 1

where 't' is the thickness of the sample. From this equation, p is

given by,

xft (RAB, CD + RBC, DA) p = - ln2 2

where 'f' is the van der Pauw correction factor with a value of

unity for uniform samples. Activation energy (EA) is calculated from

the slope of log o versus 1000/T graph, where o is the conductivity

at a temperature T of the sample. The Hall mobility (I) can be

determined by measuring the' change of resistance RBD,AC when a

magnetic field is applied perpendicular to the sample. Hall mobility

is given by,

Fig.2.12. Connections on the sample (film) A, B, C, D - contacts. I - constant current source, V - voltmeter

where 'B' is the magnetic induction and ARBD,AC is the change in

resistance RBD,AC due to the magnetic field. By conductivity~

measurements, the number of charge carriers and their mobility cannot

be determined. The sign of the prominent charge carriers also cannot

be determined using the conductivity measurements. 'Hall effect is

used to determine the mobility, carrier concentration and the sign

of the prominent charge carriers. When a magnetic field is applied

at right angles to a conductor carrying electric current, an e.m.f is

developed across the sample in a direction perpendicular to both

the magnetic field and the current. This is Hall effect and the

developed voltage is Hall voltage (17). For an n-type semiconductor

having an electric current density Jx in the x-direction and a magnetic

field in the z-direction, the Hall field, Ey is given by,

The sign of the equation depends on the sign of the charge

carriers. Carrier concentration is given by,

where 'RH' is the Hall constant given by,

Hot probe method is used here for identifying the type of

carriers present in a semiconductor. This is based on the thermoelectric

property of the semiconductor. The distribution of thermal velocities

of the charge carriers in a small region of the semiconductor depends

on the temperature of that region. If the crystal is heated with hot

probe, charge carriers near the hot probe have higher velocities than

those near the cold probe. This disturbance in the equilibrium

distribution of the charge carriers produces an electric field. The field

will be positive with respect with to the cold end if the charge

carriers are electrons (n-type) and negative if the carriers are holes

(p-type). To test the conductivity type, one probe of a digital

multimeter is heated with both probes touching the film. The deflection

in the meter indicates the conductivity type of the film.

2.11 Double beam spectrophotomete~

The optical spectra of the films in the UV-VIS-NIR region

have been taken in a Shimadzu UV 160A spectrophotometer. It is

a double beam spectrophotometer which sends the light beam from

the monochromator equally through the sample and the reference

substrate using a beam splitter. The light sources used for dksired

wavelength are deuterium lamp and halogen lamp. The light beam

emitted from the light source is reflected by the mirror MI and is

directed into the monochromator. The deuterium lamp produces

wavelength upto 2000A. The halogen lamp produces wavelength upto

11000~. These lamps are automatically interchanged for desired

wavelengths. All the optical elements except the light source are

isolated from the external atmosphere by the window plate W so

as to be dust free. The slit width of the monochromator is fixed

at 20A. G is a 900 line/nm aberration corrected concave holographic

grating. The light beam from the monochromator is passed through

the respective cell to the detector. Two voltages are produced by

the detector which are proportional to the light intensities of the

reference and the sample beams respectively. These two voltages are

amplified and is fed to A/D converter. The output ,absorbance or

transmittance obtained on the side monitor can be printed out using

a chart recorder. A simplified block diagram and optical diagram of

the spectrophotometer is given in figure 2.13 and 2.14 respectively.

The optical spectra of the samples in the infrared region have

been taken by nujol method in a Shimadzu IR 470 spectrophotometer.

It is a double beam spectrophotometer using direct method with the

use of independent dual frequencies modulation. It has a globar

source with automatic wattage control. Samples can be scanned in

the range 4000cm-I - 400cm-I and the spectra displayed on the CRT

are stored into the spectrum memory at the same time. The zooming

feature can expand the spectra on CRT with the use of data stored

in the spectrum memory. It has a built-in data processing facility

and print out can be obtained using a chart recorder.

2.12 X-ray diffractometer

For structural characterization of the films, Shimadzu XD 610

x-ray diffractometer has been used. CU kal (h=1.542A) is used as

the source with 25kV/20mA rating. The scattered intensities are angle

dependent in the Bragg-Brentano geometry where the x-ray beam

falls at an angle O to the substrate and the detector is placed at

F1g.2.lL .A slmphfied block d ~ a g r a m of the

spectrophotometer

reierence beam sample beam detector log amplifier sw'itch bus line printer driver printer keyboard

input/output port video RAM CRT controller lamp wavelength scan lamp switching iilter clock external 1/0

Fig.2.14. Optical d iagram of the Shimadzu UV 160A spectrophotometer

D2 : Deuter iumlamp G : Grating W : Window plate Sam : Sample cell W1 : Halogen lamp S1 : Entrance s11t MI-M5 : Mirrors S2 : Exit slit

M 3 : Half-mirror Ref : Reference cell F Filter D . : Photo d ~ o d e L Lens

an angle 20 . The specimen and the detector are rotated at angular

velocities o and 2w respectively to get the various diffracting planes.

In this geometry, when thin films are used, the effective thickness

where t is the thickness of the film the x-ray beam sees is - sin O '

and O is the angle of incidence . Consequently, scattered intensities

will be angle dependent and this has to be taken into account while

comparing the intensities with ASTM data. Detector is a proportional

counter connected to a pulse height analyzer. A chart recorder running

synchronous with the goniometer gives the recorded spectra.

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