METHODS AND APPARATUS USED IN THE PRESENT STUDY

2.1. Introduction

The performance of polycrystalline thin films as solar cells is related to

methods used for the fabrication of the films. Polycrystalline thin films may be

prepared by vacuum evaporation, screen-printing, close spaced sublimation,

spray pyrolysis, pressure printing, molecular beam epitaxy, electrodeposition, d

chemical bath deposition etc. Among these, chemical bath deposition deserves

special attention because it has been shown to be an inexpensive, low temperature

and non-polluting method. It is well suited for producing large area thin films.

Thus the optimization of the experimental parameters for thin film fabrication by

chemical bath deposition technique is a key factor in solar cell technology

development.

Phthalocyanjnes are the most extensively studied materials among the

1.2.3 organic semiconductors after anthracenc4 They are very stable and can be

sublimed without decomposition. They retain high purity, and are relatively

easily crystallised. Copper phthalocyanine has wide range of applications in gas

sensors', artificial solar cells6 and pigment industry. In this chapter, the apparatus

and experimental techniques used in the present study are dealt with.

2 0

Methods of preparing thin films may be divided into (i) chemical methods

(ii) physical methods and (iii) sputtering. Chemical methods include (1) chemical bath

deposition (2) chemical vapour deposition (3) spray pyrolysis (4) electrodeposition

(5) anodisation (6) solution growth and (7) screen printing. The ibrrnation of very

pure and well-defined film is possible using physical vapour deposition

techniques. Physical vapour deposition is classified into (1 ) thermal evaporation

(2) electron beam evaporation (3) molecular beam epitaxy (4) activated reactive

evaporation and (5) ion plating. Sputtering can be further classified into (i) d.c.

sputtering, (ii) r f sputtering, (iii) magnetron sputtering and (iv) ion beam

sputtering.

Each of the above mentioned methods has its own advantages and

disadvantages and we will restrict our discussion only to the method which we

have used in this thesis. We have employed chemical bath deposition technique

and thermal evaporation technique for the preparation of films.

2.2 Chemical Bath Deposition Technique

Chemical bath deposition (CRD) is a convenient and low cost technique

for producing large area thin film semiconducting The basic

principles underlying the chemical bath deposition of semiconductor thin films

and early research work in this area is reviewed by Chopra et. a1'6q7, which has

inspired many researchers to initiate work in this area. The subsequent progress

in this area is reported by ~okhande." Recipes for the chemical bath deposition

of a number of such compounds are given by ~rozdonov." Mane et. all9 and

Mahmoud et. a120 describe the chemical deposition method for metal chalcogenide

thin films, using a variety of substrates such as insulators, semiconductors and

metals. Since this is a low temperature process, this avoids oxidation and corrosion

of the substrate. These are slow processes, which facilitate better orientation of

crystallites with improved grain structure. The number of possible materials to be

produced through this technique is bound to multiply in subsequent in

this thesis we describe the chemical bath deposition of sulphide thin films on

pyrex glass substrates by the decomposition of thiourea in an alkaline

s o ~ u t i o n . ~ ~ . ~ ~ Pyrex glass slides (4cm x lcrn) are used as substrates. The cleaned

glass slides are kept vertically in the container with the deposition mixture. The

25-27 sulphide films are chemically deposited at room temperature for 24 hours.

After the deposition the films are dried in air.

Consider CdS thin film formation. The basic mechanism of CdS thin film

formation was supposed to be either (1) an ion-by-ion condensation of cd2' and

s2' on the substrate from an aqueous basic medium, containing thiourea and

cadmium ions in the form of a complex species as indicated in the following

equation,

Cd (L),, 2-t + ~ d " + nL 2.1

or, (2) the result of the adsorption of colloidal particles of CdS onto the substrate

surface2! CdS film formation may be achieved using one of the following three

complex methods2'.

1. The tetra mine complex in which the filrn formation follows

the reaction represented below.

[Cd (NY)~] "+ SC(NI 12)2+2 OH- -+ CdS +4NH3 -tOC (N H2)2+kb0 2.2

2. The cyano complex based on the reaction,

[Cd (CN)4] "+SC (nH2),+2 OH- + CdS +4CN +OC (NH2)*+ H20 2.3

3. The triethanolarnine complex m ~ t h o d ) ~ (TEA), in which film formation is

based on the reaction,

[Cd (TEA)] 2 ' + ~ ~ ( N ~ 2 ) 2 + 2 OH' + CdS +TEA+OC(NH2)2+ HzO 2.4

34-36 It has been shown that the TEA process leads to high quality films .

The mechanism involving the chemical bath deposition of cadmium sulphide thin

films Srom the ammonia - thiourea system has been studied by means of the

quartz crystal microbalance technique( QCM))'. It has been shown that CdS

formation result from the decomposition of adsorbed thiourea molecules via the

formation of an intermediate surface complex with cadmium hydroxide. This

mechanism is different from the dissociation mechanism involving the formation

o f liee sulphide ions in solution which had previously becn found. The influence

of growth parameters such as bath temperature, deposition rate, bath composition

etc, on various filrn propertics have been studied by many a u t h ~ r s ' ~ " ~ . The main

parameters which determined the quality of the films were deduced.

Reaction mechanism for the deposition of CdS films

The chemical bath deposition of CdS thin films generally consists of the

decomposition of thiourea in an alkaline solution containing a cadmium salt.

23

The deposition process is based on the slow release of cd2+ and s2- ions in

solution, which then condenses on an ion-ion basis on the substrate. The reaction

process for the formation of CdS by the tetran~ine complex method may be

described by the following steps4'.

[a] Ammonium ion formation:

N H 3 + H Z 0 N H ~ + O H - - K = l . g x 1 0 - ~ 2.5

[b] Cadmium salt reaction with the anions to form the complex compound

[c] DiffusionofthecomplexionOH~andthioureaonthecatalyticsurSaceof

CdS.

In the case where thiourea is the s2- source in an alkaline medium, the

sulphide ions are released as l'ollows3'.

SC(NH2)2 + 3 OH' -+ 2NH3 +COi 2-+ HS'

HS' +01-I' + s2- +HZO

In the case where H2S as s2- source in an acid medium, the dissociation

proceeds as follows40.

[dl Formation of CdS

c ~ ( N H ~ ) 42' + s2- + CdS +4NH3 k3 = 7.1 x 1 028

Thus the decomposition of CdS occurs when the ionic product of [cd2+]

and [s2-] exceeds the solubility product [Ks] of CdS. The very low value of the

solubility product of CdS [ I .4x 1 029 at 25'~] 'O implies that CdS precipitation can

take place evcn at the lowest s'- ion concentration that is possible to obtain in

solution. There are many ways oT obtaining suitable complex species of ~ d "

ions in solution, but most of the literature dealing with the chemical bath

deposition process is based on the tetra-amine complex method. Here ammonia is

the complexing agent and the hydroxide source and NH4Ac/ NH3 serves as the

buffer. Thus the global reaction for the process is given by

c~(NH~)F + SC(NH2)2 + 40K+ CdS + 6NH3+ C03 2' + HzO 2.13

At a given temperature, the rate of formation of CdS is determined by the

concentration of cd2' provided by c d ( ~ ~ - i , ) ~ ~ + a n d the concentration of s2- from

the hydrolysis of (NH2)2CS. The rate of hydrolysis of (NH2)2CS changes with the

pH and the temperature of the electrolyte. From the various equations above, the

presence of an ammonium salt in solution will increase the concentration of

c ~ ( N H ~ ) ~ ~ ' and reduce the concentration of Cd 2tand S ". As a result the rate of

CdS is reduced. By contrast, an increase in the concentration of ammonia will

increase the pII of the solution, promoting the formation of s2-. This will also

increase the concentration of c~(NH,):+ and reduce the concentration of ~ d "

and the rate of CdS formation.

25

As a result of the above points, the rate of deposition of CdS may be

controlled by varying the concentration of ammonia and of the ammonium salt and

the temperature of the electrolyte. The solution grown CdS films can take on a

substrate (heterogeneous deposition) or in solution (hon~ogeneous precipitation).

Homogeneous precipitation makes poorly performing films because they are

formed from the adsorption ol' CdS particles and not by ion -ion deposition. The

homogeneous process may be suppressed by causing the conditions for the

formation of CdS to occur at low rates, such as low concentrations of C ~ ( A C ) ~

and (NH2)2CS ,high concentrations of NH3 and NH~Ac, low temperature,

vigorous shaking etc. The heterogeneous process may be optimized by adequate

preparation of the substrate surface to nucleation by cleaning and etching the

conductive glass surface.

Based on the above considerations, CdS chemical deposition is influenced

by various parameters. However, the film properties may be improved by surface

treatment activation in an appropriate electrolyte. For example, a CdC12 solution

saturated in methanol may be used to activate the CdS surface before annealing in

air at 200-400'~ for 5 - 30 minutes2'. By contrast, the stability of the deposited

films still needs to be improved. This lack of stability is the key problem to be

solved in the development of CdS based thin film solar cells.

Use of the precipitate as a precursor for other coating techniques

The real challenge in the optimization of a chemical bath deposition

technique is to reduce the particulate precipitate in the bath obtain in high thin film

yield. Nevertheless, even in the best-optimized bath, precipitate presents itself as a

26

major product of the condensation process. The precipitate may be filtered and

reacted with acids to produce the starting materials for deposition. But, in many cases

the precipitate may be rinsed well, dried and stored to serve as precursor for other

deposition techniques. Following are three specific examples:

(1) Screen printing technique: The screen printing process involves the

preparation of a paste containing the semiconductor pigment, a flux

material which will fuse at a temperature much below the melting point of

the pigment, and a binder usually ethylene glycol or propylene The

paste is printed on suitable substrates using a silk screen, dried, and then

sintered at a temperature higher than the melting point of the flux.

(2) Use in the production of composite coatings: It is found that composite

coatings made of CuS pigments precipitated liom a chemical bath used for

thin film deposition possess a sheet resistance nearly 100 C2 1 I I1 42. The

conductivity does not degrade considerably when the coatings are annealed

at a temperature upto 2 5 0 ' ~ .

(3) Use of the precipitate as the source for vapour phase deposition: The

semiconductor precipitate produced in the chemical deposition bath is, in

general, stoichiometric. The purity of the precipitate is superior to that of

the starting chemicals in the bath because the impurities whose

concentrations are below those required for precipitation into solid phasc

(ionic product < solubility product) are left behind in the solution. Thus, the

precipitate can serve as a relatively pure source of semiconductor material

for vapour phase deposition43.

Chemically deposited semiconductor thin films for solar energy related applications:

Two major characteristics of solar energy are well recognized.(l) It is

abundant in most part of the world: and (2) it is available in a relatively dilute

form. Hence collection of solar energy over a large area is inevitable.

(1) Application in solar control coatings:

The basic purpose of this application is to reduce space cooling expenses

in buildings and in automobiles by selectively control ling the amount of visible

and infrared radiation entering through glazings. Chemically deposited CuS thin

films have been found to possess near-ideal solar control characteristics. The

coatings can be made on sheet glass, acrylic or polester sheet and foils44. A major

problem in the application of these films is the need for a protective coating.

Laminated PbS-CuS coatings are found to be suitable for architectural glazing

applications.

(2) Application as solar absorber coatings:

Chemically deposited S~S-CLIS~', PbS-CuS and Bi2S3 -CuS coatings have

been investigated for application as absorber coating in all-glass tubular solar

collectors. The efficiency of such coatings have been analysed in a theoretical

model. It was apparent that the absorber coating must be located on the outside of

the inner glass tube. The advantage of chemical bath deposition in producing

uniform coatings on glass tubes is substantial over vacuum techniques. Further

work on the thermal stability of chemically deposited multilayer films in vacuum

at 300- 4 0 0 ' ~ is worth considering to develop this application.

(3) Photodetector and photovoltaic applications:

The dependence of photocurrent on the intensity is found to be nearly

linear in the case of CdSe thin films. The photo generation of such magnitude in

the depletion layer of a Schottky barrier or p-n junction leads to the build up of

photo voltage. Thcre exists the possibility of creating new materials through

interfacial diffusion of atoms in chemically deposited multilayer stacks as

demonstrated in PbS-CuS, ZnS-CuS and Bi2S3-CuS films46. Further, new

materials may also be created by combining chemical deposition technique with

thermal evaporation or sputtering of mctals. The creation of new thin film

materials by the above approaches is vital for further advancement of the

chemical bath deposition technique and integration of the films produced in this

way into solar energy technologies.

Toxicity considerations

The chemical deposition technique involves the use of dilute solutions of

compounds involved in the reaction. This offers minimum toxicity and

occupational hazards since the vapour phase of the reactants are avoided. It is

well known that toxicity hazards associated with lead, cadmium. mercury,

selenium etc are severe when inhaled. Further, the unreacted ions can be

precipitated in the bath as sulphides or selenides and the solid can be separated

and stored for use or recycled to produce starting material.

The formation of multinary compounds by interracial diffusion and

reclystal lization in mu1 tilayer films can be considered as an environmentally

29

sound process since very few effluents are produced. Overall, the large-area

capability and the ease of scaling up with complcte control of material handling

in solid or liquid phase offers perspective toward the industrial production of

coatings and devices by the chemical bath deposition technique.

2.3 Thermal Evaporation Technique

One of the most widely used techniques for depositing thin films is

thermal evaporation.47 Basically it involves three steps; boiling or sub1 iming of

source to form its vapour, transport of the vapour from the source to the substrate

and condensation of the vapour on the substrate. The basic theory of the process

contains elements of thermodynamics, kinetic theory of gases and condensation

phenomena.

Solid matcrials are sublimed under high vacuum when heated to

sufficiently high temperature. The condensation of the vapour onto a cooler

substrate yields thin solid f i l r n ~ ~ ~ . ~ ' . This method has the following advantages.

1. lmpurity concentration in the film will be minimum.

2 . Materials boil at lower temperature ul~der vacuum.

3. Growth can be effectively controlled.

4. Mean free path of the vapour atom is considerably larger at low pressure

and hence a sharp pattern of the film is obtained.

5. Selection of the substrate is wide.

30

The evaporation rate and hence condensation have wide limits, depending

upon the type of source and material used. Characteristics of the prepared film

can be varied by parameters such as temperature, type of substrate, deposition

rate and residual atmosphere. All these parameters can be controlled in the

thermal evaporation method. More than that single evaporation can give films of

different thickness. We have used here molybdenum boats and tungsten baskets

for evaporation of phthalocyanine thin films. Films of high purity can readily be

produced with a minimum of interfering conditions. The different factors, which

are to be expected to have some influence on the nature and properties of an

evaporated film, are the following.

1. Nature and pressure of residual gases.

2. Vapour beam intensity

3. Nature and conditions of'substrate

4. Temperature of the vapour source and velocity of the impinging molecule.

5 . Material contamination from vapour source.

2.4 Production of Vacuum

The evaporation of thin films with controlled properties requires an

operating environment which interferes as little as possible with the process of

film formation. it is possible to obtain high vacuum in a closed chamber to

minirnise interaction between residual gases and the surface of growing films. A

wide variety of vacuum components, materials and assembly techniques are now

available. Various degrees of vacuum are given below according to their pressure

ranges. so

Coarse

Vacuum

rough Vacuum

Medium Vacuum

High Vacuum

Very High Vacuum

Ultra High Vacuum

1-10" Tom

1 - 1 oa6 Torr

1 o - ~ - 1 Torr

1 o - ~ Torr and below

The advantages of using a vacuum system for thermal evaporation are:

1. The material will boil at lower temperature in vacuum

2. Oxide formation in the boiling surface can be reduced.

2 Impurities in the deposited film will be little.

To reduce the pressure in a vacuum enclosure, two different principles are

employed (1) Physical removal of gases from the vessel by exhausting the gas

load to outside and (2) the condensation or trapping of gas molecules on some

part of the inner surface of the enclosure without discharging the gas. Oil sealed

rotary pump is the former type and din-usion pump, the latter.

2.5 Vacuum Coating Unit

The type of vacuum equipment needed obviously depends on the desired

purity of the film. Detailed reviews on various types of vacuum systems and their

ultimate pressures are given by ~ o l l a n d , ~ ' asw well,^' ~ u s h r n a n ~ ~ and ~ 0 t h . ~ ~

The vacuum system employed to deposit and characterize thin films in the present

work contain an assortment of pumps, tubings, valves and gauges to establish and

measure the required reduced pressure as shown in Figure 2.5.1. Basically the vacuum

32

system, "Hind Hivac" Vacuum coating unit model No. 12A 4 consists of 0.4 m

difhsion pump backed up by an oi! sealed rotary pump. Ultimate pressure obtained in

a 0.3 m diameter steel be1 1 jar is of the order of 8 x I 0' mbar. It has set ups for electron

beam evaporation and flash evaporation. Most of the evaporations are carried out at a

pressure of (1 -2) x 10 -5 Torr. It has provisions for ion bombardment cleaning of

substrates. The pressure measurement in the system is done by means of Pirani

and Penning Gauges (7 and 12 in figure 2.5.1) provided with the system.

The Pirani gauge model Hind HiVac-A6STM is used for measuring

vacuum in the range 0.5-1 0" Tom 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 as in figure 2.5.2. The filament is often reconditioned if the gauge behaves

erratically when it is fiilcd with any contaminants. The gauge head is flushed with

acetone and thoroughly dried. 10V AC or DC is applied across the filament to

volatise the deposits on the filament. The Penning gauge model STM4 is used to

measure vacuum in the range to 10 " 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 as given in figure 2.5.3. A potential

difference of about 2.3 kV is applied between the cathode and anode through current

limiting resistors. A magnetic field is introduced at right angles to the plane of

electrodes by a permanent magnet having nearly 800 gauss magnetic field, which

will increase the ionisation 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 perlbrm similar osci l l ations and the rate of ionisation increases rapidly.

Eventually the electrons are captured by anode and equilibrium is reached when the

number of electrons produced per second by ionisation is the sum of positive ion

current and electron current to the anode and is used the pressure of the gas. If the

gauge shows unstable pressure reading due to the contamination of the 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. Figure 2.5.4 shows the photograph of the coating unit along with

the accessories. We have used oil sealed rotary and diffusion for

the production of high vacuum. In our coating unit the diffusion pump has an

evacuating speed of 500 lit./sec. The coating unit has provisions for ion

bombardment cleaning of substrates.

Figure 2.5.1 Schematic diagram of a vacuum coating unit

Bell jar

Thickness monitor

Electron beam gun

Penning Gauge

Baffle valve

High vac valve

Fore-l ine trap

Rotary pump

2. Substrate

4. Source shutter

6. Current feed through

8. Roughing valve

10. Diffusion pump

12. PiraniGauge

14. Isolation valve

Figure 2.5.2 Schematic representation of Pirani gauge

1. High Vacuum 2. Reference wire

3. Sensing wire 4. Vacuum System

Figure 2.5.3 Schematic representation of Penning gauge

1. Magnetic field 2. Anode (+)

3. Cathodes (-) 4. Ballast resistor

2.5.4 Photograph o f the coating unit along with the accessories

2.6 Substrate Cleaning

For deposition of films, highly polished and thoroughly cleaned substrates

are required. A variety of cleaning processes are available. First the substrates

are cleaned using liquid detergent. Then it is kept in dilute nitric acid for some

time. After this the substrates are cleaned using distilled water. Then the

substrates are agitated ultrasonically in acetone. They are then rinsed in isopropyl

alcohol and dried in hot air. Inside the bell jar the substrates are subjected to ionic

bombardment for five minutes as a final cleaning before deposition. The ions are

produced inside the bell jar by H.T. discharge at medium pressure.

2.7 Thickness Measurement

There are different techniques to determine the film thickness.54 We employ

the optical techniques for measuring the thickness of the films. It can be used for both

opaque and transparent films. The basic principle underneath this technique is the

interference of two or more beams of light reflected or transmitted fi-om the bottom and

top of the film whose thickness is to be measured. The condition for maxima in

reflection will be the condition for minima in transmission and vice versa. In the case

of multiple beam interference by reflection, the interference pattern is formed. There

are sharp bright tiinges on a dark background in the case of transmission whereas

sharp dark fringes on bright background in reflection.

For opaque films a sharp step down to substrate plane must be first generated

either by a deposition through a mask or by subsequent etching. For practical purposes

the fiinges formed are classified as the two cases of multiple beam interferometry.

38

Fizeau fringes are generated by monochromatic light and represent contours of equal

thickness in an area of varying thickness 't' between two glass plates. This is

accomplished by contacting the two glass plates such that they fonn a slight wedge at

an angle a so that the thickness between the two plates can be varied. The angle a is

made very small so that consecutive fkinges are spaced as far part as possible. For

normal incidence of monochromatic light, the spacing between the Fringes correspond

to a thickness difference of lJ2, h is the wavelength of the monochromatic light used.

Another method to determine the film thickness is the Fringes of Equal

Chromatic Order (FECO). In this case white light is used at angle of incidence zero

degree and reflected or transmitted white light is dispersed by a spectrograph. Here

the fringes are formed for certain values of t/h.55 FECO fringes are obtained with two

silvered glass surfaces parallel to each other, where the plate is adjusted to get Fizeau . fringes. The spac ing between the interferogram is inversely pro port ional to the

thickness ' t' . We have used the Tolansky 's multiple beam interference method for

the determination of the thickness of the film.

2.8 Tolansky's Multiple Beam Interference Method

The schematic representation of Fizeau fringes produccd by multiple

beam interference is as shown in figure 2.8.1. The technique can be employed

when the film to be studied remains in vacuum and can be coated with highly

retlective layerms6 The film is deposited onto the glass substrate. A sharp edge

within the film is produced by shadowing with sharp masks during deposition.

The film is then coated with a highly reflecting silver layer. A second glass plate

with a silver coated surface and having some percentage of transmission is

lowered onto the glass substrate and the whole system is illuminated with a

parallel beam of monochromatic light of wavelength ( h = 5893 A) from a vapour

lamp. At small distance between two glass plates, when the cover glass is tilted

slightly, multiple beam interference fringes appear with a distance 'x'. In the

region of sharp edge, the fringes are shifted by a distance 'Ax'. A shift 'Ax' in 'x'

corresponds to a thickness step of h/2 and hence the thickness of the film.

Figure 2.8.1 Schematic representation of the multiple beam interference technique b

(a) Fringe pattern (b) Arrangement ( c ) Sample with step and a match-flat

P ~ c e

,i .

;+&O - ; +- \

S o u ~ 1. -; I I

Partial Rcfkctor

L

..

2.9 Sample &ding

The samples have been annealed in a specially designed furnace. It

consists of a coil of Kanthal (A1 grade temperature range 1150-1350'~). To

avoid heat loss it is surrounded by a thick package of firebrick silica whose

working temperature is 1 1 0 0 ' ~ and the melting point is 17 1 O'C. The width of the

heating element is about 20 cm. The filament is also covered with sillrnate (A1203

-Si02) tube, maximum working temperature is 1 5 0 0 ' ~ and melting point is 171 0'

C. It helps to provide uniform heating region at the centre of the tube. In addition

it avoids any thermal shock during the annealing process. The temperature of the

heater is controlled and recorded by a digital temperature controller cum recorder.

Figure 2.9.1 shows the block diagram of the temperature controller. The

thermocouple used is chromel-alumel type. The output of the thermocouple is

calibrated to 0 . 0 4 r n ~ l O ~ and fed directly to the comparator circuit as shown in

the block diagram. The comparator consists of the 1C LN324 and its associated

circuitry. By adjusting the hysteresis loop of the comparator, using a hysteresis

voltage regulator one can control, set and reset voltage for the relay switch. The

voltage corresponding to the setting temperature is referred by the comparator.

The heater coil is connected through the relay switch and the power to the heater

and thereby the temperature is controlled by the comparator circuitry.

The analog signal from the thermocouple is converted to a digital one with

the help of an N D converter, using a 3 digital single chip AID converter IC 7107

(inters; 1) having high accuracy. The AID converter provides a built-in-seven segment

display unit. The temperature can be displayed digitally. When the temperature

reaches the pre-set temperature, the heater cuts off automatically, by action of the

relay switch. After a few seconds, the heater is again switched on and the process is

repeated, thus maintaining a constant temperature at the centre of the fumace. Figure

2.9.2 shows the photograph of the furnace and the annealing experimental set up.

Figure 2.9.1 Blockdiagram ofthe temperature controller cum recorder

.-

'I. @

Figure 2.9.2 Photogaptl of thc annealing filmace and controller c u m recorder set up

2.10 Electrical Conductivity Measurements

A schematic diagram of the conductivity cell fabricated is as shown in

figure 2.1 0.1. The cell consists of a thick walled cylindrical chamber with a

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

closed air-tight with glass windows and are used in spectroscopic studies. The

remaining side tube is connected to a rotary vacuum pump and the chamber can

be evacuated to a low pressure of lop3 rnbar. The inner tube is made of stainless

steel pipe, which has been welded to a large copper finger. The liquid nitrogen

cavity and thc heater coil help the sample to attain the required temperature very

quickly. The outer enclosure is made leak proof by using a neoprene '0' ring,

which rests inside the groove on the flanges. A sample holder fixed at the copper

finger can hold the film on a substrate in the form of a strip with the help of

screws. Mica sheets are placed in between the sample holder and the substrate.

The outer surface of the copper finger is covered with mica sheets and the heater

coil is wound over it. The electrical leads are taken out through teflon insulation.

A d.c. power supply is used to heat the heatcr coil. The electrical leakage current

through the mount is by-passed to earth by b~ounding the inner tube. The leads of the

electrodes are taken out using BNC connector. A copper-constantan thermocouple in

contact with the sample senses the temperature. Temperature of the sample in the cell

can be varied from liquid nitrogen temperature to 400'~.

Figure 2.10.1 Schematic diagram of the cross section of the conductivity cell

Cylindrical chamber

Liquid nitrogen cavity

Mica insulator

Sample holder

Side tube

To rotary pump

BNC

Connecting leads to BNC

Inner tube

Copper finger

Heater Coil

Glass window

Bottom flange

Neoprene "0" ring

Thermocouple

Substrate with film

45

2.1 1 Keit hley Programmable Electrometer

Electrical conductivity measurements are carried out using Keithley

programmable electrometer model N0.617. It is a highly sensitive instrument

designed to measure voltage, current, charge and resistance. The instrument has a

sensitivity of 0.05%, 0.15%, 0.4% and 0.15% of the reading for the measurement

of voltage, current, charge and resistance. The very high input resistance, low

input offset current and sensitivity allows accurate measurement. The measuring

range is between 10 pV and 200V for voltage measurements, 0.1 pA and 20 mA

in the current mode and 10 f C to 20 NC in coulomb mode. The resistance can be

measured in two modes (i) constant current mode and (ii) constant voltage mode.

Due to the high input resistance, a resistance as high as 200 GR can be measured

in the constant current mode. Using constant voltage mode resistance as high as

1016 i2 can be measured. In this mode, the measured resistance is automatically

calculated from the applied voltage. The built in voltage source of the instrument

can be used to apply a current I, through the unknown resistance R. The

insulation resistance is then automatically calculated by the instrument as R =

VJI; where I is the current through the resistance and V is the programmable

voltage. The schematic diagram for the measurement of electrical resistance by

two probe method and four probe method are as shown in figure 2.1 l.l(a) and

2.1 l.l(b). Figure 2.1 1.2 gives the photograph of electrical conductivity

experimental set up used in the laboratory.

The model 61 7 has a built-in voltage source. The voltage can be programmed

between -102.35V and +102.4V in steps of 50mV and the maximum measurable

output current is 2mA. The instrument is capable of an il~ternal 100 point data

store mode that can be used to log in a series of readings. The fill rate of data

store can be set to specific intervals according to the experimental conditions

Sample film

(a)

Sample film

Figure 2. J 1.1

+ Thick Ag film

Schematic d i a p n of electrical corlductivi~ measurement dimensions are in rnrn)

(all

(a) Two probe method

(b) Four probe method

Figure 2.1 1.2 Pl~otograph of the electrical conductivity experimental set up

2.12 Optical Measurements Using UV-Visible Spectrophotometer

To study the optical absorbance and transmittance of the films in the ultraviolet-

visible range, Shimadzu lGOA spectrophotometer has been employed. It is a double

beam system employing a static bean splitting half mirror, which sends the light beam

Srom the monochromator equally through the sarnplc and the reference substrate. The

light bcam emitted from the light sourcc (Deuterium lamp 4 or halogen lanlp WI) is

reflected by the mirror MI and is directed into the monochromator. The deuterium lamp

produces wavclcngth from 200 MI. 'Ihc halogen lamp produces wavelength upto 1 100

nm. The light source-switching wavelength can be sct to any valuc within the mnge of

295 to 364 nm. Its initial value is 350.5 nm. The lan~ps can be automatically

interchanged according to the wavelength range needed. All the optical elements except

the light source are isolated from thc external atmosphere by the window plate W so as

to be dust free. 'lhe slit width of the monochron~ator is fixed at 2 nrn. G is a 900 lineslnm

aberration corrected concave holographic gating. The light beam From the

monochromator is paqsed through the stray light cut off filter I:, reflected by the mirror

M2 and split by the half rnim~r M3 into the sarnplc and thc reference beams. Fach beam

passes through the respective cells to the detector. Two voltages are produced by the

detector, whch are proportional to the light inte~~sities ofthe reference and sample beams

respcctivcly. Thesc two voltages are amplified and fed to the electrical system. Figures

2.12.1 and 2.1 2.2 give simplified block diagram of the optical systcm of the Shimadzu

160A spectrophotomcter and photograph of the optical set up. Tl~e output absorbance or

transmittance can be seen in the video display and printed out using the chart recorder.

P.D.

P.D.

Figure 2.12.1 Block diagram of the optical system of the spectrophotorneter (S hirnadzu 1 60A)

D2 :Deuteriurnlamp

W 1 : Halogen lamp

M3 : Half' mirror

L : Lens

Sam : Sample cell

Sz : Exit slit

P.D. : Photodiode

W : Window plate

M 1-M5: Mirrors,

F : Filter

C; : Grating

s 1 : Entrance slit

Ref : Reference cell

Figure 2.1 2.2. Photograph of the Sbimadzu 160A spectrophotometer

2.13 Structural Studies Using X-ray Diffractometer

Philips Analytical X-ray B.V, PW3710 BASED diniactorneter is used for the

X-ray diffraction measurements. The block diagam of the XD PW37 10 BASED is

shown in fibwe 2.13.1. The main components of the system are the X-ray tube, the

X-ray generator, goniometer, controller/counter and recorder. X-ray tube has a copper

anode target. X-ray power generator provides a high voltage upto 40 kV and current

upto 25 nlA. The goniometer can scan in the range between 5" and 80'. In the

dihctorneter X-rays are diffracted fiom the samplc and are concentrated on the

dctcction slit located at a position symmetrical to the sample about the X-ray focus of

the tuk. The X-rays are detected by scintillation detectors and converted into electrical

signals. The signal is picked up by the pulse height analyser after eliminating its noise

components. A chart recorder running synchronous with the goniorneter gives the

recorded spectra. The scattered intensities are angle dependent in the Bragg-Brentano

geometry where the X-ray beam falls at an angle 0 to the substrate and the detector is

placed at an angle 28. The specimen and the detector are rotated at anguiar velocities o

and 2w respectively to get various diffracting planes. In this geometry when the films

of' thickness 't' arc used atomic spacing between planes can be calculated using

Rragg' s condition.

where 8 is the angle of incidence and h the wavelength of the Cu K, radiation.

Conscquently the scattered intensities will be angle dependent and this has to be

taken into account while comparing the intensities with ICDD-PDF data. Since

space between atoms arc fixed by atoms and ions, the material with which the

sample is composed can be detected.

Control 1 CYU

DP 61 Syslem

ScatlerIRate Mcter Detector

Proper id=

I

HVIPHA

Key Board

Printer P Recorder r-7

Figurc2.13.1 Blockdiagramof XDPW 3710 BASEDdiffractomcter

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