University of Nigeria Research Publications

AIDAN, Joseph

Aut

hor

PG/M. Sc/98/25887

Title

Growth and Characterization of Thin FeS2 Film by Solution Deposition Technique

Facu

lty

Physical Sciences

Dep

artm

ent

Physics and Astronomy

Dat

e

August, 2001

Sign

atur

e

GROWTH AND CI3ARACTERI%A'T'I4)1N OF FeS2 'TIXLN

F1 L,M BY SOLUTION DEPOSITION TECHNIQ1JK

(AIDAN, JOSEPH)

PG/M.Sc/98/25883

DEPARTMENT OF PHYSICS AND ASTRONORW

FACULTY OF PHYSICAL SCIENCES

UNIVERSITY OF NIGERIA, NSUEUCA

ENLJGU STATE, NZ@ ERBA

GROWTH AND CHAPCACrPTERIZATZON OF FcSz TI IIN

FlLM BY SOLUTION DEPOSITTON TECliNlQUE

(AIDAN, JOSEPH)

A PROJECT REPORT SUBMITTED TO DEPARTMENT

OF PHYSICS AND ASTBQNOMY 1N PARTIAL

FULFILMENT OF THE REQUIkKMENTS FOR THE

AWARD OF THE DEGREE OF MASTER OF SCIENCE

(M.Sc) SOLAR ENERCY OF

UNlVEKSlTY OF NIGERIA, N S U K U

ENUGXJ STATE, NIGERIA

We certily that this prdject work was carried o u t by JOSEI'II AIIIAN ill

. the Department of Physics and Astrononly, ilnivcrsity ol'Nigcria ~ s ~ k k n I I I , ' l t 1

and has been approved hylthe examiners.

(Supervisor) Dr. J. IJ. Chukwudebelu

Date:

'-' (External Examiner j

Date: 200 1

This piece or work is dedicated lo my:

i. Daddy, I ,ale Mr. Aidan 'i'hlaic Shuwn

. . 11. Mummy, Mrs. l3atricia Aidan

.., 111. Brothers & Sisters

iv. Beloved Wife

seen me through my entire study in this University especially i n the

writing of his project.

A work like h i s would not have been cotnplcled without the help, advicc

and patience from a f w individid. Notably among illem is r ~ y

supervisor, Prof. C. 1:. Okelce. I would like to express my sincere 111mks

have in no little way contributed to the success ol'this work.

My equal thailk-s go to the Ag. I I . O . l ) . . my lect~~rers and a11 other

members ofthe Departmenl for their acadcnlic :~ssistancc and pntiencc.

I appreciate the ef'fbrts of Dr. rahian Ezema and that ol' Mr. Nnabuchi on

the thin film research work.

Finally, I would like to thank my C O U ~ S ~ imtcs 1i)r t1~;ir COOPC'I .~L~~() I I and

kindness and Mr. Ogw, Elemele I'or typesetting this work.

Joseph Aidr~n

TOPIC ,

Tillc pagc

Certification

Dedication

Acknowledgrnen t

List of figures

List of lahles

Abstract

CHAPTER ONE

Introduction

Justification

Objectives

CI-IAITE 1< T W 0

Literature Review

Nature and compositiotl of solar radiation 10-1 1

Interaction of light with sen~iconductor 11-14

Absorption of light in semiconductor

Direct-Band-Gap Semiconductor 14-17

Indirect-Rand-Gap Seniiconductor

Kecom bination process^

Radiative Reconlbination

Auger IZecombination

Recombination Through Traps

Recombination At Surfaces

Thin Film Growth 'I'echniques

Physical Vapour Deposition (PVD)

Spraying

Electrochemic~il Methods

Screen - Printing

Solution C3rowtli Technique

CIIAPTER 'TIIREJ'?

lixperunental Iktails

'l'he Ihposition Technicluc IJsed 3 0

Sample Preparation 36-3X

Reaction Equations Of lh in Film I )epositioll 38

Deterrnirlation Or'l'he 13and Grip 3 9

Measure~iicnts And IXscwsiun Of Rcsults

Physical Appeaxance Of 'l'hc Deposited I;iln~

Filnl Thickness

Study Of Ttic Absorbance Of 'l'he I ; i ln~

Conclusion

Suggestion For Further Study

Figure 2.1

Figure '2.2

Figure 3.1

Figure 4.0

Figure 4.1

Figure 4.2

Figure 4.3

Figure 4.4

Figure 4.5

Figure 4.6

Figure 4.7 Graph of absorbance againsf wavclength h r sanrples

A19 '% H19

Graph of absort~ance against wavelength lbr sanlples

A20 &

Table 2.1

Table 4.1 ,

I Table 4.2

1 ' . Table 4.4

'l'able oSvalues for ccluation 2.20

Variation in lilrn thickness with dcpositio~l time

Variation in absorbaim with wavelengih for san~ples

A,.

Variation in absorbance with wavclcngth Sol- s:lmples

Chemical bath required for the deposition oi' FeSz thin iIlm sing

FeSQ.71-120, Na2S2O3.5I-120 and E13'1'A as complexing agent has t~ecii ,

prepared and successfirlly deposited on a glass slide substrate.

The films showed high absorbance in the ultra-violet, low in the visible

and the infrared regions. The energy bmd gaps, as calculated. f i~ r the,

various film samples used fall within the range 2.48 3. lOeV.

The thin film grown would make good window layers for solar cclls.

The alternative approach using FeS04.71 I&) instead of I:c(NO>)~ 1i)r thc

cation source lias been economical because only a, total of 91111 volume oS

chenlicals per bath were used instead of 1 71111 as reporled [ 1 1. 'rlic

solution growth technique is simple and cheap.

Introduction

. Energy is the firndamental input in the develop~ricnt of any known

human society. The a~nount oC ciiclgy izquired pcr capila to hstes or to

maintain the developnlent of' such sociely depend largely on the

developmenla1 stage, the local resources available, the socio-economic

model chosen try the society (or country) and several other fi~ctots.

Coirn tries, which rely on local US irnpostcd lbssi l l i d s , 11iig21t fact c~iesgy

crisis as the re3ource depletes. Its replncc~wnt lry nucle;lr ciicrgy source is

potentially catastrophic i'or humanity. Massivc burning of cod to generate

energy is itself' not a viable long-Ierni solution. 11 appears theil that the era

of rqxwabie sources of energy, like solar,energy, has come a id i~ceded

to be hurried up.

Researches are now on Ihe way into the use 01' solar energy resource,

the fi-ec gifi of nature, to the conventional energy rcsoirrccs. At present

solar electricity has been universally recognised as an a l n ~ o s ~ ideal source

of energy because its relevant characteristics being that a high qmlity

form ol'eiaergy is generated in a nlodular way, with low nlaintenunce cost

and with no negative environmental risk.

Solar electricity is the direct conversion of daylight or the

electromagnetic radiation associated with the sun into electricity by

photovoltaic solar cells and it is the most promising for111 of renewable

energy to have emerged in recent years. In terms of its potential benefits

to humanity the invention in the early 1950s of this completely new pay i

of generating electricity might come in rank in iiuportance with Faraday's

discovery of electromagnetic induction that led to the development .of b

rotary electric generators and motors.

'The question that arises tl~erefore is why this cnergy generating

technology, in spite of all its good promises, not widely used in all places

especially where other conventional energy sources do not exist, or in

places where other energy sources at hand'are environmentally risky. 'Ihe

usual answer to this question is the high cost of solar electricity. The cost

of solar electricity depends on many factors, but one of the inost

important is the cost of the

depends on the cost of

photovoltaic modules. The cost of n~odules

semi-conducting materials used in their t

construction and of course the cost of the manuhcturing 'process.

However, these compounding costs can be seduced by the use of thin

films of semi-conducting materials for the cells instead of the usual slices

oC crystalline silicon. Therefore, with the reccnt developi~lcn t in the thin

film technology, it is expected that a downward cost trend will be

obtained. If so, photovoltaic generatim will o f f r a way o l helpillg to

meet the increasing world wide deimiid ibr electricity \i I t Ilo~it

accelerating the deplelion of our finite resoiirccs of' fossil fuels, adding to

the contamination of the atmosphere or builtling hundreds of Iwnlth

hazardous nuclear power stations. Additionally, solar power l~as the b

advantage of being generated as and where it is needed. I lcncc, s;lving the

cost and avoiding the losses of transmission lines.

A thin film is a layer of material with a thickness oS the order of a

micron that is deposited on a substrate, its mechanical suppol-t. Thin lilrns

can range in tl~ickness from a l~undredth of a micron (about 100 atomic

layers) to a few tens of micron (about the thickness of a layes of paint). .

Films of consideration here are the polycrystallinc films, that is, tillus

lying between crystalline and amorphous. Iron pyrite is a conlil-med

example of it [I] . Polycrystalline, as the name implied, are lilms -

composed of a large number of small crystals. Within thcsc c~yst:lllites, . .

the atoms are regularly ordered and their elcctric and optical properties

are similar to those of a bulk single crystal of the material. 'I'he

crystallites in the film are packed together in a pirrely random fashion, so

that any electric current flowing across the film must cross many.

. boundaries between.crystallites, and these grain boundaries exert a strong .

influence on the characteristics of the films. t ,

Polycrystalline film materials seem at first sight, unlikely to be useful

for efficient solar cells since their electrical properties are so inrerior to

those of crystalline materials. However, the semi-conductor which arquse

in polycrystalline cells absorb light much more strongly than crystalline

silicon, about 95% of the light can be absorbed in only a few microns of

the semi-conductor material, so the material can be made hundrcd times

thinner than a silicon wafer [2]. F~~rthermore, if the film is deposited

under carefully controlled conditions, each crystallite can be a small

column extending through the whole thickness of the film; current then

flow up these crystalline columns malting the device bchaving retiler like

a large number of tiny crystalline cells connected in parallel. The cl'l'ect of

the grain boundaries, though not said to be eliminated, but is reduced; and

the larger the diameter of the colun~nar crystallites or grain, the smaller

the effect of the grain boundaries.

For solar cells, such films i ~ u s t have a high absorption coefficient and

an o p t i ~ m m energy gap (about 1.4eV Tor single junction); i t must have a

diffusion length many times longer than the inverse absorption coeflicient .

so that all photo generated carriers can be collected; it should be stable

during operation and non-toxic. Unfortunately, solar I ! . cells have always

been faced with stability problems, which may degrade the cells. These

problems that arise from the structural, micro-structural and stoichiometry b

changes in the cells during operations have, as good-news, been foi~ncl to

be surmountable by doping with impurities [3 1.

In solar photo-thermal converters, it is desired to maximise the

efficiency of the conversion system by minimising the thetmal losses by

surface radiation and at the same time enhancing the solar absorptivity. In

other words, the collector windows should.be transparent to infrared but

should absorb radiation in the solar range. This is to allow the infrared to

go through to the absorber unabsorbed. Surfaces exhibiting these

characteristics are referred to as selective surfaces. Technically, these

surfaces should have absorptivity, a close to 1.0 in the solar range 0.3 -

2.3pm and low thermal emissivity, E close to 0.0 in the infrared (X ,>

2pm). This low emittance requirement hecomes inore of an advantage as

the workmg temperature of the system increases. These requirements are

comfortably met with the development in thin film technology. For

therinal collectors operating at high temperatures, the development is a

gateway opened to dropping the ideas of smoking matt black paint

application. Although, selective surfaces cost much more but has the

advantage of withstanding years of fluctuating high temperatures and at

the same time retaining its figure of merit, a/& as high as possible. , I n the case of collector windows used in conjunction with solar cells

the layers should be transparent to visible radiation to allow sunlight . . : '

through to the absorber, it should form an efficient hetero-junction with

the absorber, it should have a low sheet resistivity so that the photocurrent

can be collected with minimum loss. Ideally, it should have band gap of

over 3eV to allow visible radiation to be fully transmitted.

There are several ways of depositing polycrystalline films on a

substrate, each with i t s advantages and disadvantages. Some technic~ues

produce films, which are perfectly crystalline with electrical properties,

that are superior to those of slices of single crystal material. 'l'he choice

therefore,.is governed firstly by its proven ability to produce, for instance,

efficient cell from material, but then secondly but equally

7

important, by the cost per cell CIS manufac~uring a1 a given production

rate. Some techniques are relatively slow and labour intensive, but have

low capital cost. These techniques could producc cclls chcaply at qui~e

low production rates. Others :u-c capital intcilsivc, h i t would proctucc

cells at low cost at high producIion rlitcs. 'l'hesc techniijiics \vliich

include:

1.

. . 11.

... 111.

iv.

v.

I'l~ysical Vapour Deposition, 6

Spray - . Pysolysis,

I':lcctroche~nical Dcpositicm,

Screen-Printing and

Solution Growth, are fully discussed in Chapter 2.

In this research work, the solution growth technique would bc used

because of its simplicity and cheapness.

Thin films, apart from its use in solar cclls and as window coatings of

various forlns, are also of use in: oplics as reflecti~lg surli~ces,

microelectronics as inputs for the fabrication of integrated ciscuik (IC),

computenas memories, etc.

Justification

The immediate renewable energy alternative to the depleting

conventional energy .sources available is the solar energy; solar energy i,

converted to electricity using silicon wafers are expensive and

unaffordable by the poor majority of the developing countries that needed

it most. However, with thin film semi-conducting materials replacing

silicon wafers, they become cheap source of electrical energy and eyen

cheaper if the solution growth technique is used in the deposition. 'I'his is I I

because the solution growth technique is the easiesl a i d cheapest of all

the deposition techniques available. 'The solid-solid iron pyrite, FeS2 is an

example of a very good photovoltaic thin film grown by O s ~ ~ j i et a1 [ I ]

using this technique. Unfortunately, the cation source, Fe(N03)3.91-120, as

used by the researchers is higllly hyyroscopic and therefore,

uneconomical inpr profitable) for retailers to buy and display for sale .

because of fear of turning into liquid withill a short time. Hence, diflicult

to get and if available in the liquid form it will be difficult to deternine its

concentration. The alternative approach using the durable hydrate,

FeS04.71-120, as highlighted in this research work will no doubt

increase the film's popularity and si~nultaneously simplifying the

9

immediate-reach-assessment of its uses pal-titularly h r distmt

. researchers.

Objectives

i. To prepare, using the altcrnativc cation source, the cl~enlical bath

required for the deposition of the Iron pyrite (FeS2) thin film.

ii. To determine the energy band gaps of thc Glrns at di l lrent

deposition limes. @

CHAPTER TWO

Nature And Composition Of Solar ltitdia tion:

As the key player, the spectral distribution of the radiant energy l'som

the sun is very important, particularly in the functioning of solar dev~ces.

At the surface (temperature r 5800K) the solar energy radiatcd into space .

falls in the wavelength range of 0.2 - 3pm. But from the viewpoint of @

terrestrial application only radiation in the wavelength range 01' 0.20 -

2.5p.m needed consideration duc to rrbsosption. 'l'he spectral ;rnalysis 01'

such radiation within the wavelength range rcvealed that 4 1% 01' t llc

radiation is made of visible light rays, 9% conlprising of very short

gamma rays, alpha rays, x-rays and ultra-violet (UV) rays and the

remaining 50% house the infrared and heat rays (therrnal radiation).

'The attenuation of the sun's radiatiol" due to absorption depends upon

the thickness of the atmosphere through which the ray is travelling. 'I'he

thickness of the layer of atmosphere is called Air Mass (AM) and is the

path traversed by the direct solar beam defined as

0 is the angle of the sun to the zenith.

The recommended standard value of the solar intensity is obtained at

AM 1.5 and corresponds to 1 0 0 0 ~ / m ' [4].

Interaction Of Iight With Senri-conductor:

If light is incident perpendicularly onto a flat section of a semi-

conductor, certain fraction of the incident power, R, would be reflected b

and the remainder, T, transmittcd into the semi-conductor.

r 7 Ihc tru~lstnitlcd liyhl btu1 bc :rt)sorl)eti witl~irr :I semi-co~d~~ctor hy

wing its energy to excite electrons from occupied low energy states to

unoccupied higher energy states. Since there are a large number of

occupied states within the valence band of a semi-conductor separated by

the forbidden band from largely unoccupied states in llle conduction

band, absorption is particularly likely when the energy of the pholons

making up the light is larger than the Sbrbidden band gap, Eg, O S the semi-

conductor.

For absorbing materials of refractive index n, (-- n-ilc, complex). the

fraction of light reflected for normal incicfe~we is givcn by 15-7 1

Where lc is the extinction cocflicient and n the rcfi.activc index obtained

from [8]

Where n, is substrate refractive index (glass, n, = 1.5),

and T,,,,,, are envelope functions of transmission ~ n a x i m ~ ~ ~ n and

minimum in the transmission spectrum with interference peaks.

The light transmitted is attenuated as it passes through the semi-

conductor. The rate of absorption of light is proportional to the intensity

(the flux of photons) for a given wavelength. This common physical

occurrence leads to an exponential decay ir, intensity of monochromatic

light as it passes through the semi-conductor, described lnatheinatically as

Where a is a function of wavelength apd is known as the absorption

coefficient. This parameter is important in the solar cell design because it

determines how far below the surface of the cell light of a given

wavelength is absorbed.

The absorption coefficient a and the qxtinction codlicient k are

related. If the light is described by a plane wave of li-equency v

propagaling in the x-direction with velocity, v, it would have 311

associated electric Geld strength, E, given by [7]

The velocity in the semi-conductor is related to the velocily in

Hence,

1 .- n i k - - --- -. ........................................... 2.8 V C C

Substituting (2.8) into (2.6) gives

The last tenn is an attenuation factor.

Power will attenuate as the square of electric field strength

Comparing eq~~atiorls (2.5) and (2.9) gives the relationship

Absorption Of Light In Semiconductor

Direct - Band - Gap Semiconductor:

Fundamental absorption rclers to the annihilation or absorption o l

photoqs by the excitation of an electron from the valence band up into the

conduction band. Both cnergy arid mon~entum must be conse~-vcd in such

a transition. A photon has quite a largc eiicrgy (hv) but a snlafl

moinentuin (hlh).

The form o l the absorption process fbr a direct-band-gap'

semiconductor is shown in the energy-momentun1 sketch of fig. 2 1.

Because the photon niomenlum is snlall compared to the crystal

momentum, the latter essentially is conserved i11 the transition.

Fig. 2.1:- Energy- Crystal nmmm t z m diagram o f a dircct band-gay semiconductor, showing the absorption o f a photo11 by ~e e.ucitatim o f an elcctron from the valence to rhe conducbon band.

The energy difference between the initial and final state is equal to .

the energy of the original photon.

In terms of the parabolic bands

The specific value of crystal inonxntum at which thc tr-nnsilion

occurs is given by

As ihe photon energy hv increases, so does the value of crystal

momentun1 at which the transitian occurs. l'he probability ol' ;lbsorption

depends on the density of electrons at the energy corresponding to thc

initial state as well as the density of empty states at the final energy, since

both these quantities increase with energy away from the band edge. It is

not surprising that the absorption coefficient increases rapidly with

increasing photon energy above B,. A simple theoretical treatment gives

the result [9]

u(hv) z A(hv - E ~ ) ' ' ~ ... ... ... ... ... . ... . .. . . . . .. ... .. 2.1 5

here^* is a constant (= 2x10' cm-') with 1,v and E, in oV.

The direct optical absprption edge is determined from equation (2.1 5 )

by plotting a2 against h v and interpolating 2 - 0 and I?, is the energy gap

a t k = O .

Indirect - Band - Gap Semiconductor-:

In the case of an indirect-band-gap semiconductor, the minimum 4

energy i n the conduction band and the rnaslmunl energy i11 ht: valence .

band occur at dilTererll values 01' cryskil nlopcnlum (lig. 2.2). 1'1wto11

energies ~riucll larger than the forbidden gap are rcquircd to give direct

transition of electrons liom the valence to the conduction band.

However, transitions can .occur a t . lowcr ' energies by a two-step

process involving not only photons and electrons bill also a third particle,

a phonon. In the same way as light can be thought of as eithcr waves or

particles, so can the coordinated vibration of' the atorris making up the

crystal structure. A phonon is just a quantum or fi~ndamentrrl particle,

corresponding to the coordinated vibration, opposed to photons; phonons

have low energy but relatively high momentum.

As indicated in the energy - in omen tun^ sketch (fig. 2.2) an electron

can make a transition from the maxin~um energy in the valence bnnd'to

the minimum energy in the valence band in the presence 01' photons of

suitable energy by emission or absorption of a phono11 of the required

momentum. Hence, the minimun~ photon energy required to cxcite an

electron from the valence to the conduction bi~nd is

h v = E - EP ........................................ 2-16 B

b

Where Ep is the energy sf aii i i h i i i b d photon with the rzrjuired

Fig. 2.2:- Energy -- CIy~tiiI m o m n t i n diagr~im ot 'at] indirect hmd gap serniconductclr, sho wikg the a &sorption of pho tons by two-step process in v(11ving phonon cxnissio~~ or ahso~ptio~~.

Since the indirect -- band gap absorption proccss requires that an

extra "particle" be involved, the probability of light being absorbed by ,

this process is much less than in the direct - band gap case.

Hence, the absorption coei'licient is low a i d light can pass a

reasonable distance into the semiconductoi- prior to absorption. An

analysis of the theoretical value of the absorption coefficient gives the

result [7]

for a transition involving phonon absorption, and

for one involving phonon emission.

Since both photon emission and absorption are possible for hv, > '

Eg++Ep, the absorption coeXicient is then'

An empirical expression of cc to a high degree of accuracy over the

photon energy rungc 1 . I --- 4.0eV and for ~ h c tcnlperaturc; r-angc 20 SOOK

is given by [ 1 01 ......... 2.20

[ h v . . 3 (I) .I- fi ,]' .. ... ( q = 4,. ---L-I'L_ -1- li#,, ( 7))' '

. . 1-1.2 exp(Epi I KY} -- 1 j11,2

Where the values of the constants Aq, E, and E,, arc: as given in table 2.1

Table 2.1 . . . . . . .....-.............. .....

1 -

Quantity - ............................... ...-. ...

E g o ) 1.1.557eV ..................................................

&2(0 > 2.5eV

against photon energy h v [9]

Graphically, the indirect optical gap is determined by plotting a(1lv)"'

. .

The average solar absorptivity, a, can be obtained from [I 11

Where cx, is the fixlion ol' the solrlr radirltion absorbed. &(A) is the

radiant energy emitted by the sun at a particular wavelength which can be

obtained from distorted h - plots assuming air mass 2 for the solar input.

R(h) is the reflectance at wavelength h. .

Recombination Processes

If light of appropriate wiivelengtl~ is shonc on to a semiconductor an

electron - hole pairs arc created. 'l'he coilccntration of carriers in an .

illuminate material is in excess of their values in the dark. If the lighr is

switched ofx these coilccntrations decay back to their eq~~ilibriuin values.

, ? Ihe process by which this decay occurs is known as recoi~bination.

Three difTerent recombination mechanisms arc described i ~ i d can occur ip

parallel.

Radiative 1ieconlbi.a tion:

Radiative recombination is the reverse of :tbsorp~ion process. An

electron occupying a higher energy statc than it would under tliermal

equilibrium makes a transition to an empty lower -- energy state with all

(or most) of the encrgy difference between the statcs emitted as light.

Radiative recombination occurs more rapidly in direct band gap

semiconductors than in indirect types because a two -- stcp process

involving a phonon is required in the latter.

'The total radiative recombination rate R K is prc)porhial to the

product of Ihe concentration oE occupied slatcs (electr-ons) ill the

conduction band and that o'f unoccupied states in t11c valelice band

(holes).

Where B is a constant for a given sen~iconductor.

In thermal equilibrium when np - n:, this recombination rate is

obtained by an eclual and opposite generation rate.

In the absence of externally stin~ulated generated, the net b

recombination rate, 1JlX corresponding to ItR ribove is given by

The carrier lifetimes, z, for electrons and q, for holes call be dclined as

An and Ap are the disturbaixes of the respective carriers from heir

equilibrium values no and p,.

For the ra'diative recombination mechanism with An -. Ap the

Where I3 has the valuc of 2 x 1 ~ - ' ~ c i n ~ / s h r silicon.

Auger Kecom bina tion:

Here, the electron recombining gives its cscess cnergy to a nearby '

electron instead of emitting as light that relaxes to tlw o r i g i d state by

emitting phbnons.

The characteristic liletirne z for this process arc [7 1

'I'he first and thc second tcrins on the right dcscribc respectivcly the

electron excitation in the minority and the majority carrier bands. ' h i s

process occurs in relatively highly doped inaterial due to the sccond term.

Recombination ?'hrough Traps:

Inlpurities and defects in semiconductors can give rise to allowed.

energy levels within the hrbidden gaps. 'These then creates a very

efficient two-step recombination process. in which electrons relax lion1

higher energy state to the defect level a~rd later to the vriletice band

annihilating a licjk.

'I'lrc net rwombiria~ioi~ ratc by traps lJ,r, can bc writtcn as

Where zh, and z,, are lifetime parameters' whose values depend on the

type of trap and volume density 01' trapping dd'ects, 11, and pi are 4

parameters arising from the analysis which introduce a dependency of the

recombination ratc upon the energy ol'thc trapping lcvel, ItrI..

Where Nc is the efkctive density of states in the conduction band. .

If defects (impurities) exist near the middle of the forbidden gap (i.e.

when pl=nl), eiTective recombination centres are created.

Recombination At Surfaces:

Surface represents rather severe defects in the crystal structure arid

therefore a site for same type of recombination process described above.

The net recombination rate per unit area, I),, f i x a single-level surface

states is

.sea S*" (np - -- 11; ) -. - . - - -. . -- . . . .- . . . . . . .

S,,(n -t- n,) + S&(p i p , )

Where S,, and Sho are surface - rcconlbination velocities.

Thin Fihu Growth 'I'ecluLiyucs

There are severd methods of thin lilm growths each will, its

advantages and disadvantages. Thc choice thcreli~rc is :I Iiinction of' thc

nature of the de~nand h r the thin film, b u t gcncsally, thin lilm grc)\ytll

have some common aspects that includes:

i. the tlux ol'atonls and molecules to thc surl'ace ol'thc substr-ate.

. . 11. the energetics of the incoming species on the substrate which :ire

determined by the-heats of condensation and i or Ibrn~ation

... 111. and iinally, the organisatiunrtl inllucnce t11:11 are prcsent at. the

substrate.

The ability of the incoming molecules to accommodate these

organisational forces depends on the atomic mobility at the growing

surface, which in turn depend on the energy of the atoms or molecules,

and impurities and the stsuctural defects present on the susl'acc. 'I'he bettcr

the accominodatian 011 the substrate, the closer the layer is to bcing a

single crystal. In hct , the growth of thin films is siinilar in inany w;iys to

the settling of immigrants in a new land. If the inconling numbers are

large and the resources (energy) arc limited, thc resulting scttleme~lt will

likely be chaotic. On the other hand, if the i n h s is reasonably slow and

the resources ample, an orderly (epitaxial) settleinent call result a f t p

some rearrangement. Below are the deposition techniques applicable Sir

polycrystalli~~e thin films.

Physical Vapour Deposition (PVII):

'I'his is the lllost usual nletllod of depositing thin films. The deposition .

takes place in an air-tight chamber of nletal or glass in which thc air

pressure has been reduced to about one thousandth of a millionth of one

atmosphere (1 o - ~ 'l'orr). The material to be deposited is in the lbrm oC

solid pieces, powder or pellets. It is heated inside the vacuum chamber

until it begins to vaporize and the vapour stream impinges on thc

substrate on which he film is 10 be deposited. Since thc substrate: is

cooler, the vaporised material condenses onto it and layers of the lilin

2 8

build up at the rate typically of one micron pps minute to one micron per

hour. Heating of the source material can be done by heating electrically or

by using electron bean, laser beam, radio freqircl~cy coils or micsownvc.

Each with its advantages.

The PVL) teclmique is easier for metallic elements because their

composition remain unaltered. For compound semiconductos materials,

transition Srom solid to vapour is niore colnplcx becausc the solid

material rarely beconles molten and the vapour is dir-ectly produced li-om

the solid surfacc. 'l'his m a n s that the vapour hardly consist ol' the

molecules of the compound, instead the compound is usu:~lly spl~t inlo its

constituent elements, and if the vapoirr pressure of these elements at the

source temperature is vely diflcrent, then the ratio of the tonis is in the

vapour stream will be quite diEeree !i'cm the ratio of the solid. 'l'he thin

iilm that condenses Gom the vapour is then likely to have stoichionletry

(atom ratio) that is different from that of the sourcc material. I t is

therefore not uncommon that only 5 -- 10% of the source material bc

deposited on the substrate, so it is a very uscSu1 technique.

Sputtering is another PVD technique in which the source materiuI*is a

large fllat plate (the target). The atoms arc: vaporised not bv heating but by '

bonlbardment of the target by ions of Argon. Since the substrate is ilat as

the target, they form the plates of a capacitor. If a 1.X or l iI: discharge is

set up in argon gas at low pressure betweell these plates with the energetic'

argon ioil bombarding the targets, atoms arc knocked out 01' its surl'zlce

and then condense 011 the substrate.

This technique has the advantage of uniform coating over- 1h:tt of

thermal or electron beam evaporation. *

Spraying:

Semiconductor lilnls can be deposited by spraying a solution 01' the

elements onto a hot substrate. The solvent evaporates and the clenlcnts

react to leave a film of the semicondirctor material. l'his is, in principle, a

very simple technique similar to spraying paint on to a surIace and

requires. only small capital investment to coat large areas. In practice,

however, the technique has a number of problenls that have prc~unicd it

from fulfilling its apparent promises.

Most elements arc not soluble in cheap, rlon-toxic solvent, so they

must be combined into soluble compounds that are tllemselvcs cheap liiiil

. non-toxic and which decompose completely at tllc teinperatirre 01' the

substrate into the elenlent and a gas. 'l'his gas must be non-reactive and

stable at the substrate temperature so that it does not contaminate the

semiconductor lilm. 'l'he temperature at which the reaction occur must not

be too high, so that cheap substrate nlalerials such as glass may be used:

but it should be high enough to promote h e reaction of the clernents into

the required semiconductor coinpound and to promote thc growth oi:

crystalline of'the illaterials in a reasonable ordcscd way. * .

Elec trochenlical Methods:

Electroplating is a well-known technique lbr producing th in films or

metals (e.g. chrome plating). It is also a powerful method of deposititlg '

semiconductor films. The element to be .deposited are just made into ionic

compounds and dissolved in an appropriate solution. l 'he cornpoirnds

ionise in the solution so that the element exist as electsically charged ions.

If an electric field is applied, the charged ions arc forced to either anode

or cathode depending on their charge, and are deposited there when their

charge is neutralised. If the electric field is ~miiorni and the

concentrations of the ions in the solution are unilixrn, then a unifi)rm

deposit of the elements or compounds can be produced over large area.

Electroplating can be used for some or all the stages of production of

solar cells. It can be used to deposit metals tbr the front and / or. back

contacts to the cells, as well as for the deposition of the semiconductor

materials the~l~selves, so it is possible to envisage the production of a

complete module in a series of electroplating bathes. Unfortunately, the

use of' water as solvent h i t s the deposition process-taking place well b

below 100°C. Hence, extra heat treatment is ileeded to promote the

growth of the lsrger columnar, close-packed cryst:dlitcs needed lbr solar

cells.

An alternative to electroplating is electrophoretic deposition; a

suspension 01' vcry fine particlcs of t l ~ c semicond~rctor in a licli~id is

placed in an elect-ric iield. The electric field induces a11 electric charge on

the particles, which then move under the inlluence of the lield and deposit

on a conducting substrate, which neutralises their charge. 'l'he deposit is

in the forin 01 a layer of these very fine parlicles, held looscl\i together,

but a subsequent heat. treatment can trnnslorm the deposit into a

. polycrystalline film. It is dii'ficult to produce large columnar grains lvith

good electrical propertics, but lilnls with good oplic:lI propcrtics call bc

produced which may bc: suitable iiw window layers.

Screen - Printing:

Here, ink is produced which consists 01' a suspension ol' veiy line

particles of the seiniconductor or the elements in a carrier fluid. 'The

substrate is held just undenleath a fine wire mesh, and the ink is spread

over the mesh by a flat blade called a squeegee. The pressure of the

squeegee on the wire mesh pushes il in contact with the substrate and b

squeezes the ink through the mesh on to the substrate. The release oi'the

pressurc on the mesh allows it to spring back away froin the substrate

leaving ink on the substrate surhce. The ink is dried and then Grccl at

high temperature to produce the final film.The subsequent heat treatment

promotes the reaction between the elements to form the coinpound and

helps to produce large crystallites and good electricd propel-lies.

Solution Growth Technique:

The solution growth technique is based on the solubility product

principle which states that in a saturated solution of a weakly soluble

compound, the product of the molar concentration of its ions (each

concentration term being raised to a power equal to the number of ions of

that kind) called the ionic product, is a constant at 3 given tempcr:ltiire. '

For instance Cd(OH)2 when added to water will hydrolyse according to

the reaction

Cd(OI-I)2 f ~ d " + 2(OH)

The ionic product, 1P is given by

[ed2+] x - constant, called the solubility product, SI'

The above condition must be satisfied for equilibrium to occur. When

the ionic product is greater than the solubility product, precipi1:rtion.

occurs. 13ut when IP is lcss tliun thc solirbility product, thc: solid pl~i~sc:

will dissolve until equilibrium is reached.

If precipitation spontaneously occurs, thc

certainly not a thin f h . Thereliose, eliminate by a

ion reaction. This is achieved using a f'airly stable

ions called the ligand or complexing agent to provide controlled nunibcr

of the free ions. For a reaction with appropriate ligand, the concei~tsa~ion

of the resulting ions would be controlled by the conccntratioti 01' ~ l l c

complexing agent itself and the solution nlicrostruct~rrc.

In the solution growth technique, the substrates :ire immersed

vertically in the solution bath and a continuous stirring is obtained with a

magnetic stirrer. The temperature of the bath is monitored by contact

thermometer that f o r m a part of a fecdback circuit controlling the heater

to maintain a constant temperature. When the TI' 01' the metal and the

chalcogen ions exceeds the SP of the corresponding chalcogct~itlc. film is

formed on the sul~strate by an ion - by - ion coildensation proccss. b

Cornmoiily with most deposition tcchniqucs inci uding tllc solu lion

growth technique, thc growth 01' the I'irst fkw atomic layers has 'a11

enol-mous elrect on the final structusc 01% the lilnl. As atonis begin to be

deposited on the bare substrate, they do not remain at the spot where they

arrived, but n~cailder arouiid the substratc iintil they come to a site wllen

bonding to the substrate is a irt~ximum. These nucleation states trap the

wandering atoms, and it is at these sites that the film stmrj to grow. At

first the monolayers are arranged in just about the same way as in a

c~ystal of the semiconductor, but the monolayer liom one nucleation site.

has no orderly relationship with thosc of ncighbouring sites. The

boundaries where the nlonolayers meet therefi~rc remain distinct a i d no

mesging of the layers. As more atoms arrive, the crystalline layer around

each nucleation site gets thicker at a rate, which depends partly on the

crystallographic orientation of the layer. Some clytirlal directions allow the.

layers to grow more rqpidly than others, and the cryslallite layers with

these . o-6enta tions grow more quickly than those with other oricntn tions

until crystallites with the "fast growth" orientation swamp all others.

The way in which the crystallites mect at their bou~ldariw can

strongly influence the properties of the film. The ciystallites (grains) di)

not completely fill all the volume of the iilnl, and there are pores between

. the grains, which sange in s i x from a few Angslroin to a Ikw ~cnths ol',a

micron. The small pores fill with liquid watkr when the film is in a

normal atmosphere, so the optical properties are a inixqure of those of the

serniconductor and water, weighted according to the relative ainounts of

each. These pores can occupy from 0 to 20% of the total vol~rme of' h e

film; and the higher the temperature of the substrate; the better is the

crystallite structure of the thin film.

The Deposition Tecl~~ique Uscd :

In this research work, an aspect of solution growth technique called

chemical bath deposition has been used. Chen~ical bath dcposition is the

simplest of all known technique for fhbrication of thin lilnls. Con~pai-cd to

other forms of chelnical dcposition tcchniq~ws, it requires only very B

simple and modest equipments that are alli,rdable. 1 Icnce, cost clkctive.

'Other advantages include:

= easy scaling up ibr large area

high ins tel-ial u~ilization

low temperature required fix- deposi~ion

'uniformity in the film thickness and above all

comparable oplo-electronic properties with tl~osc obtaincd by

other techniques

Sample Prepars tion:

For the thin tilm of interest, Iron pyrite (i:eS2), used was made of

hydrated Fcrrous Sulphate, h'cSU4.7H20 (nlolccular weight,

MW-278.01) as the cation source; Sodium Thiosulphote, Nn2Sz03.riJ-120'

37

(MW=-248.17), the anion source and . t;,lhylene 1)iarnine l'etra Acetic

Acid, EDTA (MW-372.24), the con~plexing ageu t. 'The* cornplexing

agent is the most important parameter in chemical bath deposition

because it greatly inlluenced the stl-uctur-a1 and clectro-optical propel-ties

of the film.

Solutions with the required lM, 1 M and 0.1 M conccnlralioiis jbr-

FeSO4.7Hz0, Na2S2(J3.5H20 ard W T A respectively were prepared by 4 -

dissolving respectively 27.80 1 g, 24.8 17g and 3.7242g, each separately, in

lOOml of distilled water. Chemical bath for the deposition, after several

trials, was formed with 2ml of the 1M fer.rous sulphate solution to which

2mlo l the 0.1 M EDTA was added and stirred for 2rninutes. 5ml of the

1 M Na2S2O3.51-I20 was then added and finally 3 I n-11 of distilled watcr was

added to bring up the total volunle of the mixture to 40ml and the

resulting solution vigorously stirred

The solution was observed to be clear with no im~nediate visible reaction.

The EDTA in the solution bath serves as complexing agent tor the slow .

release of metallic ions in the solution. A previously wcll clemed and

drip-dried microscope glass slide substrate was then suspcndcd vertically

along its length in the chemical bath with the aid of'a supporting device

38

as shown in fig. 3.1. The set up was then left ~~ndisturbed at room

temperature for 16 lmirs, a meaningful deposition time t i~r h'eS?. 'l'he .

experiment was separately repeated h r 17, 18, 19 and 20 Ilours . .

deposition times to observe film growth rates. At thc end 01' tlie

deposition times, the slides were respectively taken out of the chemical

bath, rinsed with distilled water and allowed to dry in hot air. ?'he slides

were observed to have been coated with brownish yellow deposits. * Variation in film growth paranlctcrs and the optical absotptibn

measurements of the film deposited fix dilflcnt reaction baths were also

studied.

Reaction Equations Of The Film Deposition:

In the deposition 01' FeS; films described here, the S-' ions were released

[rom the hydrolysis of Sodium l'lliosulphate and Fe2' iiom t h a ~ of

Ferrous Sulphate in the presence of EDTA acid and the ion-by-ion

condensation of FeS2 onto the substrate represented by the equations

FeS04 '+ Na2S203 -t- EDTA ---+ FcS203 -t Na2S04 1- EDTA . . . . . 3.1

FeS203 -(; 6Xt FeS -t- S + 3fI20 ... ... ... ... ... ... ... ..... .. 3.2

FeS + S ----+ FeS2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3.3

/ r 7 n / J G l a s Slide (Substrate)

- - -

~;iii;:+-------50 ml Beaker

Fig. 3.1:- Sketch o f the e.~perin~cntal set up

Determination Of ?'he 13and Gap:

The direct energy gap E, is deterniined li-om equation (2.15) by plolling

u2 against h v (= hcil;) and interpolating a'- 0 callcd thc absorption edge.

At this point

Where c is the velocity of radiation, h, is the minimum cut-ofT

wavelength at the absorption edge and h is the Planck's constant.

Alternatively, E, is calculated from equation (3.4) by determining h, ii-om

the plot of a against h and exqrapolating a =: 0, the absorption edge

wavelength.

CI-IAP'TEII FOUR

Measurements And Discussion Of' Kcsults

Physical Appearance Of The Deposited Film

'The film deposited was brownish yellow in colour and adhered lirnlly

onto the glass substrate. It was insoluble in the chemical bath and water

but soluble in hydrochloric acid. The deposits were unifbrm through out

the dipped portion of the glass slide. &

Nevertheless, the films are not said to be free from pinholes arising from

such thi~igs as dust particles on thc subslr:lte during dcposi t ion, wliicli

subsequently fd l out of the film, or from defects on the substrate, which

prevent nucleation and growth o f ~ h e lilrii.

Film Thickness

'The inethod used in the ~neasurement oftlie thickness of the lihn was the

gravimetric method. In this method, the length and breadth o f ~ l i c iiliti on

the glass substrate were measured with Ihe aid of a Vernier Calliper arid

the mass of the deposited film obtained fiorn the dilkrence between the .

masses of the substrate before and after deposition. The thiclcness ol'the

film is then determined -fioni the ratio of the mass of the film to the

product of its density and area.

4 1

i.e. Thickness of film - (Mass oS Glin)/(Density of Glm x Area of film) ,

The density that was used was bulk density of the Iron pyrite, which is

5000kdm3 [12].

The variation in the FeS; film thickness with deposition time was studied

with variations in:

i. Sodium Thiosulphate concentration and

. . 11. Ethylene Diamine Tetra Acetic Acid concentration

4

Which' are represented in the vchrne ratio FcSO.~ : Na7_S7_03 : 111)'I'A as

seen in t j ~ e table. 'I'hc values obtaincd are as show11 in tablc 4. I

Table 4.1:- Variahn in FeS' film t/ikk~~css with dqmsitim tinx tbs differc~i t corn bina tiom1 volume 121 tios

Deposition Times (hrs)

16

17

18

19

20

Thickness (x 1 0-8m) for dilkrcnt volumt: ratios .

'The e&cts of varying the concentration of sodiuin thiosulpliatc and

ED'I'A acid on the thickness of FeS2 over the deposition pcriod has been

investigated and represgnted graphically on fig. 4.1. It could bt: sccn liom

this figure that the best con~binational volumc ratio with the best growth

and that that gave the greater film thickness is that obtained f'ronl

2ml:5nd:21nl (that is column B, on the table 4.1). Closely followed by that

in column A, 2inl:2ml:2nil. On the other hand, column C,, 2ni1:5in1:4in17 b

gave poor film deposition. This not surprising but an indicator of' the

effects of vitryirig ligand concentrution have on the film fiornmation. For

this particular case, is a response to the incseasc in the volumc 01 the

EDTA tioin 2nd to 4mi and of course pointing dircctly to the increased

"stinginess" in the role the EDTA played. Moreover, as observed in some

trial experiments, deviations particulilrly in the concentration or volirme

of EDTA from the optimum values often results in the formation oS very

thin and non-uniform films or totally into no iiim growth at all. 111 one

such example where 2ml:5ml:lml volume ratio was used, no Iilm was

deposited. This however, could not be seen as "stingjncss" rather- an

incomplete complex formation of the cations that, instead, resulted into

precipitation than deposition.

Study Of 'I'he Absorbance Of The Film

The optical absorption measurements of the film deposited on the glass

substrates were made on PYE UNICAM SP8- 100 UV spectrophoto~~~eter

in the range 200nm 2 h 2 800nm spanning the ultra-violet, the visible and

the near infrared regions of the solar spectra. The measurements were

such that the coated glass substrate was positioned in the path of one of

the beams and a blank glass substrate in the reference bean path. The

absorbance, a, as a iitnction of the wavelength or the incident radiation

was s t~~d ied and recorded in table 4.2 for the two best combinational

volume ratio samples, A, and B,, where t = 16, 17, 18, 19 and 20 hours.

Table 4.2:- Variation of Absorbance with wuvelength for the sample A,

Wavelength (nm>

~

Absorbance, a, for A, - 2ml:2ml:2inl

200

-- 220

240

260

-

A20 A16

0.083

0.086

0.089

A18 --

A17 .

A19

0.092

.

0.095

--

0.096

0.095

0.091 0.100

0.064

0.094 0.105

---

0.088

-

0.097

0.089

0.084 0.089

0.092 0.100 0.086

--

.-

b o o

Absorbance, a, for 1'3, -- 21111:5n11:2rnl

Figures 4.2 and 4.3 represent the superimposed plots of the graphs of the

Absorption Edge, 1,

thin films absorbance as a function of the wavclcngth of the striking .

lhcrgy Iland Gap, I:,:

'photons for each of the samples in and 1'3, respectively. I n e:ich casc, . .

maximum absorbance ibr the individual samples involved occurmd at one

lleposition ,- .- ........... ---...--....-.................... ...... llllle (hrs) : .--

.......................................

16 442 400 2.81 3.1 0

- 17 461 ' 2.92 2.69

,

. 18 500 2.48

- ......

19 406 438

......

20 453 460 2.70

.-... .- ....-...... ....

single wavdength of'34Onm close to the ultni violet rangc (350-400111~).

At this point all the thin film samples have shown high absorb:lnce.

Unfort~inately, the highest absorbency obtained in each of the A, and 13,

did not correspond, as expected, to samples with the 11 ighest lllickness

(A20 and B20); but their broadening out, pafli-ticulasly of 1320 into the visible

region (390 -- 770nm) represmls in tolaiity its superiority as absorbing

film than any one of them. Also are some observable rises ill absorbance

around 600nm in the visible region shown by A17? AIB, Bl6, lLO folhwing

falls at some points between 500 - 560nnl. 'I'!lis rise could possibly go

higher with increasing lilrn thickness; it may evcn extend inlo the infrared

region.

Figures 4.4 -- 4.8 represent comparison made in the absorbance 01' tl\e

individual samples in At and 13, with same deposition time. 1316 and 1320

showed higlier absorbance. in the ultra-violet lhan their corresponding A,,

better absorbing film in the visible region than the At. This change cannot

be coinpletely separated from the concentration chaoge eikct 01' sodir~n~

thiosulphate fiom 21111 to 5ml in the l3t that still allowed deposition to

continue ahead of the terniinal phase ofA2(,. That is to say, more layers af

the film would have been a d d d to b0 il' only its deposition time had

been extended.

Never mind the optical characteristic differences displayed of all filnis in .

4 and B,, as they are, make good invited guess for solar cell window

layers party with B16 o~cupying the high tablc.

B16 have shown alrilost the ideal qelality recluircd ol'a vely good so1tl1- cell

window layers. With the energy gap of 3. lOeV, it will transmit all solar

radiation of waveleiigths greater than. its nbsol-ytion edge u~avelength

(400nm) and this iiiclude substmtial part of the visible radiation; and

simultaneously absorbing all solar radiation of energy band gap gl-cater-

than 3.1OeV

A17 and A19 are equally good, but Bls bccnose of' tllc vciy hbh '

absorbance it has in the ultra-violet region will transmit only negligible

quantity of the ultra-violet thereby saving tllc cell more fi-om ovcr

heating.

With these results, FeS2 thin filn~ has gotten a dual lnerilbership of good

photovoltaic [l ] film and good selective window coatings.

Fig. 4.0:- Picture of f3m samples on glass didcrs

Absorbance, a

Absorbance, a

Absorbance, a

0 P +

P z 9 tQ 4- z P

V\

Sununary

The chemical bath required fix thc deposition of the solid iron pyritc,

FeSj, thin film has been prepared and successhlly deposited on a glass

slide substrate. The chemical bath is made of 21111 of' 1M VcSO,,, 5ml of

1M Na2S203 and 21111 of 0.1 M EDTA, the complexillg agent. 'I'he film * deposition required longer period to be appreciated. The lilms thickness

range lium 4.070 x 1 - 6.523 x 1 O-xnl liw s:mplcs with 2m1:5nil:2ml

volume ratio and 3.998 x 10" --- 6.129 x I0-~1n for 2nd:2n11:2ml of the

respective FeS04:Na2S703:EDrl'A over 16 - 20 hours. 'I'he fil111s were

generally ibund to haw high absorbance in the ultra-violet (0.35 ---

4.0pm), low in the visible (0.39 - 0.771~1~1) and the near infiai-ed

(>0.7pm). The band gaps for dl the saniples used liave becn detem~itied

from their absorption spectra. They range fiom: 2.74 - 3.06eV fbr A, and

2.48 - 3.10eV for

Application Of result

The thin films grown are generally siritable lor use as selective window

coatings for solar cells. This is because the lilnis have shown high

absorbence in the ultra-violet, low in the visible and the inhared. 'I'he

film with E, = 3.10eV has almost all the ideal qualities required for this

purpose.

b

Conclusion

Thin film of 17eS2 l~ris been succcssfurlly growil and deposited 011 glass

slide substrate using the solution growth technique. Details of the .

deposition procedure have been reported. Its possible practical application

as selective window coatings for solar cells has been discussecl. l'he

alternative approach using Z;eSO4.7I-120 has been proven Letter and

econonlical. The che~nical bath constitute a total v o l u n ~ of only 9ml

instead 1 71111 using F c ( N O ~ ) ~ as contained in Osuji et a1 I I], excluding

distilled water. The deposition technique used remained simple and

modest.

Suggestions For Further Studies

Material properties such as resistivity, temperature stability, etc. of the

film has not been determined for a complete assessment of its behaviour

as good window layers6for solar cells. In addition, more work could be

carried out using various volumes of the bath parameters to determine the

dependency of the film growth on them to obtain the volume combination

that could give a better terminal thickness.

REFERENCES [ I ] Osuji, R. U., Olteke, C. E. and Muokebe, I . S. 0. (1993). Nigeria.11

Journal of Solar Energy. 12, 80-86

[2] Fill, R. (1991). In Generating Electricity from the Sun, Treble, F. C. (Ed.), Pergamon Press, Great Britain. 47- - 124-

[3] Fraas, L. M. (1.978). J. Applied Physics 49, 871

[4] Report ERDAINASAII 022 - 7711 6, June. (1 977).

[5] Andrews, C. L. (1960). Optics of'the electromagnetic spectrum, Prentice I-Iall, New Jersey.

b

[6] Heavens, 0. S. (1955). Optical properties of thin solid film, . Butterworths, London.

[7] Pankove, J. 1. (1 97 1). Optical processes in semiconductors, N. J. Prentice iiall, Eaglewood Cliffs.

[8] Clark, A. H. (1980). In polycrystalline and amorphous thin films and devices, Kazmerslti, L. L. (Ed.), Academic Press, New York, 138 - 140

[9] Vavilor, V. S. (1965). Ejfect of radiation on semicondzictors, Consultant Bureau, New York.

[ lo] Rdjkanan, K., Singh, R., Shewchun, J. (1979). Solid State Electronics 22, 793.

[I I] Buhrman, R. A. and Craighead, H. C. (1980). In solar materials sciences, Murn, L. L. (Ed.), Academic Press, New York, 277.

[12] Langes, J. A. (1973). In Handbook of Chemistry, Mcgraw Hill, '

New Y orlc.

[13] Alhassan, J. A. (1992). M.Sc. pro-ject report.