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University of Nigeria Research Publications
CHIGBO, Nnolim Cosmas
Aut
hor
PG/M. SC/87/5933
Title
The Growth of Lead Sulphide (PbS) Thin Film on Glass Substrate by Chemical Bath Method
Facu
lty
Physical Sciences
Dep
artm
ent
Physics and Astronomy
Dat
e
November, 1989
Sign
atur
e
DEPARTMENT O F PHYSICS AND ASTRONOMY
UNIVERSITY OF NIGERIA
NSUKKA
RESEARCH PROJECT TOPIC:
THE GROWTH OF LEAD SULPHIDE ( P b S ) THIN
FILM ON GLASS SUBSTRATE BY CHEMICAL BATH
METHOD
CHIGBO NNOLIM COSMAS REG. NO. ~ ~ / ~ . S C / 8 7 / 5 9 3 8
. .
A RESEARCH PROJECT PRESENTED TO THE
DEPARTMENT OF PHYSICS AND ASTRONOMY
UNIVERSITY OF NIGERIA, NSUKKA, I N PARTIAL
FULFILMENT OF THE REQUIREMENTS FOR THE
AWARD OF MASTER OF SCIENCE (M.SC) DEGREE
I N PHYSICS AND ASTRONOMY.
$--<A /! / /
SUPERVISOR: DR. CoEo OKEKE
DEPARTMENT OF PHYSICS
AND ASTRONOMY,
UNIVERSITY OF NIGERIA,
NSUKKA.
NOVEMBER 1989.
T I T L E -
THE GROWTH OF LEAD SULPHIDE ( P b S ) THIN b
FILM ON A GLASS SUBSTRATE BY CHEMICAL
BATH OR SOLUTION GROWTH METHOD.
CERTIFICATION
Chigbo Nnolim Cosmas, a P o s t g r a d u a t e s t u d e n t i n t h e
Depa r tmen t of P h y s i c s and Astronomy and w i t h t h e Reg. No.
PG/M. Sc /87 /5938 h a s s a t i s f a c t o r i l y c o m p l e t e d t h e r e q u i r e -
m e n t s f o r c o u r s e and r e s e a r c h work f o r t h e d e g r e e of
M a s t e r of S c i e n c e (M.SC) i n S o l i d S t a t e and M a t e r i a l
S c i e n c e . The work embodied i n t h i s P r o j e c t R e p o r t 4s
o r i g i n a l and h a s n o t b e e n s u b m i t t e d i n p a r t o r f u l l f o r
any o t h e r d i p l o m a o r d e g r of t h i s or a n y o t h e r \'P -' r w -
U n i v e r s i t y .
D r . P.M. Okelfe, A s s o c i a t e P r o f e s s o r and (Head o f D e p a r t m e n t ) .
D r . c.F &eke, Associ te P r o f e s s o r of P h y s i c s and Associate Dean o f P h y s i c a l S c i e n c e
( S u p e r v i s o r 1.
DEDICATION
I am d e d i c a t i n g t h i s to my l o v i n g w i f e
Mrs. E l i z a b e t h Ukamaka C h i g b o a n d o u r c h i l d r e n
y e t ' to be b o r n . E q u a l l y r emembered i n t h i s
d e d i c a t i o n a r e t h e a m b i t i o u s y o u n g e r o n e s ,
f o l l o w i n g b e h i n d m e , who s h o u l d see s c i e n c e
a n d t e c h n o l o g y a s a b i g c h a l l e n g e f o r t h e 4
d e v e l o p m e n t a n d s e r v i c e o f mankind.
ACKNOWLEDGEMENT
I c a n h a r d l y e x p r e s s a d e q u a t e l y , my p r o f o u n d
g r a t i t u d e to D r . C.E. Okeke, my p r o j e c t s u p e r v i s o r , whose
w e a l t h of a c a d e m i c e x p e r i e n c e , c h e e r f u l n e s s , a c c e s s i b i l i t y
and f a t h e r l y a d v i s e i n moments of u n c e r t a i n t y a n d d e s p a i r ,
h e l p e d me, i n n o small m e a s u r e , to be w h a t I am t o d a y . To
h i m , I am v e r y g r a t e f u l .
I a l s o a p p r e c i a t e t h e e n c o u r a g e m e n t a n d c o n c e r n shown,
a t v a r i o u s s t a g e s of my s t u d y h e r e , b y t h e Head o f D e p a r t -
m e n t of P h y s i c s a n d As t ronomy, D r . P.N. Okeke , ~ p o f . A.O.E.
A n i m a l u , D r . I.S.O. Muokebe, D r . I .R .N. Awach ie , D r . S.E.A.
Awachie , P. I. O b i a k o , C.M. I. Okoye a n d Mrs. R. O s u j i . I
t h a n k you immensely .
E q u a l l y remembered h e r e i s my d e a r w i f e , Mrs.
E l i z a b e t h Ukamaka C h i g b o f r o m whom I d r a w i n s p i r a t i o n to
g e t g o i n g when t h e g o i n g becomes tough . To you d a r l i n g ,
I owe you e v e r y t h i n g .
I t h a n k a l l t h o s e who h a v e c o n t r i b u t e d i n o n e way or
t h e o t h e r , to make t h i s a s u c c e s s .
Above a l l , I a m most g r a t e f u l to God A l m i g h t y who
made e v e r y t h i n g p o s s i b l e and h e l p e d t h o s e a f o r e m e n t i o n e d
t o h e l p m e . 0 God, bless them a n d r e w a r d them more
a b u n d a n t l y .
TABLE OF CONTENTS
PAGE - T I T L E a * *
CERTIFICATION a * . . o m
DEDICATION 0.0
ACKNOWLEDGEMENT o o . 0.0
TABLE OF CONTENTS
A B S T R A C T . . . ..a
L I S T C F T A B L E S . . , L I S T ? F ! ? F G G i ? R C , .
CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW
1.1 I n t r o d u c t i o n . . . ... 1.2 Metals, S e m i m e t a l , S e m i c o n d u c t o r s and
I n s u l a t o r s . . . ... 1.3 D i f f e r e n c e s Between I n s u l a t o r s a n d
S e m i c o n d u c t o r s ... ... 1.4 D i f f e r e n c e s Between S e m i c o n d u c t o r s
a n d Metals . . . ... 1.5 F e r m i L e v e l ... ...
CHAPTER 2 APPLICATIONS OF THIN FILMS ... 2.1 D e f i n i t i o n o f T h i n F i l m . . . 2.2 Need for T h i n F i l m s ... 2.3 Metal, S e m i c o n d u c t i n g a n d I n s u l a t i n g
F i l m s 0 . . 0 . 0
2.4 A p p l i c a t i o n o f T h i n F i l m s ... CHAPTER 3 METHODS OF DEPOSITING THIN FILMS
3.1 Major Methods o f D e p o s i t i o n ... 3.2 Modes o f D e p o s i t i o n a n d C o n s e q u e n c e s
3.3 E v a p o r a t i o n ... . o .
3.4 E p i t a x i a l D e p o s i t i o n ...
i
ii
iii
v i i v i i i
T a b l e o f C o n t e n t s
P a g e
3.5 S p u t t a r i n g T e c h n i q u e s ... 3.6 S c r e e n P r i n t i n g ... 3.7 C h e m i c a l Vapor D e p o s i t i o n ... 3.8 D e c o m p o s i t i o n ... ... 3.9 R e d u c t i o n ... ...
... 3.10' P o l y m e r i z a t i o n ... . 3.11 P lasma D e p o s i t i o n . . ... . 3.12 E l e c t r o d e p o s i t i o n . . ...
3.13 Electroless o r A u t o c a t a l y t i c P l a t i n g
3.14 A n c d i z a t i o n ... ... + ... 3.15 E l e c t r o p h o r e s i s ... 3.16 Me1 t S p i n n i n g T e c h n i q u e ...
CHAPTER 4 THE GROWTH OF LEAD SULPHIDE
4.1 The C h c i c e o f S o l u t i o n Growth Method
4.2 C r y s t a l S t r u c t u r e of odium ~ h l o r i ' c m 4.3 ,, * > - ! L ! S t r u c t ~ r c ,of L e ~ d Lu:;~!.idc
4.4 The C h e m i c a l s / R e a g e n t Used a n d
P r e p a r a t i o n o f S o l u t i o n s ... 4.5 The Growth K i n e t i c s a n d C h e m i s t r y o f
R e a c t i o n ... ...
CHAPTER 5 CHARACTERIZATION A N D CONCLUSION .a. ... 5.1 T h e o r y .-. 5.2 C a l c u l a t i o n s ... . O .
5.3 A n a l y s i s o f R e s u l t s ..- 5.4 Uses o f Lead S u l p h i d e F i l m ... 5.5 Recommenda t ions f o r F u r t h e r R e a d i n g
5.6 C o n c l u s i o n . . - ... R E F E R E N C E S 0 0
~ c ( i ~ l i , ) , a:: t scdrct- of su lp ! - . t r ions* Sodium t ; h i ~ , u ? p h i t e L L
of the L ~ l t k ' r
The o&t i ca l &r;dr;>ticri d.J cc;::duckiviiy K I C ~ I L L C ~ ~ G ~ L b
w i t h t h e v c 7 , ~ e c ; ~ c t e d i n books JW t o unsu;taY;.lt t,:cz?.rc-
p h o t o m e t e r u s e d , o t lhcr >cirarnr.tc;rc L i k e c o l o u r and L C 3
r e a c t i o n w i t k , &= id skewed that: it was PbS Zilm cic;..~r;ited,
The s~ac,Lu:ly ref lectir,,; : ;~ i t~r ii of t h e film , x > v i d : ~ s
a n o t h e r striking use to t i n t khc glasses of vehicl;s m d
houses .
T i t l e - ;rou;;z i:) t k n ycriodic t d b l e
fl -!zc;c~ -i Je - l ccnduc t o r s a n d Semi-
for Y i l ? 7
Page .
9
T i t l e
sc!;r,iconduc t o r s insulators
for
ix.
CHAPTER I
I N T R O O U C T I C N A N D LITERATURE REVIEW
I n view of t h e r a p i d d e p l e t i o n of e x i s t i n g c o n v e n t i o n a l
ene rgy r e s o u r c e s , due to t h e h i g h r a t e of consumption t h e
wor ld o v e r , i t is a p p a r e n t mankind i s head ing towards an
energy crisis. Today energy consumption p e r c a p i t a i s
synonymous w i t h t h e s t a n d a r d of l i v i n g of a n a t i o n and riace
w o r l d p o p u l a t i o n , s t a n d a r d of l i v i n g of deve loped and d e v e l o h
i n g n a t i o n 6 and t h e t r e n d towards h i g h e r t e c h n o l o g i e s u e b
on t h e i n c r e a s e , i t i s e v i d e n t t h a t o u r o i l w e l l a , the
s o u r c e of t h e world well-known major e n e r g y r e a o w c e -
petroleum w i l l s o o n e r or later d r y up. 2b a v e r t t h i a
impending energy crisis and f u r t h e r meet up w i t h e n e r g y
n e e d s of n a t i o n s , renewable energy technologic.1 r e v o l u t i o n
i s t h e o n l y answer.
Renewable e n e r g y a o u r c e e t h a t c a n be deve loped i n c l u d e
c o n t i n u o u s n a t u r a l e n e r g y f l o u r aacb a s s u n l i g h t , ocean
c u r r e n t s , waves, f a l l i n g water, wind er ra tora l energy ,-
s t o c k s whore r e p l e a i a b n e n t is f rr g r e a t e r tbm p r o j e c t e d
buman use. Of t h i a wide raege of e n e r g y c h o i c e , s u n l i g h t
or sslar e n e r g y i s c e r t a i a l y o n e of tbe most a t t r a c t i v e .
Simce t h e solar energy is more abundan t i n tlro mrer and
less developed c o u n t r i e s of t h e world, i t i s hoped tba t i f
t h i n p a r t i c u l a r s o u r c e of emcrg j l a rerearchael i n t o ~ R Q
developed the b r i d g i n g of t h e t ackno log ica l gap between
t h e t h i r d world cewatries and rost of t h e develeped cam-
tries is then f e a s i b l e ,
The =)or p r o b l u f a c i n g s c i e n t i s t s is t h e r e f o r e the
way w e car we s r s l i q h t to p rov ide p r e f e r a b l y high g r a d e
energy i n a very u s e f u l f e r n which is ~ c o n o m i c a l l y coape t i -
t i v e , r e c o g n i s i a g t h e added c e r r t r a i n t t h a t tbe materials * we use must a lse be r e a d i l y a v a i l a b l e and abundant. To
a c h i e v e the c a a v e r s i a ~ s ef t h e solar enerQy to a t h e r u s e f u l
f o r n s l i k e e l e c t r i c a l energy which can be e t a r e d , q u i t e a
good number of ratbods rucb a s photothermal , pbetaehemical , p h o t ~ l e c t r o c h w i a a l , p h o t e b i e c h a r i c a l and p h o t s v o l t a i c u e
poss ib l e . Among there, t h e c l e a n e s t and most d i r o c t and
efficient method of convers ion to electrical energy is wi th
t h e h e l p o f p h o t o v o l t a i c (PV) or solar cell deviees. B t b u
s o c i a l u d t e c h n a l o g i c a l advantages t h e p h o t o v o l t a i c tech-
nolegy h a s over other8 are t h a t t h e systems are q u i e t , rmpil
l i t t l e m a i n t a a u a e , Lave no critical a i w and iadoed s ize can
be matched to load, and u e e r v i r o n n a n t a l l y benign i n epuat-
ion.
?or e f f i c i e n t p h o t o v o l t a i c convers ion however, mst of
t h e o p t i m a l l y designed d e v i c e s are made at t h i n f i l m s &ere e
t h e band gap energy is ve ry smal l f o r g r e a t e r curier
gene ra t i o n and, c o o s e q u e ~ t l y h i g h e r phe t o c u r e n t. Thus, f a r
h igb e f f i c iomcy solar cells w i t h h igh p h o t a c v r e n t s and high
open c i r c u i t v o l t a g e a b s o r p t i e a and carrier g e n e r a t i o n Oc-r
wst for pbrahons of energy q r e a t e r t b u t h e band gap of t h e
m a t e r i a l which is i n t b e r ange of 1.1 to 1.S oV. Apar t from 8
t b e u u of t h i n f i l m s i n t h e wlar cells, t h i n f i l m s have a
very wide r anqe of p r a c t i c a l and i n d u s t r i a l a p p l i c a t i o n s .
They also have maay advantages over tbe bulk forms ef t h e
s u e mater ia l s . Recant t r e n d s towards the u s e e f polycryskl-
l i n e t h i n f i l m 8 u i c o n d u c t o r s i n v a r i o u s e l e c t r o n i c and
e l e c t r o - o p t i c a l d w i c e s i n p l a c e of s imgle balk c r y s t a l s
have l e d b a drastic c u t i n t h e cost of p roduc t ion of t h e s e
dev icer . We muet amphasine t b a t a l l t h e s e p r o c e r r e r based oa
t h i n f i l m tocbaology a r e no t on ly cheaper b u t also sirplu
As a r e s u l t of t h e s e and pore .pp l i . c8 t ions te k out-
l i n e d i n subsequent c h a p t e r s , tho importance ef t h i n t i la
techaology uneet k ever-d.d.
~at-iaa.. . .
E l u t r o n i c e n e r g y band th .a ry p rov ides a c o n c e p t m l f r a m
war& f u c o n s t r w t i n g a r a t i o n a l e x p h m t i o a of tha electriec
properties of selids, b-ce it provides the baris f e r elmctr:
-1 c l a s s l f i c r t i o c r of -Aids ma metals (good conduc to r s ef
e h c t r i c i t y ) , ~ i c o r d w t o r r a d i a s u l a t o r s ( p e r cenductorm
of e l e c t r i c i t y ) . Tue energy band c s n c e p t s are involved i n
such a c l a s s i f i c a t i o n . These are (i) t h e concept of energy
band gap betveen d i f f e r e n t bands and ( i l l t h e cmacept of
occupa t ion or r e l a t i v e f u l l n e s s of t h e a l l swed bands wi th 4
e l e c t r o n i c ~ t P t e ~ a
me bands ef g r e a t e s t imterert to u s are t h e two neigh-
b o w i n g ones s a p o r a t i n g t h e b i g h e a t occupied bard (called
t h e v a l l e n c e band w i t h maximum energy Ev) and t h e lowest
empty band (called t h e cenduc t i en band wi th niwimwr energy
Ec). h e d i f f e r e n c e Eg 8 Sc - Ev d e f i n e s the forbiddam gap
or energy band gap Fig. 1, and t h i r r a s u l t s from t h e i n t u -
a c t i o n of the conduct ion e l e c t r o n waves wi th t h e i o n cores
of the c r y s t a l v i a Braqg r e f l e c t i o n . h e cbaracteristlc from
energy t t m t separate. :dccupied pmpty l e v e l s is c a l l e d t h e - F e r d l e v e l or i s t h e energy of the topmost f i l l e d l e v e l i n
t h e ground state. The c r y s t a l behaver l i k e a wtal i f oae
or more bands are p a r t l y f i l l e d , s a y 10 te 90 p e r c e n t f i l l e d
or when t h e va l ence h a d ( n ~ r m a l l y f i l l e d ) and t h e conduct-
i o n band (momal ly e m p t y ) o v e r l a p so t h a t ' b o t h are p a r t i a l l y
The s p r e a d i n g o f d i s c r e t e a t o m i c l e v e l s i n t o
b a n d s i n a c r y s t a l , a s a f u n c t i o n o f i n t e r a t o m i c
s e p a r a t i o n , a, Energy b a n d s a r i se i n s o l i d s a s
a r e s u l t of t h e a t o m i c e n e r g y l e v e l s o f t h e
i s o l a t e d a tom when a l a r g e number o f a t o m s are
b r o u g h t i n t o close p r o x i m i t y i n t h e c r y s t a l ( s o l i d ) .
I n t h e i s o l a t e d a tom t h e l e v e l s are f i l l e d t o some
p o i n t and a l l h i g h e r l e v e l s are empty. F o r examp le ,
2 6 i n sod ium (Na), t h e ls2 2 s 2p 3 s l e v e l s a r e f i l l e d
w h i l e t h e 3 p 4 s ..,. a r e empty i n t h e f r e e a tom i n i t s
g r o u n d s t a t e , I n t h e c r y s t a l , t h e c o r r e s p o n d i n g bands
w i l l be f i l l e d a n d h i g h e r b a n d s w i l l b e empty.
f i l l e d , or else the h i g h e s t band i s o n l y h a l f f i l l e d b e c a u s e
t h e atoms e;g, Na, have an odd number of e l e c t r o n s per
primitive cell, The a l k a l i e a r t h metals h a v e t w o v a l e n c e
e l e c t r o n p e r p r i m i t i v e cell, t h e y c o u l d be i n s u l a t o r s b u t
t h e bands o v e r l a p i n e n e r g y to g i v e metals b u t n o t v e r y good
metals. The electrical r e s i s t i v i t y of p u r e metal may be as
low as 10 -10 ohm - cm a t a t e m p e r a t u r e @f I k , a p a r t from t h e
p o s s i b i l i t y of s u p e r c o n d u c t i v i t y , me r e s i s t i v i t y 'of a good 22 i n s u l a t o r may also be as h i g h a. b a t r e e n loi4 and 20 ohm -
ca. h i s r a n g e of may be the widest of any c o u o n
p h y s i c a l p r o p e r t y of solid8. The r e s i s t i v i t y of t h e seal- 9 conductor . lies i n between l o w 2 and 10 oh~a-ca.
. A .
I n mariaretals t h e c o n d u c t i o n band e d g e is v e r y s l i g h t l y
lower i n e n e r g y thm t h e v a l e n c e band edge. A small o v e r l a p
i n e n e r g y of the c o n d u c t i o n and v a l e n c e bands l e a d s te mall
c o n c e n t r a t i o n of h o l e s i n t h e v a l e n c e band and o f e l e c t r o n s
i n t h e c o n d u c t i o n band, Three of t h e ~ u i m e t a l s , A r s e n i c ,
As, Antimony, Sb and Bismuth B i are i n g roup V of t h e per iodic
table. T h e i r atoms a s s o c i a t e i n p a i r s i n the c r y s t a l lattice
w i t h t w o i o n s and t o n v a l e n c e e l s c t r o n s p e r p r i m i t i v e cell,
Smal l e v e r l a p s of t h e f i f t h and s i x t h a n e r g y bands create
e q u a l numbers of h o l e s and e l e c t r o n s i n mall n e a r l y el l ip-
s o i d a l p o c k e t s i n t h e B r i l l o u i n sone, L i k e semiconduc to r s ,
t h e semi m e t a l e l e m e n t s may be doped w i t h s u i t a b l e i m p u r i t l e
t o v a r y t h e r e l a t i v e c o n c e n t r a t i o n s of h o l e s and e l e c t r o n s ,
The a b s o l u t e c o n c e n t r a t i o n s may alse be v a r i e d by a p p l i c a t i o
o f p r e s s u r e , for t h e o v e r l a p of t h e band e d g e s v a r i e s w i t h
I n i n t r i n s i c semlcemductors ( i n s u l a t o r s a t OK) and good
i n s u l a t o r s where t B e allowed e n e r g y bands are e i t h e r f i l l e d
o r empty c o m p l e t e l y ( i n p u r e c r y s t a l s ) a t Ok) e l e c t r o n i c #
c o n d u c t i o n c a n n o t be possible s i n c e electric f i e l d caasot
c a u s e t h e e l e c t r o n s to move when t h e r e are no a l l s w e d e n e r g y
levels a v a i l a b l e . A s m a l l d i s t o r t i o n of t h e bands u s u a l l y
o c c u r s , however, g i v i n g rise to p o l a r i s a t i o n a s i n dielectri
m a t e r i a l s , Thus s s n i c o n d u c t o r s and i n s u l a t o r s may be class-
i f i e d as d i e l e c t r i c s , I n m e t a l s when an elsctric f i e l d is
a p p l i e d , e l e c t r o n s e x p e r i e n c e a f o r c e and ate a c c e l e r a t e d by
t h e electric f i e l d , They r e q u i r e ene rgy and occupy a v a i l -
a b l e h i g h e r ene rgy l e v e l s , T h i s p r o c e s s d o e s n o t c o n t i n u e
i n d e f i n i t e l y , however, s i n c e t h e e l e c t r o n s w i 11 e v e n t u a l l y
be s c a t t e r e d either v i a Bragg r e f l e c t i o n a t t h e eone boundry
i n k-space o r by i m p u r i t i e s and o t h e r l a t t i c e i m p e r f e c t i o n s ,
Thus t h e y l o s e energy and u l t i m a t e l y a t t a i n an a v e r a g e speed
i n t h e d i r e c t i o n of the e l e c t r i c f i e l d and we say t h a t t h e
c r y s t a l i s c o n d u c t i n g e l e c t r i c i t y ,
I t i s s i g n i f i c a n t t h a t t h e electrical c o n d u c t i v i t i e s
o f t h e s e materials c a n be v a r i e d o v e r w i d e r a n g e s by c h a n g e s
i n t e m p e r a t u r e , o p t i c a l e x c i t a t i o n a n d i m p u r i t y c o n t e n t ,
T h i s v a r i a b i l i t y of e l e c t r i c a l p r o p e r t i e s makes t h e seai-
c o n d u c t o r m a t e r i a l s n a t u r a l c h o i c e s f o r e l e c t r o n i c d e v i c e
i n v e s t i g a t i o n s , Semiconduc to r m a t e r i a l s are f o u n d i n
column I V and n e i g h b o u r i n g co lumns o f t h e p e r i o d i c table,
T h e r e a re o n l y t h r e e s e m i c o n d u c t i n g e l e m e n t s i n t h e column * I V c a l l e d e l e m e n t a l s e m i c o n d u c t o r s namely , S i l i c o n , S i ,
Germanium, Ge and g r a y t i n , Sn, They are so c a l l e d b e c a u s e
t h e y a r e composed o f s i n g l e s p e c i e s o f a toms,
I n a d d i t i o n t o t h e e l e m e n t a l mater ia ls , there are
numerous 8 e m i c o n d u c t i n g compounds o f column I11 and column
V atoms as w e l l as c e r t a i n c o m b i n a t i o n s from I1 and VY t h a t
make u p t h e i n t e r m e t a l l i c or compound s e m i c o n d u c t o r s , As a
matter of n o m e n c l a t u r e , a b i n a r y s e m i c o n d u c t i n g compound
AB, whe re A i s any e l e m e n t of Group I11 (8, A l , G a , in, Ti)
s a y , and B is any e l e m e n t of Group V ( N , - P , A s , Sb, BI) s a y ,
i s c a l l e d a 1 1 1 - V compound: t h u s Ga As is 1 x 1 - V compotjnd,
SIC,a I V - I v compound, ZnS, a X I - V I compound and Pb T e , a
I V - V I compound. Also PbSe a n d ' t e n a r y a l l o y s s u c h a& PbiOx
Sn, Te and P x I n where x i s t h e a t o m i c f r a c t i o n o f Sn
and P, r e s p e c t i v e l y , a r e examples of t h e s e m i c o n d u c t i n g com-
pounds or compound setmiconductors .
Groups i n t h e Per iodic Table
Table 1.
Only a few c r y s t a l lat t ice s t r u c t u r e s a r e f a v o u r a b l e
to t h e semiconduct ing mechanism, The most t y p i c a l a r e
s t r u c t u r e s of t h e diamond l a t t i c e t y p e i n which each atom
i s sur rounded by four n e a r e s t n e i g h b o u r s t h a t l ie w i t h equal
d i s t a n c e a t the c o r n e r s of a t e t r a h e d r o n , The s t r u c t u r e s
a r e s a i d to be t e t r a h e d r a l l y co -ord ina ted and to have co-
o r d i n a t i o n number four . Examples a r e Germanium and S i l i c o n
though Diamond itself i s more an i n s u l a t o r r a t h e r t h a n a n y
o t h e r . 8
A s t a b l e s 1 and 2 i n d i c a t e t h e r e are nwaerous semi-
c o n d u c t o r compounds, Among t h e s e S i is used f o r t h e m a j o r i t y
of semiconductor d e v i c e s , Rectif iers, t r a n s i s t o r s and
i n t e g r a t e d c i r c u i t s a r e u s u a l l y made of a i l i c o n . The
compounds are used most wide ly i n d e v i c e s r e q u i r i n g t h e
e m i s s i o n and a b s o r p t i o n of s u n l i g h t , For exampJe, s e m i -
c o n d u c t o r l i g h t emitters are commonly made of such conpounds
as G a As, Ga P and mixed compounds as GaAsP, F l u o r e s c e n t
materials such a s t h o s e used i n t e l e v i s i o n s c r e e n s u s u a l l y
are 1 1 - V I compound semiconduc to r s such a s 2nS. L i g h t d e t e c t 0
are c o m o n l y made w i t h InSb, CdSe or o t h e r compounds sucb a s
t h e l e a d s a l t s PbTe and PbSe, S i and G s a r e a l s o w i d e l y used
a s i n f r a r e d and n u c l e a r r a d i a t i o n d e t e c t o r s , An i m p o r t a n t
microwave d e v i c e , t h e Gunn d i o d e is u s u a l l y made ef 6 a h ,
Table For Clas se s o f Semiconductors and
Seiniconduc t ing Compounds.
I Elemental ! IV Compound
sic I I 111-V Compounds 1 1 - V i Compounds ,
I
A1P
A1 AlS
Ga
A1 Sb
Ga P
Zns
Znse
#
ZnTe
CdS
Cd se
Cd Te
Table 2.
Thus t h e wide range of semiconductor materials o f f e r s con-
siderable v a r i e t y i n p r o p e r t i e s and p rov ides d e v i c e and
c i r c u i t e n g i n e e m wi th much f l e x i b i l i t y i n t h e des ign of
e l e c t r o n i c func t ions .
1.2 Diffarences Between I n s u l a t o r s and Semiconductors,
There are t h r e e major Q i f f a r e n c e s between i n s u l a t o r s and
semiconductors : B
Semiconductors normally Rave a Band gap of order
1 ev or less whi l e i n s u l a t o r s have a band gap of
s e v e r a l e l e c t r o n v o l t s , For example, s i l i c o n h a s
a band gap of 1,1 e V and S i02 h a s Eg = 8 eV.
I n s u l a t o r s u s u a l l y have deep t r a p s (energy l e v e l
fu from the band edges i n t h e c e n t r e of t h e
forb idden gap) which t r a p c a r r i e r s t h a t may be
p r e s e n t due to i m p u r i t i e s ar thermal e x c i t a t i o n s ,
Electrical d o o d u c t i v i t y sf i n s u l a t o r s i s ve ry low
of the o r d e r of 10 -22 oh;' c;' w h i l e t h a t of
semiconductor i s much h ighe r a11 a t very l o w
temperatures.
A n Dif fe rences Between Semiconductors and Metals. . T h e r e are two major d i f f e r e n c e s between semiconductors
and metalst
(i) A t h i g h t e m p e r a t u r e s , semiconduc to r s are good
c o n d u c t o r s , whereas as t h e t e m p e r a t u r e is lowered,
t h e i r c o n d u c t i v i t y d r o p s to zero. Metals u e g w d
c o n d ~ c t e s r a t both low and h i g h t e m p e r a t u r e s t h e
c o n d u c t i v i t y i n c r e a s e s r a p i d l y ( 64 T ' ~ ) as T
g o e s to zero.
(ii ) The i m p u r i t y c o n t e n t u s u a l l y enhances c o n d u c t i v i t y
i n semiconduc to r s b u t lowers c o n d u c t i v i t y 13 metals.
1.5 Fermi Level
When a semiconductor i s doped w i t h donor or a c c e p t o r
i m p u r i t i e s , and t h e c o n c e n t r a t i o n of each k i n d of i m p u r i t y
varies from one r e g i o n of t h e sample to a n o t h e r , t h e
e l e c t r o n i c ene rgy levels w i l l do l i k e w i s e . I n t h e a b r e n c e
of an e x t e r n a l electric f i e l d ( a p p l i e d p o t e n t i a l ) , t h e
q u a n t i t y t h a t r emains c o n s t a n t ' is t h e Fermi l e v e l , EF. If
t h e P e m i l e v e l were to v a r y e l e c t r o n s would f l o w from
r e g i o n s where EP is h i g h e r to r e g i o n s where EF i s lower
u n t i l a c o n s t a n t v a l u e i s achieved.
For a n i n t r i n s i c semiconductor t h e number of e l e c t r o n s
i s e q u a l to t h e number of holes and t h e Ferrni l e v e l i s
d e t e r m i n e d by the c o n d i t i o n n = p
T h i s p l a c e s EF e x a c t l y a t t h e c e n t r e of t h e f o r b i d d e n gap
i f me b u t s i n c e mh > rn , g e n e r a l l y , t h e Permi l e v e l e rises s l i g h t l y w i t h temperature .
The Fermi level i n f i g . 2 can s h i f t n e a r e r t h e
conduc t ion band edge f i g . 3, or t h e v a l a n c e band edge
f i g . 4, d e p e n d h g on whether t h e serniconductor i s of n-
t y p e r doping o r t h e p-type doping as shown below. jn f i g . 3
t h e donor i m p u r i t y l e v e l of e l e c t r o n s i s c r e a t e d i n t h e fig. 4
f o r b i d d e n band and i n , an i m p u r i t y a c c e p t o r l e v e l of h o l e 8 - i s c r e a t e d i n t h e f o r b i d d e n band. A s p e c i a l case arises
when t h e s tmiconduc tor i s so h e a v i l y doped t h a t a t even
Ok t h e a l lowed energy l e v e l s f a l l w i t h i n t h e conduc t i on
(n- type) band, which b r i n g s t h e Fermi l e v e l w i t h i n t h e
conduc t i on band as shown i n fig.5. Such a semiconductor
i s c a l l e d d e g e n e r a t e semiconductor .
The c o n d i t i o n whereby n w p w n i sets a l i m i t on
t h e o p e r a t i o n of many e l e c t r o n i c semiconductor dev ices .
A t room t empe ra tu r e i n which n i << N.d 6 Nc most d e v i c e s
o p e r a t e best b u t a s t h e s e m a t e r i a l s a r e warmed n i i n c r e a s e s
and becomes l a r g e r than Rd and once t h e c o n d i t i o n n i 2 Nd
i s ach ieved , t h e c a r r i e r c o n c e n t r a t i o n i n t h e c r y s t a l , which
4 Fig.?. 1 shows t h e s c h e m a t i c e n e r g y b a n d '
d i a g r a m f o r ( a ) i n s u l a t o r (b 1 s e m i c o n d u c t o r
( c ) metal t o p o i n t o u t f u n d a m e n t a l d i f f e r e n c e s .
Energy 1 LI
F i g u r e 1.2
F ig . 1.2.
shows a l s o t h e s c h e m a t i c e l e c t r o n
o c c u p a n c y o f a l l o w e d e n e r g y b a n d s f o r a n
i n s u l a t o r , metal, semimetal and s e m i c o n d u c t o r .
The v e r t i c a l e x t e n t o f t h e b o x e s i n d i c a t e s t h e
a l l o w e d e n e r g y r e g i o n s , t h e s h a d e d areas i n d i c a t e
t h e r e g i o n s f i l l e d w i t h e l e c t r o n s . I n a semi-
m e t a l o n e band i s a l m o s t f i l l e d a n d a n o t h e r b a n d
i s n e a r l y empty a t a b s o l u t e z e r o .
? .
, , F i g * 3.
F e r r n i l e v e l E i n a s e m i c o n d u c t o r a s a f u n c t i o n F
o f d o p i n g , F i g u r e s 2 , 3 , 4 a n d 5 show t h e p o s i t i o n s
o f Ferrni l e v e l i n t h e i n t r i n s i c , n - t y p e d o p i n g ,
p - t y p e d o p i n g and h e a v y n - t y p e d o p i n g r e s p e c t i v e l y *
F i g . 5 i s :he c a s e o f a d e g e n e r a t e s e r n i c o n d u c t o r .
now c o n s i s t s of n e a r l y e q u a l number of e l e c t r o n s a n d h o l e s ,
i s p r o d u c e d b y t h e r m a l e x c i t a t i o n across t h e g a p r a t h e r
t h a n b y t h e i m p u r i t i e s , w h e r e n i i s t h e i n t r i n s i c carrier
c o n c e n t r a t i o n a n d Nd i s t h e t o t a l d o n o r d e n s i t y ( ' l o n i s e d
a n d u n i o n i s e d C o n s e q u e n t l y , a n y v a r i a t i o n s o f carrier > c o n c e n t r a t i o n b u i l t i n t o t h e c r y s t a l b y s p e c i f i c d o p i n g
p r o f i l e s are w i p e d o u t o n c e n i ) Nd. T h e w h o l e c r y s t a l
b e c o m e s i n t r i n s i c a n d e q u a l l y t h e d e v i c e ceases to
o p e r a t e . Thus t h e c o n d i t i o n n i ,,v Nd sets a n u p p e r
t e m p e r a t u r e l i m i t o n t h e o p e r a t i o n o f m o s t s e m i c o n d u c t o r
d e v i c e s . I t i s t h i s c o n d i t i o n t h a t , i n p a r t makes s i l i c o n
a p r e f e r a b l e m a t e r i a l t o g e r m a n i u m f o r many a p p l i c a t i o n s .
T h e F e r m i s u r f a c e i s t h e s u r f a c e o f c o n s t a n t e n e r g y E i n F
K s p a c e . The e l e c t r i c a l p r o p e r t i e s o f t h e ma te r i a l a re
d e t e r m i n e d b y t h e s h a p e o f t h e F e r m i s u r f a c e b e c a u s e t h e
c u r r e n t i s d u e t o c h a n g e s i n t h e o c c u p a n c y of s t a t e s n e a r
t h e F e r m i s u r f a c e .
CHAPTER 2
APPLICATIONS OF THIN FILMS
2 , 1 Def i n i t i s n .
A t h i n f i l m i s d e f i n e d as a t h i n material created 'ab
i n i t i s ' by an atom/melecule/ien/cluster ef s p a c i a s cen-
d e n s a t i e n process . Thin m a t a r i a l s may alse be fermed frem
a l i q u i d er a p a s t e , i n which c a s e i t i s called a t h i c k 4
f i l m , It is n e t t h e t h i c k n e s s t h a t is i m p e r t a n t i n d e f i n -
i n g a f i l m b u t r a t h e r t h e way i t is c r e a t e d w i t h t h e cen-
s e q u e n t i a l effects en i ts n i c r e s t r u c t u r e and p r e p e r t i e s ,
2.3 Reed Per R l i n Film Techneleqy,
I have e a r l i e r made ment ien e f t h e a v a i l a b i l i t y ef raw
m a t e r i a l s a s an i m p e r t a n t facter i n f a b r i c a t i n g selar cells
fer t h e best d e v i c e s f e r s e l a r ene rgy cenve r s i en . h i n
f i l m d e v i c e s w u l d t y p i c a l l y be a b e u t 5 te SOY t h i c k ,
i n c e n t r a n t te bu lk d e v i c e s which are a b e u t 150 te 250 p t h i c k , It s h e u l d be p e i n t e d e u t t h a t t h e u l t i m a t e lewer
l i m i t of t h e cost of bu lk d e v i c e s i s d e f i n e d by t h e cost
of t h e w a f e r i t s e l f and t h u s cost lower ing below t h e p r i c e
o f w a f e r s i s t h u s n o t p o s s i b l e . Hence, even for S i , a
t h i n f i l m t echno logy needs to be developed to m e e t - t h e cost
Apar t from s a v i n g cost as g i v e n abave c o n v e r s i o n
e f f i c i e n c i e s of t h e s e cells have been improved upon, I n
r e c e n t y e a r s , e f f i c i e n c i e s of 10 to 17% or more i n
c r y s t a l l i s e d and /o r e p i t a x i a l t h i n S i and G A A s solar cells
r e s p e c t i v e l y , have been made p o s s i b l e ,
I n comparison to t h i n f i l m CuS/CdS cells f o r example,
t h e p r o d u c t i o n of bulk S i ( p r i m a r i l y s i n g l e c r y s t a l ) solar
cells is i n t h e r a n g e of 5 ' M W i n t e r n a t i o n a l l y f o r l a r g e * s c a l e a p p l i c a t i o n s , S i p r o d u c t i o n c a p a b i l i t y does n o t
e x i s t and t h e t r e n d today i s toward r i b b o n technology.
However, s i n g l e c r y s t a l r i b b o n t echno logy is energy-
i n t e n s i v e and c o s t l y and r e q u i r e s a l a r g e m a t e r i a l i n p u t ,
Although t h e s c o p e f o r immediate a p p l i c a t i o n e x i s t s , t h e
long-range p o t e n t i a l i s low. On t h e o t h e r hand, t h i n f i l m
a-Si solar cells o f f e r a v e r y p romis ing f u t u r e , A t p r e s e n t
Sanyo, F u j i and Sharp of Japan a r e p roduc ing 2 to 3 PrW/yeer
o f such cells for l o w - p o w e r e l e c t r o n i c a p p l i c a t i o n s ,
The s i m p l e p r o d u c t i o n i n v o l v e d and t h e s i m p l e module/
p a n e l f a b r i c a t i o n make t h i n f i l m solar cells t h e o n l y
v i a b l e system. To f u r t h e r stress t h e i m p o r t a n c e , I q u o t e
E h r e n r c i c h , "me p o t e n t i a l payoff would be immense i f t r u l y
i n e x p e n s i v e t e c h n o l o g y based on t h i n f i l m sys tem were to be
d e v e l o p e d e n F i n a l l y , l o o k i n g a t t h e u s e s , b e n e f i t s and
a p p l i c a t i o n s of t h e t h i n f i l m s i n d i f f e r e n t areas of
t e c h ~ o l o g y to be g i v e n s h o r t l y , o n e does n e t have any
other a l t e r n a t i v e for t e c h n o l o g i c a l b r e a k t h r o u g h t h a n the
' t h i n f i l m t echno logy ' ,
? 7 . Metals, Semiconducting and I n s u l a t i n g Fi lms,
F i lms of metals, semi-metals, s e m i c o n d u c t o r s and
i n s u l a t o r s can be grown b u t t h e f i l m s of metals and semi-
c o n d u c t o r s are of paramount impor tance to u s a s t h e y f i n d
v e r y u s e f u l a p p l i c a t i o n s i n o p t o - e l e c t r o n i c , m i c r o e l e c t r o n i
and o t h e r dev ices . I n i n s u l a t i n g f i l m s - , carriers may be
g e n e r a t e d or modulated i n s i d e t h e i n s u l a t i n g f i l m (bu lk
l i m i t a d p r o c e s s e s ) or i n j e c t e d from t h e aetal e l e c t r o d e
( i n j e c t i o n l i m i t e d p r o c e s s e s ) , These v a r i o u s mechanisms
of c u r r e n t t r a n s f e r th rough a t h i n i n s u l a t o r f i l m sardwiche
between two metal electrodes a r e t h e r a i o n i c emiss ion ,
S c h o t t k y e m i s s i o n of e l e c t r o n s o v e r t h e m e t a l / i n s u l a t o r
i n t e r f a c e barrier i n t o t h e c o n d u c t i o n band of t h e i n s u l a t o r
d i r e c t quantum mechanica l t a r n e l l i n g of e l e c t r o n s from one
m e t a l e l e c t r o d e t o a n o t h e r and t h e t u n n e l i n g of carriers
th rough t h e i n s u l a t o r barrier gap a t b i g h applied f i e l d
( f i e l d or cold e a i a r i o n ) . These are i n j e c t i o n p r ~ c e s ' s e r ~
Bulk l i m i t e d p r o c e s s e s i n c l u d e s p a c e - c h a r g e - l i m i t e d c u r r e n t
a n d P o o l e l i e n k e l e m i s s i o n ,
I n s e m i c a n d u c t i n g f i l m s , when t r a n s p o r t occurs t h r o u g h
t h e s p e c i m e n s , the carriers are s u b j e c t e d to c o n s i d e r a b l e
s c a t t e r i n g by t h e bounda ry s u r f a c e s i n a d d i t i o n to t h e
no rma l budk s c a t t e r i n g , The e f f e c t i v e carrier m o b i l i t y i s
t h u s r e d u c e d below t h e b u l k v a l u e b e c a u s e t h i s a d d i t i o n a l
s c a t t e r i n g g i v e s rise to c o n d u c t i v i t y s i z e eff ectr,
A n a l y s i s of t h e s i z e effects i n s e m i c o n d u c t o r s i s modif ied
from t h a t i n metals owing to t h e a d d i t i o n a l f e a t u r e of
s u r f a c e space c h a r g e and t h e r e s u l t a n t s u r f a c e p o t e n t i a l
barrier i n s e m i c o n d u c t o r s , The m o b i l i t y is s i g n i f i c a n t l y
affected by v a r i o u s s c a t t e r i n g mechanisms, e a c h of which is
o p e r a t i v e i n a p a r t i c u l a r t e m p e r a t u r e r a n g e , The s c a t t e r i n g
c a n be d u e to ( i) i m p u r i t y i o n s , (ii) t h e r m a l la t t ice
v i b r a t i o n s or phonons , (iii) i m p u r i t y atoms, ( i v ) v a c a n c i e s
and p o i n t d e f e c t s ( v ) d i s l o c a t i o n s , ( v i ) g r a i n b o u n d a r i e s ,
c l e a v a g e p l a n e s and c r y s t a l s u r f a c e s and , ( v i i ) c h a r g e
carriers,
The m o b i l i t y of t h e c h a r g e carrier c a n be e x p r e s s e d i n
terms of t h e r e l a x a t i o n t i m e r as
f l = e(r ) / m e = p (TI
/cc ar; ~ - ~ / 2
where t h e r e l a x a t i o n t i m e is d e f i n e d a s t h e t i m e t h e non
e q u i l i b r i u m s t a t e p e r s i s t s , on t h e a v e r a g e , a f t e r t h e
f i e l d s r e s p o n s i b l e f o r i t have been s w i t c h e d off and me is
t h e a f f e c t i v e mass.Semiconductor f i l m s used i n photo- -
v o l t a i c d e v i c e s a r e , i n g e n e r a l p o l l y c r y s t a l l i n e and have
a h i g h d e n s i t y of inhomogenei ties i n c o r p o r a t e d i n t h e form
o f g r a i n b o u n d a r i e s , d i f f e r e n t p h a s e s , and i n some c a s e s +
d i f f e r e n t m a t e r i a l components, The c o n d u c t i o n p r o c e s s e s
i n t h e s e f i l m s are markedly d i f f e r e n t from t h o s e i n s i n g l e
c r y s t a l homogeneous m a t e r i a l s ,
E l e c t r i c a l c o n d u c t i o n i n metal f i l m s depends on whe the r
t h e f i l m is g r a n u l a r o r i s l a n d l i k e , porous o r network t y p e
o r p h y s i c a l l y c o n t i n u o u s , h e e l e c t r i c a l c o n d u c t i v i t y of a
g r a n u l a r f i l m i s many o r d e r s of magni tude s m a l l e r t h a n t h a t
o f t h e c o r r e s p o n d i n g bulk and i s c h a r a c t e r i z e d by a n e g a t i v e
t e m p e r a t u r e c o e f f i c i e n t of r e s i s t i v i t y (TCR), A t l a r g e
( 100R) s e p e r a t i o n d between m e t a l p a r t i c l e s , c o n d u c t i o n is
d o m i n a t e d by t h e r m i o n i c emiss ion , Very t h i n metal f i l m 8
g e n e r a l l y have a g r a n u l a r s t r u c t u r e and show s e v e r a l
a n o m a l i e s i n e l e c t r i c a l c o n d u c t i o n w i t h r e s p e c t to bu lk
m e t a l s , The most i m p o r t a n t of t h e s e a n o m a l i e s a r e ( i ) a
lower c o n d u c t i v i t y t h a n t h e bulk m e t a l s ( i l l a n e g a t i v e
t h e r m a l c o e f f i c i e n t o f r e s i s t i v i t y w i t h g e n e r a l l y a r a t h e r
1 h i g h v a l u e (iii) t h e p r e s e n c e o f c u r r e n t n o i s e of 7 t y p e ,
showing t h e e x i s t e n c e o f s p o n t a n e o u s c o n d u c t i v i t y f l u c t u a t -
i o n n o t d i r e c t l y bound to t h e t h e r m a l mot ion of t h e
e l e c t r o n s , I n g e n e r a l r e c e n t t h e o r i e s h a v e assumed t h a t
e l e c t r i c a l c o n d u c t i o n t a k e s p l a c e i n t h e s e f i l m s by tunne-
l l i n g from o n e m e t a l i s l a n d to a n o t h e r t h r o u g h t h e sub- i
strate, however, a t h e r m o n i c p r o c e s s i s n o t e x c l u d e d ,
The r e s i s t a n c e ( r e s i s t i v i t y ) i n the body o f t h e f i l m
i s d u e m a i n l y to t h e e l e c t r o n - p h o n o n i n t e r a c t i o n and t h e
s c a t t e r i n g by t h i s mechanism is a n i s o t r o p i c . E x p e r i m e n t a l
measurements i n d i c a t e t h a t t h e e l e c t r i c a l c o n d u c t i v i t y o f
t h e t h i n metallic f i l m i s less t h a n t h e c o n d u c t i v i t y of
t h e b u l k material and d e c r e a s e s a s t h e t h i c k n e s s of t h e
f i l m d e c r e a s e s w h i l e i t s dependence on t h e t e m p e r a t u r e o f
t h e m a t e r i a l i s d i f f e r e n t f rom t h a t o f t h e b u l k material.
The p h y s i c a l r e a s o n f o r t h e r e d u c e d c o n d u c t i v i t y o f a f i l m
i s t h e c o l l i s i o n o f t h e e l e c t r o n s w i t h t h e s u r f a c e of t h e
f i l m and t h e r e s u l t a n t s c a t t e r i n g , I f t h i s s c a t t e r i n g i s
n o t p e r f e c t l y s p e c u l a r , i t c o n s i s t u t e s a s o u r c e o f resis-
t a n c e n o t p r e s e n t i n t h e b u l k m a t e r i a l , As a r e s u l t o f
t h e s e c o l l i s i o n s , i n a v e r y t h i n f i l m most o f t h e c u r r e n t
i s c a r r i e d by t h e e l e c t r o n s which move p a r a l l e l t o t h e
s u r f a c e . T h i s e x p l a i n s t h e d i f f e r e n c e be tween t h e
t e m p e r a t u r e dependence of t h e c o n d u c t i v i t y of a t h i n f i l m
and t h a t o f t h e bu lk m a t e r i a l , I n t h e b u l k material , small
a n g l e s c a t t e r i n g o f t h e e l e c t r o n s by t h e phonons d o e s n o t
l e a d t o a l a r g e r e s i s t i v i t y b u t i n a t h i n f i l m i t w i l l c a u s e
e l e c t r o n s p r e v i o u s l y moving p a r a l l e l t o t h e s u r f a c e to
c h a n g e t h e i r d i r e c t i o n s of mot ion and c o l l i d e w i t h t h e 4
s u r f a c e , l e a d i n g to an a p p r e c i a b l e i n c r e a s e i n t h e
r e s i s t i v i t y ,
n . A p p l i c a t i o n s of Th in Fi lms.
M e t a l l i c f i l m s a r e u s e d a s e lectr ical c o n d u c t o r s i n
numerous m i c r o e l e c t r o n i c d e v i c e s , For most a p p l i c a t i o n s
of s i l i c o n s e m i c o n d u c t o r d e v i c e s t h e c o n d u c t i n g f i l m s
u s e d ( u s u a l l y r e f e r r e d to a s m e t a l l i z a t i o n ) a r e 1 - 2 y
t h i c k , The f i l m s c a n be a s na r row a s 1 o r ' 2 p b u t
o c c a s i o n a l l y t h e y are s e v e r a l millimeters wide,
Th in f i l m s c a n a l s o b e used i n i n f o r m a t i o n r e c o r d i n g
s y s t e m s b a s e d on l i g h t or e l e c t r o n r e c o r d i n g . The r e c o r d e d
d a t a a p p e a r s as a c h a n g e i n e l e c t r i c a l or o p t i c a l p r o p e r t i e s
o f t h e f i l m , R e f l e c t i v e f i l m s h a v e been used i n photo-
p l a s t i c r e c o r d i n g , T h i s i s a method o f r e c o r d i n g l i g h t
images i n t h e form of s u r f a c e d e f o r m a t i o n s i n a photo-
c o n d u c t i v e t h e r m o p l a s t i c f i l m , A l s o f e r r o e l e c t r i c f i l m s
a r e used i n d i s p l a y d e v i c e s l i k e t h e microwave c a p a c i t o r s ,
t h e r m i s t o r s , b o l o m e t e r s , p y r o e l e c t r i c s , p i e z o e l e c t r i c
t r a n s d u c e r s and o p t i c a l d i s p l a y d e v i c e s ,
Thin f i l m s of s i l v e r and l e a d h a l i d e s as w e l l a s
a r s e n i c s u l p h i d e s a r e made u s e of a s p h o t o g r a p h i c
i n f o r m a t i o n s t o r a g e d e v i c e s . b
I t i s e a s y to c a r r y o u t a b s o r p t i o n , r e f l e c t i o n ,
e m i s s i o n and g e n e r a l l y o p t i c a l s t u d i e s f o r v i s i b l e o r n e a r
u l t r a v i o l e t r a d i a t i o n b e c a u s e of a v a i l a b i l i t y of good
f i l ters , compensa to r s or t r a n s m i s s i o n p o l a r i s e r s . S t u d i e s
below t h e 2000g wavelength a r e n o t p o s s i b l e b e c a u s e of
l a c k of e q u i v a l e n t f i l t e r s , compensators and p o l a r i s e r s
b u t t h e u s e of t h i n f i l m inultraviole~spectroscopy
e l i m i n a t e s some o f t h e d i f f i c u l t i e s e n c o u n t e r e d i n d e t e r -
mining t h e o p t i c a l c o n s t a n t s of m a t e r i a l s . With t h e t h i n
f i l m s , t h e d e s i g n of good f i l t e r s , p o l a r i s e r s and compen-
s a t o r s i s p o s s i b l e f o r o p t i c a l s t u d i e s under h igh i n t e n s i t y
s y n c h r o t r o n r a d i a t i o n and for t h e u s e of t h e hydrogen l a s e r
which o p e r a t e s a t 1600R by s o l i d s t a t e p h y s i c i s t s and bio-
p h y s i c i s t s i n r e s e a r c h c e n t r e s and l a b o r a t o r i e s .
A p r a c t i c a l a p p l i c a t i o n of Yt t r ia (Y203) f i l m s as a
d i e l e c t r i c f o r t h i n f i l m c a p a c i t o r s h a s - b e e n - r e p o r t e d from
Japan . Dielectric t h i n f i l m s have a t t r a c t e d a t t e n t i o n as
c o n v e n i e n t media fo r s u r f a c e p r o p a g a t i o n of o p t i c a l waves,
t h e t e c h n i q u e of which i s u s e d i n two d i m e n s i o n a l i n t e -
g r a t e d o p t i c a l c i r c u i t s l i k e i n t h e t w o d i m e n s i o n a l
p r o c e s s o r s . Wave g u i d e c h a r a c t e r i s t i c s o f t h e f i l m s
( r e f r a c t i v e i n d e x and a t t e n u a t i o n ) p e r m i t a c o l l e r a t i o n
between t h e o p t i c a l and electrical p r o p e r t i e s of f i l m s
t o be made.
It i s v e r y i n t e r e s t i n g and e x c i t i n g to o b s e r v e t h a t
s u p e r c o n d u c t i n g t r a n s i t i o n t e m p e r a t u r e s , T of some
m a t e r i a l s are r a i s e d a f t e r d e p o s i t i n g t h e s e m a t e r i a l s on
s u b s t r a t e s . For i n s t a n c e t h e t r a n s i t i o n t e m p e r a t u r e s of
Cd and Zn h a v e been r a i s e d from 0.3 k to 0.9 k and 0.8 k
t o 1.5 k r e s p e c t i v e l y . me Tc f o r Cd condensed on a sub-
strate h e l d a t t h a t 0.3 k i s a l m o s t twice t h e o n e o f t h e
b u l k Cd which i s 0.53 k. Though t h e i n c r e a s e i n t h e Tc
i s small, i t c a n be n o t e d t h a t i f e f f o r t s by s o l i d s ta te
p h y s i c i s t s are g e a r e d t o w a r d s t h i s d i r e c t i o n , d e f i n i t e l y
t h e r e s h o u l d be a r e m a r k a b l e b r e a k t h r o u g h i n s u p e r c o n d u c t i -
v i t y s t u d i e s i n no d i s t a n t f u t u r e , Then t h e problem of
electric power crisis w i l l have been s o l v e d and man w i l l
t h e r e f o r e l i v e to enjoy ,
CHAPTER 3
METHODS OF DEPOSITION OF THIN FILMS
There are many methods of s u c c e s s f u l d e p o s i t i o n of t h i n
f i l m s some-of which a r e r -
s p u t t e r i n g ,
e v a p o r a t i o n (vacuum) or t h e r m a l ,
plasma d e p o s i t i o n ,
e l e c t r o l y s i s ,
drawing from t h e m e 1 t ,
s p r a y p y r o l y s i s t e c h n i q u e ,
d c glow
chemica l
chemica l
r.f. pol
i s c h a r g a t e c h n i q u e ,
vapour d e p o s i t i o n ,
b a t h d e p o s i t i o n t e c h n i q u e ,
m e r i z a t i o n ,
pho to - in i t i a t a d p o l y m e r i z a t i o n ,
the rmal o x i d a t i o n ,
( x i i i ) ion-exchange r e a c t i o n s ,
( x i v ) glow d i s c h a r g e decompos i t ion ,
( X V ) e l e c t r o p h o r e s i s ,
( x v i ) s c r e e n p r i n t i n g . ,
( x v i i ) e l e c t r o d e p o s i t i o n ,
( x v i i i ) e l e c t r o l e s s or a u t o c a t a l y t i c p l a t i n g ,
( x i x ) a n o d i z a t i o n , ( X X ) e l e c t r o h y d r o d y n a m i c ~ ,
( x x i ) thermal e x p a n s i o n s h e a r s e p a r a t i o n ,
Dependong on how t h e atoms/rnolecles/ions/clusters of
s p e c i e s a r e c r e a t e d f o r t h e c o n d e n s a t i o n p r o c e s s , t h e
methods fo r d e p o s i t i n g t h i n f i l m s a r e te rmed p h y s i c a l
v a p o r d e p o s i t i o n (PVD), Chemical v a p o r d e p o s i t i o n (CVD) , electrochemical d e p o s i t i o n (ECD) , or m i x t u r e s of PVg and
CVB ( h y b r i d ) . Vapor atoms i m p i n g i n g on a s u b s t r a t e lose
t h e i r k i n e t i c e n e r g y and a r e a b s o r b e d on t h e s u r f a c e as
ad-atoms, The movement o f t h e s e ad-atoms d e p e n d s o n
numerous c o n d i t i o n s i n p a r t i c u l a r , t h e e n e r g y o f t h e v a p o r
atoms, t h e rate of impingement , t h e a b s o r p t i a n and desorp-
t i o n a c t i v a t i o n e n e r g i e s , t h e t e p o q r a p h y and c h e m i c a l
n a t u r e sf t h e substrate and . l a s t l y , the oubrtrete tomper-
ature. h a4-at.l La m t stab&. by itmat. A8 diwrr,
trimerr and coltimeua rrce famed by randm c o l l i s i o n s
processas' oa the rmbmtrata, t h e i r r t a b i l i t y i n c r e a s e s
simply becaam of the ircreaairg number ef Wads be tween
t h e ad-atoms which overcome t h e d i r r u p t i v e s u r f a c e ene rgy .
As a cr i t ica l s i z e of t h e s e monamers i s reached and
%mu l e a t i e n barrier crossed t h e ad-a tom c l u s t e r becomes
s table and is c h e m i c a l l y absorbed, The s i z e of t h e c r i t i ca l
n u c l e u s i n most cases i s of atomic d imens ions . The growth
o f a t h i n f i l m c a n t a k e p l a c e b y o n e o f t h r e e modes : on
1) l a y e r by l a y e r which o c c u r s i f e i t h e r ~ o n e e x t r e m e t h e
ad-atoms h a v e l i t t l e m o b i l i t y (as i n amorphous d e p o s i t s )
o r u n d e r t h e e x t r e m e c o n d i t i o n s o f v e r y l o w su p e r s a t u r a t -
i o n , S i n g l e c r y s t a l s u b s t r a t e and u l t r a h i g h vacuum
d e p o s i t i o n .
2 ) S t r a n s k i - K a i t c h e v mode i n which case t h e f i lm ,g rows
j u s t a s i n t h e l a y e r - b y - l a y e r mode and t h e n c o n v e r t s
itself i n t o t h r e e - d i m e n s i o n a l n u c l e i ; a n d
3 ) Three d i m e n s i o n a l g rowth of t h e discrete n u c l e i . The
l a s t mode i s most common for o r i e n t e d t h i n f i l m s i f t h e
s i z e of n u c l e a t i o n c e n t e r s i s small and ad-atom m o b i l i t y
i s l a r g e . The g rowth y i e l d s a p l a t e l e t t y p e of g rowth
which resembles two d i m e n s i o n a l g rowth ,
F o r o u r p u r p o s e , i t s u f f i c e s to s a y t h a t g rowth takes
p l a c e a t n u c l e a t i o n - s i t e s , b o t h l a t e r a l l y ( i n t h e p l a n e
o f t h e f i l m s ) and p e r p e n d i c u l a r to t h e f i l m s , The number
of g r a i n s and g r a i n s i z e are more or less d e t e r m i n e d by t h e
d e n s i t y of n u c l e a t i o n c e n t e r s u n l e s s l a r g e s c a l e coalescence
o r r e c r y s t a l l i z a t i o n l e a d s to s i n t e r i n g and i n c r e a s e of
g r a i n s i z e s . The growth i n t h e p e r p e n d i c u l a r d i r e c t i o n
t a k e s p l a c e i n a colummar f a s h i o n s and t h u s i s a n i s o t r o p i c
w i t h a g r a i n s i z e p e r p e n d i c u l a r t o t h e s u b s t r a t e s d e t e r -
mined by t h e f i l m t h i c k n e s s , and of c o u r s e , by r e c r y s t a l -
l i z a t i o n / c o a l e s c e n c e processes. G e n e r a l l y t h e l a t e r a l
g r a i n s ize w i l l be a b o u t t h e f i l m t h i c k n e s s for small
t h i c k n e s s e s or a f r a c t i o n of t h e t h i c k n e s s i n case o f
v e r y t h i c k f i l m s . T h i s mode o f g rowth c o n s i s t i n g o f
n u c l e a t i o n and ad-atom m o b i l i t y domina ted c o a l e s c e n c e h a s
t h e f o l l o w i n g c o n s e q u e n c e s : * 1, D i f f e r e n t t e c h n o l e g y f o r l a r g e area c o a t i n g r a n g i n g
from t h e v e r y s i m p l e s p r a y p r o c e s s t o t h e h i g h l y s o p h i s -
t i c a t e d m o l e c u l a r beam e p i t a x y (HBE) e x h i b i t s imilar s t a g e
of growth , The f i l m i s f i r s t d i s c o n t i n u o u s n e x t a ne twork
and f i n a l l y c o n t i n u o u s . The c r i t i c a l t h i c k n e s s tc a t
which t h e f i l m becomes c o n t i n u o u s depends marked ly o n
n u c l e a t i o n s i te and s u b s t r a t e t e m p e r a t u r e Tc.
2. N u c l e a t i o n c e n t e r s c a n be m o d i f i e d for example b y
d e p o s i t i n g S i 0 2 o n g l a s s , t h e n u c l e a t i o n c e n t e r s are
i n c r e a s e d and T i s lowered. Thus a l a y e r - b y - l a y e r C
g rowth i s p o s s i b l e . A l s o by d e p o s i t i n g a t l o w s u b s t r a t e
t e m p e r a t u r e which k i l l s ad-atom m o b i l i t y , a layer-by-
l a y e r g rowth i s p o s s i b l e .
3 , Mismatch of randomly formed n u c l e i , r e s u l t s i n a l a r g e
v a r i e t y of s t r u c t u r a l d e f e c t , p o i n t ( v a c a n c i e s ) l i n e
( d i s l o c a t i o n s ) and p l a n n e r ( s t a c k i n g f a u l t s ) , These
d e f e c t s are o b v i o u s l y connec ted w i t h t h e d e n s i t y of
n u c l e a t i o n c e n t e r s and hence t h e g r a i n s i z e w i t h g r a i n
s i z e of a b o u t 100R, one may e x p e c t f r o z e n - i n v a c a n c i e s of
a b o u t 10% i n t h i n (f-1 3000R) f i l m s , S i m i l a r l y , a d i s -
l o c a t i o n d e n s i t y of a b o u t
obse rved , I n o r i e n t e d o r
d e n s i t i e s a r e lower-about
4. Owing t o t h e g r a n u l a r
l o 2 l i n e s a n o 2 i s conundnly
e x p i t a x i a l f i l m s d i s l o c a t i o n
s t r u c t u r e and mismatch of g r a i n s ,
l a r g e stresses a r e developed b o t h compress ive and t e n s i l e , -2 o f t h e o r d e r 10" dynes c n ,
5, The e f f e c t i v e s u r f a c e a r e a of a f i l m depends on i t s
m i c r o s t r u c t u r e , I f t h e f i l m h a s a porous or columnar
n a t u r e ( a s o b t a i n e d by o b l i q u e d e p o s i t i o n ) t h e a r e a
i n c r e a s e s w i t h f i l m t h i c k n e s s , For growth a t h i g h T
w i t h l a r g e g r a i n s and f o r e p i t a x i a l l a y e r s , t h e e f f e c t i v e
a r e a approaches t h e g e o m e t r i c a l a r e a ,
6, The s m a l l e r t h e g r a i n s i z e ( m i c r o p d y c r y s t a l l i n e ) t h e
l a r g e r t h e changes i n l a t t i c e c o n s t a n t expec ted ,
7, S o l u b i l i t y c o n d i t i o n s a r e n o t r e s t r i c t i v e i n t h e vapor
phase , T h e r e f o r e c o - d e p o s i t i o n of v a p o r s r e s u l t s i n
a l l o y s and compounds, Over an ex tended r a n g e of s o l u -
b i l i t y , a-SI:H i s a good example,
8, S i n c e t h e f r e e e n e r g i e s of d i f f e r e n t s t r u c t u r e s a r e
v e r y s i m i l a r , t h e a d d i t i o n a l e n e r g i e s due to s u r f a c e ,
e l e c t r o s t a t i c , and s t r a i n e n e r g i e s , etc, r e s u l t i n t h e
e a s y f o r m a t i o n of m e t a s t a b l e s t r u c t u r e s - p r i m a r i l y
polymorphs which a r e u s u a l l y o b t a i n e d a t h igh t e m p e r a t u r e s
a n d / o r h igh p r e s s u r e s . B-Ta i s an example of such a
m e t a s t a b l e s t r u c t u r e .
9. Co-deposi t i o n and /o r mu1 t i p l e d e p o s i t i o n a l l o w s a
s p a t i a l and t h i c k n e s s g r a d i e n t of compos i t ion and t h u s
f i l m s w i th s p a t i a l l y v a r i a b l e p r o p e r t i e s ,
10, By u s i n g a p p r o p r i a t e s u b s t r a t e s and d e p o s i t i o n
c o n d i t i o n s , t h e r e s u l t i n g f i l m m i c r o s t r u c t u r e c a n be
v a r i e d from amorphous a t one ex t reme t o e p i t a x i a l a t t h e
o t h e r , Thus amorphous, m i c r o p o l y c r y s t a l l i n e o r i e n t e d
and e p i t a x i a l d e p o s i t s a r e p o s s i b l e i n f i l m s of t h e same
m a t e r i a l a s i n t h e c a s e of S i ,
11, S u r f a c e roughness depends on mode of growth, Layer-
by- layer (Low ad-atom m o b i l i t y ) growth y i e l d s a t o m i s t i c a l l y
smooth f i l m s , O r i e n t e d growth, where g r a i n s d e v e l o p o r i e n t -
a t i o n , g e n e r a l l y y i e l d s r a t h e r rough s u r f a c e s , a s fo r
example i n cdS.
3 . 1 Methods of Dcpos i t inq Thin Fi lms,
The n i n e t e e n methods for t h e d e p o s i t i o n of t h i n
f i l m s a r e b u t o n l y f e w methods known t o be used for t h e
d e p o s i t i o n of t h i n f i l m s of resistors, i n s u l a t o r s ,
ferri ter, c o n d u c t o r s and semiconduc to r s , Two d i m e n s i o n a l +
m a t e r i a l s of t h i c k n e s s e s r a n g i n g from f e w angs t roms to
hundreds of micrometerr c a n be p r e p a r e d by t h e h o s t of
so c a l l e d t h i n f i l m a s w e l l a s t h i c k f i l m t e c h n i q u e s .
Brief e x p l a n a t i o n s of some of t h e methods a r e g i v e n
w h i l e a d e t a i l e d d e s c r i p t i o n and e x p l a n a t i o n of t h e most
i m p o r t a n t s i m p l e s t and most u s e f u l of them a11 t h e
c h e m i c a l b a t h d e p o s i t i o n t e c h n i q u e or a o l u t i o n growth
method is given.
2.3 Evapora t ion : T h i s i s a p r o c e s s whereby a material
i s h e a t e d t o a s u f f i c i e n t l y h i g h t e m p e r a t u r e to p roduce
t h e d e s i r e d vapour p r e s s u r e so a s to t u r n t h e atoms/
m o l e c u l e s of t h e s u b s u r f a c e i n t o vapour. The t e m p e r a t u r e
of t h e m a t e r i a l f o r e v a p o r a t i o n may be r a i s e d by d i r e c t or
i n d i r e c t h e a t i n g .
7 . fl . E p i t a x i a l D e p o s i t i o n : T h e r e ore two or more forms
o f t h i s . The major t y p e s a r e t h e v a p o u r p h a s e e p i t a x y
and t h e l i q u i d p h a s e e p i t a x y . B a s i c a l l y t h e l i q u i d p h a s e
e p i t a x y i n v o l v e s t h e p r e c i p i t a t i o n o f a m a t e r i a l f rom a
c o o l i n g s o l u t i o n o n t o a n u n d e r l y i n g s i n g l e c r y s t a l sub-
strate. The s o l u t i o n and t h e s u b s t r a t e are k e p t a p a r t
a n d c o n t a c t i s made by " t i p p i n g * ' t h e f u r n a c e w i t h s o l u t i o n
or by d i p p i n g t h e s u b s t r a t e i n t o t h e s o l u t i o n i n a , ver t ica l f u r n a c e . The s o l u t i o n i s s a t u r a t e d w i t h t h e
growth m a t e r i a l a t t h e d e s i r e d growth t e m p e r a t u r e and
t h e n a l l o w e d to cool i n c o n t a c t w i t h t h e s u b s t r a t e s u r f a c e
a t a r a t e and f o r a t i m e i n t e r v a l a p p r o p r i a t e for t h e
g e n e r a t i o n o f t h e d e s i r e d l a y e r . Under optimum c o n d i t i o n s ,
t h e l a y e r grows a s a n e x t e n s i o n of t h e s i n g l e c r y s t a l
s u b s t r a t e . I t i s o b v i o u s l y n o t e a s y to grow p o l y c r y s t a l l -
i n e l a y e r s on g r o s s l y d i s s i m i l a r s u b s t r a t e s . The t e c h n i q u e
of growing a n o r i e n t e d s i n g l e c r y s t a l l a y e r o n ' a s u b s t r a t e
i s c a l l e d e p i t a x i a l growth. An example o f t h i s i s t h e
G a A s whose m e l t i n g p o i n t i s 1 2 3 8 ~ ~ grown o n t o a G a A s sub-
s t r a t e t h a t s e r v e s a s a w a f e r f rom a s o l u t i o n o f GaAs and
m o l t e n m e t a l G a which i s d o n e a t a t e m p e r a t u r e f a r below
1 2 3 8 ~ ~ .
r . . S p u t t e r i n q Techniques . Vapour s p e c i e s may be c r e a t e d
b y k i n e t i c e j e c t i o n from t h e s u r f a c e of a m a t e r i a l
( c a l l e d t a r g e t or c a t h o d e ) by bombardment w i t h e n e r g e t i c
a n d non r e a c t i n g i o n s . The e j e c t i o n p r o c e s s , known a s
s p u t t e r i n g t a k e s p l a c e a s a r e s u l t of momentum t r a n s f e r
be tween t h e i m p i n g i n g i o n s and t h e atoms of t h e t a rge t
s u r f a c e , The s p u t t e r e d a toms are c o n d e n s e d o n a sub-
s trate to form a f i l m , Both t h e rf s p u t t e r i n g a t l o w 6
p r e s s u r e s ( g a s i o n i z a t i o n w i t h t h e h e l p of a n i n d u c t i v e l y
c o u p l e d e x t e r n a l f i e l d ) and t h e glow d i s c h a r g e s p u t t e r i n g
( i o n s p r o v i d e d by bombardment of a r g o n a t r e d u c e d r e s i d u a l
p r e s s u r e and h i g h p o t e n t i a l d i f f e r e n c e of a b o u t 3 K V ) are
a l l p o s s i b l e .
? r - . - S c r e e n P r i n t i n g : I n t h i s a p p r o a c h p a s t e s c o n t a i n i n g
t h e d e s i r e d material a r e s c r e e n p r i n t e d by c o n v e n t i o n a l
me thods o n t o a s u i t a b l e s u b s t r a t e . The s u b s t r a t e i s
b r o u g h t b e n e a t h t h e s c r e e n i n a c a r r i a g e so t h a t t h e sub-
s t r a t e i s i n a c c u r a t e r e g i s t r a t i o n w i t h t h e p a t t e r n on t h e
s c r e e n , Then t h e p a t t e r n on t h e s c r e e n i s p h o t o l i t h o g r a -
p h i c a l l y d e f i n e d 80 t h a t open mesh a r e a s i n t h e s c r e e n
c o r r e s p o n d t o t h e c o n f i g u r a t i o n to be p r i n t e d , When t h e
s u b s t r a t e i s a t t h e correct d i s t a n c e b e n e a t h t h e s c r e e n
c a l l e d t h e snap-off d i s t a n c e , a s m a l l amount of t h e p a s t e
i s d i s p e n s e d o n t o t h e u p p e r s u r f ace of t h e s c r e e n . A
f l e x i b l e w i p e r c a l l e d t h e s q u e e g e e moves a c r o s s t h e s c r e e n
s u r f a c e , d e f l e c t i n g t h e s c r e e n v e r t i c a l l y and f i n a l l y
b r i n g i n g t h e s c r e e n i n c o n t a c t w i t h t h e s u b s t r a t e and
f o r c i n g t h e p a s t e t h r o u g h t h e open mesh areas, On remova l
of t h e squeegee , t h e s c r e e n r e g a i n s i t s o r i g i n a l p o s i t i o n 4
l e a v i n g b e h i n d t h e p r i n t e d p a s t e p a t t e r n on t h e s u b s t r a t e ,
?." Chemical Vapour D e p o s i t i o n : T h i s i n v o l v e s e s s e n t i a l l y
e x p o s u r e of t h e s u b s t r a t e to o n e or s e v e r a l of v a p o r i z e d
c o n s t i t u e n t s of t h e d e s i r e d d e p o s i t e d s u b s t r a te. A
c h e m i c a l r e a c t i o n which c o u l d be d e c o m p o s i t i o n , r e d u c t i o n ,
p o l y m e r i z a t i o n , t r a n s p o r t r e a c t i o n s etc. i s i n i t i a t e d a t
or n e a r t h e s u b s t r a t e s u r f a c e p r o d u c i n g t h e d e s i r e d
m a t e r i a l as a s o l i d - p h a s e r e a c t i o n p r o d u c t which c o n d e n s e s
on t h e s u b s t r a t e . I n some c a s e s , t h e s u b s t r a t e may t a k e
p a r t i n t h e r e a c t i o n mechanism i f t h e t e m p e r a t u r e is
s u f f i c i e n t l y h i g h , Fo r example S i o r A 1 s u b s t r a t e s when
exposed to an oxygen a t m o s p h e r e grow S i O or A l 2 o 3 l a y e r s
r e s p e c t i v e l y J The r e a c t i o n k i n e t i c s depend on s e v e r a l
compounds or r e a g e n t g a s e s , some o r a l l of which c o n t a i n
f a c t o r s , n o t a b l y f l o w r a t e s , p a r t i a l g a s p r e s s u r e s ,
d e p o s i t i o n t e m p e r a t u r e , t e m p e r a t u r e g r a d i e n t s and n a t u r e
and p r o p e r t i e s of t h e s u b s t r a t e s u r f a c e ,
7 0 - . .- ~ e c o m p o s i t i o n : The d e c o m p o s i t i o n p r o c e s s a l s o termed
p y r o l y s i s c a n be c l a s s i f i e d as h i g h t e m p e r a t u r e p y r o l y s i s
r e q u i r i n g a substrate t e m p e r a t u r e e x c e e d i n g 6 0 0 ~ ~ or a
l o w e r - t e m p e r a t u r e p r o c e s s a t t e m p e r a t u r e be tween room t
t e m p e r a t u r e and 600'~. For example i n t h e p y r o l y s i s o f
s i l a n e a t e m p e r a t u r e o f a b o u t 1 2 5 0 ~ ~ i s r e q u i r e d .
Compounds which decompose a t low t e m p e r a t u r e s i n c l u d e
m e t a l h y d r i d e s , m e t a l c a r b o n y l , most o r g a n o m e t a l l i c
compounds, m e t a l b o r o h y d r i d e s and some o f t h e more un-
s t a b l e m e t a l h a l i d e s and c a r b o n y l h a l i d e s .
n Reduc t ion : I n r e d u c t i o n ) hydrogen o r metal v a p o u r s - . are employed as r e d u c t a n t s a t lower t e m p e r a t u r e s t h a n t h e
p y r o l y s i s t e m p e r a t u r e s w h i l e m e t a l h a l i d e s , c a r b o n y l
h a l i d e s , o x y h a l i d e s or o t h e r oxygen c o n t a i n i n g compounds
are used to o b t a i n t h e m a t e r i a l to be d e p o s i t e d . A
t y p i c a l example o f t h e c h e m i c a l d e p o s i t i o n by r e d u c t i o n
i s t h e p r e p a r a t i o n of S i from t h e c o r r e s p o n d i n g h a l i d e
v a p o r s u s i n g H o r Zn a s t h e r e d u c i n g a g e n t s a c c o r d i n g to
t h e equa t ion .
SIC14 + 2H2 + S i + 4 HC1
Hydrogen h a s t h e a d v a n t a g e of n o t having ' . p remature
r e a c t i o n when premixed w i t h m e t a l h a l i d e s w h i l e m e t a l s
may c o n t a m i n a t e t h e d e p o s i t when used as a r e d u c i n g
agen t . 8
- . 4 p P o l y m e r i z a t i o n : I n t h i s t e c h n i q u e o r g a n i c and
o r g a n i c - i n o r g a n i c c o m p o s i t e s form monomers whose m o l e c u l e s
l i n k t o g e t h e r by one of t h e f o l l o w i n g a c t i v a t i o n p r o c e s s e s
(1) e l e c t r o n or i o n bombardment, (ii) I r r a d i a t i o n w i t h
l i g h t , X-rays or Gamma r a y s , (iii) electrical d i s c h a r g e i n
t h e monomer v a p o u r , ( i v ) s u r f a c e c a t a l y s i s or s u r f a c e
r e c o m b i n a t i o n of monomers having f r e e r a d i c a l s ,
2,11 Pla sma Depos i t ion : T h i s t e c h n i q u e a l s o c a l l e d t h e
glow d i s c h a r g e d e p o s i t i o n t e c h n i q u e i s e s s e n t i a l l y a
p l a s m a - a s s i s t e d CVD t e c h n i q u e , G l o w d i s c h a r g e plasma
( d c or r f ) i s used t o b reak t h e vapours up i n t o d i f f e r e n t
s p e c i e s t h a t r e a c t to d e p o s i t a s a f i l m , For example a
r e a c t i o n between t h e vapor s p e c i e s of SiH4 and NH4 y i e l d s
SIN f i lms . S i m i l a r l y a r e a c t i o n between SiH4 and H 2 0
v apou r s r e s u l t s i n t h e fo rma t ion of S i02 f i l m s .
3 . 3 2 E l e c t r o d e p o s i t i o n : The occu r r ence of chemica l
changes owing t o t h e pa s sage of electric c u r r e n t through
an e l e c t r o l y t e i s termed e l e c t r o l y s i s and t h e d e p o s i t i o n
of any s u b s t a n c e on an e l e c t r o d e is a consequence of
e l e c t r o l y s i s c a l l e d e l e c t r o d e p o s i t i o n .
3 . " '2 Electroless or A u t o c a t a l y t i c Plat ing, : S i m i l a r ,to
e l e c t r o d e p o s i t i o n , t h i s t u h n i q u e a l s o i n v o l v e s t h e
r e d u c t i o n o f metal i o n s to form d e p o s i t s b u t no e x t e r n a l
power supp ly is r e q u i r e d to p r o v i d e e l e c t r o n s . I ~ s t a ~ d
a c a t a l y t i c s u r f a c e is used to i n i t i a t e t h e d e p o s i t i o n
and then t h e m e t a l i t s e l f must be c a t a l y t i c to f u r t h e r
t h e d e p o s i t i o n . The e l e c t r o n s are prov ided by a chemica l
r e d u c i n g a g e n t i n t h e s o l u t i o n .
7 . 4 A Anodizat ion: Anodiza t ion is a f i e l d - a s s i s t e d form
of thermal growth. The me ta l to be anod ized i s made a n
anode and immersed i n an oxygen-containing e l e c t r o l y t e ,
which may be aqueous , non-aqueous or f u s e d salt . Growth
may t a k e p l a c e a t c o n s t a n t v o l t a g e or c o n s t a n t c u r r e n t .
I n t h e former case i f t h e metal is left i n t h e a n o d i z i n g
b a t h for a s u f f i c i e n t t i m e , a r e p r o d u c i b l e t h i c k n e s s
w i l l be o b t a i n e d depend ing on t h e appl ied v o l t a g e , We
c a n t h e r e f o r e d e f i n e a q u a n t i t y called t h e a n o d i z i n g
c o n s t a n t as t h e t h i c k n e s s o f t h e l a y e r grown per u n i t
v o l t a g e . To p r e v e n t t h e d i s a d v a n t a g e of h a v i n g t o p r o v i d e
h i g h c u r r e n t d e n s i t i e s d u r i n g t h e i n i t i a l g r o w t h t h e
c o n s t a n t c u r r e n t mode i s n o r m a l l y p r e f e r r e d . +
7 n r -. - E l e c t r o p h o r e s i s : I n t h i s t e c h n i q u e , e l e c t r i c a l l y *
c h a r g e d pa r t i c l e s s u s p e n d e d i n a l i q u i d medium are
d e p o s i t e d on an electrode. The a s - d e p o s i t e d f i l m s a r e
u s u a l l y l o o s e l y a d h e r e n t c o a t i n g s of powder. F u r t h e r
p o s t d e p o s i t i o n t r e a t m e n t i s r e q u i r e d to p r o d u c e a d h e r e n t
compac t and m e c h a n i c a l l y s t r o n g s u r f a c e c o a t i n g , The p o s t -
d e p o s i t i o n t r e a t m e n t u s u a l l y c o n s i s t s of p r e s s u r i z e d com-
p a c t i o n and a h e a t t r e a t m e n t t o d r y o u t traces of t h e
s u s p e n s i o n medium and s i n t e r t h e p a r t i c l e s w i t h i n t h e f i l m ,
. Melt S p i n n i n g Technique : Melt s p i n n i n g is v e r y promis-
i n g t o p r o d u c e r a p i d l y quenched r i b b o n s a t h i g h speeds of
s e v e r a l meters per second. By b r i n g i n g t h e m o l t e n material
i n a n o z z l e w i t h c o n t a c t w i t h a s p i n n i n g whee l , t h e l i q u i d
drop i s pulled out i n the form o f a ribbon, The r a t e
of s o l i f f c a t i o n of t h e m e l t and the ribbon t h i c k n e s s a r e
dependent on the n o z z l e dimensions,
CHAPTER 4
THE GROWTH OF LEAD SULPHIDE (PbS) THIN FILM ON A GLASS SUBSTRATE BY CHEMICAL DEPOSITION
OR SOLUTION GROWTH METHOD
.", 1 The Choice o f S o l u t i o n Growth Method,
The method i s c h o s e n b e c a u s e o f t h e f o l l o w i n g r e a s o n s ,
A t p r e s e n t , t h e method i s a t t r a c t i n g c o n s i d e r a b l e a t t e n t i o n
i n t h a t i t o f f e r s t h e a d v a n t a g e s o f (i 1 economy ( t h e r e * i s a d r a s t i c c u t i n t h e cost of p r o d u c t i o n ; h e n c e mass
p r o d u c t i o n i s p o s s i b l e a t r e a s o n a b l y c h e a p c o s t ) , ( i i ) con-
v e n i e n c e , s i n c e t h e e x p e r i m e n t c a n be e a s i l y c a r r i e d o u t
i n t h e l a b o r a t o r y a t room t e m p e r a t u r e s , (iii) u n i f o r m i t y
i n t h e f i l m t h i c k n e s s and t h e r e f o r e t h e t h i c k n e s s o f a n y
f i l m d e p o s i t e d c a n e a s i l y be d e t e r m i n e d by t h e g r a v i m e t r i c
method u s i n g s i m p l e and modes t equ ipmen t ( i v ) c a p a c i t y f o r
l a r g e a r e a d e p o s i t i o n and h i g h m a t e r i a l u t i l i z a t i o n ,
The C h e m i c a l s / R e a g e n t s Used.
The r e a g e n t s u s e d a r e l e a d n i t r a t e Pb(N03)2 which
r e l e a s e s l e a d i o n s ( p b 2 + ) , t h e t h i o u r e a ~ z c ( N H ~ which
2- i s t h e s o u r c e o f s u l p h u r i o n s ( S 1, t h e sodium t h i o s u l p h a t e
( N ~ ~ S ~ O ~ . S H ~ O ) which I u s e d as t h e complexing a g e n t f o r t h e
c o n t r o l and t h e g r a d u a l r e l e a s e o f t h e l e a d i o n s ( p b 2 + ) and
sodium h y d r o x i d e ( N ~ O H ) u s e d to p r o v i d e t h e optimum pH v a l u e
1 am-
Research work in progress a t t h e 1 -
Solid State/Matsriulm Science laboratory,
.m,. Fig. 6 (J.:. ::sure shol#;s o n e c o n v e n t i o n a l
Zdce c e n t r e d ruhic (fccl c e l l of Sodium
Chloride c r y s t a l s t r a c t u r e . I n the L r c L v l F i ,
Fi3. 6 L s just a m c l c i e l of sod ium C h l o r i d e .
The s c ~ d i u - i.or:s ;Ire smc;ller t h ~ r ! the C h l o r i n e
i o n s . ( C o u r t e s y c.f A.2. i!c.lder. a n d P. S i n g e r ,
from crystals a n d c r y s t a l g r o w i n g ) .
Fig, ' 7a and b' show'
t h e c r y s t a l s t r u c t u r e
of galena (PbS) which
h a s t h e NaCl c r y s t a l
s t r u c t u r e , Nhile t h e
cube e d g e a of (PbSl
i s 5.328 kh$k of NaCl
Fig. " 7a and b. show'
t h e c r y s t a l d z r u c t u r e
of g a l e n a (PbS) which
h a s t h e NaCl c r y s t a l
s t r u c t u r e . While t h e c u b e e d g e a of ( P ~ s )
is 5.32R t h a t of N a C l
is 5.632.
F i g . 7c
Na2S203.5H20 t o 50 rnls of . 1 M P b ( ~ 0 ~ ) ~ c o n t a i n e d i n a
s u c r u p u l o u s l y c l e a n e d b i g b e a k e r (500 m l s , 1, Some w h i t e
p r e c i p i t a t e was fo rmed which r e f u s e d to d i s o l v a e v e n a f t e r
s h a k i n g , 250 m l s o f d i s t i l l e d w a t e r was added to t h i s
clear s o l u t i o n and t h e n f i l t e r e d u s i n g c l e a n w h i t e b l o t t e n
p a p e r , When 30 m l s o f . 1 M t h i o u r e a was added to t h e
s o l u t i o n , t h e s o l u t i o n a f t e r s t i r r i n g and s h a k i n g v e r y w e l l ,
s t i l l r ema ined c l e a r , A s soon as a b o u t 1 0 m l s o f 1 , M
NeOH were added w h i t e p r e c i p i t a t e was formed which dot d i s o l v e d a f t e r s h a k i n g , From t h i s p r e p a r e d clear s o l u t i o n
i n t h e l a r g e 500 m l b e a k e r p o r t i o n s were q u i c k l y t r a n s f e r e d
t o f o u r small b e a k e r s c o n t a i n i n g v e r t i c a l l y c lamped
s u c r u p u l o u s l y c l e a n e d g l a s s s l i d e s . The s o l u t i o n s s t a r t e d
t o t u r n y e l l o w brown a f t e r a b o u t 20 m i n u t e s showing t h a t
r e a c t i o n h a s s t a r t e d , The pH o f t h e s o l u t i o n was a b o u t
1 0 t a k e n b y t h e pH meter i n t h e l a b o r a t o r y b e c a u s e p r e v i o u s
e x p e r i m e n t s showed t h a t pH o f 1 0 was most f a v o u r a b l e f o r
best d e p o s i t s , After SO m i n u t e s t h e s o l u t i o n t u r n e d d a r k
brown and v e r y d a r k a f t e r a t o t a l time o f 1 h r , However
t h e f i r s t g l a s s s l i d e was removed a f t e r 50 m i n u t e s , f o l l o w e d
b y t h e 2nd a f t e r 1 hr . , t h e n t h e 3 r d a f t e r 1 hr . 10 mins ,
a n d l a s t l y t h e l a s t a f t e r 1 h r . 20 mins , The g l a s s s l i d e s
were l a b e l l e d A, B, C and D r e s p e c t i v e l y .
The G l a s s S l i d e s : B e f o r e c l amping t h e g l a s s s l i d e s ,
t h e s l i d e s and t h e s m a l l g l a s s b e a k e r s were d i p p e d i n
c o n c e n t r a t e d s t r o n g h y d r o c h l o r i c a c i d , washed w i t h Omo
s o a p and c l e a n w a t e r and f i n a l l y r i n s e d w i t h d i s t i l l e d
w a t e r and t h e n d r i e d . After t h e d e p o s i t s were made, t h e
now l o a d e d g l a s s s l i d e s were r i n s e d a g a i n w i t h d i s t i l l e d
water and d r i e d . When t h e c o n d u c t i v i t i e s w e r e t e s t e d w i t h
t h e K e i t h l e y 1 7 3 A m u l t i m e t e r i n t h e l a b o r a t o r y t h e y were *
a l l c o n d u c t i n g t h e d e p o s i t on s l i d e A showing t h e g r e a t e s t
r e s i s t a n c e o f a b o u t 200 k fl w h i l e t h a t on D g l a s s s l i d e
s h o e d t h e least r e s i s t a n c e of a b o u t 46 k J L . 2nd Obser-
v a t i o n : I o b s e r v e d t h a t d e p o s i t s were made on b o t h s i d e s
o f e a c h s l i d e and on t h e w a l l s and bo t tom o f e a c h b e a k e r
a n d t h a t t h e s i d e s o f t h e b e a k e r a r e s p e c u l a r l y r e f l e c t i n g .
One c a n e a s i l y see t h r o u g h , f rom t h e i n s i d e of t h e b e a k e r s
b u t can n e v e r see a n y t h i n g p l a c e d i n s i d e t h e b e a k e r f rom
o u t s i d e ,
C h e m i s t r y o f R e a c t i o n : When t h e l i g a n d o t h e r w i s e
c a l l e d t h e complex ing a g e n t was added to t h e l e a d n i t r a t e
which r e a d i l y g o t d i s o l v e d i n d i s t i l l e d w a t e r t h e sodium
t h i o u s u l p h a t e formed complex i o n s w i t h l e a d p b ( N ~ ~ S ~ O ~ )+T2*.
The w h i t e p r e c i p i t a t e c o u l d d i s o l v e i n e x c e s s sodium
t h i o s u l p h a t e b u t t h e n , t h e r a t e of r e a c t i o n i ,e, t h e r a t e
a t which l e a d i o n s a r e r e l e a s e d w i l l be s o f a s t t h a t good
f i lms c a n h a r d l y be formed. S i m i l a r l y if t h e pH of t h e
f i n a l s o l u t i o n i s more t h a n 10,O t h e n t h e r a t e a t which
s u l p h u r i o n s are r e l e a s e d from t h i o u r e a w i l l be so f a s t
t h a t b e a u t i f u l f i l m s c a n h a r d l y be formed slso. However,
a b a l a n c e h a s to be s t r u c k whe re t h e s o l u b i l i t y p r o d u c t
i s a l w a y s less t h a n t h e i o n i c p r o d u c t . I.P. > S.P.
The e q u a t i o n o f t h e whole r e a c t i o n i s a s f o l l o p s :
- PbS + 2Na2S2o3 + O C ( N H ~ ) ~ + H20
T h i r d O b s e r v a t i o n : It was o b s e r v e d t h a t a t t e m p t s to u s e
t r i e t h a n o l a m i n e ( T E A ) a s t h e complex ing a g e n t were n o t as
good a s u s i n g Sodium t h i o u s u l p h a t e and E t h y l e n e d i a m i n e t e -
t racet ic Acid (EDTA) d i d n o t g i v e a n y p o s i t i v e r e s u l t a t
a l l . A s l e a d n i t r a t e i s r e a d i l y s o l u b l e i n d i s t i l l e d water)
i t i s n o t a s e a s y to meet t h e c o n d i t i o n I.P. 7 SoPo a s i t
would h a v e b e e n i f i t were t o be weak ly s o l u b l e i n d i s t i l l e d
w a t e r . As a r e s u l t , care was t a k e n t h a t more t h a n enough
of t h e w h i t e p r e c i p i t a t e was n o t d i s o l v e d i n e x c e s s
t h i o s u l p h a t e , Also t h e d i s o l v e d sodium Hydrox ide , whose
i n i t i a l t e m p e r a t u r e w a s a s h i g h a s 40°c was a l l o w e d to cool
to room temperature , a s i n c r e a s e i n temperature i n c r e a s e s
t h e r a t e of r e l e a s e of lead i o n s or r a t e of r e a c t i o n .
The r e s u l t g o t from t h e u s e of Ammonia s o l u t i o n was n o t
a s good as t h a t o b t a i n e d u s i n g sodium hydrox ide .
CHAPTER 5
CHARACTERIZATION AND CONCLUSION
The o p t i c a l a b s o r p t i o n measurements of t h e f i l m s
were t a k e n w i t h a P y r e Unicam SP 8000 U l t r a v i o l e t
R e c o r d i n g ( B u e h l e r I s o m e t s p e c t r o p h o t o m e t e r and i n d o i n g
t h i s a s i m i l a r b l a n k g l a s s s l i d e was u s e d as a r e f e r e n c e .
T h a t means t h a t w h a t e v e r t h e v a r i a t i o n s of t h e a b s o r b a n c e +
w i t h t h e w a v e l e n g t h s o f t h e i n c i d e n t p h o t o n s , i t mus t be
a s a r e s u l t o f t h e f i l m d e p o s i t e d on t h e s l i d e , The
o p t i c a l band g a p s f o r t h e f i l m s were c a l c u l a t e d knowing
t h e i r a b s o r p t i o n e d g e e x t r a p o l a t e d f rom t h e a b s o r p t i o n
s p e c t r a . The a b s o r p t i o n c o e f f i c i e n t s o(' a t d i f f e r e n t
w a v e l e n g t h s were computed and a g r a p h of t h e d p l o t t e d
a g a i n s t pho ton e n e r g y to g e t t h e band gap,
5.1 Theory, The a b s o r p t i o n c o e f f i c i e n t a lso cal led
a b s o r p t i o n f a c t o r i s d e f i n e d a s t h e r a t i o of t h e i n t e n s i t y
loss by t h e a b s o r p t i o n t o t h e t o t a l i n t e n s i t y of r a d i a t i o n .
I f 10 r e p r e s e n t s t h e o r i g i n a l i n t e n s i t y , Ir t h e i n t e n s i t y
o f t h e r e f l e c t e d r a d i a t i o n , I t t h e i n t e n s i t y o f t r a n s m i t t e d
r a d i a t i o n t h e n , t h e a b s o r p t i o n c o e f f i c i e n t i s g i v e n by
t h e e x p r e s s i o n
d . = 10 - (Ir + I t )
I0 .
From t h e r e a d i n g s of t h e a b s o r b a n c e and t r a n s -
m i t t a n c e i n p e r c e n t a g e s g i v e n by t h e s p e c t r o p h o t o m e t e r ,
i t c a n be shown t h a t most of t h e r a d i a t i o n i s t r a n s -
m i t t e d w h i l e t h e a b s o r b e d r a d i a t i o n is least ( n o t up
t o 1% i s a b s o r b e d ) ,
From t h e above e q u a t i o n i t f o l l o w s t h a t t h e 4
a b s o r b e d r a d i a t i o n i s g i v e n by
Ia = 10 - (Ir + I t )
Now t h i s v a l u e o f a b s o r p t i o n c o e f f i c i e n t i s p e r
u n i t wave leng th ( - lo-' nm) b u t t h e v a l u e s of to b e
p l o t t e d a g a i n s t t h e i n c i d e n t pho ton e n e r g y are t o b e
p e r u n i t metre and to g e t t h e s e , w e d i v i d e e a c h v a l u e
o f t h e p e r c e n t a g e a b s o r p t i o n c o e f f i c i e n t by i t s corres-
ponding w a v e l e n g t h ( i n n a n o m e t e r s ) . Thus, t h e r e q u i r e d
a b s o r p t i o n c o e f f i c i e n t i s g i v e n by
T h e r e f o r e knowing Ie which is n o t up to I%> , t h e
w a v e l e n g t h and t h e t o t a l i n t e n s i t y , I o which i s 100 i n
Table 8
Table F o r F i l m A
Wave- 1 eng t h
( n m > 300
3 10
320
330
340
350
360
370
380
390
400
4 10
420
430
440
450
460
470
480
490
500
510
520
530
5 40
550
A b s o r b a n c e ( arb i t rary
units
0m30
0.40
( 5 5 )
ABSORBANCE VS WAVELENGTH FOR FI LM A
T a b l e <: T a b l e F o r F i l m B
Wave- l e n g t h
(nm)
300
3 10
320
330
340
350
360
370
380
390
400
410
420
430
440
450
460
470
480
490
500
5 10
520
530
540
550
A b s o r b a n c e ( a r b i t r a r y
u n i t s
ABSORBANCE VS WAVELENCT FOR FILM B
Table 5 Table For F i l m C
Wave- 1 eng th
(nm)
Absorbance ( a r b i t r a r y
u n i t s 1
( 5 9 )
ABSORBANCE VS WAVELENGTH FOR FILM C > -
Y
Wavelength [ A ] in nm.
T a b l e 6 T a b l e For F i lm D
Jave- . eng t h : nrn)
b s o r b a n c e a r b i t r a r y
u n i t s
0.65
0.75
0.90
0.95
0.94
0.90
0.84
0.77
0.70
0.64
0.58
0. 53
0.48
0,44
0,40
0.37
0.34
0.32
0.30
0.28
0.27
0.26
0.25
0.24
0.23
0.23
0.22
0.21
1
Ene rgy o f I n c i d e n t r a d i a t i o n h (eV)
A b s o r p t i o n I
c o e f f i c i e n t
(611
ABSORBANCE VS WAVELENGTH FOR FILM 0
Wavelength [A] in nm ---
Incident radiation energy h$ ( r v )
p e r c e n t a g e , t h e a b s o r p t i o n c o e f f i c i e n t d c a n b e c a l -
c u l a t e d and t a b u l a t e d .
5 .2 C a l c u l a t i o n : From t h e g r a p h s i n f i g s . 8~~8B98.C: a n d 8D
we h a v e t h e same v a l u e o f t h e a b s o r p t i o n e d g e wave-
l e n g t h f o r t h e f o u r f i l m s d e p o s i t e d . T h a t means t h a t ,
t h e e n e r g y band g a p f o r t h e m a t e r i a l l e a d s u l p h i d e
( P b S ) , d e p o s i t e d on t h e f o u r g l a s s s l i d e s as s u b s t r a t e s , t
i s t h e same as shown i n t h e c u l c u l a t i o n below.
A b s o r p t i o n band e d g e w a v e l e n g t h = 512.5 nm
The c o r r e s p o n d i n g f r e q u e n c y i s g i v e n by
The band g a p i n ( e V ) i s g i v e n by
w h e r e c and h are t h e v e l o c i t y o f l i g h t a n d P l a n c k ' s
c o n s t a n t r e s p e c t i v e l y .
E x t r a p o l a t i n g t h e s t r a i g h t l i n e p o r t i o n o f t h e o(
2 V s h S a n d v s h d i n f i g s . 9 A and 9B, t h e v a l u e s
o f Eq a r e 2.4 e V a n d 2.71 e V r e s p e c t i v e l y . .- - -
5 , ? A n a l y s i s o f R e s u l t s :
The c a l c u l a t e d v a l u e o f t h e e n e r g y b a n d g a p d o e s n o t
a g r e e w i t h t h e v a l u e known and q u o t e d i n t h e l i t e r a t u r e B
a s 0.37 e V . me d i f f e r e n c e i s a t t r i b u t a b l e t o t h e f a c t s
t h a t (i d u r i n g t h e d e p o s i t i o n t h e r e a g e n t s m i g h t h a v e
b e e n c o n t a m i n a t e d ( ii) t h e o n l y a v a i l a b l e s p e c t r o p h o t o -
meter i n good o r d e r , u s e d , o p e r a t e s o n l y i n t h e u t r a -
v i o l e t r a n g e , The v a l u e s would h a v e a g r e e d b e t t e r i f
a s p e c t r o p h o t o m e t e r o f t h e i n f r a - r e d r a n g e h a d b e e n
u s e d b e c a u s e t h e l o w p h o t o n e n e r g y o f t h e i n c i d e n t
r a d i a t i o n would h a v e ma tched t h e low band g a p o f 0.37 e V .
However, p h y s i c a l l y , t h e c o l o u r o f t h e d e p o s i t e d
s p e c u l a r l y r e f l e c t i n g f i l m i s d e e p d a r k brown o r b l a c k
r e m i n i s c e n t o f PbS and when c l e a n e d o f f w i t h c o n c e n t r a t e d
h y d r o c h l o r i c a c i d t h e g a s e v o l v e d was v e r y p u n g e n t l i k e
t h a t o f a r o t t e n e g g s u g g e s t i n g a h y d r o g e n s u l p h i d e gas
b e i n g r e l e a s e d , On t h e w h o l e t h e r e f o r e , i t c a n b e d e d u c e d
t h a t l e a d s u l p h i d e f i l m h a s been s u c c e s s f u l l y d e p o s i t e d
b y t h e s o l u t i o n me thod ,
F u r t h e r m o r e , t h e f a c t t h a t t h e same v a l u e s of t h e
b a n d g a p f o r a l l t h e f o u r f i l m s A, 5, C a n d D d e p o s i t e d ob t a i s f &
were e v e n d i f f e r e n t m e t h o d s of c a l c u l a t i o n , t h e
non a g r e e m e n t o f t h e d e t e r m i n e d b a n d g a p w i t h t h a t f o u n d
i n t h e l i t e r a t u r e i s b a s i c a l l y d u e t o t h e c h o i c e of
s p e c t r o p h o t o m e t e r a n d n o t b e c a u s e of t h e i m p u r i t y c o n t e n t
of t h e d e p o s i t e d l e a d s u l p h i d e , The f i l m s are h o d e v e r ,
n o t a m o r p h o u s b u t e p i t a x i a l b e c a u s e when a g r a p h of
( 4 h 3 a g a i n s t h 3 w a s p l o t t e d n o r e a s o n a b l e g r a p h
w a s o b t a i n e d ,
I: . - ,8 Uses of Lead S u l p h i d e F i lm.
T h i s i s f o u n d s u i t a b l e a s a b s o r b e r - a n t i r e f l e c t i o n
c o a t i n g f o r s e l e c t i v e s u r f a c e s . A p a r t f r o m t h i s a n d
from t h e p h y s i c a l a p p e a r a n c e o f t h e f i l m w h i c h i s v e r y
s p e c u l a r l y r e f l e c t i n g , i t c a n be u s e d to t i n t g l a s s e s
o f v e h i c l e s a n d h o u s e s . T h i s i s b e c a u s e i f t h e i n s i d e
s u r f aces o f t h e g l a s s e s are f i l m e d , t h o s e p e o p l e w i t h i n
c a n see p e o p l e o u t s i d a w i t h o u t b e i n g s e e n . Also s i n c e
v e r y small q u a n t i t y of c h e m i c a l s i s u s e d to make l a r g e
s o l u t i o n s f o r d e p o s i t s , i t may e v e n be more e c o n o m i c a l
to p r o d u c e s p h e r i c a l a n d p l a n e mirrors b y d e p o s i t i n g s u c h
f i l m s t h a n s i l v e r i n g them, I m a g e s p r o d u c e d i n g l a s s
s l i d e s w i t h d e p o s i t e d f i l m s a r e b e a u t i f u l a n d clear,
Recommenda t ions f o r F u r t h e r Read ing .
I t i s i n t e r e s t i n g t o n o t e t h a t s i n c e w e a r e b l e s s e d
w i t h a b u n d a n t f ree s o l a r e n e r g y , b e i n g v e r y close t o t h e
e q u a t o r , a l l h a n d s m u s t b e on d e c k to r e s e a r c h i n t o
v a r i o u s ways o f t a p p i n g t h i s e n e r g y fo r a p o s s i b l e a n d
e f f e c t i v e r e p l a c e m e n t o f t h e w o r l d w e l l known m a j o r
e n e r g y r e s o u r c e - p e t r o l e u m t h r e a t e n e d b y w o r l d p o p u l a t i o n + e x p l o s i o n . S p e c i f i c a r e a s o f r e s e a r c h s h o u l d i n c l u d e
d e v e l o p m e n t o f f i l m s o f a l l c h a l c o g e n i d e s and h a l o g e n s
of m e t a l s and a l k a l i m e t a l s e s p e c i a l l y t h e cadmium s u l p h i d e
f o r p h o t o v o l t a i c cells. Of s p e c i a l i n t e r e s t i s t h e
d e v e l o p m e n t o f h a l o g e n s of a l k a l i metals known to
p o s s e s s e n v i a b l e p r o p e r t y o f s u p e r i o n i c c o n d u c t i v i t y
i n t h i n forms, A p o s s i b l e b r e a k t h r o u g h i n s u p e r -
c o n d u c t i v i t y i n m e t a l s c a n a l so b e p o s s i b l e if t h i n
f i l m s o f some metals l i k e z i n c a r e d e p o s i t e d o n c e r t a i n
s u b s t r a t e s , T h i s i s b e c a u s e t h e t r a n s i t i o n t e m p e r a t u r e
i s u s u a l l y r a i s e d when d e p o s i t i o n i s a c h i e v e d . L a s t
b u t n o t t h e l e a s t , d o p i n g p r o f i l e s or m u l t i p l e l a y e r t r i e d by
s t r u c t u r e s i n c l u d i n g h e t e r o j u n c t i o n s o f f i l m s s h o u l d be L s u i t a b l y m o d i f y i n g s o l u t i o n c o m p o s i t i o n a n d c o n c e n t r a t i o n .
5.6 C o n c l u s i o n .
S i n c e f r o m t h e a n a l y s i s o f r e s u l t s , w e h a v e s e e n
t h a t t h e p h y s i c a l a p p e a r a n c e o f t h e d e p o s i t e d l e a d
s u l p h i d e ( P b S ) t h i n f i l m d e e p d a r k brown i n c o l d u r
c o n f o r m s w i t h b u l k p h y s i c a l f o r m o f G a l e n a a n d a l s o
r e l e a s e s h y d r o g e n s u l p h i d e when r e a c t e d w i t h h y d r o -
c h l o r i c a c i d , I c a n c o n c l u s i v e l y s a y t h a t I h a v e
s u c c e s s f u l l y d e p o s i t e d a t h i n f i l m o f l e a d s u l p h i *
w i t h i n l i m i t s o f e x p e r i m e n t a l error. With a s p e c t r o -
p h o t o m e t e r o f t h e i n f r a - r e d r a n g e , t h e o p t i c a l e n e r g y
b a n d g a p c a n b e c a l c u l a t e d t o a g r e e w i t h t h e v a l u e
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