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A STUDY OF A NEW HETEROJUNCTION MADE OF n-TYPE SILICON AND p-TYPE AMORPHOUS
SEMICONDUCTOR
Y. Sawan, M. El-Gabaly, F. Wakim, S. Atari
To cite this version:
Y. Sawan, M. El-Gabaly, F. Wakim, S. Atari. A STUDY OF A NEW HETEROJUNCTION MADE
OF n-TYPE SILICON AND p-TYPE AMORPHOUS SEMICONDUCTOR. Journal de Physique
Colloques, 1981, 42 (C4), pp.C4-511-C4-514. �10.1051/jphyscol:19814109�. �jpa-00220724�
JOURNAL DE PHYSIQUE
CoZZoque C4, suppZ6ment au nOZO, Tome 42, octobre 1381
A STUDY O F A NEN HETEROJUNCTION MADE OF h-TYPE S I L I C O N AND p-TYPE AMORPHOUS SEMICOKDUCTOR
Y. Sawan, M. El-Gabaly, F. Wakim and S. Atari University of Kuwa;t, Kuwait
A b s t r a c t . - A p-n h e t e r o j u n c t i o n was p r e p a r e d by d e p o s i t i n g a t h i n f i l m o f p - t y p e amorphous As2Se3Cux ( X = 0 . 2 , 0 . 4 and 1 . 0 ) on n - t y p e c r y s t a l l i n e s i l i c o n w a f e r s . The I - V c h a r a c t e r i s t i c s o f t h e j u n c t i o n were t y p i c a l o f t h o s e of p-n j u n c t i o n s b o t h i n t h e d a r k and under i l l u m i n a t i o n . The p r e l i m i n a r y optimum c h a r a c t e r i s t i c s were o b t a i n e d w i t h an a n n e a l e d p-type amorphous A S ~ S ~ ~ C U ~ , ~ w i t h an o p t i c a l gap of 1 . 4 3 eV. A maximum open c i r c u i t v o l t a g e (Voc) of 0 . 3 7 V and s h o r t c i r c u i t c u r r e n t ( I s c )
2 2
o f 10 mA/cm was o b s e r v e d under s o l a r r a d i a t i o n o f 800 w/m
.
I n t r o d u c t i o n . - I n t h e amorphous A s , S e , Cu s y s t e m it was found 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 c a n be v a r i e d from t h a t o f an i n s u l a t o r t o a m e t a l depending on t h e c o n c e n t r a t i o n o f c o p p e r ( 1 ) . These changes i n e l e c t r i c a l c o n d u c t i v i t y a r e accompanied by s u b s t a n t i a l changes i n t h e m o b i l i t y gap which s u g g e s t s t h a t one can choose any c o n d u c t i v i t y and m o b i l i t y gap t h a t one needs o v e r a r e l a t i v e l y wide r a n g e . I n a d d i t i o n , a l l t h e c o m p o s i t i o n s s t u d i e d i n t h i s system were p-type s e m i c o n d u c t o r s .
Due t o t h e s e e n c o u r a g i n g p r o p e r t i e s , we a t t e m p t e d t o check t h e f e a s i b i l i t y o f making a s o l a r c e l l of c h a l c o g e n i d e As2Se3Cux g l a s s on c r y s t a l l i n e n - s i l i c o n h e t e r o j u n c t i o n . Not a l l t h e p a r a m e t e r s of such a j u n c t i o n can be o p t i m i z e d due t o t h e g r e a t number o f v a r i a b l e t h a t one e n c o u n t e r s . These i n c l u d e t h e c o n c e n t r a t i o n o f c o p p e r ( v a l u e of
X ) , c h a l c o g e n i d e f i l m t h i c k n e s s , t y p e of c o n t a c t , h e a t t r e a t m e n t and o t h e r s . I n t h i s p a p e r , o n l y a few p a r a m e t e r s were checked and t h e r e s u l t s were e n c o u r a g i n g b u t n o t i d e a l .
P r e p a r a t i o n and P r o p e r t i e s o f As2Se3Cux.- A p p r o p r i a t e r a t i o s o f h i g h p u r i t y A s , Se and Cu were h e a t e d i n f u s e d s i l i c a a t 9 5 0 ~ ~ under vacuum f o r a b o u t 6 h o u r s . D e t a i l e d d e s c r i p t i o n o f p r e p a r a t z o n condi- t i o n s was d e s c r i b e d e l s e w h e r e ( 2 ) . S i n c e a l l t h e t h r e e p r e p a r e d compo- s i t i o n s As2Se3Cux ( X = 0 . 2 , 0 . 4 and 1 . 0 ) c o n t a i n e d c o p p e r l e s s t h a n 28 a t . % , t h e y were a l l o b t a i n e d i n t h e amorphous s t a t e . 6 ) .
Sandwich e l e c t r o d e c o n f i g u r a t i o n w i t h two ohmic c o n t a c t s was used t o i n v e s t i g a t e t h e dark c o n d u c t i v i t i e s i n t h e t e m p e r a t u r e r a n g e 70 K up t o 400 K a t low e l e c t r i c f i e l d s o f 10-100 V/cm. Photoacousti'c t e c h n i q u e u s i n g powdered samples a s w e l l a s o p t i c a l a b s o r p t i o n t e c h - n i q u e f o r amorphous f i l m s were used t o measure t h e o p t i c a l gap Eg
( E c
-
E v ) . The r e s u l t s o f d a r k c o n d u c t i v i t y and o p t i c a l measurements f o r t h e i n v e s t i g a t e d amorphous m a t e r i a l s a r e g i v e n i n t h e f o l l o w i n gArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814109
JOURNAL DE PHYSIQUE
t a b l e .
No. Composition a t % - l o g o / E o p t i c a l E Thermal copper 20°c (Ec
-
Ev) (Ef-
E v )S i n c e a l l copper c o n t a i n i n g samples a r e p - t y p e , t h e main c u r r e n t i s c a r r i e d o u t by h o l e s and t h e r m a l a c t i v a t i o n e n e r g y i s g i v e n by (Ef-EJ i n t h e e x p r e s s i o n :
A l l t h e c o n d u c t i v i t y v e r s u s t e m p e r a t u r e c u r v e s were found t o e x h i b i t one s l o p e i n t h e t e m p e r a t u r e r a n g e s t u d i e d . The c h o i c e of t h e s e com- p o s i t i o n s f o r t h e p r e p a r a t i o n o f t h e h e t e r o j u n c t i o n was d e c i d e d because t h e i r o p t i c a l gaps match w e l l w i t h t h e maximum s o l a r d i s t r i - b u t i o n and a t t h e same t i m e t h e i r e l e c t r i c a l c o n d u c t i v i t i e s a r e i n t e r - mediate between h i g h l y c o n d u c t i n g doped semiconductors and poor con- d u c t i n g amorphous m a t e r i a l s . A t t h e same t i m e s i n c e t h e amorphous f i l m w i l l s e r v e a s a window i n t h e h e t e r o j u n c t i o n , i t s o p t i c a l gap must be g r e a t e r t h a n t h e o p t i c a l gap o f t h e n-Si a b s o r b e r C 3 ) . P r e p a r a t i o n o f t h e H e t e r o j u n c t i o n s . - N-type c r y s t a l l i n e s i l i c o n 200.1.1m w a f e r s ( p = 1-3 ohm-cm) were e t c h e d and c l e a n e d w i t h t h e p r o p e r r e a g e n t s and t h e n d r i e d . A t h i c k aluminum f i l m was d e p o s i t e d on one s u r f a c e of t h e w a f e r s which s e r v e d a s t h e back c o n t a c t . The w a f e r s w i t h t h e aluminum c o n t a c t were a n n e a l e d i n a n i t r o g e n atmosphere a t
2 0 0 ~ ~ f o r a b o u t 30 m i n u t e s . The o t h e r s u r f a c e o f t h e s i l i c o n w a f e r s was a g a i n e t c h e d and c l e a n e d . T h i s was f o l l o w e d by d e p o s i t i n g amor- phous As3Se3Cux ( X - = 0 . 2 , 0 . 4 and 1 . 0 ) f i l m whose t h i c k n e s s ranged from 300-to 3000
X.
A t h i n t r a n s p a r e n t g o l d f i l m was t h e n d e p o s i t e d on t h e amorphous m a t e r i a l f o l l o w e d by a g o l d g r i d which s e r v e d a s a f r o n t c o n t a c t . I - V c h a r a c t e r i s t i c s f o r t h e p r e p a r e d h e t e r o j u n c t i o n s were p l o t t e d b o t h i n t h e d a r k and under i l l u m i n a t i o n . I n l a b o r a t o r y measurements a xenon lamp (1 KW power) was used a s a l i g h t s o u r c e , b u t i n some o t h e r measuremnts d i r e c t s o l a r r a d i a t i o n was used whose i n t e n s i t y was a b o u t 800 w/n2.The I - V C h a r a c t e r i s t i c s . - Although t h e I - V c h a r a c t e r i s t i c s v a r i e d from d e v i c e t o d e v i c e , a l l showed t h e same g e n e r a l b e h a v i o r . The I - V c h a r a c t e r i s t i c s were found t o be h i g h l y dependent on t h e composition of t h e amorphous window and i t s t h i c k n e s s . The b e s t i l l u m i n a t e d I - V c h a r a c t e r i s t i c s o f t h e h e t e r o j u n c t i o r s w i t h p-type amorphous l a y e r w i t h minimum copper c o n t e n t (sample 1 i n t a b l e w i t h Eg=1.5 eV and p/20°c = 5 x 1 0 8 ohm-cm) gave an open c i r c u i t v o l t a g e o f 0.26 V and s h o r t c i r c u i t c u r r e n t of 2 . 7 mA/cm 2 under l a b o r a t o r y c o n d i t i o n s (2400 w / m 2 ) . The t h i c k n e s s o f t h e amorphous l a y e r i n t h i s j u n c t i o n was a b o u t 1200 A . 0 The h e t e r o j u n c t i o n c o n t a i n i n g t h e maximum copper c o n t e n t amorphous l a y e r (sample 3 i n t a b l e w i t h Eg = 1 . 3 eV and
p / 2 0 ° ~ = 4 X 1 0 4 ohm-cm) gave under t h e same l a b o r a t o r y i l l u m i n a t i o n c o n d i t i o n s Voc = 0.24 and Isc = 2.4 mA/cm2. The t h i c k n e s s o f t h e amorphous l a y e r i n t h i s j u n c t i o n was t h e same a s i n t h e j u n c t i o n w i t h minimum Cu c o n t e n t . The b e s t i l l u m i n a t e d I - V c h a r a c t e r i s t i c s were n o t i c e d f o r t h e h e t e r o j u n c t i o n - w i t h p-type amorphous l a y e r w i t h 7 . 4 a t . % Cu (sample 2 i n t a b l e ) w i t h Eg = 1 . 4 3 eV and p/20Qc = 8 X 106 ohm-cm and a t h i c k n e s s r a n g i n g between 1200
-
1500 A . 0 These c h a r a c - t e r i s t i c s a r e g i v e n i n t h e f o l l o w i n g f i g u r e under l a b o r a t o r y i l l u m i n a - t i o n c o n d i t i o n s .(Volt
1
F i g u r e . - The d a r k ( d ) and i l l u m i n a t e d ( i ) I - V c h a r a c t e r i s t i c s of t h e p-type amorphous A S ~ S ~ ~ C U ~ . ~ -n-type c r y s t a l l i n e s i l i c o n b e f o r e a n n e a l i n g ( c u r u e s l ( d ) and l ( i ) and a f t e r a n n e a l i n g ( c u r v e s 2 ( d ) , 2 ( i ) and 3 ( i ) ) .
Area o f t h e sample = 2 . 7 cmL L i g h t i n t e n s i t y = 2400w/m 2
JOURNAL DE PHYSIQUE
I n t h i s f i g u r e t h e d a r k and i l l u m i n a t e d I - V c h a r a c t e r i s t i c s a r e g i v e n by c u r v e s l ( d ) and l ( i ) r e s p e c t i v e l y . From t h e s e c u r v e s we n o t i o e d
2 2
t h a t Voc=0.37 V and I s c = 2 3 mA/2.7cm =8.5 mA/cm
.
We b e l i e v e t h a t - -p r o p e r t h e r m a l a n n e a l i n g o f t h e j u n c t i o n may improve t h e I s c . When t h i s sample was a n n e a l e d f o r a maximum p e r i o d o f 30 min. a t 1 0 0 ~ ~ i n n i t r o g e n atmosphere we n o t i c e d l a r g e changes i n t h e I - V c h a r a c t e r i s - t i c s . T h i s i s g i v e n by c u r v e s 2 ( d ) and 2 ( i ) i n t h e f i g u r e f o r d a r k and i l l u m i n a t e d c h a r a c t e r i s t i c s r e s p e c t i v e l y . The i l l u m i n a t e d Voc was t h e same a s b e f o r e a n n e a l i n g b u t t h e I i n c r e a s e d by a f a c t o r
S C
of 3.7 a f t e r a n n e a l i n g f o r 30 min. The v a l u e o f t h e s h o r t c i r c u i t
-
c u r r e n t was 83 mA/2.7 cm = 30.7 mA/cmL
.
When t h i s sample was checked under s o l a r r a d i a t i o n o f 800 w/m2 i n t e n s i t y t h e Voc was .37 V and Isc = 10 mA/cm 2.
T h i s was t h e b e s t c h a r a c t e r i s t i c s o b t a i n e d f o r a l l t h e s t u d i e d h e t e r o j u n c t i o n s . F u r t h e r t h e r m a l a n n e a l i n g of t h i s sam- p l e f o r 30 min. a t 200°C i n n i t r o g e n atmosphere was c a r r i e d . A l a r g e d e c r e a s e i n t h e V o c a s w e l l a s i n I s c was n o t i c e d ( c u r v e 3 ( i ) i n f i g u r e ) .C o n c l u s i o n . - From t h e s e r e s u l t s , it seems f e a s i b l e t o make a hetero- j u n c t i o n s o l a r c e l l of p - t y p e amorphous As2Se3Cux and n - t y p e s i l i c o n . The i d e a l c e l l c h a r a c t e r i s t i c s w i l l depend on many v a r i a b l e s t h a t have t o be o p t i m i z e d i n d e p e n d e n t l y and c o l l e c t i v e l y , which r e q u i r e e x t e n d e d r e s e a r c h on t h i s s u b j e c t .
T h i s r e s e a r c h was s u p p o r t e d by Kuwait I n s t i t u t e f o r S c i e n t i f i c R e s e a r c h .
R e f e r e n c e s .
-
1- SAWAN Y . , BORISOVA Z.U., J . o f I n o r g a n i c M a t e r i a l , Academy o f S c i e n c e , USSR, 4 ,
12
(1968) 2089.2- SAWAN Y . , WAKIM F . G . , EL-GABALY M . and EL-RAYES M . K . , J. o f Non- c r y s t a l l i n e S o l i d s ,
%
(1980) 319.3- FONASH S . J . , ASHOK S . , P r o c e e d i n g s o f t h e 1 4 t h P h o t o v o l t a i c s S p e c i a l i s t s C o n f e r e n c e , I E E E N . Y . , N . Y . (1980) 591.