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Submitted on 1 Jan 1981
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SUB-GAP AND BAND EDGE OPTICAL
ABSORPTION IN a-Si:H BY PHOTOTHERMAL DEFLECTION SPECTROSCOPY
W. Jackson, N. Amer
To cite this version:
W. Jackson, N. Amer. SUB-GAP AND BAND EDGE OPTICAL ABSORPTION IN a-Si:H BY
PHOTOTHERMAL DEFLECTION SPECTROSCOPY. Journal de Physique Colloques, 1981, 42
(C4), pp.C4-293-C4-296. �10.1051/jphyscol:1981461�. �jpa-00220918�
SUB-GAP AND BAND EDGE O P T I C A L ABSORPTION I N a - S i : H B Y PHOTOTHERMAL D E F L E C T I O N SPECTROSCOPY
W.B. Jackson and N.M. Amer
Applied Physics and Laser Spectroscopy Group, Lawrence BerkeZey Laboratory, University o f California, Berkeley, California
94720, U.S.A.A b s t r a c t . - Using p h o t o t h e r m a l d e f l e c t i o n s p e c t r o s c o p y , we have i n v e s t i g a t e d t h e o p t i c a l a b s o r p t i o n of v a r i o u s a-Si:H f i l m s i n t h e r a n g e of 2.1-0.6 eV. An a b s o r p t i o n s h o u l d e r which depends o n d e p o s i t i o n c o n d i t i o n s and on doping was found and was a t t r i b u t e d t o d a n g l i n g bonds. We a l s o o b s e r v e d t h a t t h e exponen- t i a l edge broadens w i t h i n c r e a s i n g s p i n d e n s i t y .
INTRODUCTION:
The n a t u r e of t h e o p t i c a l a b s o r p t i o n i n amorphous semiconductors a t and below t h e a b s o r p t i o n edge i s of i n t e r e s t . I n p a r t i c u l a r , t h e a b s o r p t i o n below t h e band gap i s a s t r o n g f u n c t i o n of d e f e c t s and i m p u r i t i e s s i n c e i t i s n o t masked by t h e s t r o n g e r band-to-band a b s o r p t i o n . I n t h e c a s e of hydrogenated amorphous s i l i c o n
(a-Si:H), s i n c e t h e f i l m t h i c k n e s s i s t y p i c a l l y o n e micron t h i c k , c o n v e n t i o n a l methods o f r e f l e c t i o n and transmission do n o t r e l i a b l y measure a b s o r p t i o n c o e f f i - c i e n t s ( a ) below % 50 cm-l. And w h i l e p h o t o c o n d u c t i v i t y d a t a have been used t o i n f e r low o p t i c a l a b s o r p t i o n , u n v e r i f i e d a s s u m p t i o n s a b o u t t h e t r a n s p o r t p r o p e r t i e s of t h e m a t e r i a l had t o b e made. To overcome t h e s e l i m i t a t i o n s , we have r e c e n t l y developed t h e t e c h n i q u e o f p h o t o t h e r m a l d e f l e c t i o n s p e c t r o s c o p y (') (PDS which e n a b l e s t h e d i r e c t measurement of low a b s o r p t i o n c o e f f i c i e n t s (a 9. %
lo-' -
lo-').Using t h i s t e c h n i q u e , we have measured t h e sub-gap a b s o r p t i o n of v a r i o u s a-Si:R f i l m s down t o 0.6 eV.
EXPERIMENTAL COIISIDERATIONS :
The p h y s i c a l b a s i s of PDS i s t h a t when a n i n t e n s i t y - m o d u l a t e d t u n a b l e l i g h t beam (pumpbeam) i s a b s o r b e d , h e a t i n g w i l l o c c u r . T h i s h e a t i n g c a u s e s a p e r i o d i c i n d e x of r e f r a c t i o n g r a d i e n t i n a t h i n l a y e r a d j a c e n t t o t h e sample s u r f a c e . A second beam ( p r o b e beam), p r o p a g a t i n g t h r o u g h t h i s t h i n l a y e r , w i l l t h e n e x p e r i - e n c e a p e r i o d i c d e f l e c t i o n c h c a n b e q u a n t i t a t i v e l y r e l a t e d t o t h e o p t i c a l a b s o r p t i o n . We have shown r y t h a t f o r t h e r m a l l y t h i n m a t e r i a l s , t h e power- normalized s i g n a l S i s g i v e n by
S = A [ 1
-
exp (- a x ) ]where A i s a c o n s t a n t which c a n b e d e t e r m i n e d e m p i r i c a l l y , and i s t h e f i l m t h i c k n e s s . For a!.>> 1, S = A and h e n c e A c a n b e d e t e r m i n e d .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981461
JOURNAL DE PHYSIQUE
E x p e r i m e n t a l l y , t h e o u t p u t of a 1kW Bg-Xe a r c lamp was monochromatized (0.01 eV bandwidth), m e c h a n i c a l l y chopped, and f o c u s s e d o n t h e sample which was immersed i n CCl4. The d e f l e c t i o n d u e t o a b s o r p t i o n was measured w i t h a He-Ne l a s e r (probe) beam w h o s e d e f l e c t i o n w a s d e t e c t e d w i t h a p o s i t i o n s e n s o r . The o u t p u t of t h i s s e n s o r was f e d i n t o a l o c k - i n a m p l i f i e r and t h e pump beam power was monitored w i t h a pyro- e l e c t r i c d e t e c t o r . I n t e r f e r e n c e f r i n g e s were averaged o u t u s i n g c o n v e n t i o n a l t e c h - n i q u e s . Using r a d i a t i o n t r a n s f e r t h e o r y , we h a v e a l s o d e m o n s t r a t e d t h a t h e PDS s i g n a l i s h i g h l y i n s e n s i t i v e t o t h e s c a t t e r i n g p r o p e r t i e s o f t h e f i l m s F 2 ) . To i n s u r e t h a t t h e o b s e r v e d PDS s i g n a l o r i g i n a t e d from t h e a-Si:H f i l m and n o t from t h e s u b s t r a t e o r t h e CC14, we monitored t h e p h a s e of t h e s i g n a l . Theory shows t h a t s i g n a l s from t h e s u b s t r a t e and from t h e l i q u i d , should b e +45O and -135' o u t of p h a s e w i t h t h a t from t h e f i l m , r e s p e c t i v e l y . No s u c h p h a s e s h i f t s were o b s e r v e d . We a l s o v a r i e d t h e s u b s t r a t e m a t e r i a l and t h e l i q u i d c h e m i c a l n a t u r e and found t h a t n e i t h e r a f f e c t e d o u r r e s u l t s .
The a-Si:H f i l m s were undoped r . f . and d . c . glow d i s c h a r g e 1-2um t h i c k . Sub- s t r a t e d e p o s i t i o n t e m p e r a t u r e ranged from 100° - 3 0 0 ° C , and t h e r . f . power was 2
-
4 0 W. One phosphorus doped sample pH3) was a l s o measured.RESULTS AND DISCUSSION:
The r e s u l t s a r e shown i n F i g u r e s 1 and 2. As t h e s u b s t r a t e t e m p e r a t u r e i s i n - c r e a s e d , t h e s h o u l d e r a t % 1 . 3 eV d e c r e a s e s . Then above 230°c t h e s t r e n g t h o f t h e a b s o r p t i o n i n c r e a s e s . F u r t h e r m o r e , a s t h e r . f . power g o e s up, t h e a b s o r p t i o n s h o u l d e r i n c r e a s e s i n a f a i r l y monotonic f a s h i o n . Even though t h e phosphorus-doped f i l m was d e p o s i t e d a t 230°C and 2 W power, i t e x h i b i t s t h e l a r g e s t a b s o r p t i o n s h o u l d e r ( o u r r e s u l t s from a s y s t e m a t i i n v e s t i g a t i o n of single-doped and compens- a t e d f i l m s w i l l b e r e p o r t e d e l s e w h e r e f 3 ) ) . Another c h a r a c t e r i s t i c of t h e a b s o r p t i o n s p e c t r a i s that t h e s l o p e o f t h e e x p o n e n t i a l band edge v a r i e s w i t h t h e r . f . power, w i t h t h e s l o p e b e i n g w e l l c o r r e l a t e d w i t h t h e a b s o r p t i o n s h o u l d e r .
1 1 ~ 1 1 1 1 1 1 1 1
0. 4 0.8 1. 2 1.6 2 2. 4 0.4 0.8 1 . 2 1.6 2 2.4
ENERGY (eV) ENERGY CeV)
F i g . ( 1 ) Dependence o f A b s o r p t i o n Shoulder on S u b s t r a t e Temperature
F i g . (2) Dependence of A b s o r p t i o n Shoulder o n r .f
.
powerHence, t o a t t e m p t t o i d e n t i f y t h e o r i g i n of t h i s a b s o r p t i o n , w e p l o t t h e e q u i l i b r i u m s p i n d e n s i t y of t h e v a r i o u s f i l m s ( a s deduced from ESR measurements) v s . Eo. A s can be s e e n from F i g u r e ( 3 ) , t h e c o r r e l a t i o n i s v e r y s t r o n g . There a r e t h r e e p o s s i b l e r e a s o n s f o r such a c o r r e l a t i o n : 1) s i n c e t h e a b s o r p t i o n shoulder i s a t t r i b u t e d t o s p i n r e l a t e d d e f e c t s
('1 ,
such a shoulder may extend i n t o t h e expo- n e n t i a l r e g i o n . Thus a n i n c r e a s e i n t h e s p i n d e n s i t y would i n c r e a s e t h e a b s o r p t i o n , and i n e f f e c t f l a t t e n t h e e x p o n e n t i a l t a i l ; 2) i t i s b e l i e v e d t h a t t h e s l o p e of t h e e x p o n e n t i a l edge i s c o n t r o l l e d by disorder-induced f i e l d s o r s t r a i n s caused by s p i n d e f e c t s ; o r 3) t h e s p i n d e f e c t s and t h e d i s o r d e r a r e produced under t h e same depos- i t i o n c o n d i t i o n s .. '
URBACH ENERGY V S. . . ' . .
SPIN- ' . l
1 0 ' ~ :
es
0 0
8
0y o l 7
0
z
01016 : 0
F i g . ( 3 ) Eo v s . Equilibrium Spin F i g . ( & ) Schematic R e p r e s e n t a t i o n of
Density. t h e E f f e c t of I n c r e a s i n g
Number of S p i n s on D e n s i t y of S t a t e s
Photoinduced a b s o r p t i o n i s a n o t h way of probing t h i s r e g i o n of t h e o p t i c a l a b s o r p t i o n spectrum. It i s given by
6 )
h e r e a = kT/Ec ( o r q ) , where Ec ( q ) i s t h e s l o p e of t h e e x p o n e n t i a l t a i l of t h e conduction (valence) band. Hence, one would expect a t o b e r 1 t e d t o t h e s l o p e of t h e a b s o r p t i o n edge. P r e l i m i n a r y r e s u l t s i n o u r l a b o r a t o r y f 5 r i n d i c a t e a good c o r r e l a t i o n between t h e photoinduced parameter a and a b s o r p t i o n edge s l o p e .
A model t o e x p l a i n our r e s u l t s i s s c h e m a t i c a l l y p r e s e n t e d i n F i g u r e ( 4 ) . A s t h e s p i n d e n s i t y i n c r e a s e s , t h e v a l e n c e band d e f e c t s t a t e s i n c r e a s e and f l a t t e n i n g of t h e v a l e n c e band t a i l o c c u r s . Evidence f o r t h e e x i s t e n c e of t h i s s t r u c t u r e i n
JOURNAL DE PHYSIQUE
t h e d e n s i t y of s t a t e s comes from t h e s t r o n g c o r r e l a t i o n between t h e a b s o r p t i o n and s p i n d e n s i t y i n undoped, s i n g l e - d o p e d , and compensated samples ( 3 ,
,
f i e l d e f f e c t measurements ( 6 ) , and DLTS ( 7 ) . T h i s p i c t u r e i s c o n s i s t e n t w i t h t h e o r e t i c a l and e x p e r i m e n t a l e v i d e n c e i n d i c a t i n g t h a t t h e v a l e n c e band i s more s e n s i t i v e t o t h e e f f e c t s of d i s o r d e r t h a n t h e c o n d u c t i o n band (9 .
F i n a l l y , i t h a s been s u g g e s t e d that t h e l u m i n e s c e n c e i n v o l v e s t r a n s i t i o n s from t h e c o n d u c t i o n band t o a peak i n t h e d e n s i t y o f s t a t e s . Such a model r e q u i r e s a p o s i t i v e c o r r e l a t i o n between a b s o r p t i o n and luminescence. From o u r r e s u l t s we f i n d t h a t t h e a b s o r p t i o n and luminescence a r e n o t a t a l l c o r r e l a t e d (')
.
Indeed, o n e h a s t o c o n c l u d e t h a t t h e peak i n t h e d e n s i t y o f s t a t e s quenches t h e luminescence r a t h e r t h a n c u a s e s i t .CONCLUSION:
We summarize our r e s u l t s a s f o l l o w s :
1 ) A s i g n i f i c a n t a b s o r p t i o n s h o u l d e r a t % 1.3 eV i s o b s e r v e d which i s s t r o n g l y c o r r e l a t e d w i t h s p i n d e n s i t y .
2) The growth of t h i s a b s o r p t i o n f e a t u r e i s accompanied by a broadening of t h e a b s o r p t i o n edge.
3) T h i s b r o a d e n i n g p r o v i d e s e v i d e n c e t h a t t h e v a l e n c e band edge b r o a d e n s a s t h e d e n s i t y of d e f e c t grows.
ACKNOWLEDGEMENTS:
We t h a n k Drs. R. S t r e e t and J . Knights o f Xerox and D . Carlsori o f RCA f o r pro- v i d i n g u s w i t h w e l l c h a r a c t e r i z e d samples. T h i s work was s u p p o r t e d by t h e
A s s i s t a n t S e c r e t a r y f o r C o n s e r v a t i o n and S o l a r Energy, P h o t o v o l t a i c Systems D i v i s i o n of t h e US Department o f Energy under C o n t r a c t W-4705-ENG-48.
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0,
1333, (1981).2. Z . A. Yasa, W. B. J a c k s o n , and N. 1%. Amer, s u b m i t t e d t o Appl. Opt.
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