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OPTICAL STUDIES IN a-Si : H/a-SiNx : H SUPERLATTICES
S. Kalem
To cite this version:
S. Kalem. OPTICAL STUDIES IN a-Si : H/a-SiNx : H SUPERLATTICES. Journal de Physique
Colloques, 1987, 48 (C5), pp.C5-191-C5-194. �10.1051/jphyscol:1987538�. �jpa-00226742�
JOURNAL DE PHYSIQUE
Colloque C5, supplement au nall, Tome 48, novembre 1987
OPTICAL STUDIES IN a-Si:H/a-SiN,:H SUPERLATTICES
S. KALEM
University of Sheffield, Department of Physics, GB-Sheffield S3 7RH, Great-Britain
R6sumB.- P r o p r i 6 t 6 s o p t i q u e s d e s s u p e r f i s e a v x de a - S i : ~ / a - S ~ N :H
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ont Q t 6 BtudiBes en f o n c t i o n d 9 8 p a i s s e u r p a r l a s p e c t r o s c o g i e de l a d6f l e c t i o n photothermique e t phot olumine scence. Nous sugg&
r o n s contamination, p l u t 6 t que d e s e f f e t s q u a n t i q u e s peut d e t e r miner l e s p r o p r i i t 6 s o p t i q u e s l o r s q u e l ' 6 p a i s s e u r e s t r 6 d u i t e a u dessous de
N30 8.
Abstract.- O p t i c a l p r o p e r t i e s of a - ~ i : ~ / a - ~ i I ' i ~ : ~ s u p e r l a t t i c e s have been i n v e s t i g a t e d as a f u n c t i o n of ' s u b l a y e r t h i c k n e s s by photothermal d e f l e c t i o n spectroscopy and photoluminescence mea- surement s. We suggest contaminat i o n , r a t h e r t h a n quantum con- f inement e f f e c t s, may determine o p t i c a l p r o p e r t i e s when sub- l a y e r t h i c k n e s s i s reduced below
N30 a.
Changes i n t h e o p t i c a l and e l e c t r o n i c parameters of a - s i : ~ / a - S i N ~ : ~ composi- t i o n modulated s u p e r l a t t i c e s ( S 1 ) have been commonly observed when t h e a-Si:H sub- l a y e r t h i c h e s s ( d s ) i s reduced below about 40 8. However, t h e r e i s not a c l e a r evi- dence f o r . quantum confinement e f f e c t s i n t h e s e structures(1-4). I n t h i s paper, we have s t u d i e d i n d e t a i l t h e a b s o r p t i o n and emission p r o p e r t i e s of t h e s e s u p e r l a t t i c e s u s i n g photothermal d e f l e c t i o n s p e c t r o s c o P y ( ~ ~ S ) and p h o t o l u m i n e s c e n c e ( ~ ~ ) measure- ments. The r e s u l t s a r e compared w i t h t h o s e of unlayered a-Six :H a l l o y s w i t h diffe--
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r e n t a l l o y composition and hydrogen r i c h a-Si:H d e p o s i t e d a t low t e m p e r a t u r e s i n o r d e r t o c l a r i f y t h e o r i g i n of t h e s i z e e f f e c t s .
S u p e r l a t t i c e samples of a l t e r n a t i n g ~ - s ~ : H / ~ - s ~ N ~ : H l a y e r s were prepared by glow d i s c h a r g e decomposition of SiH and NH / S ~ H g a s mixture a t 3 3 0 ' ~ i n a s i n g l e
4 3 4
chamber system. Unlayered a-SiN :H and a-Si:H have been prepared i n s i m i l a r condi- X
t i o n s . PDS measurements were c a r r i e d out u s i n g 450 W xenon a r c lamp a t 20 Hz and photoluminescence e x c i t a t i o n used 6328 a He-Ne l a s e r l i n e .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987538
JOURNAL DE PHYSIQUE
F i g u r e l a shows t h e v a r i a t i o n of t h e o p t i c a l gap Eopt (deduced from t r a n s m i s s i o n measurements) of SL samples a s a f w c t i o n of d e c r e a s i n g t h i c k n e s s (l/dS). The s o l i d l i n e i s t h e f i t by a KronigPen- ney model f o r a f r e e e l e c t r o n i n one dimensional square p o t e n t i a l well. I n t h e c a l c u l a t i o n , t h e con- d u c t i o n and valance band d i s c o n t i n u i t i e s , e l e c t r o n and hole e f f e c t i v e masses a r e assumed a s 1.0 eV ,
0.6 eV , O . h o , 1 .Omo , r e s p e c t i v e l y . Thickness r a t i o &/dm , a-Si:H and n i t r i d e band gaps a r e 0.9, 1.7 eV and 3.3 eV , r e s p e c t i v e l y . A good agreement i s obtained w i t h experimental d a t a .
The upward i n c r e a s e i n E w i t h d e c r e a s i n g opt
t h i c k n e s s i s followed by a broadening of t h e Urbach and by a d e c r e a s e in t h e d e f e c t s t a t e a+
s o r p t i o n measured a t 1.1 e ~ ( ~ i g u r e I b and I c ) . Note
t h a t due t o t h e opening gap, t h e i n t e g r a t e d d e f e c t
*
q t
a b s o r p t i o n i n t e n s i t y i s i n c r e a s e d i n t h e s m a l l e s t
0.5 1.0
x t h i c k n e s s s u p e r l a t t i c e s. Despite of s i m i l a r t r e n d s w i t h Eopt, E i s probably n o t c o r r e l a t e d w i t h con- F i g u r e 1. ( a ) O p t i c a l band
finement e f f e c t , i n s t e a d suggest d i s o r d e r o r s t r u c gap Eopt , (b) Urbach energy t u r a l changes a t i n t e r f a c e s o r i n t h e quantum w e l l Eo and defect state ab-
l a y e r . Bond a n g l e d i s t o r t i o n s a t i n t e r f a c e s h a s been s o r p t i o n W D a t 1.1 eV of a l r e a d y r e p o r t e d from Raman s c a t t e r i n g measurements a-Si: ~ / a - S ~ N x : H s u p e r l a t t i c e s (5). But, it i s d i f f i c u l t t o d i s t i n g u i s h between in- a s a f u n c t i o n of s u b l a y e r t e r f a c e and w e l l l a y e r whose contamination may a l s o thickness(e)' *Iso shown induce s i m i l a r changes i n Eo. Here one important a r e o p t i c a l parameters of
parameter i s B i n t h e Tauc law (4fia) '/*=B(*W- E i)
a-SiN :H a l l o y s v e r s u s x OP
x The a n a l y s i s shows t h a t B i s almost gap independent
(dashed l i n e s ) . i n n i t r i d e a l l o y s and d e c r e a s e s w i t h H q u a n t i t y f o r
'
a-Si:H. In s u p e r l a t t i c e s , B d e c r e a s e s from dS+ 8
w i t h i n c r e a s i n g E and f o r s m a l l e s t t h i c k n e s s SL, it i s c l o s e t o t h o s e of hydrogen o p t
r i c h a-s~:H( 35 % H ) and a-SiNx:H a l l o y s . The d e c r e a s e of B suggest probably t h a t --
important s t r u c t u r a l m o d i f i c a t i o n s t a k e p l a c e i n t h e quantum w e l l l a y e r w i t h d e c r e a s i n g dS. One reason of such m o d i f i c a t i o n may be an i n c r e a s e d q u a n t i t y of H and N in t h e w e l l and t h i s i s c o n s i s t e n t w i t h r e c e n t o b s e n r a t i o n s r e p o r t i n g a n e x c e s s H and N i n t h e s e s t r u c t u r e s (6,7).
O p t i c a l parameters of a range of n i t r i d e a l l o y s a r e summarised on F i g u r e 1,
and r e s u l t s a r e compared w i t h SL. We f i n d t h a t t h i c k n e s s and a l l o y i n g induced
changes i n o p t i c a l parameters occur a t about same r a t e s . T h i s p a r a l l e l i s m between
SL and a l l o y s obviously makes d i f f i c u l t t h e i n t e r p r e t r a t i o n of t h e changes i n t e r m s
of confinement e f f e c t s .
I n F i g u r e 2, we r e p o r t t h e gap s t a t e
loL -
a b s o r p t i o n s p e c t r a of a s e r i e s of H r i c h a-Si:H d e p o s i t e d a t d i f f e r e n t temperatures.
Also shown i s t h e spectrum of a 1 2 2 t h i c k SL. Despite of some o v e r l a p p i n g between t h e s p e c t r a of SL and H r i c h a-Si:H, it i s hard t o prove t h a t t h e p r o p e r t i e s of SL a r e determined by H contamination. The ex- c e s s a b s o r p t i o n i s s e n s i t i v e t o hydrogena- t i o n . T h i s b e h a v i o r of t h e d e f e c t t a i l i s
10"
9 I to p p o s i t e t o t h o s e of s u p e r l a t t i c e s ( ~ i g . 1 c ) 1 .O 1 5 2 0 Moreover, t h e a b s o r p t i o n edge i n a-Si:H i s f i w (eV) r e l a t i v e l y i n s e n s i t i v e t o hydrogenation
F i g u r e 2. Gap s t a t e a b s o r p t i o n s p e c and t h e b l u e s h i f t i n t h e a b s o r p t i o n edge
t r a of a-Si:H d e p o s i t e d a t 3 0 0 ( ~ ) , of ~ ~ ( 1 2 2 ) i s much l a r g e r .
Photoluminescence was e x c i t e d w i t h 1 5 0 ( ~ ) , ~ o ( B ) , and ~o'c(A). samples 1.96 e V photons. The PL emission p r o p e r c o n t a i n 15, 22, 27 and 35 % H, r e p
p e c t i v e l y . Dashed l i n e shows t h e t i e s of 12 8 SL a r e r e p o r t e d i n F i g u r e 3.
Low temperature e x c i t a t i o n i n t h e SL sample spectrum of t h e 12 8 SL.
r e s u l t s i n a main band a t 1.46 eV w i t h a
weak s h o u l d e r a t 1.3 eV. Note t h a t PL peak e n e r g i e s a r e almost independent of ten?
p e r a t u r e . Moreover, t h e i n t e n s i t y of t h e s h o u l d e r a t 1.3 eV i n c r e a s e s w i t h tempera- t u r e . F i g u r e 3 a l s o summarises t h e r e s u l t s of PL measurements t a k e n on unlayered n i t r i d e alloy(x=0.8) and hydrogen r i c h a-Si:H ( 35 % H). The PL e x c i t a t i o n i n t h e n i t r i d e a l l o y i s c h a r a c t e r i z e d w i t h only one emission band a t 1.46 eV and i t s peak energy i s a l s o independent of temperature. In a-Si:H, t h e e x c i t a t i o n produces a band a t 1.3 eV and a t e l e v a t e d t e m p e r a t u r e s , we observe t h e appearance of t h e O.9eV d e f e c t band. Note t h a t t h e main PL emission band of t h l s sample i s a l s o temperature i n s e n s i t i v e ( p e a k energy). One remarkable f e a t u r e i s t h e absence of t h e d e f e c t band i n b o t h SL and n i t r i d e a l l o y . The comparison between t h e PL p r o p e r t i e s of SL and H r i c h a-Si:H and a l l o y s u g g e s t s t h a t t h e lowest t h i c k n e s s SL can n o t be simulated w i t h H r i c h a-Si:H a l o n e ( i n t h e temperature range e x p l o r e d ) o r n i t r i d e a l l o y . The emission bands a t 1.46eV and 1.3 eV i n SL a r e probably due t o n i t r o g e n ( f o r example N d a n g l i n g bonds) and ~ ~ d r o ~ e n ( ~ i d a n g l i n g bonds) induced recombination c e n t r e s ,
r e s p e c t i v e l y .
In summary, t h e changes i n a - ~ i : ~ / a - s i N :H s u p e r l a t t i o e p r o p e r t i e s w i t h d e c
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