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LUMINESCENCE IN PLASMA DEPOSITED AMORPHOUS SixC1-x ALLOYS
R. Sussmann, E. Lauder
To cite this version:
R. Sussmann, E. Lauder. LUMINESCENCE IN PLASMA DEPOSITED AMORPHOUS SixC1-x ALLOYS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-1029-C4-1032.
�10.1051/jphyscol:19814225�. �jpa-00220855�
JOURNAL DE PHYSIQUE
CoZloque C4, suppl6ment au nO1O, Tome 4 2 , octobre 1981 page C4-1029
LUMINESCENCE I N PLASMA D E P O S I T E D AMORPHOUS S~,C~-, A L L O Y S
R.S. Sussmann and E.H. Lauder
Plessey Research (CasweZZ), Limited AlZen CZark Research Centre, CasweZZ, Towcester,Northamptonshire, England
Abstract - Results are presented on the composition and'temperature dependence of t h e luminescence i n plasma deposited amorphous SixC1-x alloys using films of com- position ranging from pure Si t o 90% C in t h e 77
Kt o 450
Ktemperature range.
There i s evidence t o suggest t h a t t h e radiative recombination r a t e increases by over a f a c t o r of 103 from pure
S it o C rich films. This increase i s shown t o be a1 so consistent
w i t hthe composition dependence of the quantum efficiency. These r e s u l t s a r e discussed i n terms of a radiative tunnel ling model.
Introduction - A previous study on t h e optical and luminescence properties of plasma deposited amorphous SixC1-x ( 1 ) showed two d i s t i n c t features. One was the increase by a f a c t o r of about 103 i n the r a t i o of t h e luminescence a t 300 K t o t h a t a t 77
Kwith carbon content. This was interpreted
i nterms of enhancement i n the Coulombic electron-hole binding energy due t o the decrease i n t h e d i e l e c t r i c constant with
Cconcentration ( 1 ) . There was however no d i r e c t experimental evidence f o r t h i s change i n binding energy and other mechanism, such a s an increase i n radiative r a t e , could not be ruled out. The second feature was a r e l a t i v e l y high quantum e f f i c i e n c y f o r C r i c h films. The composition dependence of t h e quantum efficiency was discussed i n terms of a model based on the tunnelling of electrons t o defect centres (2). The experimental r e s u l t s were consistent with an increase i n the defect density t o about 1017 t o 101* a s the carbon concen- t r a t i o n was increased and with a reduction i n t h e c r i t i c a l tunnelling distance by a f a c t o r between 2 and 3. In t h i s work we present f u r t h e r r e s u l t s on t h e tempera- t u r e and composition dependence of
theluminescence i n a-SixC1-x which allows a b e t t e r understanding of the d i f f e r e n t mechanisms which control the luminescence i n these a1 loys.
Experiment - The experimental d e t a i l s regarding sample preparation and measuring techniques have been reported previously (1
). A1 l the r e s u l t s i n t h i s work r e f e r t o films grown a t a substrate deposition temperature of 3000C. These films a r e hydrogenated as determined by infra-red absorption. The Si/C composition, deter- mined by electron microprobe analysis, i s described by the parameter X which stands f o r t h e number of Si in respect t o the t o t a l number of Si plus
C atoms. The temperature dependence of the luminescence was measured using a "DN 704 Oxford Instrument cyostat and a DTC2 temperature c o n t r o l l e r operating in the 77 t o 450 K temperature range.
Results - The spectral dependence of the luminescence emission f o r samples of d i f f e r e n t compositions is shown i n Fig, 1. For most compositions the emission i s i n the form of a single band which broadens and s h i f t s towards higher photon
energies a s the carbon content of the films i s increased. The composition dependence of the peak emission energy and bandwidth have been discussed in some d e t a i l e l s e - where ( l ) . The emission efficiency a s a function of composition i s shown i n Fig.
2.I t should be recalled t h a t the uncertainty i n determining the r e l a t i v e value of the quantum efficiency i s large ( 1 ) . The expected corrections, however, would i n principle accentuate the features shown in Fig. 2 and i t can be concluded t h a t f o r
Crich specimens t h e emission efficiency i s of t h e same order o r l a r g e r than i n e i t h e r a-Si:H or Si rich samples.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814225
C4-1030 JOURNAL DE PHYSIQUE
The temperature dependence o f t h e luminescence i s shown i n F i g . 3a f o r samples o f t h r e e d i f f e r e n t compositions. The i n t e n s i t y i n t h i s p l o t has been normalized w i t h r e s p e c t t o t h e i n t e n s i t y a t low temperatures ( 3 ) . I n agreement w i t h previous p u b l i c a t i o n s (4,5) pure S i samples show a decrease o f over a f a c t o r o f 103 f r o m 77 t o 300 K. T h i s can be taken as an i n d i c a t i o n t h a t o u r our a-Si specimens have a low d e n s i t y o f d e f e c t centres (6,7). On t h e o t h e r hand i n 90% C f i l m s t h e corresponding decrease i s l e s s than a f a c t o r o f f o u r . It can be argued t h a t t h i s weaker tempera- t u r e dependence i s s i m i l a r i n o r i g i n t o t h a t observed i n h i g h d e f e c t d e n s i t y a-Si:H (6,7).
However these a-Si:H f i l m s a l s o show a much lower low temperature quantum e f f i c i e n c y and as discussed w i t h r e f e r e n c e t o F i g . 2 t h i s i s n o t t h e case i n C
r i c h samples which show t h e weakest temperature . . . . . . -. . . . . . . . dependence. Furthermore, i f an e s t i m a t e i s made
u s i n g t h e model proposed by S t r e e t ( 7 ) o f t h e d e n s i t y o f d e f e c t centres needed t o account f o r t h e observed weaker temperature dependence o f
samples i n t h e mid composition range, an o r d e r o f 1.2 1.6 2.0 2.4
magnitude h i g h e r r e d u c t i o n i n t h e low temperature Photon Energy (=V)
quantum e f f i c i e n c y i s p r e d i c t e d than t h a t a c t u a l l y
observed. F i p . 1 Emission spectra for
S i # l - , f i l m s o f d i f f e r e n t We have suggested t h a t t h e d i f f e r e n c e i n tempera-
t u r e dependence f o r d i f f e r e n t compositions c o u l d o r i g i n a t e from an increase i n the e l e c t r o n - h o l e
b i n d i n g energy as t h e carbon c o n c e n t r a t i o n i s
-
increased ( 1 ) . I f t h e r e i s a s i n g l e a c t i v a t i o n energy t h e quantum e f f i c i e n c y
QL
i s expected t ovary as QL = PR/(PR
+
wo exp-
Ea/kT) where PR i s1
t h e r a d i a t i v e recombination r a t e , wo an a p p r o p r i a t e
E
phonon frequency and E, a b i n d i n g energy which c o n t r o l s t h e thermal d i s s o c i a t i o n r a t e . I f t h e parameter w0/PR i s independent o f temperature and
5
composition a change i n b i n d i n g energy w i l l r e s u l t
g
i n a p r o p o r t i o n a l change i n t h e temperature T1
a t which a value Q L ~ i s a t t a i n e d . An i n s p e c t i o n 10- .
0 2 0 4 0 6 0 8 1 0
o f Fig. 3a shows t h a t , depending on the value o f C O ~ P O I , ~ , ~ parameter I
Q L ~ , t h e value Ea can vary by a - f a c t o r o f 1.5 t o
3 going from pure S i t o 90% C f i l m s . T h i s i s Fig. 2 Composition dependence of w e l l w i t h i n t h e f a c t o r of four chanqe expected from the photoluminescence efficiency an increase i n Coulombic energy owing t o t h e decrease i n d i e l e c t r i c constant w i t h carbon c o n t e n t ( 1 ) . However t h i s e x p l a n a t i o n i s l i k e l y t o be an o v e r - s i m p l i f i c a t i o n . Changes i n quenching temperature can a l s o o r i g i n a t e from v a r i a t i o n s i n the r a d i a t i v e recombination r a t e which i s u n l i k e l y t o remain c o n s t a n t along t h e composition range.
a l s o t h e temperature dependence o f t h e quantum e f f i c i e n c y w i l l n o t n e c e s s a r i l y f o l l o w a s i n g l e a c t i v a t e d regime. As shown i n F i g . 3b, t h e r e i s ( i n s i m i l a r i t y w i t h a-Si:H) (4,5) evidence f o r a double exponential regime o f t h e form
QL-l = 1
+
A exp-
(Ea/kT)+
B exp-
(EbIkT) (1 ) The h i g h temperature a c t i v a t i o n energy E,
i s thought t o r e p r e s e n t t h e thermal d i s s o c i a t i o n of e l e c t r o n - h o l e p a i r s and t h e pre-exponential f a c t o r A i s associated w i t h w o / P ~ .The composition dependence o f t h e parameters A, B, Ea and Eb as obtained from p l o t s l i k e t h a t i n F i g . 3b i s shown i n F i S . 4 and 5. It would appear t h a t a g a i n s t t h e e x p e c t a t i o n s of t h e s i m p l i f i e d mode? considered above, t h e a c t i v a t i o n energy E, i s o n l y a weak f u n c t i o n o f composition and i n f a c t seems t o decrease w i t h i n c r e a s i n g
carbon content. The h i g h e r t h r e s h o l d quenching temperatures i n h i g h carbon content samples seem t o o r i g i n a t e mostly from a pronounced decrease i n t h e parameter A.
I f A = w o / P ~ t h i s means an increase i n t h e r a d i a t i v e recombination r a t e o f a f a c t o r o f 103 t o 104 (wo i s o n l y expected t o v a r y by l e s s than a f a c t o r o f two).
F u r t h e r support f o r t h e i n t e r p r e t a t i o n o f t h e data o f F i g . 4 i n terms o f an increase i n the r a d i a t i v e recombination r a t e comes from t h e composition dependence o f t h e quantum e f f i c i e n c y shown i n F i g . 2. T h i s p a t t e r n was i n t e r p r e t e d i n ( l ) i n terms o f t h e model proposed by Tsang and S t r e e t ( 2 ) based on a n o n - r a d i a t i v e process i n which t h e e l e c t r o n t u n n e l s t o a nearby d e f e c t centre. For a random d i s t r i b u t i o n o f centres t h i s model p r e d i c t s a low temperature quantum e f f i c i e n c y given by
QL
= exp-
( - 4 .rr RC3 Ns)3 (2
where R i s d e f i n e d such t h a t t h e p r o b a b i l i t y o f t u n n e l l i n g t o a s i t e a d i s t a n c e RC equaFs t h e r a d i a t i ve recombination p r o b a b i l i t y and i s given by
RC =
I
R, I n ( w o / P ~ )2 ( 3 )
where R. i s t h e Bohr r a d i u s and NS t h e d e n s i t y o f n o n - r a d i a t i v e d e f e c t centres. I t was argued ( 1 ) t h a t as t h e carbon c o n c e n t r a t i o n i s increased t h e d e f e c t d e n s i t y a l s o increases t o l e v e l s o f t h e o r d e r o f 1017 t o 1018 cm-3 so t h a t a t compositions
1000/~(K")
F i g . 3a P h o t o l u m i n e s c e n c e i n t e n s i t y IL n o r m a l i z e d t o t h e l o w t e m p e r a t u r e v a l u e I. vs T' f o r t h r e e S i g l - x f i l m s
F i g . 3b ( I o / I L - I ) vs T 1 f o r t h e same
-
f i l m s a s F i g . 3a.
0.2 0 4 0.6 0 8 1.0
Composition Parameter r
F i g . 4 C o m p o s i t i o n dependence o f t h e p r e - e x p o n e n t i a l f a c t o r s A and B
Compostlion Parameter S
F i g . 5 C o m p o s i t i o n dependence o f t h e a c t i v a t i o n e n e r g i e s E , and Eb
C4-1032 JOURNAL DE PHYSIQUE
o f about 40 t o 60% S i t h e e f f i c i e n c y , as p r e d i c t e d by equation 2 i s reduced by about an o r d e r o f magnitude. I t was f u r t h e r suggested t h a t f o r h i g h e r C c o n c e n t r a t i o n s t h e d e f e c t d e n s i t y d i d n o t increase a p p r e c i a b l y b u t t h e r e was a r e d u c t i o n i n t h e c r i t i c a l d i s t a n c e RC by a f a c t o r between 2 and 3. An i n s p e c t i o n o f e q u a t i o n (3) shows t h a t such a r e d u c t i o n i n RC i s indeed o b t a i n e d i f t h e parameter w /PR i s reduced by a f a c t o r 103 t o 104 from t h e value o f 106 found i n pure S i fi?ms. T h i s r e d u c t i o n i n wo/PR i s i n good agreement w i t h t h e change i n t h e pre-exponential parameter A as shown i n F i g . 4.
I n a model i n which t h e band-gap luminescence i s i n t e r p r e t e d i n terms o f r a d i a t i v e tunnel l i n g t h e r a d i a t i v e r a t e w i l l be given by ( 4 )
PR = uo exp ( - 2 R /Ro) ( 4 )
where R i s t h e e l e c t r o n - h o l e separation. For a-Si:H a most probable value o f R/Ro
= 5 has been estimated from time r e s o l v e d luminescence measurements (4). I n o r d e r t o account f o r an increase i n P o f a f a c t o r o f 103 i t i s necessary according t o equation ( 4 ) t o assume values o! R/Ro between 1 and 2. Such a small s e p a r a t i o n i s p o s s i b l e i f t h e e l e c t r o n - h o l e p a i r i s e x c i t e d e i t h e r near t h e bottom o f t h e band o r i n t o l o c a l i z e d s t a t e s so t h a t t h e c a r r i e r s a r e n o t a b l e t o d i f f u s e a p a r t b e f o r e recombining. A f u r t h e r consequence o f t h i s small s e p a r a t i o n w i l l be a l a r g e r b i n d i n g energy and t h i s appears i n p r i n c i p l e t o c o n t r a d i c t o u r experimental r e s u l t s which show a small decrease i n t h e h i g h temperature a c t i v a t i o n energy f o r C r i c h f i l m s . I t should be p o i n t e d o u t however t h a t o u r measurements a r e l i m i t e d t o a maximum temperature o f 450 K. I t can be e a s i l y appreciated from F i g . 3b t h a t f o r s i m i l a r values o f A as those found i n C r i c h specimens (about 102) an a c t i v a t i o n energy f a c t o r o f t h r e e o r f o u r times l a r g e r than t h a t a c t u a l l y measured c o u l d n o t be observed unless temperatures i n excess o f 1000 K are used. This i s c l e a r l y i n p r a c t i c a b l e i n these hydrogenated amorphous a l l o y s . I t c o u l d t h e n be argued t h a t t h e a c t i v a t i o n energy estimated from t h e h i g h temperature regime i n C r i c h specimens i s s t i l l s t r o n g l y i n f l u e n c e d by t h e low temperature regime and i s n o t a good rep- r e s e n t a t i o n o f t h e t r u e b i n d i n g energy.
Conclusions
-
Our measurements o f t h e temperature and composition dependence o f t h e luminescence i n amorphous SixC1-x f i l m s suggests t h a t t h e r a d i a t i v e recombinationr a t e increases by over a f a c t o r o f 103 from pure S i t o 90% C specimens. T h i s increase i s t e n t a t i v e l y e x p l a i n e d as a consequence o f a s m a l l e r e l e c t r o n - h o l e s e p a r a t i o n i n C r i c h sample c a r r i e r s being e x c i t e d near o r i n t o l o c a l i s e d s t a t e s . A most probable value o f t h e e l e c t r o n - h o l e separation R i s estimated a t between 1 and 2 times t h e Bohr r a d i u s .
Acknowledgements
-
We would l i k e t o thank A W M a b b i t t f o r h i s support and encour- agement. We are a l s o g r a t e f u l t o S Deppina from t h e U n i v e r s i t y o f H u l l f o r s u p p l y i n g some o f t h e low temperature luminescence data.References
(1) Sussmann R S and Ogden R, P h i l o s Mag, (1981) ( i n Press) ( 2 ) Tsang C and S t r e e t R A, P h i l o s Mag,
37
(1978) 601 ( 3 ) Deppina S, P r i v a t e Communication(4) Tsang C and S t r e e t R A, Phys Rev, (1979) 3027
( 5 ) A u s t i n I G, Nashashibi T S, Searle T M, Le Comber P A and Spear W E, J N o n - C r y s t a l l i n e Solids,
2
(1979) 373(6) Nashashibi T S, A u s t i n I G and Searle T M, P h i l o s Mag,
