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Effect of deposition temperature and annealing on optically detected magnetic resonance in GD a-Si
Journal of Non-Crystalline Solids 35 & 36 (1980) 633-638 © North-Holland Publishing Company
EFFECT OF D E P O S I T I O N T E M P E R A T U R E AND A N N E A L I N G ON O P T I C A L L Y D E T E C T E D M A G N E T I C R E S O N A N C E IN GD a-Si K. M o r i g a k i I n s t i t u t e for S o l i d State P h y s i c s U n i v e r s i t y of T o k y o Roppongi, Tokyo 106, Japan B.C.
Cavenett,
P. Dawson
D e p a r t m e n t of Physics, U n i v e r s i t y Hull, HU6 7RX, U. K. S. N i t t a
of Hull
and K. S h i m a k a w a
F a c u l t y of Engineering, G i f u U n i v e r s i t y Kakamihara, G i f u 504, Japan
O p t i c a l l y d e t e c t e d m a g n e t i c r e s o n a n c e of GD a-Si has been e x a m i n e d as a function of d e p o s i t i o n t e m p e r a t u r e at 2 K and 9 GHz. A b r o a d line was o b s e r v e d only in samples p r e p a r e d above 260°C. S a m p l e s p r e p a r e d b e l o w 260°C e x h i b i t e d only two n a r r o w lines, but w h e n a n n e a l e d at h i g h temperatures such as 350°C, the b r o a d line a p p e a r e d and the n a r r o w lines d e c r e a s e d in intensity. The b r o a d line m a y be i n t e r p r e t e d in terms of a nonb o n d i n g state (T ° ) of a t h r e e - c e n t r e bond.
INTRODUCTION The gap states and r e c o m b i n a t i o n p r o c e s s e s in g l o w - d i s c h a r g e a-Si have been i n v e s t i g a t e d using v a r i o u s kinds of techniques (Spear1977). In o p t i c a l l y d e t e c t e d m a g n e t i c r e s o n a n c e (ODMR) experiments, resonance signals are d e t e c t e d by m o n i t o r i n g the i n t e n s i t y or p o l a r i z a tion of luminescence. Therefore, from these m e a s u r e m e n t s , we can obtain i n f o r m a t i o n about the r e c o m b i n a t i o n centres and the correlation b e t w e e n l u m i n e s c e n c e and m a g n e t i c centres r e s p o n s i b l e for ODMR. This t e c h n i q u e has been a p p l i e d to GD a-Si by M o r i g a k i et al (1978), and i n d e p e n d e n t l y by B i e g e l s e n et al (1978) and by Lampel et a i ( 1 9 7 8 ) In this p a p e r we p r e s e n t e x p e r i m e n t a l results of O D M R in GD a-Si w i t h d i f f e r e n t d e p o s i t i o n temperatures. The e l e c t r o n i c p r o p e r t i e s of GD a-Si are a f f e c t e d by v a r i o u s p a r a m e t e r s of sample preparation, for example, the d e p o s i t i o n temperature. It is w e l l known that the SiH b o n d i n g e n v i r o n m e n t depends on the d e p o s i t i o n t e m p e r a t u r e of sample preparation [6 - i0]. Therefore, we can d i s c u s s the nature of the gap states r e s p o n s i b l e for l u m i n e s c e n c e and O D M R signals on the basis of the e x p e r i m e n t a l results on the c o r r e l a t i o n b e t w e e n O D M R spectra and d e p o s i t i o n temperatures. A n n e a l i n g e x p e r i m e n t s p r o v i d e also useful i n f o r m a t i o n to clarify the n a t u r e of the gap states. Thus we i n t e r p r e t the e x p e r i m e n t a l results in terms of the r a d i a [ i v e and n o n r a d i a t i v e r e c o m b i n a t i o n p r o c e s s e s i n v o l v i n g gap states such as d a n g l i n g bonds and t h r e e - c e n t r e bonds [11,12].
633
634
K. Morigaki et al. / Optically Detected Magnetic Resonance in GD a-Si
a Si
D2 i
I
3.0
i
l
i
I
I
i
3.5 KG
Fig. l. O D M R s p e c t r u m o b t a i n e d at 9.04 GHz and 2 K m o n i t o r i n g the total i n t e n s i t y of the e m i t t e d light for a sample with TD=300°C. The m i c r o w a v e p o w e r used was 4 W. EXPERIMENTAL The O D M R m e a s u r e m e n t s were c a r r i e d out at 2 K using the X band microwave e q u i p m e n t w i t h the m i c r o w a v e source of a k l y s t r o n (VA297) and a 16 W t r a v e l l i n g w a v e tube (EEl095). The O D M R signal was d e t e c t e d by m o n i t o r i n g all of the e m i t t e d light from the sample under unfocused argon ion laser e x c i t a t i o n at 514.5 nm. The m i c r o w a v e p o w e r was c h o p p e d at 1 kHz and the O D M R signal was d e t e c t e d using a phase sensitive d e t e c t o r and a N i c o l e t signal averager. The luminescence e f f i c i e n c y d e p e n d e d on the d e p o s i t i o n t e m p e r a t u r e s of samples, but the l u m i n e s c e n c e i n t e n s i t y was kept at 2 V at the output of an S1 tube (EMI9684B). S a m p l e s w e r e p r e p a r e d by glow d i s c h a r g e decomposition of silane d i l u t e d to 10% in argon w i t h an inductive coupling scheme. The d e p o s i t i o n t e m p e r a t u r e s were 104, 142, 200, 210, 240, 255, 300, and 359°C. The t h i c k n e s s of the a-Si films ranged between 0.4 and 1 ~ m . The surface of the substrate was r o u g h e n e d before d e p o s i t i o n of a-Si to avoid i n t e r f e r e n c e effects in the luminescence. RESULTS
AND D I S C U S S I O N
As was p r e v i o u s l y r e p o r t e d [2,3], the O D M R s p e c t r u m of samples with T D = 3 0 0 ° C c o n s i s t e d of two n a r r o w lines D1 and D 2, and a broad line A. A typical e x a m p l e of this s p e c t r u m is shown in Fig. l. The D1 and D 2 lines c o r r e s p o n d to d e c r e a s e s in the luminescence intensity, w h o s e gvalues are 2.018 and 2.006, respectively. The A line c o r r e s p o n d s to an increase in the l u m i n e s c e n c e ~ n t e n s i t y , whose g-value and line w i d t h d e p e n d on the m i c r o w a v e power. At r e l a t i v e l y low m i c r o w a v e powers, the A line c o n s i s t e d of two components w h i c h had G a u s s i a n shapes w i t h g-values of 2.00 and 2.012 and w i t h line w i d t h s (the full h a l f - a m p l i t u d e w i d t h ~ H I / 2 ) of 230 G and 40 G, respectively. A b r o a d line w i t h g=2.00 and ~ H I / 2 = 2 3 0 G was e n h a n c e d at high microwave powers. In p r e v i o u s papers [2,3], we i n t e r p r e t e d the m a g n e t i c centres responsible for the O D M R as follows: The A line arises from an acceptorlike centre c o n t r i b u t i n g to the d o n o r - a c c e p t o r pair type radiative recombination. The D 1 and D 2 centres are d o n o r - l i k e centres acting
K. Morigaki et al. / Optically D e t e c t e d M a g n e t i c Resonance in GD a-Si
635
D2
3,2
33
KG
3:2
31a
KG
(b)
(c)
Fig. 2. O D M R s p e c t r a o b t a i n e d at 9.04 G H z a n d 2 K for s a m p l e s w i t h various deposition temperatures, T D ; (a) T D = 2 0 0 ° C , (b) T D = I 4 2 ° C , (c) T D = I 0 4 ° C . The m i c r o w a v e p o w e r u s e d w a s 600 m W for (a) and (c), a n d i00 mW for (b).
as e m i t t i n g s t a t e s a n d n o n r a d i a t i v e recombination centres, respectively. The D 2 c e n t r e w a s i d e n t i f i e d as d a n g l i n g b o n d s in m u l t i v a c a n cies or v o i d s since its g - v a l u e c o i n c i d e d w i t h t h a t of m a g n e t i c c e n t r e s r e s p o n s i b l e for the o r d i n a r y E S R s i g n a l ( g = 2 . 0 0 5 5 ) in e v a p o r a t e d a-Si [13]. As w a s m e n t i o n e d above, the A c e n t r e r e s o n a n c e e x h i b i t e d a s i g n i f i c a n t b r o a d e n i n g and a c h a n g e in g - v a l u e s w i t h i n c r e a s i n g t h e m i c r o w a v e power. T h i s r e s u l t m a y be i n t e r p r e t e d in t e r m s of an e x c h a n g e i n t e r a c t i o n b e t w e e n an A c e n t r e a n d a d o n o r - l i k e c e n t r e p a r t i c i p a t i n g in the d o n o r - a c c e p t o r p a i r type r a d i a t i v e r e c o m b i n a t i o n in a s i m i l a r w a y to the case of Z n O : L i [14]. Thus, the A c e n t r e i s o l a t e d f r o m o t h e r c e n t r e s seems to h a v e g = 2 . 0 1 2 and ~ H I / 2 = 4 0 G. F r o m this p o i n t of view, d o n o r - l i k e c e n t r e s a c t i n g as the e m i t t i n g s t a t e s m a y be c o u p l e d w i t h a c c e p t o r - l i k e c e n t r e s , so t h a t the D 1 line s h o u l d be s i g n i f i c a n t l y b r o a d e n e d w i t h i n c r e a s i n g the m i c r o w a v e power. H o w e v e r , this w a s n o t the case for the D 1 c e n t r e r e s o n a n c e , a l t h o u g h this r e s o n a n c e line w a s s l i g h t l y b r o a d e n e d at h i g h m i c r o w a v e power. T h e r e f o r e , t h e r e is a p o s s i b i l i t y t h a t the D 1 c e n t r e is a l s o a n o n radiative recombination c e n t r e a n d c a u s e s a d e c r e a s e in the lumin e s c e n c e i n t e n s i t y at r e s o n a n c e in a s i m i l a r w a y to the D 2 c e n t r e resonance. The O D M R m e a s u r e m e n t s w e r e d o n e at 2 K for s a m p l e s w i t h d i f f e r e n t deposition temperatures. Fig. 2 s h o w s the O D M R s p e c t r a w h i c h c o n s i s t of the D 1 and D 2 lines. S a m p l e s w i t h T D = 2 0 0 ° C and 142°C e x h i b i t e d v a r i a t i o n s of the O D M R s p e c t r u m w i t h t i m e a f t e r the a r g o n ion laser e x c i t a t i o n l i g h t (514.5 nm) w a s s w i t c h e d on at 2 K [15]. Therefore, the s t e a d y s t a t e O D M R s p e c t r a are s h o w n in Fig. 2 for those samples. The i n t e n s i t i e s of the D 1 a n d D 2 lines as w e l l as the A line are p l o t t e d as a f u n c t i o n of the d e p o s i t i o n t e m p e r a t u r e as s h o w n in Fig. 3. As seen f r o m t h e s e figures, the s a m p l e s p r e p a r e d at low deposition temperatures (below 260°C) d i d n o t e x h i b i t the A line. H o w e v e r , w h e n a n n e a l e d at h i g h e r t e m p e r a t u r e s s u c h as 350°C, the A line a p p e a r e d a n d the D 1 a n d D 2 l i n e s d e c r e a s e d in i n t e n s i t y as s h o w n
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K. Morigaki et al. / Optically Detected Magnetic Resonance in GD a-Si
Ifh
ID
0.2
IDI
x
o
0.1
]10
/
O.
5
tOO
200
300
0
TD(°C )
Fig. 3. Relative intensities of the ODMR signals of different centres D I, D2 and A m e a s u r e d at 2 K and the m i c r o w a v e power of 600 mW. in Fig. 4. In the following, we discuss the nature of the A centre on the basis of the above results. F r o m i n f r a r e d absorption studies [6-10], it 5as been c o n c l u d e d that the low deposition temperature samples contain large numbers of h y d r o g e n atoms in the c o n f i g u r a t i o n s such as SiH 2 and/or (SiH2) n chains, while the high deposition temperature samples contain the h y d r o g e n atoms mostly in the form of Si-H bonds. The fact that the A centre resonance is a s s o c i a t e d with the high d e p o s i t i o n temperature samples suggests that the A centre is related to the existence of the Si-H bond. A n n e a l i n g at high t e m p e r a t u r e s of samples p r e p a r e d at low t e m p e r a t u r e s results in r e a r r a n g e m e n t of the h y d r o g e n atom configuration. This can explain the observation that annealing of the low deposition temperature samples at higher
% :2~Cc AS-DEPOSITED
To = 210°C TA= 350°C ANNEALED
i
3.0
,
i
,
,
i
35
,
,
KG
Fig. 4. O D M R spectra obtained at 9.04 GHz and 2 K under u n f o c u s e d argon ion laser excitation with 200 mW for samples; (a) a s - d e p o s i t e d with TD=210°C, (b) annealed at 350°C(TD=210°C) The m i c r o w a v e power used was 1.5 W for (a) and 3.4 W for (b).
K. Morigaki et al. / Optically Detected Magnetic Resonance in GD a-Si
temperatures
caused
the A centre
resonance
637
to recover.
A p o s s i b l e m o d e l for the A centre is the n o n b o n d i n g state (T ° ) of the t h r e e - c e n t r e bond (Si-H-Si), s u g g e s t e d by F i s c h and Licciardello(1978) and also i n d e p e n d e n t l y by K o m a t s u b a r a and U d a (1978). The h y p e r f i n e i n t e r a c t i o n w i t h central h y d r o g e n n u c l e u s is e x p e c t e d to exist as a r e s u l t of the a d m i x t u r e of is o r b i t a l of the h y d r o g e n atom into the wave function of an u n p a i r e d electron. If one assumes the d i s t r i b u tion of g e o m e t r i c a l c o n f i g u r a t i o n of the t h r e e - c e n t r e bond, that is, the d i s t r i b u t i o n of is o r b i t a l ' s a d m i x t u r e degree of the T ° state, the h y p e r f i n e s p l i t t i n g is a v e r a g e d out, so that one can e x p l a i n the l i n e - b r o a d e n i n g of the A centre r e s o n a n c e ( ~ H I / 2 = 4 0 G). A c o r r e l a t i o n b e t w e e n the l u m i n e s c e n c e and the A centre is d i s c u s s e d in the following. F r o m our p r e v i o u s O D M R e x p e r i m e n t s [2,3], it has been c o n c l u d e d that the A centre r e s o n a n c e is a s s o c i a t e d w i t h an emission b a n d p e a k e d at 1.31 eV (950 nm) for a sample w i t h T D = 3 0 0 ° C and is the r a d i a t i v e r e c o m b i n a t i o n centre of the d o n o r - a c c e p t o r p a i r type. The peak p o s i t i o n of the l u m i n e s c e n c e spectra s h i f t e d to h i g h e r p h o t o n energy w i t h d e c r e a s i n g the d e p o s i t i o n temperature. The i n t e n s i t y of the 1.31 eV e m i s s i o n band d e c r e a s e d w i t h this change in the spectra. This is c o n s i s t e n t w i t h the model of the A centre d e s c r i b e d above, a l t h o u g h the main feature of the change in the lumin e s c e n c e spectra w i t h the d e p o s i t i o n t e m p e r a t u r e is e x p l a i n e d by the band gap change a s s o c i a t e d w i t h the h y d r o g e n c o n t e n t [16,17] and the p o s i t i o n of the energy level of the t h r e e - c e n t r e bond may shift w i t h the h y d r o g e n content. A n n e a l i n g of a sample w i t h T D = 2 1 0 ° C at 350°C c a u s e d the peak p o s i t i o n of the l u m i n e s c e n c e s p e c t r u m to shift from 1.52 eV (818 nm) to 1.42 eV (874 nm). A l s o the 1.31 eV e m i s s i o n band i n c r e a s e d in i n t e n s i t y w i t h this annealing. This is also c o n s i s t e n t w i t h the above m o d e l of the A centre. The d e p e n d e n c e s of the i n t e n s i t i e s of the D 1 and D 2 r e s o n a n c e s on the d e p o s i t i o n t e m p e r a t u r e are u n d e r s t o o d in terms of a c o m b i n a t i o n of s p i n - d e p e n d e n t and - i n d e p e n d e n t n o n r a d i a t i v e r e c o m b i n a t i o n p r o c e s s e s i n v o l v i n g the D 1 and D 2 centres. CONCLUSION F r o m the O D M R m e a s u r e m e n t s for GD a-Si samples w i t h v a r i o u s deposition temperatures, it is c o n c l u d e d that the A centre is r e l a t e d to the e x i s t e n c e of the Si-H bond and the n o n b o n d i n g state (T ° ) of the t h r e e - c e n t r e b o n d is a c a n d i d a t e for the A centre. A correlation b e t w e e n the 1.31 eV e m i s s i o n b a n d and the A centre r e s o n a n c e is c o n f i r m e d from the v a r i a t i o n of the l u m i n e s c e n c e spectra w i t h d e p o s i tion temperatures. ACKNOWLEDGEMENTS The O D M R e x p e r i m e n t s for samples w i t h T D = 3 0 0 ° C were done in collaboration w i t h Dr. D.J. Dunstan, to w h o m we are grateful. One of us (KM) is i n d e b t e d to the Science R e s e a r c h Council for a S e n i o r V i s i t ing F e l l o w s h i p w h i c h made it p o s s i b l e to do the p r e s e n t e x p e r i m e n t s at the M a g n e t o - O p t i c s group, U n i v e r s i t y of Hull and also thanks Dr. K. K o m a t s u b a r a and Dr. T. Uda, Hitachi C e n t r a l R e s e a r c h Laboratory, for v a l u a b l e d i s c u s s i o n s on the t h r e e - c e n t r e bonds. P.D. is grateful to SRC for a R e s e a r c h A s s i s t a n t s h i p and we a c k n o w l e d g e the generous support of this w o r k by SRC.
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Spear, W.E., Adv.Phys. 26 (1977) 811. Morigaki, K., Dunstan, D.J., Cavenett, B.C., Dawson, P., Nicholls, J.E., Nitta, S., and Shimakawa, K., Solid State Commun 26 (1978) 981. Morigaki, K., Dawson, P., Cavenett, B.C., Dunstan, D.J., Nitta, S. and Shimakawa, K., Proc. 14th Int. Conf. Phys. Semicond., Edinburgh, (1978) p.i163. Biegelsen, D.K., Knights, J.C., Street, R.A., Tsang, C., and White, R.M., Phil. Mag. B37 (1978) 477. Lampel, G., Rosso, M., and Solomon, I., Private communication (1978). Brodsky, M.H., Cardona, M., and Cuomo, J.J., Phys. Rev. BI6 (1977) 3556. Tsai, C.C., Fritzsche, H., Tanielian, M.H., Gaczi, P.J., Persans, P.D., and Vesaghi, M.A., Proc. 7th Int. Conf. Amorph. & Liq. Semicond., Edinburgh, (1977) p. 339. Knights, J.C., Lucovsky, G. and Nemanich, R.J., Phil. Mag. B37 (1978) 467. Lucovsky, G., Nemanich, R.J., and Knights, J.C., Phys. Rev. BI9 (1979) 2064. Knights, J.C., Lucovsky, G., and Nemanich, R.J., J. Non-Cryst. Solids 32 (1979) 393. Fisch, R., and Licciardello, D.C., Phys. Rev. Lett. 41 (1978) 889. Komatsubara, K. and Uda, T., Private communication (1978). Brodsky, MH. and Title, R.S., Phys. Rev. Lett. 23 (1969) 581. Cox, R.T., Block, D., Herv~, A., Piccard, R., Santier, C., and Helbig, R., Solid State Commun. 25 (1978) 77. Morigaki, K., Cavenett, B.C., Dawson, P., Nitta, S., and Shimakawa, K., to be published in Solid State Commun. Solomon, I., Perrin, J. and Bourdon, B., Proc. 14th Int. Conf. Phys. Semicond., Edinburgh, (1978) p. 689. Dunstan, D.J. and Rosso, M., to be published.
EFFECT OF D E P O S I T I O N T E M P E R A T U R E AND A N N E A L I N G ON O P T I C A L L Y D E T E C T E D M A G N E T I C R E S O N A N C E IN GD a-Si K. M o r i g a k i I n s t i t u t e for S o l i d State P h y s i c s U n i v e r s i t y of T o k y o Roppongi, Tokyo 106, Japan B.C.
Cavenett,
P. Dawson
D e p a r t m e n t of Physics, U n i v e r s i t y Hull, HU6 7RX, U. K. S. N i t t a
of Hull
and K. S h i m a k a w a
F a c u l t y of Engineering, G i f u U n i v e r s i t y Kakamihara, G i f u 504, Japan
O p t i c a l l y d e t e c t e d m a g n e t i c r e s o n a n c e of GD a-Si has been e x a m i n e d as a function of d e p o s i t i o n t e m p e r a t u r e at 2 K and 9 GHz. A b r o a d line was o b s e r v e d only in samples p r e p a r e d above 260°C. S a m p l e s p r e p a r e d b e l o w 260°C e x h i b i t e d only two n a r r o w lines, but w h e n a n n e a l e d at h i g h temperatures such as 350°C, the b r o a d line a p p e a r e d and the n a r r o w lines d e c r e a s e d in intensity. The b r o a d line m a y be i n t e r p r e t e d in terms of a nonb o n d i n g state (T ° ) of a t h r e e - c e n t r e bond.
INTRODUCTION The gap states and r e c o m b i n a t i o n p r o c e s s e s in g l o w - d i s c h a r g e a-Si have been i n v e s t i g a t e d using v a r i o u s kinds of techniques (Spear1977). In o p t i c a l l y d e t e c t e d m a g n e t i c r e s o n a n c e (ODMR) experiments, resonance signals are d e t e c t e d by m o n i t o r i n g the i n t e n s i t y or p o l a r i z a tion of luminescence. Therefore, from these m e a s u r e m e n t s , we can obtain i n f o r m a t i o n about the r e c o m b i n a t i o n centres and the correlation b e t w e e n l u m i n e s c e n c e and m a g n e t i c centres r e s p o n s i b l e for ODMR. This t e c h n i q u e has been a p p l i e d to GD a-Si by M o r i g a k i et al (1978), and i n d e p e n d e n t l y by B i e g e l s e n et al (1978) and by Lampel et a i ( 1 9 7 8 ) In this p a p e r we p r e s e n t e x p e r i m e n t a l results of O D M R in GD a-Si w i t h d i f f e r e n t d e p o s i t i o n temperatures. The e l e c t r o n i c p r o p e r t i e s of GD a-Si are a f f e c t e d by v a r i o u s p a r a m e t e r s of sample preparation, for example, the d e p o s i t i o n temperature. It is w e l l known that the SiH b o n d i n g e n v i r o n m e n t depends on the d e p o s i t i o n t e m p e r a t u r e of sample preparation [6 - i0]. Therefore, we can d i s c u s s the nature of the gap states r e s p o n s i b l e for l u m i n e s c e n c e and O D M R signals on the basis of the e x p e r i m e n t a l results on the c o r r e l a t i o n b e t w e e n O D M R spectra and d e p o s i t i o n temperatures. A n n e a l i n g e x p e r i m e n t s p r o v i d e also useful i n f o r m a t i o n to clarify the n a t u r e of the gap states. Thus we i n t e r p r e t the e x p e r i m e n t a l results in terms of the r a d i a [ i v e and n o n r a d i a t i v e r e c o m b i n a t i o n p r o c e s s e s i n v o l v i n g gap states such as d a n g l i n g bonds and t h r e e - c e n t r e bonds [11,12].
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Fig. l. O D M R s p e c t r u m o b t a i n e d at 9.04 GHz and 2 K m o n i t o r i n g the total i n t e n s i t y of the e m i t t e d light for a sample with TD=300°C. The m i c r o w a v e p o w e r used was 4 W. EXPERIMENTAL The O D M R m e a s u r e m e n t s were c a r r i e d out at 2 K using the X band microwave e q u i p m e n t w i t h the m i c r o w a v e source of a k l y s t r o n (VA297) and a 16 W t r a v e l l i n g w a v e tube (EEl095). The O D M R signal was d e t e c t e d by m o n i t o r i n g all of the e m i t t e d light from the sample under unfocused argon ion laser e x c i t a t i o n at 514.5 nm. The m i c r o w a v e p o w e r was c h o p p e d at 1 kHz and the O D M R signal was d e t e c t e d using a phase sensitive d e t e c t o r and a N i c o l e t signal averager. The luminescence e f f i c i e n c y d e p e n d e d on the d e p o s i t i o n t e m p e r a t u r e s of samples, but the l u m i n e s c e n c e i n t e n s i t y was kept at 2 V at the output of an S1 tube (EMI9684B). S a m p l e s w e r e p r e p a r e d by glow d i s c h a r g e decomposition of silane d i l u t e d to 10% in argon w i t h an inductive coupling scheme. The d e p o s i t i o n t e m p e r a t u r e s were 104, 142, 200, 210, 240, 255, 300, and 359°C. The t h i c k n e s s of the a-Si films ranged between 0.4 and 1 ~ m . The surface of the substrate was r o u g h e n e d before d e p o s i t i o n of a-Si to avoid i n t e r f e r e n c e effects in the luminescence. RESULTS
AND D I S C U S S I O N
As was p r e v i o u s l y r e p o r t e d [2,3], the O D M R s p e c t r u m of samples with T D = 3 0 0 ° C c o n s i s t e d of two n a r r o w lines D1 and D 2, and a broad line A. A typical e x a m p l e of this s p e c t r u m is shown in Fig. l. The D1 and D 2 lines c o r r e s p o n d to d e c r e a s e s in the luminescence intensity, w h o s e gvalues are 2.018 and 2.006, respectively. The A line c o r r e s p o n d s to an increase in the l u m i n e s c e n c e ~ n t e n s i t y , whose g-value and line w i d t h d e p e n d on the m i c r o w a v e power. At r e l a t i v e l y low m i c r o w a v e powers, the A line c o n s i s t e d of two components w h i c h had G a u s s i a n shapes w i t h g-values of 2.00 and 2.012 and w i t h line w i d t h s (the full h a l f - a m p l i t u d e w i d t h ~ H I / 2 ) of 230 G and 40 G, respectively. A b r o a d line w i t h g=2.00 and ~ H I / 2 = 2 3 0 G was e n h a n c e d at high microwave powers. In p r e v i o u s papers [2,3], we i n t e r p r e t e d the m a g n e t i c centres responsible for the O D M R as follows: The A line arises from an acceptorlike centre c o n t r i b u t i n g to the d o n o r - a c c e p t o r pair type radiative recombination. The D 1 and D 2 centres are d o n o r - l i k e centres acting
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Fig. 2. O D M R s p e c t r a o b t a i n e d at 9.04 G H z a n d 2 K for s a m p l e s w i t h various deposition temperatures, T D ; (a) T D = 2 0 0 ° C , (b) T D = I 4 2 ° C , (c) T D = I 0 4 ° C . The m i c r o w a v e p o w e r u s e d w a s 600 m W for (a) and (c), a n d i00 mW for (b).
as e m i t t i n g s t a t e s a n d n o n r a d i a t i v e recombination centres, respectively. The D 2 c e n t r e w a s i d e n t i f i e d as d a n g l i n g b o n d s in m u l t i v a c a n cies or v o i d s since its g - v a l u e c o i n c i d e d w i t h t h a t of m a g n e t i c c e n t r e s r e s p o n s i b l e for the o r d i n a r y E S R s i g n a l ( g = 2 . 0 0 5 5 ) in e v a p o r a t e d a-Si [13]. As w a s m e n t i o n e d above, the A c e n t r e r e s o n a n c e e x h i b i t e d a s i g n i f i c a n t b r o a d e n i n g and a c h a n g e in g - v a l u e s w i t h i n c r e a s i n g t h e m i c r o w a v e power. T h i s r e s u l t m a y be i n t e r p r e t e d in t e r m s of an e x c h a n g e i n t e r a c t i o n b e t w e e n an A c e n t r e a n d a d o n o r - l i k e c e n t r e p a r t i c i p a t i n g in the d o n o r - a c c e p t o r p a i r type r a d i a t i v e r e c o m b i n a t i o n in a s i m i l a r w a y to the case of Z n O : L i [14]. Thus, the A c e n t r e i s o l a t e d f r o m o t h e r c e n t r e s seems to h a v e g = 2 . 0 1 2 and ~ H I / 2 = 4 0 G. F r o m this p o i n t of view, d o n o r - l i k e c e n t r e s a c t i n g as the e m i t t i n g s t a t e s m a y be c o u p l e d w i t h a c c e p t o r - l i k e c e n t r e s , so t h a t the D 1 line s h o u l d be s i g n i f i c a n t l y b r o a d e n e d w i t h i n c r e a s i n g the m i c r o w a v e power. H o w e v e r , this w a s n o t the case for the D 1 c e n t r e r e s o n a n c e , a l t h o u g h this r e s o n a n c e line w a s s l i g h t l y b r o a d e n e d at h i g h m i c r o w a v e power. T h e r e f o r e , t h e r e is a p o s s i b i l i t y t h a t the D 1 c e n t r e is a l s o a n o n radiative recombination c e n t r e a n d c a u s e s a d e c r e a s e in the lumin e s c e n c e i n t e n s i t y at r e s o n a n c e in a s i m i l a r w a y to the D 2 c e n t r e resonance. The O D M R m e a s u r e m e n t s w e r e d o n e at 2 K for s a m p l e s w i t h d i f f e r e n t deposition temperatures. Fig. 2 s h o w s the O D M R s p e c t r a w h i c h c o n s i s t of the D 1 and D 2 lines. S a m p l e s w i t h T D = 2 0 0 ° C and 142°C e x h i b i t e d v a r i a t i o n s of the O D M R s p e c t r u m w i t h t i m e a f t e r the a r g o n ion laser e x c i t a t i o n l i g h t (514.5 nm) w a s s w i t c h e d on at 2 K [15]. Therefore, the s t e a d y s t a t e O D M R s p e c t r a are s h o w n in Fig. 2 for those samples. The i n t e n s i t i e s of the D 1 a n d D 2 lines as w e l l as the A line are p l o t t e d as a f u n c t i o n of the d e p o s i t i o n t e m p e r a t u r e as s h o w n in Fig. 3. As seen f r o m t h e s e figures, the s a m p l e s p r e p a r e d at low deposition temperatures (below 260°C) d i d n o t e x h i b i t the A line. H o w e v e r , w h e n a n n e a l e d at h i g h e r t e m p e r a t u r e s s u c h as 350°C, the A line a p p e a r e d a n d the D 1 a n d D 2 l i n e s d e c r e a s e d in i n t e n s i t y as s h o w n
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Ifh
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/
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Fig. 3. Relative intensities of the ODMR signals of different centres D I, D2 and A m e a s u r e d at 2 K and the m i c r o w a v e power of 600 mW. in Fig. 4. In the following, we discuss the nature of the A centre on the basis of the above results. F r o m i n f r a r e d absorption studies [6-10], it 5as been c o n c l u d e d that the low deposition temperature samples contain large numbers of h y d r o g e n atoms in the c o n f i g u r a t i o n s such as SiH 2 and/or (SiH2) n chains, while the high deposition temperature samples contain the h y d r o g e n atoms mostly in the form of Si-H bonds. The fact that the A centre resonance is a s s o c i a t e d with the high d e p o s i t i o n temperature samples suggests that the A centre is related to the existence of the Si-H bond. A n n e a l i n g at high t e m p e r a t u r e s of samples p r e p a r e d at low t e m p e r a t u r e s results in r e a r r a n g e m e n t of the h y d r o g e n atom configuration. This can explain the observation that annealing of the low deposition temperature samples at higher
% :2~Cc AS-DEPOSITED
To = 210°C TA= 350°C ANNEALED
i
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Fig. 4. O D M R spectra obtained at 9.04 GHz and 2 K under u n f o c u s e d argon ion laser excitation with 200 mW for samples; (a) a s - d e p o s i t e d with TD=210°C, (b) annealed at 350°C(TD=210°C) The m i c r o w a v e power used was 1.5 W for (a) and 3.4 W for (b).
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temperatures
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to recover.
A p o s s i b l e m o d e l for the A centre is the n o n b o n d i n g state (T ° ) of the t h r e e - c e n t r e bond (Si-H-Si), s u g g e s t e d by F i s c h and Licciardello(1978) and also i n d e p e n d e n t l y by K o m a t s u b a r a and U d a (1978). The h y p e r f i n e i n t e r a c t i o n w i t h central h y d r o g e n n u c l e u s is e x p e c t e d to exist as a r e s u l t of the a d m i x t u r e of is o r b i t a l of the h y d r o g e n atom into the wave function of an u n p a i r e d electron. If one assumes the d i s t r i b u tion of g e o m e t r i c a l c o n f i g u r a t i o n of the t h r e e - c e n t r e bond, that is, the d i s t r i b u t i o n of is o r b i t a l ' s a d m i x t u r e degree of the T ° state, the h y p e r f i n e s p l i t t i n g is a v e r a g e d out, so that one can e x p l a i n the l i n e - b r o a d e n i n g of the A centre r e s o n a n c e ( ~ H I / 2 = 4 0 G). A c o r r e l a t i o n b e t w e e n the l u m i n e s c e n c e and the A centre is d i s c u s s e d in the following. F r o m our p r e v i o u s O D M R e x p e r i m e n t s [2,3], it has been c o n c l u d e d that the A centre r e s o n a n c e is a s s o c i a t e d w i t h an emission b a n d p e a k e d at 1.31 eV (950 nm) for a sample w i t h T D = 3 0 0 ° C and is the r a d i a t i v e r e c o m b i n a t i o n centre of the d o n o r - a c c e p t o r p a i r type. The peak p o s i t i o n of the l u m i n e s c e n c e spectra s h i f t e d to h i g h e r p h o t o n energy w i t h d e c r e a s i n g the d e p o s i t i o n temperature. The i n t e n s i t y of the 1.31 eV e m i s s i o n band d e c r e a s e d w i t h this change in the spectra. This is c o n s i s t e n t w i t h the model of the A centre d e s c r i b e d above, a l t h o u g h the main feature of the change in the lumin e s c e n c e spectra w i t h the d e p o s i t i o n t e m p e r a t u r e is e x p l a i n e d by the band gap change a s s o c i a t e d w i t h the h y d r o g e n c o n t e n t [16,17] and the p o s i t i o n of the energy level of the t h r e e - c e n t r e bond may shift w i t h the h y d r o g e n content. A n n e a l i n g of a sample w i t h T D = 2 1 0 ° C at 350°C c a u s e d the peak p o s i t i o n of the l u m i n e s c e n c e s p e c t r u m to shift from 1.52 eV (818 nm) to 1.42 eV (874 nm). A l s o the 1.31 eV e m i s s i o n band i n c r e a s e d in i n t e n s i t y w i t h this annealing. This is also c o n s i s t e n t w i t h the above m o d e l of the A centre. The d e p e n d e n c e s of the i n t e n s i t i e s of the D 1 and D 2 r e s o n a n c e s on the d e p o s i t i o n t e m p e r a t u r e are u n d e r s t o o d in terms of a c o m b i n a t i o n of s p i n - d e p e n d e n t and - i n d e p e n d e n t n o n r a d i a t i v e r e c o m b i n a t i o n p r o c e s s e s i n v o l v i n g the D 1 and D 2 centres. CONCLUSION F r o m the O D M R m e a s u r e m e n t s for GD a-Si samples w i t h v a r i o u s deposition temperatures, it is c o n c l u d e d that the A centre is r e l a t e d to the e x i s t e n c e of the Si-H bond and the n o n b o n d i n g state (T ° ) of the t h r e e - c e n t r e b o n d is a c a n d i d a t e for the A centre. A correlation b e t w e e n the 1.31 eV e m i s s i o n b a n d and the A centre r e s o n a n c e is c o n f i r m e d from the v a r i a t i o n of the l u m i n e s c e n c e spectra w i t h d e p o s i tion temperatures. ACKNOWLEDGEMENTS The O D M R e x p e r i m e n t s for samples w i t h T D = 3 0 0 ° C were done in collaboration w i t h Dr. D.J. Dunstan, to w h o m we are grateful. One of us (KM) is i n d e b t e d to the Science R e s e a r c h Council for a S e n i o r V i s i t ing F e l l o w s h i p w h i c h made it p o s s i b l e to do the p r e s e n t e x p e r i m e n t s at the M a g n e t o - O p t i c s group, U n i v e r s i t y of Hull and also thanks Dr. K. K o m a t s u b a r a and Dr. T. Uda, Hitachi C e n t r a l R e s e a r c h Laboratory, for v a l u a b l e d i s c u s s i o n s on the t h r e e - c e n t r e bonds. P.D. is grateful to SRC for a R e s e a r c h A s s i s t a n t s h i p and we a c k n o w l e d g e the generous support of this w o r k by SRC.
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