PHOTOLUMINESCENCE DECAY IN a-Si : INFLUENCE OF EXCITATION DENSITY AND n-TYPE DOPING

HAL Id: jpa-00220734

https://hal.archives-ouvertes.fr/jpa-00220734

Submitted on 1 Jan 1981

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

PHOTOLUMINESCENCE DECAY IN a-Si : INFLUENCE OF EXCITATION DENSITY AND

n-TYPE DOPING

W. Czaja, S. Kinmond

To cite this version:

W. Czaja, S. Kinmond. PHOTOLUMINESCENCE DECAY IN a-Si : INFLUENCE OF EXCITA-

TION DENSITY AND n-TYPE DOPING. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-543-

C4-546. �10.1051/jphyscol:19814117�. �jpa-00220734�

PHOTOLUMINESCENCE DECAY IN a-Si : INFLUENCE OF EXCITATION DENSITY AND n-TYPE DOPING

W. Czaja and S . ~inmond"

I n s t i t u t e of Applied Physics,

EPFL,

CH-1015 Lausanne, Switzerland

" Carnegie Lab. o f Phys., Universi%y o f Lhndee, Scotland, U.K.

A b s t r a c t . - The luminescenceofundoped GD a-Si d i s p l a y s a t s h o r t times an e x p o n e n t i a l decay. I n t h i s a r t i c l e , we show t h a t t h e eecay time c o n s t a n t de- c r e a s e s w i t h i n c r e a s i n g e x c i t a t i o n d e n s i t y . The power law-decay a t l o n g e r t i m e s i s a l s o , b u t o n l y weakly changed by i n c r e a s e d e x c i t a t i o n d e n s i t y . Doping, however, h a s a pronounced and remarkable e f f e c t i n t h i s time domain which i s e x p l a i n e l w i t h t h e e x i s t e n c e of c?efect s t a t e s induced by t h e doping.

I n t r o d u c t i o n . - Time r e s o l v e d photoluminescencehasbeen observed i n t h e p a s t by seve- r a l groups [1][2][3][4]. I n t h i s c o n t r i b u t i o n we c o n s i d e r e x c i t a t i o n d e n s i t y e f f e c t s a s w e l l a s t h e dependence o f t h e decay behaviour on n-type doping. We use glow d i s - charge a-Si samples c o n t a i n i n g about 5% hydrogen. The d e p o s i t i o n temperature i s gene- r a l l y 550K. I n t h e s e measurements, t h e samples were immersed i n l i q u i d n i t r o g e n and e x c i t e d by a p u l s e d , n i t r o g e n pumped dye l a s e r (Sopra, t y p e G03A) s e t a t an e n e r g y o f 2.16 eV. A s mentioned i n o u r e a r l i e r p u b l i c a t i o n s [1:[2] we observe a t z e r o d e l a y a luminescence spectrum c o n t a i n i n g a f a s t decaying peak a t 1.45 eV and a slow decaying peak a t 1.35 eV. The f a c t t h a t o t h e r a u t h o r s [3][4] do not observe t h e f a s t decaying peak i s ?robably r e l a t e d t o d i f f e r e n c e s i n t h e time r e s o l u t i o n , s i n c e a l r e a d y a f t e r 1 0 n s d e l a y t h e i n t e n s i t y of t h e f a s t decaying peak i s reduced d r a s t i c a l l y . I n our measurements we use a s d e t e c t i n g e l e c t r o n i c s a P h i l i p s P M 3400 sampling o s c i l l o s c o p e w i t h 200 p s r i s e time. A d e t a i l e d a n a l y s i s of t h e s i g n a l which i s l i m i t e d i n r e s o l u - t i o n by t h e d y e - l a s e r p u l s e shape and t h e response o f t h e p h o t o m u l t i p l i e r , shows t h a t down t o a d e l a y of 1 t o 2 ns from t h e luminescence maximum we observe t h e t r u e luminescence decay of t h e sample. For a l l decay measurements, t h e luminescence has been d e t e c t e d a t 1 . 3 eV, which i s 0.05 eV below t h e slow decaying maximum t o reduce i n t e r f e r e n c e w i t h t h e f a s t decaying peak.

E x c i t a e i o n d e n s i t y e f f e c t s . - We f i r s t c o n s i d e r t h e e x p o n e n t i a l decay a t s h o r t times a s d i s p l a y e d i n t h e i n s e r t of Fig. 1 f o r a n-type doped sample. T h i s decay i s b a s i - c a l l y t h e same a s r e p o r t e d e a r l i e r [1][21[5] f o r undoped samples. S i n c e t h e samples were immersed i n l i q u i d n i t r o g e n , we e x p e c t h e a t i n g e f f e c t s t o b e low. I n f a c t , t h e r e s u l t s o f o u r e x p e r i n e n t s , although performed a t 77K, correspond t o t h e low tempe- r a t u r e l i m i t d e f i n e d i n E5J. For t h e change of t h e luminescence decay w i t h i n c r e a - s i n g temperature t h e t h e o r y o f Ref.[5] i s a p p l i c a b l e . I n F i g . 2 we show t h e change of t h e e x p o n e n t i a l decay time c o n s t a n t w i t h e x c i t a t i o n d e n s i t y f o r an undoped sample a s compared w i t h t h e luminescence response a t z e r o d e l a y . Even i n t h e range of l i n e a r response, one o b s e r v e s a s t r o n g i n c r e a s e of t h e time c o n s t a n t w i t h d e c r e a s i n g e x c i t a t i o n . We do n o t c o n s i d e r t h e n o n l i n e a r response r e g i o n s i n c e s a m p l e , h e a t i n g i n t h i s range cannot be excluded. The a b s o l u t e e x c i t a t i o n d e n s i t y s c a l e i s b e l i e v e d t o be c o r r e c t t o w i t h i n a f a c t o r of two. The v a l u e s seem t o be i n r e a s o n a b l e agreement w i t h t h e o b s e r v a t i o n s of Shah e t a l . f 4 1 . With t h e a u t h o r s of Ref. 4, we assume an i n i t i a l c a r r i e r c o n c e n t r a t i o n no of about 1 0 ~ * c m - ~ f o r an e x c i t a t i o n d e n s i t y of 3 p ~ / c m 2 . I n t h e range o f l i n e a r response, (no depends l i n e a r l y on t h e e x c i t a t i o n d e n s i t y ) ^{~ ~}can be decomposed t o y i e l d ^{f f}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814117

JOURNAL DE PHYSIQUE

Fig. 1 : Photoluminescence decay f o r an n-type a-Si Fig. 2 : Time c o n s t a n t o f t h e Sample. The i n s e r t shows t h e f i r s t e x p o n e n t i a l de- f i r s t e x p o n e n t i a l decay o f cay i n a semilogarithmic p l o t . t h e p u r e a-Si sample a s a

f u n c t i o n of e x c i t a t i o n densi- t y . Lower h a l f : photolumines- cence response w i t h t h e same e x c i t a t i o n s c a l e .

An e x p o n e n t i a l decay w i t h a time c o n s t a n t given by Eq. (1) f o l l o w s from t h e r a t e e q u a t i o n f o r t h e c a r r i e r c o n c e n t r a t i o n n

T h i s can b e seen when t h e s o l u t i o n

i s expanded f o r t < T which corresponds w e l l t o t h e r e g i o n o f v a l i d i t y o f Eq. (1) a s found e x p e r i m e n t a l l y . Thus o u r experiments s u g g e s t a r a t e e q u a t i o n f o r t h e f a s t decay a s g i v e n i n Eq. ( 2 ) . We propose t h a t .I b e i n t e r p r e t e d a s t h e i n t r i n s i c

(Onsager t y p e ) r a d i a t i v e recombination l i f e time of t h e e l e c t r o n - h o l e p a i r s c r e a t e d by t h e e x c i t a t i o n . Using t h e arguments of Sinha and Didomenico [6] f o r a g e n e r i c a l l y r e l a t e d problem t h e term an2 i n Eq. ( 2 ) might b e i n t e r p r e t e d a s a non r a d i a t i v e Auger-effect, however, o t h e r i n t e r p r e t a t i o n s cannot be r u l e d o u t . Based on d i f f e r e n t arguments S t r e e t [ 7 j h a s r e c e n t l y a l s o proposed an Auger-effect f o r a-Si.

Furthermore, independent of t h i s i n t e r p r e t a t i o n o u r d a t a a l l o w s u s t o reduce t h e e x c i t a t i o n energy dependence of t h e f a s t e x p o n e n t i a l decay r e p o r t e d e a r l i e r [81[2], t o a n e x c i t a t i o n d e n s i t y e f f e c t w i t h t h e known energy dependent a b s o r p t i o n c o e f f i c i e n t . I n f a c t , i f s o reduced, t h e d a t a r e p o r t e d i n Ref. 8 f o l l o w s e x a c t l y t h e curve o f Flg. 2.

-

pendent of e x c i t a t i o n d e n s i t y .

E f f e c t of doping.- Fig. 3 demonstrates t h e e f f e c t of n-type doping on t h e lumines- cence time decay. The c h a r a c t e r i s t i c ' s h i f t of t h e slow decaying component t o s h o r t t i m e s h a s been proposed a l r e a d y e a r l i e r 121. I t i s i n t e r e s t i n g t o n o t e t h a t t h e s h i f t i s a l r e a d y v i s i b l e a t r a t h e r small dopings. We d e f i n e a c h a r a c t e r i s t i c

o o o a-54 P I(N)

...

^{a-51 P,B}

^{l m}

l I

o no 200 300 LOO 5w

N d o p i n g l ~ ~ ~ m l

F i g . 4 : C h a r a c t e r L s t i c time

?

and E = - E a s f u n c t i o n o f doping c o n c e n t r a t i o n . f

Fig. 3 : I n f l u e n c e of n-type doping on t h e luminescence decay.

time ?, t h e e x t r a p o l a t e d i n t e r s e c t i o n o f t h e two power-law-behaviours and p l o t i n Fig. 4 ? a s a f u n c t i o n o f t h e doping c o n c e n t r a t i o n N added t o t h e SiHq e n t e r i n g t h e glow d i s c h a r g e d e p o s i t i o n system. The dependence ? ( N ) i s compared w i t h t h e f u n c t i o n E ~ ( N ) which i s t h e a c t i v a t i o n energy d e s c r i b i n g t h e t e m p e r a t u r e dependence of t h e e l e c t r i c r e s i s t i v i t y

I n Eq. ( 4 ) E, i s t h e m o b i l i t y edge and E f ( N ) t h e Fermi energy, compare e.g.[9]. The two dependences

(?(NI

and E 0 ( N ) ) do n o t s u p p o r t an i n t e r p r e t a t i o n of t h e s h i f t a s b e i n g due t o a change i n t h e p o s i t i o n o f t h e Fermi-energy w i t h doping. To e l u c i d a t e t h e dependence of

? ( N I

on t h e Fermi energy, we have measured two compensated samples [lO]. The r e s u l t i n g ? a r e a l s o given i n Fig. 4 . T h i s d a t a shows beyond doubt t h a t t h e v a l u e of 't is independent o f t h e Fermi l e v e l p o s i t i o n and depends, f o r t h e compensated samples, on t h e t o t a l dopant c o n c e n t r a t i o n N = ND

+

^Ni. I t i s tempting t o assume t h a t d e f e c t s t a t e s i n t r o d u c e d w i t h t h e doping a r e r e s p o n s i b l e f o r t h e change i n ?. T h i s assumption i m p l i e s t h a t p and n-type doping a r e e q u a l l y e f f i c i e n t i n t h e c r e a t i o n of d e f e c t s which a r e r e s p o n s i b l e f o r t h e luminescence decay. The importance of d e f e c t s t a t e s r e l a t e d t o doping h a s r e c e n t l y been s t r e s s e d by S t r e e t e t al.1111.

JOURNAL DE PHYSIQUE

Since we observe the same decay for an n-type sample with a donor concentration N = ND as for a compensated sample with N = ND/2

+

~ ~ / 2 , an explanation of the time decay as recombination via donor acceptor-pairs of varying distance seems to be ruled out.

Furthermore, Fig. 3 suggests that one could explain the quenching of cw- luminescence observed for increased doping L81 by the shift of ?(N). However, sys- tematic changes in the zero delay photoluminescence spectra indicate that this might only be part of the process; additional effects seem to contribute to the quenching in an important way.

Finally, the results presented here seem to demand a reinterpretation of the states responsible for the luminescence decay. The original idea of the unique importance of intrinsic band tail states [1][12] should probably be changed by adding the assumption that doping creates defect states which increase the concen- tration of band tail states without drastically changing their energy distribution.

Since doping decreases the structural short range.order even without defect creation, one might speculate that it is the increase in disorder that one is sen- sitive to in these experiments.

We are grateful for clarifying discussions with K. Maschke, EPF-Lausanne.

References.-

1) S. KURITA, W. CZAJA and S. KINMOND, Solid State comm.

2

(1979) 879, 1345 and papers quoted therein.

2) W. CZAJA and S. KURITA, Helv. Phys. Acta (1981) in press.

3) hl. REHM and R. FISCHER, Phys. Stat. Sol. (b)

94

(1979) 595.

4) J. SHAH, B.G. BAYLEY and F.B. ALEXANDER, Jr., Solid State Comm.

36

(1930) 199.

5) J. NOOLANDI, K.M. HONG and R.A. STREET, Solid State Comm.

34

(1980) 45, and Phys. Rev.

B23

(1981) 2967.

6) K.P. SINHA and M. DIDOMENICO, Jr., Phys. Rev. (1970) 2623.

7) R.A. STREET, Phys. Rev. (1981) 861.

8) R. FISCHER, W. REHM, J. STUKE and U.VOGET-GROTE, J. Noncryst. Solids

35/36

(1980) 687.

9) W.E. SPEAR, Adv. Phys.

26

(1977) 811.

10) X-Si(BP)-64 : 255 vppm PH3/SiH4, 250 vppm B2H6/SiHq E

.

=

0,8 eV, ~ R T 10-10 ( k m ) -l, Td = 550K

X-Si(BP)-38 : 51 vppm PH3/SiH4

,

50 vppm B2H6/SiH4, E. = 0.85 eV

11) R.A. STREET, D.K. BIEGELSEN and J.C. KNIGHTS, PhyS. Rev. B (1981), in press.

12) R.A. STREET, Phys. Rev.

B21

(1980) 5775.