PHOTOLUMINESCENCE VERSUS BORON DOPING AND PREPARATION CONDITIONS

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PHOTOLUMINESCENCE VERSUS BORON DOPING AND PREPARATION CONDITIONS

J. Bruyère, A. Deneuville

To cite this version:

J. Bruyère, A. Deneuville. PHOTOLUMINESCENCE VERSUS BORON DOPING AND PREPA- RATION CONDITIONS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-339-C4-342.

�10.1051/jphyscol:1981472�. �jpa-00220929�

JOURNAL DE PHYSIQUE

CoZZoque C4, suppldment au nO1O, Tome 42, oetobre 1981 page C4-339

PHOTOLUMINESCENCE VERSUS BORON DOPING AND PREPARATION CONDITIONS

J . C . BruySre and A. Deneuville

Groupe des Transitions de Phases, C.N.R.S., B.P. 166, 38042 GrenobZe Cedex, France

A b s t r a c t . -

The v a r i a t i o n s of t h e i n t e n s i t i e s of t h e v a r i o u s components o f t h e photo- luminescence spectrum of undoped and boron doped a-Si:H a r e r e p o r t e d ver- sus p r e p a r a t i o n parameters. They a r e d i s c u s s e d i n t h e framework of two phases ( g r a i n l i k e and g r a i n boundary l i k e ) of hydrogenated amorphous s i l i c o n . We conclude t h a t t h e g r a i n l i k e zone i s weakly modified by t h e d e p o s i t i o n r a t e o r s u b s t r a t e temperature i n undoped a-Si:H w h i l e i t i s s t r o n g l y changed by boron doping and t h a t t h e g r a i n boundary l i k e zone i s very s e n s i t i v e t o any p r e p a r a t i o n parameters.

I. INTRODUCTION

Photoluminescence i n undoped amorphous hydrogenated s i l i c o n (a-Si:H) was f i r s t r e p o r t e d by Engemann and F i s h e r ( 1 ) . I t h a s been g e n e r a l l y found i n b o t h glow d i s - charge and r e a c t i v e l y s p u t t e r e d undoped a-Si:H. However, t h e number of t h e lumines- cence peaks and t h e i r r e l a t i v e i n t e n s i t i e s v a r y widely with t h e c o n d i t i o n s of prepa- r a t i o n (2-5). From t h e v a r i a t i o n s of t h e luminescence i n t e n s i t y w i t h t h e t e m p e r a t u r e of measurement o r w i t h t i m e , s e v e r a l models have been proposed w i t h t h e i m p l i c i t a s -

sumption of an homogeneous m a t e r i a l with a s i n g l e d i s t r i b u t i o n of s t a t e s . The photon e n e r g i e s corresponding t o t h e peaks i n photoluminescence have been a t t r i b u t e d t o a number of d i f f e r e n t mechanisms : c o n v o l u t i o n of t h e s t a t e d i s t r i b u t i o n s i n t h e con- d u c t i o n (unoccupied) and v a l e n c e (occupied) band t a i l s ( I ) , band t o band t r a n s i t i o n s w i t h p o l a r o n i c e f f e c t s ( 4 ) , t r a p p i n g on d e f e c t s and subsequent r a d i a t i v e recombina- t i o n towards t h e valence band ( 3 ) . The d e c r e a s e of luminescence i n t e n s i t y w i t h borum doping (5) was a t t r i b u t e d t o a p a r a l l e l i n c r e a s e i n t h e d a n g l i n g bonds d e n s i - t y . I n t h e same t i m e , t h e i n t e n s i t y of t h e l i n e a t 0.93 eV ( a t t r i b u t e d t o c a t c h and l o s s of a n a d d i t i o n a l e l e c t r o n by d a n g l i n g bond) i n c r e a s e s .

We b e l i e v e t h a t many r e s u l t s such a s t h o s e o f photoluminescence, t h e r m a l l y sti- mulated c u r r e n t s , o p t i c a l a b s o r p t i o n and e l e c t r i c a l c o n d u c t i v i t y a r e n o t c o n s i s t e n t w i t h t h e d e s c r i p t i o n of t h e a-Si:H by a s i n g l e d i s t r i b u t i o n of s t a t e s ( 6 ) . On t h e c o n t r a r y , t h e y a r e q u i t e c o n s i s t e n t w i t h t h e e x i s t e n c e of a t l e a s t two d i s t r i b u t i o n s of s t a t e s i n an inhomogeneous m a t e r i a l . TEN and W ( 7 ) s t u d i e s g i v e evidence f o r two zones of d i f f e r e n t physicochemical p r o p e r t i e s , one g r a i n l i k e (G.L.) and t h e o t h e r g r a i n boundary l i k e (G.B.L.) and s o independent s u p p o r t of t h i s p i c t u r e . We have d i s - cussed t h e constancy o f t h e energy of t h e luminescence components i n a n o t h e r paper ( 6 ) and proposed a p i c t u r e a c c o r d i n g t o which i n photoluminescence experiments, t h e l i g h t a b s o r p t i o n t a k e s p l a c e i n t h e G.B.L. zone, and t h e c a r r i e r s a r e t r a n s f e r r e d t o t h e G.L. zone where t h e luminescence t a k e s p l a c e . I n t h i s p a p e r , we s t u d y t h e v a r i a t i o n s of t h e i n t e n s i t i e s of t h e v a r i o u s peaks due t o changes i n t h e d e p o s i t i o n r a t e s v , t h e d e p o s i t i o n temperature Ts and i n boron doping c o n c e n t r a t i o n t o s e e i f t h e y a r e con- s i s t e n t w i t h our p i c t u r e .

11. PREPARATION AND CHARACTERISATION

The f i l m s a r e prepared by RF d i o d e c a t h o d i c s p u t t e r i n g of a S i t a r g e t i n a r e a c - t i v e g a s 20 % H2 / 80 % A r ando,, 10-5, 10-4, 10-3 of B2Hg. About 1 of a-Si:H was d e p o s i t e d o n t o roughened c-Si t o remove t h e i n t e r f e r e n c e s f r i n g e s . We have s e l e c t e d t h r e e t y p i c a l d e p o s i t i o n r a t e s 30 f / m n , 70

Umn,

¹⁰⁰ &/mn corresponding t o d i f E e r e n t RF powers (8) and s u b s t r a t e temperature o f 190°C and 250°C.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981472

C4-340 JOURNAL DE PHYSIQUE

The t o t a l hydrogen c o n t e n t i s determined by r e s o n a n t n u c l e a r r e a c t i o n w i t h boron (8) o r n i t r o g e n ( 9 ) . The boron c o n t e n t i s determined by t h e r e s o n a n t n u c l e a r r e a c t i o n w i t h hydrogen ( 9 ) . The argon and S i d e n s i t i e s a r e determined by Rutherford backscat- t e r i n g w i t h ~e~ ( 9 ) . R e s u l t s f o r undoped a-Si:H were a l r e a d y p u b l i s h e d (8).

A s v i n c r e a s e s , t h e hydrogen c o n t e n t d e c r e a s e s . RMN (7) experiments s u g g e s t t h a t t h i s i s due mainly t o changes i n G.B.L. zone i n undoped m a t e r i a l . O p t i c a l a b s o r p t i o n experiments confirm t h i s p i c t u r e ( 6 ) . R e s u l t s f o r B doping a r e given on F i g . 1 .

Fig. 1. B 2 H 6 / ( ~ r + H 2 )

There i s an important i n t e r a c t i o n between boron, hydrogen, s i l i c o n and argon ( 9 ) r e s u l t i n g , f o r i n s t a n c e , from boron atoms which do not g i v e a d d i t i o n a l c a r r i e r s a s they would have done i f they were i n s u b s t i t u - t i o n a l p o s i t i o n s .

The f i l m s a r e e x c i t e d by a D.C.

argon l a s e r e m i t t i n g a t 5328 w i t h 1.3 w on 3 mm2. The l i g h t e m i t t e d by t h e sample i s d i s p e r s e d by a g r a t i n g monochromator and d e t e c t e d by a coo- l e d PbS c e l l . We r e p o r t h e r e t h e measurements a t 4 . 2 O ~ .

111. RESULTS AND DISCUSSION

A t y p i c a l photoluminescence spectrum i s shown i n Ref. 6 . A computer program de- t e r m i n e s t h e b e s t f i t f o r t h e p o s i t i o n s and i n t e n s i t i e s of f o u r l i n e s w i t h i d e n t i c a l width. A t 4.2OK, we g e t 1.36 eV, 1.2 e v , 1.08 eV and 0.93 eV w i t h a f u l l width a t middle h e i g h t of 0.3 eV. The p o s i t i o n s and width a r e s i m i l a r t o t h o s e r e p o r t e d i n t h e l i t e r a t u r e ( 1-5)

.

a ) Undoped a-Si:H

The v a r i a t i o n s of t h e i n t e n s i t i e s of t h e v a r i o u s l i n e s v e r s u s v a r e shown on Fig. 2a f o r Ts = 190°C. A t 190°C, when v i n c r e a s e s t h e i n t e n s i t i e s of t h e 1.08 and 0.93 eV components remain n e a r l y c o n s t a n t , w h i l e t h a t o f t h e 1.36 eV component decrea- s e s and t h a t of t h e 1.2 e v component i n c r e a s e s slowly t h e n d e c r e a s e s . A t 250°C ( ~ i g . Z b ) when v i n c r e a s e s , t h e i n t e n s i t y of t h e 0.9 eV component remains c o n s t a n t , while t h a t of t h e 1.36 and 1.2 e v components d e c r e a s e s and t h a t of t h e 1.08 eV l i n e slowly in- c r e a s e s t h e n decreases.

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F i g - 2 - I n t e n s i t i e s of components of luminescence vs d e p o s i t i o n

The v a r i a t i o n of t h e o p t i c a l gap v e r s u s v a t 190°C and 250°C a r e shown on Pig. 2c (8).

The o p t i c a l gap d e c r e a s e s a s v i n c r e a s e s and it d e c r e a s e s more r a p i d l y a s T, increa-

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0.93 eV w i l l be always below t h e ~ o b i l i t y Fig. 2c : V a r i a t i o n of t h e o p t i c a l gap

w i t h t h e d e p o s i t i o n r a t e . T, = 190°C ; edge and s o uniformly s u p p l i e d w i t h addi- T, = 250°C. t i o n a l c a r r i e r s . This w i l l g i v e a c o n s t a n t

i n t e n s i t y f o r t h e 0.9 eV peak a s v o r Ts i n - c r e a s e . On t h e c o n t r a r y , t h e h i g h e r s t a r t i n g l e v e l w i l l have always poor c a r r i e r sup- p l y t h a t w i l l become p o o r e r and p o o r e r a s t h e m o b i l i t y gap narrows. As t h e gapnarrows t h e 1.2 and 1.08 eV s t a r t i n g l e v e l s w i l l change from t h e f i r s t t o t h e second beha- v i o u r . This simple phenomenological p i c t u r e a g r e e s w i t h a l l our experimental r e s u l t s .

b) Boron doped f i l m s

The e n e r g i e s o f t h e l i n e s d e c r e a s e a s t h e boron doping i n c r e a s e s . The e f f e c t is l a r g e r and l a r g e r when t h e energy of t h e l i n e d e c r e a s e s . Nearly t h e same energy v a l u e s a r e o b t a i n e d f o r high boron doping f o r any v. The v a r i a t i o n of t h e luminescence i n - t e n s i t i e s of t h e v a r i o u s components w i t h i n c r e a s i n g B doping i s shown on Fig. 2a f o r v a 30

Umn

and Fig. 4a f o r 100 &/m. The corresponding v a r i a t i o n s of t h e m o b i l i t y gaps a r e shown on Fig. J b and 4b. For a l l components, a t b o t h v ' s t h e luminescence i n t e n s i t y d e c r e a s e s r a p i d l y a s t h e doping l e v e l i n c r e a s e s , much more r a p i d l y t h a n t h e d e c r e a s e we have with v i n t h e undoped m a t e r i a l . However, t h e v a r i a t i o n of t h e o p t i c a l gap i s q u k t e d i f f e r e n t . A t 30

Vmn,

it goes through a maximum between 10-5 and lod4 while it i n c r e a s e s slowly w i t h t h e doping l e v e l a t v 100 %/mn. The h y p o t h e s i s of n e a r l y c o n s t a n t G.L. s t r u c t w e and luminescence i n t e n s i t y c o n t r o l l e d by t h e t r a n s f e r of c a r r i e r s from t h e G.B.L. zone cannot be r e t a i n e d h e r e . The s i m p l e s t h y p o t h e s i s i s t h e occurrence o f an i n c r e a s i n g number of recombination c e n t e r s i n t h e G.L. zone when t h e doping i n c r e a s e s . These a d d i t i o n a l d e f e c t s may be induced f o r i n s t a n c e by non s u b s t i t u t i o n a l doping i n t h e G.L. zone whose c o n c e n t r a t i o n w i l l i n c r e a s e w i t h doping.

A I P ~

Fig. 3a. I n t e n s i t i e s of components F i g . 3b : V a r i a t i o n of t h e o p t i c a l gap of of luminescence vs boron doping. t h e oorresponding f i l m s .

C4-342 JOURNAL DE PHYSIQUE

v : 100 A/mn Ts: 190 O C

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F i g . %b : V a r i a t i o n o f t h e o p t i c a l gap o f t h e c o r r e s p o n d i n g f i l m s .

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The s h i f t o f t h e p e a k s t h a t we o b s e r v e w i t h B doping i s c o n s i s t e n t w i t h t h i s p i c t u r e .

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I V . CONCLUSION

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We have shown t h a t t h e g r a i n l i k e

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g r a i n F i g . 4a : V a r i a t i o n of t h e i n t e n s i t i e s boundary l i k e model f o r a-Si:H w i t h t h e c o r - of t h e v a r i o u s components o f t h e lumi- r e s p o n d i n g model o f two d i s t r i b u t i o n s o f n e s c e n c e v s boron doping T, = 190°C ; s t a t e s i n t h e m a t e r i a l c a n g i v e a good d e s -

v

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100 x/mn. c r i p t i o n o f o u r luminescence results. A b s o r p

t i o n o f l i g h t which p r o d u c e s c a r r i e r s , t a k e s p l a c e i n t h e G.8.L. zone. For undoped a-Si:H, t h e e f f i c i e n c y o f t h e c a r r i e r t r a n s f e r t o t h e G.L. zone where t h e luminescence t a k e s p l a c e c o n t r o l s t h e i n t e n s i t y of t h e v a r i o u s peaks whose e n e r g y r e m a i n s a b o u t c o n s t a n t r e f l e c t i n g t h e c o n s t a n c y o f theG.L.

s t r u c t u r e . The s i t u a t i o n i s s i m p l e r i n t h e c a s e o f B d o p i n g where t h e dominant f e a - t u r e i s t h e i n c r e a s e o f non r a d i a t i v e c h a n n e l by t h e d e f e c t s i n t r o d u c e d by non sub- s t i t u t i o n a l doping i n t h e g r a i n l i k e zone a s w e l l a s t h e g r a i n boundary l i k e zone.

ACKNOWLEDGMENTS

-

We wish t o t h a n k B.K. C h a k r a v e r t y f o r f r u i t f u l d i s c u s s i o n s , and M. Le C o n t e l l e c , C.N.E.T. Lannion, f o r h i s h e l p i n luminescence measurements.

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$

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