MULTIPHASES DESCRIPTION OF a-Si : H

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MULTIPHASES DESCRIPTION OF a-Si : H

A. Deneuville, J. Bruyère, H. Hamdi

To cite this version:

A. Deneuville, J. Bruyère, H. Hamdi. MULTIPHASES DESCRIPTION OF a-Si : H. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-733-C4-736. �10.1051/jphyscol:19814160�. �jpa-00220784�

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JOURNAL DE PHYSIQUE

ColZoque C4, suppl&ment au nO1O, Tome 42, octobre 1981 page C4-733

MULTIPHASES D E S C R I P T I O N OF a - S i : H

A. D e n e u v i l l e , J . C . BruyZre and H . Hamdi

Groupe des Transitions de Phases, Centre National de Za Recherche S c i e n t i f i q u e , B.P. 166, 38042 Grenoble Cedex, Ekance

A b s t r a c t - Photolurninescencc, t h e r m a l l y s t i m u l a t e d c u r r e n t s (TSC), 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 r e p o r t e d v e r s u s d e p o s i t i o n tem- p e r a t u r e and d e p o s i t i o n r a t e o f undoped a-Si:H. These r e s u l t s a r e c o h e r e n t w i t h a t e n t a t i v e model a c c o r d i n g t o which t h e r a d i a t i v e r e c o m b i n a t i o n and

t h e r m a l e m i s s i o n t a k e p l a c e i n a n e a r l y c o n s t a n t and weakly d i s o r d e r e d g r a i n l i k e zone, w h i l e o p t i c a l a b s o r p t i o n o c c u r s m a i n l y i n a w i d e l y v a r y i n g and l a r g e l y d i s o r d e r e d g r a i n boundary l i k e zone ; i n t h i s p i c t u r e , t h e e l e c t r i c a l c o n d u c t i v i t y i n v o l v e s b o t h z o n e s .

I . INTRODUCTION

Up t o now, t h e e l e c t r o n i c p r o p e r t i e s o f amorphous h y d r o g e n a t e d s i l i c o n (a-Si:H) have been m a i n l y d e s c r i b e d i n t h e framework o f a s i n g l e d i s t r i b u t i o n o f l o c a l i z e d s t a t e s i n a n homogeneous m a t e r i a l ( I ) . R e c e n t l y t h e Xerox group gave e v i d e n c e ( 2 ) f o r t h e e x i s t e n c e of two zones i n a-Si:H, a g r a i n l i k e zone (GL) w i t h few non i n t e r a c t i n g hydrogen (< 3 %) embedded i n a g r a i n boundary l i k e zone (GEL) w i t h a l a r g e c o n c e n t r a - t i o n o f i n t e r a c t i n g hydrogen (> 10 % ) . T h i s q u i t e n a t u r a l l y s u g g e s t s t h a t two d i f f e - r e n t DOS have t o o r i g i n a t e from t h e GL and GEL z o n e s . As a c o n s e q u e n c e , some s i g n a l s might o c c u r p r e f e r e n t i a l l y from one o f t h e s e two zones which a r e e x p e c t e d t o p r e s e n t v e r y d i f f e r e n t e l e c t r o n i c p r o p e r t i e s . P r e v i o u s a t t e m p t s t o i n t e r p r e t t h e a-Si:H pro- p e r t i e s a s t h o s e of a n inhomogeneous m a t e r i a l have been made by Brodsky ( 3 ) and Anderson and P a u l ( 4 ) . We choose h e r e t o s t u d y v e r s u s t h e d e p o s i t i o n t e m p e r a t u r e (TS) and t h e d e p o s i t i o n r a t e ( v ) , t h e o p t i c a l a b s o r p t i o n , t h e e l e c t r i c a l c o n d u c t i v i t y , t h e p h o t o l u m i n e s c e n c e and t h e t h e r m a l l y s t i m u l a t e d c u r r e n t s o f a-Si:H.

11. PREPARATION AND CHARACTEKISATION

The f i l m s were p r e p a r e d by KF d i o d e s p u t t e r i n g o f a C-Si t a r g e t i n a ( c o n s t a n t ) 8 0 % Ar / 20 % H2 r e a c t i v e m i x t u r e o n t o f u s e d s i l i c a o r C-Si s u b s t r a t e s . Ke measure t h e t o t a l hydrogen c o n t e n t ( 5 , 6 ) and i t s d i s t r i b u t i o n among t h e v a r i o u s s i t e s (j), t h e S i and t h e Ar c o n c e n t r a t i o n s ( 6 ) w i t h i n t h e f i l m s . The p h o t o l u m i n e s c e n c e a p p a r a - t u s i s d e s c r i b e d i n a n o t h e r p a p e r ( 7 ) . T h e r m a l l y s t i m u l a t e d c u r r e n t s a r e measured du- r i n g a h e a t i n g r a t e o f 0 . 2 K/s by a n e l e c t r o m e t e r K e i t h l e ~ 602B undcr sandwich con- f i g u r a t i o n ( w i t h a-Si and Pd ohmic c o n t a c t s ( 1 5 ) ) . E x c i t a t i o n i s done a t 120 K by a monochromatic l i g h t o f 1.94 eV. The Iwasurements o f t h e o p t i c a l a b s o r p t i o n ( 5 ) and c o n d u c t i v i t y ( 15) were p u b l i s h e d p r e v i o u s l y .

111. RESULTS AND DISCUSSION

1 . Photoluminescence

A l l t h e s p e c t r a e x h i b i t s t r u c t u r e s a t t h e same e n e r g i e s w i t h v a r i o u s r e l a t i v e i n t e n s i t i e s ( F i g . l a ) . We l o o k f o r t h e b e s t f i t o f t h e s e s p e c t r a t h r o u g h t h e i n t e n s i - t i e s and p o s i t i o n s (around t h o s e of t h e s t r u c t u r e s ) of f o u r component l i n e s o f i d e n - t i c a l w i d t h . We d i s c u s s h e r e t h e e n e r g y p o s i t i o n o f t h e s e l i n e s v e r s u s t h e p r e p a r a t i o n p a r a m e t e r s . T h e i r r e l a t i v e i n t e n s i t i e s a r e d i s c u s s e d i n a n o t h e r p a p e r ( 7 ) . F i g . Ib

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814160

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C4-734 JOURNAL DE PHYSIQUE

Fig. 1. Photoluminescence in undoped a-Si:H.

, .

. a

la. Three typical spectra and one example OJIL

- ³⁰ ⁷⁰ ¹⁰⁰ ^{~ ( m ?}

of decomposition.

Ib. Energy position of the lines vs shows the energy of these lines versus v at - v at Ts = 190°C (01 a,d 250°C (01.

Ts = 190 (0) and 250°C ( 0 ) . There is a very

slight systematic decrease as v increases around 1.36 eV, 1.2 eV, 1.08 eV and 0.93 eV.

These lines are found in all the 20 undoped a-Si:H studied so far. For undoped glow discharge or sputtered a-Si:H all these lines (8) or some of them (9) have been found by other authors.

2. Thermally stimulated currents

Fig. 2 shows the TSC signals vgrsus temgerature for $ heating rate of 0.2 K.of undoped a-Si:H deposited at v = 30 A/mn, 70 A/mn and 100 Alum. !*Je obtain current peaks at 2 12 and 278 K (30 A/-), 275 and 315 K (70 /I/mn) and a very large peak at 310 K (100 A/=). These curves are typical of those obtained over a larger range of excitation, heating rates and samples which will be discussed elsewhere. The current peaks appear in all cases around the same temperatures 210 K, 275 K, 315 K. Similar TSC peak temperatures for similar heating rates were reported by other authors in both sputtered and glow discharge a-Si:H (10).

Fig. 2. Thermally absorption coefficient versus photon energy.

Ts = 190°C, v = 20, 49, 73 and 105 A/mn.

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Dc -4

c r n

3. Optical absorption a(hv)

10 5 -

a(hv) is shown on Fig. 3 foroTs = I

190°C and v = 20, 49, 73 and 105 A/mn. 4 They exhibit a main component (which 10 I -

20 $mn

defines the optical gap) which is

3 ^{- - L 3} "

strongly dependent on v (5,7) and a low :O - -- 73 ,.

energy tail which on the contrary is - :05 ,

nearly constant (11). 2'

*o -

Fig. 3. Optical absorption coefficient

versus photon energy. T s = 190°C,

-_

^-^.^.^.^{I - _}

-_--

>

v = 20, 49, 73 and 105 h m n . 5 1 ',5 2 2,s h y e v

4. Electrical conductivity versus temperature

The activation energy of the conductivity for coplanar measurements decreases from 0.69 eV to 0.62 eV as v increases ( a ) , while it decreases from 0.9 eV to 0.7 eV in sandwich configuration on similar samples.

5. Discussion

We have obviously two sets of behaviours for the electronic properties of a-Si:H.

The optical absorption and the electrical conductivity depend strongly on v and Ts, while the energy of the peaks of photoluminescence or thermally stimulated currents are independent of v and Ts. This suggests that each set takes place in a different zone of the material.

If the hydrogen atoms are mainly linked to Si structural defects (12), there is a weak disorder and so few localized defects in the grain like zone.The corresponding localized levels appear almost discrete and give well defined peaks in photolumines- cence and TSC. Their energies give respectively their distances from the valence band edge (or from a large density of hole traps near it) and from the conduction band edge. For the three upper levels we find their sum nearly constant around 1.7 eV which would be the gap of the GL zone. It was suggested that this energy corresponds

to a gap associated with the tetrahedral bonding symmetry, which is observed prefe- rentially in disordered silicon (13). The constancy of all these energies indicates that the G L zone is weakly dependent on v in the undoped a-Si:H.

As a consequence, the optical absorption would occur preferentially in the more disordered GBL zone where the selection rules have been completely lost. This gives a main absorption component very sensitive to the ~ r e ~ a r a t i o n conditions as the GBL zone (2). On the contrary some short range order remains and so the selection rules are not completely removed in the GL zone. This results in a weak absorption compo- nent nearly constant with v and Ts (11).

Finally electrical conductivity needs thermal generation of carrier in the GI.

zone, then thermal emission in (or over) the GBL zone which introduce additional energy on Eo and a loss coefficient on 00. In particular, the different hydrogensites in the vicinity of the substrate or of the metal contact (14) result in different GBL zone which might explain the differences in electrical conductivity between coplanar and sandwich configurations (8) .

Our tentative picture of the two zones behaviour of the material is summarized on Fig. 4. Actually, while the GL zone would have smooth variations, the GBL zone is very sensitive to the preparation parameters and to the boundary materials. So a-Si:H is rather a multiphase than a two phase material.

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JOURNAL DE PHYSIQUE

Fig. 4. Schematic energy band diagram for the grain like zone

(GLI and the grain boundary

like zone ( G B L I .

\

^ld;1 : f , i 2 9

(,,,

^'

Y L * Y

-. 0

G 3'- G L G 3 C

IV. CONCLUSION

We have described photoluminescence, thermally stimulated currents, optical absorption and conductivity-temperature measurements of sputtered a-Si:H films. Their interpretation is coherent with a model according to which there are two different distributions of states in the two zones description of the material originally pro- posed by the Xerox group.

The grain like zone exhibiting weak disorder would have smooth dependence with the preparation conditions. Pholuminescence and thermally stimulated current whose peak positions are-very similar with very different a-Si:H preparation will originate from this region. The grain boundary like zone exhibiting a large disorder would be very dependent on the condition preparation. Optical absorption would occur mainly in this region. Conductivity would need carrier transfer between these two regions which would be very dependent on the preparation through the connection of the bandsbetween

these two regions.

ACKNOWLEDGEMENTS - We wish to thank Dr. LE CONTELLEC (CNET, Lannion) for his help in the photoluminescence measurements.

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