HYDROGEN PROFILING IN GAS PHASE DOPED AND ION IMPLANTED AMORPHOUS SILICON FILMS

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HYDROGEN PROFILING IN GAS PHASE DOPED AND ION IMPLANTED AMORPHOUS SILICON

FILMS

F. Demond, G. Müller, H. Damjantschitsch, H. Mannsperger, S. Kalbitzer, P.

Le Comber, W. Spear

To cite this version:

F. Demond, G. Müller, H. Damjantschitsch, H. Mannsperger, S. Kalbitzer, et al.. HYDROGEN PROFILING IN GAS PHASE DOPED AND ION IMPLANTED AMORPHOUS SILICON FILMS.

Journal de Physique Colloques, 1981, 42 (C4), pp.C4-779-C4-782. �10.1051/jphyscol:19814170�. �jpa-

00220795�

JOURNAL DE PHYSIQUE

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HYDROGEN P R O F I L I N G I N GAS PHASE DOPED AND I O N IEIPLANTED ArlORPHOUS S I L I C O N F I L P I S

F.J. Demond, G. Miller, H. Damjantschitsch,

H.

Mannsperger,

S.

Kalbitzer, P . G . ~ e ~ o m b e r * and

W.E.

spear*

Max-PLanck-Institut

fur

Kemphysik, 0-6900 HeideZberg,

F.R. G.

* ~ n i v e r s i t ~ of Dundee, Dundee

D D I 4HN,

Scotland

Abstract.- The hydrogen concentration CH and its spatial distribution in a series of glow discharge a-Si specimens, doped in the gas phase or by ion im- plantation, have been investigated by the "N nuclear reaction. The results show: (i) CH in the bulk of gas phase doped material depends on the gaseous doping ratios; (ii) CH in the surface generally deviates from CH in the bulk within a depth of about 500

A;

(iii) in specimens doped by ion implantation

~f can be moved throughout the mobility gap without producing changes in CH.

Introduction.- In the last few years it has been firmly established that hydrogen in concentrations of o few at.% is incorporated in a-Si produced by the glow discharge technique. Although it is still uncertain whether the major fraction of it is really needed for the compensation of electrically active defect centres, it is nevertheless of great interest to investigate the total hydrooen content CH, its spatial distribu- tion and how it is influenced by factors such as doping, annealing and ion implanta- tion. In an earlier paper (1) we reported preliminary data on the effect of doping on CH and on its spatial dependence. This paper presents further results and also deals with the effect of ion implantation on the hydrogen content.

The experimental techniques used in the present work for the production of the glow discharge a-Si films ( 2 ) , for the ion implantation experiments (3) and for the determination of the hydrogen content and its depth profile using the nuclear reac- tion 'H('~N,~~)'~c ( I ) , have all been described in nrevious papers and reference should be made to these for further details.

Results and discussion.- The variation of CH with doping is shown in figure 1. Each point represents the results of several measurements from at least two different depths at distances greater than 1000

A

from the surfaces. The statistical error in the points is about the size of the symbol used. The scatter of the data points is

: Hydrogen concentration vs.

ratio of gd-a-Si samples. n,p means bulk material of the rcspective type, whereas i-n-i, i-p-i refers to a three-layer structure in which a cen- tral zone of n- or p-type material is sandwiched between two intrinsic lay- ers. Fermi level positions corrcspond- ing to the doping ratios are given with reference to the conduction band edge.

loL lo3 102 10' 0 10' 102 lo3 lo4

PH3/S~HL

-

doplng (vppm)- B2Hg/S~H4

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814170

JOURNAL DE PHYSIQUE

mainly due t o s y s t e m a t i c e r r o r s a r i s i n g from sample-to-sample v a r i a t i o n s i n t h e dep- o s i t i o n p r o c e s s . A s i s s e e n from t h e m a j o r i t y of t h e p o i n t s , t h i s s p r e a d amounts t o a b o u t +1 a t . % . The s i n g l e p o i n t f o r undoped s a m p l e s r e p r e s e n t s an a v e r a g e o v e r 15 specimens from d i f f e r e n t d e p o s i t i o n r u n s , t h e s c a t t e r b e i n g a l s o a b o u t f l a t . % .

I t i s a p p a r e n t from f i g u r e 1 t h a t even t h e a d d i t i o n of r e l a t i v e l y s m a l l amounts of B 2 H g o r PH3 p r o d u c e s v a r i a t i o n s i n C H , up t o '5 a t . % r e l a t i v e t o t h e i n t r i n s i c l e v e l . T h i s i s b a s i c a l l y t h e c o n c l u s i o n we drew from a s m a l l e r body o f informa- t i o n (1). The s i t u a t i o n now h a s changed i n s o f a r a s we o b s e r v e an a d d i t i o n a l s t r o n g peak i n t h e weak boron d o p i n g regime from a b o u t 10 t o 100 vppm B2Hg.

A s i t i s u n l i k e l y t h a t t h e s m a l l c h a n g e s i n g a s c o m p o s i t i o n d u r i n g d o p i n g c o u l d produce s u f f i c i e n t l y l a r g e c h a n g e s i n t h e plasma c o n d i t i o n s t o c a u s e t h e s e e f f e c t s , one c o n c l u d e s t h a t t h e induced s h i f t of t h e Ferrni l e v e l , c f , c a u s e s t h e v a r i a t i o n o f c ~ . C l e a r l y , t h e p o s i t i o n of c f w i t h i n t h e l o c a l i s e d d e n s i t y o f s t a t e d i s t r i b u t i o n , g ( c ) , d e t e r m i n e s t h e a b s o l u t e d e n s i t y , g ( ~ f ) , o f d e f e c t c e n t r e s i n a g i v e n c h a r g e s t a t e and t h u s v e r y l i k e l y t h e amount o f hydrogen t a k e n up a t t h e g a s - s o l i d i n t e r - f a c e . I f we a c c e p t t h i s argument t h e v a r i a t i o n o f c ~ w i t h d o p i n g s h o u l d e s s e n t i a l l y r e f l e c t t h e d e n s i t y o f d e f e c t c e n t r e s a t t h e i n d i v i d u a l v a l u e s o f c f . A s f i g u r e 1 r e v e a l s t h i s c o u l d b e t r u e f o r weakly doped samples b u t n o t f o r t h e more h e a v i l y doped o n e s i n which g ( c f ) i s h i g h and where a t t h e same t i m e C H is r e l a t i v e l y low. C l e a r l y . f u r t h e r work i s needed b e f o r e a d e f i n i t e e x p l a n a t i o n can be g i v e n of t h e s e f e a t u r e s .

I F i g . 2 : Normalized hydrogen c o n c e n t r a t i o n

1.5

.

^{gd-a-SI T}

-

'\ - v s . d e p t h w i t h t h e d o p i n g r a t i o a s p a r a m e t e r .

We now t u r n t o t h e q u e s t i o n o f how hydrogen i s d i s t r i b u t e d i n t h e specimens n e a r t h e f r e e s u r f a c e . F i g u r e 2 shows d e p t h p r o f i l e s o f s e v e n samples w i t h w i d e l y d i f f e r i n g c o n c e n t r a t i o n s o f B and P. The c u r v e s a r e n o r m a l i z e d t o t h e r e s p e c t i v e b u l k v a l u e s , c,, which a r e l i s t e d i n t h e g r a p h . D e v i a t i o n s from c, a r e s e e n t o o c - c u r i n t h e d e p t h i n t e r v a l from 300-1000 f i . Two d i f f e r e n t t r e n d s p r e v a i l : f i r s t , t h e samples w i t h z e r o o r weak d o p i n g a r e c h a r a c t e r i z e d by c o n t i n u o u s l y i n c r e a s i n g H l o s s - e s a s t h e s u r f a c e i s approached. The e f f e c t i s l a r g e and amounts t o r e l a t i v e l o s s e s up t o 98%. With i n c r e a s i n g d o p i n g l e v e l a second e f f e c t i s superimposed. I n t h e r e - g i o n v e r y n e a r t h e s u r f a c e , minima i n C H a r e o b s e r v e d a f t e r which CH i s enhanced a s compared w i t h t h e f i r s t group o f s a m p l e s . I n t h e extreme c a s e o f d o p i n g w i t h l o 3 vppm o f B2Hg a n e t enhancement by a b o u t 50% o v e r t h e bulk v a l u e o c c u r s .

Using a d i f f u s i o n c o n s t a n t D H = 2 . 5 ~ 1 0 - l 5 cm2s-' a t t h e d e p o s i t i o n t e m p e r a t u r e o f 280°C ( 4 , s ) and assuming an e f f e c t i v e time o f 400 s f o r t h e d e p o s i t i o n o f a 1000

8

l a y e r we o b t a i n a d i f f u s i o n l e n g t h L D = 200

8

which i s o f t h e o b s e r v e d o r d e r of ma

?

-4-

n i t u d e o f t h e s p a t i a l e x t e n s i o n o f t h e s u r f a c e d r o o p . The v a l u e o f D H = 1 0 - l 6 cm s r e p o r t e d by C a r l s o n and Magee ( 6 ) would o n l y g i v e L D = 40

&

which c a n n o t e x p l a i n t h e s e e f f e c t s on t h e b a s i s o f r e g u l a r , non-enhanced d i f f u s i o n . The second e f f e c t , t h c r e l - a t i v e enhancement o f C H n e a r t h e s u r f a c e of t h e most h e a v i l y doped s p e c i m e n s , sug- g e s t s t h a t d o p i n g c o u l d modify t h e g a s - s o l i d i n t e r a c t i o n s a p p r e c i a b l y . The r e a s o n s f o r t h i s a r e n o t , however, c l e a r a t p r e s e n t .

An i m p o r t a n t q u e s t i o n a r i s i n g from t h e s e r e s u l t s i s t h e e x t e n t t o which t h e de- c r e a s e i n C H n e a r t h e s u r f a c e a f f e c t s 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 gd-a-Si. As l o n g a s s u f f i c i e n t bonded H i s i n c o r p o r a t e d d u r i n g d e p o s i t i o n t o s a t u r a t e d e f e c t s i t e s i n t h e s u r f a c e r e g i o n , t h e n one would n o t e x p e c t an a p p r e c i a b l e d e t e r i o r a t i o n i n prop- e r t i e s . However, i f t h e o b s e r v e d d e c r e a s e i n c ~ i s a s s o c i a t e d w i t h t h e b r e a k i n g o f

Si-H bonds, the density of defect states is very likely to be higher at the surface which could have marked effects on the electronic properties. This has clearly been established for the specimen volume after effusion of H at annealing temperatures above 400°C ( 7 )

.

Fig. 3 : Hydrogen depth profiles of gd- a-Si samples before and after ion im- plantation.

0 annealed e l TA:2800C

1

0 1

^{I I I I}

0 500 1000 1500 2000 Depth

(I!)

In recent work we have used ion implantation techniques in the development of photovoltaic devices ( 8 ) . Since the bombardment inevitably introduces defects over the particle range, it is natural to ask to what extent a given H profile will be modified. Figure 3 shows the surface profiles of a-Si samples bombarded with 1016

ions/cm2 of B, P, C and Si as indicated. Two reference samples, one as deposited and the other with an additional anneal, are included in the graph. These data clearly show that in all samples the initial H profile is retained after implanta- tion.* However, this is no longer true if the ion dose is further increased as shown in figure 4. Here the normalized concentration, cH/c,, as in figure 2, is plotted vs. ion dose at five different depths. It is seen that the initial H distribution remains essentially unchanged up to doses of 1016 ions/cm2. Plajor losses of H are seen to take place for implantations with doses of 10" ions/cm2 over a depth scale given by the particle range. This effect has been reported before by Carlson and Magee ( 9 ) , who studied the loss of H from glow discharge material under bombardment

1 5 0

120

*On the basis of the H diffusion coefficient of 2 . 5 x lo-'' cm2 s-', which has been extrapolated from 350°C to 280°C, a broadening of the H-depleted surface layer by about a factor of 5 would have been expected. In analogy to the K diffusion in this material (4) DH may be suspected to depend on time due to trapping processes.

I

'

^I

'

I

' " '

^I

' " '

^I

'

L _{z Flg. 4 :} Normalized hydrogen concentra- - _{Ion induced H - release ~n gd} - a - 51 under

- 2

tlon of g d - a - ~ i samples vs. ~mplanta-

- _{5 0 keV} 51 and P lrradlat~on tlon dose at different distances below

I

^{TI =} 2 8 O 0 C 1 - 1 0 4 s - 9 the fllm surface.

depth(%)

I -

^{1 0 0 0}

v

- 30

-

_-

2 0 0 -

- -

O L ' - - L + ' " I I ' ' I

'

^I " I I

'

o

¹⁰¹⁵

loq6 loi7

1 o n s l c m 2

C4-782 JOURNAL DE PHYSIQUE

w i t h 0 . 7 5 k e V d e u t e r o n s . A s r e g a r d s t h e consequences f o r t h e appli'cation of i o n i m - p l a n t a t i o n t o doping purposes, i t should be noted t h a t a dose of 1016 ions/cm2 c o r - responds t o a volume c o n c e n t r a t i o n of about 1021 atoms/cm3. T h i s doping l e v e l ap- p e a r s t o be adequate f o r most p r a c t i c a l c a s e s , s i n c e it i s s u f f i c i e n t t o s h i f t ~f

throughout t h e m o b i l i t y gap. The a b i l i t y t o dope by ion i m p l a n t a t i o n without modify- i n g CH i s i n d i r e c t c o n t r a s t t o t h e d a t a f o r t h e gas-phase-doped specimens shown i n f i g u r e 1 and c l e a r l y r e v e a l s t h a t v a r i a t i o n s i n C H a r e n o t necessary t o allow ef t o be s h i f t e d i n a-Si. T h i s must be taken i n t o account i n any model proposed t o e x p l a i n t h e e f f e c t s o f doping on CH.

References

1 ) MULLER G., DEMOND F . J . , KALBITZER S., DAMJANTSCHITSCH H . , MANSPERGER H . , SPEAR W.E., LeCO14BER P.G. and GIBSON R . A . , P h i l . Elag.

841

(1980) 571.

2 ) SPEARW.E. and LeCOMBER P.G., P h i l . Mag. (1976) 935.

3 ) KALBITZER S . , MULLER G . , LeCOMBER P.G. and SPEAR W.E., P h i l . Mag.

B41

(1980) 439.

4 ) REINELT M . , T h e s i s ( 1 9 8 1 ) , Heidelberg.

5 ) ZELLAMA K., GERMAIN P.,

SQUELA~D

S . , MONGE J . and LIGEON E . , J . Non-Cryst. S o l i d s 35,36 (1980) 22.

6 ) CARLSON D.E. and MAGEE G.W., Appl. Phys. L e t t .

2

(1978) 81.

7 ) JONES D . I . , GIBSON R . A . , LeCOhlBER P.G. and SPEAR W.E., S o l a r Energy ater rials

2

(1979) 93.

8 ) GIBSON R . A . , YANG D. LeCOMBER P.G., SPEAR W . E . , PIULLER G. and KALBITZER S . , t h i s conference.

9 ) CARLSON D.E. and MAGEE C.W., Proc. 2nd EEC Conference on P h o t o v o l t a i c S o l a r En- e r g y , B e r l i n 1979, p.312.