STABLE AMORPHOUS GERMANIUM FILMS PREPARED IN ULTRA HIGH VACUUM AND MEASURED IN-SITU : STRUCTURE AND ELECTRONIC PROPERTIES

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STABLE AMORPHOUS GERMANIUM FILMS PREPARED IN ULTRA HIGH VACUUM AND

MEASURED IN-SITU : STRUCTURE AND ELECTRONIC PROPERTIES

P. Viščor, A. Yoffe

To cite this version:

P. Viščor, A. Yoffe. STABLE AMORPHOUS GERMANIUM FILMS PREPARED IN ULTRA HIGH

VACUUM AND MEASURED IN-SITU : STRUCTURE AND ELECTRONIC PROPERTIES. Journal

de Physique Colloques, 1981, 42 (C4), pp.C4-691-C4-694. �10.1051/jphyscol:19814153�. �jpa-00220776�

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STABLE AMORPHOUS GERMANIUM F I L M S PREPARED I N ULTRA H I G H VACUUM AND MEASURED I N - S I T U

:

STRUCTURE AND ELECTRONIC PROPERTIES

P. ~ i & ? o r and A.D. Y o f f e

Cavendish LaSoratory, Mac!ingZey Rond, Cambridge, England.

A b s t r a c t . - S t a b l e , homogeneous germanium f i l m s h a v e been p r e p a r e d by slow evapor- a t i o n o n t o h e a t e d s a p p h i r e s u b s t r a t e s i n u l t r a h i h vacuum.

B

T h e i r p r o p e r t i e s do n o t c h a n g e o n a n n e a l i n g u n t i l t h e y c r y s t a l l i s e a t 250 C ( I , ? ) . The s t r u c t u r e o f t h e s e f i l m s i s c h a r a c t e r i s e d by low d e n s i t y , by a s h i f t o f t h e f i r s t d i f f r a c t i o n peak t o h i g h e r s c a t t e r i n g a n g l e , w i t h r e s p e c t t o t h e c r y s t a l and by t h e e x i s t a n e e of a d i f f - r a c t i o n pre-peak. The o p t i c a l a b s o r p t i o n e d g e i s r e l a t i v e l y s h a r p , h a s a s h o u l d e r a t 0 . 8 5 eV, and t h e low photon e n e r g y r e f r a c t i v e i n d e x h a s a lower v a l u e t h a n f o r t h e c r y s t a l . The t e m p e r a t u r e T dependence o f t h e d . c . e l e c t r i c a l c o n d u c t i v i t y o i s c h a r a c t e r i s e d by a n "S" s h a p e d c u r v e , when p l o t t e d a s l o g o v e r s u s T-!. The photo- c o n d u c t i v i t y shows marked l o n g time ( t >

l o 3

s e c ) r e l a x a t i o n e f f e c t s a t 77K and e x h i b i t s p h o t o c o n d u c t i v i t y f a t i g u e a t 5 K . These r e s u l t s a r e compared w i t h amorphous germanium (a-Ge) f i l m s p r e p a r e d u n d e r d i f f e r e n t c o n d i t i o n s and w i t h o t h e r amorphous and g l a s s y m a t e r i a l s . Some common t r e n d s i n t h e s t r u c t u r e and e l e c t r o n i c p r o p e r t i e s a r e p o i n t e d o u t .

P r e p a r a t i o n . - A-Ce f i l m s were p r e p a r e d by slow e v a p o r a t i o n ( 0 . 1

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^{0 . 5}

8

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a t normal i n c i d e n c e ( d i s t a n c e between t h e s o u r c e and t h e s u b s t r a t e q, 50 cm), o f h i g h p u r i t y germanium (1013 of e l e c t r i c a l l y a c t i v e i m p u r i t i e s ) from a t u n g s t e n b o a t

( d e g a s s e d under u l t r a h i g h vacuum c o n d i t i o n s ) o n t o s i n g l e c r y s t a l s a p p h i r e s u b s t r a m h e l d a t 400K d u r i n g t h e e v a p o r a t i o n s , The p r e s s u r e d u r i n g p r e p a r a t i o n was k e p t below 5.10-lo t o r r . The t h i c k n e s s of t h e d e p o s i t e d f i l m s was v a r i e d between two monolayers and one m i c r o n t o s u i t v a r i o u s e x p e r i m e n t a l r e q u i r e m e n t s . For comparizon, o t h e r f i l m s were a l s o p r e p a r e d , w i t h v a r y i n g r a t e s o f d e p o s i t i o n s (up t o 40

8

^{s e c}l ) and s u b s t r a t e t e m p e r a t u r e (down t o Ts = 77K). However, u n l e s s s p e c i f i c a l l y s t a t e d , t h e p r o p e r t i e s d i s c u s s e d i n t h e f o l l o w i n g s e c t i o n s r e f e r t o s t a b l e f i l m s f o r which t h e e v a p o r a t i o n r a t e was 0.1 A/sec and TS = 400K.

D e n s i t y and E l e c t r o n D i f f r a c t i o n measurements.- I n - s i t u d e n s i t y measurements ( 3 ) w e r e made on 500

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f i l m s . I n c o n s t r a s t t o ~ r e v i o u s work concern-

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i n g t h e d e n s i t y o f a n n e a l e d , s t a b l e f i l m s , a

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r e l a t i v e l y l a r g e d e n s i t y d e f i c i t of approxim-

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a t e l y 1 6 . 5 2 w i t h r e s p e c t t o t h e c r y s t a l was

D f o u n d , somewhat d e p e n d e n t o f t h i c k n e s s ( 4 ) . Of

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2. found i n b u l k , c o v a l e n t g l a s s e s s u c h a s a-As, a-SiO and some c h a l c o g e n i d e g l a s s e s . I t

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: & a p p e a r s t o be a g e n e r a l f e a t u r e o f t h e topology and c o n n e c t i v i t y of c o n t i n u o u s , random n e t - works, which a r e t h o u g h t t o r e p r e s e n t t h e s t r u c t u r e o f t h e s e m a t e r i a l s . The r e s u l t s o f h i g h e n e r g y e l e c t r o n e l a s t i c s c a t t e r i n g exper- i m e n t s a r e s u m a r i s e d i n F i g u r e 1. Here t h e e l a s t i c a l l y s c a t t e r e d i n t e n s i t y o f 40 keV 9 8 7 6 5 1 3 2 1

e l e c t r o n s t h r o u g h a 5 0 0

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t h i c k a-Ge 4 i l m i s 5 / h“ p l o t t e d a g a i n s t s c a t t e r i n g angle s = s i n 8.

The i m p o r t a n t new f e a t u r e i s t h e s h i f t by ca F i g . 1: E l a s t i c a l l y s c a t t e r e d 6% o f t h e f i r s t d i f f r a c t i o n peak ( f ) towards i n t e n s i t y o f 40 keV e l e c t r o n s f o r a h i g h e r s c a t t e r i n g a n g l e r e l a t i v e t o t h e t h i n a-Ge f i l m . Dashed line

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s c a t t e - c r y s t a l . 4 s i m i l a r o b s e r v a t i o n was made i n r e d i n t e n s i t y from t h e s u b s t r a t e . a-As (5) and t h e phenomenon maybe .due t o t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814153

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C4-692 JOURNAL DE PHYSIQUE

more m o l e c u l a r n a t u r e o f s t a b l e g l a s s e s ( 6 ) . f i n d i n g o f a d i f f r a c t i o n pre-peak ( p ) a t a s c a t t e r i n g a n g l e s F u r t h e r e v i d e n c e comes from- he new ^%1 . 2

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£ .

As f a r a s we a r e aware t h i s h a s n o t p r e v i o u s l y been r e p o r t e d f o r amorphous f i l m s o f group I V e l e m e n t s .

O p t i c a l P r o p e r t i e s . - The s i m u l t a n e o u s measurements o f o p t i c a l r e f l e c t i o n and t r a n s - m i s s i o n haw g i v e n a n i n d e p e n d e n t d e t e r m i n a t i o n o f b o t h p a r t s o f t h e d i e l e c t r i c r e s p o n s e f u n c t i o n between 0 . 4 5 and 3.0 eV. The r e s u l t s a r e shown i n f i g u r e s 2 and 3 , t o g e t h e r w i t h p r e v i o u s r e s u l t s by P a u l e t a l . ( 7 ) and Donovan e t a l . ( 8 ) . The

I 1 I I 1

F i g . 2: R e a l p a r t o f t h e d i e l e c t r i c r e s p o n s e f u c t i o n v e r s u s e n e r g y : a- p r e s e n t work, b,c- P a u l e t a l . ( 7 ) i d-Donovan e t a l . ( 8 ) .

F i g . 3: Imaginary p a r t o f t h e d i e l e c t - r i c r e s p o n s e f u n c t i o n v e r s u s e n e r g y : a - p r e s e n t work, b , c - P a u l e t a l . (7) i d-Donovan e t a 1

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main new f e a t u r e , a p a r t from t h e s h i f t o f € 2 t o h i g h e r e n e r g i e s i s t h e v a l u e of t h e r e f r a c t i v e i n d e x n a t low e n e r g i e s

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n (hw-to) = 3.6. T h i s i s a p p r e c i a b l y lower t h a n i n t h e c r y s t a l , b u t a g a i n a g r e e s w i t h t h e g e n e r a l t r e n d of s i m i l a r o b s e r v a t i o n s i n a-As a-Si02 and some c h a l c o g e n i d e g l a s s e s . The "main o p t i c a l gap" ( t h e i n t e c e p t Of h w 6 w i t h t h e e n e r g y a x i s ) i s found t o be 1 . 0 eV, w i t h an a b s o r p t i o n s h o u l d e r a t 0 . 8 5 eV ( a t t h e v a l u e s o f a b s o r p t i o n c o e f f i c i e n t a 6103cm

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e x t e n d i n g down 0 . 6 5 eV.

T h i s "minimum o p t i c a l gap" h a s a l s o been c o n f i r m e d from measurements o f t h e s p e c t r a l r e s p o n s e of t h e p h o t o c o n d u c t i v i t y . These r e s u l t s d i f f e r from t h o s e o b t a i n e d by P a u l e t a 1 . ( 7 ) and Donovan e t a 1 . ( 8 ) , b u t a r e i n agreement w i t h t h e work o f E l l i o t t ( 9 ) o n a-Ge f i l m s , p r e p a r e d i n u l t r a h i g h vacuum o n c o l d s a p p h i r e s u b s t r a t e s (Ts = 77K), a t h i g h e v a p o r a t i o n r a t e s (a30

2

s e c I ) , b u t a n n e a l e d t o 400K. They a l s o a g r e e w i t h measurements by C o n n e l l and Pawlik (10) o n h y d r o g e n a t e d , s p u t t e r e d a-Ge ( w i t h 2.8%

h y d r o g e n ) . I t would seem t h a t b o t h a n n e a l i n g t o h i g h enough t e m p e r a t u r e s o r p a r t i a l h y d r o g e n a t i o n o f a-Ge f i l m s c a n l e a d t o t h e same ( a s s y m p t o t i c a l l y ) o p t i c a l a b s o r p t b q c h a r a c t e r i s t i c o f t h e s t a b l e , amorphous s t a t e o f germanium.

D.C. e l e c t r i c a l c o n d u c t i v i t y . - Measurements o f t h e d . c . e l e c t r i c a l c o n d u c t i v i t y r e p o r t e d e a r l i e r + ( l l ) , have now been e x t e n d e d t o lower t e m p e r a t u r e s ( n o i s e c u r r e n t o f t h e o r d e r o f

-

5 . 1 0 - ' ~ Amps) and t o l a r g e r f i l m t h i c k n e s s e s (up t o o n e m i c r o n ) . These measurements have c o n f i r m e d t h e p r e v i o u s l y r e p o r t e d h i g h t e m p e r a t u r e b e h a v i o u r , and h a v e e s t a b l i s h e d t h e d e v i a t i o n from l i n e a r i t y o f t h e l o g a v e r s u s T-4 p l o t a t t h e l o w e s t t e m p e r a t u r e s r e a c h e d (below 40K) ( 1 2 ) . A t y p i c a l r e s u l t ( r e p r e s e n t i n g t h e b e h a v i o u r o f f i l m s r a n g i n g from 500

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t o one micron) i s shown i n F i g u r e 4 . The curve

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p r e s e n t work, 2 - E l l i o t t ( 9 ) ; 5- Lewis ( 1 2 ) ; 6- Beyer e t a l . ( 1 0 ) .

h a s a s t r o n g (on a s c a l e o f kT) "S" s h a p e , t h e c o n d u c t i v i t y d e v i a t i n g downwards a t h i g h t e m p e r a t u r e s and upwards a t t h e l o w e s t t e m p e r a t u r e s . The c o n d u c t i v i t y d a t a i s r e p l o t t e d i n F i g u r e 5 t o g e t h e r w i t h t h e r e s u l t s o f o t k e r i n v e s t i g a t i o n s . The most i n t e r e s t i n g f e a t u r e i n F i g u r e 5 i s a g a i n t h a t a-Ge f i l m s p r e p a r e d under v e r y d i f f e r - e n t p r e p a r a t i o n c o n d i t i o n s , when a n n e a l e d t o h i g h enough t e m p e r a t u r e s , o r hydro- g e n a t e d (by a p p r . 2.8% o f h y d r o g e r ~ ) s h o ~ q u a l i t a t i v e l y t h e same b e h a v i o u r , a s s t a b l e UHV p r e p a r e d f i l m .

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i d ~ / ~

ed when t h e c h o p p i n g r a t e o f t h e i n c i d e n t l i g h t was i n c r e a s e d from 20 t o 850 Hz. However, a F i g . 6: C o n d u c t i v i t y v e r s u s r e c i p -

rocal ^darkconductivity much more pronounced l o n g d u r a t i o n ( t >

l o 3

sec) s t r o n g l y t e m p e r a t u r e d e p e n d e n t e f f e c t was o f c r y s t a l l i n e (c-Ge) and amorphous

o b s e r v e d , when t h e c o n t i n u o u s w h i t e l i g h t i l l u - (a-Ge) germanium; photoconductivity m i n a t i o n was i n t e r r u p t e d by a s h u t t e r . T y p i c a l of amorphous germanium ( ~ ~ 1 , ) . r e s u l t s a r e shown i n F i g u r e 7 (77K) and 8 (55K)- P h o t o c o n d u c t i v i t y . - The s p e c t r a l r e s p o n s e of t h e s t e a d y s t a t e ( c h o p p i n g f r e q u e n c y %

1 3 8 Hz) p h o t o c o n d u c t i v i t y showed, i n agreement w i t h t h e o p t i c a l measurements, a c u t - o f f a t 0.65 eV, s u g g e s t i n g a r e l a t i v e l y s h a r p o p t i c a l a b s o r p t i o n e d g e . The t e m p e r a t u r e dependence of t h e s t e a d y s t a t e p h o t o c o n d u c t i v i t y was measured between 1 5 K and 300 K , f o r a n e x c i t a t i o n rad- i a t i o n o f 1 . 3 eV (low- h o t o n f l u x l e v e l s % 0.5 1012 p h o t o n s cm-2 s e c

P,

and t h e r e s u l t i s s b i n F i g u r e 6 . , t o g e t h e r w i t h t h e t e m p e r a t u r e dependence o f t h e d a r k c o n d u c t i v i t y i n amorphous

( p r e s e n t s t u d y ) and c r y s t a l l i n e ( n - t y p e ;

l o i 3

d o n o r s cm 3 , germanium. The p h o t o c o n d u c t i v i t y i s a l m o s t t e m p e r a t u r e i n d e p e n d e n t and h a s a v a l u e which i s h i g h e r o r comparable t o t h e v a l u e s found i n h y d r o g e n a t e d germanium ( 1 3 ) , s i l i c o n ( 1 4 ) and o t h e r p h o t o c o n d u c t i v e g l a s s e s s u c h a s AS2Te2 ( 1 5 ) . The t i m e r e s p o n s e o f t h e p h o t o c u r r e n t was a l s o measured and a s l i g h t d e c r e a s e o f t h e s i n e a l (% UD t o 5 % ) was d e t e c t -

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C4-694 JOURNAL DE PHYSIQUE

F i g . 7: P h o t o r e s p o n s e o f a-Ge a t 77K. F i g . 8: P h o t o r e s p o n s e o f a-Ge a t 5.5K a f t e r p h o t o r e s p o n s e f a t i g u e .

At 77K, t h e s a t u r a t i o n o f t h e p h o t o c u r r e n t was n o t a c h i e v e d a f t e r 1 0 3 s e c , a n d a l s o t h e d e c a y o f t h e p h o t o c u r r e n t was c h a r a c t e r i s e d by a l o n g t i m e c o n s t a n t o f t h e o r d e r o f 1 0 3 s e c . C o o l i n g t h e s a m p l e t o l i q . He l e m p e r a t u r e d e c r e a s e d t h e

a b s o l u t e v a l u e o f t h e p h o t o c u r r e n t f r o m a p p r . 5 . 1 0 1 2 Amps t o a p p r . 1.10-13 Amps.

However, c o n t r a r y t o t h e b e h a v i o u r a t 77K, s t r o n g f a t i g u i n g o f t h e p h o t o - s i g n a l was now o b s e r v e d o n a t i m e s c a l e o f 6 0 s e c . , a f t e r w h i c h a r e l a t i v e l y c o n s t a n t

" f a s t " ( r e s p o n s e a n d d e c a y t i m e s l e s s t h a n 0 . 1 s e c ) p h o t o - s i g n a l o f 6.10-14 Amps was o b s e r v e d o n r e p e a t e d , a p p e r i o d i c a l l y i n t e r r u p t e d i l l u m i n a t i o n s , a s shown i n F i g u r e 8 . T h e s e e f f e c t s a r e a t p r e s e n t b e i n g i n v e s t i g a t e d by e x - s i t u t i m e r e s o l v e d p h o t o c o n d u c t i v i t y a n d p h o t o l u m i n e s e n c e e x p e r i m e n t s .

R e f e r e n c e s :

1. ~ i E r o r P . , C a t t e l l R., F e r r i e r R. a n d Y o f f e A.D., P r o c e e d i n g s o f 6 t h I n t . Conf.

o n Amorphous and L i q u i d S e m i c o n d u c t o r s , L e n i n g r a d (1975) 5 8 .

2. Laude L . D . , W i l l i s R.F. a n d F i t t o n B., S o l i d S t . Connnunications

20

(1973) 1007.

3 . v i g E o r P . a n d A l l a n D., T h i n S o l i d F i l m s 62 (1979) 259, 4. viHEor P . , T h i n S o l i d F i l m s ( t o be p u b l i s h e d ) .

5. G r e a v e s G.N., E l l i o t t , S.R. and D a v i s E.A. Advances i n P h y s i c s

8

(1979) 49.

6 . v i i z o r P., Phys. Rev. L e t t . ( t o b e p u b l i s h e d ) .

7. P a u l W., C o n n e l l G.A.N. a n d Temkin R . J . , Advances i n P h y s i c s

22

(1973) 49.

8 . Donovan T.M., S p i c e r W.E., B e n n e t t J . M . a n d A s h l e y E . J . , P h y s . Rev. (1970) 197.

-. . .

9 . E l l i o t t P . , Ph.D. T h e s i s , U n i v e r s i t y o f Cambridge ( 1 9 7 4 ) . 10. C o n n e l l G.A.N., P a w l i k , J . R . , Phys. Rev. B13 (1976) 787.

11. v i g E o r P. and Y o f f e A.D., J . Non-Cryst. S x d s

35-36

( 1 9 7 8 ) 409.

12. S z p i l k a A.M. a n d ~ i Z E o r P . , P h i l . Mag. ( t o b e p u b l i s h e d ) .

1 3 . Moustakas T.D., C o n n e l l G.A.N. a n d P a u l W . , P r o c e e d i n g s o f 6 t h I n t . Conf. o n Amorphous a n d L i q u i d S e m i c o n d u c t o r s , L e n i n g r a d (1975) 310.

1 4 . L o v e l a n d R . J . , S p e a r W.E. and A l - S h a r b a t y A., J . Non-Cryst. S o l i d s

12

(1973174) 55.

15. M o u s t a k a s T.D. a n d W e i s e r K., P h y s . Rev.

012

(1975) 2448.

1 6 . Lewis A . J . , P h y s . Rev. ( 1 9 7 6 ) 658.

1 7 . B e y e r W. and S t u k e J . Phys. S t . S o l i d i

2