THE STRUCTURE AND THE MEDIUM RANGE ORDER IN THIN AMORPHOUS GERMANIUM FILMS PREPARED IN UHV

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THE STRUCTURE AND THE MEDIUM RANGE ORDER IN THIN AMORPHOUS GERMANIUM

FILMS PREPARED IN UHV

P. Viščor

To cite this version:

P. Viščor. THE STRUCTURE AND THE MEDIUM RANGE ORDER IN THIN AMORPHOUS

GERMANIUM FILMS PREPARED IN UHV. Journal de Physique Colloques, 1982, 43 (C9), pp.C9-

39-C9-42. �10.1051/jphyscol:1982907�. �jpa-00222400�

THE STRUCTURE AND THE MEDIUM RANGE ORDER IN THIN AMORPHOUS GERMANIUM FILMS PREPARED IN UHV

P. Viscor

Université de l'Etat, 7000 Mons, Belgium and Cavendish Labovatoyy, Madingley Road, Cambridge, U.K.

Résumé, — Des couches minces stables de germanium amorphe (a-Ge) ont été préparées par évaporation lente, sur des supports en saphir chauffés en ultravide. La structure de ces couches a été étudiée in situ par diffusion élastique des électrons de haute énergie, en transmission à 77 K. Les mesures de densité et de l'indice de réfraction de basse énergie ont été également faites in situ. Celles-ci indiquent que les couches m i n c e s , stables de a-Ge sont bien moins denses que les phases cristallines. Les expériences de diffusion des électrons aux basses températures ont montré quelques différences dans la position des maximas d'interférence par rapport aux travaux antérieurs. Pour la pre- mière fois, un pré-pic "aigu" vers s = 1,2 A-' a été o b s e r v é , comme il est habituel dans les systèmes désordonnés. On discute les résultats en termes de coordination et de l'étendue des liaisons électroniques dans le verre.

Abstract.- Stable amorphous germanium (a-Ge) films were prepared by slow evaporation onto heated sapphire substrates in ultra high vaccuum ( U H V ) . The structure of these films was investi- gated by in-situ transmission high energy electron elastic diffraction performed at 77K, by in-situ density measurements and by in-situ determination of the low energy refractive index.

The density and the refractive index measurements indicated that thin, stable a-Ge films are appreciably less dense than

the crystalline counterpart. The low temperature electron diffraction experiments showed some changes in the positions of the reciprocal space interference m a x i m a , when compared with work of others. A l s o , for the_first time, a "sharp"

diffraction pre-peak (at s = 1.2 A ) has been observed, a diffraction feature generally seen in disordered systems.

The results are discussed in terms of the coordination and the range of the electronic bonding interactions in glass.

Preparation.

A-Ge films were prepared by slow evaporation (0.1-0.5A sec ) , at normal incidence (distance between the source and the substrate ~ 5 0 c m ) , of high purity germanium (10 cm of electrically active impurities) from a tungsten boat (degassed under UHV conditions) onto single crys- tal sapphire substrates (optical measurements) or onto amorphous thin sapphire substrates (25A° of a - A l203 deposited onto 1 00A° of a A-C [1] - the electron diffraction m e a s u r e m e n t s ) . The substrate temperature during the deposition was 400 K and the pressure in the system was kept below 5-10 torr.

Experimental techniques.

In-situ simultaneous measurements of the optical reflection R and transmission T were performed in order to obtain trie low energy refrac- Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982907

C9-40 JOURNAL DE PHYSIQUE

tive index. The exact thickness of the studied a-Ge f i l m s w a s deter- mined using a g e n e r a l f o r m u l a f o r the interference m a x i m a and/or mini- ma in R and T in an energy r e g i o n of n o n z e r o o p t i c a l absorption 121.

I t has b e e n s h o w n [3] that the density

y

of an unknown f i l m c a n be calculated f r o m a formula

w h e r e Af is the t o t a l frequency change of the quartz crystal thickness monitor during the deposition of the f i l m and t i s the thickness of t h e film. T h i s t e c h n i q u e h a s b e e n used i n the present study for in- situ d e t e r m i n a t i o n of Q . T h e constant of proportionality K in equa- tion ( 1 ) w a s obtained by measuring Af,t and

7

i n gold f i l m s of various thicknesses.

T h i n a-Ge f i l m s , prepared on thin composite substrates w e r e a l s o studied through in-situ high energy (40 keV) e l e c t r o n elastic scatte- ring (energy r e s o l u t i o n of t h e scanning electron d i f f r a c t i o n c a m e r a

-

was appr. 2 eV)

.

Results.

L o w energy refractive i n d e x and the thickness. T h e normalised o p t i c a l r e f l e c t i o n and transmission i n studied f i l m s i s s h o w n i n figure 1 , where i t is plotted as a f u n c t i o n of e n e r g y E A r e f l e c t i o n interference m a x i m u m at 1.35 e V a n d a minimum at 2.15 e V are clearly visible. T h e v a l u e s of R(E) and T(E), together w i t h the interference condition f o r R were solved numerically i n terms of the refractive index n(E), the e x t i n c t i o n coefficient k(E) and the thickness of t h e f i l m t. The v a l u e of n(k+o) i s lower than in the crystal, indicating a f i n i t e density deficit of studied a-Ge films. T h e measurements of Keat [4] o n the density dependence of the r e f r a c t i v e index in S i 0 2 support this conclusion.

1.0

u

08

a t 0s- E,

0

.= ^OL-

;- 0.2

0.0 -02

-

t =

510.4 A

3oooo

-

-

n(k-0)

= 3.7

. \.,

_{\ /} ^/

^.--\

^\ _\ R, X K ) o -

v, 8

,/ \,

3

,

^{/ \ \}

- '. . . _{--- ----_}

^loo00

^-

---

T

-

'

1 I I I I

05 1.0

1.5

2.0 2

5 30

1 " " l " " l '

WXX) 2000 3000

E/eV ' / A

Fig. 1 : Normalised optical reflec- Fig. 2 : L i n e a r d e p e n d e n c e of the tion R and transmission T a s a frequency shift A £ of the quartz f u n c t i o n of e n e r g y for a 5 1 0 A' crystal oscillator v e r s u s the thick a-Ge film. thickness t of measureda-Ge films.

The density. T h e constant of proportionality K in e q u a t i o n ( 1 ) was found to be K=0.435+0.010. The standard deviation o f 0.01 deter- mined t h e overall accuracy w i t h w h i c h t h e density of a-Ge could be

calculated (the errorsin A f and t w e r e small compared to t h e error in K determination). The results of in-situ density measurements are shown i n f i g u r e 2 f o r two f i l m s of various thicknesses. T h e ratio

e q u a t i o n ( 1 ) t o b e 4 . 4 6 k 0 . 1 6 , s h o w i n g a d e n s i t y d e f i c i t of some 1 6 . 5 % w i t h t h e r e s p e c t t o c r y s t a l . A r e l a t i v e l y l a r g e s t a n d a r d d e v i a t i o n o f 0 . 1 6 i s c a u s e d p a r t i a l l y by t h e t h i c k n e s s d e p e n d e n c e o f s t u d i e d a-Ge f i l m s ( a n e x p e r i m e n t , i n c l u d i n g t h i c k f i l m s , t o c l a r i f y t h i s p o i n t m o r e q u a n t i t a t i v e l y i s u n d e r w a y ) . T h i s r e s u l t a g r e e s q u a l i t a t i v e l y w i t h r e c e n t n e u t r o n d i f f r a c t i o n m e a s u r e m e n t s [ 5 ] on a - G e , f r o m w h i c h a d e n s i t y d e f i c i t o f 1 0 % was i n f e r r e d . I t a l s o a g r e e s w i t h t h e d e n - s i t y m e a s u r e m e n t s o n some 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 S i 0 , [ 4 ] a n d a-As [ 6 ] a n d g i v e s s u p p o r t t o t h e t o p o l o g i c a l a r g u m e n t s of P h i l l i p s [ 7 ] c o n c e r n i n g t h e b e s t c o o r d i n - a t i o n f o r g l a s s f o r m a t i o n ( o v e r c o o r d i n a t e d r a n d o m n e t w o r k s

-

c o o r d i n a t i o n >3- s h o u l d l e a d t o l e s s p e r f e c t p a c k i n g a n d t h e r e f o r e i n c r e a s i n g d e n s i t y d e f i c i t ) .

The t r a n s m i s s i o n 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 . The p r i n c i p a l r e s u l t o f t h e p r e s e n t i n v e s t i g a t i o n i s shown i n f i g u r e 3 . H e r e t h e s c a t t e r e d i n t e n s i t y ( n o t n o r m a i i s e d ) f r o m a-Ge f i l m ; m e a s u r e d a t 77K, i s p l o t t e d a g a i n s t t h e s c a t t e r i n g v e c t o r s . I n o r d e r t o b e a b l e t o c o m p a r e t h e p r e s e n t r e s u l t s w i t h o t h e r s t r u c t u r a l d a t a i n t h e l i t e r a t u r e , t h e d i f f r a c t i o n d a t a i n f i g u r e 3 was a n a l y z e d t o y i e l d t h e t o t a l i n t e r f e r e n c e f u n c t i o n F ( s ) . T h i s was t h e n a p p r o p r i a t e l y t r a n s f o r m e d i n t o t h e r e a l s p a c e ( a m o d i f i c a t i o n f u n c t i o n d u e t o L o r c h [ 8 ] h a s b e e n u s e d i n t h e F o u r i e r i n t e g r a l t o r e d u c e t h e s p u r i o u s

r i p p l e s i n r e a l s p a c e r e s u l t s d u e t o t h e t e r m i n a t i o n o f F ( s ) a t f i n i t e s ) i n o r d e r t o o b t a i n e d t h e t o t a l d i f f e r e n t i a l c o r r e l a t i o n f u n c t i o n D ( r ) . B e c a u s e of t h e t e r m i n a t i o n e f f e c t s , t h e r e a l s p a c e d a t a c a n s e r v e o n l y a s i n d i c a t i o n of a u a l i t a t i v e t r e n d s i n r e a l s p a c e r a t h e r t h a n a s a d e t a i l e d q u a n t i t a t i v e e v a l u a t i o n o f t h e r e a l s p a c e p a r a m e - t e r s . To s u m m a r i s e b r i e f l y t h e e l e c t r o n d i f f r a c t i o n r e s u l t s , i t c a n b e s a i d t h a t

a ) n e a r e s t - n e i g h b o u r c o o r d i n a t i o n i s w e l l b e l o w t h a t o f t h e c r y s t a l a n d no d i s o r d e r b r o a d e n i n g o f t h e f i r s t c o o r d i n a t i o n p e a k i s o b s e r v e d

( t h i s i s i n a good q u a l i t a t i v e a g r e e m e n t w i t h t h e n e u t r o n d i f f r a c t i o n r e s u l t s o n a-Ge [ 51

,

b ) t h e s e c o n d n e a r e s t n e i g h b o u r c o o r d i n a r i o n i s i n c r e a s e d a n d t h e r e s e e m t o b e

c ) g r o u p i n g o f t h e c h a r a c t e r i s t i c d i s t a n c e s i n b o t h t h e r e a l a n d

t h e r e c i p r o c a l s p a c e .

-

¹

F u r t h e r m o r e , a n e x t r a f e a t u r e i s a p p a r e n t a t s - 1 . 2 A i n t h e r a w d i f f r a c t i o n p a t t e r n shown i n f i g u r e 3 . T h i s i s t h e f i r s t t i m e t h a t w h a t i s known i n t h e l i t e r a t u r e a s a " f i r s t s h a r p d i f f r a c t i o n p e a k "

( F S D P ) h a s b e e n a l s o i d e n t i f i e d i n a n e l e m e n t a l , t e t r a h e d r a l l y b o n d e d a m o r p h o u s s e m i c o n d u c t o r .

D i s c u s s i o n .

The l o w d e n s i t y ( r e s u l t s u p p o r t e d b y t h e m e a s u r e m e n t s o f low e n e r g y r e f r a c t i v e i n d e x ) , t o g e t h e r w i t h t h e n e a r e s t a n d t h e s e c o n d n e a r e s t n e i g h b o u r c o o r d i n a t i o n s h o u l d s e r v e a s i m p o r t a n t p o i n t s o f r e f e r e n c e f o r a n y man o r c o m p u t e r b u i l t s t r u c t u r a l m o d e l of a-Ge.

F u r t h e r m o r e , a n y s u c h m o d e l h a s t o r e p r o d u c e t h e FSDP b o t h i n i t ' s p o s i t i o n a n d i t ' s s t r e n g h t .

The e x i s t e n c e o f t h e FSDP s e e m s t o b e a g e n e r a l f e a t u r e , common t o many d i s o r d e r e d s y s t e m s , i r r e s p e c t i v e o f t h e i r e l e c t r o n i c s t r u c - t u r e , n e a r e s t n e i g h b o u r c o o r d i n a t i o n a n d t h e c h e m i c a l c o m p o s i t i o n

(FSDP h a s b e e n o b s e r v e d 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 As, S ,

,

As ( S , S e ) 3 , G e ( S , S e ) z , SiOz b u t a l s o i n e l e m e n t a l a m o r p h o u s S b , A s , P a n d e v e n i n some m e t a l l i c g l a s s e s and l i q u i d m e t a l a l l o y s ) . I t i s p r o b a b l y c o n n e c t e d w i t h t h e l o c a l n a t u r e of t h e r e a l s p a c e

c o r r e l a t i o n s I n a l s o r d e r e a s y s t e m s . I n c a s e o f p r e s e n t s t u d y , t h i s i s d e m o n s t r a t e d i n f i g u r e 4 , w h e r e a "wave p a c k e t " l i k e n a t u r e o f t h e t o - t a l d i f f e r e n t i a l c o r r e l a t i o n f u n c t i o n D ( r ) i s a p p a r e n t , t h e r e a l s p a c e

C9-42 JOURNAL DE PHYSIQUE

correlations dying away fast with increasing r. The existence of the FSDP might be therefore an important "range" parameter of an elec- tron interactions in a glass, indicating the importance of bonding interactions beyond the nearest neighbour. If so, this would be of relevance, when the electronic structure of a glass is to be calculated [ 9,101.

-

UI

=

^I,

-

C =!

-

I

-

i

;

_ _ - -

-4

-

1 1 1 1 1 1 1 1 1 I I I

9 8 7 6 5 4 3 2 1 ₀ 2 4 ^I6

s

/

A-'

r / A

Fig. 3 : Plot of elastically scatte- Fig. 4 : The total differential red unnormalised electron intensity correlation function D(r) for versus scattering vector s for 510~' 5 1 0 A@ thick a-Ge film.

thick a-Ge film.

References.

1 . ViZzor P., Cattell R., Ferrier R. and Yoffe A.D., Proceedings of 6th Int. Conf. on Amorphous and Liquid Semiconductors, Leningrad (1975) 58.

2. ~ i g z o r P., Journal of Applied Optics (to be published).

3. Vi::or P. and Allan D., Thin Solid Films

62

(1979) 259.

4. Keat P.P., Science

120

(1954), 328.

5. Etherington G., Wright A.C., Wenzel J.T., Dore J.C., Clarke J.M.

and Sinclair R.N., Journal of Non-Crystalline Solids (1982- in press).

6. Greaves G.N., Elliott, S.R. and Davis E.A.,Advances in Physics

28

(1979) 49.

7. Phillips J.C., Journal of Non-Crystalline Solids

2

^{(1979), 153;}

ibid 35-36 (1 980), 1 1 57.

8. Lorch E.A., J. Phys. C 2 (1969), 229.

-

9. Heine V., Bullett D.W. and Kelly M.J., Solid State Physics

35

(1980). 1 .

10.bnderson P.W., La Matisre ma1 CondensQe (I11 Condensed Matter), ed. by Balian R.,

Maynard R. and Toulouse G. (North Holland Publishing Comp., New York 1979), 180; Physical Review Letters

21

(1968), 13;

Physical Review

181

(1969), 25.