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MICROSCOPIC PROPERTIES OF ORDERED AND DISORDERED ZnSe-Ge INTERFACES+
C. Quaresima, F. Patella, F. Sette, C. Capasso, A. Savoia, P. Perfetti, G.
Margaritondo
To cite this version:
C. Quaresima, F. Patella, F. Sette, C. Capasso, A. Savoia, et al.. MICROSCOPIC PROPERTIES OF ORDERED AND DISORDERED ZnSe-Ge INTERFACES+. Journal de Physique Colloques, 1984, 45 (C5), pp.C5-395-C5-399. �10.1051/jphyscol:1984560�. �jpa-00224179�
JOURNAL DE PHYSIQUE
Colloque C5, supplkrnent a u no&, Tome 45, a v r i l 1984 p a g e C5-395
MICROSCOPIC PROPERTIES OF ORDERED AND D 1 SORDERED Z n S e - G e INTERFACES+
C . Quaresima, F. P a t e l l a , F. S e t t e , C. Capasso, A . S a v o i a , P . P e r f e t t i and G. ~ a r ~ a r i t o n d o *
PULS, GNSM-INFN, k b o r a t o r i NazionaZi d i Frascati, ItaZy
*~epartrnent of Physics, U n i v e r s i t y o f Wisconsin, Madison, WIS 53706, U.S.A.
Rksum& - Nous donnons une dkmonstration experimentale d i r e c t e que l a d i s - c o n t i n u i t k de l a bande de valence du syst&me ZnSe(ll0)-Ge n ' e s t pas mo- d i f i e e de p l u s de 0 , l eV p a r d e s p r o c e s s u s thermiques. Ce r k s u l t a t e s t en f a v e u r d e s t h k o r i e s pour une d e s c r i p t i o n approchke de c e t t e d i s c o n t i n u i - t k q u i n k g l i g e n t l e s e f f e t s locaux.
A b s t r a c t - We p r o v i d e d i r e c t e x p e r i m e n t a l e v i d e n c e t h a t a n n e a l i n g p r o c e s s e s do n o t change t h e ZnSe(ll0)-Ge v a l e n c e band d i s c o n t i n u i t y b y more t h a n 0. l eV. T h i s r e s u l t s u p p o r t s r e c e n t a t t e m p t s t o f o r m u l a t e a p p r o x i m a t e d i s c o n t i n u i t y t h e o r i e s which n e g l e c t l o c a l e f f e c t s .
The n a t u r e o f t h e h e t e r o j u n c t i o n band d i s c o n t i n u i t i e s i s one o f t h e most c r u c i a l p r o b l e m s i n s e m i c o n d u c t o r i n t e r f a c e p h y s i c s . f S u b s t a n t i a l p r o g r e s s was made i n r e c e n t y e a r s w i t h t h e u s e o f p h o t o e m i s s i o n t e c h n i q u e s t o perform measurements o f t h e valence-band d i s c o n t i n u i t y , AEv. These measurements have p r o v i d e d t h e n e c e s s a r y e x p e r i m e n t a l b a s i s t o t e s t e x i s t i n g d i s c o n t i n u i t y t h e o r i e s and t o d e v e l o p new t h e o r e t i c a l a p p r o a c h e ~ . * - ~
One p o i n t i n common t o most c u r r e n t t h e o r i e s which p r e d i c t AEv i s t h e i r n e g l e c t o f t h e l o c a l f a c t o r s i n f l u e n c i n g t h i s .2* 3 T h i s a p p r o x i m a t i o n r e s u l t s i n an u n d e r l y i n g a c c u r a c y l i m i t f o r t h e c o r r e s p o n d i n g t h e o r e t i c a l p r e d i c t i o n s o f AEv. There i s a t p r e s e n t some u n c e r t a i n t y a b o u t t h e m a g n i t u d e o f t h i s a c c u r a c y l i m i t . For example, Katnani and ~ a r g a r i t o n d o 3 s y s t e m a t i c a l l y a n a l y z e d p h o t o e m i s s i o n measurements o f AE, f o r a l a r g e number o f i n t e r f a c e s , c o n c l u d i n g t h a t t h e above a c c u r a c y l i m i t i s c l o s e t o + 0.15 eV. But o t h e r a u t h o r s p r e s e n t e d e v i d e n c e f o r l o c a l e f f e c t s o f a b i g g e r magnitude i n s e l e c t e d i n t e r f a c e s . 5 - 7 A c r u c i a l p o i n t i n a s s e s s i n g t h e magnitude o f l o c a l f a c t o r s i n f l u e n c i n g AEv i s t h e r e l i a b i l i t y o f t h e p h o t o e m i s s i o n methods t o measure t h i s 3 For many i n t e r f a c e s , p h o t o e m i s s i o n measurements o f AEv r e q u i r e an e s t i m a t e o f t h e band bending c h a n g e s d u r i n g t h e i n t e r f a c e f o r m a t i o n . I n t u r n , t h i s e s t i m a t e i s b a s e d on t h e o b s e r v e d c o r e - l e v e l e n e r g y s h i f t s . However, t h e e n e r g y p o s i t i o n o f e a c h c o r e l e v e l can be i n f l u e n c e d by o t h e r f a c t o r s b e s i d e s t h e band b e n d i n g . e . g . b y i n t e r f a c e c h e m i c a l r e a c t i o n s -- and t h i s i n t r o d u c e s a n e l e m e n t o f u n c e r t a i n i t y i n t h e c o r r e s p o n d i n g e s t i m a t e s o f AEv. T h e r e f o r e , t h e few i n t e r f a c e s f o r which AEv i s l a r g e enough t o b e s e e n and measured d i r e c t l y i n t h e p h o t o e m i s s i o n s p e c t r a a r e e x t r e m e l y v a l u a b l e t e s t c a s e s . We used one o f t h e s e i n t e r f a c e s , Ge on c l e a v e d Z n S e ( l l 0 ) . t o p r o b e t h e magnitude o f one o f t h e p o t e n t i a l l o c a l e f f e c t s c o n t r i b u t i n g t o AE, -- i n t e r f a c e d i ~ o r d e r . ~
+ w o r k s u p p o r t e d i n p a r t by t h e NSF. G r a n t DMR-82-00518. The i n t e r n a t i o n a l c o l l a b o r a t i o n F r a s c a t i - W i s c o n s i n i s s u p p o r t e d by t h e I t a l i a n N a t i o n a l R e s e a r c h C o u n c i l and by t h e NSF, G r a n t INT-81-22013.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984560
C5-396 JOURNAL DE PHYSIQUE
I t i s w e l l known t h a t room-temperature d e p o s i t i o n o f Ge on c l e a v e d Z n S e ( l l 0 ) r e s u l t s i n an amorphous o v e r l a y e r , 3 * 6 a s s e e n from t h e d i s a p p e a r a n c e o f t h e LEED p a t t e r n s and from t h e shape o f t h e Ge valence-band p h o t o e m i s s i o n s p e c t r a . Kowalczyk e t a1.6 r e p o r t e d t h a t a n n e a l i n g f o r a few m i n u t e s a t t e m p e r a t u r e s o f t h e o r d e r of 300' C r e s u l t s i n a c r y s t a l l i z a t i o n o f t h e o v e r l a y e r a s s e e n from LEED p a t t e r n s . Our p r e s e n t e x p e r i m e n t s c o n f i r m e d t h a t a 1x1 LEED p a t t e r n r e a p p e a r s upon a n n e a l i n g . But we a l s o found e v i d e n c e f o r s t r o n g d e p l e t i o n o f Ge c a u s e d by t h e a n n e a l i n g p r o c e s s . Valence-band p h o t o e m i s s i o n s p e c t r a were used t o measure 4Ev d i r e c t l y b e f o r e and a f t e r t h e a n n e a l i n g p r o c e s s . Our r e s u l t s showed n o c h a n g e s i n AEv, w i t h i n t h e e x p e r i m e n t a l a c c u r a c y o f * 0.1 eV. These
r e s u l t s s u p p o r t t h e t h e o r e t i c a l a c c u r a c y l i m i t deduced by K a t n a n i and M a r g a r i t o n d o . 3
The p h o t o e m i s s i o n e x p e r i m e n t a l p r o c e d u r e t o d i r e c t l y m e a s u r e AEv i s s c h e m a t i c a l l y i l l u s t r a t e d i n F i g . 1. The valence-band p h o t o e m i s s i o n s p e c t r a o f a t h i n G e o v e r l a y e r on ZnSe e x h i b i t two e d g e s , r e l a t e d t o t h e valence-band e d g e s o f t h e two components. AE,, i s s i m p l y g i v e n by t h e d i f f e r e n c e between t h e e n e r g y p o s i t i o n s o f t h e two s p e c t r a l e d g e s . The e x p e r i m e n t s were e n t i r e l y performed u n d e r u l t r a h i g h vacuum c o n d i t i o n s , a t p r e s s u r e s o f 2 X 10-'O T o r r o r b e t t e r . w i t h t h e e x p e r i m e n t a l a p p r o a c h d e s c r i b e d i n Ref. 3. I n p a r t i c u l a r , t h e p h o t o e l e c t r o n - e x c i t i n g p h o t o n s were e m i t t e d by s y n c h r o t r o n r a d i a t i o n s o u r c e s (Wisconsin and F r a s c a t i ) . and t h e i r e n e r g y was i n t h e r a n g e 10-200 eV.
P h o t o e m i s s i o n s p e c t r a t a k e n a t t h e s e photon e n e r g i e s d o c l e a r l y e x h i b i t t h e two valence-band e d g e s due t o AE,,, a s shown i n F i g . 2 f o r b o t h unannealed and a n n e a l e d ZnSe-Ge. We e s t i m a t e d t h e d i s c o n t i n u i t y from t h e s e s p e c t r a , b y l i n e a r e x t r a p o l a t i o n o f t h e two e d g e s w i t h an o v e r a l l a c c u r a c y o f + 0.1 e v . 3 A s one c a n c l e a r l y s e e from F i g . 2, n o d i f f e r e n c e i n A& can b e d e t e c t e d between t h e a n n e a l e d - i n t e r f a c e and a s - g r o w n - i n t e r f a c e s p e c t r a . The measured v a l u e o f AEv i s 1 . 4 ( 2 ) + 0.1 eV.
F i g . 1 - D i r e c t p h o t o e m i s s i o n measurement o f t h e ZnSe-Ge i n t e r f a c e v a l e n c e band d i s c o n t i n u i t y , LIEv. Valence-band p h o t o e m i s s i o n s p e c t r a t a k e n on ZnSe c o v e r e d by a t h i n Ge o v e r l a y e r e x h i b i t two e d g e s , a n d A E v i s g i v e n by t h e d i f f e r e n c e between t h e i r p o s i t i o n s i n e n e r g y .
F i g . 2
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Valence-band p h o t o e m i s s i o n s p e c t r a t a k e n on c l e a v e d Z n S e ( l l 0 ) w i t h a Ge o v e r l a y e r . ( a ) 5 8 o f Ge, as-grown. (b) 3 0 8 o f Ge.a n n e a l e d a t 2 8 0 ~ ~ f o r 5 min. (c) Same i n t e r f a c e a s f o r s p e c t r u m ( b ) , a f t e r f u r t h e r a n n e a l i n g f o r 5 min a t 300'~. The s p e c t r a were s h i f t e d i n e n e r g y w i t h r e s p e c t t o e a c h o t h e r t o compensate f o r t h e a n n e a l i n g - i n d u c e d band-bending c h a n g e s , and t h e z e r o o f t h e e n e r g y s c a l e i s t h e t o p o f t h e Ge v a l e n c e band. We s e e no m e a s u r a b l e c h a n g e i n AEV due t o t h e a n n e a l i n g p r o c e s s .
The p o s s i b i l i t y o f d i r e c t l y o b s e r v i n g and m e a s u r i n g AE, i s p a r t i c u l a r l y i m p o r t a n t f o r t h e ZnSe-Ge i n t e r f a c e . I n f a c t , X-ray p h o t o e m i s s i o n e x p e r i m e n t s 6 on t h i s same system had r e v e a l e d a 0.3 eV c h a n g e i n t h e Zn3d-Ga3d b i n d i n g e n e r g y d i f f e r e n c e d u r i n g t h e a n n e a l i n g p r o c e s s . T h i s c h a n g e c o u l d b e e r r o n e o u s l y a t t r i b u t e d t o a change i n AEv, a l t h o u g h Kowalczyk e t c o r r e c t l y warned t h a t t h i s c o n c l u s i o n c o u l d n o t b e drawn on t h e b a s i s o f t h e i r d a t a . I n f a c t , o u r p r e s e n t d a t a e x c l u d e a n n e a l i n g - i n d u c e d d i s c o n t i n u i t y c h a n g e s o f t h a t magnitude.
Another p o s s i b l e c a u s e f o r t h e above 0.3 eV c h a n g e i n t h e b i n d i n g e n e r g y d i f f e r e n c e is t h e s p e c t r a l c o n t r i b u t i o n from atoms i n chemical environments d i f f e r e n t from t h o s e o f t h e c l e a n s u r f a c e s o f Ge and ZnSe. N o t i c e t h a t i n F i g . 2 t h e Ge s i g n a l i s o f r o u g h l y e q u i v a l e n t m a g n i t u d e f o r 5 8 o f as-grown Ge and f o r 30 8 o f Ge a f t e r a n n e a l i n g . T h i s i s due t o a d i f f u s i o n o f Ge atoms d u r i n g a n n e a l i n g , l e a d i n g t o a d e p l e t i o n o f t h e o v e r l a y e r . The d i f f u s e d Ge a t o m s a r e l i k e l y t o c o n t r i b u t e t o t h e X-ray Ge3d p h o t o e m i s s i o n peak, and t o i t s a v e r a g e p o s i t i o n i n e n e r g y .
We made p r e l i m i n a r y a t t e m p t s t o measure t h e d i s c o n t i n u i t y by means o f a d e c o n v o l u t i o n o f t h e ZnSe-Ge s p e c t r a . The as-grown s p e c t r a were f i t t e d w i t h a sum o f t h e valence-band s p e c t r a o f c l e a n Z n S e ( l l 0 ) and o f a t h i c k o v e r l a y e r of amorphous Ge. The f i t t i n g p a r a m e t e r s were t h e p o s i t i o n i n e n e r g y and t h e
JOURNAL DE PHYSIQUE
F i g . 3 - The s p e c t r a o f Ge-covered ZnSe c a n b e e m p i r i c a l l y r e p r o d u c e d b y a sum o f t h e s p e c t r a o f c l e a n ZnSe and o f a t h i c k Ge o v e r l a y e r . ( a ) S p e c t r u m o f 5 2 o f Ge on c l e a v e d ZnSe, as-grown. ( b f B e s t f i t o f c u r v e ( a ) , o b t a i n e d by summing t h e ZnSe s p e c t r u m ( c ) and t h e Ge s p e c t r u m ( d ) . The f i t t i n g p a r a m e t e r s were t h e i n t e n s i t y and r e l a t i v e p o s i t i o n i n e n e r g y o f t h e s p e c t r a ( c ) and ( d ) . The good q u a l i t y o f t h e f i t r u l e s o u t a l a r g e d e n s i t y o f l o c a l i z e d e l e c t r o n i c s t a t e s a t t h i s i n t e r f a c e .
i n t e n s i t y o f t h e two c l e a n - s u r f a c e s p e c t r a . The r e s u l t s o f t h e f i t , a s shown i n F i g . 3, w e r e v e r y g o o d . T h i s i m p l i e s t h a t t h e as-grown i n t e r f a c e i s r e a s o n a b l y a b r u p t , and i n p a r t i c u l a r t h a t t h e r e i s n o s t r o n g c o n t r i b u t i o n from l o c a l i z e d e l e c t r o n i c s t a t e s a s , f o r e x a m p l e , i n t h e Si-Ge i n t e r f a c e . 8 P r e l i m i n a r y a t t e m p t s t o e x t e n d t h i s a n a l y s i s t o t h e a n n e a l e d i n t e r f a c e s c o n f i r m e d t h e a b o v e c o n c l u s i o n s a b o u t t h e i n s e n s i t i v i t y o f AE,, t o t h e a n n e a l i n g p r o c e s s e s .
I n c o n c l u s i o n , o u r test r e v e a l e d no c h a n g e i n t h e h e t e r o j u n c t i o n v a l e n c e band d i s c o n t i n u i t y upon a n n e a l i n g t o c r y s t a l l i z e a Ge o v e r l a y e r on c l e a v e d ZnSe.
T h i s r e s u l t c a n n o t b e e x t e n d e d t o o t h e r i n t e r f a c e s w i t h o u t p e r f o r m i n g t h e c o r r e s p o n d i n g e x p e r i m e n t s -- n o r t o o t h e r p o s s i b l e n l o c a l l * f a c t o r s i n AEv s u c h a s t h e s u b s t r a t e o r i e n t a t i o n and t h e s e q u e n c e o f d e p o s i t i o n . A g e n e r a l c o n c l u s i o n , h o w e v e r , c a n b e drawn from i t -- it i s e x t r e m e l y i m p o r t a n t t o p e r f o r m t h e s e t e s t s on s y s t e m s f o r which t h e d i s c o n t i n u i t y i s d i r e c t l y v i s i b l e . e . g . t h o s e i n v o l v i n g ZnSe. Gap a n d CdS s u b s t r a t e s . We a r e i n v o l v e d a t p r e s e n t i n t h e s t u d y o f a n n e a l i n g e f f e c t s a n d o f o t h e r l o c a l c o n t r i b u t i o n s t o AEv i n s e v e r a l s y s t e m s o f t h i s k i n d .
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