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STRUCTURE OF CLEAN III-V SEMICONDUCTOR SURFACES
M. Sauvage-Simkin
To cite this version:
M. Sauvage-Simkin. STRUCTURE OF CLEAN III-V SEMICONDUCTOR SURFACES. Journal de
Physique Colloques, 1989, 50 (C7), pp.C7-119-C7-128. �10.1051/jphyscol:1989711�. �jpa-00229684�
COLLOQUE DE PHYSIQUE
Colloque C 7 , Suppl6ment au nolO, Tome 50, octobre 1989
STRUCTURE OF CLEAN 1 1 1 - V SEMICONDUCTOR SURFACES
M. SAWAGE-SIMKIN'
LURE, CNRS-MEN-CEA, Bat.209d, F-91405 Orsay, France
RCsum6 - Les structures atomiques de surfaces reconstruites de serniconducteurs III-V ont CtC rCsolues par la technique de diffraction des rayons X sous incidence rasante en ultra-vide pour deux orientations cristallographiques importantes:
{l l l
}et (001
) .I1a et6 montrC que dans le premier cas, le processus dominant la reconstruction est la rehybridation des orbitales tandis que dans le second, la dimCrisation des atomes de la couche de surface est 5 l'origine de la nouvelle symktrie. La coexistence de deux structures ordonnies a
CttobservCe dans le cas de la reconstruction ~ ( 4 x 4 ) de GaAs (001) ce qui met en Cvidence le degrC de IibertC additionnel lie i une possible variation de la stcechiomCme de surface..
Abstract- The atomic structures of III-V semiconductor reconstructed surfaces have been solved by grazing incidence X-ray diffraction under ultra-high vacuum for two important cristallographic orientations:
{11 1) and (001
).It was shown that the dominant effect in the first case was orbital rehybridization, whereas in the second example, dimer formation between the atoms of the surface layer is at the origin of the new symmetry . The coexistence of two ordered structures has been observed for the ~ ( 4 x 4 ) reconstruction of GaAs (001) which outlines the additional degree of freedom induced by a possible variation of the surface stoichiometry.
S u r f a c e s t u d i e s o n 111-V c o m p o u n d s h a v e b e e n g r e a t l y s t i m u l a t e d b y t h e d e v e l o p m e n t o f u l t r a - h i g h v a c u u m (UHV) g r o w t h a n d p r e p a r a t i o n t e c h n i q u e s . R e s t r i c t e d a t f i r s t t o t h e a n a l y s i s o f f r e s h l y c l e a v e d ( 1 10) s a m p l e s , e l e c t r o n s p e c t r o s c o p i c and s t r u c t u r a l p r o b e s a r e now a p p l i e d t o a v a r i e t y of s u r f a c e o r i e n t a t i o n s w h e n t h e y a r e c o u p l e d t o m o l e c u l a r b e a m e p i t a x i a l (MBE) g r o w t h c h a m b e r s . R e c e n t l y , s u r f a c e a t o m i c s t r u c t u r e d e t e r m i n a t i o n h a s b e e n s u c c e s s f u l l y a c h i e v e d b y t h e u s e o f g r a z i n g i n c i d e n c e X - r a y d i f f r a c t i o n (GIXD) w i t h 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 ; a p r e s e n t a t i o n o f GIXD i s b e y o n d
+and Laboratoire de Minkralogie-Cristallographie,associC au CNRS et aux Universitis P. et M .Cwie et Paris 7 , 4 place Jussieu, F-75252 Paris-Cedex 05, France
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989711
t h e s c o p e o f t h i s p a p e r a n d t h e r e a d e r i s r e f e r r e d t o a d e d i c a t e d c h a p t e r w r i t t e n b y ~ . ~ . ~ o b i n s o n ( ' ) . I n t h i s r e v i e w w e w i l l d e s c r i b e t h e r e s u l t s o f e x p e r i m e n t s p e r f o r m e d on t w o d i f f e r e n t c l a s s e s o f m a t e r i a l s : ( 1 11) o r i e n t e d s a m p l e s o f I n S b a n d GaSb s t u d i e d b y a c o l l a b o r a t i n g t e a m w o r k i n g a t ~ a s ~ l a b * ( ~ a m b u r g - F R G ) a n d data c o n c e r n i n g t h e GaAs (001) r e c o n s t r u . c t e d s u r f a c e s c o m i n g f r o m t h e LURE f a c i l i t y * * (Orsay-F). S i n c e a m a j o r e f f o r t i s p r e s e n t l y d i r e c t e d t o w a r d s MBE g r o w t h of 11-V1 c o m p o u n d s , a l t h o u g h GIXD e x p e r i m e n t s a r e o n l y i n a . p r e p a r a t i o n phase, a b r i e f r e v i e w o f t h e s u r f a c e r e c o n s t r u c t i o n s i n t h e s e c o m p o u n d s i s g i v e n i n t h e l a s t s e c t i o n .
11-BASIC FFATURFS OF 111
-
V SURF ACESWhen c o m p a r e d t o e l e m e n t a l s e m i c o n d u c t o r s , 111-V c o m p o u n d s u r f a c e s p r e s e n t a n a d d i t i o n a l d e g r e e o f f r e e d o m r e l a t e d t o t h e s t o i c h i o m e t r y a n d p o l a r i t y . The b u l k c r y s t a l s t r u c t u r e b e l o n g s t o t h e z i n c b l e n d e t y p e a n d t h e l o w i n d e x f a c e s a r e v e r y d i f f e r e n t f r o m one a n o t h e r . T h e { l 10) c l e a v a g e f a c e i s s t o i c h i o m e t r i c , w i t h one d a n g l i n g b o n d p e r s u r f a c e a t o m a n d e x h i b i t s a 1x1 r e c o n s t r u c t i o n i n v o l v i n g r e h y b r i d i z a t i o n f r o m sp3 t o s p 2 f o r t y p e I11 a t o m s a n d f r o m s p 3 t o s Z p 3 f o r t y p e V a t o m s The c l e a n ( 1 1 l } s u r f a c e o b t a i n e d b y s p u t t e r a n d a n n e a l i n g c y c l e s o r b y MBE, e x p o s e s e i t h e r t y p e 111 a t o m s ( o r i e n t a t i o n l a b e l l e d ( 1 1 ) o r t y p e V a t o m s ( o r i e n t a t i o n l a b e l l e d
( i T i ) B
w i t h one d a n g l i n g b o n d p e r s u r f a c e a t o m . S e v e r a l r e c o n s t r u c t i o n 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 h a v e been o b s e r v e d f o r b o t h cases, e a c h o f w h i c h h a v e a s u b s t a n c i a l amount o f o r b i t a l r e h y b r i d i z a t i o n ( 3 ) . The (001) s u r f a c e , o f m a j o r t e c h n o l o g i c a l i m p a c t , has b e e n t h e s u b j e c t o f n u m e r o u s s t u d i e s p a r t i c u l a r l y i n t h e c a s e o f GaAs (001). The b u l k t e r m i n a t i o n l a y e r i s p o p u l a t e d b y e i t h e r Ga o r A s a t o m s w i t h t w o d a n g l i n g b o n d s p e r atom, b u t a n i d e a l 1x1 s t r u c t u r e i s n e v e r o b s e r v e d . The s u r f a c e p h a s e d i a g r a m , a s a f u n c t i o n o f t e m p e r a t u r e o r (and) A s p a r t i a l p r e s s u r e , i s s u m m a r i z e d i n t a b l e 1 w h i c h i s t a k e n f r o m r e f e r e n c e 4 . S e v e r a l s u r f a c e s y m m e t r i e s h a v e b e e n i d e n t i f i e d b y r e f l e x i o n h i g h e n e r g y e l e c t r o n d i f f r a c t i o n (RHEED) a n d l o w e n e r g y e l e c t r o n d i f f r a c t i o n (LEED) a n d t h e i r a p p r o x i m a t e s t o i c h i o m e t r y , c h a r a c t e r i z e d b y t h e As/Ga r a t i o , h a s b e e n e s t i m a t e d on t h e b a s i s o f A u g e r o r c o r e l e v e l p h o t o e m i s s i o n s p e c t r o s c o p i c H o w e v e r , e x t e n d e d c o m p o s i t i o n r a n g e s a r e u s u a l l y f o u n d f o r e a c h r e c o n s t r u c t i o n a n d i n m a n y p h a s e s , t h e n a t u r e a n d e x a c t l o c a t i o n o f t h e a t o m i c s p e c i e s i n t h e t o p l a y e r s a r e n o t k n o w n .T a b l e 1
Effects of surface stoichiometry on GaAs (001) surface reconstruction
Temperature
range Surface structun Reparation mode Surface cov- BA,CC)
(after ref.
[5 1)
c(8 X
2) MBE
or annealing0.22
(4
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ofc(2
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d6 X 4) 0.37
(l X 6) 0.5 2
d2 X 8 ) 0.6
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adsorption or0.86 (1
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?A s f a r as t h e t w o a r s e n i c r i c h p h a s e s a r e c o n c e r n e d , p r i o r t o t h e G X I D r e s u l t s d e s c r i b e d l a t e r i n t h i s p a p e r , a s t r u c t u r a l m o d e l h a d b e e n p r o p o s e d f o r t h e A s s a t u r a t e d ~ ( 4 x 4 ) p h a s e on t h e b a s i s o f c o r e l e v e l p h o t o e l e c t r o n s p e c t r o s c o p y , i n v o l v i n g t h e c h e m i s o r p t i o n o f A s d i m e r s on an a r s e n i c b u l k t e r m i n a t i o n l a y e r ( 7 ) . The s o - c a l l e d A s s t a b i l i z e d 2 x 4 ( o r
~ ( 2 x 8 ) ) r e c o n s t r u c t e d s u r f a c e h a s b e e n s t u d i e d b y s c a n n i n g t u n n e l l i n g m i c r o s c o p y (STM) (8) a n d RHEED('), t h e b u i l d i n g b l o c k s o f t h e s u r f a c e s t r u c t u r e a r e a l s o A s d i m e r s b u t t h e y a r e a s s u m e d h e r e t o b e b o n d e d t o a
0
Unreconstructed Reconstructed
Fig. l a)Asymmetric unit of the Patterson map calculated from the InSb (1 11) 2x2 half-order peaks b)interatomic vectors in a reconstructed and ideal asymmemc unit
c)projected interatomic vectors in the distorted and ideal hexagonal bonding scheme.
(after ref. l l )
w h i c h r e v e a l s t h e i m p o r t a n t i n t e r a t o m i c v e c t o r s i n t h e u n i t c e l l . C o n s i d e r i n g a n i d e a l s u r f a c e o n e c a n i n f e r t h e a t o m i c d i s p l a c e m e n t s p r o d u c i n g t h e s a m e v e c t o r s e t . F i g . l a t o c , e x t r a c t e d f r o m t h e PhD t h e s i s o f ~ . ~ e i d e n h a n s ' l ( ' l), s h o w s t h e d i s t o r t i o n s o f t h e p r o j e c t e d h e x a g o n a l b o n d n e t w o r k b e t w e e n I n a n d Sb a t o m s , f u l f i l l i n g t h e r e q u i r e m e n t s o f t h e P a t t e r s o n m a p .
H o w e v e r , s t r u c t u r e f a c t o r s c a l c u l a t e d w i t h a 2 x 2 u n i t c e l l c o n t a i n i n g o n l y t h e s i x a t o m s o f t h e d i s t o r t e d h e x a g o n d i d n o t y i e l d a g o o d f i t w i t h t h e e x p e r i m e n t a l d a t a . I n o r d e r t o r e v e a l t h e e r r o r s i n t h e m o d e l o n e c a l c u l a t e s a F o u r i e r d i f f e r e n c e m a p a c c o r d i n g t o t h e f o r m u l a :
Ga b u l k t e r m i n a t i o n l a y e r . T h i s s t r u c t u r a l d e s c r i p t i o n i s s u p p o r t e d b y t o t a l e n e r g y c a l c u l a t i o n s ( 9 ) b u t i t does n o t g i v e a p r o p e r a c c o u n t o f t h e o b s e r v e d X - r a y d i f f r a c t e d i n t e n s i t i e s c o l l e c t e d b y GIXD as w i l l b e d i s c u s s e d l a t e r .
I n g e n e r a l , i t c a n b e s a i d t h a t t h e m a i n f e a t u r e o f { l 1 0 ) a n d { l 1 l } r e c o n s t r u c t e d 111-V s u r f a c e s i s o r b i t a l r e h y b r i d i z a t i o n w h e r e a s d i m e r o r d e r i n g a l o n g d i f f e r e n t g e , o m e t r i c a l p a t t e r n s a c c o u n t s f o r t h e v a r i o u s s y m m e t r i e s p r e s e n t e d by t h e { l o o ) A s r i c h g a l l i u m a r s e n i d e s u r f a c e s .
111-ATOMIC STRUCTURE OF
11
1 1 ) 111-V COMPOUND SURFACEST h e GIXD e x p e r i m e n t , p e r f o r m e d on a I n S b ( l 1 l ) A 2 x 2 r e c o n s t r u c t e d s u r f a c e ( l O ) , i s t h e f i r s t e x a m p l e o f t h e a p p l i c a t i o n of t h e s t a n d a r d c r y s t a l l o g r a p h i c m e t h o d s t o a t w o - d i m e n s i o n a l p r o b l e m . The s u c c e s s i v e s t e p s l e a d i n g t o t h e f i n a l a s s e s s m e n t o f t h e s t r u c t u r a l p a r a m e t e r s w i l l t h u s b e d e s c r i b e d i n some d e t a i l as a m o d e l c a s e .
s a m p l e o r e p a r a t i o n : t h e s a m p l e s w e r e c l e a n e d b y r e p e a t e d c y c l e s o f A r s p u t t e r i n g a n d annealing: t b e s u r f a c e r e c o n s t r u c t i o n w a s o b s e r v e d b y LEED a n d t h e a b s e n c e o f i m p u r i t y w a s c o n t r o l l e d b y VUV p h o t o e l e c t r o n s p e c t r o s c o p y . The s a m p l e w a s t h e n t r a n s f e r r e d t o a p o r t a b l e UHV c h a m b e r , e q u i p p e d w i t h a v a c u u m t i g h t b e r y l l i u m w i n d o w a d a p t a b l e t o t h e X + - a y d i f f r a c t o r n e t e r a n d d i f f r a c t e d i n t e n s i t i e s w e r e c o l l e c t e d a t t h e c r i t i c a l i n c i d e n c e a n g l e f o r t o t a l r e f l e c t i o n .
D a t a a n a l u s i s : a f t e r c o r r e c t i o n o f t h e g e o m e t r i c a l f a c t o r s a n d s y m m e t r y a v e r a g i n g , t h e e x p e r i m e n t a l i n t e n s i t i e s w e r e F o u r i e r t r a n s f o r m e d t o p r o d u c e a P a t t e r s o n map,
w h e r e
1
F c a l c l a n d ahk a r e , r e s p e c t i v e l y , t h e m o d u l u s a n d t h e p h a s e o f t h e s t r u c t u r e f a c t o r s c a l c u l a t e d w i t h t h e m o d e l s t r u c t u r a l p a r a m e t e r s . P o s i t i v e p e a k s i n t h e F o u r i e r d i f f e r e n c e m a p ( f i g 2) m e a n t h a t a t o m s h a v e b e e n o m i t t e d i n t h e d e s c r i p t i o n . C l e a r l y a s e v e n t h a t o m a t a n u n d i s p l a c e d b u l k p o s i t i o n ( a r r o w i n f i g . 2 ) m u s t be i n c l u d e d i n t h e 2 x 2 s u r f a c e u n i t c e l l . L e a s t s q u a r e r e f i n e m e n t o f t h e a t o m i c p o s i t i o n s y i e l d e d a v a l u e o f 2 . 1 . A t t h i s s t a g e , no a s s u m p t i o n w a s m a d e on t h e n a t u r e o f t h e a t o m i c s p e c i e s w h i c h w e r e r e p l a c e d b y a n " a v e r a g e " Sn a t o m ( 2 = 5 0 ) . H o w e v e r , upon r e f i n i n g t h e a t o m i c n u m b e r s , I n (Z=49) a n d Sb (2=51) a t o m s h a v e b e e n i d e n t i f i e d o n t h e d i f f e r e n t s i t e s w i t h a f i n a l v a l u e o f 1 . 8 . T h e s a m e p r o c e d u r e h a s b e e n a p p l i e d t o t h e GaSb ( 1 l l ) A 2x2(12) w h i c h s h o w s a v e r y s i m i l a r s t r u c t u r e . B e c a u s e o f t h e l a r g e d i f f e r e n c e i n Ga a n d Sb f o r m f a c t o r s , i n t e r c h a n g i n g Sb f o r Ga o n t h e s e v e n t h a t o m s i t e l e d i n t h a t c a s e t o a n i n c r e a s e o fx2
f r o m 2 t o 17.Fig.2 Electron density difference map calculated with a 6-atom basis: arrow points to the positive peak revealing
a
missing atom in the basis (after ref. 10)Fig.3 Model for the reconstructed 2x2 unit cell : large displacements for Gal and Sbl in the first bilayer; smaller displacements for Ga2 and Sb3 in the second bilayer (after ref.14)
The GaSb d a t a s e t h a s b e e n r e c e n t l y r e v i s i t e d t w i c e ; t h e f i r s t i m p r o v e m e n t s u g g e s t e d a p a r t i a l o c c u p a n c y o f t h e Ga v a c a n c y c o r n e r s i t e ( 1 3 ) w h i c h b r o u g h t
X 2
d o w n t o 0 . 4 2.
The a u t h o r s o f t h e o r i g i n a l p a p e r , a l t h o u g h n o t d i s r e g a r d i n g t h i s p o s s i b i l i t y , s h o w e d t h a t a s i m i l a r r e d u c t i o n o f w a s o b t a i n e d b y i n c l u d i n g d i s p l a c e m e n t s i n t h e s e c o n d a t o m b i l a y e r , a s s h o w n i n f i g 3 ( l 4 ) . By p r e s e r v i n g t h e m a i n f i n d i n g s o f t h e s t r u c t u r e a n a l y s i s n a m e l y t h e a l m o s t p l a n a r sp2 c o n f i g u r a t i o n f o r t h e g r o u p 111 a t o m s a n d t r i h e d r a l p3 b o n d i n g f o r t h e g r o u p V a t o m w i t h a l o n e s 2 p a i r , t h i s f i n a l d e s c r i p t i o n c o n v e y s m o r e p h y s i c a l s i g n i f i c a n c e . H o w e v e r t h e s t a t i s t i c a l e q u i v a l e n c e o f t h e t w o s o l u t i o n s a d r e s s e s t h e u n i c l t y o f t h e s t r u c t u r a l d e s c r i p t i o n .D a t a h a v e b e e n c o l l e c t e d b y t h e s a m e g r o u p on t h e o p p o s i t e ( i i i ) B 3 x 3 r e c o n s t r u c t e d s u r f a c e s o f InSb and GaSb t h a t w e r e p r e p a r e d b y an i d e n t i c a l p r o c e d u r e ( 1 l ) ' A l t h o u g h t h e d a t a s e t s w e r e r e p r o d u c i b l e a n d s h o w e d a s i m i l a r h i e r a r c h y i n t h e d i f f r a c t e d i n t e n s i t i e s w h i c h s t r e n g h e n e d t h e b e l i e f i n a u n i q u e s u r f a c e u n i t c e l l i n b o t h m a t e r i a l s , no s a t i s f a c t o r y m o d e l c o u l d b e d e r i v e d a n d i t w a s s u g g e s t e d t h a t t h e 3 x 3 a p p a r e n t s y m m e t r y c o u l d r e s u l t f r o m t h e s u p e r p o s i t i o n o f d i f f e r e n t d o m a i n s w i t h 1x3, 3 x 1 and\j3x\l3 r e c o n s t r u c t i o n s . P h a s e c o e x i s t e n c e i s i n d e e d t h e k e y w o r d t o u n r a v e l l i n g s u r f a c e s t r u c t u r e s i n m a n y i n s t a n c e s as w i l l b e d e m o n s t r a t e d i n t h e n e x t s e c t i o n .
I V - G a A s (001) RECONSTRUCTED A s RICH SURFACES
A s a l r e a d y m e n t i o n e d i n s e c t i o n 11, o n l y t h e t w o m o s t A s r i c h s u r f a c e s h a v e b e e n t h e o b j e c t o f o u r GIXD s t u d i e s n a m e l y t h e A s s a t u r a t e d ~ ( 4 x 4 ) a n d A s s t a b i l i z e d 2 x 4 r e c o n s t r u c t i o n s .
S a m p l e p r e w a r a t i o n : t h e s u r f a c e s h a v e b e e n p r e p a r e d i n - s i t u b y t h e s t a n d a r d
M B E
p r o c e d u r e s i n a g r o w t h c h a m b e r d i r e c t l y c o u p l e d t o t h e X - r a yUHV d i f f r a c t o m e t e r ( 1 5 ) . A t t h e g r o w t h t e m p e r a t u r e (580'C) a n d u n d e r an A s p r e s s u r e o f 1 0 - ~ m b a r , t h e s t a b l e p h a s e i s t h e 2 x 4 r e c o n s t r u c t e d s u r f a c e w h i c h t r a n s f o r m s t o t h e ~ ( 4 x 4 ) w h e n c o o l e d b e l o w 300°C u n d e r t h e s a m e A s p r e s s u r e . I n o r d e r t o r e t a i n t h e 2 x 4 s y m m e t r y a t r o o m t e m p e r a t u r e , t h e A s p r e s s u r e m u s t b e l o w e r e d w h i l e t h e s a m p l e i s k e p t a t 45O0C, f r o m w h e r e t h e s t r u c t u r e r e m a i n s u n c h a n g e d down t o l o w t e m p e r a t u r e i n UHV c o n d i t i o n s .
D a t a a n a l u s i s .: i n t h e c a s e o f t h e ~ ( 4 x 4 ) s t r u c t u r e , 38 i n d e p e n d e n t f r a c t i o n a l s u r f a c e p e a k s h a v e b e e n c o l l e c t e d a n d u s e d t o p r o d u c e a t w o d i m e n s i o n a l P a t t e r s o n m a p ( f i g . 4 ) . T h e A s c h e m i s o r b e d m o d e l d e r i v e d f r o m p h o t o e m i s s i o n data(7) and c o r r e s p o n d i n g t o an A s c o v e r a g e o f 0 . 5 d i d n o t a c c o u n t f o r a l l t h e i n t e r a t o m i c v e c t o r s p r e s e n t i n t h e P a t t e r s o n map; a m o d e l i n v o l v i n g s i x A s a t o m c l u s t e r s a r r a n g e d on t h e s a m e ~ ( 4 x 4 ) s q u a r e l a t t i c e (As c o v e r a g e 0 . 7 5 ) w a s t h e n i n t r o d u c e d ( f i g . 5a a n d b).
Fig 5.
---
Cherniaorbed As 0 As bulk termination layer
@
0 Ga in the JalaYerFig.4
Experimental Patterson
mapfor
the GaAs (001) ~ ( 4 x 4 )surface;
comparison withthe interatomic vectors present in models 5a and 5b.
Fig.5 a) structural model proposed in ref.
7on the basis of photoelectron spectoscopy data.
b) structural model with a higher As coverage proposed in ref.16.
L e a s t s q u a r e f i t t i n g o f t h e o b s e r v e d s t r u c t u r e f a c t o r s s h o w e d t h a t t h e a c t u a l s u r f a c e w a s a 50-50 m i x t u r e o f b o t h s t r u c t u r e s l e a d i n g t o a n a v e r a g e A s c o v e r a g e o f 0 , 6 2 5 ( ~ ~ ) . T h i s r e s u l t e n a b l e s u s t o u n d e r s t a n d t h e b r o a d
s t o i c h i o m e t r y r a n g e f o u n d f o r t h e ~ ( 4 x 4 ) s y m m e t r y s i n c e 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 m i g h t c h a n g e t h e p a r t i t i o n b e t w e e n t h e t w o s t a b l e a t o m i c a r r a n g e m e n t s .
S i x i n d e p e n d e n t i n t e g e r o r d e r d i f f r a c t i o n p e a k s h a v e b e e n u s e d t o a s s e s s t h e r e g i s t r y o f t h e c h e m i s o r b e d l a y e r w i t h r e s p e c t t o t h e u n d e r l y i n g b u l k , s i n c e b o t h t h e b u l k a n d t h e s u r f a c e c o n t r i b u t e c o h e r e n t l y t o t h e a m p l i t u d e d i f f r a c t e d a t t h e s e l o c a t i o n s i n r e c i p r o c a l space(17). I n t h e c a s e o f 111-V (001) s u r f a c e s , i n t e g e r o r d e r s hkO w h i c h a r e b u l k f o r b i d d e n b u t s u r f a c e a l l o w e d ( h a n d k b o t h odd), a r e v e r y s e n s i t i v e i n d i c a t o r s o f t h e c h e m i c a l n a t u r e o f t h e b u l k t e r m i n a t i o n l a y e r : o w i n g t o t h e n o n - e q u i v a l e n c e o f d i r e c t i o n s C l
l 0 1
a n d [ lTO1
i n d u c e d b y t h e d a n g l i n g b o n d o r i e n t a t i o n( c o n v e n t i o n a l l y , t h e d a n g l i n g b o n d s a r e p r o j e c t e d a l o n g
Elm]
i n a n A s t e r m i n a t e d i d e a l (001) s u r f a c e ) , it c a n b e s h o w n t h a t t h e b u l k c o n t r i b u t i o n t o hkO r e f l e c t i o n s a r e a l t e r n a t i v e l y s t r o n g a n d w e a k a c c o r d i n g t o t h e f o l l o w i n g r u l e :s t r o n g f o r an A s t e r m i n a t e d b u l k h + k = 4 n
weak f o r a Ga t e r m i n a t e d b u l k weak f o r an A s t e r m i n a t e d b u l k h + k = 4 n + 2
s t r o n g f o r a Ga t e r m i n a t e d b u l k .
A s s h o w n i n f i g . 6 , w h e r e t h e i n t e g e r o r d e r s 310 ( p r o f i l e a) and
3m
( p r o f i l e b) a r e d i s p l a y e d , t h e ~ ( 4 x 4 ) r e c o n s t r u c t i o n c o r r e s p o n d s t o an A s t e r m i n a t e d b u l k . The L o r e n z i a n f i t l e a d s t o a c o h e r e n c e l e n g t h o f~ o o A ,
w h e r e a s t h e f u l l w i d t h a t h a l f m a x i m u m o f t h e p u r e s u r f a c e p e a k s i n d i c a t e s a c o h e r e n t s i z e f o r r e c o n s t r u c t e d d o m a i n s o f about 500.A.
T h e d a t a c o l l e c t e d o n t h e 2 x 4 s u r f a c e d o n o t a l l o w a s i m p l e i n t e r p r e t a t i o n ( 1 8 ) : t h e i n t e n s i t y r a t i o s b e t w e e n f r a c t i o n a l q u a r t e r o r d e r s d i s a g r e e w i t h t h e STM d e r i v e d ~ t r u c t u r e ' ~ ) a n d t h e i n t e g e r o r d e r s do n o t
Fig.6 Dissymmetry between integer orders 310 and 310 in the ~(4x4) structure demonstrating the As nature of the bulk termination layer.
Fig.7 unpredicted strong bulk-like 310 reflexion (a) and multicomponent 310 reflection (b) for
a
2x4GaAs (001) phase showing the presence of both Ga and As bulk top layers.
o b e y e i t h e r r u l e s t a t e d a b o v e (fig.7). The m o s t p r o b a b l e e x p l a n a t i o n f o r t h i s i s t h a t A s t e r m i n a t e d a r e a s w i t h no o r d e r e d o v e r l a y e r a r e a l s o p r e s e n t b e s i d e r e c o n s t r u c t e d r e g i o n s w i t h a p a r t i a l A s l a y e r o v e r a Ga t e r m i n a t e d b u l k w h i c h e x p l a i n s t h e s h a r p n e s s o f p r o f f l e 7 a a r i s i n g o n l y f r o m b u l k d i f f r a c t i o n
.
T h i s e x a m p l e i l l u s t r a t e s t h e l i m i t a t i o n s and a d v a n t a g e s o f GIXD c o m p a r e d t o STM : GIXD p r o b e s a l a r g e s a m p l e a r e a and t h u s d e l i v e r s a v e r a g e s t r u c t u r a l r e s u l t s , w h e r e a s STN i s a b l e t o i d e n t i f y s m a l l p a t c h e s o f d i f f e r e n t l y o r d e r e d r e g i o n s . H o w e v e r , GIXD d a t a i n c l u d e b u l k contributions w h i c h a r e r e q u i r e d f o r a f u l l d e s c r i p t i o n o f t h e s t r u c t u r e s .
.Y CONCLUSION
GIXD i s p r o b a b l y t h e m o s t p o w e r f u l t o o l t o i n v e s t i g a t e t h e a t o m i c r e a r r a n g e m e n t s w h i c h t a k e p l a c e on c l e a n s u r f a c e s . O u r e x a m p l e s , d e a l i n g w i t h c o m p o u n d m a t e r i a l s , h a v e s h o w n t h a t GIXD p r o v i d e s b o t h t h e l o c a t i o n a n d c h e m i c a l i d e n t i f i c a t i o n o f a t o m s n o t o n l y i n t h e t o p m o s t l a y e r b u t a l s o i n t h e u n d e r l y i n g b u l k . I n a d d i t i o n t o i t s f u n d a m e n t a l i n t e r e s t , t h e k n o w l e d g e o f t h e a t o m i c s t r u c t u r e o f 111-V c o m p o u n d s u r f a c e s i s a p r i m a r y i m p o r t a n c e f o r t h e u n d e r s t a n d i n g o f 111-V i n t e r f a c e s ; a p a r t i c u l a r l y e x c i t i n g f i e l d i s c o n c e r n e d w i t h t h e h e t e r o e p i t a x i a l g r o w t h o f 11-V1 c o m p o u n d s o n GaAs s u b s t r a t e s w h e r e a' c l e a r c o r r e l a t i o n h a s b e e n e s t a b l i s h e d b e t w e e n t h e r e c o n s t r u c t i o n o f t h e GaAs (001) s u b s t r a t e a n d t h e o r i e n t a t i o n o f t h e CdTe e p i l a Y e r ( l g ) . D e t a i l e d s t u d i e s o f t h e p r e c u r s o r s t a t e s T ~ / G ~ A s ( " ) h a v e b r o u g h t a d d i t i o n a l l i g h t o n t h i s s u b j e c t c o n f i r m i n g t h a t ( 1 11) o r i e n t e d CdTe, r e c o n s t r u c t e d 2 x 1 , i s p r o d u c e d on t h e 4 x 2 ( o r ~ ( 8 x 2 ) ) G a - s t a b i l i z e d GaAs (001) w h e r e a s (001) o r i e n t e d CdTe g r o w s on t h e 2 x 4 A s - s t a b i l i z e d s u r f a c e . GIXD e x p e r i m e n t s on t h e s e s y s t e m s a r e b e i n g s t a r t e d t o c o m p l e m e n t t h e i n f o r m a t i o n d e r i v e d f r o m e l e c t r o n s p e c t r o s c o p y o n t h e a t o m i c a r r a n g e m e n t s a t t h e i n t e r f a c e .
A k n o w l e d a e m e n t s : t h a n k s a r e d u e t o a l l m y c o l l e a g u e s a n d c o w o r k e r s who p a r t i c i p a t e d a c t i v e l y t o t h e r e s e a r c h e s q u o t e d i n t h e p r e s e n t r e v i e w :
* R. Feidenhans'l, M. Nielsen, J. Bohr, F. Grey and
M.Toney - Riso National Laboratory,DK-4000 Roskilde, Denmark; R. L. Johnson -
Max Planck Institiit furFestkorperjorschung, 0-7000 Stuttgart, Federal Republic of Germany; I. K. Robinson -AT
&T Bell Laboratories, Murray Hill, New Jersey 07974, USA.
** R. Pinchaux, LURE and Universite' P. et M. Curie, 4 Place Jussieu, F-75252 Paris Cedex 05;
J. Massies, LURE and LPSES-CNRS, rue B. Gregory, 06950 Valbonne, France;
N.Jedrecy, P.
Claverie and J. Bonnet, LURE; I. K. Robinson, AT &T Bell Laboratories.
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