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DIRECT OBSERVATION OF ATOMIC STEP
STRUCTURE AT GaAS-AlAs HETEROINTERFACES IN TRANSMISSION ELECTRON MICROSCOPY AND IMPROVED LATTICE IMAGE TO DETECT
THE INTERFACE BY MATERIAL-DEPENDENT PATTERNS
M. Tanaka, H. Ichinose, T. Furuta, Y. Ishida, H. Sakaki
To cite this version:
M. Tanaka, H. Ichinose, T. Furuta, Y. Ishida, H. Sakaki. DIRECT OBSERVATION OF ATOMIC
STEP STRUCTURE AT GaAS-AlAs HETEROINTERFACES IN TRANSMISSION ELECTRON
MICROSCOPY AND IMPROVED LATTICE IMAGE TO DETECT THE INTERFACE BY
MATERIAL-DEPENDENT PATTERNS. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-101-
C5-104. �10.1051/jphyscol:1987517�. �jpa-00226721�
JOURNAL DE PHYSIQUE
Colloque C5, supplkment au noll, Tome 48, novembre 1987
DIRECT OBSERVATION OF ATOMIC STEP STRUCTURE AT GaAs-AlAs
HETEROINTERFACES IN TRAElSMISSION ELECTRON MICROSCOPY AND IMPROVED LATTICE IMAGE TO DETECT THE INTERFACE BY MATERIAL-DEPENDENT PATTERNS
M. TANAKA, H. ICHINOSE, T. FURUTA, Y. ISHIDA and H. SAKAKI
Institute of Industrial Science. University of Tokyo 7-22-1 Roppongi, Minato-ku, ~ o k y o 106, Japan
Abstract:
E l e c t r o n microscopic o b s e r v a t i o n was made on GaAs-AlAs h e t e r o i n t e r a c e s grown by m o l e c u l a r beam e p i t a x y w i t h a c c u r a t e 1 y c o n t r o l l e d l ayer t h i c k n e s s and growth i n t e r -
r u p t i o n s f o r smoothing i n t e r f a c e roughness. For [ l 1 0 1 e l e c t r o n beam (EB) incidence, a c l e a r l a t t i c e image was obtained, i n which one-atomic-layer s t e p s t r u c t u r e was d i r e c t l y observed a t t h e t o p (AlAs-on-GaAs) i n t e r f a c e . Furthermore, by t a k i n g [ l 0 0 1 EB i n c i d e n c e and o p t i m i z i n g t h e o b s e r v a t i o n c o n d i t i o n s . we have demonstrated f o r t h e f i r s t t i m e t h a t GaAs-AlAs i n t e r f a c e can be d e t e c t e d i n the l a t t i c e image n o t by i t s c o n t r a s t b u t by i t s c h a r a c t e r i s t i c p a t t e r n s which depend on m a t e r i a l s .
Our r e c e n t photoluminescence (PL) s t u d i e s o f GaAs-AlAs quantum we1 1s (QW) [ l ] - [ 3 ] have i n d i c a t e d t h a t t o p (AlAs-on-GaAs) i n t e r f a c e s grown by m o l e c u l a r beam e p i t a x y (MBE) have mono-atomic step s t r u c t u r e s whose l a t e r a l s i z e fstep i s about 200A. b u t such roughness can be smoothed by t h e growth i n t e r r u p t i o n , l e a d i n g t o t h e i n c r e a s e o f
pstep
up t o more than a few thousands o f angstroms. I n contrast. bottom (GaAs-on-AlAs) i n t e r f a c e s a r e pseudo-smooth when sensed by e x c i t o n s w i t h ,@step b e i n g 30-40A. I n t h i s paper, we r e p o r t f o r t h e f i r s t t i m e t h e d i r e c t o b s e r v a t i o n o f such atomic s t e p s t r u c t u r e s a t t h e t o p i n t e r f a c e o f QWs i n t h e l a t t i c e image o f t r a n s m i s s i o n e l e c t r o n microscopy (TEM) taken w i t h t h e i n c i d e n t e l e c t r o n beam (EB) a1 ong b o t h [ l 101 and [ l 0 0 1 d i r e c t i o n s . P a r t i c u l a r l y , we demonstrate t h a t GaAs-A1 As i n t e r f a c e s can be detected n o t by i t s c o n t r a s t b u t b y i t s c h a r a c t e r i s t i c p a t t e r n s i n t h e l a t t i c e image.( G ~ A S ) ~ ~ - ( A ~ A S ) ~ ~ QW s t r u c t u r e s w i t h 50 p e r i o d s were grown on (001) GaAs s u b s t r a t e - a t 580C under t h e c o n d i t i o n s i d e n t i c a l w i t h a p r e v i o u s papers[2][3]. The i n t e n s i t y o f t h e s p e c u l a r beam i n r e f l e c t i o n h i g h energy e l e c t r o n d i f f r a c t i o n (RHEED) p a t t e r n s was monitored t o c o n t r o l the l a y e r thicknesses o f GaAs and AlAs p r e c i s e l y a t 17 and 21 monolayers. The d e p o s i t i o n was i n t e r r u p t e d t o smooth t h e t o p i n t e r f a c e as shown i n Fig.l(a). F i g u r e s I ( b - l ) and (b-2) show t h e PL spectra measured a t 77K from two d i f f e r e n t p o s i t i o n s o f t h e sample..A s i n g l e sharp peak w i t h t h e l i n e w i d t h o f 4meV and two s p l i t t e d peaks which correspond t o t h e d i f f e r e n t w e l l w i d t h s L, o f n and n+l monolayers a r e seen. i n d i c a t i n g t h a t t h e s t e p i n t e r v a l o f the t o p i n t e r f a c e i s much l a r g e r than e x c i t o n diameter. By comparing t h e measured PL w a v e l e n g t h w i t h t h e Kronig-Penney c a l c u l a t i o n . n i s found t o be c l o s e t o 17.
An improved v e r s i o n o f JEM-200CX o f JEOL w i t h 2OOkeV a c c e l e r a t i o n v o l t a g e was employed t o e v a l u a t e t h e i n t e r f a c e s o f t h e grown s t r u c t u r e . F i r s t . Fig.Z(a) shows a c r o s s s e c t i o n a l l a t t i c e image taken w i t h E0 incidence a l o n g [110]. D i r e c t (000)
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987517
(3-102 JOURNAL DE PHYSIQUE
beam, and f o u r { l 1 1) and two {ZOO) d i f f r a c t e d beams f o r m t h e l a t t i c e image. GaAs and A l A s can be d i s t i n g u i s h e d b y t h e c o n t r a s t due t o t h e i n t e n s i t y d i f f e r e n c e o f {ZOO) beams i n each c r y s t a l . Note i n Fig.Z(a) t h a t t h e a b r u p t n e s s o f i n t e r f a c e i s w i t h i n 1 a t o m i c l a y e r , and t h e l a y e r t h i c k n e s s e s o f GaAs and A l A s a r e e x c e l l e n t agreement w i t h RHEED o s c i l l a t i o n d a t a and PL r e s u l t s .
A widened i n t e r - s p o t d a r k l i n e (WISL) i s c l e a r l y seen a t t h e t o p i n t e r f a c e as i n d i c a t e d b y A a n d A' i n Fig.Z(a). The WISL i s n o t s t r a i g h t a 1 l t h e way b u t h a s a s t e p l i k e s t r u c t u r e where t h e t h i c k n e s s o f GaAs changes from 1 7 t o 1 8 monolayers.
Fig.Z(b) i s an e n l a r g e d p h o t o g r a p h o f t h e s t e p - l i k e s t r u c t u r e . Me b e l i e v e t h a t t h i s s t r u c t u r e i n WISL c o r r e s p o n d s t o mono-atomic l a y e r s t e p a t t h e i n t e r f a c e . Such a s t e p s t r u c t u r e , i s n o t found a t t h e b o t t o m i n t e r f a c e b u t i s found e x c l u s i v e l y a t t h e t o p i n t e r f a c e . These f i n d i n g s a r e c o n s i s t e n t w i t h o u r p r e v i o u s PL study, because i t i s e s t a b l i s h e d [ 2 ] [ 3 ] t h a t
IStep
i s much l a r g e r t h a n t h e t h i c k n e s s c-100A) o f TEM specimen o n l y a t t h e t o p i n t e r f a c e s p r e p a r e d w i t h g r o w t h i n t e r r u p t i o n s o t h a t t h e ys h o u l d be d e t e c t e d b y h i g h r e s o l u t i o n TEM. Indeed
itep
o f s t e p s t r u c t u r e i n WISL o b s e r v e d i n l a r g e s c a l e TEM p i c t u r e s i s more t h a n 500A a t l e a s t . Note a l s o t h a t t h es t e p edge i s n o t c l e a r b u t i t ranges o v e r t h e d i s t a n c e o f 40-50A ( f r o m S t o S' i n Fig.1).
To g e t f u r t h e r i n f o r m a t i o n , we extended o u r l a t t i c e image s t u d y t o t h e case o f t h e e l e c t r o n beam (EB) i n c i d e n c e a l o n g [ l 0 0 1 d i r e c t i o n . R e c e n t l y . Suzuki and Okamoto[4] p o i n t e d o u t t h a t c o n t r a s t between GaAs and AlGaAs c o u l d be enhanced i n t h i s case, s i n c e t h e c o n t r i b u t i o n o f 1200) d i f f r a c t e d beams t o t h e l a t t i c e image was much l a r g e r h e r e t h a n t h e c a s e of i n c i d e n t EB a l o n g [ I l O ] . T h e i r l a t t i c e image, however. g i v e s s t i l l l i m i t e d i n f o r m a t i o n s on t h e a t o m i c - s c a l e s t r u c t u r e s o f t h e i n t e r f a c e s . I n Fig.3(a), we show t h e l a t t i c e image o f o u r SL sample t a k e n f r o m [ l 0 0 1 i n c i d e n c e u s i n g o n l y {OOO) and {ZOO} beams. C o n t r a s t between GaAs and A1 As i s t h e c l e a r e s t t h a t have e v e r been r e p o r t e d .
Next. we l e t { 2 2 0 \ beams as we1 l as 1000) and
{
200) beams pass t h r o u g h t h e o b j e c t i v e a p e r t u r e . I n A l A s b o t h {ZOO) and 1220) beams c o n t r i b u t e t o t h e l a t t i c e image, w h i l e i n GaAs {ZOO) d i f f r a c t i o n s a r e v e r y weak a 1 t h o u g h C2201 ones a r e i n t e n s e enough t o c o n t r i b u t e t o t h e l a t t i c e image. T h e r e f o r e . i f one a c h i e v e s s u i t a b l e c o n d i t i o n s b y a d j u s t i n g specimen t h i c k n e s s , amount o f d e f o c u s and o b j e c t i v e a p e r t u r e s i z e . t h e r e c o u l d be a s i t u a t i o n where t h e l a t t i c e images o f GaAs and A l A s a r e c o m p l e t e l y d i f f e r e n t i n t h e i r p a t t e r n s . as we r e p o r t e d i n d e t a i le l s e w h e r e [ 5 ] [ 6 ] . We h a v e o p t i m i z e d t h e o b s e r v a t i o n c o n d i t i o n s [ 6 ] and h a v e succeeded i n o b t a i n i n g a l a t t i c e image h a v i n g m a t e r i a l - d e p e n d e n t p a t t e r n s f o r t h e f i r s t time.
Here, we a p p l y t h i s t e c h n i q u e t o d e t e c t t h e h e t e r o i n t e r f a c e and t o c h a r a c t e r i z e t h e i n t e r f a c e s t r u c t u r e . s i n c e t h e i n t e r f a c e can be r e c o g n i z e d as t h e boundary between t h e s e two d i f f e r e n t images.
F i g u r e 3(b) i s one example o f o u r such l a t t i c e images, w h i c h shows A1As-on-GaAs i n t e r f a c e . Note t h a t t h e image o f A l A s ( u p p e r h a l f ) and t h a t o f GaAs ( l o w e r h a l f ) a r e q u i t e d i f f e r e n t f r o m each o t h e r . Note a l s o t h a t i n GaAs r e g i o n w h i t e s p o t s f o r m
a square l a t t i c e , which i s o b t a i n e d b y e x c l u d i n g e v e r y o t h e r w h i t e s p o t f r o m t h e image o f AlAs. The p o s i t i o n o f i n t e r f a c e i s e a s i l y d e t e c t e d as t h e t r a n s i t i o n p o i n t f r o m one image t o another. T h i s method was a p p l i e d t o t h e d e t e c t i o n o f a t o m i c s t e p s a t t h e i n t e r f a c e . Note i n Fig.3(b) t h a t t h e i n t e r f a c e i n d i c a t e d b y t w o h o r i z o n t a l arrows appears t o be n o t s t r a i g h t b u t has a s t e p - l i k e s t r u c t u r e j u s t as i n Fig.2 t a k e n w i t h [ l 1 0 1 EB incidence. S i n c e w h i t e s p o t s l o c a t e d a t t h e i n t e r f a c e i n Fig.3(b) i s found t o be f a d i n g from p o s i t i o n S t o S'. t h e s t e p edge seen from [ l 0 0 1 d i r e c t i o n has a f i n i t e s p a t i a l e x t e n s i o n as i n t h e case o f Fig.Z(b). W h i l e we h a v e r e p o r t e d p a r t o f t h e s e m a t e r i a l dependent p a t t e r n s i n t h e l a t t i c e image f o r t h e f i r s t t i m e [ 5 ] [ 6 ] , Ourmazd[7] e t a l . has r e c e n t l y s t u d i e d i n d e p e n d e n t l y t h e s i m i l a r method on InGaAs-InP i n t e r f a c e .
I n summary, o u r l a t t i c e image s t u d y on GaAs-AlAs QW has shown t h a t t h e abruptness o f i n t e r f a c e s i s w i t h i n one a t o m i c l a y e r and t h e t h i c k n e s s o f GaAs and A l A s i s i n good agreement w i t h t h e RHEED and PL r e s u l t s . A t t h e t o p i n t e r f a c e . a mono-atomic s t e p s t r u c t u r e w i t h t h e i n t e r v a l o f a t l e a s t more t h a n 500A was observed. F o r [ l 0 0 1 e l e c t r o n beam i n c i d e n c e . an i m p r o v e d l a t t i c e image scheme i s demonstrated where two m a t e r i a l s , GaAs and AlAs, and t h e i r i n t e r f a c e s a r e d e t e c t e d n o t b y t h e c o n t r a s t b u t b y t h e d i f f e r e n t image p a t t e r n s .
References
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[ 6 ] H. I c h i n o s e . T. F u r u t a , H. S a k a k i and Y. I s h i d a , J. E l e c t r o n M i c r o s c o p y 36. 82 (1987).
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Goes-* QW
-1 1 77.
5
F i g u r e 1 (a) shows a t i m e e v o l u t i o n o f RHEED s p e c u l a r beam i n t e n s i t y t a k e n f r o m (001)-2x4 r e c o n s t r u c t e d surface. [ l 1 0 1 azimuth. d u r i n g t h e MBE g r o w t h o f (GaAs) 7- ( A l A s ) QW a t 580C. Note t h a t t h e t h i c k n e s s e s o f GaAs and A l A s a r e s e t p r e c i s e l y a t 17 a n 8 5 1 rnonolayers. ( b - l ) and (b-2) a r e PL s p e c t r a o f t h e QW measured a t 77K.
w h i c h a r e t a k e n f r o m d i f f e r e n t p o s i t i o n s o f t h e same sample.
C5-104 3 0 U R N A L DE PHYSIQUE
( b) S S'
F i g u r e 2 (a) shows l a t t i c e image o f GaAs-A1As QW t a k e n w i t h t h e e l e c t r o n beam i n c i d e n c e o f [110]. (b) i s an e n l a r g e d p h o t o g r a p h o f t h e s t e p - l i k e s t r u c t u r e a t t h e t o p i n t e r f a c e o f t h e SL d e n o t e d b y A a n d A'.
F i g u r e 3 ( a ) shows a l a t t i c e image of GaAs-A1As QW w i t h t h e e l e c t r o n beam i n c i d e n c e o f [100], c o n s t r u c t e d by {OOO) and {ZOO} d i f f r a c t i o n s . whereas (b) shows an e n l a r g e d
l a t t i c e image formed by t000}, {ZOO} and {220} d i f f r a c t i o n s , i n d i c a t i n g an a t o m i c s t e p s t r u c t u r e a t t h e t o p i n t e r f a c e o f GaAs-A1As QW.
