Evidence for the Diffusion of Au Atoms into the Te UPD Layer Formed on an Au(111) Substrate A s i n g l e a t o m i c l a y e r c a n b e f o r m e d b y e l e c t r o c h e m i c a l p r o c e s s e s u s i n g u n d e r p o t e n t i a l d e p o s i t i o n ( U P D ) , w h i c h i s a s u r f a c e - l i m i t e d r e a c t i o n t o a s u b - m o n o l a y e r o r m o n o l a y e r ( M L ) coverage. In UPD, the electrochemical deposition of foreign metal atoms onto substrate is performed a t a p o s i t i v e p o t e n t i a l r e l a t i v e t o t h e r e v e r s i b l e Nernst potential for bulk deposition ( Fig. 1(a) ). By a l t e r n a t i n g t h e U P D o f t w o d i f f e r e n t e l e m e n t s w e can obtain a binary compound with a layer-by-layer grown structure, that is the electrochemical atomic l a y e r e p i t a x y ( E C A L E ) ( F i g . 1 ( b ) ) [ 1 ] . E p i t a x i a l g r o w t h C d T e l a y e r s o n a n A u s i n g l e c r y s t a l s u b s t r a t e a r e v e r y i m p o r t a n t a n d i n t e r e s t i n g f r o m the viewpoint of their applications to solar cells. Since it is considered that the deposition of Cd o n a n A u s u b s t r a t e e a s i l y p r o d u c e s i n t e r m e t a l l i c compounds, the first step to the formation of CdTe films by ECALE is the UPD of the Te. The structure exchanging electrolyte and applying UPD potential for element B exchanging electrolyte and applying UPD potential for element A UPD of element A on substrate UPD of element B on element A UPD of element A on element B (b) approaching deposition potential not-deposition approaching deposition potential u n d e r p o t e n t i a l d e p o s i t i o n ( U P D ) bulk deposition (a) Fig. 1. Schematic illustrations of (a) underpotential deposition ( UPD ) and (b) electrochemical atomic layer epitaxy ( ECALE ). of Te UPD layer has been studied using a scanning t u n n e l i n g m i c r o s c o p y ( S T M ) [ 2 , 3 ] a n d a n a t o m i c f o r c e m i c r o s c o p y ( A F M ) [ 4 ] . H o w e v e r , s t r u c t u r a l analysis using STM and AFM is still limited to two- dimensional surface structure. In the present study, the structure which is normal to the surface of the Te UP D la ye r wa s re ve al ed by in si tu sp ec ul ar X- ray reflectivity measurements [5]. T h e i n s i t u s p e c u l a r X - r a y r e f l e c t i v i t y m e a s u r e m e n t s w e r e c a r r i e d o u t u s i n g a κ - t y p e m u l t i - a x i s d i f f r a c t o m e t e r i n s t a l l e d a t b e a m l i n e BL 14 B1 . Th e wa ve le ng th ut il iz ed wa s λ = 0. 11 0 nm. The Te UPD layer is formed on a pre-treated A u ( 1 1 1 ) d i s k e l e c t r o d e i n a n e l e c t r o l y t i c s o l u t i o n contained 0.1 mM TeO 2 and 10 mM H 2 SO 4 as the s u p p o r t i n g e l e c t r o l y t e . T h e p o t e n t i a l f o r t h e T e U P D i s a p p l i e d t o t h e A u ( 1 1 1 ) e l e c t r o d e d u r i n g reflectivity measurements. The theoretical specular reflectivity for the electrode surface is given by the k i n e m a t i c a l a p p r o x i m a t i o n . I n t h e q u a n t i t a t i v e de te rm in at io n of th e ne ar -s ur fa ce st ru ct ur e of Te - a n d A u - l a y e r s , t h e p a r a m e t e r s r e p r e s e n t i n g t h e electron density profiles of each layer are optimized s o a s t o a d e q u a t e l y d e s c r i b e t h e o b s e r v e d reflectivity. 36 (b) -6 -2 0 2 4 6 8 10 0.0 0.2 0.4 0.6 0.8 1.0 second layer under lying layer Position (Å) Te Au Te Au under lying layer (a) second layer 0.0 0.2 0.4 0.6 0.8 1.0 N o r m a l i z e d e l e c t r o n d e n s i t y Reflectivity 0 1 2 3 4 5 6 7 L 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 T h e m e a s u r e m e n t s w e r e c a r r i e d o u t f o r t h e samples which were kept at the UPD potential for 4 t o 5 9 h o u r s . T h e s p e c u l a r r e f l e c t i v i t y f o r t h e T e U P D l a y e r f o r m e d o n a n A u ( 1 1 1 ) s u b s t r a t e i s shown as solid circles in Fig. 2 . The data collection w a s p e r f o r m e d b e t w e e n 3 9 t o 5 9 h o u r s a f t e r t h e UPD potential was applied. The dashed red line is c a l c u l a t e d p r o f i l e b a s e d o n a m o d e l t h a t t h e T e UPD layer is formed just on the Au(111) substrate; we assume the electron density profile in Fig. 3(a) , a n d t h e T e - c o v e r a g e o f 0 . 3 3 M L w h i c h i s determined from the electrochemical measurement. H o w e v e r , t h e c a l c u l a t e d p r o f i l e d o e s n o t f i t t h e o b s e r v e d r e f l e c t i v i t y . T h e r e f o r e , w e a s s u m e another electron density profile shown in Fig. 3(b) ; here, the first layer consists of 0.33 ML Te and 0.08 ML Au, while the second layer consists of 0.92 ML A u . T h e c a l c u l a t e d r e f l e c t i v i t y ( s o l i d b l u e l i n e i n F i g . 2 ) r e p r o d u c e s t h e e x p e r i m e n t a l r e s u l t s v e r y well. From the X-ray reflectivity measurements we can conclude that a portion of Au atoms migrates from the top layer of the Au(111) substrate into the top-most Te layer. In consequence, an ideal pure a t o m i c l a y e r f o r m e d b y t h e U P D p r o c e s s i s n o t always stable for a long period of time, but atomic i n t e r d i f f u s i o n m a y o c c u r w i t h i n t h e n e a r - s u r f a c e layers. References [1 ] J. L. St ic kn ey , ‘E le ct ro ch em ic al At om ic La ye r E p i t a x y ’ , i n E l e c t r o a n a l y t i c a l C h e m . , A . J . B a r d and I. Reubenstein, Editors, Marcel Dekker, New York 21 (1999) 75. [2] B.E. Hayden and I.S. Nandhakumar, J. Phys. Chem. 101 (1997) 7751. [ 3 ] T . A . S o r e n s o n e t a l . , S u r f . S c i . 4 7 0 ( 2 0 0 1 ) 197. [4] N. Ikemiya et al. , Surf. Sci. 369 (1996) 199. [5] H . K a w a m u r a , M . T a k a h a s i , N. Hojo, M. Miyake, K. Murase, K. Tamura, K. Uosaki, Y. Awakura, J. Mizuki and E. Matsubara, J. Electrochem. Soc. 149 (2002) C83. H i r o y u k i K a w a m u r a , M a s a m i t u T a k a h a s i a n d Jun’ichiro Mizuki SPring-8 / JAERI E-mail: kawamura @ spring8.or.jp Fig. 3. Normalized electron density profile normal to the surface for fit to the observed specular X-ray reflectivity. A negative value of position means that a layer is out of the ideally terminated Au surface. (a) Model for the top la ye r co ns is ti ng of 0. 33 ML Te . (b ) Mo de l fo r th e to p layer consisting of 0.33 ML Te and 0.08 ML Au, and the second layer consisting of 0.92 ML Au. Fig. 2. Specular X-ray reflectivity from the Te UPD lay er on the Au( 111 ) sub str ate . The dat a col lec tio n was performed between 39 to 59 hours after the UPD p o t e n t i a l h a d b e e n a p p l i e d . T h e d a s h e d r e d l i n e s h o w s t h e r e f l e c t i v i t y e x p e c t e d f o r a T e l a y e r imme diat ely abov e the Au(1 11) subs trat e. The soli d b l u e l i n e s h o w s t h e r e f l e c t i v i t y e x p e c t e d f o r t h e t o p layer consisting of the UPD Te atoms and Au atoms which diffuse from the Au(111) substrate. 37
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