PHOTOEMISSION PROCESS OF A LOCALIZED ELECTRON IN CARRIER TRAP AND ITS APPLICATION TO SITE-SELECTIVE X-RAY ABSORPTION FINE STRUCTURE MEASUREMENT

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F i g . 1 . T e m p e r a t u r e d e p e n d e n c e o f capacitance XAFS at the Ga K-edge. X - r a y a b s o r p t i o n f i n e s t r u c t u r e ( X A F S ) m e a s u r e m e n t i s a n i m p o r t a n t e x p e r i m e n t a l te ch ni qu e, wh ic h us es sy nc hr ot ro n ra di at io n ( SR ) f o r l o c a l s t r u c t u r e a n a l y s e s . I n X A F S a n a l y s i s , microscopic information, such as bond lengths and t h e c o o r d i n a t i o n n u m b e r , i s d e r i v e d f r o m t h e m a c r o s c o p i c a b s o r p t i o n p r o p e r t y , a n d t h u s structural homogeneity of the sample is necessary. W h e n t h e s a m p l e i s h e t e r o g e n e o u s w i t h v a r i o u s local structures, the XAFS spectrum provides only the average information of these local structures. A meaningful analysis using this average information is impossible, except in the rare case in which the spectrum can be deconvoluted into a few spectra of s o m e w e l l - k n o w n s t r u c t u r e s . A h e t e r o g e n e o u s system is not only common but also is of interest in current material science. For instance, the defects in a si ng le cr ys ta l an d he te ro in te rf ac es fa br ic at ed PHOTOEMISSION PROCESS OF A LOCALIZED ELECTRON IN CARRIER TRAP AND ITS APPLICATION TO SITE-SELECTIVE X-RAY ABSORPTION FINE STRUCTURE MEASUREMENT In this resea rch, the X-ray induc ed photo emiss ion p r o p e r t y o f t h e l o c a l i z e d e l e c t r o n a n d t h e s i t e - s e l e c t i v i t y o f c a p a c i t a n c e X A F S m e a s u r e m e n t based on this process are discussed. T h e d e t a i l e d c o n c e p t a n d e x p e r i m e n t a l a p p a r a t u s o f c a p a c i t a n c e X A F S m e t h o d w e r e d e s c r i b e d i n p r e v i o u s p a p e r s [ 1 , 2 ] . I n o u r e x p e r i m e n t s p e r f o r m e d a t t h e H i g h B r i l l i a n c e X A F S b e a m l i n e BL10XU , the capacitance involved in the Schottky d i o d e o f a c o m p o u n d s e m i c o n d u c t o r , S e - d o p e d AlGaAs, was measured under SR irradiation. It is well know that a deep level of the electron trap, the D X c e n t e r , i s f o r m e d i n t h i s s a m p l e d u e t o t h e i n t r i n s i c p r o p e r t y o f t h e d o n o r i m p u r i t y i n z i n c - blende semiconductors. Figure 1 shows the capacitance XAFS spectra a t t h e G a K - e d g e ( 1 0 . 3 7 5 k e V ) . T h e s a m p l e Applied bias: – 1.5 V 0 10 20 30 40 50 10.36 10.37 10.38 10.39 10.40 300 K 200 K 120 K 100 K 80 K 60 K Ga K -edge Sample temperature Se-doped AlGaAs Metal electrode SR C Photon energy ( keV ) Capacitance ( arb. units) b y e p i t a x i a l g r o w t h h a v e s i n g u l a r s t r u c t u r e s i n t h e c r y s t a l . T h e a s s u m p t i o n o f s a m p l e homogeneity restricts the applicability of XAFS measurement. Recently, we proposed a new XAFS method, c a p a c i t a n c e X A F S m e a s u r e m e n t [ 1 ] . I n t h i s method, since the amount of X-ray absorption is e v a l u a t e d b y a c a p a c i t a n c e c h a n g e d u e t o X - r a y i n d u c e d p h o t o e m i s s i o n o f a l o c a l i z e d ele ctr on, a sit e-s ele cti ve XAF S ana lys is of the sp ec if ic at om s wi th th e lo ca li ze d el ec tr on ma y be achieved. It is well known that the defects a n d d a n g l i n g b o n d s a t t h e h e t e r o i n t e r f a c e i n semiconductors localize electrons owing to their characteristics as trap centers, suggesting that t h e m i c r o s c o p i c a b s o r p t i o n p r o p e r t y o f t h e s e sin gul ar ato ms can be sel ect ive ly ana lyz ed by capacitance XAFS measurement. Fig. 2. Arrhenius’ plot estimation of thermal activation energy of the photoemission process in the capacitance XAFS method. t e m p e r a t u r e w a s v a r i e d f r o m 6 0 K t o 3 0 0 K . A s shown in this figure, the edge jump decreases with i n c r e a s i n g s u b s t r a t e t e m p e r a t u r e , a n d f i n a l l y , i t a l m o s t d i s a p p e a r s a t r o o m t e m p e r a t u r e . T h i s tende ncy is in contr ast to that of the conve ntion al XAFS method. In conventional fluorescence XAFS m e t h o d , s i n c e t h e X - r a y p e n e t r a t i o n d e p t h a n d e s c a p e d e p t h o f t h e f l u o r e s c e n c e , w h i c h a r e independent of the sample temperature, determine t h e n u m b e r o f d e t e c t a b l e a t o m s , n o t e m p e r a t u r e dependence of the edge-jump is observed. On the other hand, the valence electronic states are easily modified by thermal energy of the order of ~ meV. The temperature dependence of edge-jump in Fig. 1 p r o v i d e s e v i d e n c e t h a t t h e v a l e n c e e l e c t r o n i s r e l a t e d t o t h e s i g n a l - d e t e c t i o n p r o c e s s i n c a p a c i t a n c e XAFS measurement. In fact, the activation energy of the photoemission process of trapped electron is estimated to be 7.33 meV from a fitted result of the tem per atu re dep end enc e bas ed on the Arr hen ius ’ equation as shown in Fig. 2 . 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.004 0.008 0.012 0.016 0.020 0.024 Sample: Se-doped AlGaAs Bias: – 1.5 V 300 200 100 50 1/T (K -1 ) Temperature (K) Activation energy = 7.33 meV De-localization efficiency Moreover, a capacitance-voltage (C-V) analysis w h i c h p r o v i d e s t h e c a r r i e r c o n c e n t r a t i o n i n t h e semiconductor, indicates that the activation energy of the extra donor generation process under SR is e v a l u a t e d t o b e 7 . 6 8 m e V [ 2 ] . A l m o s t t h e s a m e a c t i v a t i o n e n e r g i e s f o r t h e p h o t o e m i s s i o n i n ca pa ci ta nc e XA FS me as ur em en t (7 .3 3 me V ) an d this extra donor generation indicate that the signal a m p l i t u d e o f c a p a c i t a n c e X A F S m e a s u r e m e n t i s determined by the X-ray induced photoemission of t h e l o c a l i z e d e l e c t r o n t o c o n d u c t i o n b a n d i n t h e semiconductor. This is consistent with the concept of capacitance XAFS method. F i g u r e 3 s h o w s a p o s s i b l e p h o t o e m i s s i o n dynamics based on these finding. Only the energy diagram around the Fermi level, E f , is illustrated in this figure. The Fig. 3 (a) indicates a steady state b e f o r e t h e X - r a y i r r a d i a t i o n . T h e l o c a l i z e d electrons occupy the trap level in the band gap of the semiconductor. When the X-ray is absorbed at t h e d e f e c t a t o m , ( b ) e x c i t a t i o n o f t h e l o c a l i z e d Fig. 3. Possible photoemission dynamics in capacitance XAFS measurement. (a) (b) Conduction band Trap level (c) E f - ∆ ∆ E E f Depletion layer Metal electrode Semiconductor Conduction band Trap level E f Depletion layer Metal electrode Semiconductor Trap level E c + ∆ ∆ E E c X-ray photoemission + Masashi Ishii SPring-8 / JASRI E-mail: ishiim @ spring8.or.jp electrons into a conduction band is expected ( Fig.3 (b) ). The de-localized electrons are swept out from t h e d e p l e t i o n l a y e r s o t h a t t h e p o s i t i v e l y c h a r g e d t r a p c e n t e r p u s h e s d o w n t h e E f t o E f – ∆ E . T h e Fermi level of the semiconductor should be equal to that of the metal electrode used for the capacitance formation. Consequently, (c) the energy level of the c o n d u c t i o n b a n d i s i n c r e a s e d b y ∆ E ( F i g . 3 ( c ) ) . The level modification reduces the thickness of the d e p l e t i o n l a y e r w i t h o u t t h e l o c a l i z e d e l e c t r o n , r e s u l t i n g i n t h e c a p a c i t a n c e i n c r e m e n t b y t h e X - r a y a b s o r p t i o n . T h i s d y n a m i c s i n d i c a t e s t h e microscopic X-ray absorption at the defect, not the m a c r o s c o p i c a b s o r p t i o n a t b u l k , i s s e l e c t i v e l y o b t a i n e d b y c a p a c i t a n c e X A F S m e t h o d . T h e d e t a i l s o f s t r u c t u r a l a n a l y s i s u s i n g X A F S m e t h o d a r e d e s c r i b e d i n S P r i n g - 8 R e s e a r c h F r o n t i e r s 1997/1998 [3] . References [1] M. Ishii et al. , Appl. Phys. Lett. 74 (1999) 2672. [ 2 ] M . I s h i i , Y . Y o s h i n o , K . T a k a r a b e a n d O . Shimomura, J. Appl. Phys. 88 (2000) 3962. [ 3 ] M . I s h i i , S P r i n g - 8 R e s e a r c h F r o n t i e r s 1997/1998, p.94.