3 - D I m a g i n g a r o u n d a D o p a n t i n C r y s t a l b y X - r a y F l u o r e s c e n c e H o l o g r a p h y 3 - D I m a g i n g a r o u n d a D o p a n t i n Si C r y s t a l b y X - r a y F l u o r e s c e n c e H o l o g r a p h y Fig. 1 T h e d o p i n g t e c h n i q u e i n c r y s t a l l i n e S i h a s p l a y e d a n i m p o r t a n t r o l e i n t h e f a b r i c a t i o n o f a d v a n c e d s e m i c o n d u c t o r d e v i c e s , w h i c h r e q u i r e s s tate-of-the-art tailoring of a band gap. In order to understand the nature of doping-induced electronic st at e s , it is es se nt ia l to st ud y th e lo ca l st ru ct ur es around impurities in a doped semiconductor. The 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 t h o d i s com mon ly use d for stu dyi ng the loc al str uct ure of particular elements. X -ray fluorescence holography ( X F H ) [ 1 , 2 ] i s a p r o m i s i n g n e w t e c h n i q u e f o r i m a g i n g 3 - D a t o m i c a r r a n g e m e n t s a r o u n d a t o m s em it ti ng fl uo re sc en ce ph ot on s. Th is te ch ni qu e is a l s o a p p l i c a b l e t o i n v e s t i g a t e t h e e n v i r o n m e n t around a dopant [3]. The “normal” and “inverse” modes exist in the X F H m e t h o d . I n t h e n o r m a l X F H m e t h o d , f l u o r e s c e n t X - r a y s f r o m a t o m s i n a s a m p l e w i t h an d without being scattered by surrounding atoms s e r v e a s t h e o b j e c t a n d r e f e r e n c e w a v e s , respectively. A holographic pattern is recorded by s c a n n i n g a d e t e c t o r a r o u n d t h e s a m p l e . I n t h e inver se mode, f l u o r e s c e n c e i s u s e d t o d e t e c t a n i n t e r f e r e n c e f i e l d o r i g i n a t i n g f r o m i n c i d e n t a n d scattered X-rays ( ). The holographic pattern is obtained by detecting the fluorescence while the s a m p l e ’ s o r i e n t a t i o n i s v a r i e d r e l a t i v e t o t h e incident beam. The inverse mode was preferably u s e d i n t h e X F H e x p e r i m e n t a t t h i s s y n c h r o t r o n r a d i a t i o n f a c i l i t y b e c a u s e t h e h o l o g r a m s c a n b e r e c o r d e d a t a n y i n c i d e n t e n e r g y a b o v e t h e a b s o r p t i o n e d g e o f a n emitter. Fig. 1. The XFH principle for the inverse mode. 38 F l u o r e s c e n t X - r a y s I n c i d e n t X - r a y s X F H p a t t e r n s w e r e m e a s u r e d a t b e a m l i n e B L 4 7 X U . A n S i 0. 99 9 G e 0. 00 1 s a m p l e g r o w n b y t h e C z o c h r a l s k i m e t h o d w a s u s e d a s t h e s a m p l e [ 4 ] , and its d imensions w ere 5 × 5 × 2 mm 3 . Incident energies were 14.5 - 17.0 keV with 0.25 keV steps. Figure 2 shows an experimental setup for recording inverse XFH hologram s . The Ge K α (9.87 keV ) X- r a y f l u o r e s c e n c e v i a a c y l i n d r i c a l L i F c r y s t a l w a s detected by an avalanche photodiode. The X-ray f l u o r e s c e n c e c o u n t r a t e w a s a b o u t 2 0 0 , 0 0 0 c p s . T he fl uo re sc en ce in te ns it ie s we re me as ur ed as a f u n c t i o n o f a z i m u t h a l a n g l e φ a n d p o l a r a n g l e θ 1 θ 1 within the ranges of 0 φ 360 and 0 76 . The X-ray exit angle of θ 2 was fixed at 45 . In t h i s e x p e r i m e n t , w e r e c o r d e d 1 1 h o l o g r a m s a t different energies. To handle the data , we incorporated extension o f t h e h o l o g r a m t o t h e f u l l s p h e r e b y u s i n g m e a s u r e d s y m m e t r i e s a n d l o w - p a s s f i l t e r i n g . Fi gu re 3 sh ow s th e re su l t i ng ho l og ra m pa tt er n at 1 4 . 5 k e V . M u l t i p l e e n e r g y r e c o n s t r u c t i o n b y t h e H e l m h o l t z - K i r c h h o f f t r a n s f o r m a t i o n m o d i f i e d according to Barton was applied to these hologram data. The real space image was depicted in Fig. 4 . T h e a t o m i c i m a g e s w e r e e x t r e m e l y f i n e , a n d artifacts, which were obvious in the reconstruction b y t h e s i n g l e e n e r g y X F H , w e r e s u f f i c i e n t l y s u p p r e s s e d . T h e a t o m s u p t o t h e s e v e n t h c o o r d i n a t i o n s h e l l w e r e r e c o g n i z e d , t h o u g h w e F i g . 2 . P h o t o g r a p h ( a ) a n d s c h e m a t i c d i a g r a m ( b ) o f t h e e x p e r i m e n t a l s e t u p i n s t a l l e d a t S P r i n g - 8 f o r t h e s t r u c t u r a l analysis of condensed systems by XFH. Fig. 3. Ge X-ray fluorescence hologram o f S i 0 . 9 9 9 G e 0 . 0 0 1 r e c o r d e d a t 1 4 . 5 k e V . T h e d i s p l a y e d p a t t e r n w a s o b t a i n e d b y symmetrization and low-pass filtering. 39 I n c i d e n t b e a m S a m p l e C r y s t a l a n a l y z e r F l u o r e s c e n c e p h o t o n s θ 1 φ A P D θ 2 ( b ) A P D C y l i n d r i c a l L i F a n a l y z e r S a m p l e ( a ) Fig. 4. 3-D image of the atomic environment around Ge in Si 0.999 Ge 0.001 . F i g . 4 Kouichi Hayashi, Yukio Takahashi and Ei-ichiro Matsubara Tohoku University E-mail: khayashi @ imr.tohoku.ac.jp References [1] M. Tegze and G. Faigel, Nature 380 (1996) 49. [2] T. Gog et al. , Phys. Rev. Lett. 76 (1996) 313. [ 3 ] K . H a y a s h i , M . M a t s u i , Y . A w a k u r a , T . Kaneyoshi, H. Tanida and M. Ishii, Phys. Rev. B 63 (2001) R41201. [ 4 ] I . Y o n e n a g a , J . C r y s t . G r o w t h 1 9 8 - 1 9 9 ( 1 9 9 9 ) 4 0 4 . [5] K. Hayashi, J. J. Soc. Synchrotron Rad. Res. 15 (2002) 267. h a v e o n l y d i s p l a y e d t h e a t o m s u p t o t h e f o u r t h c o o r d i n a t i o n s h e l l i n t h e b e c a u s e o f t h e complication of the figure . The arrangement of the a t o m s i n t h e r e c o n s t r u c t i o n r e m a r k a b l y s h o w s a s u p e r p o s i t i o n of two asso ciat ed envi ronm ents of a di am on d st ru ct ur e, re ve al in g th at Ge at om s li e in two distinct crystallographic sites. Thus, we f ou nd that Ge atoms are substituted for Si sites. Taking i n t o a c c o u n t t h e G e c o n c e n t r a t i o n , m o s t o f t h e a t o m i c i m a g e s w e r e r e g a r d e d a s b e i n g o f S i . O r i g i n a l l y , X F H w a s w e a k a t i m a g i n g l i g h t a t o m s such as Si and O. The present result revealed that ou r ex pe ri me nt al an d da ta pr oc es si ng te ch ni qu es significantly improved the results [5]. 40
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