O 2 Incident Energy ( eV ) 1 2 0 3 S a t u r a t e d O x y g e n C o v e r a g e ( a r b . u n i t s ) T N = 1400 K O 2 /He/Ar mixture T N = 940 K O 2 /He mixture T N = 1160 K O 2 /He mixture 1st threshold H 2 O-chemisorbed level I II III F i g . 1 . T h e s a t u r a t e d o x y g e n c o v e r a g e o n t h e p a r t i a l l y - o x i d i z e d ( H 2 O - c h e m i s o r b e d ) Si(001) surface as a function of translational kinetic energy of incident O 2 molecules. The symbol represents the results obtained by varying mixing ratio of O 2 /He/Ar keeping a nozzle temperature (T N ) of 1400 K. The symbol and represent the results obtained by different gas mixing ratios and nozzle temperatures of 940 K and 1160 K, respectively. Photoemission Study on the Surface Reaction Dynamics of Si(001) Oxidation by Supersonic O 2 Molecular Beams In the surface reaction dynamics study, the roles o f t r a n s l a t i o n a l k i n e t i c e n e r g y ( E t ) o f i n c i d e n t m o l e c u l e s a r e a n a r e a o f c u r r e n t i n t e r e s t . F o r i n s t a n c e , t h e d i s s o c i a t i v e c h e m i s o r p t i o n o f O 2 molecules takes place on Si(001) surfaces at room t e m p e r a t u r e ( p a s s i v e o x i d a t i o n ) . I n o r d e r t o un de rs ta nd th e ro le s of th e in ci de nt en er gy in O 2 m o l e c u l e s , E t d e p e n d e n c i e s o f p a s s i v e o x i d a t i o n , i n d u c e d b y s u p e r s o n i c O 2 m o l e c u l a r b e a m s ( SSMBs ), have been investigated by photoemission s p e c t r o s c o p y u s i n g h i g h - e n e r g y - r e s o l u t i o n synchrotron radiation ( SR ). A l l e x p e r i m e n t s w e r e p e r f o r m e d a t t h e e x p e r i m e n t a l s t a t i o n f o r s u r f a c e c h e m i s t r y : S U R E A C 2 0 0 0 [ 1 , 2 ] a t B L 2 3 S U . T h e b a s e p r e s s u r e is less than 5 × 10 -9 Pa when liquid N 2 is filled in the sh ro ud an d th e ma ni pu la to r. Mo no ch ro ma te d SR beams of about 400 eV and 830 eV were used for Si-2 p and O-1 s photoemission measurements with a s u r f a c e s e n s i t i v e c o n d i t i o n ( e s c a p e d e p t h : 0 . 3 n m ) . T h e O 2 S S M B s a r e g e n e r a t e d b y t h e adiabatic expansion of a mixture of O 2 , He and Ar using a high temperature nozzle. The maximum O 2 i n c i d e n t e n e r g y w a s c a l c u l a t e d t o b e 3 e V . T h e typical SSMB flux density and the photon flux were e s t i m a t e d t o b e 2 × 1 0 1 4 m o l e c u l e s c m - 2 s - 1 a n d 2 × 10 10 photons s -1 , respectively. First, an H 2 O-chemisorbed Si(001) surface was formed before O 2 exposure. Dangling bonds of the topmost Si dimers were terminated by H and OH in t h e H 2 O - c h e m i s o r b e d S i ( 0 0 1 ) s u r f a c e . I t w a s exposed to O 2 gas up to saturation. The oxidized surface was irradiated again by the O 2 SSMBs with r e s p e c t t o s u r f a c e n o r m a l u n t i l t h e s a t u r a t i o n coverage was achieved. The E t dependence of the s a t u r a t i o n c o v e r a g e i s s h o w n i n F i g . 1 [ 3 ] . Referring to the first-principles molecular dynamics calculation [4], the lower break was assigned to a potential energy barrier for the backbond oxidation of the top mos t Si ato ms, and the hig her one was a s s i g n e d t o a p o t e n t i a l e n e r g y b a r r i e r f o r t h e b a c k b o n d o x i d a t i o n o f t h e s e c o n d l a y e r (subsurface) Si atoms. 48 Different oxidation states are expected to appear i n S i - 2 p p h o t o e m i s s i o n s p e c t r a i n e a c h i n c i d e n t en er gy re gi on . Re pr es en ta ti ve Si -2 p ph ot oe mi ss io n s p e c t r a a r e s h o w n i n F i g . 2 [ 5 ] . T h e y w e r e d e - convoluted into bulk Si ( Si-2 p 1 /2 and 2 p 3 /2 ), interface Si, H-terminated Si and oxidized Si: Si n+ (n = 1 - 4). The photoemission peak intensity ratio for the 2 p 1 /2 a n d 2 p 3 / 2 c o m p o n e n t s w a s m e a s u r e d t o b e 1 : 2 u n d e r a b u l k s e n s i t i v e c o n d i t i o n s o t h a t t h e r a t i o was used for the de-convolution. The LS-coupling scheme may be no longer good due to crystal field effects even for the Si 2 + component [6]. Therefore, t h e s p i n - o r b i t s p l i t t i n g w a s t a k e n i n t o a c c o u n t f o r bulk Si, distorted interface Si, H-terminated Si, and S i 1 + c o m p o n e n t s e x c e p t S i 2 + , S i 3 + a n d S i 4 + c o m p o n e n t s . T h e S i - 2 p s p e c t r u m o f t h e S i ( 0 0 1 ) 0 0 1 2 3 4 3/2 1/2 3/2 Si n+ n-value: (c) Et = 2.0 eV (Region II ) Tox = 0.36 nm (d) Et = 3.0 eV (Region III ) Tox = 0.53 nm (b) Et = 0.04 eV (Region I) Tox = 0.17 nm (a) Before O 2 exposure Relative Kinetic Energy ( eV ) -6 -5 -4 -3 -2 -1 0 1 Si 1+ Si-H & Interface Si Bulk Si Observed Si 2+ Si 3+ Si 4+ Interface Si Si-2 p photoemission intensity ( arb. units) Fig. 2. Si-2p photoemission spectra for Si(001) surfaces oxidized up to saturation coverage at room te mp er at ur e by O 2 mo le cu le s wi th va ri ou s in ci de nt en er gi es (E t ) : (a ) fo r th e in it ia l (p ar ti al ly - o x i d i z e d ) S i ( 0 0 1 ) s u r f a c e , ( b ) f o r E t = 0 . 0 4 e V , ( c ) f o r E t = 2 . 0 e V a n d ( d ) f o r E t = 3 . 0 e V , respectively. T ox represents the oxide layer thickness. surface before O 2 exposure consists of bulk Si, Si- H an d Si -O H ( Si 1+ ) co mp on en ts as sh ow n in Fi g. 2 ( a ) . T h e s p e c t r a l p r o f i l e o b t a i n e d a f t e r O 2 ex po su re wi th E t = 0. 04 eV , re pr es en ta ti ve in th e region I, is very close to that before O 2 exposure. T h i s f a c t i m p l i e s t h a t t h e S i - O H a n d S i - H b o n d s prohibit further oxidation by O 2 molecules. On the o t h e r h a n d , t h e s p e c t r a l p r o f i l e c h a n g e d d r a m a t i c a l l y w h e n t h e O 2 S S M B s w i t h i n c i d e n t energy larger than the first threshold, 1.0 eV, were irradiated to the surface. An Si-2 p spectrum for E t = 2.0 eV is shown in Fig. 2(c) as a representative in regi on II, indi cati ng the Si 4+ form atio n. The dire ct ox id at io n of Si di me r ba ck bo nd s du e to en er ge ti c O 2 collisions can take place in this incident energy r e g i o n s o t h a t t h e t o p m o s t S i a t o m s c a n b e s u r r o u n d e d 49 O Si H H 2 O -adsorbed dimer Buckled Dimer Bridge Site Subsurface Bulk (a) Before O 2 exposure (b) E t = 0.04 eV (c) E t = 2.0 eV (d) E t = 3.0 eV Dimer Backbond Site Subsurface Backbond Site by up to four oxygen atoms. The Si-2 p spectrum f o r E t = 3 . 0 e V s h o w s t h a t S i 3 + a n d S i 4 + c o n t r i b u t i o n s s h a r e a l a r g e p a r t o f t h e s a t e l l i t e p e a k s , a s s h o w n i n F i g . 2 ( d ) . S u c h a l a r g e c o n t r i b u t i o n i s i n t e r p r e t e d a s t h e i n c i d e n t - e n e r g y - induced dissociative chemisorption of O 2 molecules at the subsurface backbonds in the energy region III as well as the topmost Si dimers bridge sites and t h e i r b a c k b o n d s i t e s . T h e o x i d e l a y e r t h i c k n e s s w a s e s t i m a t e d a p p r o x i m a t e l y t o b e 0 . 5 3 n m . C o n s e q u e n t l y , t h e t h i c k n e s s o f u l t r a - t h i n o x i d e l a y e r , l e s s t h a n 1 n m , c a n b e c o n t r o l l e d a t r o o m te mp er at ur e by co nt ro ll in g th e tr an sl at io na l ki ne ti c energy of incident O 2 molecules in a hyperthermal energy region. Reaction models are also presented in Fig. 3 . Yuden Teraoka and Akitaka Yoshigoe SPring-8 / JAERI E-mail: yteraoka @ spring8.or.jp F i g . 3 . R e a c t i o n m o d e l s o f i n c i d e n t - e n e r g y - i n d u c e d o x i d a t i o n p r o c e s s e s : ( a ) f o r t h e partially-H 2 O-chemisorbed Si(001) surface, (b ) for the residual Si dimer oxidation by O 2 exposure, (c) for the Si dimer backbond oxidation by O 2 incident energy larger than the first threshold (1.0 eV ), (d) for the subsurface backbond oxidation by O 2 incident energy larger than the second threshold (2.6 eV ), respectively. References [ 1 ] Y . T e r a o k a a n d A . Y o s h i g o e , J p n . J . A p p l . Phys. 38 , Suppl. 38-1 (1999) 642. [2] Y. Ter aok a and A. Yos hig oe, App l. Sur f. Sci . 169-170 (2001) 738. [3] A. Yoshigoe, M. Sano and Y. Teraoka, Jpn. J. Appl. Phys. 39 (2000) 7026. [4] K. Kato and T. Uda, Phys. Rev. B 62 (2000) 15978. [ 5 ] Y . T e r a o k a a n d A . Y o s h i g o e , A p p l . S u r f . S c i . 190 (2002) 75. [6] Y. Miyamoto and A. Oshiyama, Phys. Rev. B 44 (1991) 5931. 50
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