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INVESTIGATION OF THE L NEAR-EDGE
STRUCTURE IN CRYSTALLINE AND AMORPHOUS GaAs
S. Guita, E. Belin, C. Senemaud, D. Udron, A. Gheorghiu, M. Theye
To cite this version:
S. Guita, E. Belin, C. Senemaud, D. Udron, A. Gheorghiu, et al.. INVESTIGATION OF THE L
NEAR-EDGE STRUCTURE IN CRYSTALLINE AND AMORPHOUS GaAs. Journal de Physique
Colloques, 1986, 47 (C8), pp.C8-427-C8-430. �10.1051/jphyscol:1986886�. �jpa-00226217�
INVESTIGATION OF THE L NEAR-EDGE STRUCTURE IN CRYSTALLINE AND AMORPHOUS GaAs
S. GUITA, E. BELIN, C. SENEMAUD, D. UDRON* , A. GHEORGHIU' and M.L. THEYE"
Laboratoire de Chimie-Physique, U A 176, Universite Pierre et Marie Curie, 11, Rue Pierre et Marie Curie, F-75231 Paris Cedex 0 5 , France
"Laboratoire d1Optique des Solides, U A 040-781 CNRS, ~ n i v e r s i t e Pierre et Marie Curie, 4, Place Jussieu, F-75320 Paris cedex 05, France
Resume : Une e t u d e c a m p a r e e d e s s p e c t r e s de p h o t o a b s o r p t i o n a u v o i s i - n a g e d e s s e u i l s L g de As e t d e Ga d a n s GaAs a m o r p h e e t c r i s t a l l i s e a m o n t r e q u e s e u l s l e s B t a t s s d u Ga s o n t p r e s e n t s d a n s l e b a s d e l a b a n d e d e c o n d u c t i o n . L e d e s o r d r e n l i n d u i t p a s de m e l a n g e s e n s i b l e d e s e t a t s d e As e t d e Ga a u v o i s i n a g e d u s e u i l .
A b s t r a c t : A c o m p a r a t i v e s t u d y o f t h e p h o t o a b s o r p t i o n s p e c t r a n e a r t h e As a n d Ga L e d g e s i n a m o r p h o u s a n d c r y s t a l l i n e GaAs h a s s h o w n t h a t o n l y Ga s - s 2 a t e s a r e f o u n d a t t h e b o t t o m o f t h e conduction b a n d . D i s o r d e r d o e s n o t i n d u c e n o t i c e a b l e a d m i x t u r e o f As a n d Ga s t a t e s c l o s e t o t h e b a n d e d g e .
INTRODUCTION
I n o r d e r t o a n a l y s e m o s t o f t h e p h y s i c a l p r o p e r t i e s o f s e m i c o n d u c t o r s , i t . i s i n t e r e s t i n g t o know t h e d e n s i t y o f s t a t e s a t t h e b a n d e d g e s a s w e l l a s o v e r a l a r g e e n e r g y r a n g e o n b o t h s i d e s o f t h e e n e r g y g a p . The g r e a t a d v a n t a g e o f X - r a y s p e c t r o s c o p y i s t h a t i t a l l o w s t o p r o b e s e p a r a t e l y t h e e l e c t r o n i c d i s t r i b u t i o n s o f t h e v a l e n c e a n d c o n d u c t i o n b a n d s . I n a d d i t i o n , i t g i v e s i n f o r m a t i o n o n t h e s y m m e t r y o f t h e e l e c - t r o n i c s t a t e s a n d , i n . t h e c a s e o f a compound, o n t h e i r s p a t i a l l o c a - l i z a t i o n .
We a r e i n t e r e s t e d h e r e i n t h e m o d i f i c a t i o n s o f t h e d e n s i t y o f s t a t e s i n t r o d u c e d b y d i s o r d e r when g o i n g f r o m c r y s t a l l i n e ( c ) t o a m o r p h o u s
( a ) GaAs. S u c h m o d i f i c a t i o n s h a v e a l r e a d y b e e n i n v e s t i g a t e d i n d e t a i l f o r t h e v a l e n c e b a n d b y p h o t o e l e c t r o n s p e c t r o s c o p y e x p e r i m e n t s ( 1 , 2 , 3 ) . We c o n c e n t r a t e h e r e o n t h e c o n d u c t i o n b a n d , w h i c h we s t u d y b y X - r a y p h o t o a b s o r p t i o n e x p e r i m e n t s p e r f o r m e d o n b o t h c r y s t a l l i n e a n d
a m o r p h o u s m a t e r i a l s . The s a m p l e s a r e p r e p a r e d u n d e r i d e n t i c a l c o n d i - t i o n s a s t h e o n e s a l r e a d y s t u d i e d b y o p t i c a l ( 4 ) a n d X - r a y p h o t o - e l e c t r o n ( 3 ) s p e c t r o s c o p i e s . B o t h t h e As a n d Ga L 3 h a v e b e e n i n v e s t i - g a t e d ; t h e y g i v e t h e e l e c t r o n d i s t r i b u t i o n s w i t h s a n d d c h a r a c t e r a r o u n d As a n d Ga a t o m s r e s p e c t i v e l y . I n e a c h c a s e , t h e F e r m i l e v e l c o u l d b e l o c a t e d o n t h e X - r a y t r a n s i t i o n e n e r g y s c a l e w i t h t h e h e l p o f s e p a r a t e XPS ( 3 ) a n d X - r a y e m i s s i o n d a t a .
EXPERIMENT
T h e p h o t o a b s o r p t i o n e x p e r i m e n t s a t t h e L 3 e d g e s o f Ga a n d As w e r e p e r - f o r m e d u s i n g a c l a s s i c a l X - r a y t u b e w i t h a W a n o d e , c o u p l e d w i t h a
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986886
C8-428 JOURNAL DE PHYSIQUE
bent-crystal vacuum spectrometer equipped either with a gypsum (020) or a beryl (10T0) crystal. The energy resolution is about 0.3 eV.
It must be pointed out that the width of the inner core level invol- ved in the transitions (2pgI2) is about 1 eV (5). The amorphous GaAs .-, samples were 0:75 )rm thick films deposited under high vacuum ( N lo-' torr) by flash-evaporation of crystalline powder onto aluminium foils.
Fig. 1 As L3 photoabsorption spectra -. Fig. 2 Ga L3 photoabsorption spectra
The film composition, controlled by electron microprobe, was close to stoechiometry within 1 % . The crystalline GaAs samples were ob- talned by annealing the amorphous films in situ at 400°C.
In order to locate the Fermi level E F on the energy scale of the experimental photoabsorption spectra, which correspond to
+
s,d empty states transitions, the energy separation between :hP~'??p core level and E F was determined by combining (i) the measurzkgnt of the Ld (3d \ 2p ) emission line energy, (ii) the determination of the 3 6 core la@l energy with respect t o E F by XPS experiments on the same samples. This procedure allows to study the relative positions of the As and Ga electronic distribu- tions for the same sample.RESULTS AND DISCUSSION
Figure 1 shows the As L g photoabsorption spectra in crystalline and amorphous GaAs, as well as in pure bulk amorphous As for comparison.
The intensities of all these curves have been normalized in the flat region at about 18 eV above E , although the experimental con- trast for pure As was lower than !or the compounds.
- The spectrum for a-As exhibits an absorption edge at the Fermi level with rather small amplitude,followed by a more important jump at about 4eV above E The first contribution corresponds to a re- gion where density-oglstates calculations predict the presence o f p antibonding states (6,7) our data suggest that these p states at the
- The As s p e c t r a f o r c - a n d a-GaAs h a v e v e r y s i m i l a r s h a p e s . The a b s o r p t i o n e d g e , w h i c h i s s h i f t e d t o h i g h e r e n e r g i e s w i t h r e s p e c t t o a - A s , i s s t e e p e r a n d h a s a l a r g e r a m p l i t u d e ; i t i s l o c a t e d a t t h e same e n e r g y i n b o t h c a s e s w i t h i n e x p e r i m e n t a l u n c e r t a i n t i e s a t a b o u t 2 eV a b o v e E F . The s e c o n d j u m p a l s o o c c u r s r o u g h l y a t t h e same e n e r g y . T h e e f f e c t o f d i s o r d e r i s j u s t t o s m o o t h o u t t h e f i n e s t r u c t u - r e s d e t e c t e d i n t h e c-GaAs.
F i g u r e 2 s h o w s t h e Ga L3 p h o t o a b s o r p t i o n s p e c t r a i n c r y s t a l l i n e a n d a m o r p h o u s GaAs, a s w e l l a s i n p u r e GaAs f o r c o m p a r i s o n . The same n o r - m a l i z a t i o n p r o c e d u r e a s b e f o r e h a s b e e n u s e d .
- The a b s o r p t i o n e d g e i n p u r e Ga e x h i b i t s a n a r c - t a n g e n t s h a p e c h a r a c - t e r i s t i c o f a f r e e e l e c t r o n m e t a l .
- The o v e r a l l s h a p e o f t h e s p e c t r u m i s m o d i f i e d a n d the, a b s o r p t i o n e d g e s h i f t e d t o h i g h e r e n e r g i e s i n t h e c o m p o u n d s . The d i f f e r e n c e s b e t w e e n c - a n d a-GaAs a r e m o r e i m p o r t a n t f o r t h e Ga s p e c t r a t h a n f o r t h e As s p e c t r a . I n c-GaAs, t h e r e i s a w e l l - d e f i n e d p e a k , a b o u t 2eV w i d e , a t t h e t o p o f t h e a b s o r p t i o n e d g e . T h i s p e a k i s f o l l o w e d b y a b r o a d e r s t r u c t u r e , t h e n b y a s e c o n d j u m p s t a r t i n g a t a b o u t 6eV a b o v e E , w i t h a f a i n t d i p a r o u n d 11 eV. I n a - G a A s , t h e a b s o r p t i o n e d g e i s s h i f t e d t o h i g h e r e n e r g i e s b y a b o u t 0 . 4 eV w i t h r e s p e c t t o i t s p o s i t i o n i n c - G a A s , b u t r e t a i n s t h e same s l o p e . The p e a k a n d t h e b r o a d s t r u c t u r e h a v e d i - s a p p e a r e d a n d a r e r e p l a c e d b y a p l a t e a u . The s e c o n d j u m p i s p r a c t i c a l - l y n o t m o d i f i e d i n l o c a t i o n , s h a p e a n d a m p l i t u d e .
F i g u r e s 3 a a n d 3b show a c o m p a r i s o n b e t w e e n t h e d i s t r i b u t i o n s o f u n o c - c u p i e d s t a t e s w i t h s s y m m e t r y a r o u n d Ga a n d As a t o m s , i n c r y s t a l l i n e a n d a m o r p h o u s GaAs r e s p e c t i v e l y . As a l r e a d y e m p h a s i z e d , s u c h a compa- r i s o n i s a l l o w e d b y t h e p o s i t i o n i n g o f t h e F e r m i l e v e l i n e a c h c a s e . The m o s t s t r i k i n g r e s u l t i s t h a t , i n b o t h c a s e s , Ga s t a t e s a r e p r e s e n t a t t h e b o t t o m o f t h e c o n d u c t i o n b a n d , w h i l e As s t a t e s c a n o n l y b e f o u n d a t h i g h e r e n e r g i e s . T h i s means t h a t t h e l o w e s t l y i n g p e a k i n t h e c o m p u t e d c o n d u c t i o n b a n d d e n s i t y o f s t a t e s o f c - GaAs ( 8 ) m u s t c o r r e s p - o n d e s s e n t i a l l y t o Ga s s t a t e s .
The e n e r g y s e p a r a t i o n b e t w e e n
t h e Ga a n d As L a b s o r p t i o n A x 4 e d g e s i s o n l y s ? i g h t l y s m a l l e r
i n a-GaAs t h a n i n c - G a A s , a b o u t l e v i n s t e a d o f 1 . 5 e V , e s s e n - t i a l l y d u e t o t h e s h i f t u p w a r d s
o f t h e Ga e d g e . The d i s o r d e r i s (arbun~ts) t h e r e f o r e u n a b l e t o c o m p l e t e l y
m i x t h e s t a t e s a t t h e b o t t o m o f t h e c o n d u c t i o n b a n d . F i g u r e s 3a a n d 3 b a l s o show t h a t , i f t h e As s - s t a t e s d i s t r i b u t i o n i s r a t h e r f r e e - e l e c t r o n l i k e , a s p r e d i c t e d f o r c o n d u c t i o n e l e c t r o n s b y e l e c t r o n i c c h a r g e d e n s i t y c a l c u l a t i o n s (9), t h e Ga s s t a t e s a t t h e b o t t o m o f t h e conduction b a n d a r e r a t h e r s t r o n g l y l o c a l i z e d , a t l e a s t i n c - G a A s .
F i g 3 As L 3 ( - - - ) a n d Ga L (-1 p h o t o a b s o r p t i o n s p e c t r a i n a) C - S a ~ s b) a-Ga As
C8-430 JOURNAL DE PHYSIQUE
T h i s e f f e c t is s i g n i f i c a n t l y r e d u c e d by t h e d i s o r d e r i n a-GaAs. T h e s a m e c o n c l u s i o n w a s a l s o r e a c h e d f o r t h e c o n d u c t i o n s t a t e s w i t h p sym- m e t r y f r o m a c o m p a r i s o n o f t h e Ga a n d A s K p h o t o a b s o r p t i o n s p e c t r a i n c-GaAs ( 1 0 ) .
REFERENCES
( 1 ) N . J . S h e v c h i k . AIP c o n f C e p r o c . v o l 2 0 . i n " T e t r a h e d r a l l y b o n d e d a m o r p h o u s s e m i c o n d u c t o r s " e d . by M . H . B r o d s k y , S . K i r k p a t r i c k a n d D . W e a i r e , A m e r i c a n I n s t i t u t e o f P h y s i c s , New y o r k , 1 9 7 4 p . 7 2 .
( 2 ) N . J . S h e v c h i k , J . T e j e d a , M . C a r d o n a , P h y s . R e v .
E,
2 6 2 71 9 7 4
( 3 ) C . S e n e m a u d , E . B e l i n , A . G h e o r g h i u , M . L . ThBye, S o l . S t Comm.
55,
9 4 7 , 1 9 8 5 .( 4 ) A . G h e o r g h i u , M.L. ThGye, P h i l - M a g .
844,
2 8 5 , 1 9 8 1 ( 5 ) 0 . K e s k i - R a h k o n e n , M . O . K r a u s e .A t . d a t a a n d N u c l . d a t a t a b l e s
2,
1 3 9 , 1 9 7 4( 6 ) M.J. K e l l y , D . W . B u l l e t t , S o l . S t . Comm. - 1 8 , 5 9 3 , 1 9 7 6 ( 7 ) M.J. K e l l y , D . W . B u l l e t t , P h i l . Mag.
838,
4 2 3 , 1 9 7 8 ( 8 ) J . R . C h e l i k o w s k y , M . L . C o h e n , P h y s . R e v . B-
1 4 , 5 5 6 , 1 9 7 6 ( 9 ) J . P . W a l t e r , M . L . C o h e n , P h y s . Rev. - 8 4 , 1 8 7 7 , 1 9 7 10
( 1 0 ) J . D r a h o k o u p i l , H . K l o k o c n i k o v a , A . S i m u n e k , J . P h y s . C S o l . st. P h y s . 9 2 6 6 7 , 1 9 7 6 . -
