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AES AND EELS MEASUREMENTS ON THE Si-Au-Ag INTERFACE
A. Cros, F. Salvan, J. Derrien
To cite this version:
A. Cros, F. Salvan, J. Derrien. AES AND EELS MEASUREMENTS ON THE Si- Au-Ag INTERFACE. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-1081-C4-1083.
�10.1051/jphyscol:19814237�. �jpa-00220868�
JOURNAL DE PHYSIQUE
CoZZoque C4, suppldment au nOIO, Tome 42, octobre 1981 page C4-1081
AES AND EELS MEASUREMENTS ON THE S i - A u - A g INTERFACE
FacuZte' des Sciences de Lwniny, Gdpartement de Physique, Case 901, 13288 MarseiZZe Cedex 9, France
Abstract.- Surface techniques under UHV conditions have been applied t o the study of the Si-Au-Ag i n t e r f a c e . The gold layer induces strong s t r u c t u r a l modifications
:some s i l i c o n atoms diffuse i n t o the s i l v e r layer which is found amorphous. This i s i n contrast with t h e non diffusive Si-Ag i n t e r f a c e where ordered s t r u c t u r e s a r e ob.served.
Introduction.- In t h e past few years, many detailed investigations on metal- semiconductor systems have provided a b e t t e r understanding of t h e Schottky b a r r i e r formation.
Acentral point of i n t e r e s t i s the s t r u c t u r a l properties of the interface:
interdiffusion between elements leads t o the formation of alloys o r compounds i n most cases. As a complement t o our previous Si-Ag and Si-Au i n t e r f a c e s t u d i e s
(1)(2)we present here some r e s u l t s on the Si-Au-Ag one.
Experimental.- All experiments have been carried out i n a
UHVchamber equipped
w i t hAES, EELS,
LEEDand ion sputtering f a c i l i t i e s . Metal evaporation i s made i n s i t u and calibrated by conventional quartz and Auger techniques ( r e f .2).
Summary of the main properties of Si-Ag and Si-Au i n t e r f a c e s . - These systems have been extensively studied by AES,
LEED (3), RHEED (4),EELS (ref.1 and 2 ) ,
UPS(5) techniques. I t i s generally agreed t h a t both Ag and Au, deposited on a s i l i c o n s u b s t r a t e a t room temperature, grow in a layer by layer mode. However d e t a i l e d studies have revealed s t r i k i n g differences
:i ) the Ag layers a r e ordered
:a f t e r the completion of a f i r s t monolayer, one observes the e p i t a x i a l growth of (111) Ag planes. In c o n t r a s t , LEED and
RHEEDana- l y s i s have shown t h a t the gold layers a r e amorphous.
i i ) the junction between s i l v e r and s i l i c o n i s atomically abrupt, p r a c t i c a l l y one bond length wide. This i s clearly different from the SS-Au interface which i s diffusive. In,the l a t t e r case, we successively f i n d from the s u b s t r a t e
:an alloyed region
(*15
Awide), a pure gold layer and f i n a l l y a surface monolayer where Si atoms have segregated and formed an alloy with gold.
The Si-Au-Ag i n t e r f a c e . - We show here t h a t when one adsorbs successively gold and s i l v e r atoms on a clean Si (111) surface, diffusion of s i l i c o n atoms i n t o t h e s i l v e r layer takes place.
"ERA C.N.R.S. 070373
""ERA C.N.R.S. 070899
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814237
JOURNAL DE PHYSIQUE
AES r e s u l t s taken d u r i n g t h e i n t e r f a c e growth are presented i n f i g u r e 1.
: AES s p e c t r a recorded d u r i n g i n t e r f a c e rowth : s u r f a c e (S~?LVVI a t 92 ev) b ) a f t e r t h e d e p o s i t i o n of 1.5 monolayer
d Au (NVV) a t 69 ev
c ) d ) e ) i n c r e a s i n g s i l v e r coverage : 0 = 7 , 8 = 12, 8 = 50.
Note t h e i n c r e a s i n g Ag (MNN) peaks a t 325 ev.
- a
I GO 65 70 75 80 85 90 95
L
+ . . . * ~ ~ .
ELECTRON ENERGY (ev)
Curve a) corresponds t o the well-known S i (LVVI t r a n s i t i o n o f t h e clean S i surface.
A f t e r t h e d e p o s i t i o n o f 1.5 g o l d monolayers ( c u r v e b ) ) , Au (NVV) peak appears a t 69 ev. Previous EELS s t u d i e s ( r e f . 2 ) have shown t h a t a t t h i s stage, displacement o f s i l i c o n s u r f a c e atoms has a l r e a d y occurred and induced t h e formation o f an a l l o y e d monolayer. Uhen Ag i s evaporated on t h i s surface, two regimes can be d i s t i n g u i s h e d : t h e f i r s t one (curves c ) d ) ) corresponds t o t h e f o r m a t i o n o f an Si-Au-Ag a l l o y as evidenced by t h e simultaneous presence o f t h e Auger peaks o f these elements. One i m p o r t a n t p o i n t l i e s i n t h e s p l i t t i n g o f t h e S i (LVV) t r a n s i t i o n i n two peaks a t 90 and 94 ev. T h i s f e a t u r e , which i s a l s o observed on Si-Au and Si-Ag 3D a l l o y s , r e v e a l s t h e presence o f s i l i c o n atoms which have a m e t a l l i c c h a r a c t e r i n a m e t a l l i c environment. The second regime ( c u r v e e ) ) i s c h a r a c t e r i z e d by t h e absence o f g o l d atoms and t h e presence o f an Ag-S1 a l l o y evidenced by the s p l i t S i (LVV) t r a n s i t i o n . Depth p r o f i l i n g a n a l y s i s by
~ r + s p u t t e r i n g has c l e a r l y shown t h a t t h i s a l l o y i s s u p e r f i c i a l . S i m i l a r l y t o t h e case o f t h e Si-Au i n t e r f a c e , t h e Si-Ag a l l o y s t a y s on t o p o f a pure Ag l a y e r .
F i g u r e 2 gives a schematic p i c t u r e o f t h e Si-Ag, Si-Au and Si-Au-Ag
i n t e r f a c e s . I t i s i m p o r t a n t t o mention t h a t a t each stage o f t h e i n t e r f a c e growth, t h e s t r u c t u r e s are amorphous (no LEED p a t t e r n ) .
SUBSTRATE
a
.. . .
SUBSTRATE
SUSSTRATE
Fig. 2 : Schematic s t r u c t u r e o f t h e Si-Ag, Si-Au and Si-Au-Ag i n t e r f a c e s .
I n summary, t h e p r e d e p o s i t i o n o f some g o l d atoms (% one dense (111) Au plane) on t h e c l e a n S i (111) s u r f a c e can induce d r a s t i c s t r u c t u r a l changes i n t h e Ag l a y e r s subsequently deposited : i ) they are amorphous i n c o n t r a s t t o t h e Si-Ag case.
i i ) t h e i n t e r f a c e becomes d i f f u s i v e . One observe t h e formation o f Au-Ag-Si and Ag-Si a l l o y s . The Au-Ag i n t e r m i x i n g i s n o t s u r p r i s i n g . The observation o f S i m e t a l l i c atoms i n a s i l v e r environment i s more i n t e r e s t i n g . To our knowledge, t h e o n l y mean t o get such s t r u c t u r e was t o quench Ag-Si l i q u i d s o l u t i o n s . A mechanism already evoked f o r s i l i c i d e f o r m a t i o n ( r e f . 6 ) can account f o r t h e s i l i c o n d i f f u s i o n : g o l d atoms, by occupying i n t e r s t i t i a l p o s i t i o n s a t t h e S i surface, can weaken t h e s t r o n g c o v a l e n t S i - S i bonds
.
These S i atoms have a h i g h e r m o b i l i t y . They s t a y i n t h e i r m e t a l l i c s t a t e when s i l v e r atoms are s u b s t i t u t e d t o g o l d ones d u r i n g t h e i n t e r f a c e growth.References.
-
( 1 ) DERRIEN J., LE LAY G. and SALVAN F., J. Phys. L e t t . ( P a r i s ) 39 (1978)287.
( 2 ) SALVAN F., CROS A. and DERRIEM J., J. Phys. L e t t . ( P a r i s ) 4 1 ( m 8 0 ) 337.
( 3 ) GREEN A.K. and BAUER E., J. Appl. Phys. 47 (1976) 1284). - ( 4 ) LE LAY G., MANNEVILLE M. and KERN R., SuyT. S c i .
$5
(1977) 261.( 5 ) BRAICOVITCH L., GARNER C.M., SKEATH P.R., SU C.Y., CHYE P.W., LINDAU I. and SPICER W.E., Phys. Rev. B 20 (1980) 5131.
( 6 ) TU K.N., Appl. Phys. L e t m 7
