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FORMATION AND OXIDATION OF A
METAL/SEMICONDUCTOR INTERFACE : Pb/Ge (111)
G. Le Lay, J. Metois
To cite this version:
G. Le Lay, J. Metois. FORMATION AND OXIDATION OF A METAL/SEMICONDUCTOR INTERFACE : Pb/Ge (111). Journal de Physique Colloques, 1984, 45 (C5), pp.C5-427-C5-433.
�10.1051/jphyscol:1984564�. �jpa-00224183�
JOURNAL DE PHYSIQUE
Colloque C5, suppl6rnent au n04, Tome 45, avril 1984 page C5-427
FORMATION AND O X I D A T I O N OF A METAL/sEMICONDUCTOR INTERFACE : ~ b / ~ e ( l l l ) G. Le ~ a and J.J. Metois ~ +
C.R.M.C.2 , CNRS, Campus d e I,umuminy, Case 913, 13288 MarseiZZe Cedex 0 9 , France
RCsurn6 - Nous presentons une a n a l y s e d e t a i l l e e de l a dynamique de f o r m a t i o n m n t e r f a c e P b / G e ( l l l ) P t u d i g e en DEL s p e c t r o s c o p i e Auger e t m i c r o s c o p i e 2 balayage. On s 1 i n t P r e s s e specialement 5 une t r a n s i t i o n de phase 2D
$ 3 x f l . ~ ( 3 0 0 ) ? 1x1 observee dans l e domaine de l a sous-monocouche. Des ex- pPriences de c i n g t i q u e de d P s o r p t i o n isotherme p e r m e t t e n t de d 6 t e r m i n e r l l & n e r g i e de c o h g s i o n de l a phase 1x1. E n f i n on i n d i q u e quelques r C s u l t a t s p r g l i m i n a i r e s s u r l ' o x y d a t i o n de l a monocouche de plomb.
A b s t r a c t - We p r e s e n t a d e t a i l e d a n a l y s i s o f t h e dynamics o f t h e f o r m a t i o n o f t h e P b / G e ( l l l ) i n t e r f a c e s t u d i e d b LEED, AES and SEM. We d i s c u s s es- p e c i a l l y a 20 phase t r a n s f o r m a t i o n : d x f l - R ( 3 0 0 ) f 1x1 observed i n t h e mono- l a y e r range. I s o t h e r m a l d e s o r p t i o n k i n e t i c experiments p e r m i t t h e determina- t i o n o f t h e cohesive energy o f t h e 1x1 phase. F i n a l l y we i n d i c a t e a few p r e l i m i n a r y r e s u l t s on t h e o x i d a t i o n o f t h e l e a d monolayer.
1 - INTRODUCTION
A c o n s i d e r a b l e e f f o r t has been undertaken i n t h e l a s t t e n y e a r s t o determine t h e s t r u c t u r a l and e l e c t r o n i c p r o p e r t i e s o f t h e metal/semiconductor i n t e r f a c e i n t h e course o f i t s f o r m a t i o n . I n t h i s r e s p e c t , a l a r g e number o f s t u d i e s concern t h e n o b l e m e t a l / e l e m e n t a l semiconductor s e r i e s ( f o r a r e c e n t r e v i e w see / 1 / and r e f e r - ences t h e r e i n ) . B u t t h e s t u d y o f group I V m e t a l s on e l e m e n t a l semiconductor i s a l s o i n t e r e s t i n g i n i t s e l f ; w h i l e t h i s was undertaken v e r y e a r l y (1964) f r o m t h e s t r u c - t u r a l p o i n t o f v i e w f o r s i l i c o n s u b s t r a t e s (LEED s t u d y o f t h e a d s o r p t i o n o f Pb and Sn on S i ( l l 1 ) /2/), a s i m i l a r study, t o o u r knowledge was c a r r i e d o u t o n l y r e c e n t l y (1981) f o r germanium s u b s t r a t e s (RHEED o b s e r v a t i o n s o f t h e a d s o r p t i o n o f Sn on
~ e ( 1 1 1 ) / 3 / ) . I n t h e l a s t p e r i o d we have undertaken a thorough i n v e s t i g a t i o n o f t h e
~ b / G e ( l l l ) i n t e r f a c e f o r m a t i o n u s i n g s e v e r a l complementary t e c h n i q u e s i.e. AES, LEED o b s e r v a t i o n s and i n t e n s i t y measurements, i s o t h e r m a l d e s o r p t i o n spectroscopy (ITDS) o f monolayer overgrowths, t o g e t h e r w i t h SEM o b s e r v a t i o n s o f t h i n l e a d films/4,5/.
I n d e p e n d e n t l y a RHEED s t u d y o f t h e same system was c a r r i e d o u t by Ichikawa / 6 / a t t h e same t i m e and i t was concluded t h a t t h e two-dimensional (2D) l e a d monolayers on t h e germanium (111) surfaces undergo a 2D s o l i d t o 2D l i q u i d t r a n s f o r m a t i o n a t f i x e d temperatures, a t v a r i a n c e w i t h o u r c o n c l u s i o n t o a r e v e r s i b l e 2D s o l i d - 2 D s o l i d t r a n s f o r m a t i o n . Beside t h e g e n e r a l i n t e r e s t o f t h e u n d e r s t a n d i n g o f t h e metal/semi- conductor S c h o t t k y - b a r r i e r f o r m a t i o n , and t h e f a c t t h a t t h i n l e a d f i l m s a r e super- c o n d u c t i v e m a t e r i a l s i m p o r t a n t i n Josephson technology, t h e Pb/Ge system w i t h n e g l i - g i b l y small mutual s o l u b i l i t i e s a t t h e l e a d m e l t i n g p o i n t , m i g h t o f f e r t h e p o s s i b i l - i t y of s t u d y i n g t h e c h a r a c t e r i s t i c s o f t h e m e l t i n g o f 2D c r y s t a l s , an i n t r i g u i n g problem i n t h e p h y s i c s o f condenced m a t t e r / 7 / . I n f a c t t h i s p o s s i b i l i t y was used by W i l l e n s e t a1 / 8 / who s t u d i e d t h e m e l t i n g b e h a v i o u r o f modulated Pb-Ge-foils by scanning c a l o r i m e t r y and X-ray d i f f r a c t i o n .
I n t h i s paper we f i r s t p r e s e n t t h e dynamics o f t h e f o r m a t i o n o f t h e P b / G e ( l l l ) i n t e r - f a c e and a n a l y z e i t s f e a t u r e s i n t h e l i g h t o f t h e s t r u c t u r a l and e l e c t r o n i c p r o p e r - t i e s of t h e A u / G e ( l l l ) and A g / G e ( l l l ) i n t e r f a c e f o r m a t i o n . Then we d i s c u s s t h e ques- t i o n of t h e 2D phase t r a n s f o r m a t i o n b e f o r e p r e s e n t i n g p r e l i m i n a r y r e s u l t s on t h e oxygen a d s o r p t i o n r a t e a t d i f f e r e n t temperatures as a f u n c t i o n o f t h e coverage o f t h e preadsorbed metal. T h i s l a s t work has been undertaken as r e l e v s n t s t u d i e s o f t h e ox-
'ALSO UER de Physique, UniversitS de Provence, Marseille
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984564
C5-428 JOURNAL DE PHYSIQUE
i d a t i o n process o f u l t r a - t h i n l e a d o v e r l a y e r s on p o l y c r y s t a l l i n e s i l v e r / 9 / a n d Copper / l o / s u b s t r a t e s on t h e one hand, as we1 1 as thorough s t u d i e s o f t h e oxygen a d s o r p t i o n on a t o m i c a l l y c l e a n /11/ and a1 k a l i - m e t a l covered ( 1 11) s u r f a c e s o f g e r - manium /12/ on t h e second hand have been p u b l i s h e d r e c e n t l y which p e r m i t s a d i r e c t comparison o f t h e behaviours.
2 - EXPERIMENTAL
Experiments a r e performed i n a UHV chamber, t h e r e s i d u a l p r e s s u r e n e v e r exceeding 5x10-10 t o r r ; n - t y p e germanium s i n g l e c r y s t a l s a r e c l e a v e d i n - s i t u , Lead (5N) i s eva- p o r a t e d f r o m a Knudsen c e l l a t a r a t e o f 8x10-38 s-1 as determined w i t h a q u a r t z c r y s - t a l o s c i l l a t o r . Assuming u n i t y s t i c k i n g c o e f f i c i e n t a t RT a monolaycr i n s u b s t r a t e u n i t s : @=I (i.e. 7.21 1014 atoms ~ m - ~ ) i s completed w i t h i n 300 s d e p o s i t i o n t i m e (N.B. t h e atomic d e n s i t y i n a (111) b u l k Pb p l a n e i s 9.42 1014 atoms cm-2 i . e . a close-packed Pb a r r a y would correspond t o 0=1.31). Exposure o f t h e s u r f a c e s t o oxy- gen was c a r r i e d o u t t h r o u g h a l e a k v a l v e and t h e gas p r e s s u r e s were measured w i t h a c o l d - c a t h o d e gauge. Auger e l e c t r o n spectroscopy i s performed w i t h a g r a z i n g incidence gun and t h e f o u r - g r i d LEED o p t i c s . The sample i s p l a c e d i n a small oven p e r m i t t i n g i t s h e a t i n g ; i t s temperature i s c o n t r o l l e d w i t h a Pt,Pt,Rh thermocouple; t h e absolute e r r o r i n t e m p e r a t u r e r e a d i n g b e i n g l e s s t h a n 10°C.
The i s o t h e r m a l d e s o r p t i o n experiments a r e c a r r i e d o u t w i t h AES. A h i g h h e a t i n g r a t e and a s t a b i l i z e d power s u p p l y e n a b l e a c o n s t a n t temperature (*l°C) t o be reached w i t h i n l e s s t h a n one minute.
F i n a l l y t h e morphology o f t h i c k l e a d f i l m s were observed a f t e r removal f r o m t h e vessel by SEM.
3 - FORMATION OF THE P b / G e ( l l l ) INTERFACE: RESULTS AND DISCUSSION 3-1. Growth mode
Between RT and 3000C t h e P b / G e ( l l l ) system obeys t h e S t r a n s k i - K r a s t a n o v (SK) growth mode i.e. 3D l e a d c r y s t a l l i t e s develop on t h e s u b s t r a t e a f t e r c o m p l e t i o n o f a 2D l a y e r which i s s a t u r a t e d a t 0=1 as i n d i c a t e d by t h e b r e a k s i n t h e a d s o r p t i o n c u r v e s o f f i g u r e 1. The p l a t e a u s correspond t o t h e o n s e t o f 3D n u c l e a t i o n . A t h i g h coverages new s p o t s i n t h e LEED p a t t e r n s c o r r e s p o n d t o a? e p i t a x i a l arrangement o f t h e l e a d c r y s t a l 1 i t e s a c c o r d i n g t o ( 1 l l ) ~ b / / ( l l l ) G e , <1 1 0 > p b / / < l l O > ~ e ( p a r a l l e l e p i t a x y ) , a r e s u l t f u r t h e r c o n f i r m e d by t h e o b s e r v a t i o n o f l a r g e f l a t c r y s t a l l i t e s w i t h s h a r p o r i e n t e d edaed i n SEM.
LEED o b s e r v a t i o n s d u r i n g d e p o s i t i o n showed two k i n d s o f 6 x E - ~ ( 3 0 ~ ) ( i n s h o r t c)
s u ~ e r s t r u c t u r e s .
1 1 2 2
-i- as l o n g as 0<1/3 f r a c t i o n a l o r d e r s p o t s o f t h e f o r m (3.3) and d3$ a r e observed f r o m RT and p e r s i s t u n t i l a l l l e a d i s evaporated i n t h e d e s o r p t i o n t e m p e r a t u r e do- main (T>3500C)
F i g . 1 - SK growth mode o f Pb on G e ( l l 1 ) between RT and 3 0 0 ~ ~ as determined w i t h AES:
peak t o peak a m p l i t u d e s o f t h e 94 eV Pb NO0 and 52 eV Ge MMM l i n e s .
-ii- beyond 8=1/3 and u p t o 0=1, w h i l t h e i n t e n s i t y o f t h e (2/3,2/3) fi s p o t s increases w i t h coverage, t h e i n t e n s i t y o f t h e (g,&) s p o t s s t r o n g l y decreases i n such a manner t h a t a t 0=1 t h e s e s p o t s a r e h a r d l y observed ( f i g u r e 2, c u r v e a ) . T h i s s i t u a t i o n e x i s t s up t o T%280°C. I n a narrow range o f temperatures around 280°C ( t h u s markedly b e l o w t h e d e s o r p t i o n temperatures) we observe a r e v e r s i b l e t r a n s i t i o n t o a 1x1 s t r u c - t u r e . Now i f we e v a p o r a t e t h e metal on t h e germanium s u b s t r a t e beyond t h i s t r a n s i - t i o n temperature, we f i r s t observe t h i r d o r d e r s u p e r s t r u c t u e s p o t s w i t h i n c r e a s i n g i n t e n s i t y up t o 0=1 t h e p o s i t i o n o f t h e maximum; beyond 0 7 , r t h e s e s p o t s v a n i s h l e a v i n g t h e 1x1 pa?iern ( f i g u r e 2, c u r v e b ) .
F i g . 2 - LEED i n t e n s i t y v a r i a t i o n s ( o p t i c a l d e n s i t y measurements) o f s u p e r s t r u c t u r e s p o t s versus coverage: a(a) and C(B) s t r u c t u r e s . a ) RT-40V, b ) 320'~-40V (beyond t h e t r a n s i t i o n temperature).
I c h i k a w a has observed a s i m i l a r b e h a v i o u r w i t h RHEED; a d o p t i n g h i n o t a t i o n we name
JS
i n s h o r t T 3 ( a ) , t h e f i r s t s t a b l e s t r u c t u r e completed a t 0=1 and 3 ( 6 ) t h e second one which undergoes t h e r e v e r s i b l e 3(6) f 1x1 t r a n s i t i o n . ~ e d r t h e l e s s t h e coverages a t c o m p l e t i o n o f t h e s e two s t r u c t u r e s t h t were guessed by Ichikawa f r o m t h e RHEED h
p a t t e r n changes ( i . e . r e s p e c t i v e l y 0 3 and 0=1.31) a r e t o o h i g h as c l e a r l y i n d i c a t e d by t h e more a c c u r a t e d e t e r m i n a t i o n o f t h e maximum i n t h e LEED i n t e n s i t y c u r v e o f f i g u r e 2b and t h e c l e a r break p o i n t s i n t h e Auger a d s o r p t i o n c u r v e s o f f i g u r e 1.
3-2. 3D s o l i d - 3 D l i q u i d t r a n s i t i o n
F o r 8<1 t h e Pb 94 eV s i g n a l does n o t change when t h e temperature i s i n c r e a s e d beyond t h e m e l t i n g p o i n t o f b u l k l e a d ( ~ ~ = 3 2 7 O ) , which means t h a t t h e adsorbed monolayer i s s t a b l e and t h a t no agglomeration, e v a p o r a t i o n , d i f f u s i o n o r i n t e r m i x i n g w i t h t h e s u b s t r a t e occurs beyond t h e Pb b u l k m e l t i n g p o i n t .
For 0>>1 (e.g. m 3 0 0 ) we n o t i c e an a b r u p t decrease o f t h e Auger s i g n a l when t h e m e l t i n g p o i n t i s reached. T h i s e f f e c t i s t h e consequence o f t h e s o l i d - l i q u i d t r a n s i - t i o n o f t h e l e a d f i l m which i s s o l i d and c o n t i n u o u s below TM and becomes d i s c o n t i n u - ous beyond TM w i t h a p o p u l a t i o n o f l i q u i d d r o p l e t s on a l e a d monolayer, as i s con- f i r m e d by t h e f a c t t h a t t h e Auger i n t e n s i t y i s n e a r l y equal t o t h a t o f one d e p o s i t e d l e a d monolayer ( @ = I ) , t h e c o n t r i b u t i o n o f t h e 3D l i q u i d d r o p l e t s t o t h e Auger s i g n a l b e i n g t h u s n e g l i g i b l e .
Besides, comparison o f t h e Auger a m p l i t u d e o f t h e c o n t i n u o u s t h i c k l e a d f i l m w i t h t h e s i g n a l a t one monolayer coverage p e r m i t s t h e d e t e r m i n a t i o n o f t h e escape d e p t h o f t h e 94 eV e l e c t r o n s i n l e a d f i l m s : 4.6t0.58.
3-3. I s o t h e r m a l d e s o r p t i o n k i n e t i c s
F i g u r e 3 shows s e v e r a l d e s o r p t i o n i s o t h e r m s measured w i t h AES, s t a r t i n g f r o m an i n - i t i a l coverage 0=1. A l l a l o n g t h e d i f f e r e n t curves t h e LEED p a t t e r n shows t h e same 1x1 s t r u c t u r e . We n o t i c e t h a t t h e d a t a a r e f a i r 1 w e l l s i t u a t e d a l o n g a s t r a i g h t l i n e f o r each temperature. Obviously, t h e s l o p e 0 = dg i s independent o f coverage and i s a f u n c t i o n o f temperature o n l y , t h a t i s t h e d t d e s o r p t i o n o r d e r i s zero. An A r r h g n i u s p l o t of t h e s l o p e s y i e l d s an a c t i v a t i o n energy o f 46.8+0.5 Kcal mole-1 and a d e s o r p t i o n f r e q u e n c y o f (1.2?0.5)~1012 s-1.
J O U R N A L DE PHYSIQUE
F i g . 3 - D e s o r p t i o n i s o t h e r m s o f t h e l e a d monolayer (1x1 LEED p a t t e r n a l o n g t h e p l o t s ) .
3-4. Scanning e l e c t r o n microscopy o b s e r v a t i o n s
- i - we have a l r e a d y i n d i c a t e d t h a t a f t e r d e p o s i t i o n t h e l e a d c r y s t a l l i t e s e x h i b i t k i n e t i c shapes: t h e s e a r e t a b u l a r c r y s t a l s w i t h l a r g e (111) f a c e t s and sharp a1 i g n e d edges.
-ii- we have a l s o examined t h e e q u i l i b r i u m shape o f t h e l e a d c r y s t a l l i t e s o b t a i n e d by a n n e a l i n g a t 2 5 0 ~ ~ f o r 5 h o u r s a p o p u l a t i o n o f c r y s t a l l i t e s which e x h i b i t e d i n i t i - a l l y t h e morphology o f a f r a c t i o n o f a sphere /13/ . These " l e n s " c r y s t a l l i t e s were o b t a i n e d a f t e r h e a t i n g a t h i c k (%3000A) l e a d filn! above t h e m e l t i n g p o i n t o f b u l k l e a d and t h e n quenching a t RT. The e q u i l i b r i u m shape i s a cubo-octahedron w i t h c u r - ved zones between f a c e t s . The s i t u a t i o n i s v e r y s i m i l a r t o t h e e q u i l i b r i u m shape o f l e a d on (0001) g r a p h i t e b u t f o r t h e w e t t i n g angle: a = 580k1° f o r P b / G e ( l l l ) i n s t e a d o f a = 117' f o r P b / g r a p h i t e /14/ . N e g l e c t i n g t h e s u r f a c e a n i s o t r o p y , t h e adhesion energy B o f t h e l e a d c r y s t a l l i t e s on G e ( l l 1 ) i s deduced f r o m t h e Young and D u p r s ' s f o r m u l a :
B = o ( l + c o s a ) = 91829 e r g s cmd2 where 0 = 600 e r g s cm-2 i s t h e l e a d s u r f a c e energy.
3-5. D i s c u s s i o n
The s a l i e n t f e a t u r e s o f t h e f o r m a t i o n o f t h e P b / G e ( l l l ) i n t e r f a c e a r e -i- t h e SK growth mode a l r e a d y a t RT w i t h c o m p l e t i o n o f t h e i n t e r m e d i a t e 2D l a y e r a t 0=1, -ii- th e extreme abruptness o f t h e i n t e r f a c e w i t h o u t any evidence o f i n t e r m i x i n g . P o i n t -i- i s a t v a r i a n c e w i t h t h e RT growth mode o f t h e n o b l e m e t a l s on G e ( l l 1 ) : f o r t h e systems A u / G e ( l l l ) /15/ and A g / G e ( l l l ) 1 1 6 1 a m e t a s t a b l e l a y e r l i k e growth f a s h i o n preceeds t h e h i g h temperature SK growth mode. For A u / G e ( l l l ) t h e r a p i d quenching o f t h e Ge 52 eV Auger s i g n a l t o g e t h e r w i t h UPS r e s u l t s showing d i s t i n c t i v e s p e c t r o s c o p i c f e a t u r e s i n t h e EDC's a t 2.6 eV w i t h hv=21 eV and a t 10 eV below EF w i t h hv=80 and 150 eV, due t o Ge p- Au d h y b r i d i z a t i o n r e v e a l e d t h e f o r m a t i o n o f a mixed s u r f a c e phase. I n f a c t , i n many r e s p e c t s , t h e growth o f P b / G e ( l l l ) appears s i m i l a r t o t h e h i g h temperature growth mode o f A g / G e ( l l l ) where t h e AES r e s u l t s i n - d i c a t e a sharp i n t e r f a c e .
The d e s o r p t i o n experiments a r e remarkable i n t h e sense t h a t a z e r o - o r d e r d e s o r p t i o n process i s c l e a r l y evidenced i n a l a r g e coverage range (0=1 + W0.1) where a s i n g l e phase (1x1 LEED s t r u c t u r e ) desorbs. B u t f o r t h e A u / G e ( l l l ) system which p r e s e n t s such a s t r o n g a f f i n i t y between Au and Ge t h a t no d e s o r p t i o n below t h e b u l k m e l t i n g p o i n t o f germanium was p o s s i b l e , we c o u l d p e r f o r m such ITDS experiments f o r a l l o t h e r n o b l e metal/semiconductor systems, and t h u s s t u d y t h e e n e r g e t i c s o f t h e i r d i f - f e r e n t s u r f a c e phases. However t h e number o f s u c c e s s i v e 20 phases (e.g. 3 f o r
A u / S i ( l l l ) , 2 f o r Ag/Si(111) and A g / G e ( l l l ) ) i n t h e submonolayer domain made t h e ex- p e r i m e n t s d i f f i c u l t , t h e i r a n a l y s i s r a t h e r complex. Here t h e much more f a v o u r a b l e s i t u a t i o n o f a s i n g l e d e s o r b i n g phase y i e l d s much more c o n v i n c i n g r e s u l t s . We ana- l y z e t h e s e ITDS d a t a i n t h e framework o f t h e model we had proposed some y e a r s ago /l9/ : we assume t h a t t h e e v a p o r a t i o n o f t h e 2D phase proceeds i n d i r e c t l y f r o m t h e edge o f 2D i s l a n d s o f t h e d e s o r b i n g phase v i a a d i l u t e 2D gas adsorbed on t h e s u r - f a c e . Two regimes c o u l d be i n f e r r e d a c c o r d i n g t o t h e v a l u e o f t h e mean square d i s - placement XS o f t h e ad-atoms -i- i f X i s much h i g h e r t h a n t h e mean square d i s t a n c e between t h e 2D i s l a n d s 0-112 (P= i s l a n a d e n s i t y ) a thermodynamic e q u i l i b r i u m i s es- t a b l i s h e d between t h e 2D gas phase (ad-atoms) and t h e 2D i s l a n d s . I n t h i s s i t u a t i o n t h e ad-atom p o p u l a t i o n on t h e s u b s t r a t e depends o n l y on t h e temperature and t h e r e - f o r e a l s o t h e d e s o r p t i o n f l u x : t h e r e a c t i o n i s o f z e r o - o r d e r -ii- i f on t h e c o n t r a r y Xs i s i n f e r i o r t o t h e mean d i s t a n c e between t h e i s l a n d s ( x s < p - l l 2 ) then t h e desorp- t i o n process i s c o n t r o l l e d by t h e d e p a r t u r e o f t h e atoms f r o m k i n k p o s i t i o n s on t h e edge o f t h e 2D i s l a n d s and t h e r e a c t i o n i s a h a l f - o r d e r process. The e x p r e s s i o n de- r i v e d f o r a z e r o - o r d e r process was:
- 0 = a -1 -1 kT f* E2
o ns T 5 exp - K
where aQ i s t h e i n t e r a t o m i c d i s t a n c e i n t h e 20 phase, n i s t h e number o f s u b s t r a t e a d s o r p t ~ o n s i t e s p e r cm2, f * and f 2 t h e p a r t i t i o n s f u n c 2 i o n s o f an a c t i v a t e d complex i n t h e 2D gas phase and of an atom i n t h e 2D i s l a n d , E2 t h e detatchment energy f r o m t h e h a l f c r y s t a l p o s i t i o n i n t h e 2D i s l a n d t o t h e vacuum. A c c o r d i n g t o t h e e x p r e s s i o n t h e p r e e x p o n e n t i a l f a c t o r i s t h e d e s o r p t i o n frequency: v =(I. 2*0.5)x1012 s-1 and t h e a c t i v a t i o n energy determined e x p e r i m e n t a l l y i s a d i r e c t measure o f t h e c o h e s i v e en- e r g y o f t h e 2D phase t h a t desorbs. Hence, E2=46.8'0.5 Kcal mole-1. T h i s c o h e s i v e energy i s t h e sum o f t h e bond energy @pb-Ge o f one l e a d atom on an a d s o r p t i o n s i t e of ~ e ( l l 1 ) p l u s t h e l a t e r a l bond energy o f one l e a d atom i n a k i n k s i t e o f a 2D i s - l a n d ~ f g - ~ ~ . T h i s l a s t q u a n t i t y can be e s t i m a t d u s i n g t h e Morse p o t e n t i a l o f G i r i f a co e t a1 /20/ f o r b u l k l e a d , y i e l d i n g @%g-pb5 16 Kcal mole-I. As
+ @ 2D -1
E2 = @Pb-Ge pb-pb We @Pb-Ge = 31'2 Kcal mole .
4 - THE 2D PHASE TRANSFORMATION
A s i n d i c a t e d above, t h e G ( B ) 20 s o l i d phase undergoes a r e v e r s i b l e phase t r a n s i t i o n t o a 1x1 s t r u c t u r e . I n o u r LEED o b s e r v a t i o n s t h e t r a n s f o r m a t i o n a t 0=1 was f o u n d t o o c c u r i n a temperature i n t e r v a l o f a b o u t 10 degrees around 280'~. The 1x1 p a t t e r n a t 300°C showing sharp i n t e g e r s p o t s we concluded t o a second o r d e r 2D s o l i d - 2 D s o l i d t r a n s i t i o n . The RHEED o b s e r v a t i o n s o f Ichikawa / 3 / a r e somewhat d i f f e r e n t : t h e tem- p e r a t u r e o f t h e t r a n s i t i o n appears t o depend c r i t i c a l l y on t h e coverage: f o r
0.5<0<1.3 i t o c c u r s a t r a t h e r l o w temperature: 1 9 2 ' ~ w h i l e a t 0=1.4 i t s u d d e n t l y i n - creases beyond t h e b u l k l e a d m e l t i n g p o i n t (TM=3270C) up t o 333OC. Moreover d i f f u s e r o d s a r e observed which l o o k a l i k e d u r i n g a z i m u t h a l r o t a t i o n o f t h e sample. T h i s was c o n s i d e r e d as m e e t i n g t h e c h a r a c t e r i s t i c s o f r e f l e c t i o n s f r o m an i d e a l 2 D l i q u i d . C h a l l enging o u r c o n c l u s i o n s , t h e G(B) 2 1 x 1 t r a n s f o r m a t i o n was regarded as t h e m e l t i n g of t h e 2D Pb m o n o c r y s t a l l i n e l a y e r . I n a d d i t i o n t h e a z i m u t h a l i n v a r i a n c e o f t h e d i f f u s e r o d s would i m p l y t h a t t h e l i q u i d Pb i s n o t under s t r o n g i n f l u e n c e o f t h e G e ( l l 1 ) s u b s t r a t e . T h i s p o i n t i s o f g r e a t i n t e r e s t as i t would s i g n i f y t h a t t h e mono- l a y e r of Pb on G e ( l 1 l ) c o u l d b e a p r o t o t y p e f o r t h e s t u d y o f t h e m e l t i n g o f 2D s o l ids as t y p i c a l l y a r e now t h e r a r e gases adsorbed a t l o w temperatures on g r a p h i t e . I n f a c t , as mentioned i n t h e i n t r o d u c t i o n , t h e m e l t i n g b e h a v i o u r o f Pb-Ge-foils, as Pb approaches a 2D s o l i d has been i n v e s t i g a t e d by scanning c a l o r i m e t r y and X-ray d i f - f r a c t i o n / B / . The t r a n s i t i o n has t h e a t t r i b u t e s o f a second o r d e r phase t r a n s i t i o n , t h e m e l t i n g temperature decreases w i t h r e s p e c t t o b u l k l e a d b u t t e n d s t o a s y m p t o t i - c a l l y approach a l i m i t i n g v a l u e (e.g. s t a r t 45' below TM f o r a 1 0 8 - t h i c k Pb f i l m ) and m e l t i n g t r a n s i t i o n s broadens o v e r a range o f tempgratures when t h e t h i c k n e s s o f t h e Pb i s reduced (i.e. t h e t r a n s i t i o n b r e a d t h i s 40 C f o r a 1 0 8 - t h i c k Pb f i l m ) . I f we c o n s i d e r t h e g e n e r a l tendency, t h e s e l a s t r e s u l t s appear t o be i n c l o s e r agreement w i t h o u r LEED o b s e r v a t i o n s t h a n w i t h t h e RHEED ones. Why such a d i s c r e p a n c y between b o t h t y p e s o f o b s e r v a t i o n s ? From t h e w i d t h o f t h e d i f f u s e r o d s a p p e a r i n g a f t e r t h e G ( B ) 3 1x1 t r a n s i t i o n beyond 1 9 2 ~ ~ i t was i n f e r r e d t h a t o n l y a s h o r t range c o r r e l a - t i o n i n a t o m i c geometry o f t h e 2D s t r u c t u r e e x ' s t e d : t h e c o r r e l a t i o n l e n g t h c o u l d be t y p i c a l l y 5 2 5 8 . However we s t i l l observe t h e &(B) s t r u c t u r e i n t h e LEED p a t t e r n s up t o 280'C.~ p o s s i b l e e x p l a n a t i o n i s t h a t i n LEED t h e coherence l e n g t h i s t y p i c a l l y
C5-432 JOURNAL DE PHYSIQUE
10 nm w h i l e i t i s a f e w 100 nm i n RHEED. To c o n t r i b u t e t o a d i f f r a c t i o n p a t t e r n d i f f e r e n t domains w i t h i n t h e coherence l e n g t h o f t h e e l e c t r o n beam must be commensurably spaced. If w i t h i n c r e a s i n g temperatures t h e s i z e o f the$3(8) domains s h r i n k s t o some 108 i n d i a m e t e r t h i s c o m m e n s u r a b i l i t y i s l i k e l y t o o c c u r f o r t h e f e w n e i g h b o u r i n g domains i n a 102nm2 zone; t h i s i s however
v e r y u n l i k e l y f o r an a r e a e x t e n d i n g t o s e v e r a l 104 nm2. We a l s o s t r e s s . t h a t d u r i n g and beyond t h e t r a n s i t i o n t h e i n t e g e r s p o t s remain r a t h e r sharp. We t h i n k t h a t t h i s would be u n l i k e l y i f t h e l e a d o v e r l a y e r was m o l t e n and i t s atom would b e a r no r e - g i s t r y w i t h t h e G e ( l l 1 ) s u b s t r a t e . We t h u s f a v o r , as i n d i c a t e d above, a
2D s o l i d 2 2D s o l i d t r a n s i t i o n . F u r t h e r s u p p o r t t o t h i s v i e w comes f r o m t h e i n t e n - s i t y measurements d e p i c t e d i n f i g u r e 4. F o r t h e i n t e g e r o r d e r s p o t s o f t h e C ( B ) s t r u c t u r e we n o t e a decrease o f t h e i n t e n s i t y versus t e m p e r a t u r e s i m i l a r t o t h a t o f t h e i n t e g e r as w e l l as s u p e r s t r u c t u r e s p o t s o f t h e $3(a) s t r u c t u r e . We a t t r i b u t e t h i s v a r i a t i o n t o t h e thermal v i b r a t i o n s o f t h e l e a d atoms which must have a l a r g e r a m p l i t u d e t h a n t h o s e o f t h e s u b s t r a t e germanium atoms as i s t e s t i f i e d b y t h e com- p a r i s o n o f t h e r e s p e c t i v e Debye temperatures: 88OK f o r 1 ead, 2 9 0 ' ~ f o r germanium. I n a d d i t i o n l e t us u n d e r l i n e t h a t t h e h y p o t h e s i s o f a m e l t i n g t r a n s i t i o n r e l i e s s t r o n g l y t o t h e assumption t h a t t h e f l ( c ) phase i s s a t u r a t e d a t 0%4/3 i .e. corresponds t o a s l i g h t l y compressed (111) p l a n e o f l e a d t h a t s h o u l d be r o t a t e d by 30° ( o r 90') w i t h r e s p e c t t o t h e s u b s t r a t e , i n such a manner t h a t < l l Z > ~ b / / < l T O > G e ( p e r p e n d i c u l a r o r i e n t a t i o n ) , t o g i v e a $ 3 x 6 - ~ ( 3 0 ~ ) p a t t e r n w i t h t h e observed i n t e n s i t i e s o f t h e s u p e r s t r u c t u r e s s o t s . I n s t e a d we have c l e a r l y e s t a b l i s h e d t h a t t h e s a t u r a t i o n coverage o f t h e R(B) phase i s 8 - 1 f r o m t h e AES condensation curves. Moreover we can u n d e r l i n e t h a t i f t h e v a l u e 0=4/3 was r e t a i n e d t h e model o f t h e r o t a t e d monolayer o f Pb would appear q u i t e u n r e a l i s t i c when i t has been shown t h a t t h e o r i e n t a t i o n o t t h e 3D l e a d c r y s t a l l i t e s i n e p i t a x y i s t h e p a r a l l e l one ( u n r o t a t e d ) w i t h <110> //<llO>Ge.
F i n a l l y we can a l s o i n d i c a t e t h a t i n such an h y p o t h e s i s one would e x p e c t f e b t h e r growth on t h i s phase t o proceed i n a l a y e r l i k e f a s h i o n / 21/ .
Fig. 4 - Normalized ( D ( T ) / D ( Z ~ O ~ C ) ) o p t i c a d e n s i t y measurements f r o m EED p a t t e r n photographs (40 V ) versus temperature. a) k 3 ( a ) s t r u c t u r g 8=1/3, b ) 3 ( ~ ) s t r u c t u r e 8=1 showing t h e t r a n s i t i o n t o t h e 1x1 s t r u c t u r e a t % 280 C.
i-
Indeed we do n o t c l a i m t o have g i v e n a d e f i n i t e p r o o f t h a t t h e t r a n s i t i o n i s n o t a m e l t i n g one, which would be more e x c i t i n g ! Yet we have t h e f e e l i n g t h a t t h i s i s
r a t h e r unprobable. N e v e r t h e l e s s we t h i n k t h a t t h e i n t e r e s t o f t h e s u b j e c t m e r i t s f u r t h e r developments and we a r e p e r f o r m i n g new experiments t o c l a r i f y t h e q u e s t i o n . 5 - OXIDATION OF THE LEAD MONOLAYER ON G e ( l l 1 )
We j u s t i n d i c a t e h e r e general t r e n d s as work i s s t i l l underway: a more d e t a i l e d p r e - s e n t a t i o n w i l l b e t h e s u b j e c t o f a f o r t h c o m i n g paper. A t f i x e d oxygen p r e s s u r e s
5x10-7 t o r r range)and temperatures (RT t o 3200C) we record t h e Auger s i g n a l s of oxygen (512 eV lead (94 eV) a d Ge (46 eV) a s a function of exposure time f o r d i f f e r e n t lead coverages ( ~ = o , ~ , ~ , l ) . 12 Our prel'minary r e s u l t s show a higher i n i t i a l oxygen uptake f o r t h e lead covered s u r f a c e (0- 1 ) than f o r t h e clean G e ( l l 1 ) one: t h e Pb peak i s markedly damped but t h e Ge one i s 3 l y s l i g h t l y a f f e c t e d ; which i n d i c a t e s a p r e f e r e n t i a l oxidation of t h e metal, w i t h most probably formation of Pb-0 bonds.
Moreover t h e oxygen adsorption r a t e decreases notably with i n c r e a s i n g l e a d coverage.
For comparison, t h e presence of a l k a l i metal a l s o r e s u l t s in an enhancement of t h e oxygen adsorption r a t e but a t variance with l e a d , t h e s e metals f a v o r t h e oxidation of t h e s u b s t r a t e i.e. t h e formation of Ge-0 bonds / 1 2 / . The decrease of t h e oxida- t i o n r a t e of lead f o r 1 Opb>l i s t o be r e l a t e d t o t h e two d i f f e r e n t regimes observed f o r t h e lead overlayer? on AJ and Cu s u b s t r a t e s /9,10 / : a high r a t e a t low Pb coverages (0<0.5) f a l l s r a p i d l y t o reach a constant low r a t e a t monolayer completion.
This two regimes were i n t e r p r e t e d a s i n d i c a t i n g a 2D growth of PbO upon i n t e r a c t i o n of oxygen with edge atoms of t h e 2D Pb i s l a n d s . The f a l l i n r e a c t i v i t y beyond @=0.5 possibly r e s u l t i n g from a decreasing proportion of edge atoms a s t h e i s l a n d s i z e i n c r e a s e s of from i s l a n g aggregation.
Thus these f i r s t r e s u l t s i n d i c a t e a behaviour towards oxidation of t h e lead mono- l a y e r on a germanium (111) s u b s t r a t e analogous t o t h a t on a m e t a l l i c s u b s t r a t e . 6 - CONCLUSION
Combining d i f f e r e n t experimental approaches f o r t h e c h a r a c t e r i z a t i o n of t h e
Pb/Ge(lll) i n t e r f a c e formation, we have obtained t h e following r e s u l t s : from RT t h e growth proceeds according t o a SK mode; i n t h e course of i t s completio (reachgd a t 0=1) t h e 2D intermediate l a y e r shows two s t r u c t u r e s i t h t h e same ~ 3 x b - R (30 ) u n i t
J-
mesh but very d i f f e r e n t spot i n t e n s i t i e s , these a r e 3 ( a ) s a t u r a t e d a 0=1L3 and
j"
$ 3 ( ~ ) s a t u r a t e s a t 0=1; t h i s l a s t s t r u c t u r e undergoes a r e v e r s i b l e 3 ( 8 ) + 1x1 two- dimensional phase transformation; we have discussed t h e nature of t h i s t r a n s i t i o n , i n c l i n i n g t o a 2D s o l i d 20 s o l i d t r a n s i t i o n r a t h e r than t o a 2D melting transforma- t i o n of t h e lead monolayer; f i n a l l y we have reported a few preliminary r e s u l t s which seem t o i n d i c a t e a behaviour of t h e lead monolayer adsorbed on G e ( l l 1 ) towards oxida- t i o n bearing strong analogy t o t h a t on m e t a l l i c s u b s t r a t e s .
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