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Chemical reaction at the Ge(111)-Ag and Si(111)-Ag interfaces for small Ag coverages
Surface Science 112 (1981) L765-L769 North-Holland Publishing Company
L765
SURFACE SCIENCE LETTERS CHEMICAL REACTION AT THE G e ( l l l ) - A g AND S i ( l l l ) - A g INTERFACES FOR SMALL Ag COVERAGES
G. ROSSI, I. ABBATI *, L. BRAICOVICH *, I. LINDAU and W.E. SPICER** Stanford Electronics Laborato~, Stanford University, Stanford, California 94305, USA Received 22 July 1981 ; accepted for publication 15 September 1981
We present a detailed synchrotron radiation photoemission study of the elemental semiconductor-Ag interfaces for very small coverages (up to 2.5 monolayers). Our results provide the first evidence of a chemical reaction and intermixing be:ween Ag and the (IV) semiconductors at room temperature.
Energy dependent photoemission spectroscopy using the Cooper minimum method [1] has been exploited to obtain information on the partial contributions to the total density of states at the Ag-semiconductor(iV) interface. We report the first evidence of a chemical reaction between Ag and the elemental semiconductors with characteristics similar to that for the semiconductor-d metal "reactive" interfaces. This result modify the current models for Agsemiconductor interfaces; in fact the Si-Ag junction has been described as sharp on the basis of previous photoelectron investigation with conventional light sources [2]. Earlier LEED studies had also supported a model with an atomically abrupt interface [3-5]. On the basis of the present results the Si-Ag interface at low coverages ( < 2.5 monolayers, ML) appears more similar to the interfaces between Si and the other noble metals (Cu and Au) where intermixing is present [6-9]. this new information is of paramount importance for the understanding of the semiconductor-Ag interface growth mechanism as well as for the general systematics of the semiconductor-noble metal junctions. In this connection it should be emphasized that most of the previous models for the Ag-Si interface, based on AES and LEED results, referred to a higher coverage situation ( > 5 ML) and to measurements at temperatures where agglomeration of Ag-rich islands take place according to the Stranski-Krastanov growth * Permanent address: Istituto di Fisica del Politecnico di Milano, 1-20133 Milano, Italy. ** Stanford W. Ascherman Professor of Engineering.
0039-6028/81/0000-0000/$02.75 © 1981 North-Holland
L766
G, Rossi et aL / Ge(lll)-Ag and S i ( l l l ) - A g interfaces
mechanism. Very recent LEED, ISS and surface potential measurements on interfaces annealed at 400°C by Oura et al. [10] indicate that Ag is incorporated in the Si(l 11) surface for submonolyer coverages and the observed Ag × v/3 LEED pattern represents a Ag reconstruction beneath the top layer which is mostly Si. Our results give a direct confirmation of the incorporation of Ag into Si. Angle integrated EDCs for the A g - G e and Ag-Si interfaces were obtained at h p = 130 eV where the cross section for photoionization of the Ag 4d electrons has a Cooper minimum [1]. The intensity profiles of the Ge 3d, Si 2p and Ag 3d core lines were measured as a function of coverage at the same photon energy. The experiments were made possible by the unique tunability of synchrotron radiation at SSRL and were carried out on the four-degree beam line with the experimental set-up described by Pianetta et al. [11]. The angle integrated energy distribution curves (EDCs) were taken with a double pass cylindrical mirror analyzer (CMA) in the retarding mode. The surface of the sample was normal to the CMA axis and the monochromatic light was impinging at grazing incidence (15°). Thin films of Ag were evaporated at a pressure of 2 × 1 0 - to Torr on UHV cleaved (111) surfaces. The measurements were carried out with an operative pressure of 7 × 10- ~ Torr. A detailed description of the two experiments on G e ( l l l ) / A g and Si(111)/Ag, including higher coverage will be presented elsewhere [12,13]. In this letter we will present the evolution of the valence band EDCs at the very beginning of the interface growth and the relative core level emission intensities during this initial growth. One main conclusion from our earlier work on the Si-d metal intermixed system is that the sp 3 hybridization of Si is broken upon bond formation with the metal [1]. As a consequence of breaking the tetrahedral coordination, the deepest valence structure of Si is substituted with a structure a slightly lower binding energy. This structure is basically derived, from the s electron of Si, and has a non-bonding character [1,14]. On the basis of this well-established behavior the interpretation of the photoemission data of figs. 1 and 2 (valence band at h u = 130 eV) becomes very informative. In fact the deepest lying valence band structure (at 11.0 eV for Ge and 11.2 eV for Si) begins to shift towards lower binding energy for both Si and Ge cases at ~ 0.3 ML. In the meantime the 4d peak of Ag is still very narrow and shifted towards higher binding energies with respect to the pure metal indicating that the ~tdatomadatom interaction is small for the metal. The shift of the semiconductor structure provides evidence for the modification of the sp3 configuration in Si and Ge due to the formation of hybrid bonds with the d electrons of Ag: this behavior is remarkably similar to what has been seen in reactive Si-d metal interfaces and is the key result of this letter. The amount of this shift is about 1 eV for Si-Pd [15] and Si-Pt [16] with some minor differences ( ~ 0 . 2 eV) between the different interfaces probably due to the strength of the new bond and the morphology of the interface. In the present cases the shift is - 0 . 6 eV
G. Rossi et al. / Ge(lll)-Ag and Si(lll)-,4g interfaces
L767
%
hu = 130 eV Ge(111)+Ag, RT
A [ /
I I
O.72/
!'
ik..
0 32 ~ ~ ' / ~ I
I
I
I
-13-12-I1-10
/~,_11
I
I
I
i
I
I I
3-11-9-7-5-3-1 EF eV
Fig. I. Angle integrated photoelectron energy distribution curves (EDCs) at h u = 130 eV, Cooper minimum for the Ag 4d electrons, for G e ( l l l ) - A g interfaces at increasing coverage # (in monolayer units). The expansion shows the energy shift of the "s" structure of Ge due to the breaking of the sp3 coordination.
for Si-Ag and - 0.3 eV for G e - A g . The smaller shift observed for Ge may be due to the smaller strength of the G e - G e covalent bond compared to the Si-Si bond. This is the first time that EDCs have been taken at the Cooper minimum for a semiconductor-noble metal interface, and the first time a G e - d metal interface has been studied, so there are no data for comparison. ,%
I
hu= 13OeV
/~ ~ ' I
~
' '-,'~
'
Si(111)+Ag, RT
~
1.2
i
' -,'o ~
",;
~_//
\_'2~\
. . . . -; . . . . ~'~
eV Fig. 2. EDCs at the Cooper minimum for Si(l I 1)-Ag interfaces at increasing 0 (monolayer units). The expansion shows the energy shift of the "s" structure of Si due to the breaking of the sp3 coordination.
G. Rossi et al. / Ge(lll)-Ag and Si(lll)-Ag interfaces
L768
The new energy Position of the 3s level of the semiconductor substrate is maintained after the formation of the first monolayer, and persists for higher coverages. The signal is quickly lost above 3 monolayers of coverage due to the scarce intermixing and is followed by the growth of an Ag-rich phase on top of the interface when more Ag is deposited [13]. The study of the morphology of this growth at high coverages is beyond the purpose of the present work and is not discussed here. The results discussed above and presented in figs. 1 and 2 provide the evidence of chemical interaction but are not sufficient to clarify another relevant point, namely if the Ag remains on top of Si (Ge) or is incorporated within the surface. This point can be clarified by analyzing the core line intensities versus coverage. The increase of the Ag intensity as a function of coverage is particularly relevant and is shown for Ge in fig. 3 and for Si in fig. 4. In both cases the intensity increases linearly at low 0 but when the exposure is doubled, the Ag signal is multiplied by a smaller factor, indicating that some Ag is lost ( ~ 30%) due to incorporation into the surface. We can exclude an agglomeration of Ag for two reasons:(i) At very low coverages it is impossible to form adatoms clusters having dimensions such that the Ag signal is reduced due to an escape effect [17];(ii) the deep structure of Si (and of Ge) should still be seen due to the great fraction of uncovered surface if Ag agglomeration occurred.
1OO% ~ \, , ~
\ ~
80%
~
\. ~.,
6o%
CORE LINES INTENSITY PROFILES VERSUS COVERAGE
s~2p . . . . Ge3d
------
"~
REDUCED
l 3I-
,NTENS,TY PROFILES A9 ON G e - - - - - -
/
~ 2 oi2s
3 4 s o'.7~ 112
Re
2.s
e (MLI
Fig. 3. Core line intensity profiles versus Ag coverage (0) for Si 2p (dash-dotted line) and Ge 3d (dashed line); and Ag 3d reduced intensity profile (RI) versus reduced coverages (R8=0/0.25 ML) for the submonolayer interval. Ag on Si (I 1 I) dash-dotted line, Ag on Ge(I I I) dashed line; the solid line is the result expected in absence of intermixing at the Ag-semiconductor interfaces.
G. Rossi et al. / Ge(111)-Ag and Si(111)-Ag interfaces
L769
T h e a t t e n u a t i o n curves for Si 2p a n d G e 3d core line emissions p l o t t e d in the u p p e r p a r t o f fig. 3, is also in q u a n t i t a t i v e a g r e e m e n t with a m o d e l where A g is i n c o r p o r a t e d into the substrate. O u r e x p e r i m e n t a l results thus rule o u t all the m o d e l s b a s e d o n a a t o m i c a l l y a b r u p t interface a n d the suggestion o f o n - t o p sites for the A g atoms. I n o r d e r to e x p l a i n the loss o f A g signal it is necessary to a s s u m e that at least the first two layers of the s u b s t r a t e are i n t e r m i x e d with Ag. I n conclusion, we have p r e s e n t e d the first evidence o f a chemical r e a c t i o n b e t w e e n A g a n d the elemental s e m i c o n d u c t o r s that b r e a k s the t e t r a h e d r a l c o o r d i n a t i o n o f the semicon.ductor atoms. T h e core line analysis i n d i c a t e s that the r e a c t i o n is a c c o m p a n i e d b y i n t e r m i x i n g a n d a g r a d e d interface involving at least two layers o f m a t e r i a l is formed. Q u a l i t a t i v e l y the two i n v e s t i g a t e d systems b e h a v e in the s a m e w a y a n d the discussion has been c a r r i e d out in parallel, also if s o m e q u a n t i t a t i v e differences have b e e n noted, a n d m o r e will b e discussed in f o r t h c o m i n g p a p e r s [12,13]. This w o r k was s u p p o r t e d b y the A d v a n c e d R e s e a r c h Projects A g e n c y of the D e p a r t m e n t o f D e f e n s e u n d e r C o n t r a c t No. N00014-79-C-007s, a n d b y the G N S M of C N R , Italy. T h e e x p e r i m e n t s were p e r f o r m e d at the S t a n f o r d S y n c h r o t r o n R a d i a t i o n L a b o r a t o r y which is s u p p o r t e d b y the N a t i o n a l Science F o u n d a t i o n u n d e r G r a n t No. D M R 77-27489 in c o l l a b o r a t i o n with the S t a n f o r d L i n e a r A c c e l e r a tor C e n t e r a n d the D e p a r t m e n t o f Energy.
References [1] [2] [3] [4] [5] [6]
G. Rossi, I. Abbati, L. Braicovich, I. Lindau and W.E. Spicer, Solid State Commun., in press. A. McKinley, R.H. Williams and A.W. Parke, J. Phys. C (Solid State Phys.) 12 (1979) 2447. J. Derrien, G. LeLay and F. Salvan, J. Physique Lettres 39 (1978) L287. Y. Gotoh and S. Ino, Japan. J. Appl. Phys. 17 (1978) 2097. G. LeLay, M. Manneville and R. Kern, Surface Sci. 72 (1978) 405. L. Braicovich, C.M. Garner, P.R. Skeath, C.Y. Su, P.W. Chue, I. Lindau and W.E. Spicer, Phys. Rev. B20 (1979) 512 I. [7] I. Abbati,.L. Braicovich and A. Franciosi, Phys. LettersSOA (1980) 69. [8] I. Abbati and M. Grioni, J. Vacuum Sci. Technol. 19 (1981) No. 4. [9] P. Perfetti, S. Nannarone, F. Patella, C. Quaresima, A. Savoia, F. Cerrina and M. Capozi, Solid State Commun. 35 (1980) 151. [10] K. Oura, T. Taminaga and T. Hanawa, Solid State Commun. 37 (1981) 523; M. Saitoh, F. Shoji, Koura and T. Hanawa, Japan. J. Appl. Phys. 19 (1980) L421. [I I] P. Pianetta, I. Lindau and W.E. Spicer, in: Quantitative Surface Analysis of Materials, A Symposium Sponsored by ASTM Committee E-42 on Surface Analysis, Ed. N.S. Mclntyre, Cleveland, Ohio, 1977, ASTM Special Technical Publ. 643 (1978) pp. 105-123. [12] G. Rossi. I. Abbati, I. Lindau and W.E. Spicer, Appl. Surface Sci., to be published. [13] G. Rossi, I. Abbati, L. Braicovich, I. Lindau and W.E. Spicer, to be published. [14] G.W. Rubloff, P.S. Ho, J.F. Freeouf and J.E. Lewis, Phys. Rev. B23 (1981) 4183. [15] I. Abbati, G. Rossi, I. Lindau and W.E. Spicer, J. Vacuum Sci. Technol. 19 (1981) No. 4. [16] G. Rossi, I. Abbati, L. BraicovicJa,I. Lindau and W.E. Spicer, to be published, and abstract in 41st Phys. Electronics Conf. Program. [17] D.A. Venables, J. Derrien and A.P. Janssen, Surface Sci. 95 (1980) 41 I.
L765
SURFACE SCIENCE LETTERS CHEMICAL REACTION AT THE G e ( l l l ) - A g AND S i ( l l l ) - A g INTERFACES FOR SMALL Ag COVERAGES
G. ROSSI, I. ABBATI *, L. BRAICOVICH *, I. LINDAU and W.E. SPICER** Stanford Electronics Laborato~, Stanford University, Stanford, California 94305, USA Received 22 July 1981 ; accepted for publication 15 September 1981
We present a detailed synchrotron radiation photoemission study of the elemental semiconductor-Ag interfaces for very small coverages (up to 2.5 monolayers). Our results provide the first evidence of a chemical reaction and intermixing be:ween Ag and the (IV) semiconductors at room temperature.
Energy dependent photoemission spectroscopy using the Cooper minimum method [1] has been exploited to obtain information on the partial contributions to the total density of states at the Ag-semiconductor(iV) interface. We report the first evidence of a chemical reaction between Ag and the elemental semiconductors with characteristics similar to that for the semiconductor-d metal "reactive" interfaces. This result modify the current models for Agsemiconductor interfaces; in fact the Si-Ag junction has been described as sharp on the basis of previous photoelectron investigation with conventional light sources [2]. Earlier LEED studies had also supported a model with an atomically abrupt interface [3-5]. On the basis of the present results the Si-Ag interface at low coverages ( < 2.5 monolayers, ML) appears more similar to the interfaces between Si and the other noble metals (Cu and Au) where intermixing is present [6-9]. this new information is of paramount importance for the understanding of the semiconductor-Ag interface growth mechanism as well as for the general systematics of the semiconductor-noble metal junctions. In this connection it should be emphasized that most of the previous models for the Ag-Si interface, based on AES and LEED results, referred to a higher coverage situation ( > 5 ML) and to measurements at temperatures where agglomeration of Ag-rich islands take place according to the Stranski-Krastanov growth * Permanent address: Istituto di Fisica del Politecnico di Milano, 1-20133 Milano, Italy. ** Stanford W. Ascherman Professor of Engineering.
0039-6028/81/0000-0000/$02.75 © 1981 North-Holland
L766
G, Rossi et aL / Ge(lll)-Ag and S i ( l l l ) - A g interfaces
mechanism. Very recent LEED, ISS and surface potential measurements on interfaces annealed at 400°C by Oura et al. [10] indicate that Ag is incorporated in the Si(l 11) surface for submonolyer coverages and the observed Ag × v/3 LEED pattern represents a Ag reconstruction beneath the top layer which is mostly Si. Our results give a direct confirmation of the incorporation of Ag into Si. Angle integrated EDCs for the A g - G e and Ag-Si interfaces were obtained at h p = 130 eV where the cross section for photoionization of the Ag 4d electrons has a Cooper minimum [1]. The intensity profiles of the Ge 3d, Si 2p and Ag 3d core lines were measured as a function of coverage at the same photon energy. The experiments were made possible by the unique tunability of synchrotron radiation at SSRL and were carried out on the four-degree beam line with the experimental set-up described by Pianetta et al. [11]. The angle integrated energy distribution curves (EDCs) were taken with a double pass cylindrical mirror analyzer (CMA) in the retarding mode. The surface of the sample was normal to the CMA axis and the monochromatic light was impinging at grazing incidence (15°). Thin films of Ag were evaporated at a pressure of 2 × 1 0 - to Torr on UHV cleaved (111) surfaces. The measurements were carried out with an operative pressure of 7 × 10- ~ Torr. A detailed description of the two experiments on G e ( l l l ) / A g and Si(111)/Ag, including higher coverage will be presented elsewhere [12,13]. In this letter we will present the evolution of the valence band EDCs at the very beginning of the interface growth and the relative core level emission intensities during this initial growth. One main conclusion from our earlier work on the Si-d metal intermixed system is that the sp 3 hybridization of Si is broken upon bond formation with the metal [1]. As a consequence of breaking the tetrahedral coordination, the deepest valence structure of Si is substituted with a structure a slightly lower binding energy. This structure is basically derived, from the s electron of Si, and has a non-bonding character [1,14]. On the basis of this well-established behavior the interpretation of the photoemission data of figs. 1 and 2 (valence band at h u = 130 eV) becomes very informative. In fact the deepest lying valence band structure (at 11.0 eV for Ge and 11.2 eV for Si) begins to shift towards lower binding energy for both Si and Ge cases at ~ 0.3 ML. In the meantime the 4d peak of Ag is still very narrow and shifted towards higher binding energies with respect to the pure metal indicating that the ~tdatomadatom interaction is small for the metal. The shift of the semiconductor structure provides evidence for the modification of the sp3 configuration in Si and Ge due to the formation of hybrid bonds with the d electrons of Ag: this behavior is remarkably similar to what has been seen in reactive Si-d metal interfaces and is the key result of this letter. The amount of this shift is about 1 eV for Si-Pd [15] and Si-Pt [16] with some minor differences ( ~ 0 . 2 eV) between the different interfaces probably due to the strength of the new bond and the morphology of the interface. In the present cases the shift is - 0 . 6 eV
G. Rossi et al. / Ge(lll)-Ag and Si(lll)-,4g interfaces
L767
%
hu = 130 eV Ge(111)+Ag, RT
A [ /
I I
O.72/
!'
ik..
0 32 ~ ~ ' / ~ I
I
I
I
-13-12-I1-10
/~,_11
I
I
I
i
I
I I
3-11-9-7-5-3-1 EF eV
Fig. I. Angle integrated photoelectron energy distribution curves (EDCs) at h u = 130 eV, Cooper minimum for the Ag 4d electrons, for G e ( l l l ) - A g interfaces at increasing coverage # (in monolayer units). The expansion shows the energy shift of the "s" structure of Ge due to the breaking of the sp3 coordination.
for Si-Ag and - 0.3 eV for G e - A g . The smaller shift observed for Ge may be due to the smaller strength of the G e - G e covalent bond compared to the Si-Si bond. This is the first time that EDCs have been taken at the Cooper minimum for a semiconductor-noble metal interface, and the first time a G e - d metal interface has been studied, so there are no data for comparison. ,%
I
hu= 13OeV
/~ ~ ' I
~
' '-,'~
'
Si(111)+Ag, RT
~
1.2
i
' -,'o ~
",;
~_//
\_'2~\
. . . . -; . . . . ~'~
eV Fig. 2. EDCs at the Cooper minimum for Si(l I 1)-Ag interfaces at increasing 0 (monolayer units). The expansion shows the energy shift of the "s" structure of Si due to the breaking of the sp3 coordination.
G. Rossi et al. / Ge(lll)-Ag and Si(lll)-Ag interfaces
L768
The new energy Position of the 3s level of the semiconductor substrate is maintained after the formation of the first monolayer, and persists for higher coverages. The signal is quickly lost above 3 monolayers of coverage due to the scarce intermixing and is followed by the growth of an Ag-rich phase on top of the interface when more Ag is deposited [13]. The study of the morphology of this growth at high coverages is beyond the purpose of the present work and is not discussed here. The results discussed above and presented in figs. 1 and 2 provide the evidence of chemical interaction but are not sufficient to clarify another relevant point, namely if the Ag remains on top of Si (Ge) or is incorporated within the surface. This point can be clarified by analyzing the core line intensities versus coverage. The increase of the Ag intensity as a function of coverage is particularly relevant and is shown for Ge in fig. 3 and for Si in fig. 4. In both cases the intensity increases linearly at low 0 but when the exposure is doubled, the Ag signal is multiplied by a smaller factor, indicating that some Ag is lost ( ~ 30%) due to incorporation into the surface. We can exclude an agglomeration of Ag for two reasons:(i) At very low coverages it is impossible to form adatoms clusters having dimensions such that the Ag signal is reduced due to an escape effect [17];(ii) the deep structure of Si (and of Ge) should still be seen due to the great fraction of uncovered surface if Ag agglomeration occurred.
1OO% ~ \, , ~
\ ~
80%
~
\. ~.,
6o%
CORE LINES INTENSITY PROFILES VERSUS COVERAGE
s~2p . . . . Ge3d
------
"~
REDUCED
l 3I-
,NTENS,TY PROFILES A9 ON G e - - - - - -
/
~ 2 oi2s
3 4 s o'.7~ 112
Re
2.s
e (MLI
Fig. 3. Core line intensity profiles versus Ag coverage (0) for Si 2p (dash-dotted line) and Ge 3d (dashed line); and Ag 3d reduced intensity profile (RI) versus reduced coverages (R8=0/0.25 ML) for the submonolayer interval. Ag on Si (I 1 I) dash-dotted line, Ag on Ge(I I I) dashed line; the solid line is the result expected in absence of intermixing at the Ag-semiconductor interfaces.
G. Rossi et al. / Ge(111)-Ag and Si(111)-Ag interfaces
L769
T h e a t t e n u a t i o n curves for Si 2p a n d G e 3d core line emissions p l o t t e d in the u p p e r p a r t o f fig. 3, is also in q u a n t i t a t i v e a g r e e m e n t with a m o d e l where A g is i n c o r p o r a t e d into the substrate. O u r e x p e r i m e n t a l results thus rule o u t all the m o d e l s b a s e d o n a a t o m i c a l l y a b r u p t interface a n d the suggestion o f o n - t o p sites for the A g atoms. I n o r d e r to e x p l a i n the loss o f A g signal it is necessary to a s s u m e that at least the first two layers of the s u b s t r a t e are i n t e r m i x e d with Ag. I n conclusion, we have p r e s e n t e d the first evidence o f a chemical r e a c t i o n b e t w e e n A g a n d the elemental s e m i c o n d u c t o r s that b r e a k s the t e t r a h e d r a l c o o r d i n a t i o n o f the semicon.ductor atoms. T h e core line analysis i n d i c a t e s that the r e a c t i o n is a c c o m p a n i e d b y i n t e r m i x i n g a n d a g r a d e d interface involving at least two layers o f m a t e r i a l is formed. Q u a l i t a t i v e l y the two i n v e s t i g a t e d systems b e h a v e in the s a m e w a y a n d the discussion has been c a r r i e d out in parallel, also if s o m e q u a n t i t a t i v e differences have b e e n noted, a n d m o r e will b e discussed in f o r t h c o m i n g p a p e r s [12,13]. This w o r k was s u p p o r t e d b y the A d v a n c e d R e s e a r c h Projects A g e n c y of the D e p a r t m e n t o f D e f e n s e u n d e r C o n t r a c t No. N00014-79-C-007s, a n d b y the G N S M of C N R , Italy. T h e e x p e r i m e n t s were p e r f o r m e d at the S t a n f o r d S y n c h r o t r o n R a d i a t i o n L a b o r a t o r y which is s u p p o r t e d b y the N a t i o n a l Science F o u n d a t i o n u n d e r G r a n t No. D M R 77-27489 in c o l l a b o r a t i o n with the S t a n f o r d L i n e a r A c c e l e r a tor C e n t e r a n d the D e p a r t m e n t o f Energy.
References [1] [2] [3] [4] [5] [6]
G. Rossi, I. Abbati, L. Braicovich, I. Lindau and W.E. Spicer, Solid State Commun., in press. A. McKinley, R.H. Williams and A.W. Parke, J. Phys. C (Solid State Phys.) 12 (1979) 2447. J. Derrien, G. LeLay and F. Salvan, J. Physique Lettres 39 (1978) L287. Y. Gotoh and S. Ino, Japan. J. Appl. Phys. 17 (1978) 2097. G. LeLay, M. Manneville and R. Kern, Surface Sci. 72 (1978) 405. L. Braicovich, C.M. Garner, P.R. Skeath, C.Y. Su, P.W. Chue, I. Lindau and W.E. Spicer, Phys. Rev. B20 (1979) 512 I. [7] I. Abbati,.L. Braicovich and A. Franciosi, Phys. LettersSOA (1980) 69. [8] I. Abbati and M. Grioni, J. Vacuum Sci. Technol. 19 (1981) No. 4. [9] P. Perfetti, S. Nannarone, F. Patella, C. Quaresima, A. Savoia, F. Cerrina and M. Capozi, Solid State Commun. 35 (1980) 151. [10] K. Oura, T. Taminaga and T. Hanawa, Solid State Commun. 37 (1981) 523; M. Saitoh, F. Shoji, Koura and T. Hanawa, Japan. J. Appl. Phys. 19 (1980) L421. [I I] P. Pianetta, I. Lindau and W.E. Spicer, in: Quantitative Surface Analysis of Materials, A Symposium Sponsored by ASTM Committee E-42 on Surface Analysis, Ed. N.S. Mclntyre, Cleveland, Ohio, 1977, ASTM Special Technical Publ. 643 (1978) pp. 105-123. [12] G. Rossi. I. Abbati, I. Lindau and W.E. Spicer, Appl. Surface Sci., to be published. [13] G. Rossi, I. Abbati, L. Braicovich, I. Lindau and W.E. Spicer, to be published. [14] G.W. Rubloff, P.S. Ho, J.F. Freeouf and J.E. Lewis, Phys. Rev. B23 (1981) 4183. [15] I. Abbati, G. Rossi, I. Lindau and W.E. Spicer, J. Vacuum Sci. Technol. 19 (1981) No. 4. [16] G. Rossi, I. Abbati, L. BraicovicJa,I. Lindau and W.E. Spicer, to be published, and abstract in 41st Phys. Electronics Conf. Program. [17] D.A. Venables, J. Derrien and A.P. Janssen, Surface Sci. 95 (1980) 41 I.