Rotated incommensurate domains of Co ultrathin films on Pt(111)

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surface science

ELSEVIER

Surface Science 396 (1998) 319-326

Rotated incommensurate domains of Co ultrathin films on Pt (111 ) J.S. Tsay, C.S. S h e r n * Department of Physics, National Taiwan Normal University, 88, Sec. 4, Ting-Chou Road, Taipei, 117, Taiwan

Received 12 May 1997; accepted for publication 21 August 1997

Abstract

Low energy electron diffraction (LEED) and Auger electron spectroscopy were used to study the initial growth of cobalt ultrathin films on a Pt(111 ) surface at room and low temperatures. The films show incoherent epitaxy at room temperature. Unrotated and rotated incommensurate Co domains with two equivalent angles of rotation, +4.9 ° and -4.9 °, with respect tO the aligned substrate, are observed by LEED for 2 monolayers of Co/Pt( 111 ) after applying an annealing treatment. From a calculation of the mismatch for the corrugated surface, we confirm this reorientation angle. The evolution of the LEED satellite pattern for the deposition at 140 K is the same as for the deposition at room temperature, but a faint (2 x 2) LEED pattern was observed for higher coverages. UV photoelectron spectroscopy was used to monitor the evolution of the density of electronic states during deposition. © 1998 Elsevier Science B.V. Keywords." Auger electron spectroscopy; Cobalt; Low energy electron diffraction; Metallic films; Metal-metal nonmagnetic thin film

structures; Platinum; UV photoelectron spectroscopy

1. Introduction

T h e basic u n d e r s t a n d i n g o f early stages o f the g r o w t h o f a m e t a l film o n a m e t a l s u b s t r a t e has been o f c o n t i n u i n g interest for m a n y years. T h e interfacial p r o p e r t i e s o f u l t r a t h i n h e t e r o e p i t a x i a l films are o f f u n d a m e n t a l a n d p r a c t i c a l i m p o r t a n c e in surface science a n d m a t e r i a l s research [1-3]. This system has b e e n selected for a n u m b e r o f reasons. F r o m the v i e w p o i n t o f crystal g r o w t h , C o / P t ( l l l ) is e x p e c t e d to s h o w s o m e interesting p h e n o m e n a b e c a u s e the m i s m a t c h is r a t h e r large ( a b o u t 10%). I n this situation, i n h o m o g e n e o u s strain a n d a r o t a t e d p h a s e m a y a p p e a r at the * Corresponding author. Tel. : (+ 886) 2 934 6620; fax: (+ 886) 2 932 6408; e-mail: [email protected] 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0039-6028 (97) 00691-2

interface. I n a d d i t i o n , Pt is a very g o o d c a t a l y s t for o x i d a t i o n [4]. C o - b a s e d alloys are used extensively for m a g n e t i c recording. Recently, m u c h attention h a s been d e v o t e d to metallic u l t r a t h i n films, sandwiches a n d m u l t i l a y e r s o f the C o - P t system o w i n g to their p a r t i c u l a r m a g n e t i c p r o p e r t i e s [5-7]. F o r e x a m p l e , p o l y c r y s t a l l i n e CoPt3 alloy films h a v e been used in m a g n e t i c - o p t i c a l r e c o r d ings [8]. V a r i o u s k i n d s o f o r d e r e d C o a n d Pt alloys, such as Co3Pt, C o P t a n d CoPt3, also exist [9,10]. So the system c o n t a i n i n g Pt a n d C o is especially a p p e a l i n g t o us. W e u s e d A u g e r electron s p e c t r o s c o p y ( A E S ) a n d low energy electron diffraction ( L E E D ) to d e t e r m i n e the c o v e r a g e a n d to s t u d y the initial g r o w t h o f the C o / P t ( l l l ) system. T h e r o t a t e d i n c o m m e n s u r a t e C o d o m a i n s were o b s e r v e d after

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annealing treatment. They are verified by a calculation based on the consideration of lattice mismatch. This interesting structure was first discovered by us in this system.

2. Experiment Experiments were conducted in a stainless steel ultrahigh vacuum ( U H V ) chamber. The base pressure was better than 3 x 10 -~° Torr. The residual gases were checked using a quadrupole mass spectrometer. The U H V chamber was also equipped for AES, four-grid video L E E D and ultraviolet photoelectron spectroscopy (UPS). The electron energy analyzer for AES and UPS was the C L A M - 2 hemispherical analyzer of V G M I C R O T E C H . The crystal was oriented within 0.5 ° of the [ 111 ] direction checked by X-ray reflection. The Pt(111) substrate was cleaned by cycles of argon ion b o m b a r d m e n t and annealing, until sharp diffraction spots with low background were observed by LEED. Before sputtering, the sample was heated at 880 K in an oxygen environment at a pressure of 5 x 10 .7 Torr for about 3 min to remove the residual carbon. The cleanness of the surface was checked by AES. A Co coil 0.5 m m in diameter and of 99.997% purity was used to evaporate cobalt atoms. The background pressure was about 1 x 10 .9 Torr during Co evaporation. Co was evaporated with a slow rate about 1 monolayer ( M L ) in 840 s. The instrumentation and sample preparation processes of this experiment are described in detail elsewhere [11,12].

energy of 88 eV. Where Ak± is the m o m e n t u m transfer perpendicular to the surface and h = 2.05 is the step height of the adsorbed Co atoms. It is an out-of-phase condition of diffraction which is more sensitive to the surface step density and can oscillate in a layer-by-layer growth [13]. The profile of the L E E D peak-height intensity versus deposition time (I-t profile) of the (0, 0) beam at r o o m temperature is shown in Fig. 1. The intensity is in oscillation. Two peaks are located at t = 840 s and 1680 s. Since the specular beam is a direct measurement of Ak±, it is sensitive to the surface step formation. The m a x i m u m of the intensity means that one monolayer grows completely. Hence 1 M L occurs at 840s and 2 M L occurs at 1680 s. Scanning tunneling microscopy measurements by Grtitter and Diirig [14] showed that Co grows quasi-layer-by-layer up to a coverage of 3 M L at r o o m temperature. For coverage greater than 3 ML, a three-dimensional island growth mode is observed. AES measurements followed by calibration of the calculated inelastic mean free path of Auger electrons by Thiele et al. [15] showed that Co grows layer-by-layer to at least 2 ML. Because of these results we decided to observe the structural

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The evolution of the L E E D peak-height intensity of the specular beam and the Auger signal vs. deposition time ( A S - t ) plot can be used to determine the coverage of Co overlayer. Four-grid video L E E D with a computer-controlled image process was used to scan the peak-height intensity of a diffracted spot as a function of deposition time. The phase of L E E D i n o u r experimental conditions is (a(O,O)=Akxh=3n for the incident electron

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Fig. 1. Peak-height intensity of the specular beam of LEED as a function of deposition time (I-t profile). A beam energy of 88 eV was used. The intensity is in oscillation. Two peaks are located at t = 840 s and 1680 s. The depositing time corresponding to 1 ML is 840 s.

J.S. Tsay, C.S. Shern / Surface Science 396 (1998) 319-326 evolution with temperature after depositing 2 M L o f C o on the Pt( 111 ) surface. We d o u b l y checked the thickness o f the C o overlayer by the evolutions o f the C o 775 eV and Pt 237 eV A u g e r signals at r o o m temperature. The A S - t plot, or A u g e r u p t a k e curve, is shown in Fig. 2. The A u g e r intensity o f C o 775 eV increases linearly while the intensity o f Pt 237 eV A u g e r signal decreases linearly at the beginning. The change in the slope o f the A u g e r u p t a k e curve is interpreted as a complete g r o w t h o f each layer on a flat surface. Fig. 2 also shows that C o atoms g r o w 1 M L in a r o u n d 840 s and 2 M L in a r o u n d 1680 s. Because the A S - t plot is n o t very reliable for determining overlayer g r o w t h mode, the plot o f Fig. 2 is just the c o m p l e m e n t for that for L E E D . In fact, the g r o w t h o f C o / P t superlattices on epitaxial A g films on G a A s substrates has been studied by X - r a y photoelectron diffraction. The a u t h o r concluded that the C o film h a d mixed with Pt during g r o w t h at 373 K [16]. This m a y be the source o f the uncertainty in our A u g e r signals. A series o f L E E D patterns were observed at different Co coverages at r o o m temperature. These patterns can be clearly visible as the incident energy o f electrons is higher t h a n 56 eV. The well-

321

ordered h e x a g o n a l L E E D pattern o f the clean P t ( 1 1 1 ) surface shows that the surface is threefold symmetric. D u r i n g deposition o f Co onto the Pt(lll) surface, the L E E D pattern gradually changes to a sixfold symmetry. W h e n the coverage is over 1 M L , the L E E D pattern shows a m a r k e d change. E a c h integer spot o f the substrate develops

(a)

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1500 2000 T i m e (See.)

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Fig. 2. The Auger uptake curve, x represents the intensity of Co 775 eV Auger signal. * represents the intensity of Pt 237 eV Auger signal. The linear part is interpreted as a monolayer-bymonolayer growth. The break at t = 840 s corresponds to 1 ML of Co coverage.

(c) Fig. 3. The LEED patterns with a fine structure for different Co coverages are presented: (a) 2 ML at room temperature; (b) 2 ML after annealing to 650 K; (c) 5 ML grown at 140 K.

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J.S. Tsay, C.S. Shern / Surface Science 396 (1998) 319-326

a surrounding fine structure. The complex structures for L E E D are shown in Fig. 3. The incommensurate L E E D pattern for coverage of 2 M L is presented in Fig. 3a. (Fig. 3b and c will be discussed later.) The intensity of these satellites increases gradually and the higher order satellites develop as the coverage increases. The intensity of the fine structure reaches a m a x i m u m with a low background as the coverage of Co is 2 ML. The background of these satellites increases at higher coverages. These fine structures disappear and the L E E D pattern returns to its original sixfold symmetry with a strong background after the coverage exceeds 9 ML. The separation in k-space of the nth order of the fine structure from the original integer beam can be described by a misfit vector n(aco apt*) [6,15]. These reciprocal lattice vectors apt = 2~/aet, aco=2rc/aco are equal to 2 . 2 7 A -1 and 2 . 5 0 A -1 respectively ( a m = 2 . 7 7 A and aco = 2.51 A). The incommensurate L E E D pattern indicates incoherent epitaxy of Co overlayer on the Pt(111) surface. The geometrical structure of the L E E D satellites at the coverage of 2 M L is shown in Fig. 4. These satellites can be divided into two sets. One set surrounding (0, 1), ( 1 , - 1 ) and ( - 1, 0) looks like a honeycomb. Each spot of the satellites has other hexagonal satellites surrounding it. Another set surrounding (1,0), ( 0 , - 1 ) and ( - 1 , 1) shows a sixfold hexagonal structure. A similar fine structure of L E E D satellites has also been found by other authors in the C o / P t ( l l l ) system [6,15], and in some other thin film systems with a large lattice mismatch, such as ad/W(ll0 [17], P t / N i ( l l l ) [18] and FeO/ P t ( l l l ) [19,20]. After annealing 2 M L C o / P t ( l l l ) at 650 K for 4 rain followed by cooling to r o o m temperature, we found an interesting phenomenon. The satellite rotates about its original integer beam as shown in Fig. 3(b). Fig. 5(a) is a redrawn schematic diagram showing the new structure of satellites after this annealing treatment. The angle of 4.9 ° is obtained from experimental measurement. The surface structure forming these new hexagonal satellites can be explained by the existence of unrotated (the left part of Fig. 5(b)) and rotated (the right part of Fig. 5(b)) incommensurate Co -

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[1,-1] Fig. 4. The schematic LEED pattern for Co/Pt( 111) at the coverage of 2 ML. There are satellites around each integer LEED spot. They can be divided into two sets. The lighter the colour of the spot, the higher was the intensity of the observed LEED spot. The distance in k-space is indicated. domains of the top layer [ 18 ]. The rotated domains can have two equivalent angles of rotation, + 4.9 ° and - 4 . 9 ° with respect to the substrate. The rotated L E E D pattern can be clearly observed at temperatures between 630 K and 750 K. It persists for a longer period when the temperature is lower within this temperature range. But the L E E D pattern becomes a fuzzy (1 x 1) structure if the annealing lasts too long or the annealing temperature is higher than 780 K. Gtitter and Dtirig studied the alloy formation of Co/Pt(111) at 750 K for the coverage of 2 ML. The diffusion is very fast and the top few layers are almost pure Pt after 10 rain [14]. Thiele et al. showed that the structure of the incoherent epitaxial layer of C o / P t ( l l l ) is maintained and no interdiffusion takes place at T < 525 K. Annealing 4 M L of C o / P t ( l l l ) at 675 K for 75 min results in the formation of a Pt25Co75 surface alloy [15]. F r o m these results, the ultrathin film of 2 M L Co/Pt( 111 ) annealed at 650 K has been developing to form an alloy at the interface. We also checked using AES and showed that the intermixture of Co and Pt occurs under this condition. Since a

J.S. Tsay, C.S. Shern / Surface Science 396 (1998) 319-326

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Pb/Ag(111 ) system [21 ]. To confirm that the reorientation angle is equal to 4.9 ° for this system, we used the hard sphere model to calculate the mismatch for Co(0001) lattice points on the P t ( l l l ) surface at different angles in a real space• We chose the original point of the coordinate located on a hollow site on P t ( l l l ) . An x-axis is fixed a l o n g the direction of [-1,1,0] of the P t ( l l l ) surface and an r-axis is set along a straight row of Co adatoms in a rotated Co domain as shown in Fig. 5(b). Then the reorientation angle 0 is defined as the angle between the fixed x-axis and the raxis. For example, 0 is zero for the left part and is 4.9 ° for the right part in Fig. 5(b). A point P is confined along the nearest-neighbor row of the xaxis, i.e. the position of P is 2.77(34+ 1/2, ~ / 2 ) , where M is the number of hollow sites projected on the x-axis of OP. There are N Co atoms arranged along the r-axis. The Nth Co atom on the r-axis will be close to another hollow site on the substrate after the Co domain rotates an angle of 0. The location of the Nth Co a t o m is Q--2.51N(cos 0, sin 0). The distance d between points P and Q is:

I-1,1,01

Fig. 5. (a) The schematic LEED pattern (as shown in Fig. 3(b)) of 2 ML Co/Pt( 111 ) annealed at 650 K for 4 min. The angle of 4.9 ° is obtained from the experimental measurement. (b) Atomic arrangements with a hard sphere model of rotated and nnrotated domains corresponding to Fig. 5(a).

56 eV electron energy for L E E D is very surface sensitive, we believe that the intermixture only occurs at the interface and rotated domains are the top layer of Co. F r o m the above discussion, we can conclude that the driving force for domain rotation comes from a thermal activation. The large mismatch of this system and incommensurate adsorbed layers mean that the original structure is in a metastable state. The rotated domains can be formed under the thermal activation before an alloy state is complete. This new structural phase of the existence of the rotated domains after the annealing is the first such report in the Co/Pt( 111 ) system as far as we know. A hard sphere model, based on the wave vector of the superstructure, has been used to study the

d = IPQI-- {[2.77(M+ 1/2) --2.51N cos 0] 2 + [2.77~v/3/2 - 2.51N sin 0] 2 } 1/2. Let us define a parameter L = d-2.77z where z = 0, 1, 2, 3, .... The parameter L is the mismatch with a unit of A as defined. The range of L is assumed to be 2.77>L_>0. The plot of L vs. the reorientation angle 0 is shown in Fig. 6. We only take the part whose mismatch is less than 0.5 A because we are interested only in the minimum mismatch. The range of 0 is set to be within 0-30 ° under consideration of small reorientation. Each lowest point in Fig. 6 represents a minimum mismatch between the row of Co and the P t ( l l l ) substrate. Table 1 shows all the reorientation angles corresponding to these minimum values of L in Fig. 6. F r o m this table, one can find that N increases at the beginning to a m a x i m u m value of N = 15 at 0 = 4 . 9 °. As 0 exceeds 4.9 °, N decreases. Because the Co a t o m is smaller than the Pt atom in size, a row of Co atoms sitting on a Pt(111 ) surface must buckle up and down. I f the distance between two hollow sites of nearest matched points is larger, this buckling

J.S. Tsay, C.S. Shern / Surface Science 396 (1998) 319 326

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will be less. The buckling causes an a d d i t i o n a l term in the surface free energy and increases the surface free energy [22]. Thus the reorientation angle of 4.9 ° will cause the least mismatch and lowest free energy and it is a reasonable rotation angle for this system. F r o m this calculation we can draw a conclusion: the Co adlayer arranges itself with a row of 15 Co adatoms accumulating on the r-axis with 0 = 4 . 9 ° in which it contains 14 Table 1 All the reorientation angles corresponding to the m i n i m u m points as shown in Fig. 6. N increases to a m a x i m u m value of 15 at 0 = 4 . 9 °. When 0 is larger than 4.9 °, N decreases. Detail is shown in text Reorientation angle 0

N

Mismatch L

0.1 0.4 0.8 1.3 1.9 *4.9 6.1 7.1 8.2 9.4 10.7 12.2 14.0 16.3 19.3 23.5

11 12 13 14 15 15 14 13 12 11 10 9 8 7 6 5

0.0052 0.0234 0.0173 0.0033 0.0145 0.0056 0.0237 0.0070 0.0179 0.0338 0.0302 O.0205 0.0074 0.0077 0.0036 0.0052

Pt hollow sites along [-1,1,0] plus another one of the nearest-neighbor row. We also used L E E D to study the growth of this system at 140 K. The same fine structure was observed. These satellites are fuzzier with a stronger background than that of the deposition at r o o m temperature at the coverage above 3 ML. The insufficient diffusion at this low temperature causes the morphology of this ultrathin film which is rougher than that growing at r o o m temperature. An additional (2 x 2) L E E D pattern was observed as the coverage is larger than 4.5 M L as shown in Fig. 3(c). These (2 x 2) spots persist until the coverage is over 7 ML. A similar (2 z 2) pattern was also found in a multilayer F e O / P t ( l l l ) system and these additional (2 x 2) L E E D spots was interpreted by a particular arrangement of the step structure at the topmost layer [19]. The structure of the valence band of this system in the growing process is interesting. The evolution of the angle integrated U P spectra during deposition at r o o m temperature is shown in Fig. 7. He I resonance radiation with a photon energy of 21.2 eV was used. All U P spectra displayed in Fig. 7 were taken at the near-normal electron emission angle. A sharp Fermi edge and two main peaks located at 1.4 eV and 4.2 eV below the Fermi level were observed for the clean surface (Fig. 7(a)). After Co atoms are deposited on the Pt(111) surface, some interesting phenomena occur: (1) The Fermi edge shifts 0.2 eV to the higher binding energy as the coverage reaches 3.6 ML. This shift is the difference in work functions for Co and Pt. (2) The peak height of the Fermi edge decreases monotonically as the coverage increases. The peak height of the Fermi edge at 1 M L attenuates to 2/3 of its original value for the clean surface. (3) The peak height of the peak located at 4.2 eV decreases and the location of this peak shifts toward higher binding energy monotonically as the Co coverage increases. This shift is up to 0.3 eV as the coverage of Co is 1.2 ML. (4) Only a strong d-band near the Fermi edge is observed when the Co coverage is over

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J.S. Tsay, C.S. Shern / Surface Science 396 (1998) 319-326

(h)

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Fig. 7. UP spectra of Co/Pt( 111) corresponding to different Co coverages: (a) clean surface; (b) 0.4 ML; (c) 0.8 ML; (d) 1.2 ML; (e) 1.6 ML; (f) 2 ML; (g) 2.4 ML; (h) 3.6 ML.

3.6 ML. This is the structure of the density of electronic states for Co bulk [23]. We believe that most of these changes are due to the shift from a Co atomic state to a Co solid state at the interface.

4.

Conclusion

We determined the coverage of Co overlayer by oscillation of the out-of-phase L E E D intensity and A S - t plot. Cobalt ultrathin film on P t ( l l l ) shows incoherent epitaxy at r o o m temperature. Unrotated and rotated incommensurate Co

domains of the top layer with two equivalent angles of rotation, +4.9 ° and - 4 . 9 ° with respect to the aligned substrate, are observed by L E E D for 2 M L of Co/Pt( 111 ) after applying an annealing treatment at 650 K. The reorientation angle of 4.9 ° is confirmed from calculation of mismatch and the grade of buckling. The Co adlayer arranges with a row of 15 Co adatoms accumulating on a row of 14 Pt hollow sites along [-1,1,0] plus another one on the nearest-neighbor row. The rotated domains are caused by the thermal activation and the corresponding L E E D pattern will disappear after the alloy state is complete. This interesting structure was first discovered by us in

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this system. T h e L E E D satellite p a t t e r n s are t h e s a m e as t h a t o f t h e d e p o s i t i o n at r o o m t e m p e r ature, but a faint (2x2) LEED pattern was o b s e r v e d f o r h i g h e r c o v e r a g e s at 140 K . T h e d e n sity o f e l e c t r o n i c states o f this s y s t e m s h o w s s o m e interesting changes during deposition. Most of these changes are due to the evolution from the a t o m i c state to t h e solid state o f C o at t h e i n t e r f a c e o f C o / P t ( 111 ).

Acknowledgement This work was supported by the National Science Council of ROC under Grant No. NSC 86-2112-M-003-002.

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