Si(111) submonolayer interface

L299

Surface Science 130 (1983) L299-L306 North-Holland Publishing Company

SURFACE

SCIENCE

LETTERS

STUDY OF Ag/Si(lll) SUBMONOLAYER INTERFACE II. Atomic geometry of Si(ll1) (0 X 6)R30°-Ag surface S. KONO, H. SAKURAI, Department Received

I$ Physics, Far&y

I March

K. HIGASHIYAMA of Science, Tohoku University,

1983; accepted

for publication

11 April

and T. SAGAWA Sendai 980, .Japan 1983

Final state diffraction of Ag 3d X-ray photoelectrons from the Si(ll1) (6 X &)R30°-Ag surface has been measured. From a kinematical analysis of the diffraction patterns, it is found that a buried honeycomb framework of Ag atoms is formed on the surface with lateral displacement of the first Si layer.

The atomic geometry of the Si( 111) (6 X 6)R30”-Ag surface (called v? hereafter) has been studied extensively in recent years, Saitoh et al. [I] argued for the first time, from an analysis of the low energy ISS intensity, that Ag atoms are buried underneath the first Si layer. Terada et al. [2] measured the constant-momentum-transfer-averaged (CMTA) LEED intensity of (00) beams and anaIyzed their data kinematically. They concluded that Ag atoms are located 0.7 A below the first Si layer in accordance with the result of the ISS expecment and that the thickness of the first Si double layer is expanded by 0.75 A and that the separation between the second and the third Si layer is contracted by 1.05 A. However, the lateral arrangement of Ag and Si atoms on the 6 X 6 surface is not yet examined experimentally. If one assumes a Ag saturation coverage (8,) of 2/3 of a monolayer, a monolayer being 7.8 x lOi atoms/cm2 for an ideal Si(lll) surface, symmetry considerations lead to a so-called “honeycomb” arrangement of Ag atoms as considered by Le Lay et al. [33. A recent report by Gotoh et al. [4], however, showed that the saturation coverage depends on the amount of carbon impurity on the surface and that 0, - 1 rather than 2/3 for a clean surface. The saturation coverage was also found to be dependent on the substrate temperature while Ag is being deposited as observed by Le Lay et al. [5]. They found the saturation coverage to vary from 8, = 2/3 at 25O*C to 0.8-I at 5OO’C. Because of this dependence of the saturation coverage on the substrate temperature, Le Lay et al, favored a two-stage hypothesis: one stage at & = 2/3 with the honeycomb array of Ag atoms, and the other one at 8, = 1 which needs and activation energy to transform from the first. x v’?,

0039-6028/83/0000-0~/$03.00

0 1983 North-Holland

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S. Kono et ai. / Stud) of Ag/Si(i

11) submonolayer interface, II

It has been demonstrated in recent years that the diffraction of X-ray photoelectrons or Auger electrons from an adsorbate core.level is a promising means for the adsorbate geometry determination, especially for an underlayer adsorbate system [6,7]. In this report, we present the azimuthal angle dependence of Ag 3d X-ray photoelectron diffraction from the fi X fi surface and the kinematical analysis of the diffraction patterns. The validity of the kinematical analysis of X-ray photoelectron diffraction has already been confirmed for a Ag single crystal [7] and has been fully discussed in ref. [6], The kinematical analysis of the azimuthal dependence reported in this Letter strongly indicates that a buried honeycomb framework of Ag atoms is formed with lateral displacement of the first Si layer. In paper I of this series, an angle-resolved UPS study of the electronic structure in the fi X fi surface has been reported [S]. A VG ADES 400 spectrometer was used under a base pressure of - 5 x lo- ” Torr. A polished and pretreated [9] silicon wafer (B doped at 1.5 X lOI atoms/cm3) was mounted to a two-axis sample manipulator and the surface normal was aligned within 0.5” to the axis of azimuthal rotation. Cleaning of the silicon wafer was performed in situ by electron-bombardment flashing from the back side. Silver was deposited onto the Si( 111) 7 X 7 surface at room temperature using a quartz thickness monitor although no absolute determination of the coverage was intended. The amount of deposition was carefully controlled not to exceed too much the saturation coverage for the fi X 6 surface by checking the LEED patterns after annealing to - 300°C. The superstructures of 3 x 1, fi x 6 and 6 X 1 showed a dependence on coverage and annealing conditions similar to those of Gotoh and Ino [lo] The irradiation by Mg Ka X-rays for the photoelectron excitation was collimated to - 3 mm in diameter on the sample surface. The photoelectron intensities of the adsorbate and the substrate were measured every 4.3” in azimuthal angle + at a constant electron take-off angle 6, (with respect to the surface) by azimuthal rotation of the sample (cf. inset in fig. 1). The kinematical formalism for X-ray photoelectron diffraction has been fully described previously f6] and some modifications and simplifications have been implemented to meet the special conditions of the measurement using the VC apparatus as described in ref. [7]. In the present case of the fi X fi surface, the following conditions were applied in the calculation. No inner potential at the surface was included since the precise inner potential is not known and the inclusion of an inner potential merely changes the take-off angle by - 2’ at very low take-off angles. The atomic scattering factor used for Si was the mean of those for Al and P as tabulated by Fink and Ingram [ 1 l] and Gregory and Fink [ 121 and quadratically interpolated to a scattering factor at an electron energy of 894 eV. The scattering factors are then devided by 2 for the reasons described [6]. and Ag layer were included in the Only scattering from the first Si layer

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calculation since scattering by atoms below the photoelectron emitters did not contribute to the diffraction patterns because of the forward-peaking of the scattering [6]. The size of a cluster used in the calculation was checked to be sufficient; it consisted of 32 Ag atoms and 48 first layer Si atoms. The solid lines with filled circles in figs. 1 and 2 are the azimuthal photoelectron-diffraction patterns for Ag 3d electrons as measured at constant experimental take-off angles 0, from 7“ to 20”. Statistical errors in the patterns are less than 2% as shown with the error bars. However, there exist monotonic

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Fig. 1. Azimuthal-angle photoelectron-diffraction patterns of Ag 3d photoelectrons as excited by Mg Ka X-rays from the Si(ll1) (0 X&)R30”-Ag surface. Pertinent angles are defined in the inset. Filled circles with solid lines are experimental patterns and open circles with broken lines are patterns as calculated for the model in fig. 3 (see text). Ordinate scales are anisotropies defined as they are 20% for experiment and 40% for calculation. (I,,, - 1)/I,,,;

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(112-1

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Study

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)(/TX J3)R30°-Ag

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Fig. 2. As in fig. 1 but for the higher take-off angles.

but systematic intensity variations not related to the diffraction effect that are as large as 5-10%. These variations are found to be due to the differences in Ag coverage over the sample surface. Namely, as the azimuthal angle C$ is changed by rotation of the sample, X-rays slightly irradiate different portions of the sample surface and the saturation coverage for the fix fi structure differs slightly depending on the degree of “cleanness” of the irradiated surface. This is consistent with what Gotoh et al. [4] found for the saturation coverage, although the carbon contamination level in our case was below the detection limit of XPS. Therefore, the monotonic and systematic intensity variations are to be ignored. The experimental azimuthal diffraction patterns for 0, = 7’-12’ are similar to one another showing diffraction peaks A and B (equivalently, A’ and B’). The diffraction peak B moves to the symmetry axis (172) as 19,increases from

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7” to 14’. The diffraction peaks A and B disappear for 0,~ 16” and weak peaks at I#J= O”, - 30” and 60” appear. For ~9,> 20” no appreciable diffraction peaks are found. To analyze the diffraction patterns in figs. 1 and 2 kinematically, we have considered the following two types of structural model. Type 1 is a honeycomb framework of Ag atoms with the first-layer Si atoms distorted laterally; this corresponds to the saturation coverage of 2/3 monolayer. Type 2 is a triangular arrangement of three Ag atoms on every 6 X fi point with laterally distorted first-layer Si atoms; this corresponds to the saturation coverage of one monola~e-. For the two types of structural model the positions of the Ag and Si atoms are varied in physically plausible ranges including both overlayer sites and underlayer sites. Good agreement between the calculation and the experiment is reached for the type 1 model, which is illustrated in fig. 3. The model consists of Ag atoms in the hollow sites of the first Si layer; the longitudinal distance is 0.2 A below the first Si layer. The first-layer Si atoms are contracted toward the center of the hollow of the honeycomb; the lateral distance S of a side of the Si triangle is 2.6 A. The azimuthal photoelectron-diffraction patterns as calculated for the model in fig. 3 are shown by the dashed lines in figs. 1 and 2. Since the determination

I -\

Fig. 3. Honeycomb model of the Si(lll)(fi xfi)R30°-Ag surface used for the kinematical calculation in figs. 1 and 2. Hatched large and small circles are Ag and the first-layer Si atoms, respectively, whose diameters are chosen to be their ionic radii for convenience of drawing. The double layer framework of the ideal Si( 111) surface is shown with dashed circles and dashed lines.

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of the experimental take-off angle contained an uncertainty of - 2” and since a possible inner potential is not included in the calculation, comparison of the calculation with the experiment was made with an allowance in take-off angle fluctuation of f 2”. It appeared that good agreement is reached when 8, = 0, + lo, where 19, is the take-off angle in the calculation. Although the fine structures and the relative intensities of the patterns are not well reproduced by the calculation, there exists good general correspondence between the calculation and the experiment. Especially, the positions of the diffraction peaks A and B (equivalently, A’ and B’) are well reproduced for 0, = 6’, 9” and 11 O. For 8, > 13”, the theoretical diffraction patterns become subtle with fine structures, reproducing the general tendency of the experiment. The very fine structures of the theoretical patterns might be smeared put in the experiment due to the finite angle resolution in the experiment. For example, a sharp dip at $ = 43” for 8, = 11’ can be eliminated by the angle-broadening of - 4O, which is a reasonable degree of angle-broadening for the experimental condition. It was already fully discussed why the theoretical anisotropy is generally higher than that of experiment [6]. In view of this, we consider that good agreement has been reached between the experiment and the theory in figs. 1 and 2. We estimate the precision of the model, i.e., the parameter S and the longitudinal displacement (d,) of Ag atoms with respect to the first Si layer in fig. 3, as follows. The arrows A, A’, B and B’ in fig. 3 indicate the directions along which the diffraction peaks A, A’, B and B’ are found in fig. I, respectively. Namely, the four diffraction peaks are parts of the forward-peaking of the diffraction by the first-layer Si atoms. Therefore, the 9 position of the four diffraction peaks are sensitive to the parameter S, and S turns out to be 2.6 + 0.1 A (for an ideal surface, S = 3.84 A). The determination of the precision in d, is not straightforward. By judging from the degree of agreement of the diffraction patterns as a whole between the experiment and the calculation with and without inclusion of inner potentials, we have concluded that the inner potential is nearly zero and that d, is - 0.2 f 0.1 A (minus sign means “below”). The bond length thus determined between the first-layer Si and Ag atoms is 2.6-2.7 A which is close to the mean of those for Si-Si (2.35 A) and Ag-Ag (2.89 A) in their pure substances. Since S is - 30% less than that for the ideal Si( 111) surface, the first Si double layer must be broken to make a surface compound with Ag. The expansion of the thickness of the first Si double layer found by Terada et al. [2] is consistent with this. The disappearance of a surface dangling-bond surface state as found with UPS [S] may be consistent with this as well. The longitudinal displacement d, = - 0.2 -C 0.1 A may not be in contradition with the CMTA-LEED determination (d, = -0.7 A) in ref. [2] since we learned that d, is not very sensitive to the CMTA-LEED curve 1131.

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A very recent study by Stohr and Jaeger [14] who used the EXAFS technique revealed the Si-Ag nearest neighbor distance to be 2.45 A. This bond length seems to be rather short. However, the polarization dependence of the Si-Ag amplitude is in accordance with the present model [14]. The relationship between the present model and the two-stage hypothesis of Le Lay et al. [5] or the determined saturation coverage 19,= 1 by Gotoh et al. [4] is an unsolved problem. Unfortunately, in our experiment the absolute saturation coverage for the fi x 6 surface was not determined and the annealing temperature after Ag deposition was crudely determined. However, we think that, if the two-stage hypothesis holds true, the atomic geometries of the two stages should differ only slightly from each other. This is because in our experiment the Ag coverage as monitored with XPS scattered as much as 50% but no noticeable differences were found in the diffraction patterns among the several individual scans. This is in agreement with the fact that no modifications in the I- I’ curve were observed for a coverage near 2/3 or near 1 [5]. In relation to the two-stage hypothesis, Julg and Allouche [ 151 predicted theoretically that a centered hexagonal array of Ag atoms with 8, = 1 is formed not underneath but on the first Si layer. We state that this type of centered hexagonal overlayer is unlikely as the last stage of the two stages since the azimuthal diffraction patterns for this overlayer are very different from those in figs. 1 and 2. Addition of Ag atoms of l/3 of a monolayer onto the center of the honeycomb framework of fig. 3 would imply substantial modifications in the diffraction patterns. We, therefore, do not favor the two-stage hypothesis, although further experiments on photoelectron diffraction with a fine control of coverage and annealing temperature are needed to clarify this point. In a recent angle-resolved photoemission measurement for the fix fi surface, Houzay et al. [16] favored the embedment of Ag atoms. Their argument is based on their finding that the bulk feature at - 3.4 eV persists for the 6 X 6 surface. We think this argument is not feasible since the embedment of Ag atoms as in the model of fig. 3 must imply the breakage of the tetrahedral bonding in the first double layer of Si. Therefore, the structure at - 3.4 eV found in ref. [ 161 is likely to be a surface-related structure. In fact, the angle-resolved UPS measurements in paper I of this series [8] exhibit quite different electronic structures from those of the bulk for energies between the Fermi level and the Ag 4d derived peaks. A theoretical study of the surface electronic structure for an embedded Ag model cluster is performed by Hoshino [17]. In his embedded model cluster, the thickness of the first Si double layer is doubled without lateral displacement and the Ag atoms are placed in the middle of the threefold hollows. The calculated electronic structure for this model cluster corresponds rather well to the result of the UPS measurements and shows that the back-bonds between the Si atoms in the double layer are broken and that new bonds between the Ag atoms and the Si atoms are formed [17]. As is mentioned by Hoshino, this is not to be taken as

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definite proof for his model cluster since the features of the calculated electronic structure are common to the hollow-site geometry of Ag atoms. A final judgment as to the preference for the embedded model should be made by an analysis of the back-bond direction between the Ag atoms and the first layer Si atoms, which, we think, is not generally verified due to the diffraction effect of the photoelectrons. Therefore, further theoretical work, which includes a calculation of the dispersion of the surface-related structures such as observed in ref. [8], is necessary to determine the atomic geometry from the viewpoint of electronic structure. In this respect, the present analysis has a weakness in that only the geometry of the first-layer Si atoms and the Ag atoms is determined, whereas the atomic geometry of the second Si layer may play a crucial role in the formation of an electronic structure. A possible use of grazing incidence Auger electron diffraction as suggested [7] may be helpful to overcome this weakness although it may encounter several practical difficulties. We acknowledge Drs. K. Oura and T. Ichikawa The experimental assistance of Mrs. T. Yokotsuka, acknowledged.

for stimulating discussions. K. Sato and T. Teruyama is

References [ 1] M. Saitoh, F. Shoji, K. Oura and T. Hanawa, Japan. J. Appl. Phys. 20 (1980) L421; Surface Sci. 112 (1981) 306. [2] Y. Terada, T. Yoshizuka, K. Oura and T. Hanawa, Japan. J. Appl. Phys. 20 (1981) L333; Surface Sci. 114 (1982) 65. [3] G. Le Lay, M. Manneville and R. Kern, Surface Sci. 72 (197X) 405. [4] Y. Gotoh, A Chauvet, M. Manneville and R. Kern, Japan. J. Appl. Phvs. LO (1981) LX53. [5] G. Le Lay, A. Chauvet, M. Manneville and R. Kern, Appl. Surface Sci. 9 (1981) 190. [6] S. Kono, S.M. Goldberg, N.F.T. Hall and C.S. Fadley, Phys. Rev. B22 (1980) 6085. [7] S. Takabashi, S. Kono, H. Sakurai and T. Sagawa, J. Phyc. Sot. Japan 51 (1982) 3296. [X] T. Yokotsuka, S. Kono, S. Suzuki and T. Sagawa, Surface Sci. 127 (1983) 35. [9] R.C. Henderson, J. Electrochem. Sot. 119 (1972) 772. (IO] Y. Gotoh and S. Ino, Japan. J. Appl. Phys. 17 (197X) 2097. [ 111 M. Fink and J. Ingram, At. Data 4 (1972) 129. [12] H. Gregory and M. Fink, At. Data Nucl. Data Tables 14 (1974) 39. [ 131 K. Oura, private communication. [ 141 J. Stohr and R. Jaeger, J. Vacuum Sci. Technol. 2 1 (1982)6 19. [15] A. Julg and A. Allouche, Intern J. Quantum Chem. 22 (1982) 739. [16] F. Houzay, G.M. Guichar, A. Cros, F. Salvan, R. Pinchaux and J. Derrien, Surface Sci. 124 (1983) Ll. [17] T. Hoshino, Surface Sci. 121 (1982) 1.