Adsorption of Sb on Ag(111) studied using LEED, AES and XPS

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Surface Science 307-309 (1994) 101-106

Adsorption of Sb on Ag( 111) studied using LEED, AES and XPS T.C.Q. Noakes *, D.A. Hutt, C.F. McConville Department of Physics, University of Wan&k,

Co~wztry CV4 7AL, UK

(Received 20 August 1993)

Abstract

The growth of Sb overlayers on Ag(lll) has been studied using LEED, AES and XPS from sub-monolayer to multilayer coverages as a function of deposition temperature. At room temperature, growth was found to be by the Franck-van der Merwe (layer-by-iayer) mechanism. Two distinct ordered structures were seen after annealing of the deposited layers, a (6 x 6)R30” at low coverage and a C~V’?x 2&)R30” at coverages above 1.5 monolayers. The first of these structures is thought to involve substituted Sb into the first layer of Ag atoms, forming a surface alloy. The second structure may be due to the formation of an Sb overlayer on top of the Sb substituted layer. Small core level shifts were observed in the XPS spectra of the deposited surfaces.

1. Introduction Recently there has been increased interest in the use of sub-monolayer quantities of Sb as a surfactant to enhance layer-by-layer growth in so-called surfactant mediated epitaxy (SME). Much of the work carried out has involved semiconductor materials, examples including the epitaxial growth of Si [l] and Ge/Si multilayers 123. There has also been some interest in the use of Sb for homoepitaxy of face centred cubic (fee) metals such as Ag, where the growth mechanism has been studied on the close packed (111) surface [3]. Typically, low coverages of antimony are used for SME and it has been proposed that preferential adsorption at step edges occurs, thus reducing the potential barrier seen by silver atoms moving over the step. The Sb appears to con-

* Corresponding

author.

stantly diffuse to the growth surface, effectively “floating” on top of the deposited Ag during epitaxy. It therefore acts as a surfactant throughout the growth process, suppressing island formation and 3D growth. However, despite the increased interest in this area, there is relatively little information available on the interaction of sub-monolayer coverages of Sb with fee metals. There has, however, been extensive work on the deposition of Sb on semiconductors, including Si [4-61 and Ge [‘7-91, since along with the interest in SME Sb has also been used for delta doped layers [51, where about one monolayer of Sb is incorporated into a Si or Si/Ge structure to provide a high dopant concentration in a 20 plane. In the only previous study of Sb adsorption on Ag(lll) [lo], using electron diffraction, amorphous overlayers were formed with no ordered structures observed until very high coverages, where there was some evidence of the bulk rhombohedral Sb(ll0) surface appearing. However,

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this early study did not investigate post annealing of the sample after deposition. There has also been some more recent work on electrochemical deposition of Sb on Au(ll1) [ll] and there are several related examples of the deposition of other metallic overlayers which can be drawn on for comparison [12-141. In this study, the growth of overlayers of Sb on Ag(ll1) has been investigated at coverages up to 4.5 monolayers. The growth mode has been determined using AES, whilst LEED has been used to obtain information on the various ordered and disordered overlayer structures formed. Some additional XPS data was also obtained.

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Sb Coverage (Monolayers)

Deposition Time (min)

2. Experimental The experiments were carried out in an ultra high vacuum chamber with a base pressure of approximately 1 X lo-” mbar. The chamber was equipped with a reverse view LEED optic and a hemispherical analyser to allow electron spectroscopy to be carried out using either an electron gun or an X-ray source for AES and XPS respectively. The analyser was interfaced to a computer to allow easier data acquisition. The sample was mounted on a manipulator and could be accurately and reproducibly positioned. Heating was achieved by electron bombardment from a W filament mounted behind the sample. The Ag(ll1) crystal was cut using spark erosion as precisely as possible to the (111) face and the orientation was checked using Laue X-ray diffraction. The crystal was then polished using progressively lower grades of diamond paste to produce a mirror finish. After insertion into the vacuum system a series of ion bombardment and annealing cycles were performed. Bombardment was carried out for approximately 30 min using 4 keV Ar ions and the sample was annealed for 10 min at 675°C. After cleaning, the AES spectrum showed no evidence of C or 0 and a clear Ag(ll1) (1 X 1) hexagonal LEED pattern could be seen from this surface. Overlayer deposition was accomplished using a small Knudsen cell, mounted perpendicular to the sample, filled with high purity Sb (6N pure,

Fig. 1. Plot of Sb (454 eV)/Ag (351 eV) Auger peak-to-peak intensity ratio as a function of deposition time, showing regular break points for each monolayer deposited.

MCP Ltd. UK) and operating at 455°C. The Sb was deposited in one minute steps after which the Ag MNN and Sb MNN Auger peaks were recorded. This time interval was always used, since this kept transient temperature effects, associated with opening and closing the Knudsen cell shutter, constant.

3. Results and discussion

A series of deposition experiments were carried out, mostly at room temperature, but also using elevated substrate temperatures. Fig. 1 shows a plot of the Sb/Ag Auger peak-to-peak height ratio against deposition time with the sample surface at room temperature. The regular breaks in the slope of this plot are indicative of the Franck-van der Merwe (layer-by-layer) growth mode. This allowed the monolayer assignments shown in the figure to be made and it can be seen that the average monolayer deposition time was around 10 min. Fig. 2 shows a comparison of deposition experiments conducted at both room and elevated sample temperatures. It was found that at - 100°C deposition appeared to be unchanged from that at room temperature, whereas at N 275°C a maximum coverage of ap-

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proximately half a monolayer was achieved, indicating either no further adsorption or dissolution of excess Sb into the bulk of the sample. Room temperature deposited layers gave very diffuse LEED patterns, which displayed no additional features to the (1 X 1) clean surface pattern and at the highest coverages the pattern became barely visible. In order to obtain information on the formation of ordered structures, annealing experiments were performed at initial coverages ranging from 0.2 to 4.5 monolayers. The sample was annealed to progressively increasing temperatures (10 min at each temperature) and following each anneal the LEED pattern was checked on cooling to room temperature. AES data was also obtained during these annealing experiments. These results are summarised in Fig. 3, which indicates the LEED patterns obtained as a function of initial Sb coverage and annealing temperature. The associated LEED patterns are shown in Fig. 4. At very low Sb coverages only the (1 x 1) LEED pattern was observed, regardless of annealing temperature. For annealed deposits in the initial coverage region of between 0.5 and 1.5 monolayers the (6 x 6)R30” structure was seen and higher temperature annealing showed that

Sb Coverage (Monolayers)

0' 0

6

IO

15

Deposition Time (min) Fig. 2. A comparison of deposition experiments carried out at different substrate temperatures. Note the change in behaviour at 275°C.

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2

3

4

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Initial Coverage (Monolayers) Fig. 3. regions LEED (1 x l),

A plot showing the initial coverage and temperature where the three surface structures observed with occurred during the deposition of Sb on Ag(ll1). (0) (0) (6 x fi)R30”, ( v ) (2& x 2fi)R30”.

this was stable up to around 400°C. Above this temperature the Sb concentration at the surface was reduced, either by desorption or by diffusion into the bulk and the LEED pattern reverted to the clean (1 X 1) structure. On completion of Sb deposition at 275°C the (6 X &)R30“ LEED pattern was also observed. Annealing of Sb deposits in excess of 1.5 monolayers led to the formation of another distinct ordered structure which displayed a (26 X 2fi)R30” LEED pattern. This structure was stable to about 200°C after which the surface Sb concentration dropped below 1.5 monolayers and the (6 x fi)R30 structure reappeared. At very high coverages it might be expected that another structure would have been observed corresponding to a bulk Sb surface [10,15]. However, annealing the sample with more than four monolayers of deposited Sb only yielded a diffuse (1 X 1) LEED pattern. No change in the LEED pattern occurred with increasing anneal temperature, until the surface Sb coverage was significantly reduced and the (6 X fi)R30” pattern was again observed. XPS data from surfaces having (1 X 11, (fi X &)R30” and (26 x 26)R30” structures was also obtained. Detailed scans were carried out in the region of the Ag 3d,,, and 3d,,, in order to determine whether core level shifts occurred as a

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function of increasing Sb content. The XPS data showed a slight increase in the 3d silver binding energy of about 0.2 eV on formation of the (6 x fi)R30” structure. This suggests some shift in electron density from the silver to the antimony due to the bonding between them. No apparent change in binding energy occurred when going from the (16 x fi>R30° to the (2& X 2fi)R30 structure, although this may be because the resolution of the equipment was not sufficiently high to detect such a shift. During room temperature growth, it appears that the Sb is not mobile enough to form any long

a)

b)

c)

Fig. 4. The three LEED patterns observed for Sb deposition on Ag(ll1); (a) (1 x 1) at 93 eV, (b) (6 X fi)R30” at 82 eV, Cc)(26 x 2fi)R30” at 132 eV.

Fig. 5. Suggested structures for the observed reconstructions; (a) (6 x &)R30” overlayer, (b) (& x fi)R30” inlaid structure, (c) (26 x 2J?;)R30” composed of (2 x 2) overlayer above (fiX&)R30” inlaid structure, showing the three surface nets. (0) Ag, (0) Sb.

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range structures that are visible in LEED. Bulk Sb has the rhombohedral (pseudo-cubic) structure, of which the closest packed face is the (110) [151 and this would be the most likely face to form. The breaks in Fig. 1 are therefore assigned to the formation of individual Sb(ll0) layers with a density of N 0.75 of an Ag(ll1) layer. The Sb(ll0) plane was observed in the previous electron diffraction study of high coverages of Sb on Agtlll) [lo]. Two possible atomic arrangements are proposed for the (6 x fi)R30” structure and these are shown in Figs. 5a and b. One arrangement (Fig. 5a) has the Sb atoms occupying the three fold hollow sites on the silver surface, resulting in a coverage of l/3 of a monolayer relative to the Ag(ll1) substrate. The alternative structure (Fig. 5b) is the incorporation of Sb atoms into the top Ag layer to give a “surface alloy” of composition Ag,Sb. This structure has been observed for several other metal overlayers on fee (111) surfaces [13,14]. The most common alloy of Sb and Ag is Ag,Sb which is orthorhombic in structure [16], however, the inlaid structure proposed is not related in any way to a bulk terminated surface of this material. It is interesting to compare this system with work carried out on the deposition of Sn on Pt(lll), Ni(ll1) and Cu(lll) 1131where, although the bulk alloy structure is somewhat different (despite identical stoichiometry), a similar effect is observed. The bulk structure of these alloys is fee of composition X,Sn (X = Pt, Ni, Cu), which would be expected to give a p(2 X 2) termination on the (111) face, however, these Sn overlayers have been shown to form (6 X fi)R30 surface alloys instead. It has been proposed that the (fi x &)R30” incorporated structure is more stable than the expected bulk termination when it is a surface alloy. For the case of Sn on these fee (111) metals, overlayer material in excess of l/3 ML dissolved into the bulk of the sample leaving a constant overlayer composition and it is possible that this may be analogous to the high temperature growth experiment shown here. In a recent theoretical study, ab initio calculations of Sb adsorption on Ag(ll1) [17] have been performed and show that, for Sb, the substitutional adsorption site is found to be greatly

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favoured with respect to the on surface fee and subsurface sites, so that a surface alloy layer is formed. In addition, a comparison of the (& x &)R30”-Sb and p(2 x 2)-Sb substitutional sites indicated that the first of these structures is the more energetically stable, in agreement with the results presented here. The structure responsible for the (26 x 2&)R30” LEED pattern is likely to be more complex. Since the surface Sb concentration is higher than for the (6 X &)R30”, this structure cannot arise from a single atom at each surface net point, but rather must be due to either multiple atoms at each net point, or multiple layers with the appropriate symmetries. The first of these suggestions is the least probable of the two, since to give the observed Sb/Ag Auger peak height ratio, it would involve Sb atoms being too tightly packed together on the surface. In contrast, a multiple layer arrangement allows a higher surface Sb content to be achieved, in agreement with the observed data. Since the atomic radii of Ag and Sb are similar, it might be expected that an alloy surface would be quite smooth and the formation of a regular overlayer structure on top of this layer would seem feasible. The simplest multiple layer arrangement would be a p(2 x 2) overlayer of Sb on top of the (6 X filR30 alloy and this structure is shown in Fig. 5c. However, there are other structures which could give rise to the observed LEED pattern and further experimental data is required before any positive identification can be made.

4. Summary Overlayers of Sb up to 4.5 monolayers thick were grown on Ag(ll1). The growth was shown to be by the Franck-van der Merwe (layer-by-layer) mechanism. Two distinct surface structures were seen, the (fi X fiJR30” and the (20 X 2&)R30”. The first of these structures appears to be caused by a single antimony atom at the appropriate registry, most probably incorporated into the surface layer. One possible explanation of the higher coverage structure is a p(2 X 2)

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of Sb on &)R30” structure.

overlayer x

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of the

inlaid

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5. Acknowledgements [6]

The authors would like to thank the Science and Engineering Research Council for providing funding for this work in the form of a studentship (T.C.Q.N.) and a research fellowship (D.A.H.). AEA Technology Ltd. (Harwell Laboratory) are acknowledged for a CASE contribution (T.C.Q.N.).

6. References [l] B. Voigtlander and A. Zinner, Surf. Sci. Lett. 292 (1993) L775. [2] M. Horn-von Hoegen, M. Pook, A.Al Falou, B.H. Muller and M. Henzler, Surf. Sci. 284 (1993) 53. [3] H.A. van der Vegt, H.M. van Pinxteren, M. Lohmeier, E.

[l] [X] [9] [lo] [II] [12] [13] [14] [15] [16] [ll]

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