Giant faceting of vicinal Si(001) induced by Au adsorption.

by F.-J. Meyer zu Heringdorf (a), R. Hild (a), P. Zahl (a), Th. Schmidt (b), S. Heun (c), B. Ressel (c), E. Bauer (d), and M. Horn-von Hoegen (a);
a) Institut fuer Festkoerperphysik, Universitaet Hannover, Appelstrasse 2, 30167 Hannover, Germany;
b) Sinchrotrone Trieste, AREA Science Park, 34012 Basovizza TS, Italy; present address: Institut fuer Experimentelle Physik II, Universitaet Wuerzburg, Am Hubland, 97074 Wuerzburg, Germany;
c) Sinchrotrone Trieste, AREA Science Park, 34012 Basovizza TS, Italy;
d) Department of Physics and Astronomy, Arizona State University, Tempe, USA.

(contact: stefan.heun@elettra.trieste.it )

Adsorbate induced faceting and step arrangement control are key techniques for the self organized formation of one-dimensional mesoscopic structures. Using Au as adsorbate on a 4 deg vicinal Si(001) substrate the initially double-stepped surface decomposes into a hill and valley structure consisting of flat (001) terraces and facets with a slope depending on the adsorption conditions. At high temperature deposition (850 C) solely (119) type facets are observed in common with huge (001) terraces extending over a mm range along the step edges of the initial surface. The typical distance between two adjacent terraces is of the order of several µm perpendicular to the step edges. The huge structure size provides an ideal system for the analysis with the spectroscopic photoemission low energy electron microscope (SPELEEM) at ELETTRA. The morphological transition during Au adsorption can be observed by low energy electron microscopy (LEEM), while x-ray photoemission electron microscopy (XPEEM) allows a time and laterally resolved determination of the local Au coverage.

Fig. 1: LEEM investigation during Au adsorption at 850 C. The movie starts after several minutes of Au deposition. Terraces appear in bright, step bands are imaged dark. (119) facets, formed during the last stage of the faceting process, appear light grey. The field of view is 12µm.
Links to movie 1: Quick Time Format (5939 kB)

A series of LEEM images during adsorption at 850 C is shown in movie 1. The field of view (FoV) corresponds to a circle with 12µm diameter on the surface. Initially the LEEM image appears grey without any contrast. Due to the lateral resolution of the LEEM instrument of several nm, the double step train of the surface cannot be resolved. After several minutes of deposition a (001) terrace (imaged in bright in movie 1) nucleates outside the FoV and propagates anisotropically along the step edges with an average speed of 20µm/s. With further Au coverage more terraces are formed and induce all steps in between two adjacent terraces to be bunched into "step bands". This is to compensate for the macroscopic miscut of the sample, which must be conserved. With high resolution electron diffraction (Spot Profile Analysis-Low Energy Electron Diffraction SPA-LEED) a maximum slope of the step bands of 16 deg relative to (001) could be observed [1]. In the LEEM movie (001) terraces appear bright while step band areas are imaged dark.
With further deposition, however, the step band is transformed to (119) facets with a slope of 8.93 deg in a second stage. In the LEEM movie facet areas appear light grey in contrast to the dark step band. The facets are very well ordered and show kinks, that compensate a small azimuthal miscut of the sample. Driving force for the faceting process is a very complex '5x3.2' reconstruction formed on the terraces. If Au diffuses onto a terrace, it is collected in this superstructure, as shown by soft x-ray photoemission measurements shown in movie 2.

Fig. 2: XPEEM investigation during Au adsorption at 850 C using the Au 4f 7/2 photoemission peak for imaging. Intensity scaling is proportional to the local Au coverage. Initially Au is collected in a lattice gas before (001) terraces are formed and collect the gold. As in movie 1 the field of view is 12µm.
Links to movie 2: Quick Time Format (5275 kB)

In movie 2 the Au 4f 7/2 photoemission peak was used for imaging, representing the local Au coverage on the surface since no shift of the Au 4f peak could be observed during deposition with the given energy resolution of 0.4 eV. In movie 2 terraces appear bright again, but this is not due to a topographic contrast, but is a chemical contrast: bright terraces collect gold from their neighbourhood and the Au coverage on terraces is increased compared to the step band regions. Finally, when the step band is transformed to (119) facets, the contrast disappears, for the whole surface is homogeneously covered with gold.
From both, movie 1 and movie 2, the faceting process can be summarized as follows: in the first stage of faceting, Au forms a lattice gas on the surface. After a critical Au coverage of approximately 1/3 ML is reached [2], (001) terraces with a '5x3.2' reconstruction are formed and propagate anisotropically along the step edges of the initial substrate. Au necessary for '5x3.2' reconstruction formation is collected from the lattice gas. Due to expansion of the (001) terraces perpendicular to the step edges steps in between two adjacent terraces are bunched and form step bands, which are only slightly covered by Au. When the (001) terraces extend to their maximum size it is not possible to further bunch the steps. The excess Au cannot be used to enlargen the terraces any longer, and so the step band areas increase. This is the driving force to transform the step band into well ordered (119) facets.

References
[1] H. Minoda, K. Yagi, F.-J. Meyer zu Heringdorf, A. Meier, D. Kaehler, and M. Horn-von Hoegen, 'Gold induced faceting on a Si(001) vicinal surface: Spot-profile-analyzing LEED and reflection-electron-microscopy study', Phys. Rev. B 59(3), p. 2363-2375 (1999).
[2] F.-J. Meyer zu Heringdorf, D. Kaehler, M. Horn-von Hoegen, Th. Schmidt, E. Bauer, M. Copel, H. Minoda, 'Giant faceting of vicinal Si(001) induced by Au adsorption', Surf. Rev. and Lett. 5(6), p. 1167-1178 (1998).