STRUCTURE OF DOMAIN WALLS IN Al/Si(111) GAMMA-PHASE A.A. Saranin, V.G. Kotlyar, A.V. Zotov, T.V. Kasyanova, M.A. Cherevik, V.G. Lifshits Institute of Automation and Control Processes, RAS, Vladivostok, Russia. Using LEED and high-resolution STM observations, atomic structure of the Al/Si(111) gamma-phase formed upon saturating adsorption of Al at about 675oC has been studied. The obtained results in conjunction with the published data provide the description of the Al/Si(111) gamma-phase structure as follows. The gamma-phase contains about 0.65 ML of Al, which substitutes for the Si atoms in the outermost atomic layer of the almost bulk-like Si(111)1x1 surface. Due to a greater atomic radius of Al as compared to Si, incorporation of Al atoms leads to the increase of the surface lattice constant by about 10%. The misfit between the Al-incorporated layer and the Si bulk is relieved through the formation of the domain wall net, which decomposes the surface into quasi-periodic incommensurate superlattice of the triangle-shaped domains. The domains have a close, but not identical size, hence, the surface is lacking the well-defined long-range ordering. Like the well-known 7x7 DAS (dimer-adatom-stacking fault) structure, every second triangular subunit contains a stacking fault in the top surface layer. However, in contrast to the DAS structure and recently proposed DS (dimer-stacking fault) model of the gamma-phase [1], almost all domain boundaries are represented not by the dimer rows, but by the Si zigzag chains (see Fig.1). These boundaries comprise the light domain walls depeleted of Al. Upon the growth under condition of the Al supersaturation, the incorporation of the extra Al at the surface results in the formation of the Al-rich heavy domain walls and possibly induces the distortion in the underlying Si(111) substrate layers. Fig.1. (a) High-resolution empty-state STM image of the Al/Si(111) gamma-phase with the light domain wall (indicated by arrow). (b) Schematic sketch illustrating the zigzag arrangment of Al atoms, which border the domain wall (shown by grey circles). (c) ZS-model (zigzag chain-stacking fault model), which accounts for the domain wall structure. [1] M. Yoshimura, K. Takaoka, T. Yao, T. Sueyoshi, T. Sato and M. Iwatsuki, J.Vac.Sci.Technol. B 12, 2434 (1994).