LOW-ENERGY ELECTRON-DIFFRACTION STUDIES OF SI(111)-(SQUARE-ROOT-3XSQUARE-ROOT-3)R30-DEGREES-BI SYSTEM - OBSERVATION AND STRUCTURAL DETERMINATION OF 2 PHASES

The initial epitaxial growth of Bi on a Si(111)-7 x 7 surfaces has been studied as a function of overlayer coverage and deposition conditions using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Experiments show for the first time that two equilibrium phases are formed in the growth process, each displaying a (square-root 3 x square-root 3)R30-degrees LEED symmetry: an alpha-phase occurring at 1/3 ML Bi coverage and substrate temperature of 360-degrees-C, and a beta-phase at 1 ML and 300-degrees-C. Quantitative structural information for each phase was determined by multiple scattering analysis of I-V curves. The analysis is facilitated by comparing LEED intensity data measured for each phase with calculated values using appropriate structural models. The T4 geometry for the alpha-phase and a trimer geometry for the beta-phase are revealed to be the "best" models; detailed atomic coordinates have been determined for each phase.