SILVER ON SILICON (111): SURFACE STRUCTURAL TRANSFORMATIONS AND TRACE CONTAMINANT EFFECTS AT SUBMONOLAYER COVERAGE
The structural details for the formation of the (SQRT(3)xSQRT(3))R30deg Silver on Silicon (111) system have eluded definitive resolution for 40 years. Wildly conflicting evidence, gathered by every available surface science technique and theoretical method, is beyond the scale customary for this field. In order for this system to progress, a comprehensive review of the state-of-the-field, a newly constructed surface structure diagram, a new set of experimentally determined atomic positions and the quantitative effects minor contaminants are presented for Ag/Si(111). From careful data mining and analysis of the nearly 600 papers in the literature database, a comprehensive Ag-(SQRT(3)x SQRT(3))R30deg/Si(111) surface structure diagram has been constructed. It includes a new beta-(SQRT(3)xSQRT(3))R30deg phase; a two atom per unit cell proto-cluster variation of the SAV structure, and also a gamma-(SQRT(3)xSQRT(3))R30deg phase; with structure similar to a later variant of the HCT model. Atomic positions for Ag in the (SQRT(3)xSQRT(3))R30deg and the (3x1) structural conformations have been determined by a number of complementary techniques; X-ray Standing Wave (XSW), Surface Extended X-ray Absorption Fine Structure (SEXAFS), Auger Electron Spectroscopy (AES), Rutherford Backscattering Spectroscopy (RBS), Low Energy Electron Diffraction (LEED) and Isothermal Desorption Spectroscopy (ITDS). Three second order influences were also evaluated; boron, surface steps and carbon. It was found that boron contamination and low surface steps densities have insignificant influences on the SQRT(3) or (3x1) structures. Minor carbon contamination, however, caused dramatic effects. First, tenth ML levels of carbon completely inhibit the formation of the Ag-(3x1)/Si(111) phase. Second, at low C contamination levels, the Ag-(SQRT(3)xSQRT(3))R30deg layer is comprised of dual, Ag-Si bond lengths (2.2 Angstrom and 2.6 Angstrom); Ag-Ag bond lengths of 3.2 Angstrom; and Ag locations 0.97 Angstrom below the extension of bulk Si(111) planes inferring a Si surface contraction of 0.2 Angstrom. Finally, carbon promoted a high degree of Ag surface order and surface relaxation. The existence of the beta-(SQRT(3)xSQRT(3))R30deg and gamma-(SQRT(3)x SQRT(3))R30deg phases is fully consistent with quantitative literature citations and helps explain the decades-long debate over model and critical coverage inconsistencies. The profound influence of C, a ubiquitous contaminant in even the best experimental systems, helps explain surface position, relaxation and coverage discrepancies.
Committee Chairman: Professor J.M. Blakely; Committee Members: Professor B.W. Batterman, Professor S.L. Sass, Professor J.D. Brock, Professor H.H. Johnson.
Surface Physics; Silver; Silicon; Surface Phases; X-Ray; Standing Wave; Rutherford Backscattering; X-Ray Absorption Fine Structure; Auger Electron Spectroscopy; Quantitation
dissertation or thesis