SUBSTITUENT EFFECTS IN SILICON HYDRIDES - IMPLICATIONS FOR MODELS OF SURFACE SITES

To establish guidelines for using small silicon-containing molecules t o model silicon surfaces, a series of substituted silanes have been st udied using post-Hartree-Fock and density functional theories. The two theories differ in their detailed predictions, but similar trends are found with both methods. Si-Si and Si-H bond lengths vary slightly am ong the compounds studied. Mulliken charges on the substituted silicon atom are altered significantly, but Mulliken charges on the other ato ms remain unaffected. The Si-Si bond energy decreases by about 0.5 kca l/mol with each silyl group that replaces hydrogen. Force constants ch ange by a few percent. The most dramatic effect of substitution is tha t the energy of Si-H bonds at the substituted Si decrease by about 3 k cal/mol with each successive replacement of hydrogen by silyl groups. Of the properties calculated, only the Si-Si bond strength correlates with substituent electronegativity. The effects of model structure on surface ionization potentials have also been determined for comparison to earlier work. In general, substitution at a surface site alters so me model properties, but more distant substitutions have little effect .