Theoretical Investigation of the Structure and Coverage of the Si(111)−OCH_3 Surface

The surface structure, strain energy, and charge profile of the methoxylated Si(111) surface, Si(111)−OCH_3, has been studied using quantum mechanics, and the results are compared to those obtained previously for Si(111)−CH_3 and Si(111)−C_2H_5. The calculations indicate that 100% coverage is feasible for Si(111)−OCH_3 (similar to the methylated surface), as compared to only ∼80% coverage for the ethylated surface. These differences can be understood in terms of nearest-neighbor steric and electrostatic interactions. Enthalpy and free energy calculations indicate that the formation of the Si(111)−OCH_3 surface from Si(111)−H and methanol is favorable at 300 K. The calculations have also indicated the conditions under which stacking faults can emerge on Si(111)−OCH_3, and such conditions are contrasted with the behavior of Si(111)−CH_3 and Si(111)−CH_2CH_3 surfaces, for which stacking faults are calculated to be energetically feasible when etch pits with sufficiently long edges are present on the surface.