Infrared spectroscopic investigation of the reaction of hydrogen-terminated, (111)-oriented, silicon surfaces with liquid methanol

Fourier transform infrared spectroscopy and first principles calculations have been used to investigate the reaction of atomically smooth, hydrogen-terminated Si(111) (H-Si) surfaces with anhydrous liquid methanol. After 10 min of reaction at room temperature, a sharp absorbance feature was apparent at ∼ 1080 cm^-1 that was polarized normal to the surface plane. Previous reports have identified this mode as a Si-O-C stretch; however, the first principles calculations, presented in this work, indicate that this mode is a combination of an O-C stretch with a CH₃ rock. At longer reaction times, the intensity of the Si-H stretching mode decreased, while peaks attributable to the O-C coupled stretch and the CH₃ stretching modes, respectively, increased in intensity. Spectra of H-Si(111) surfaces that had reacted with CD₃OD showed the appearance of Si-D signals polarized normal to the surface as well as the appearance of vibrations indicative of Si-OCD₃ surface species. The data are consistent with two surface reactions occurring in parallel, involving (a) chemical attack of hydrogen-terminated Si(111) terraces by CH₃OH, forming Si-OCH ₃ moieties having their Si-O bond oriented normal to the Si(111) surface and (b) transfer of the acidic hydrogen of the methanol to the silicon surface, either through a direct H-to-D exchange mechanism or through a mechanism involving chemical step-flow etching of Si-H step sites.