Infrared Spectroscopic Investigation of the Reaction of Hydrogen-Terminated, (111)-Oriented, Silicon Surfaces with Liquid Methanol

Abstract

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 CH_3 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 CH_3 stretching modes, respectively, increased in intensity. Spectra of H−Si(111) surfaces that had reacted with CD_3OD showed the appearance of Si−D signals polarized normal to the surface as well as the appearance of vibrations indicative of Si−OCD_3 surface species. The data are consistent with two surface reactions occurring in parallel, involving (a) chemical attack of hydrogen-terminated Si(111) terraces by CH_3OH, 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.