Investigation of the Chemical Purity of Silicon Surfaces Reacted with Liquid Methanol

The reaction of hydrogen-terminated Si(111) and oxide-terminated silicon surfaces with neat anhydrous liquid methanol (CH3OH) has been studied with Fourier transform infrared spectroscopy (FTIR) as a function of solution temperature and immersion time. At 65 °C, reaction of atomically smooth H-Si(111) surfaces with CH3OH (l) results in partially methoxylated silicon surfaces that are free of any detectable subsurface oxidation (Si-O-Si bonds); this is in contrast to observable oxidation found after similar reactions on H-Si(100) surfaces. At long reaction times (t> 3 h), the Si(111) surface saturates with Si-OCH3 sites at a coverage of approximately 30 % of a monolayer, with the residual ∼70 % comprised of unreacted Si-H sites. The lack of any detectable silicon oxide makes it possible to conclude the following: (i) Reaction mechanisms involving insertion of oxygen atoms from the CH3OH molecule into the subsurface Si-Si back bonds cannot be dominant for (111)-oriented silicon under these conditions. (ii) The vibrational modes of the oxide-free surface are very sharp and can be clearly distinguished from blue-shifted modes observed for methoxyl groups chemisorbed on oxidized surfaces. For surfaces that display subsurface oxidation, no evidence for oxygen atoms directly below atop Si-H sites has been observed. Instead, FTIR analysis demonstrates that subsurface oxidation selectively exists underneath atop Si-OCH3 sites. Finally, H-terminated oxide surfaces, prepared by reacting trichlorosilanes on OH-terminated SiO2 surfaces, react with methanol to form a methoxy-terminated oxide surface. I