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

摘要

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 (1) 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.