Vibrational study of silicon oxidation: H 2O on Si(100)

Abstract

The vibrational spectrum of water dissociatively adsorbed on Si(100) surfaces is obtained with surface infrared absorption spectroscopy. Low frequency spectra (< 1450 cm −1 are acquired using a buried CoSi 2 layer as an internal mirror to perform external reflection spectroscopy. On clean Si(100), water dissociates into H and OH surface species as evidenced by EELS results [1] in the literature which show a SiH stretching vibration (2082 cm −1), and SiOH vibrations (OH stretch at 3660 cm −1 and the SiOH bend and SiO stretch of the hydroxyl group centered around 820 cm −1). In this paper, infrared (IR) measurements are presented which confirm and resolve the issue of a puzzling isotopic shift for the SiO mode of the surface hydroxyl group, namely, that the SiO stretch of the OH surface species formed upon H 2O exposure occurs at 825 cm −1, while the SiO stretch of the OD surface species formed upon D 2O exposure shifts to 840 cm −1, contrary to what is expected for simple reduced mass arguments. The higher resolution of IR measurements versus typical EELS measurements makes it possible to identify a new mode at 898 cm −1, which is an important piece of evidence in understanding the anomalous frequency shift. By comparing the results of measurements for adsorption of H 16 2O, H 18 2O and D 2O with the results from recently performed first-principles calculations, it can be shown that a strong vibrational interaction between the SiO stretching and SiOH bending functional group vibrations of the hydroxyl group accounts for the observed isotopic shifts.