Vibrational study of silicon oxidation: H2O on Si(100)

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 CoSi2 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 Si-O-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 -OH surface species formed upon H2O exposure occurs at 825 cm(-1), while the Si-O stretch of the -OD surface species formed upon D2O 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 (H2O)-O-16, (H2O)-O-18 and D2O 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. (C) 1997 Elsevier Science B.V.