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 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 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 H¹⁶₂O, H¹⁸₂O 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.