Adsorption of H, O, and H2O at Si(100) and Si(111) surfaces in the monolayer range: A combined EELS, LEED, and XPS study

Abstract

This paper is a summary of a series of experiments studying the exposure of hydrogen, oxygen, and water, on the (2×1) surfaces of Si(100) and Si(111). While the primary focus has been on high resolution electron energy loss (EELS) results, low energy electron diffraction (LEED) and x‐ray photoelectron spectroscopy (XPS) are also used in the studies. Both the (100) and cleavage (111) surfaces form a monohydride and a dihydride exhibiting a (2×1) and a (1×1) LEED pattern, respectively. These systems exhibit saturation, which is consistent with the model of hydrogen saturation of the dangling bonds. Upon water adsorption the Si–H and Si–OH vibronic modes are observed, indicating that water is decomposed. On the cleavage surface only, there is evidence of a very weak scissor mode, allowing for the possibility of a few percent of molecular water adsorption. Oxygen adsorption is complex. For samples formed at high temperatures (∼1000 K) the observed vibronic features are similar to those known for the Si–O–Si complexes in vitreous glasses. For thin oxide layers (0.5<θ<1.3 monolayers) a linear relationship is observed between oxygen coverage and asymmetric mode frequency. These data are fit with models developed for glasses which, for the monolayer regime, yield an average bond angle of about 130° and a bond distance of 1.65 Å. The results support the model in which the oxygens are envisioned as being inserted into the Si–Si back bonds.