The adsorption of hydrogen on Si(111)-7×7 as studied by multiple internal reflection spectroscopy

Abstract

Multiple internal reflection (MIR) infrared spectroscopy has been used to study the adsorption of hydrogen on the Si(111)-7×7 surface over the temperature range of 100–400 K. The high resolution of this technique has enabled us to observe three different silicon monohydride stretch modes at 2076, 2086, and 2095 cm−1. Using the polarization selection rules for infrared spectroscopy we have assigned these modes to a silicon monohydride whose bond axis is parallel to the surface normal and two monohydrides whose bond axes are tilted with respect to the surface normal. We have also observed the temperature dependent formation of surface dihydride and trihydride species. The formation of both the dihydride, observed at 2119 cm−1, and the trihydride, seen at 2143 cm−1, is favored by low adsorption temperatures. Finally, we also report a temperature dependent competitive formation between the two higher hydrides with the formation of the dihydride species maximizing near 200 K and lower temperatures favoring the surface trihydride.