Hydrogen adsorption and desorption on Si(100) and Si(111) surfaces investigated by in situ surface infrared spectroscopy

The hydrogen adsorption and desorption on Si(100)(2×1) and Si(111)(7×7) were investigated by using in situ surface infrared spectroscopy in the multiple internal reflection geometry. At initial stages of room-temperature adsorption of atomic hydrogen, dangling bonds of surface silicon atoms are terminated by hydrogen with the surface reconstruction structure retained, producing the so-called 'double-occupied dimer' (HSi-SiH) on Si(100)(2×1) and the monohydride Si (Si-H) perpendicular to the surface on Si(111)(7×7). On Si(111)(7×7), hydrogen adsorption onto the adatom and the rest atom have been distinguished, which have two different Si-H vibration frequencies. For high hydrogen exposure, atomic hydrogen breaks the surface Si-Si bonds (dimer bonds and backbonds) to produce higher hydride species: dihydride (SiH₂) and trihydride (SiH₃). Upon annealing of the hydrogen-exposed surface at moderate temperatures, trihydride species are thoroughly etched away with monohydride and dihydride remaining. We find that the conversion from the monohydride to the dihydride phase occurs during thermal annealing of the hydrogen-saturated Si(100)surface. Thermal annealing of the hydrogen-exposed Si(111) surface produces hydrogen-terminated (1×1) domains.