Kinetics of molecular hydrogen dissociative adsorption on Si(111) studied by sum-frequency vibrational spectroscopy and second harmonic generation

Sum-frequency vibrational spectroscopy (SFVS) and optical second-harmonic generation (SHG) were used to monitor hydrogen adsorption on Si(111) by molecular-hydrogen dosing. A properly treated sample exhibited a single H-Si stretch-vibrational mode representing H adsorbed at the top sites. A fully H-terminated Si(111) surface could be obtained by H-2 dosing at sufficiently high temperatures. The observed adsorption kinetics was qualitatively the same as that reported by Hofer and co-workers, including a nearly equal adsorption-energy barrier. However, our results indicate that the initial adsorption on a nearly bare surface obeys the simple Langmuir model. And with subsequent adsorption to higher coverage range, the adsorption gradually becomes cooperative type with the sticking coefficient increasing with H coverage at sufficient high temperature. Evidences of SFVS spectroscopy revealed that it is related to the Si(lll) surface-structure change during H2 adsorption, which appeared to undergo a structural change from (7x7) to (1x1) with increasing temperature or coverage. A simple model was used to qualitatively explain our observation.