Introduction and survey of early work

The adsorption of atomic hydrogen on silicon is probably the simplest gas—semiconductor system and could provide the basis for a realistic attempt to relate theory and experiment in chemisorption on semiconductors. Early work by Law and Francois [196] and Law [197] established that atomic hydrogen is adsorbed to monolayer coverage, with an 

Ibach and Rowe found the reconstructed surface structures to be very stable to hydrogen adsorption, in that on 111 7x7 and 100 2 x 1 surfaces, no change in LEED patterns occurred up to saturation coverage. The favoured interpretation was based on the model of clean surface reconstruction proposed by Lander [76] and extended by Phillips [202], whereby surface vacancies produce warped benzene ring structures in the first and second surface layers, and such distortions are too large to be removed by hydrogen adsorption. The 111 2 x 1 structure was found to be somewhat less stable, however, and the fractional order spots disappeared on hydrogen adsorption. This was related to the Haneman clean 

More recent and detailed experiments by Hagstrum and co-workers [203—208], together with theoretical calculations by Appelbaum and Hamann [209, 210] and Pandey [211] have, however, considerably modified the results and conclusions reached by Ibach and Rowe. The reasons for this are related to control of substrate temperature and surface coverage, which have been found to be very important to the UPS spectra and LEED patterns observed. The work has also been extended to include Si 110 surfaces [206]. 

To a limited extent, these results support the contention that the H— 111 Si chemisorption system is simple, in that theory and experiment agree that H atoms bond to the single dangling orbital on each Si atom to 

The trihydride phase is energetically more favourable than the monohydride because of the relative strengths of Si—H and Si—Si bonds, the former being 50% stronger. However, in order to form Si lll SiH3, the outermost Si layer has to be removed, which can only occur by 

---