Recent experiments had revealed an unexpected diffusion behavior of hydrogen on the Si(001) surface at high temperatures and high coverages. Density-functional . theory was employed to investigate H diffusion on the flat Si(001) surface for various coverages; with the main emphasis being on the high-coverage limit of Si(001) monohydride. Three basic diffusion steps, intradimer, intrarow, and interrow were studied both for isolated H atoms on the clean Si(001) surface, as well as for isolated and paired H vacancies on the Si(001) monohydride surface. The barrier energies depended strongly on the distance between the two Si neighbours of the diffusing H atom in the transition state. It was observed that an isolated vacancy was less mobile than an isolated H atom, showing that the Si(001) monohydride surface was more rigid than the clean surface. Interestingly, two adjacent vacancies could transfer dangling-bond charge from one to another prior to a transition of one of them, which significantly lowers the transition barrier. The reaction mechanisms were visualized using maximally localized Wannier functions, and hopping rates were considered within the harmonic approximation to transition state theory in comparison with experimental data.

Ab initio Study of Atomic Hydrogen Diffusion on the Clean and Hydrogen-Terminated Si(001) Surface. J.Wieferink, P.Krüger, J.Pollmann: Physical Review B, 2010, 82[7], 075323