Ab initiostudy of atomic hydrogen diffusion on the clean and hydrogen-terminated Si(001) surface

Abstract

Recent experiments have shown an unexpected diffusion behavior of hydrogen on the Si(001) surface at high temperatures and high coverages. To shed some light on this behavior, we have employed density-functional theory to investigate H diffusion on the flat Si(001) surface for different coverages with main emphasis on the high-coverage limit of Si(001) monohydride. Three basic diffusion steps, intradimer, intrarow, and interrow have been 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 depend strongly on the distance between the two Si neighbors of the diffusing H atom in the transition state. We observe that an isolated vacancy is less mobile than an isolated H atom showing that the Si(001) monohydride surface is more rigid than the clean surface. Interestingly, two adjacent vacancies may transfer dangling-bond charge from one to another prior to a transition of one of them, which significantly lowers the transition barrier. We visualize the reaction mechanisms using maximally localized Wannier functions and we discuss hopping rates within the harmonic approximation to transition state theory in comparison with experimental data.