Abstract
The diffusion behavior of hydrogen substitutional sites on the chlorine-terminated Si(100) surface was investigated at elevated temperatures using time-lapse scanning tunneling microscopy (STM). STM movies show that each hydrogen atom undergoes Brownian motion within a monochloride dimer row. The position of a hydrogen substitutional site is exchanged directly with that of an immediate neighboring chlorine atom in either the same dimer (intradimer diffusion) or in one of the two adjacent dimers in the same row (intrarow diffusion). Accordingly, conceptual direct exchange diffusion (DED) in a two-dimensional lattice was experimentally observed. Analysis of STM movies at various temperatures yielded rather low attempt frequencies and energy barriers, leading to the suggestion that the diffusion mechanism involves an intermediate low-energy molecular state. Density-functional theory (DFT) calculations were also performed and provided partial support for the proposed diffusion mechanism. © 2009 The American Physical Society.