Diffusion of paired hydrogen on Si(001)

An understanding of the diffusion of hydrogen on Si(001) is important in the gas-source growth of silicon and silicon-germanium alloys. We have imaged the saturated monohydride surface between 600 and 700 K using an elevated-temperature scanning tunneling microscope (STM), and have observed the concerted diffusion of pairs of hydrogen atoms along the dimer rows. The barrier for this process has been estimated from the STM data to be $1.95\pm 0.2\mathrm{eV}.$ Using our experimental images, we have narrowed down the mechanism to two possible routes, which we have modeled with tight binding. Only one of these routes has a barrier (1.98 eV) which is consistent with the experiment, leading to a conclusive identification of the diffusion mechanism.