Single hydrogen atoms on the Si(001) surface

Chemisorption of a single hydrogen atom on the n-type Si(001) surface is investigated by scanning tunneling microscopy (STM) and first-principles density functional theory (DFT) calculations. The STM experiments show that the formation of a hemihydride induces static buckling of the neighboring Si-Si dimers and suggest that different buckling configurations of these dimers are observed at negative and positive biases. They also show that the appearance of an isolated Si-Si-H hemihydride on Si(001) exhibits a complex voltage dependence with the brightness of the dangling bond of the hemihydride changing significantly at negative sample bias. DFT calculations predict two stable, ground state atomic configurations for the hemihydride on Si(001). These correspond to parallel and antiparallel configurations of the Si-Si-H hemihydride with respect to the neighboring bare Si-Si dimers. In order to understand the bias-dependent appearance in the STM images of the n-type Si(001) surface, we include the effect of hemihydride charging due to tip-induced band bending. In filled state, the STM images are shown to result from the electronic and structural features that originate from the charge-dependent parallel configuration. In empty state, the energetics and STM measurements support the charge independent antiparallel configuration, while either structure can produce simulated images consistent with experiment.