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The stability of different surface reconstructions on InAs~001! is investigated theoretically and experimentally. Density-functional theory calculations predict four different surface reconstructions to be stable at different chemical potentials. The two dominant reconstructions are the b2 ~234! for high As, and the a2 ~234! for low As overpressure. This trend is confirmed by scanning tunneling microscopy of carefully annealed InAs~001! surfaces. A similar behavior is predicted for GaAs~001!.

Atomically perfect bismuth lines form on Si(001) by a selective desorption process around the temperature at which most of the bismuth desorbs from bismuth epitaxial layers. The lines are perpendicular to the silicon dimer rows; they are 1 nm wide and can be hundreds of nm long in a flat Si(001) surface. They are utterly free of kinks or other defects. In our proposed model three silicon dimers in the surface are replaced with two Bi dimers, with a rebonded missing dimer defect between them. It is suggested that the lines are straight because of kinetic reasons rather than thermodynamic ones -- they form straight and are unable subsequently to kink.

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 $1.95\ifmmode\pm\else\textpm\fi{}0.2\mathrm{eV}.$ 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.

By means of variable-temperature scanning tunneling microscopy, we have investigated systematically the formation process of Al magic cluster arrays on the $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ surface at high temperature in situ. It was found that the magic clusters form only when the Al coverage is over a critical value, $\ensuremath{\sim}0.08\ifmmode\pm\else\textpm\fi{}0.015$ ML. These clusters occupy preferentially on the faulted-half-unit cells of the $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ surface, but this preference is coverage dependent. By analyzing systematically the spatial distribution of magic clusters below the saturation coverage, an attractive interaction between clusters was found, which prevents a triangular ordering of the nanocluster array.

Ammonia adsorbs dissociatively on the Si(001) surface above $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The majority dissociation products are -H and -$\mathrm{N}{\mathrm{H}}_{2}$ groups, adsorbed on the dangling bonds of the Si dimers. At low coverages, the ammonia fragments are observed to form clumps, which extend both along and across dimer rows, suggesting interactions between molecules: either direct interactions, or those mediated by the buckled dimers on the Si(001) substrate. The detailed reactions in the adsorption and dissociation of $\mathrm{N}{\mathrm{H}}_{3}$ on Si(001) have been investigated by density functional theory calculations, and compared to data from scanning tunneling microscope images. The buckling of the Si dimers provides a mechanism for preferential adsorption next to existing adsorbed species, along a single dimer row. We find that direct H-bonding interactions between chemisorbed $\mathrm{N}{\mathrm{H}}_{2}$ groups and incoming $\mathrm{N}{\mathrm{H}}_{3}\phantom{\rule{0.3em}{0ex}}$ molecules are also significant, including an interaction across dimer rows, which explains the observed clumping behavior.