Structure of steps on As-passivated Si(111):Ab initiocalculations and scanning tunneling microscopy

Abstract

The structures of terrace steps on As-covered Si(111) are examined using ab initio calculations. In scanning tunneling microscope (STM) images, two orientations of steps are found to terminate As-covered Si islands after annealing: $[11\overline{2}]$ and $[\overline{1}\overline{1}2]$. Total energy calculations and simulations of STM images yield detailed information on the atomic structures of the steps: In the lowest-energy configuration for both orientations, surplus As atoms replace any exposed Si atoms at the steps. The As atoms rearrange to establish a threefold coordination. For $[11\overline{2}]$ steps the atom positions of the relaxed configuration are very close to the ideal bulk positions, but for $[\overline{1}\overline{1}2]$ steps the terminating As atoms form dimers in front of the steps. The different atomic structures of the two step orientations clearly show up in the simulated STM images for negative bias (occupied states imaged), the STM images for positive bias (unoccupied states) are much less distinctive. All features of existing experimental positive-bias STM images can be explained by an analysis of the electronic local density of states. With the calculated step energies we construct a phase diagram for the equilibrium shape of As-covered islands. In the accessible range of As chemical potentials between As bulk and ${\mathrm{As}}_{4}$ molecules, we expect either triangular islands terminated by $[11\overline{2}]$ steps or hexagons with long edges of $[11\overline{2}]$ orientation and short ones with $[\overline{1}\overline{1}2]$ orientation.