The kinetics of vacancy diffusion and annihilation on (001) 2 x 1 surfaces were studied by means of scanning reflection electron microscopy. It was found that SB steps, which ran across dimer rows, retreated 1.8 times faster than did SA steps during low-energy Ar-ion sputtering at high substrate temperatures. This led to single-domain stabilization in the initial stages of layer-by-layer sputtering. The various types of step maintained an equilibrium configuration, due to step-step interaction. On wide terraces, elongated vacancy islands with a height of one atom, or an alternative 2 x 1 reconstruction, formed along dimer rows. The results indicated the occurrence of preferential vacancy annihilation at SB steps, and an anisotropic vacancy diffusion, depending upon the dimer-row direction. The diffusion length of the vacancies was estimated from the width of the denuded zones of the vacancy islands which formed on both sides of the atomic steps during heating after vacancy introduction at room temperature. The activation energy for vacancy diffusion along the dimer rows was deduced to be 2.3eV (figure 2). A vacancy diffusion model which involved dimer vacancy complexes rather than single-dimer vacancies, was used to interpret these results.

H.Watanabe, M.Ichikawa: Physical Review B, 1997, 55[15], 9699-705