Anisotropic kinetics of vacancy diffusion and annihilation on Si(001) surfaces studied by scanning reflection electron microscopy

The kinetics of vacancy diffusion and annihilation on Si(001)-2x1 surfaces are studied by using scanning reflection electron microscopy. The S-B steps, which run across dimer rows, retreat 1.8 times faster than S-A steps during low-energy Ar-ion sputtering at elevated substrate temperatures. This leads to virtual single-domain stabilization in the initial stage of layer-by-layer sputtering. These different types of steps maintain an equilibrium configuration due to a step-step interaction. On wide terraces, elongated vacancy islands with a single atomic height and alternative 2x1 reconstruction are formed along the dimer rows. These results indicate preferential vacancy annihilation at S-B steps and anisotropic vacancy diffusion depending on the dimer row direction. The diffusion length of the vacancies is estimated from the width of the denuded zones of the vacancy islands, which are formed on both sides of the atomic steps by thermal heating after vacancy introduction at room substrate temperature. The activation energy for vacancy diffusion along the dimer rows is obtained to be 2.3+/-0.2 eV. A vacancy diffusion model mediated by dimer vacancy complexes rather than single-dimer vacancies, interprets our experimental results.