Structures and reactions of missing dimers in epitaxial growth of Ge on Si(100)

We present first-principles total-energy calculations for surface atomic structures which provide a natural explanation for island formation in heteroepitaxial growth of Ge on Si(100) surfaces. We first investigate structures of dimer vacancies of missing dimers (MD's) which appear on Ge overlayers on Si(100) due to lattice mismatch (∼4.3%) between the two materials. It is found that rebonded MD's aligned straight in the direction perpendicular to dimer rows (MD lines) are stable against their meandering. This result is consistent with the recent scanning tunneling microscopy measurements and the subsequent statistical analysis. Next we investigate diffusion of a Ge atom adsorbed on the Ge overlayers which exhibit the straight MD lines. The energy barrier for diffusion along the direction of the dimer rows, along which the fast diffusion of Ge adatoms on the clean Si(100) surfaces occurs increases by 0.8 eV near the rebonded MD. This value is much larger than the corresponding value of 0.36 eV from the previous empirical potential calculations. More importantly, this additional energy barrier is large enough to confine the Ge adatom in the flat region (on terrace) surrounded by the MD lines and thereby enhances the island formation of Ge atoms on the Ge overlayers on Si(100).