Structure and energetics of segregated and nonsegregated Ge(001)/Si(2x1)

We present the results of ab initio pseudopotential density functional calculations for the geometry, energetics and electronic structure of the monolayer Si covered Ge(001)(2x1) surface. A segregated structure, in which Si occupies the second layer while Ge floats to the surface, is found to be energetically favorable by 0.38 eV per dimer compared to the nonsegregated Si-capped structure. In the latter case, the Si dimers are found to be asymmetric, with a bond length of 2.26 Angstrom and a tilt angle of 16.7 degrees, while in the segregated structure, the Ge dimers have a bond length of 2.39 Angstrom, and a tilt angle of 17.5 degrees. [S0163-1829(98)00712-7].