Tellurium on Ge(001): a perfect restoration of the (1x1) symmetry?

We have performed first-principles total energy calculations to determine the atomic structure of the Ge(001) surface with up to one monolayer (ML) of tellurium. Two different phases have been observed experimentally, one with a tellurium coverage of 1 ML and the other obtained by annealing the surface to 400 degrees C and with a coverage of approximately 0.6 ML. We have found that the most favorable structure for the 1 ML coverage corresponds to tellurium atoms adsorbed on near-bridge sites. The (I x 1) symmetry is broken by a small shift of the tellurium atoms along the +/-[110] directions, giving rise to a slightly disordered surface. A similar displacement also occurs in the presence of tellurium missing rows. To determine the atomic structure of the annealed surface, we have performed calculations for different models, all having 0.5 ML tellurium coverage and c(2 x 2) periodicity. We found that the most stable configuration corresponds to a structure with mixed germanium-tellurium dimers. Structural parameters are in excellent agreement with experimental data. (C) 1999 Elsevier Science B.V. All rights reserved.