Surface geometry and STM image of the Sn/Ge(111)-3 x 3 reconstruction

A theoretical analysis of the STM image of the Sn/Ge(1 1 1)-(3 x 3) surface is presented. In a first step, the atomic structure is reached using a com bination of local-orbital and plane-wave density functional methods. We fou nd a ground state geometry presenting two different types of Sn adatoms, wi th vertical positions differing by similar to0.3 Angstrom. In a second step , this geometry was used to analyze the tunneling currents of this surface in a typical STM experiment. In our approach, a Keldysh formalism is used w ith a coupling between tip and the sample defined by Bardeen-matrix element s: the tunneling currents are found to depend on the semiconductor surface density of states and these Bardeen coupling elements. We study the STM sur face corrugation for positive and negative biases and find that both STM im ages are complementary of each other, in good agreement with the experiment al evidence. (C) 2001 Elsevier Science B.V. All rights reserved.