THE LOW COVERAGE PHASES OF Pb ON Ge(001): SCANNING TUNNELING MICROSCOPY AND FIRST PRINCIPLES

Abstract

Using scanning tunneling microscopy and first principles total energy calculations, we have studied the atomic structure of the low coverage phases of Pb on Ge(001): the (2 × 2) and the c(4 × 8)- α reconstructions. Based on the analysis of the experimental bias-dependent STM images and supported by the ab initio pseudopotential density-functional calculations, we present models for each structure. In both cases, there is a coexistence of Ge and Pb dimers. The (2 × 2) phase is formed by asymmetric Pb dimers parallel and in the trenches between the underlying Ge dimers. The Pb coverage is 0.5 ML and it corresponds to the maximum packing density of Pb dimers without breaking Ge dimers. The c(4 × 8) - α reconstruction has a Pb coverage of 0.75 ML and it is characterized by asymmetric Pb dimers forming chains along the [110] direction. However, in this case half of the Ge dimers are broken. The other half remain intact and they are perpendicular to the Pb dimers.