Exchange mechanisms at the Ge/Si(001) interface from a multiple-scattering analysis of the Ge L3 absorption edge

Intermixing in Ge/Si(001) systems has been recently observed and has emerged as a crucial and controversial topic in the study of Ge first stages of interface formation. In this paper we investigated the structure of Ge overlayers epitaxially grown on Si(001) substrates by a refined approach to x-ray absorption spectroscopy (XAS) at the Ge L-3 edge. XAS experimental spectra, recorded in situ using plane polarized synchrotron radiation, have been directly compared to several Ge L-3 near-edge spectral features calculated for different Ge/Si(001) growth models making use of a multiple-scattering approach. This analysis clearly excludes a layer-by-layer Ge laminar growth and confirms the occurrence-of intermixing processes even in the case of Ge room temperature deposition. Moreover, information on the interface structure has been evidenced and the occurrence of a preferential double-layer ordering mechanism along the [111] crystallographic directions has been singled out.