Exchange mechanisms at the Ge/Si(001) interface from a multiple-scattering analysis of the GeL3absorption 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.