Electronic density of empty states of Ge/Si(111) epitaxial layers: Theory and experiment

The structural and morphological properties of thin Ge layers deposited on $\mathrm{Si}\mathrm{}\left(111\right)7\times 7$ substrate kept at room temperature and subsequently annealed at several temperatures have been investigated using near-edge x-ray absorption spectroscopy (XAS) at the Ge L edges. Experimental data have been compared with spectra calculated by a multiple-scattering approach in order to obtain the best-fitting interface model. Our results indicate an amorphous growth at room temperature, for coverages ranging from 3 to 14 ML. Two different behaviors are observed after annealing for Ge coverages below and above 3 ML, the so-called critical thickness. For very thin layers, XAS suggests the formation of a continuous crystalline structure driven by strong intermixing processes. Ge layers thicker than the critical thickness have been found to form three-dimensional islands characterized by a lattice-parameter relaxation and dispersed on a wetting layer.