Strain relaxation during the surfactant modified epitaxial growth of Ge/Si(001)

Abstract

The initial strain relaxation of Ge on Si(001) has been investigated during epitaxial growth modified by a “surfactant” layer of Sb. Grazing-incidence X-ray diffraction was used to measure the strain relaxation due to its high sensitivity to the in-plane distribution of lattice spacings. We have observed the critical thickness for strain relaxation in the Ge overlayer to be ~ 11 monolayers (ML), with further relaxation developing gradually and in stages. A meta-stable, partially relaxed layer forms up to a coverage of ~ 30 ML, after which a more pronounced relaxation takes place. Even at a coverage of ~ 55 ML, complete strain relief has not been reached, and was only achieved after a 700°C thermal anneal. Concurrent specular reflectivity measurements also reveal that the Ge overlayer grows in a layer-by-layer fashion, and not in the Stranski-Krastanow mode expected for the Ge/Si system. These data show dramatically how the modification of surface energies through the presence of a “surfactant” can affect the morphology of, and strain in, a lattice-mismatched heteroepitaxial system.