Strain relief by microroughness in surfactant-mediated growth of Ge on Si(001)

The deliberate use of surfactants during the growth of Ge on Si(001) prevents the formation of three-dimensional clusters and allows the deposition of continuous and smooth Ge films. This, however, is not valid for the regime of strained film growth prior to the generation of misfit-relieving defects. Using Sb as the surfactant, an 8-ML-thick pseudomorphic Ge film exhibits a pronounced microroughness on an angstrom scale. The average terrace width is only {~}10 {}. Up to 4--5 vertical layers are simultaneously visible at the surface. This microroughness allows the Ge atoms (which are under compressive stress) to relieve lattice-misfit-related strain by partial lateral relaxation towards their bulk lattice constant. This would not be possible for a flat and continuous film. Now the microrough surface is energetically favored and the influence of the growth kinetics is therefore observed in an increase of the roughness with temperature. Strain-relieving defects are generated at a coverage of {~}12 ML and finally lead to heavily defected films consisting of small-angle mosaics.