Surfactant-mediated epitaxy of thin germanium films on SiGe(001) virtual substrates

We report on the impact of a surfactant on the growth mode and strain relaxation of thin Ge films on Si_0.21Ge_0.79 virtual substrates grown by surfactant mediated epitaxy on Si(001) wafers. Ge epitaxy without surfactant results in island formation after deposition of only 5 nm Ge. A certain part of the strain in the Ge islands is relaxed via interfacial misfit dislocations, which are located within the core part of the islands. We discuss the possibilities for the occurrence of three-dimensional growth at low Ge layer thickness. The use of Sb as a surfactant suppresses three-dimensional islanding and enables the growth of smooth pseudomorphically strained Ge films on Si_0.21Ge_0.79(001) virtual substrates up to a thickness of 10 nm. At thicknesses higher than 20 nm, the films relax via the formation of a misfit dislocation network at the Ge/ Si_1−xGe_x interface. The surface roughness of up to 30 nm thick layers is below 1.6 nm. Our experimental results corroborate the calculated thickness for plastic relaxation of Ge on Si_1−xGe_x. The effect of the surfactant on the growth of the virtual substrate and on the subsequent growth of Ge on Si_0.21Ge_0.79 is discussed.