Strain relaxation of GeSi/Si(001) heterostructures grown by low-temperature molecular-beam epitaxy

Abstract

Plastic relaxation in GexSi1−x∕Si(001) heterostructures with x=0.18–0.62, grown at temperatures of 300–600 °C with the use of a low-temperature (350 °C) Si buffer layer, is considered. It is shown that the use of low-temperature Si and low temperature of growth of GeSi films decreases the density of threading dislocations to the value of 105–106cm−2 in heterostructures with a germanium content x<¯0.3, whereas the density of the threading dislocations in heterostructures with a higher content of Ge remains at the level of ∼108cm−2 and higher. By means of transmission electron microscopy, it is shown that the origination of dislocation half-loops from the surface in the case of a high content of germanium in the film is the main reason for the high density of threading dislocations. Growing of GeSi films with a two-step change in composition is considered. The fact that the density of the threading dislocations in the first step of the film is significantly higher than that in the substrate is noted. Because of their presence, the real thickness of insertion of misfit dislocations into the second step of the film is in ten times less than for the first layer. With an allowance for this effect, almost complete plastic relaxation of the second and further heterostructure steps can be reached at low temperatures and at a smaller thickness of GeSi films. It is concluded that the main factors of low-temperature epitaxy of GeSi, which reduce the density of the threading dislocations in heterostructures are (i) a decrease in the initial threading dislocation density and (ii) an increase in the rate of expansion of dislocation loops, which facilitates plastic relaxation with a smaller number of threading dislocations.