Critical layer thickness of MOVPE-grown GaAs on In xGa 1− xAs

Abstract

In this study we report investigations of strain and relaxation in MOVPE-grown GaAs layers on In-alloyed GaAs LEC substrates, with an In-content between 0.1% and 1.15%. The experimentally determined critical layer thickness for the formation of misfit dislocations in the GaAs layers are compared with theoretical calculations of the Gibbs free energy of the epilayer, containing a half loop dislocation which has released a certain amount of elastic energy. A very good matching between theory and experiments is obtained, even in the range of very small misfits. The shift of the bandgap energy as induced by the strain in the epilayer is given by the deformation potential theory. These shifts have been measured using photoluminescence at liquid He temperatures. It appears from these measurements that for layers grown beyond the critical thickness the amount of relaxation is exactly determined by the theoretical equilibrium between elastical deformation and the formation of misfit dislocations. In the region of low misfit dislocation density the dislocations can be distinguished separately by high spatial resolution photoluminescence. When the density becomes too high the depletion regions around the dislocations are overlapping each other. The individual dislocations are not discernable anymore.