Band-edge alignment of SiGe/Si quantum wells and SiGe/Si self-assembled islands

We report on the energy band gap and band lineup of SiGe/Si heterostructures either in the case of coherently strained quantum wells or in the case of SiGe/Si self-assembled islands. We take into account the strain field and the quantum confinement effects through an accurate description of the conduction band including the Delta and L bands. The strain field is calculated using a microscopic valence force field theory. The conduction-band diagram and energies are obtained from a 30-band k center dot p Hamiltonian accounting for the strain through the Bir-Pikus Hamiltonian. The band-edge description is first given for biaxially strained pseudomorphic SiGe layers. In SiGe quantum wells grown on relaxed silicon, the band line-up switches from type I to type II depending on the value of the average valence band offset. Applying the 30-band formalism to the case of heterostructures grown on relaxed silicon germanium buffer layers indicates that a better agreement with experimental data is obtained for a valence-band offset value Delta E-v=0.54x where x is the Ge composition. For this parameter, a type-II band lineup is thus expected for all compositions of pseudomorphic SiGe/relaxed Si heterostructures. For GeSi/Si islands, we take into account the strain relaxation in the surrounding Si matrix. A type-II band lineup is predicted for all Ge compositions. The near-infrared interband recombination energy of the islands is calculated as a function of their SiGe composition.