Si/SiGe quantum cascade structures

Quantum Cascade (QC) structures offer a viable route for the fabrication of a Si based laser, since this device relies on intersubband transitions within one band and thus circumventing the problem of the indirect nature of interband transitions in Si. In a previous study we demonstrated the electroluminescence of the heavy hole 2 (HH/sub 2/) to HH/sub 1/ transition in a Si/SiGe QC structure grown pseudomorphically on Si [100] substrates. However, due to the huge amount of strain incorporated into the QC structures with every cascade deposited, this approach of a pseudomorphic Si/SiGe QC structure appears to be rather limited in respect to the number of possible cascades and the incorporation of the waveguide. In addition a compromise between the Ge concentration and the well width has to be considered to achieve the maximal confinement energy without exceeding the critical thickness for the individual layers. The growth of strain compensated Si/SiGe QC structures on relaxed SiGe buffer layers offers a much wider window in the design of the structure, including multiple quantum well structures in the injector and collector regions with a fully developed miniband structure. Here we present results on strain compensated QC structures deposited on relaxed buffer layers. The substrates contain a step graded buffer layer topped with a thick film grown by low pressure chemical vapour phase deposition The substrate was subsequently polished by chemical-mechanical polishing and than overgrown with the cascade structure by molecular beam epitaxy.