Confinement and interwell coupling effects in Ge double quantum wells pseudomorphic to a Si (001) substrate

We present a theoretical study of the electronic properties of two coupled Ge quantum wells separated by a variable number of Si monolayers, epitaxially grown on a Si (001) substrate. We adopt the real space decimation-renormalization method and a sp3d5s* nearest neighbors tight binding Hamiltonian for the description of the electronic states of bulk Si and Ge crystals. Strain, band offsets and spin-orbit interactions are properly taken into account. From the Green's function of the multilayer structure considered, the energy spectrum and partial and total densities of states projected on each layer and orbital are obtained. This has allowed us to investigate the nature of the valence and conduction confined states and the effect of interwell coupling on the optical properties. In particular we show that position and red shift with Ge width of the experimental no phonon luminescence line of these structures are well interpreted by the present calculation and that additional luminescence lines are predicted.