Electronic structure of Ge/Si monolayer strained-layer superlattices

We report the results of a study of Ge/Si strained-layer superlattices grown on (001) Si substrates. These results allow us to study the transition between superlattice and bulk states. We have examined samples whose superlattice period lies between 3 and 15 Å, similar to the lattice parameter of the crystalline unit cell. All of the samples in this study are ordered superlattices with an average composition of ${\mathrm{Ge}}_{0.5}$ ${\mathrm{Si}}_{0.5}$. Intentional ordering on a monolayer scale was achieved by molecular-beam epitaxy. The optical energy-level spectra of these structures at critical points in the Brillouin zone were measured by Schottky-barrier electroreflectance in the energy range 0.6 to 4 eV. Some features of these spectra can be attributed to the creation of new band-to-band optical transitions that are induced by the artificial periodicity imposed on the sample during growth. These new energy levels are derived from bulk Si and Ge energy levels modified by heterojunction offset, strain, and the lower symmetry of the new unit cell. Several of the new optical transitions observed between 0.6 and 1.5 eV are normally forbidden or weakly allowed. The observation of relatively strong transition amplitudes in electroreflectance suggests that electric field effects and deviations from an ideal diamond-lattice structure may play an important role in the enchancement of transition probabilities in superlattice structures.