Theory of optical transitions in SiGe(001) strained layer superlattices.
01 January 1987
Superlattices alternating ultra-thin (two, four and six atoms) Si and Ge layers form artificial compound semiconductors. Trends and accurate transition energies for these materials are calculated based on the local density functional and quasiparticle self energy approaches. The lowest transition for the Si substrate material is found to be indirect, but direct transitions to a zone folded final state occur at slightly higher energy. The strain can be transferred to the Si layers with opposite sign by growing the superlattice on a Ge substrate. This reverses the order of the indirect and direct zone-folded transitions which is predicted to yield an approximately direct gap material. However, the allowed dipole matrix elements are small for these new transitions. The description of the near band edge states in these materials in terms of quantum wells in an effective mass approach is found to be reasonable.