Direct Band-Gap Si-Based Semiconductors, Principals and Prospects.

New direct optical transitions that occur at energies below the fundamental gap of Si have been measured in Si-based structures. The new energy levels have been induced in implanted Si as well as pseudo-morphic Ge-Si atomic layer superlattices. Important features that affect the bandstructure of strained-layer superlattice structures are: superlattice period and symmetry, substrate crystallographic orientation and strain. A strategy for manipulating these parameters to produce a direct bandgap semiconductor out of Ge and Si is discussed in terms of experimental data and theoretical calculations.