Band Structure Engineering for Ultra-Low Threshold Laser Diodes

Abstract

It has been proposed that strained layer superlattices be used to reduce the effective mass of holes, thereby reducing the transparency electron density and subsequently the lasing threshold. In conventional bulk materials, the degeneracy of heavy and light hole bands at zone center means domination of the heavy hole band in hole occupation. It was known that, by applying strain to the crystal, the degeneracy can be broken such that under a biaxial compressive stress, the light hole band is lifted above the heavy hole band in the k-vector directions parallel to the applied strain. Since the heavy hole is five times heavier than the light hole, the resultant effective hole mass can be reduced by a factor of 5. The strain can be built in by growing lattice-mismatched InGaAs on GaAs substrate. Such lattice-mismatched epitaxial layers cannot be arbitrarily thick for it to be defect-free, the maximum thickness being only about 100 Angstroms.