Optical properties and valence-band masses in spontaneously ordered semiconductor alloys

Abstract

Analytical results are obtained on the valence-band structure in zinc-blende semiconductor alloys that exhibit spontaneous CuPt ordering. The case of pure ordering is considered, as well as the case of coexistance of ordering and lattice mismatch. Ordering is entered through the chemical deformations, and lattice mismatch is introduced by the epitaxial strain. The interband transition probabilities are calculated as a function of the degree of ordering and the polarization angle. It is shown that the optical response in the two possible variants of ordering is different when light is polarized along a nonsymmetrical direction. A similar situation occurs in the case of circularly polarized light. The difference is less significant in complex structures where the two variants of ordering coexist or form overlapping regions. The optical anisotropy in the plane perpendicular to the growth direction is also examined. We find that as the ordering parameter increases, the polarization dependence of the transitions involving the upper valence band becomes more significant. The polarization degree for the light-hole related transitions is a nonmonotonical function of the ordering parameter. The hole masses are calculated analytically as a function of the degree of ordering. It is shown that in the presence of lattice mismatch all hole masses, as well as the orientation of the mass ellipsoid axes, change with the ordering parameter. For weak ordering the largest axis is close to the [001] cubic direction. The anisotropy of the hole masses is governed by the epitaxial strain and is presented for finite band parameter ${\ensuremath{\gamma}}_{2},$ if the growth direction is along the [001] axis. At large-ordering parameter the largest axis of the mass ellipsoid is close to the ordering direction. The mass anisotropy in this case is mostly due to ordering and is presented for finite ${\ensuremath{\gamma}}_{3}.$