Optical properties and indirect-to-direct transition of GaP/AlP (001) superlattices

The electronic structure and optical properties of GaP/AlP superlattices are investigated theoretically to clarify the general features of the zone-folding and the band-mixing effects in the superlattice composed of indirect-band-gap semiconductors. The degeneracy of the minimum energies of the conduction band at the X points in the zinc-blende-type bulk materials cannot be lifted by the zone-folding effect alone. The band-mixing effect through the interfaces between two constituent materials plays an important role in determining the overall band lineup throughout the entire Brillouin zone. Thus the electronic structure of the superlattice over the entire Brillouin zone is calculated for the first time in order to determine whether the superlattice is an indirect- or direct-band-gap material. In these calculations, the sp3s* tight-binding method is employed which is known to yield a sufficiently accurate conduction band and to give its minimum position correctly. The electronic structure of the superlattice turns out to be quite sensitive to the combination of the well and barrier layer thickness. This sensitivity suggests the possibility of designing suitable band structures for device application. Oscillator strength in the superlattice is also estimated and is found to be larger for shorter-period superlattices in general. Because the value of the band discontinuity is quite uncertain at present, three previously reported values are applied in the calculations. The electronic structure and the optical properties are revealed for the first time to be very sensitive to the band discontinuity. To explore the possibility of designing the band structure further, the effect of applying pressure is also investigated. Applied pressure induces the changes in the electron transfer-matrix elements primarily through changes of interatomic distances and consequently modifies the electronic band structure. It is found that by applying pressure parallel to the superlattice layer, a transition can be induced from an indirect-band-gap to direct-band-gap material and thus the optical activity is enhanced. These properties suggest that the superlattice composed of indirect-band-gap semiconductors offers great potential for application to optical devices.