Direct and indirect transitions in (GaAs)n/(AlAs)n superlattices with n=1-15

Abstract

Photoreflectance and photoluminescence measurements are carried out to clarify optical properties of (GaAs)/(AlAs) (n = 1-15) short-period superlattices, placing main interest in the crossover of direct-indirect transition and zone-folded weak transition. Photoreflectance spectra of (GaAs)/(A1As) with n < 10 exhibit a weak structure below the main (strong) structure. Photoluminescence peaks appear at the photon energies corresponding to the critical points of these weak and strong structures. Energy band calculations are performed by using the empirical tight-binding method. Calculations of momentum matrix elements between the top valence band and lowest three conduction bands at the I' point show an existence of weakly allowed direct transition below the strongly allowed direct transition edge in (GaAs)/(AlAs)with n < 5. These results strongly suggest that the observed weak structures in the photoreflectance arise from the weakly allowed direct transition, indicating that the conduction band reflects the nature of the zone-folding effect (Xi).