Photoreflectance studies of optical transitions in type II (GaAs)m(AlAs)n superlattices

Photoreflectance (PR) spectra of two type II :[001](GaAs)(m)(AlAs)(n) superlattices (SLs) have been measured at 77 K. In the conventional picture of the envelope-function approximation, the lowest conduction state originates, in the first sample, from the X-z point of the AlAs Brillouin zone (z being the growth direction) whereas it originates from the X-x,X-y point in the second sample. Our spectra exhibit Franz-Keldysh oscillation (FKO) features and interband transition lines. The origin of the built-in electric field within the samples is discussed and its strength calculated from FKOs. For interpreting our spectra of interband optical transitions, a least-squares fit of the data to the Aspnes third-derivative functional form has been performed as well as computation of the optical transition energies. From the energy and amplitude of the interband transition lines in PR spectra, we showed that the two SLs are pseudo-direct, i.e. the ground optical transition in any of them is direct in the k space and takes place at the Gamma point of the SL Brillouin zone. All the other interband transitions appearing in the SL spectra are also direct in k space.