ZONE-FOLDING EFFECT IN SHORT-PERIOD (GAAS)N/(ALAS)N SUPERLATTICES WITH N IN THE RANGE 3-15

Photoreflectance and photoluminescence measurements are carried out in the temperature range from 25 to 275 K to clarify the optical properties of(GaAs)n/(AlAs)n (n = 3-15) short-period superlattices. Our primary interest is in the zone-folded weak transition. Weak signals of photoreflectance associated with the critical point of the pseudodirect transition, a weakly allowed direct transition arising from the zone-folding effect, have been found below the main signals associated with the direct allowed transitions for (GaAs)n/(AlAs)n superlattices with n < 12 at 200 K. Photoluminescence peaks appear at photon energies corresponding to the critical points of this weak structure. This assignment is supported by the temperature dependence of the photoluminescence intensity, which gives the transition probability ratio of the allowed direct-to-pseudodirect transition of about 10(4). Energy-band calculations are performed by using the empirical sp3 tight-binding method including second-nearest-neighbor interactions, where the tight-binding parameters are determined to fit the effective masses of the conduction band at the X point. These calculations show the existence of a weakly allowed direct transition below the strongly allowed direct transition edge in (GaAs)n/(AlAs)n superlattices with n < 12, which shows a good agreement with the present experimental results. These results strongly suggest that the observed weak structures in the photoreflectance arise from the pseudodirect transition, indicating that the conduction band reflects the nature of the zone-folding effect (X(z)-like state at the GAMMA-point).