Pressure-Induced Type I-Type II Crossover in AlAs/GaAs Short-Period Superlattices

We have studied the cw and time-resolved photoluminescence spectra of (001)-(AlAs)n/(GaAs)n (n=8~15) short-period superlattices under hydrostatic pressure. At atmospheric pressure, the superlattices with n≤12 are of the type-II and the lowest excited state is X exciton, while the samples with n≥13 are type-I materials and the conduction-band minimum occurs at Γ in the GaAs. It is found from decay pattern analysis that the significant Γ-X mixing occurs only near the direct-indirect crossover and dominantly at an odd number of AlAs monolayers. In the (AlAs)13/(GaAs)13 sample, where the Γ-X crossover just occurs, evidence of the Γ-X mixing can be extracted from energy- and lifetime-versus-pressure data.