Fine structure of heavy excitons in GaAs/AlAs superlattices

A splitting of the optically allowed states of the heavy exciton in GaAs/AlAs short-period, pseudodirect superlattices was recently reported. Moreover, it was shown that the sublevels are dipole active along 〈110〉 crystallographic directions. In this work, the splitting energy, of the order of a few microelectronvolts, is determined for several samples from the period of the quantum beats observed in photoluminescence excited with a 〈100〉 polarized light. The time-resolved photoluminescence is analyzed within the framework of the density-matrix formalism to obtain the lifetime and spin-relaxation time of excitons. We show that the perturbation that splits the heavy-exciton states has the symmetry of either an electric field ${\mathit{E}}_{\mathit{z}}$ along the growth axis or an ${\mathrm{\varepsilon }}_{\mathit{x}\mathit{y}}$ shear strain. This perturbation couples the heavy- and light-exciton states and gives a splitting energy proportional to the electron-hole exchange interaction and to the strength Z of the perturbation and inversely proportional to the energy difference between light and heavy excitons. The value of Z is found to be the same for all the samples studied, Z=14.5±1.5 meV. We discuss possible origins of the perturbation.