Biexciton formation in GaAs/AlAs type-II superlattices under extremely low excitation powers

We have investigated the biexciton formation in a (GaAs${)}_{12}$/(AlAs${)}_{12}$ type-II superlattice by measuring the cw-excitation-power dependence and decay profiles of the photoluminescence band of the pseudodirect exciton consisting of the AlAs-${\mathit{X}}_{\mathit{z}}$ electron and the GaAs-Γ heavy hole. The photoluminescence-line shape of the pseudodirect exciton excited at ∼1 mW/${\mathrm{cm}}^2$ exhibits a doublet feature having an energy separation of ∼3 meV. The low-energy photoluminescence band, whose line shape is well fitted by a biexciton distribution function, becomes dominant as the excitation power increases. The decay time of the low-energy photoluminescence band (∼0.7 μs) is about one-half of that of the high-energy one. These findings demonstrate the biexciton formation of the pseudodirect exciton under extremely low excitation powers.