High-density-excitation states in GaAs/AlAs type-II superlattices

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High-density-excitation effects on photoluminescence (PL) properties in GaAs/AlAs type-II superlattices (SLs) have been investigated from the aspects of biexcitons and electron-hole droplets. The structures of (GaAs)_m/(AlAs)_n SL samples are as follows : (m, n)=(10, 10), (12, 12), (13, 13), (10, 14), and (10, 20), where m and n indicate the thickness of the GaAs and AlAs layers in monolayer (ML) units (1 ML=0.283 nm), respectively. In the SLs, the lowest-energy exciton consists of the X electron and F heavy hole which are confined in the AlAs and the GaAs layers, respectively : so-called the type-II exciton. The lifetime of the type-Il exciton is of the order of *is : This leads to the advantage for the formation of the high-density-excitation state. Following results have been obtained.1. The biexciton formation is observed under the very low excitation-power condition around 10 mW/cm^2. From the line-shape analysis of the PL bands, the biexciton binding energies are estimated to be 2.5-3.5 mV.The layer-thickness dependence of the biexciton binding energy reveals the following facts : The quantum confinement of the electron and hole increases the binding energy, while the spatial separation decreases it. From time-resolved PL spectra, the transient processes of the formation and decay of the type-II biexciton are clearly detected.2. Under the excitation-power condition three-orders higher than that for the biexciton formation, a novel PL band with a threshold nature in the excitation is observed on the low-energy side of the biexciton band. The shape of the PL band dose not change with the excitation power, and its lifetime is much faster than that of the biexciton PL.These results suggest that the PL band results from the formation of electron-hole droplets. The PL-band shape is explained by a conventional model for electron-hole droplets used in indirect-transition-type semiconductors.