Confinement-enhanced biexciton binding energy in ZnO/ZnMgO multiple quantum wells

Abstract

By employing a nanosecond pump-probe method, biexciton formation process was investigated in ZnO/Zn1−xMgxO (x=0.26) multiple quantum wells (MQWs) grown on ScAlMgO4 substrate by laser molecular-beam epitaxy. Bleaching of absorption due to the saturation of excitonic states, and induced absorption related to the exciton–biexciton transition were observed in their spectra. It is demonstrated that the pump-probe method allows us to precisely determine binding energies of exciton complexes even applied to the semiconductor quantum structures where the localization effect are not negligible. This is because a transition from free-excitonic states to free-biexcitonic states is involved in the induced absorption process. The biexciton binding energy is a monotonically decreasing function of well width (Lw). For the MQWs with Lw smaller than 2.5 nm, the biexciton binding energy is larger than 25 meV, comparable to the thermal energy of room temperature.