Phonon-assisted exciton formation in ZnO/(Zn, Mg)O single quantum wells grown on C-plane oriented substrates

We report on absorption phenomena in ZnO/(Zn, Mg)O quantum wells grown along the c-axis by molecular beam epitaxy. The optical properties of such quantum wells are affected by a huge internal electric field. For wide quantum wells the absorption is driven by Quantum Confined Stark Effect. Phonon-assisted formation of excitons is observed in the case of thin quantum wells. The physical origin of these hot excitons is determined by using both low temperature (T=10K) photoluminescence excitation spectroscopy and reflectivity measurements. ► High structural quality ZnO/(Zn, Mg)O quantum wells are growth along the polar c-direction. ► Indirect phonon-assisted formation of excitons in the thin single quantum wells. ► Strong internal electric field present in polar heterostructures prevents the observation of hot excitons.