Intrinsic exciton transitions in high-quality ZnO thin films grown by plasma-enhanced molecular-beam epitaxy on sapphire substrates

High quality ZnO thin films are grown by plasma-enhanced molecular-beam epitaxy on sapphire substrates and free-exciton absorption and exciton-LO phonon absorption peaks is observed in the films at room temperature. It is indicated that the exciton states are stable even at room temperature.