Exciton ionization induced by intersubband absorption in nonpolar ZnO-ZnMgO quantum wells at room temperature

We present a systematic investigation of intersubband transitions in nonintentionally doped m-plane ZnO/ZnMgO quantum wells (QWs). The investigation is performed using photoinduced absorption spectroscopy at room temperature under optical pumping by a UV laser to generate electron-hole pairs. All samples exhibit TM-polarized intersubbandlike absorption resonances. However, the peak transition energy is largely blueshifted (>100 meV) with expectations from electronic quantum confinement simulations. Based on calculations of the exciton binding energies, we attribute the photoinduced absorption at room temperature to the dissociation of hh1-e1 excitons towards free carriers in the e2 state and not to hh1-e1 to hh1-e2 excitonic transitions induced by the intersubband absorption as previously stated by Olszakier [Phys. Rev. Lett. 62, 2997 (1989)10.1103/PhysRevLett.62.2997]. This effect is a consequence of the huge binding energy of excitons in the ZnO material system, which is further enhanced in QWs due to the quantum confinement. This may pave the way for a better understanding of semiconductors' excitonic processes as well as for developing intersubband devices with a blueshifted operating range.