Tunneling in ZnO/ZnCdO quantum wells towards next generation photovoltaic cells

摘要

We report on the structural and photoluminescence (PL) properties of ZnO-based multiple quantum wells (MQWs) by metal organic vapor phase epitaxy. Two sets of ZnO/Zn_(0.75)Cd_(0.25)O MQW structures were grown on c-and r-plane oriented sapphire substrates to elucidate the carrier dynamics by varying the well (Zn_(0.75)Cd_(0.25)O) and barrier (ZnO) thicknesses. The high crystalline quality of the structures was confirmed by X-ray diffraction measurements. Importantly, no emission related to ZnO barriers could be resolved in PL spectra implying effective carrier confinement in the wells. This fact was further supported by a prominent blue-shift (～0.5 eV) of the dominant emission from the MQWs with respect to that in a single Zn_(0.75)Cd_(0.25)O layer. The structures with thicker wells are found to exhibit conventional quantum confinement, while samples with thinner wells demonstrate signs of excitonic wave functions penetrating into barrier layers according to results of carrier life-time measurements. It has been demonstrated that the well thickness also influences the tunneling in Zn_(1-x)Cd_xO/ZnO MQWs.The estimated built-in electric field from optical transition is of the order of ～1.75 MV/cm. The observed spectral and carrier lifetime variations are discussed in terms of quantum confinement and internal electric field modulation.