Electronic coupling in ZnO/Mg_xZn_(1-x)O double quantum wells grown by pulsed-laser deposition

摘要

We report the investigation of electronic coupling in ZnO/ Mg_XZn_(1-x)O double quantum well structures grown by pulsed-laser deposition (PLD) on a-plane sapphire. A shift of the exciton transition energy to lower energy with decreasing barrier thickness between two coupled quantum wells is observed in cathodoluminescence (CL) and is consistent with effective mass theory. Time-resolved photoluminescence (TRPL) has been performed and shows that the recombination dynamics of coupled double quantum wells (CDQWs) is clearly different from that of ZnO/Mg,Zn_(1-x)O single quantum wells (SQWs). For SQWs only a monoexponential decay is observed under resonant excitation into (excited) QW states. For the CDQWs a combination between a monoexponential decay and a second process described by a power law was found, attributed to indirect excitons. Asymmetric double quantum wells (ADQWs), consisting of two quantum wells of different thicknesses separated by a Mg_XZn_(1-x)O barrier layer, show an additional luminescence peak at higher emission energies than the luminescence of the narrow SQWs. Again, time-resolved measurements provide evidence for electronic coupling. We tentatively attribute this peak to an indirect excitonic transition as a consequence of tunneling of heavy holes from the narrow well into the wide quantum well.