Asymmetric ZnO/ZnMgO double quantum well structures grown on m-plane ZnO substrates by MBE

Asymmetric ZnO/ZnMgO double quantum well (QW) structures grown on non-polar m-plane ZnO bulk substrates are investigated. Pairs of 2 and 5 nm wide ZnO QWs were grown on ZnMgO buffer/barrier layers with Mg concentrations over 20%. The QWs are separated with 7 nm or 20 nm ZnMgO barriers. Temperature dependent photoluminescence (PL) and low temperature cross-sectional cathodoluminescence allowed to identify the origin of the observed spectral lines. Well resolved and narrow luminescence lines due to localized and free excitons are observed, confirming excellent quality of the structures. The results of luminescence studies suggest that at the barrier width of 7 nm coupling between the wells enhances excitonic emission in 5 nm well via tunneling of charge carriers from 2 nm to 5 nm QW. No trace of inter-well coupling in a structure with a 20 nm thick barrier is observed. It is found that the binding energies of excitons in 2 and 5 nm wells are 52 and 101 meV, respectively. Such large enhancement of the binding energies is due to the quantum confinement in the wells.