A series of MOCVD grown MgZnO epitaxial layers (1m thick) with increasing Mg incorporation on ZnO/GaN/sapphire substrate are investigated. No remarkable change of the morphology is found with increasing Mg-content. The laterally integrated cathodoluminescence (CL) spectrum of each sample is dominated by three spectral bands. Two bands at higher energies can be assigned to the MgZnO emission, i.e. for a [Mg] = 6% E1= 3.474eV and E2= 3.442eV at 5K. These two peaks are clearly separated from the ZnO emission (E= 3.356eV). The two MgZnO peaks shift simultaneously to higher energies with increasing Mg-content. Furthermore MgZnO/ZnO/MgZnO single quantum well (SQW) structures on ZnO/GaN/sapphire substrate are investigated and two additional luminescence lines appear clearly assignable to the 2nm thick single quantum well.
Microscopic morphological features of the layers directly correlate with the local spectral CL emission characteristic. While the high energy peak of MgZnO buffer and of the SQW exhibit an almost homogeneous lateral distribution, the emission of the low energy peak of the MgZnO barrier as well as that of the SQW vanishes completely at local surface defects.
The excitation and the temperature dependence is investigated by spatially integrated photoluminescence. No spectral shift with excitation density is found for all samples. However, the intensity ratio of the both MgZnO peaks strongly changes: The intensity of the high energy MgZnO peak exhibits an almost perfectly linear dependency on excitation density whereas the low energy peak increases significantly stronger with excitation. Identical dependencies are found for the SQW structures for both, the MgZnO barriers as well as the two peaks from the ZnO-SQW. Spectral-time-resolved measurements directly evidence the transient competition between the two different recombination channels in the SQW as well as in the MgZnO barriers and the thick MgZnO layers, respectively.

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