Study of structural and optical properties of MBE grown nonpolar (10-10) ZnO/ZnMgO photonic structures

Two types of simple monolithic photonic ZnO/ZnMgO structures, differing by a factor of two in thicknesses of individual components and grown on m-ZnO substrates, are tested to determine limitations in obtaining their good quality at Mg concentrations close to the limit for phase segregation (~40–45%). The studied structures were composed of 5.5 pairs of ZnO/ZnMgO bilayers (20/22 and 40/45 nm), the central ZnO microcavity (70 and 140 nm) and 5 bilayers on top. Actual layer thicknesses were verified using Scanning Electron Microscopy. Rutherford Backscattering Spectrometry was used to determine the Mg content. The X-ray rocking curves revealed high periodicity and proved that the wurtzite structure was retained without precipitates of foreign phases. Reciprocal space maps and the calculated lattice constants indicated that the layers were strained. Excitonic lines in cross-sectional cathodoluminescence (CL) spectra from individual ZnO layers are blue-shifted compared to emissions from ZnO substrates, thus confirming the presence of strain. The CL intensity of the first ZnMgO layers deposited on ZnO substrates and on both interfaces of the central microcavities was low thus suggesting an effective generation of misfit defects, especially in thicker structures. Reflectivity measurements confirmed the existence of cavity resonance at 385 nm and stop band at 370–400 nm in the photonic structure with a 140 nm microcavity. The results show that defect engineering at the initial stages of deposition of ZnMgO layers with high Mg is critical for the optimization of all-ZnO photonic structures.