Two-dimensional confinement of excitons at the interface in nonpolar MgZnO/ZnO heterostructures

We report on the polarity-dependent excitonic emission at the interface in MgZnO/ZnO heterostructures grown on both polar and nonpolar ZnO single-crystal substrates. Structural and morphological analyses confirm that the heterostructures grow homoepitaxially on nonpolar m-plane and a-plane crystal substrates. The nonpolar heterostructures are investigated by depth-resolved cathodoluminescence spectroscopy revealing an excitonic interface emission centered at 3.12 eV, identified as the H-band signature of indirect excitons. These results reveal the formation of robust indirect excitons confined at the MgZnO/ZnO interface due to the presence of an epitaxially strained interfacial layer near the interface in the MgZnO, which is confirmed by high-order X-ray diffraction. It is also found that the H-band emission originates from the self-absorption of the ZnO free exciton emission in the proximity of the interface. Temperature-dependent studies of the H band yields a thermal activation energy of 47 meV for the indirect exciton in the nonpolar heterojunctions. This activation energy is the largest among all free and bound excitons in ZnO and is much higher than the values found in AlGaN/GaN and MgZnO/ZnO/sapphire hetrostructures. Our findings indicate that the optical properties of nonpolar heterostructures are strongly influenced by the piezoelectric effect.