Optical properties of wurtzite ZnO-based quantum well structures with piezoelectric and spontaneous polarizations

Optical properties of ZnO/MgZnO quantum well (QW) structures, considering piezoelectric and spontaneous polarizations, are investigated by using the non-Markovian gain model with many-body effects. The spontaneous polarization constant for MgO was determined from a comparison with the experiment, which gives a value of about −0.070 C/m². The optical matrix element of the ZnO/MgZnO QW structure decreases with the inclusion of Mg. This is attributed to an increase in the spatial separation between the electron and the hole wave functions due to the large internal field. However, the ZnO/MgZnO QW structure with a relatively high Mg composition (x = 0.3) is found to have a larger optical gain than that with a relatively low Mg composition. This can be explained by the fact that the quasi-Fermi-level separation ∆E_fc in the conduction band increases with the inclusion of Mg. The increase in ∆E_fc is because the QW structure with a high Mg composition has a larger energy spacing in the conduction band. We also know that the exciton binding energy of ZnO/MgZnO QW structures is much larger than that of GaN/AlGaN QW structures. This can be explained by the fact that ZnO/MgZnO QW structures have a larger matrix element than the GaN/AlGaN QW structures and by the smaller dielectric constant.