Optical gain characteristics in (1 1 \bar{2}\,0)-oriented CdZnO/MgZnO quantum wells emitting at 360–400 nm

Optical gain characteristics of a-plane CdZnO/MgZnO QW structures with a (1 1 0) crystal orientation were theoretically investigated by using the non-Markovian gain model with many-body effects. These results are compared with those of c-plane quantum-well (QW) structures with a (0 0 0 1) crystal orientation. In the case of the c-plane, the optical gain gradually decreases with increasing Cd composition. This is mainly due to the increase in the piezoelectric and spontaneous polarizations. On the other hand, in the case of the a-plane, the optical gain gradually increases with increasing Cd composition. This can be explained by the fact that the quasi-Fermi-level separation increases with the Cd composition. The a-plane shows much larger optical gain than the c-plane, in particular, for QW structures with a larger Cd composition. This means that the crystal orientation effect is much dominant for the QW structure with a large Cd composition. Also, in the case of a low Cd composition, it is found that the CdZnO/MgZnO QW structure shows much larger optical gain than that of the a-plane InGaN/AlGaN QW structure.