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Exciton binding energies in wurtzite ZnO/MgZnO quantum wells

The exciton binding energies of ZnO/MgZnO quantum well (QW) structures were investigated by considering piezoelectric (PZ) and spontaneous (SP) polarizations. These results are compared with those of GaN/AlGaN QW lasers. With increasing sheet carrier density, both QW structures show that the exciton binding energy is significantly reduced, suggesting that excitons are nearly bleached at typical densities ( approximately 1013cm-2 ) for which lasing occurs. The flat-band model of the both QW structure shows that, with decreasing well width, the exciton binding energy increases due to the increasing confinement effect in the well. However, the self-consistent model of both QW structures shows that, with the inclusion of the internal field, the exciton binding energy is substantially reduced compared to that of the flat-band value. This is resulted from the smaller overlap due to spatial separation between the conduction and the valence wave functions as the well width gets larger. We also know that the exciton binding energy of ZnO/MgZnO QW structures is much larger than that of GaN/AlGaN QW structures. The larger exciton binding energy observed in ZnO/MgZnO QW structure can be explained by the larger matrix element than the GaN/AlGaN QW stucture, in addition to its smaller dielectric constant.