Exciton localization and phonon sidebands in polar ZnO/MgZnO quantum wells

Exciton localization is studied for polar ZnO/Mg sub(0.27)Zn sub(0.73) O quantum wells (QWs) grown by pulsed laser deposition. The samples exhibit both quantum confinement as well as the quantum-confined Stark effect. Emission energies between 3.46 and 3.18 eV, depending on the QW thickness, are observed in wedge-shaped samples. Intense QW-phonon sidebands are observed as a result of a strong exciton-phonon interaction and the localization of excitons. To calculate the Huang-Rhys factor S correctly, the concept of strongly and weakly localized excitons is applied. With increasing QW thickness from 3 to 9 nm, a decrease of S from 0.67 to 0.43 and an increase of the fraction of strongly localized excitons from 0.2 to 0.63 is observed. The decrease of S is explained by a strong increase of the in-plane pseudo-Bohr radius, whereas the increase of the fraction of strongly localized excitons is expected to result from the increasing strength of localization due to the internal electric field. In this regard, a theory based on an effective mass approximation is applied in calculations to determine the shape of the excitons under the appearance of an internal electric field.