Exciton optical absorption in asymmetric ZnO/ZnMgO double quantum wells with mixed phases*

Abstract

The optical absorption of exciton interstate transition in Zn1 − xl Mg xl O/ZnO/Zn1 − xc Mg xc O/ZnO/Zn1 − xr Mg xr O asymmetric double quantum wells (ADQWs) with mixed phases of zinc-blende and wurtzite in Zn1 − x Mg x O for 0.37 < x < 0.62 is discussed. The mixed phases are taken into account by our weight model of fitting. The states of excitons are obtained by a finite difference method and a variational procedure in consideration of built-in electric fields (BEFs) and the Hartree potential. The optical absorption coefficients (OACs) of exciton interstate transition are obtained by the density matrix method. The results show that Hartree potential bends the conduction and valence bands, whereas a BEF tilts the bands and the combined effect enforces electrons and holes to approach the opposite interfaces to decrease the Coulomb interaction effects between electrons and holes. Furthermore, the OACs indicate a transformation between direct and indirect excitons in zinc-blende ADQWs due to the quantum confinement effects. There are two kinds of peaks corresponding to wurtzite and zinc-blende structures respectively, and the OACs merge together under some special conditions. The computed result of exciton interband emission energy agrees well with a previous experiment. Our conclusions are helpful for further relative theoretical studies, experiments, and design of devices consisting of these quantum well structures.