Temperature dependence of exciton localization in ZnO/Zn1-xMgxO multiple quantum wells with different barrier compositions

Temperature-dependent photoluminescence (PL) measurements are used to study two ZnO/Zn1-xMgxO multiple quantum well (MQW) structures with different barrier compositions. The PL band in the well layers is dominated by localized excitons (LEs), free excitons (FEs), and two longitudinal optical (LO) phonon replicas of the LE emission. The LE emission of higher Mg compositional barrier exhibits a significant blue shift of about 70 meV with respect to the lower one. The mechanisms of carrier dynamics and localization are investigated within the temperature range 14-300 K. As the temperature increases, luminescence from the excitons localized in the well layers shows an 'S-shaped' shift in the high-barrier MQWs, whereas a monotonic red shift is observed in the low-barrier MQWs. The 'S-shaped' shift behavior is associated with delocalization of the excitons in the potential minima induced by interface fluctuations or alloy disorder. Large exciton binding energies of 64 and 76 meV were deduced, demonstrating efficient quantum confinement in the ZnO/ZnMgO MQWs. (C) 2014 Elsevier B.V. All rights reserved.