Zn1−xMgxO/Eu-doped Zn1−yMgyO (x>y) multi-quantum well structures toward maximizing Eu3+ luminescence in oxide-based light sources

Abstract

Europium-doped wide-bandgap semiconductors are promising materials for red light-emitting devices. Here, we present a europium-doped Zn(Mg)O-based multi-quantum well (MQW) structure design consisting of undoped Zn1−xMgxO barrier layers and Eu-doped Zn1−yMgyO (x > y ≥ 0), quantum wells. A gradient in the Mg content between the layers provides the potential barrier necessary to confine Eu dopants. A reference structure consisting of 20 pairs of Zn0.9Mg0.1O/ZnO:Eu was grown on the c-ZnO substrate. The structural and luminescence properties of these MQWs grown using plasma-assisted molecular beam epitaxy technique are investigated. A systematic comparison of the luminescence intensity and decay dynamics of Zn1−xMgxO/Zn1−yMgyO:Eu MQWs with ZnMgO:Eu epilayers reveals the clear superiority of MQWs over epilayers. The photoluminescence of Eu ions in the MQWs is two orders of magnitude stronger than that of ZnMgO:Eu epilayers, when the Mg content in the Eu-doped QW layer and epilayer is comparable. This can be explained by the increased carrier density around the Eu ions, as a result of their localization in QWs. The results suggest that the Zn1−xMgxO/Zn1−yMgyO:Eu MQWs enhance Eu emission via exciton generation in both the barriers and QW layers, followed by carrier relaxation into the QWs and subsequent energy transfer to the Eu dopants.