Eu³⁺ Luminescence Enhancement via Bandgap Engineering of Zn_1–xMg_xO:Eu Epitaxial Thin Films

Abstract

Zinc oxide has long been considered a promising wide bandgap semiconductor for optoelectronic applications in the UV range of wavelengths. On the other hand, Eu-doped ZnO has appeared to be an interesting material for red light applications. In this work, we are looking for a way to increase the red emission from ZnO:Eu by adding Mg to the ZnO host. We explore the effect of tunable wide-bandgap semiconductor host-Zn 1−x Mg x O on the photoexcitation−emission properties of dopant-europium ions. Eu-doped Zn 1−x Mg x O thin films with various Mg compositions were epitaxially grown on a-Al 2 O 3 substrates via oxygen plasma-assisted molecular beam epitaxy. The elemental composition was modified during growth by varying the Zn and Mg fluxes at a constant oxygen flow or by changing the oxygen flow at fixed Zn and Mg fluxes. The Eu doping level is nearly constant at 0.6 ± 0.1 at %. X-ray photoemission spectroscopy measurements show that europium doped in Zn 1−x Mg x O thin films are present in Eu 3+ /Eu 2+ ionic states. Further, the ZnMgO:Eu thin films are explored through X-ray diffraction and scanning electron microscopy analyses. Photoluminescence (PL) analysis shows that the intensity of the 5 D 0 − 7 F 2 red emission of the Eu 3+ ions depends critically on the Mg content. When the bandgap energy of Zn 1−x Mg x O:Eu samples varies from 3.61 to 4.00 eV, the Eu 3+ PL intensity presents nearly a 60-fold increase. We interpret the increased PL efficiency of Eu 3+ as a function of the Mg composition in terms of host-exciton energy transfer to the higher energy levels of Eu ions. The efficient energy transfer to Eu ions is attributed to the increased oscillator strength of excitons and their strong localization in Zn 1−x Mg x O due to the local fluctuations in the Mg content within the material.