Optimization of the growth conditions for molecular beam epitaxy of Mg _xZn_1-xO (0 < x < 0:12) films on Zn-polar ZnO substrates

We report on optimization of the growth conditions for Mg _xZn_1-xO (x = 0; 0:04; 0:05; 0:12) thin films grown on c-plane Zn-polar ZnO single crystal substrates by using plasma-assisted molecular beam epitaxy (PAMBE). A normal vector to the ZnO substrate surfaces was angled at 0:5 ± 0:1° off from the [0001] c-axis toward the [11̄100] direction, leading to a stable step-and-terrace structure. A growth temperature (T_g) higher than 800 °C led to the ZnO films presenting the first excited state luminescence of A-free excitons in photoluminescence (PL) spectra at 12 K. A T_g higher than 800 °C enhanced optical attributes of a Mg_xZn_1-xO film. The longest PL lifetime of fast-decay components reached 3.5 ns in time-resolved PL measurement for an Mg_0.12Zn_0.88O film grown at 900 °C, indicating a concentration of nonradiative recombination centers is substantially eliminated compared to the previously reported PL lifetime of 60 ps for an Mg_0.11Zn_0.89O film grown by pulsed laser deposition.