On the thermal stability and optical properties of extreme wide-bandgap MgZnO semiconductor alloys

Mg(x)Zn(1-x)O provide by-design alloys with varying UV-bandgaps, 3.3 to 7eV, depending on the composition x. For an intermediate composition-range the different crystal structures of the end-members may result in a mixed-phase alloy, which renders the system not viable in that range. By growing the alloys away from their thermodynamic-equilibrium, the solubility limit can be extended, and single-phase alloys may be realized. However, a key issue addressed here is the thermal stability of such alloys. Alloys up to x=0.72 were grown at a low-temperature 250C. The as-grown alloys were found via XRD and transmission studies to possess a single-phased wurtzite structure with a bandgap range 3.3 to 4.42eV. The extent of their thermal stability was studied via post-growth annealing up to 900C. Alloys with low-Mg up to x=0.34 were found to be highly stable and retain their optical and material properties. Alloys with higher Mg, up to x=0.72, were found to be unstable and were phase-separated into cubic and wurtzite phases with bandgap ~ 6 and 3.5eV, respectively. It was found that the solubility-limit at x~0.30 is much higher than expected.