Impact of Thermal\nAnnealing on Properties of MgGa<sub>2</sub>O<sub>4</sub> Thin Films\nin Oxygen Ambience

In this study, pulsed laser-deposited magnesium gallate\n(MgGa<sub>2</sub>O<sub>4</sub>) thin films on sapphire substrates\nwere annealed\nat high temperatures in a continuous oxygen flow. The postgrowth thermally\nannealed MgGa<sub>2</sub>O<sub>4</sub> (MGO) films showed an increasing\ntrend in the position shift of the diffraction peaks with temperatures\nsuggesting an alloy or change in composition. The presence of aluminum\n(Al) in the photoemission survey spectra after the annealing process\nimplies Al diffusion from the substrate. This also complemented the\nprevious assumption of a compositional change in the MGO films, which\npossibly caused the position change in the diffraction peaks. Furthermore,\nthe individual photoemission lines revealed oxidation states of +2,\n+3, and +3 for Mg, Ga, and Al, respectively, in the annealed MGO films.\nIn addition, transmission electron micrographs exhibited the preferential\norientation of magnesium gallate films with an individually uniform\ndistribution of the compositional elements throughout the film layer.\nThe annealed MGO films showed an increase in the bandgap with temperatures,\nwhich can be attributed to the incorporation of Al into the formation\nof an alloy. The complex refractive index has been estimated to be\n∼1.863 at 632 nm for annealing at 700 °C, with a decreasing\ntrend observed with temperature. The annealed samples’ root-mean-square\n(RMS) roughness followed a decreasing trend with temperatures indicating\nan improved surface quality upon the postgrowth thermal treatment.