Impact of Thermal Annealing on Properties of MgGa₂O₄ Thin Films in Oxygen Ambience

In this study, pulsed laser-deposited magnesium gallate (MgGa2O4) thin films on sapphire substrates were annealed at high temperatures in a continuous oxygen flow. The postgrowth thermally annealed MgGa2O4 (MGO) films showed an increasing trend in the position shift of the diffraction peaks with temperatures suggesting an alloy or change in composition. The presence of aluminum (Al) in the photoemission survey spectra after the annealing process implies Al diffusion from the substrate. This also complemented the previous assumption of a compositional change in the MGO films, which possibly caused the position change in the diffraction peaks. Furthermore, the individual photoemission lines revealed oxidation states of +2, +3, and +3 for Mg, Ga, and Al, respectively, in the annealed MGO films. In addition, transmission electron micrographs exhibited the preferential orientation of magnesium gallate films with an individually uniform distribution of the compositional elements throughout the film layer. The annealed MGO films showed an increase in the bandgap with temperatures, which can be attributed to the incorporation of Al into the formation of an alloy. The complex refractive index has been estimated to be ∼1.863 at 632 nm for annealing at 700 °C, with a decreasing trend observed with temperature. The annealed samples’ root-mean-square (RMS) roughness followed a decreasing trend with temperatures indicating an improved surface quality upon the postgrowth thermal treatment.