The thermal stability of ε-Ga2O3 polymorph was studied by complementary methods. Epitaxial films of ε-Ga2O3 grown on c-oriented sapphire were annealed at temperatures in the range 700-1000 °C and then investigated by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). In addition, Differential Scanning Calorimetry (DSC) up to 1100 °C was carried out on fragments of pure ε-Ga2O3 taken from a very thick layer. The results clearly indicate that ε-Ga2O3 initiates modifying its crystallographic structure above 650 °C, as demonstrated by a mild endothermic bent of the DSC curves. However, the effective transition to beta-phase occurs quite suddenly at 880-920 °C, depending of the DSC heating rate. XRD and TEM results confirm this evidence. TEM investigation in particular shows that after annealing at 1000 °C and rapid cooling the film is completely made of ε-Ga2O3 grains, most of them with orientation (310) respect to the sapphire substrate. However, if the cooling rate is substantially reduced, the converted ε-Ga2O3 layer assumes the standard orientation (-201) parallel to (00.1) of the Al2O3 substrate. Based on the results of this study we conclude that ε-Ga2O3 may conveniently be used for device fabrication, exploiting its higher crystallographic symmetry and better matching to sapphire. However, all fabrication steps must be carried out at temperatures below 700 °C.
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