Thermal stability of ε-Ga2O3 polymorph

The thermal stability of epsilon-Ga2O3 polymorph was studied by complementary methods. Epitaxial films of epsilon-Ga2O3 grown on c-oriented sapphire were annealed at temperatures in the range 700-1000 degrees C and then investigated by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). In addition, Differential Scanning Calorimetry (DSC) up to 1100 degrees C was carried out on fragments of pure epsilon-Ga2O3 taken from a very thick layer. The results clearly indicate that epsilon-Ga2O3 initiates modifying its crystallographic structure above 650 degrees C as demonstrated by a mild endothermic bent of the DSC curves. However, the effective transition to beta-phase occurs quite suddenly at 880-900 degrees C, depending of the DSC heating rate. XRD and TEM results confirm this evidence. TEM investigation in particular shows that after annealing at 1000 degrees C and rapid cooling the film is completely made of beta-Ga2O3 grains, most of them with orientation (310) respect to the sapphire substrate. However, if the cooling rate is substantially reduced, the converted beta-Ga2O3 layer assumes the standard orientation (-201) parallal to (00.1) of the Al2O3 substrate. Based on the results of this study we conclude that epsilon-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 degrees C. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.