In situ TEM study of κ → β and κ → γ phase transformations in Ga2O3

The temperature-driven phase transformation of metastable kappa-Ga2O3 layers deposited on sapphire was studied by high resolution TEM. Annealing experiments up to 1000 degrees C were performed either in situ in vacuum within the TEM or ex situ in ambient air. This allowed for the detection of the atomistic mechanisms at the basis of kappa to beta phase transition. In the case of in situ TEM observations we could even record in real time the atomic rearrangement. We provide in this paper the relevant crystallographic relations between original kappa and new beta lattice. Surprisingly, the ex situ experiments demonstrated the additional formation of a gamma-Ga2O3 intermediate phase at 820 degrees C. The remarkably different behavior between in situ and ex situ annealing experiments is explained in terms of ambient (ambient air or high vacuum) and heating rate. An extensive investigation of gamma-Ga2O3, also a metastable phase, showed that it has a cubic defect spinel structure (Fd (3) over barm) with disordered vacancies. Repeated observations of the metastable gamma-Ga2O3 after two months show that the vacancies tend to order, and that the vacancies are fully ordered after one year. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.