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Phase transitions in metastable α-, κ(ε)-, and γ-Ga2O3 films to thermodynamically stable β-Ga2O3 during annealing in air, N2, and vacuum have been systematically investigated via in situ high-temperature x-ray diffraction (HT-XRD) and scanning electron microscopy (SEM). These respective polymorphs exhibited thermal stability to ∼471–525 °C, ∼773–825 °C, and ∼490–575 °C before transforming into β-Ga2O3, across all tested ambient conditions. Particular crystallographic orientation relationships were observed before and after the phase transitions, i.e., (0001) α-Ga2O3 → ( 2 ¯ 01) β-Ga2O3, (001) κ(ε)-Ga2O3 → (310) and ( 2 ¯ 01) β-Ga2O3, and (100) γ-Ga2O3 → (100) β-Ga2O3. The phase transition of α-Ga2O3 to β-Ga2O3 resulted in catastrophic damage to the film and upheaval of the surface. The respective primary and possibly secondary causes of this damage are the +8.6% volume expansion and the dual displacive and reconstructive transformations that occur during this transition. The κ(ε)- and γ-Ga2O3 films converted to β-Ga2O3 via singular reconstructive transformations with small changes in volume and unchanged surface microstructures. [ABSTRACT FROM AUTHOR]

Ga2O3 films were deposited on (100) MgAl2O4 spinel substrates at 550, 650, 750, and 850 ° C using metal-organic chemical vapor deposition and investigated using x-ray diffraction and transmission electron microscopy. A phase-pure γ-Ga2O3-based material having an inverse spinel structure was formed at 850 °C; a mixture of the γ-phase and β-Ga2O3 was detected in films grown at 750 ° C. Only β-Ga2O3 was determined in the films deposited at 650 and 550 ° C. A β- to γ-phase transition occurred from the substrate/film interface during growth at 750 ° C. The growth and stabilization of the γ-phase at the outset of film growth at 850 ° C was affected by the substantial Mg and Al chemical interdiffusion from the MgAl2O4 substrate observed in the energy-dispersive x-ray spectrum. Atomic-scale investigations via scanning transmission electron microscopy of the films grown at 750 and 850 ° C revealed a strong tetrahedral site preference for Ga and an octahedral site preference for Mg and Al. It is postulated that the occupation of these atoms in these particular sites drives the β-Ga2O3 to γ-phase transition and markedly enhances the thermal stability of the latter phase at elevated temperatures. [ABSTRACT FROM AUTHOR]