Thermal stability and phase transformation of α-, κ(ε)-, and γ-Ga₂O₃ thin films to β-Ga₂O₃ under various ambient conditions

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 and scanning electron microscopy. These respective polymorphs exhibited thermal stability to around 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., (0006) α-Ga₂O₃ ∥ (4̄02) β-Ga₂O₃, (004) κ(ε)-Ga₂O₃ ∥ (310) and (4̄02) β-Ga₂O₃, and (400) γ-Ga₂O₃ ∥ (400) 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.