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.

中文翻译：

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不同环境条件下 α-、κ（ε）- 和 γ-Ga2O3 薄膜的热稳定性和相变


通过原位高温 X 射线衍射 （HT-XRD） 和扫描电子显微镜 （SEM） 系统研究了亚稳态 α-、κ（ε） 和 γ-Ga2O3 薄膜在空气、N2 和真空退火过程中向热力学稳定的 β-Ga2O3 薄膜的相变。在所有测试环境条件下，这些多晶型物在转化为 β-Ga2O3 之前表现出 ∼471-525 °C、∼773-825 °C 和 ∼490-575 °C 的热稳定性。在相变前后观察到特定的晶体取向关系，即 （0001） α-Ga2O3 → （2 ̄01） β-Ga2O3，（001） κ（ε）-Ga2O3 → （310） 和 （2 ̄01） β-Ga2O3，以及 （100） γ-Ga2O3 → （100） β-Ga2O3。α-Ga2O3 到 β-Ga2O3 的相变导致薄膜的灾难性损伤和表面的隆起。这种损伤的主要和可能的次要原因是 +8.6% 的体积膨胀以及在此过渡期间发生的双重位移和重建转变。κ（ε） 和 γ-Ga2O3 薄膜通过体积变化很小且表面微观结构不变的奇异重建转变转化为 β-Ga2O3。