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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.
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Thermal stability of HVPE-grown (0001) α-Ga2O3 on sapphire template under vacuum and atmospheric environments
In the present study, thermal stability of α-Ga2O3 under vacuum and ambient pressure conditions was investigated in situ by x-ray diffraction and transmission electron microscopy (TEM). It was observed that the thermal stability of α-Ga2O3 increased by 200 °C when pressure was lowered from an atmospheric to a vacuum level. This finding can be explained by oxygen diffusion under different oxygen partial pressures. In addition, in situ TEM imaging revealed that, once past the decomposition temperature, the onset of phase change propagates from the top crystal surface and accumulates strain, eventually resulting in a fractural film. The mechanism of α-Ga2O3 to β-Ga2O3 transition is evaluated through experiments and is discussed in this manuscript.
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- Award ID(s):
- 2043803
- PAR ID:
- 10488375
- Publisher / Repository:
- AIP
- Date Published:
- Journal Name:
- Journal of Vacuum Science & Technology A
- Volume:
- 41
- Issue:
- 4
- ISSN:
- 0734-2101
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1116/6.0002559
