Abstract The thermal stability of e-Ga 2 O 3 polymorph was studied by complementary methods. Epitaxial films of e-Ga 2 O 3 grown on c-oriented sapphire were annealed at temperatures in the range 700–1000 °C and then investigated by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). In addition, Differential Scanning Calorimetry (DSC) up to 1100 °C was carried out on fragments of pure e-Ga 2 O 3 taken from a very thick layer. The results clearly indicate that e-Ga 2 O 3 initiates modifying its crystallographic structure above 650 °C, as demonstrated by a mild endothermic bent of the DSC curves. However, the effective transition to β-phase occurs quite suddenly at 880–920 °C, depending of the DSC heating rate. XRD and TEM results confirm this evidence. TEM investigation in particular shows that after annealing at 1000 °C and rapid cooling the film is completely made of β-Ga 2 O 3 grains, most of them with orientation (310) respect to the sapphire substrate. However, if the cooling rate is substantially reduced, the converted β-Ga 2 O 3 layer assumes the standard orientation (−201) parallel to (00.1) of the Al 2 O 3 substrate. Based on the results of this study we conclude that e-Ga 2 O 3 may conveniently be used for device fabrication, exploiting its higher crystallographic symmetry and better matching to sapphire. However, all fabrication steps must be carried out at temperatures below 700 °C.

中文翻译：

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ε-Ga 2 O 3 多晶型物的热稳定性

摘要 采用互补方法研究了e-Ga 2 O 3 多晶型物的热稳定性。在 c 取向蓝宝石上生长的 e-Ga 2 O 3 外延膜在 700-1000 °C 的温度范围内退火，然后通过 X 射线衍射 (XRD) 和透射电子显微镜 (TEM) 进行研究。此外，对取自非常厚层的纯 e-Ga 2 O 3 碎片进行了高达 1100 °C 的差示扫描量热法 (DSC)。结果清楚地表明 e-Ga 2 O 3 在 650 °C 以上开始改变其晶体结构，如 DSC 曲线的温和吸热弯曲所示。然而，在 880-920 °C 时，β 相的有效转变发生得非常突然，这取决于 DSC 加热速率。XRD 和 TEM 结果证实了这一证据。特别是 TEM 研究表明，在 1000 °C 下退火并快速冷却后，薄膜完全由 β-Ga 2 O 3 晶粒构成，其中大部分晶粒相对于蓝宝石衬底具有 (310) 取向。然而，如果冷却速率显着降低，则转化的β-Ga 2 O 3 层呈现平行于Al 2 O 3 衬底的(00.1)的标准取向(-201)。基于这项研究的结果，我们得出结论，e-Ga 2 O 3 可以方便地用于器件制造，利用其更高的晶体对称性和与蓝宝石的更好匹配。但是，所有制造步骤都必须在低于 700 °C 的温度下进行。转换后的β-Ga 2 O 3 层呈现标准取向(-201)，平行于Al 2 O 3 衬底的(00.1)。基于这项研究的结果，我们得出结论，e-Ga 2 O 3 可以方便地用于器件制造，利用其更高的晶体对称性和与蓝宝石的更好匹配。但是，所有制造步骤都必须在低于 700 °C 的温度下进行。转换后的β-Ga 2 O 3 层呈现标准取向(-201)，平行于Al 2 O 3 衬底的(00.1)。基于这项研究的结果，我们得出结论，e-Ga 2 O 3 可以方便地用于器件制造，利用其更高的晶体对称性和与蓝宝石的更好匹配。但是，所有制造步骤都必须在低于 700 °C 的温度下进行。