This paper investigates the systematic influence of thickness (136–412 nm) and temperature (300–860 K) on the refractive index and the band-edge of aluminum nitride (AlN) films. The combination of x-ray diffraction, spectroscopic ellipsometry (SE), and transmittance measurements at 300 K shows that the increase of epilayer thickness or the introduction of an AlN interlayer can improve the crystal quality. This is observed as an enlargement of the grain size, a reduction of the Urbach binding energy, and strain with a corresponding increase in the refractive index and bandgap. Moreover, the expected reduction in the bandgap and the increase of the refractive index are observed at elevated temperatures by SE. The temperature dependence of the refractive index at 632.8 nm and the bandgap were well understood and modeled using a quadratic nonlinear equation and the Bose–Einstein equation, respectively. This high-temperature phenomenological and quantitative analysis suggests that the reduction of the bandgap with temperature is more significant in thinner or noninterlayer films than expected due to the corresponding stronger electron–phonon interactions involved with larger Urbach binding energies. The thickest AlN film in this work (with an epilayer thickness of 412.9 nm and an interlayer thickness of 20.69 nm) has the smallest strength of the average electron–phonon coupling (407 meV) in the temperature range 300–860 K. On the other hand, the temperature-dependent variation of the refractive index in the transparent region is more rapid as the film thickness decreases owing to the high correlation between temperature-dependent bandgap and refractive index. These observations are critical when designing AlN-based device structures that can operate well above room temperature.

中文翻译：

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厚度和夹层对AlN薄膜室温和高温光学性能的影响

本文研究了厚度 (136–412 nm) 和温度 (300–860 K) 对氮化铝 (AlN) 薄膜的折射率和带边的系统影响。X 射线衍射、光谱椭偏仪 (SE) 和 300 K 下的透射率测量结果表明，增加外延层厚度或引入 AlN 夹层可以提高晶体质量。这被观察为晶粒尺寸的增大、Urbach 结合能的降低以及应变与折射率和带隙的相应增加。此外，通过 SE 在升高的温度下观察到带隙的预期减小和折射率的增加。632 处折射率的温度依赖性。8 nm 和带隙分别使用二次非线性方程和 Bose-Einstein 方程得到了很好的理解和建模。这种高温现象学和定量分析表明，由于相应更强的电子 - 声子相互作用与更大的 Urbach 结合能有关，因此在更薄或非夹层薄膜中带隙随温度的减小比预期的更显着。这项工作中最厚的 AlN 薄膜（外延层厚度为 412.9 nm，层间厚度为 20.69 nm）在 300-860 K 的温度范围内具有最小的平均电子 - 声子耦合强度（407 meV）。另一方面手，由于温度相关带隙和折射率之间的高度相关性，随着薄膜厚度的减小，透明区域中折射率的温度相关变化更快。在设计可以远高于室温的基于 AlN 的器件结构时，这些观察结果至关重要。