A Mueller matrix spectroscopic ellipsometry approach was used to investigate the anisotropic dielectric constants of corundum α-(Al_xGa_1−x)₂O₃ thin films in their below bandgap spectral regions. The sample set was epitaxially grown using plasma-assisted molecular beam epitaxy on m-plane sapphire. The spectroscopic ellipsometry measurements were performed at multiple azimuthal angles to resolve the uniaxial dielectric properties. A Cauchy dispersion model was applied, and high-frequency dielectric constants are determined for polarization perpendicular (${\epsilon }_{\infty ,\perp }$) and parallel (${\epsilon }_{\infty ,\parallel }$) to the thin film c-axis. The optical birefringence is negative throughout the composition range, and the overall index of refraction substantially decreases upon incorporation of Al. We find small bowing parameters of the high-frequency dielectric constants with $b_{\perp }=0.386$ and $b_{\parallel }=0.307$.

中文翻译：

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α-(AlxGa1−x)2O3(0≤x≤1)中的高频及带隙以下各向异性介电常数

穆勒矩阵光谱椭偏法用于研究刚玉α -(Al _x Ga _{1- x} ) ₂ O ₃薄膜在其低于带隙光谱区的各向异性介电常数。样品组使用等离子体辅助分子束外延在m面蓝宝石上外延生长。光谱椭偏测量是在多个方位角进行的，以解决单轴介电特性。应用柯西色散模型，并确定垂直极化的高频介电常数（${\epsilon }_{\infty ,\perp }$) 和并行 (${\epsilon }_{\infty ,\parallel }$) 到薄膜c轴。在整个组成范围内，光学双折射为负，并且在掺入 Al 后总折射率显着降低。我们发现高频介电常数的小弯曲参数为${乙}_{\perp }=0.386$ 和 ${乙}_{\parallel }=0.307$.