We explore the effect of temperatures between 80 and 720 K on the energy and linewidth of zone-center transverse (TO) and longitudinal (LO) optical phonons in bulk gallium phosphide (GaP) using Fourier transform infrared ellipsometry from 0.03 to 0.60 eV. We extract the optical phonon parameters of GaP by fitting the ellipsometric angles with the Lowndes–Gervais model, which applies two different broadening parameters to the TO and LO phonons. In GaP, the two-phonon density of states is larger for the decay of TO phonons than for LO phonons. Therefore, we observed a larger TO phonon broadening (compared to the LO phonon) and an asymmetric reststrahlen line shape. This would lead to a negative imaginary part of the dielectric function just above the LO phonon energy but the addition of two-phonon absorption avoids this. We find a temperature-dependent redshift and broadening of TO and LO phonons with increasing temperature due to thermal expansion and anharmonic phonon-phonon scattering, involving three and four phonon decay processes. We also investigate the temperature dependence of the high-frequency dielectric constant. Its variation is explained by thermal expansion and the temperature dependence of the Penn gap.

中文翻译：

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GaP中光学声子反射带的温度依赖性

我们使用从 0.03 到 0.60 eV 的傅里叶变换红外椭偏法探索了 80 到 720 K 之间的温度对体磷化镓 (GaP) 中区域中心横向 (TO) 和纵向 (LO) 光学声子的能量和线宽的影响。我们通过用 Lowndes-Gervais 模型拟合椭偏角来提取 GaP 的光学声子参数，该模型将两个不同的展宽参数应用于 TO 和 LO 声子。在 GaP 中，TO 声子衰变的双声子态密度大于 LO 声子。因此，我们观察到更大的 TO 声子展宽（与 LO 声子相比）和不对称的 reststrahlen 线形状。这将导致刚好高于 LO 声子能量的介电函数的虚部为负，但添加双声子吸收避免了这种情况。我们发现由于热膨胀和非谐声子 - 声子散射，随着温度升高，TO 和 LO 声子的温度相关红移和展宽，涉及三个和四个声子衰变过程。我们还研究了高频介电常数的温度依赖性。它的变化可以通过热膨胀和 Penn 间隙的温度依赖性来解释。