Epitaxial metal/semiconductor superlattice heterostructures with lattice-matched abrupt interfaces and suitable Schottky barrier heights are attractive for thermionic energy conversion, hot electron-based solar energy conversion, and optical hyperbolic metamaterials. HfN/ScN is one of the earliest demonstrations of epitaxial single-crystalline metal/semiconductor heterostructures and has attracted significant interest in recent years to harness its excellent properties in device applications. Although the understanding of the mechanism of thermal transport in HfN/ScN superlattices is extremely important for their practical applications, not much attention has been devoted to measuring their phonon dispersion and related properties. In this Letter, we employ non-resonant meV-resolution inelastic x-ray scattering to determine the momentum-dependent phonon modes in epitaxial metallic HfN and lattice-matched HfN/ScN metal/semiconductor superlattices. HfN exhibits a large phononic bandgap (∼40 meV) and Kohn anomaly in the longitudinal and transverse acoustic phonon modes at q ∼ 0.73 along the [100] and [110] directions of the Brillouin zone due to the nesting of the Fermi surface by the wave vector ( q). The in-plane [100] acoustic phonon dispersion of the HfN/ScN superlattices is found to be dominated by the HfN phonons, while the optical phonons exhibit both ScN and HfN characteristics. First-principles density functional perturbation theory modeling is performed to explain the experimental phonon spectra, and temperature-dependent thermal conductivity is measured using a pump-probe spectroscopic technique. These results will help understand the phonons in HfN and HfN/ScN metal/semiconductor superlattices for thermionic energy conversion.

中文翻译：

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用非弹性 X 射线散射测量 HfN 和 HfN/ScN 金属/半导体超晶格中的声子带隙和声子异常

具有晶格匹配突变界面和合适肖特基势垒高度的外延金属/半导体超晶格异质结构对于热离子能量转换、基于热电子的太阳能转换和光学双曲超材料具有吸引力。HfN/ScN 是外延单晶金属/半导体异质结构的最早示范之一，近年来引起了人们极大的兴趣，以利用其在器件应用中的优异性能。尽管了解 HfN/ScN 超晶格中的热传输机制对其实际应用极为重要，但并没有太多关注测量它们的声子色散和相关特性。在这封信中，我们采用非共振 meV 分辨率非弹性 X 射线散射来确定外延金属 HfN 和晶格匹配 HfN/ScN 金属/半导体超晶格中的动量相关声子模式。由于费米表面的嵌套，HfN 沿布里渊区的 [100] 和 [110] 方向在 q ∼ 0.73 处的纵向和横向声学声子模式中表现出大的声子带隙 (∼40 meV) 和 Kohn 异常波矢量 (q)。发现 HfN/ScN 超晶格的面内 [100] 声学声子色散以 HfN 声子为主，而光学声子同时表现出 ScN 和 HfN 特性。执行第一性原理密度泛函微扰理论建模来解释实验声子光谱，和温度相关的热导率是使用泵探针光谱技术测量的。这些结果将有助于理解用于热离子能量转换的 HfN 和 HfN/ScN 金属/半导体超晶格中的声子。