ABSTRACT Understanding the mean free path distribution of the dominant heat carriers is very important in determining the ballistic to diffusive heat transport transition in nanoscale devices. This is true for the high electron mobility transistors made from GaN where both the thickness of the buffer layer and localized heating causing ballistic-diffusive heat transfer may complicate the transport properties needed to describe the device thermal response. In this work, we obtain the mean free path–thermal conductivity relation of phonons in bulk wurtzite GaN crystals using two different, ab-initio-based calculations. While the Vienna Ab-initio Simulation Package (VASP) is used in both approaches at the initial stage, the first method does not calculate the third-order force constants (FCs) and approximates the anharmonicity with a single fitting parameter in determination of discrete phonon properties thermal conductivity and relaxation time, while the second method uses third-order force constants directly. Results show that the third-order FCs are important in modeling the contribution of high-frequency optical phonons with relatively short MFPs, to the thermal conductivity of the material. Yet these effects are more significant at high temperatures and at samples without crystallographic disorders, and they can be omitted while modeling the real samples at low temperatures.

中文翻译：

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纤锌矿氮化镓的平均自由程-热导率累积计算：两种方法

摘要 了解主要热载体的平均自由程分布对于确定纳米器件中的弹道到扩散热传输转变非常重要。对于由 GaN 制成的高电子迁移率晶体管来说，这是正确的，其中缓冲层的厚度和导致弹道扩散热传递的局部加热都可能使描述器件热响应所需的传输特性复杂化。在这项工作中，我们使用两种不同的、基于 ab-initio 的计算获得了块状纤锌矿 GaN 晶体中声子的平均自由程-热导率关系。虽然在初始阶段这两种方法都使用了 Vienna Ab-initio Simulation Package (VASP)，第一种方法不计算三阶力常数 (FCs) 并在确定离散声子特性热导率和弛豫时间时使用单个拟合参数近似非谐性，而第二种方法直接使用三阶力常数。结果表明，三阶 FC 在模拟具有相对较短 MFP 的高频光学声子对材料热导率的贡献方面很重要。然而，这些影响在高温和没有晶体学紊乱的样品中更为显着，并且在低温下模拟真实样品时可以忽略它们。结果表明，三阶 FC 在模拟具有相对较短 MFP 的高频光学声子对材料热导率的贡献方面很重要。然而，这些影响在高温和没有晶体学紊乱的样品中更为显着，并且在低温下模拟真实样品时可以忽略它们。结果表明，三阶 FC 在模拟具有相对较短 MFP 的高频光学声子对材料热导率的贡献方面很重要。然而，这些影响在高温和没有晶体学紊乱的样品中更为显着，并且在低温下模拟真实样品时可以忽略它们。