[论文解读] Effects of thermal, elastic, and surface properties on the stability of SiC polytypes

Senja Ramakers, Anika Marusczyk+5 more|arXiv (Cornell University)|Jan 14, 2022

Advanced ceramic materials synthesis参考文献 129被引用 25

一句话总结

本研究采用多方法密度泛函理论（DFT）研究SiC多型相（3C、2H、4H、6H）的热力学稳定性，比较了体相、热力学、弹性及表面性质。结果表明，表面能——特别是Si-terminated (0001) 表面——主导了3C-SiC的成核过程，而体相稳定性则略微倾向于4H/6H；因此，准确的外延生长建模必须同时考虑表面和体相效应。

ABSTRACT

SiC polytypes have been studied for decades, both experimentally and with atomistic simulations, yet no consensus has been reached on the factors that determine their stability and growth. Proposed governing factors are temperature-dependent differences in the bulk energy, biaxial strain induced through point defects, and surface properties. In this work, we investigate the thermodynamic stability of the 3C, 2H, 4H, and 6H polytypes with density functional theory (DFT) calculations. The small differences of the bulk energies between the polytypes can lead to intricate changes in their energetic ordering depending on the computational method. Therefore, we employ and compare various DFT-codes: VASP, CP2K, and FHI-aims; exchange-correlation functionals: LDA, PBE, PBEsol, PW91, HSE06, SCAN, and RTPSS; and nine different van der Waals (vdW) corrections. At $T=0$~K, 4H-SiC is marginally more stable than 3C-SiC, and the stability further increases with temperature by including entropic effects from lattice vibrations. Neither the most advanced vdW corrections nor strain on the lattice have a significant effect on the relative polytype stability. We further investigate the energies of the (0001) polytype surfaces that are commonly exposed during epitaxial growth. For Si-terminated surfaces, we find 3C-SiC to be significantly more stable than 4H-SiC. We conclude that the difference in surface energy is likely the driving force for 3C-nucleation, whereas the difference in the bulk thermodynamic stability slightly favors the 4H and 6H polytypes. In order to describe the polytype stability during crystal growth correctly, it is thus crucial to take into account both of these effects.

研究动机与目标

• 解决长期存在的实验观察（3C-SiC成核）与理论预测（0 K时4H/6H更稳定）之间的矛盾。
• 基准测试多种DFT代码（VASP、CP2K、FHI-aims）、交换关联泛函及范德华校正对SiC多型稳定性的影响。
• 评估热力学、弹性及表面效应对外延生长中3C、2H、4H和6H SiC多型相对稳定性的影响。
• 确定在外延生长过程中，表面能还是体相热力学是多型选择的主导因素。

提出的方法

• 采用三种DFT代码——VASP、CP2K和FHI-aims——结合不同的交换关联泛函（LDA、PBE、PBEsol、PW91、HSE06、SCAN、RTPSS）和基组，以确保方法的稳健性。
• 应用九种范德华校正方案，评估色散力对多型稳定性的影响。
• 计算零温下的体相能量，并通过声子计算引入有限温度下晶格振动的熵贡献。
• 计算所有多型相(0001)面的表面能，重点关注与外延生长相关的Si-terminated和C-terminated终止结构。
• 对不同DFT实现方式和泛函进行系统比较，评估结果的一致性与敏感性。
• 利用声子态密度和亥姆霍兹自由能评估温度依赖的稳定性趋势。

实验结果

研究问题

• RQ1 哪种DFT方法与泛函组合能最可靠地预测SiC多型的稳定性？
• RQ2 晶格振动的热力学贡献如何影响有限温度下3C、4H和6H SiC的相对稳定性排序？
• RQ3 范德华校正及点缺陷引起的双轴应变在多大程度上改变SiC多型的相对稳定性？
• RQ4 表面能——特别是Si-terminated (0001) 表面——在决定3C-SiC相对于4H-SiC的成核偏好中起什么作用？
• RQ5 实验观察到的3C-SiC成核能否仅由体相热力学解释，还是表面能是主导因素？

主要发现

• 在T = 0 K时，4H-SiC比3C-SiC略为稳定，体相能量差为几meV/SiC，具体数值取决于DFT方法。
• 引入晶格振动的熵贡献后，4H-SiC相对于3C-SiC的稳定性进一步增强，强化了其在高温下的热力学偏好。
• 无论是先进的范德华校正还是点缺陷引起的应变，均未显著改变多型相的相对稳定性排序。
• 3C-SiC的Si-terminated (0001) 表面显著比4H-SiC更稳定，表面能差有利于3C成核。
• 该表面能差足够大，可压倒微小的体相能量差，从而解释实验观察到的3C成核倾向。
• 综合分析表明，为准确建模SiC外延生长过程中的多型稳定性，必须同时考虑表面能和体相热力学。