Development of the ReaxFF Reactive Force Field for Inherent Point Defects in the Si/Silica System

We redeveloped the ReaxFF force field parameters for Si/O/H interactions that enable molecular dynamics (MD) simulations of Si/SiO2 interfaces and O diffusion in bulk Si at high temperatures, in particular with respect to point defect stability and migration. Our calculations show that the new force field framework (ReaxFFpresent), which was guided by the extensive quantum mechanical-based training set, describes correctly the underlying mechanism of the O-migration in Si network, namely, the diffusion of O in bulk Si occurs by jumping between the neighboring bond-centered sites along a path in the (110) plane, and during the jumping, O goes through the asymmetric transition state at a saddle point. Additionally, the ReaxFFpresent predicts the diffusion barrier of O-interstitial in the bulk Si of 64.8 kcal/mol, showing a good agreement with the experimental and density functional theory values in the literature. The new force field description was further applied to MD simulations addressing O diffusion in bulk Si at different target temperatures ranging between 800 and 2400 K. According to our results, O diffusion initiates at the temperatures over 1400 K, and the atom diffuses only between the bond-centered sites even at high temperatures. In addition, the diffusion coefficient of O in Si matrix as a function of temperature is in overall good agreement with experimental results. As a further step of the force field validation, we also prepared amorphous SiO2 (a-SiO2) with a mass density of 2.21 gr/cm3, which excellently agrees with the experimental value of 2.20 gr/cm3, to model a-SiO2/Si system. After annealing the a-SiO2/Si system at high temperatures until below the computed melting point of bulk Si, the results show that ReaxFFpresent successfully reproduces the experimentally and theoretically defined diffusion mechanism in the system and succeeded in overcoming the diffusion problem observed with ReaxFFSiOH(2010), which results in O diffusion in the Si substrate even at the low temperature such as 300 K.我们针对Si/O/H相互作用重新开发了ReaxFF力场（ReaxFF force field）参数，该参数可用于开展Si/SiO₂界面以及高温下体硅中氧扩散的分子动力学（MD）模拟，尤其适用于研究点缺陷的稳定性与迁移行为。计算结果表明，基于大规模量子力学训练集构建的新型力场框架（ReaxFFpresent），能够准确描述硅网络中氧迁移的内在机制：体硅中的氧扩散通过沿(110)晶面路径上相邻键心位点之间的跳跃实现，跳跃过程中氧原子会经历鞍点处的非对称过渡态。此外，ReaxFFpresent预测体硅中氧间隙原子的扩散势垒为64.8 kcal/mol，与文献中的实验值和密度泛函理论（Density Functional Theory）计算结果吻合良好。我们进一步将该新型力场应用于800~2400 K不同目标温度下体硅中氧扩散的MD模拟。结果显示，氧扩散在1400 K以上的温度下才会启动，且即便在高温条件下，氧原子仍仅在键心位点之间进行扩散。此外，硅基体中氧扩散系数随温度的变化规律整体上与实验结果吻合良好。作为力场验证的进一步步骤，我们还制备了质量密度为2.21 g/cm³的无定形二氧化硅（a-SiO₂），其与2.20 g/cm³的实验值高度吻合，以此构建a-SiO₂/Si界面体系模型。将该a-SiO₂/Si体系在高温下退火至低于计算得到的体硅熔点后，结果显示ReaxFFpresent成功复现了该体系中经实验与理论证实的氧扩散机制，同时解决了ReaxFFSiOH(2010)存在的扩散异常问题——后者即便在300 K这类低温下也会使氧在硅衬底中发生扩散。