Diffusion of interstitial oxygen in silicon and germanium: a hybrid functional study

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 (ReaxFF_present), 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 ReaxFF_present 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 ReaxFF_present successfully reproduces the experimentally and theoretically defined diffusion mechanism in the system and succeeded in overcoming the diffusion problem observed with ReaxFF_SiOH(2010), which results in O diffusion in the Si substrate even at the low temperature such as 300 K.