Molecular dynamics simulation of electric field effects on surface diffusion and surface oxidation.

摘要

Scope and Method of Study: Classical molecular dynamics method with charge transferable ES+EAM (Streitz-Mintmire) potential has been employed to study surface diffusion and surface oxidation on FCC metal surfaces under the influence of applied electric field. The potential parameters are firstly optimized by fitting to structural and elastic properties. Diffusion barriers for adatoms and monovacancy are calculated as the optimized energy differences at adsorption site and symmetric transition states. The static barriers are comparable to activation energy derived from Arrhenius equation in dynamic simulations. External electric field is simulated by including the interactions of ionic charges q and field E. In simulating surface diffusions, electric field is modeled as exponential decreasing function. In oxidation simulation, the field is applied via a bias voltage across the sample and electric field in each finite difference grid is calculated by solving the Kirchhoff's law for circuit equations. The field in each grid is updated with time according to the local composition. Heat diffusion equation is numerically solved and the temperature in the dynamic simulations is updated via an ad hoc loop.;Findings and Conclusions: Hopping and exchange mechanisms have different reactions to external field. Exchange barrier increases with field and hopping barrier decreases with field, but with a smaller slope. If the exchange barrier is preferred at zero field, there is a chance that at higher field the two barriers will have crossover and we will observe a mechanism change. This mechanism change is found in Pt/Pt(100) system and confirms previous experimental observations with FIM. If the hopping barrier is preferred at zero field, the diffusion will be promoted by the field but the two barriers will never have crossover at positive electric field. Electric field has limited effect on vacancy diffusion barriers, and adding solute Al to Cu(111) surface cannot improve the lifetime of the interconnects. In metal surface oxidation, electric field and temperature are both important and can enhance the oxide thickness. The miscrostructure of the oxide is dramatically changed by electric field and high temperature, including oxide density, bond length, Al-O pair distribution, coordination number. Our results show the oxide growth can be controlled via electric field and temperature. Our current simulation method provides a new method to study inhomogeneous systems.