Diffusion mechanisms at metallic grain boundaries

Abstract

As the sizes of electronic devices continue to shrink, understanding key atomic phenomena vital to macroscopic processes become increasingly important. Mass transport along grain boundaries (GBs) is such a key process. We have studied diffusion mechanisms at metallic GBs of Ag and Al with the embedded-atom method, with molecular statics (MS) and molecular dynamics (MD), as well as with massively parallel computers at Oak Ridge National Laboratory and Sandia National Laboratory. Formation and migration energies of interstitials and vacancies at the optimal symmetric tilt GBs of different angles in Ag were first obtained using MS at 0 K. Extensive MD simulations were then carried out for selected Ag GBs using massively parallel computers at different finite temperatures up to the melting point. In this way, we were able to determine the dominant diffusion mechanism within different temperature regimes by comparison of the activation energies from MS results for the identified diffusion processes with the MD simulation results. For the first time, the results of this study on Ag GBs had provided realistic explanation and simulation-based evidence for the discrepancy between the activation energies from the recent low-temperature experiments by Ma and Balluffi and those at high temperatures reported in the literature. Preliminary MD results on Al GB diffusion are in excellent agreement with experiment and on-going work on Al and Al-Cu systems aimed to further understand electromigration phenomena will be briefly discussed.