Density Functional Theory Study of the Impact of Impurities in Silicon Carbide Bulk and Grain Boundaries

Report No. ARL-TR-8836
Authors: Cassidy Atkinson, Matthew Guziewski, and Shawn Coleman
Date/Pages: October 2019; 24 pages
Abstract: Density functional theory is used to determine the favorability of defects in the silicon carbide (SiC) system. Vacancy, substitutional, and interstitial defects are evaluated for bulk crystalline SiC and symmetric tilt Σ9 {122} SiC grain boundaries. Ten impurity atoms are considered; however, impurities of carbon, silicon, and silver are initially examined to compare calculated values to those in literature and to validate parameters used in calculations. All systems are relaxed, and the resulting energy difference is related to nondefected structures to determine the formation energy of the impurity. Lower formation energies determine more-stable, and likely more-favorable systems. Generally, carbon vacancies were found to be more stable than silicon vacancies, and substitutional defects were found to be more favorable than interstitial defects. The formation energy in both substitutional and interstitial sites increased as the atomic radius of the impurity atom increased. For grain boundaries, the calculations performed were more complicated and therefore took more computational hours to complete. For some of the elements, it was found that the grain boundary interstitial energies were lower than the most-favored defect site in the bulk lattice, which indicates a preferential desire to segregate at the interface. This project was performed with the intention of predicting structures that are likely to be encountered during processing SiC. This will allow probable cases to be further examined and classified as beneficial or hazardous to the mechanical properties of the material.
Distribution: Approved for public release
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Last Update / Reviewed: October 1, 2019