Understanding of Ion-Solid Interaction and Defect Evolution in Zinc-Blende Structured Materials

Abstract

Zinc-blende structured materials have received considerable attentions due to their excellent performance in many fields. The major benefit has attributed to high power space energy systems and nuclear reactors. Their applications can expose to high energy radiation, including neutrons, ions and cosmic rays. Under these conditions, defects are generated in materials in amounts significantly exceeding their equilibrium concentrations. The accumulation of defects can lead to undesired consequences, which may alter the performance of the materials. Therefore, the fundamental understanding of ion-solid interaction and defect evolution is a key factor to the success of both nuclear and electronic materials. This thesis focuses on the study of zinc-blende materials, including GaAs, GaN, InAs, and SiC for their possible applications in both nuclear and space fields. SiC has unique capability in the applications of nuclear fuel. In tri-structural isotropic (TRISO) fuel particles, SiC coating is considered as a major barrier for the release of fission products (FPs). However, some metallic FPs (i.e. Ag, Pd, Ru, and I) release from fully intact fuel particles raises serious concern on the safety of high temperature gas-cooled reactors. This thesis first addresses atomistic process of FP diffusion in SiC. Ab initio calculations are used to determine the defects configurations, migration energy barriers and pathways of FPs in SiC. Based on the ab initio results, the interatomic potentials of FP-SiC are developed and evaluated to link between the density functional theory and next coarser level. Classical molecular dynamics (MD) simulations have been employed to investigate FP accommodation in SiC, interactions with point defects and grain boundaries (GBs), and their diffusion kinetics. These findings lead to a conclusion that the GB diffusion of FPs is faster than bulk diffusion with a strong segregation at the GBs. Analysis of the radiation enhanced diffusion obtained by experiments and diffusion by modeling work for Ru and I has suggested the interstitial migration is likely to be a major mechanism under irradiation condition. Moreover, the diffusivities can vary by GB types, whereas high energetic GBs can provide the fastest paths for FPs to diffuse. Particularly, an elevation of 1.5 J/m2 in GB energy can result in 2-3 orders of magnitude difference in Ag diffusion coefficient. We have further explored the defect production, clustering, and its evolution in GaAs, GaN, and InAs, and determined non-ionizing energy loss (NIEL) that indicates the rate of degradation in electronic devices in space applications. Nonlinear defect production is observed with an increasing of primary knock-on (PKA) energy in GaAs and InAs. This effect, which corresponds to the direct-impact amorphization, is observed for PKA energy over 2 keV. GaN is however different and presents a pseudometallic behavior resulting in a majority of surviving defects to be single interstitials or vacancies. With the damage density evaluated from MD simulations, a model to determine NIEL has been developed to qualify the radiation degradation. The NIELs for proton, alpha, and Xe particles are predicted, and provide a pathway to evaluate the capabilities of materials for the space applications. The comparisons of defect creation, density, and effective NIEL suggest GaN may be the best candidate as a radiation hard material for space applications at high-energy regime. For low incident particle energies at which the NIEL ratio of InAs-to-GaN is less than 1, the performance of InAs may be superior to that of GaN.