Papers by Author: William J. Weber

Abstract: The defect properties and atomic configurations in GaN have been comparatively investigated using density functional theory (DFT) and molecular dynamics method with two representative potentials. The DFT calculations show that the relaxation of vacancies is generally small, but the relaxation around antisite defects is large. The N interstitials, starting from any possible configurations, eventually relax into a N+-N< 0 2 11 > split interstitial. In the case of Ga interstitials, the most stable configuration is a Ga octahedral interstitial, but the Ga+-Ga< 0 2 11 > split interstitial can bridge the gap between non-bounded Ga atoms. The formation energies of vacancies and antisite defects obtained using the Stillinger-Weber potential (SW) are in reasonable agreement with those obtained by DFT calculations, whereas the Tersoff-Brenner (TB) potential better describes the behavior of N interstitials.

Abstract: Atomic-level simulations are used to determine defect production, cascade-overlap effects, and defect migration energies in SiC. Energetic C and Si collision cascades primarily produce single interstitials, mono-vacancies, antisite defects, and small defect clusters, while amorphous clusters are produced within 25% of Au cascades. Cascade overlap results in defect stimulated cluster growth that drives the amorphization process. The good agreement of disordering behavior and changes in volume and elastic modulus obtained computationally and experimentally provides atomic-level interpretation of experimentally observed features. Simulations indicate that close-pair recombination activation energies range from 0.24 to 0.38 eV, and long-range migration energies for interstitials and vacancies are determined.

Abstract: Single crystal 4H-SiC was irradiated with 2 MeV Au ions at 165 K. Ion-induced defect configurations and damage accumulation were studied by ion-channeling techniques along the <0001>, > < 3 40 4 and > < 1 20 2 directions. A nonlinear dependence of damage accumulation is observed for both the Si and C sublattices along all three directions, and the relative disorder observed along the > < 3 40 4 and > < 1 20 2 directions is much higher than that along the <0001> direction. The damage accumulation can be described by a disorder accumulation model, which indicates that defect-stimulated amorphization is the primary amorphization mechanism in SiC, and the high disorder level for the large off-axis angles is attributed to particular defect configurations. Molecular dynamics (MD) simulations demonstrate that most single interstitial configurations are shielded by Si and C atoms on the lattice sites along the <0001> direction, which significantly reduces their contribution to the backscattering/reaction yield along the <0001> direction.