Papers by Author: Fei Gao

Abstract: Using density functional theory, we have studied surface structural and electronic properties of sulfur adsorption on As-terminated and In-terminated InAs(001) surfaces with the coverage (Θ) of 0.5ML and 1ML. Based on adsorption energy calculations, we found that at Θ=0.5ML, S adatoms preferred to replace the As atoms at As-terminated InAs(001)(2×1) surface. For 1ML S adsorption on InAs(001)(2×1) surface, the most stable adsorption geometry is S-S dimers covered on the In-terminated surface. This result is different from that for 1ML S adsorption on GaP(001) and InP(001) surfaces, and it is consistent with the experimental results. The electronic band structure analysis showed that the surface state density around the Fermi level was considerably diminished for 0.5ML S adsorption on As-terminated InAs(001)(2×1) surface at substitution site. The surface state density of S-S dimer adsorption on In-terminated (2×1) surface was strengthened due to one excess valence electron on the surface.

Abstract: First-principles calculations based on density functional theory, using PW91 functional have been performed to study the adsorption of Alkali metal (AM) on C(100)(2×1) surface. The stable geometries, adsorption energies for all adsorption configurations have been calculated on half a monolayer and one monolayer. The preferred binding sites have been determined to be valley-bridge sites at the coverage of 0.5ML. At higher coverage of 1ML, two AM adsorbates were found to reside in pedestal site and valley-bridge site, respectively. Work function analysis showed that when AMs are adsorbed on C(100)(2×1) surface, the work function decreases linearly with increasing coverage and reaches a minimum at Θ=0.5ML. At higher coverage, the work function is increased again, which may be caused by depolarization effect of the adsorbate.

Abstract: Molecular dynamics (MD) methods are utilized to study the displacement cascades in α-Fe containing different concentrations of substitutional He atoms. Primary knock-on atom (PKA) energies, Ep, from o.5 keV to 20 keV are considered at a temperature of 100 K and 600 K, and the results are compared with those performed in pure α-Fe. There are distinct differences in the number and size of defect clusters within displacement cascades with and without substitutional helium atoms. Particularly, the number and size of helium-vacancy clusters generally increase with increasing helium concentration and PKA energy. However, the number of He-vacancy (He-V) clusters increases with increasing temperature, the mean size of He-V clusters is independent on temperature for the same He concentration and energy recoils.

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.