Antisites in silicon carbide

Ten years ago, deep-level-transient-spectroscopy (DLTS) signals, assigned to centers labeled as H1, H2, H3, and E2, have been detected in neutron-irradiated 3C SiC. The H centers were believed to be the primary point defects and the E2 center a secondary defect, which forms after the H centers start to migrate. A conclusive identification of these signals has not been presented so far. We present computational evidence that the H centers are due to silicon antisite defects (SiC). In both cubic (3C) and hexagonal (2H) polytypes, the silicon antisite has several ionization levels in the band gap. The positions of these ionization levels in 3C SiC have been calculated accurately with the plane wave pseudopotential method using a large 128-atom site supercell, and compared with the DLTS spectrum. A very good agreement with experimental data indicates that H centers are due to the formation of SiC during neutron irradiation. The formation energies and local geometries of the antisite defects in SiC are also reported.Peer reviewe

Ab initio study of fully relaxed divacancies in GaAs

We report calculations of the electronic and atomic structures of neutral and charged divacancies in GaAs using the first-principles Car-Parrinello method. It is found that the divacancy relaxes inwards in all charge states (2-,1-,0,1+) studied. The defect-induced electron levels lie in the lower half of the fundamental band gap. The doubly negative divacancy is the most stable one for nearly all values of the electron chemical potential within the band gap. The deep-level electron density is localized at the Ga-vacancy end of the divacancy and the ionic relaxation is stronger there than at the As-vacancy end. We have also calculated the thermodynamic concentrations for several different native defects in GaAs, and the implications for self-diffusion are discussed.Peer reviewe

Ab initio study of point defects in CdF2

The plane-wave pseudopotential method is used to study point defects in CdF2. We present comprehensive results for the native defects as well as for dominant impurities. In addition to Fi, VCd and OF were found to be easily formed compensating acceptors. For In and Ga impurities the experimentally observed large Stokes shift could not be established, and the results rule out symmetric atomic relaxation as the mechanism leading to the bistable behavior. The limitations of the present approach utilizing density-functional theory and the local-density approximation in the case of ionic materials are addressed.Peer reviewe

Chlorine-impurity-related defects in ZnSe

Defect complexes formed by chlorine-impurity atoms and native defects in ZnSe are studied by first-principles electronic-structure calculations. The strong tendency for the formation of vacancy-impurity pairs is shown. The chlorine-impurity–zinc-vacancy complex is shown to be the most important source of donor compensation. The results presented are compared with recent experimental results.Peer reviewe

Metastability of the antistructure pair in GaAs

We have studied the metastability of the antistructure (arsenic-antisite gallium-antisite) pair in GaAs using self-consistent, parameter-free total energy methods. Our calculations predict that this defect complex exhibits metastability similar to that of the isolated arsenic antisite. However, the antistructure pair has ionization levels in the band gap in the metastable configuration, unlike the isolated arsenic antisite. The ionization levels enable absorption of infrared light in the metastable state. The results are used to discuss and interpret the arsenic-antisite-type defects observed experimentally in electron-irradiated GaAs.Peer reviewe

Nitrogen-impurity–native-defect complexes in ZnSe

Total-energy calculations for defect complexes formed by nitrogen impurities and native defects in ZnSe are reported. Complexes formed by a substitutional nitrogen bound to a zinc interstitial or a selenium vacancy are shown to be the most probable candidates for the compensating defect in p-type ZnSe. Our results also show that the clustering of defects in ZnSe is an energetically favored process. This may explain the short lifetimes of ZnSe-based devices.Peer reviewe

Spin-density study of the silicon divacancy

The possible charge states of the silicon divacancy V2 are studied using the local spin-density pseudopotential method. The ionic coordinates are relaxed without any symmetry constraints. We obtain the formation and binding energies as well as the ionization levels from total-energy calculations and use them to discuss several experiments. We find using the 216-atom-site supercell that V02 and V−2 have a “mixed” structure that includes both pairing and resonant-bond characters, V02 being more of the pairing type and V−2 more of the resonant-bond type.Peer reviewe

Nitrogen Doping of Amorphous Carbon Surfaces

The surface properties of amorphous carbon ( a−C) are studied using first-principles electronic structure methods. The effect of nitrogen doping near the surface and, in particular, the effect of nitrogen on the work function is studied by doing a series of nitrogen substitutions near the surface. It is found that the work function is reduced by nitrogen doping of the a−C surface at “on top of the surface” sp1 and sp2 sites. Nitrogen doping by low energy ion bombardment is suggested as a doping method to minimize work function of the a−C surfaces.Peer reviewe

Convergence of supercell calculations for point defects in semiconductors: Vacancy in silicon

The convergence of first-principles supercell calculations for defects in semiconductors is studied with the vacancy in bulk Si as a test case. The ionic relaxations, defect formation energies, and ionization levels are calculated for supercell sizes of up to 216 atomic sites using several k-point meshes in the Brillouin-zone integrations. The energy dispersion, inherent for the deep defect states in the supercell approximation, and the long range of the ionic relaxations are shown to postpone the convergence so that conclusive results for the physical properties cannot be obtained before the supercell size is of the order of 128–216 atomic sites.Peer reviewe

Silicon vacancy in SiC: A high-spin state defect

We report results from spin-polarized ab initio local spin-density calculations for the silicon vacancy (VSi) in 3C– and 2H–SiC in all its possible charge states. The calculated electronic structure for SiC reveals the presence of a stable spin-aligned electron-state t2 near the midgap. The neutral and doubly negative charge states of VSi in 3C–SiC are stabilized in a high-spin configuration with S=1 giving rise to a ground state, which is a many-electron orbital singlet 3T1. For the singly negative VSi, we find a high-spin ground-state4A2 with S=3/2. In the high-spin configuration, VSi preserves the Td symmetry. These results indicate that in neutral, singly, and doubly negative charge states a strong exchange coupling, which prefers parallel electron spins, overcomes the Jahn–Teller energy. In other charge states, the ground state of VSi has a low-spin configuration.Peer reviewe