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.
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 (2Ϫ,1Ϫ,0,1ϩ) 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 selfdiffusion are discussed.
We report results from spin-polarized ab initio local spin-density calculations for the silicon vacancy (V Si ) 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 t 2 near the midgap. The neutral and doubly negative charge states of V Si in 3C-SiC are stabilized in a high-spin configuration with Sϭ1 giving rise to a ground state, which is a many-electron orbital singlet 3 T 1 . For the singly negative V Si , we find a high-spin ground-state 4 A 2 with Sϭ3/2. In the high-spin configuration, V Si preserves the T d 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 V Si has a low-spin configuration.
A microscopic model for polarization fatigue in ferroelectric perovskites based on dipolar defects formed by the combination of oxygen vacancies and impurity metal ions is presented. The binding energy of an oxygen vacancy to a Pt impurity in PbTiO 3 is calculated to be ϳ2.9 eV. The complex is strongly polar, is stabilized by electron capture, and pins the polarization of the surrounding lattice.
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