Nitrogen Vacancies as Major Point Defects in Gallium Nitride

We present results of ab initio calculations for vacancies and divacancies in GaN. Particular attention is paid to nitrogen vacancies and mixed Ga-N divacancies in negatively charged states, which in n-type GaN are found to be energetically comparable with gallium vacancies. We also demonstrate that the activation energy for self-diffusion over the nitrogen sublattice is lower than over the gallium one for all Fermi-level positions, which implies the nitrogen vacancies are major defects in samples annealed at high temperatures. Possibilities for direct observations of nitrogen vacancies through positron annihilation experiments are discussed.Peer reviewe

Hydrogen in beryllium: Solubility, transport, and trapping

The paper presents the results of ab initio simulation of hydrogen properties in beryllium. Both interstitial hydrogen positions in the lattice and various hydrogen positions in a vacancy have been studied. The most energetically favorable interstitial hydrogen configuration among the four considered high-symmetry configurations is the basal tetrahedral one, in agreement with the earlier predictions. The most probable diffusion pathway for hydrogen atoms in the bulk involves the exchange of octahedral and basal tetrahedral positions with the effective migration energy of ∼0.4 eV. For hydrogen atom in a vacancy, an off-center (nearly basal tetrahedral) configuration is definitely preferred. Addition of more hydrogen atoms to a vacancy remains energetically favorable up to at least five hydrogen atoms, though the binding energies fall down with the increase in the number of hydrogen atoms in the vacancy.Peer reviewe

Electronic structure of the phosphorus-vacancy complex in silicon: A resonant-bond model

Using first-principles calculations, the electronic structure of the phosphorus-vacancy pair in silicon has been studied. Detailed analysis of the atomic displacement fields associated with the atomic structure optimization after the defect formation indicates a strong dependence of the character and magnitude of relaxation both on the supercell size and the E-center charge state. Our simulation results strongly suggest that the E-center structure is of the resonant-bond type with a strong localization of an electron pair at the phosphorus atom. The energy level splitting for shared electrons in a vacancy due to the appearance of the resonance distortion is discussed, as well as the nature of and the reasons for the level occupation.Peer reviewe

Quasigraphite: Density functional theory based predictions of a structure and its properties

Based on density-functional calculations, we propose a carbon-based nanostructure which we call quasigraphite phase. The quasigraphite phase resembles carbon nanotubes welded into planes, which are arranged similar to layers in graphite. It demonstrates a strong stability with respect to temperatures and external strain. The elastic and electronic properties of the proposed structure are discussed.Peer reviewe

Modeling the compositional instability in wurtzite Ga1-x InxN

The paper deals with multiscale modeling of the minor component ordering in wurtzite Ga1−xInxN (x<0.5) alloys. The treatment combines the total-energy density-functional calculations of the metal atom interaction parameters and the atomistic description of the alloy decomposition using lattice kinetic Monte Carlo. It is demonstrated that the phase decomposition patterns in wurzite GaInN are very sensitive to the interplay of metal atom interactions at several interatomic distances (at least to the fourth nearest neighbors) on the cation sublattice. Variation of the metal interaction energies within reasonable limits resulted in pronouncedly different relaxation patterns (linear or wall ordering of In and Ga atoms along c-axis, planar ordering parallel to basal plane, spinodal decomposition). The high sensitivity of the GaInN decomposition to relatively small variations of the metal interaction energies could be the main reason for the experimentally observed versatility of the alloy decomposition patterns and their sensitivity to the particular experimental conditions.Peer reviewe

Vacancies in CuInSe(2): new insights from hybrid-functional calculations

We calculate the energetics of vacancies in CuInSe(2) using a hybrid functional (HSE06, HSE standing for Heyd, Scuseria and Ernzerhof), which gives a better description of the band gap compared to (semi)local exchange-correlation functionals. We show that, contrary to present beliefs, copper and indium vacancies induce no defect levels within the band gap and therefore cannot account for any experimentally observed levels. The selenium vacancy is responsible for only one level, namely, a deep acceptor level is an element of(0/2-). We find strong preference for V(Cu) and V(Se) over V(In) under practically all chemical conditions

Hydrogen transport on graphene: Competition of mobility and desorption

The results of molecular dynamics (MD) simulations of atomic hydrogen kinetics on graphene are presented. The simulations involve a combination of approaches based on Brenner carbon-hydrogen potential and first-principles force calculations. Both kinds of MD calculations predict very similar qualitative trends and reproduce equally well the features of hydrogen behavior, even such sophisticated modes as long correlated jump chains. Both approaches agree that chemisorbed hydrogen diffusion on graphene is strongly limited by thermal desorption. This limitation rules out long-range diffusion of hydrogen on graphene but does not exclude the short-range hydrogen diffusion contribution to hydrogen cluster nucleation and growth.Peer reviewe

Hydrogen interaction with fullerenes: From C[sub 20] to graphene

The paper presents a systematic study of the trends in the interaction of hydrogen with carbon fullerenes versus their curvature, where graphene is taken as the limit of zero curvature. The efficiency of hydrogen incapsulation in fullerenes, penetration into them, and adsorption on their surface are analyzed and discussed. The effects on magnetism are also considered; in particular, it is shown that hydrogen adsorption to some fullerenes induces magnetism to initially nonmagnetic systems. In addition, highly hydrogen-saturated fullerenes are examined and the suitability of fullerenes for hydrogen storage is discussed.Peer reviewe

Energetics of intrinsic defects and their complexes in ZnO investigated by density functional calculations

Formation energies of various intrinsic defects and defect complexes in ZnO have been calculated using a density-functional-theory-based pseudopotential all-electron method. The various defects considered are oxygen vacancy (VO), zinc vacancy (VZn), oxygen at an interstitial site (Oi), Zn at an interstitial site (Zni), Zn at VO (ZnO), O at VZn(OZn), and an antisite pair (combination of the preceding two defects). In addition, defect complexes like (VO+Zni) and Zn-vacancy clusters are studied. The Schokkty pair (VO+VZn) and Frenkel pairs [(VO+Oi) and (VZn+Zni)] are considered theoretically for the first time. Upon comparing the formation energies of these defects, we find that VO would be the dominant intrinsic defect under both Zn-rich and O-rich conditions and it is a deep double donor. Both ZnO and Zni are found to be shallow donors. The low formation energy of donor-type intrinsic defects could lead to difficulty in achieving p-type conductivity in ZnO. Defect complexes have charge transitions deep inside the band gap. The red, yellow, and green photoluminescence peaks of undoped samples can be assigned to some of the defect complexes considered. It is believed that the red luminescence originates from an electronic transition in VO, but we find that it can originate from the antisite ZnO defect. Charge density and electron-localization function analyses have been used to understand the effect of these defects on the ZnO lattice. The electronic structure of ZnO with intrinsic defects has been studied using density-of-states and electronic band structure plots. The acceptor levels introduced by VZn are relatively localized, making it difficult to achieve p-type conductivity with sufficient hole mobility.Peer reviewe