Hybrid density functional calculations of the defect properties of ZnO:Rh and ZnO:Ir

We report density functional calculations of the atomic and electronic structure of the spinel phases ZnRh2O4 and ZnIr2O4 as well as crystalline ZnO lightly doped (1 at.%) with Rh and Ir ions using the B3LYP hybrid functional. Calculations for the spinels show band gaps (∼3 eV) and lattice parameters (∼2% difference) in reasonable agreement with experimental data. Incorporation of the transition metals into ZnO induces local distortions in the lattice and the appearance of metal d levels in the low gap region and near the conduction band minimum, with a d-d splitting larger than 2 eV, which helps maintain transparency in the material. Addition of a hole to the simulation cell of both spinels and doped ZnO leads to charge localization in the neighbourhood of Rh/Ir accompanied by local lattice deformations to form a small polaron which may lead to low hole mobility. We calculate polaron diffusion barriers in the spinels and obtain values around 0.02-0.03 eV. These very low barrier energies suggest that at high Rh/Ir concentration hole conduction occurs mainly by the band conduction mechanism at room temperature. We also develop models of the amorphous spinels by means of classical molecular dynamics simulations, and observe a marked reduction in the coordination number of Rh/Ir, from 6 to 4, in the amorphous phase, which may reduce transparency in these materials.Financial support for this work is provided by the European Commission through contract No. NMP3-LA-2010-246334 (ORAMA).This is the originally submitted version of the article, and does not include any of the changes arising from peer-review. The final, peer-reviewed and edited version of the article is available at http://www.sciencedirect.com/science/article/pii/S0040609013013059

The effect of defects and disorder on the electronic properties of ZnIr2O4.

We analyze by means of ab initio calculations the role of imperfections on the electronic structure of ZnIr2O4, ranging from point defects in the spinel phase to the fully amorphous phase. We find that interstitial defects and anion vacancies in the spinel have large formation energies, in agreement with the trends observed in other spinels. In contrast, cation vacancies and antisites have lower formation energies. Among them, the zinc antisite and the zinc vacancy are the defects with the lowest formation energy. They are found to act as acceptors, and may be responsible for the spontaneous hole doping in the material. They may also induce optical transitions that would reduce the transparency of the material. Amorphization of ZnIr2O4 leads a large decrease of the band gap and appearance of localized states at the edges of the band gap region, which may act as charge traps and prevent amorphous ZnIr2O4 from being a good hole conductor.Financial support for this work is provided by the European Commission through contract No.NMP3-LA-2010-246334 (ORAMA). We acknowledge computational support from the UK national high performance computing service ARCHER, for which access was obtained via the UKCP consortium and funded by EPSRC grant EP/K014560/1.This is the final version of the article. It was originally published by AIP in The Journal of Chemical Physics here: http://scitation.aip.org/content/aip/journal/jcp/141/8/10.1063/1.4893556

Effect of trivalent dopants on local coordination and electronic structure in crystalline and amorphous ZnO

Density functional theory calculations are used to investigate the structure and binding energies of clusters formed between oxygen vacancies and trivalent dopant atoms (indium, gallium and aluminium) substituted into zinc oxide. Our results show that indium atoms form stable nearest neighbour pairs with oxygen vacancies, while gallium and aluminium atoms associate with them at next nearest neighbour sites. Using a combination of classical molecular dynamics and Reverse Monte Carlo methods, models of amorphous indium zinc oxide at different compositions up to 25 at.% indium are created. Analysis of these models indicates that, in contrast with the trend observed in the crystal phase, indium does not tend to be undercoordinated in the amorphous phase. The value of the band gap obtained for the amorphous compositions is smaller than that of crystalline undoped ZnO by about 0.8 eV and is largely independent of the indium concentration. Electron effective masses calculated in all the amorphous models decrease with increasing amount of indium due to the larger dispersion of the In-dominated conduction bands. This trend is compared to resistivity measurements on amorphous indium zinc oxide which also decrease with increasing indium concentration.Financial support for this work is provided by the European Commission through contract No. NMP3-LA-2010-246334 (ORAMA).This is the original submitted version of the article. It does not incorporate changes arising from peer-review. The final peer-reviewed version of the article has been published in Thin Solid Films as 'Effect of trivalent dopants on local coordination and electronic structure in crystalline and amorphous ZnO' and is available at http://www.sciencedirect.com/science/article/pii/S004060901301002X

Negative oxygen vacancies in HfO2_2 as charge traps in high-k stacks

We calculated the optical excitation and thermal ionization energies of oxygen vacancies in m-HfO$_2$ using atomic basis sets, a non-local density functional and periodic supercell. The thermal ionization energies of negatively charged V$^-$ and V$^{2-}$ centres are consistent with values obtained by the electrical measurements. The results suggest that negative oxygen vacancies are the likely candidates for intrinsic electron traps in the hafnum-based gate stack devices.Comment: 3 pages, 2 figure

Adsorption of atmospheric gases on cementite 010 surfaces.

We study the adsorption of a series of small molecules on the nonstoichiometric {010} surface of cementite (θ-Fe3C) by means of first-principles calculations. We find that CO, N2, H2O, and CH4 prefer to adsorb over iron atoms in an atop configuration. O2, CO2, and the OH radical prefer a configuration bridging two iron atoms and CH2O adsorbs in a configuration bridging a surface iron atom and a surface carbon atom. Adsorption energies are small for H2, CO2, and CH4, indicating a physisorption process, while those for CO, CH2O and especially for O2 and the OH radical are large, indicating a strong chemisorption process. H2O and N2 display adsorption energies between these two extremes, indicating moderate chemisorption. The dissociation of H2, CH2O, the OH radical, and O2 is favoured on this surface. Comparison with adsorption on Fe{100} surfaces indicates that most of these gases have similar adsorption energies on both surfaces, with the exception of CO and the OH radical. In addition, we find similarities between the reactivities of cementite and Mo2C surfaces, due to the similar covalent character of both carbides

Impact of amorphization on the electronic properties of Zn-Ir-O systems.

We analyze the geometry and electronic structure of a series of amorphous Zn-Ir-O systems using classical molecular dynamics followed by density functional theory taking into account two different charge states of Ir (+3 and +4). The structures obtained consist of a matrix of interconnected metal-oxygen polyhedra, with Zn adopting preferentially a coordination of 4 and Ir a mixture of coordinations between 4 and 6 that depend on the charge state of Ir and its concentration. The amorphous phases display reduced band gaps compared to crystalline ZnIr2O4 and exhibit localized states near the band edges, which harm their transparency and hole mobility. Increasing amounts of Ir in the Ir(4+) phases decrease the band gap further while not altering it significantly in the Ir(3+) phases. The results are consistent with recent transmittance and resistivity measurements

Adsorption of alcohols and hydrocarbons on nonstoichiometric cementite{010} surfaces.

We investigate the adsorption of several organic molecules on a nonstoichiometric {010} surface of Fe3C (cementite) by means of density functional theory calculations with van der Waals corrections. The molecules studied include methanol, ethanol, n-heptane, isooctane, benzene, toluene, cyclohexane, naphthalene, 1-methylnaphthalene and decalin. We find that methanol and ethanol chemisorb over the surface, with adsorption heats between 1.0 and 1.2 eV, through their OH groups. In contrast, n-heptane, isooctane and decalin physisorb over the surface, preferentially in a flat configuration, with adsorption heats between 0.19 and 0.27 eV per carbon atom in the molecule. Aromatic molecules strongly chemisorb over the surface of cementite in a flat configuration with adsorption heat of ∼0.31 eV per molecular carbon atom. Increase of the coverage reduces the adsorption heat in all the cases considered. Dehydrogenation is disfavoured in the adsorbed nonaromatic molecules, and neutral or slightly exothermic in the aromatic ones. The adsorption process induces a small spin polarization in the rings of adsorbed aromatic molecules. The relatively strong adsorption of all the molecules considered makes them possible candidates for nucleation processes on the cementite surface

Hybrid density functional calculations of the defect properties of ZnO:Rh and ZnO:Ir

We report density functional calculations of the atomic and electronic structure of the spinel phases ZnRh2O4 and ZnIr 2O4 as well as crystalline ZnO lightly doped (1 at.%) with Rh and Ir ions using the B3LYP hybrid functional. Calculations for the spinels show band gaps (~ 3 eV) and lattice parameters (~ 2% difference) in reasonable agreement with experimental data. Incorporation of the transition metals into ZnO induces local distortions in the lattice and the appearance of metal d levels in the low gap region and near the conduction band minimum, with a d-d splitting of about 2 eV, which reduces the effective transparency of the material. Addition of a hole to the simulation cell of both spinels and doped ZnO leads to charge localization in the neighbourhood of Rh/Ir accompanied by local lattice deformations to form a small polaron which may lead to low hole mobility. We calculate polaron diffusion barriers in the spinels and obtain values around 0.02-0.03 eV. These very low barrier energies suggest that at high Rh/Ir concentrations polaron hopping is not going to be detected at room temperature. © 2013 Elsevier B.V