DIMENSION CHANGES DUE TO ALIGNED VK-CENTERS AND H-CENTERS IN IONIC-CRYSTALS

When anisotropic defects are aligned, the dimensions of the host crystal parallel and perpendicular to the defect axis are changed, an effect observed previously for H centres and Vk centres in KCl. The authors have calculated the effect for H centres and Vk centres in several crystal structures. The contribution from long-range Coulomb interactions has been obtained in all cases, with less-detailed estimates of short-range repulsion and covalency effects in special cases. The predictions are in good agreement with experiment for H centres in KCl, but agree only poorly for Vk centres in the same host. The discrepancy appears to arise from modifications of the local repulsive forces near the defect. Measurements of dimension changes show great promise for studies of such interatomic forces

Interstitial muons and hydrogen in crystalline silicon

We have calculated self-consistent total energy surfaces for H+, H° and H2 present interstitially in crystalline Si. We conclude molecular hydrogen is the stable form consistent with the lack of observed electrical and optical activity. Both H+ and H° have complex surfaces, with some features sensitive to lattice distortion. The local minima are too small to give localised states when zero-point energy is included. We discuss our results in relation to earlier theories and to experiments on “normal” and “anomalous” muonium [μ+e-]

Theory of hydrogen in liquid and solid metals

A method for calculating the interatomic forces between isolated hydrogens and their host metal atoms is outlined. The method uses a semiempirical, molecular-orbital approach for a suitable cluster of atoms, with the empirical parameters fitted to experimental potential energy curves for diatomic molecules. Parameters suitable for hydrogen in liquid or solid Li and Na are given. The method is applied to the calculation of solvation energies of hydrogen in liquid Li and Na, where satisfactory agreement with experiment is obtained. Detailed potential energy surfaces are also found for H in solid Na and estimates are made of local mode frequencies, the stability of the tetrahedral sites, lattice relaxation, and effective charges, and atomic radii. Neither the anionic nor the protonic limit is appropriate. It has not proved possible to describe the potential energy surfaces in terms of a sum of twobody and volume-dependent terms alone

INTERSTITIAL MUONS AND HYDROGEN IN DIAMOND AND SILICON

The authors have calculated self-consistent total-energy surface for hydrogen present interstitially as H+, H0 and H2 in crystalline silicon and diamond. The dissimilarities of the two materials are more evident than their similarities, for they show molecular hydrogen to be the stable form in silicon, and atomic hydrogen to be the stable form in diamond in the absence of impurities. The energy surfaces for H0 and H+ are complex, with minima too small to trap the atoms when zero-point energy is taken into account. They discuss their results in relation to other theories and to the normal and anomalous muonium ( mu +e-) experiments

Stability of electronic states of the vacancy in diamond

The vacancy in diamond is a fundamental defect which has been studied theoretically and experimentally for forty years. However, although early theories (Coulson C A and Kearsley M J 1957 Proc. R. Soc. A 241 433) were extremely successful in explaining the nature of the ground state of the neutral defect and the Jahn-Teller distortion expected (Lannoo M and Stoneham A M 1968 J. Phys. Chem. Solids 29 1987), there are still several questions which have not been answered satisfactorily. in particular, the many-electron effects and configuration interaction are vital. They determine not only the order of electronic levels in the vacancy, but also the best-known optical transition. GR1, which cannot be expressed in terms of one-electron levels alone.We bring together much of the derailed recent experimental data on the different charge states and excited states of the vacancy to build up a simple empirical model of the defect. We show that the stability of the states and their photoconductivity, or lack of it, can be reproduced. We can predict that other states of the neutral vacancy, observable by EPR, lie very close above the ground state. and another high-energy optical transition might be detectable

Mesoscopic modelling of the interaction of infrared lasers with composite materials: an application to dental enamel

The mesostructure and composition of composite materials determine their mechanical, optical and thermal properties and, consequently, their response to incident radiation. We have developed general finite element models of porous composite materials under infrared radiation to examine the influence of pore size on one of the determining parameters of the stress distribution in the material: the temperature distribution. We apply them to the specific case of human dental enamel, a material which has nanometer scale pores containing water/organic, and predict the maximum temperature reached after a single 0.35 μs laser pulse of sub-ablative fluence by two lasers: Er:YAG (2.9 μm) and CO2 (10.6 μm). For the Er:YAG laser, the results imply a strong dependence of the maximum temperature reached at the pore on the area-to-volume ratio of the pore, whereas there is little such dependence for CO2 lasers. Thus, CO2 lasers may produce more reproducible results than Er:YAG lasers when it comes to enamel ablation, which may be of significant interest during clinical practice. More generally, when ablating composite materials by infrared lasers researchers should account for the material’s microstructure and composition when designing experiments or interpreting results, since a more simplistic continuum approach may not be sufficient to explain differences observed during ablation of materials with similar optical properties or of the same material but using different wavelengths

Quantum behaviour of hydrogen and muonium in vacancy-containing complexes in diamond

Most solid-state electronic structure calculations are based on quantum electrons and classical nuclei. These calculations either omit quantum zero-point motion and tunnelling, or estimate it in an extra step. Such quantum effects are especially significant for light nuclei, such as the proton or its analogue, μ+. We propose a simple approach to including such quantum behaviour, in a form readily integrated with standard electronic structure calculations. This approach is demonstrated for a number of vacancy-containing defect complexes in diamond. Our results suggest that for the NHV- complex, quantum motion of the proton between three equivalent potential energy minima is sufficiently rapid to time-average measurements at X-band frequencies

The roles of charged and neutral oxidising species in silicon oxidation from ab initio calculations

We examine the roles of charged oxidising species based on extensive ab initio density functional theory calculations. Six species are considered: interstitial atomic O, O-, O2- and molecular species: O-2, O-2(-), O-2(2-) We calculate their incorporation energies into bulk silicon dioxide, vertical electron affinities and diffusion barriers. In our calculations, we assume that the electrons responsible for the change of charge state come from the silicon conduction band, however, the generalisation to any other source of electrons is possible, and hence, our results are also relevant to electron-beam assisted oxidation and plasma oxidation. The calculations yield information about the relative stability of oxidising species, and the possible transformations between them and their charging patterns. We discuss the ability to exchange O atoms between the mobile species and the host lattice during diffusion, since this determines whether or not isotope exchange is expected. Our results show very clear trends: (1) the molecular species are energetically preferable over alo,nic ones, (2) the charged species are energetically more favourable than neutral ones, (3) diffusion of atomic species (O, O-, O2-) will result in oxygen exchange, whereas the diffusion of nzoleculai species (O-2, O-2(-), O-2(2-)) is not likely to lead to a significant exchange with the lattice. On the basis of our calculation, we predict that charging of oxidising species may play a key role in silicon oxidation process. (C) 2000 Elsevier Science Ltd. All rights reserved

The structure and motion of the self-interstitial in diamond

We have made self-consistent semi-empirical molecular orbital calculations for various possible self-interstitial geometries in diamond, both with and without lattice distortion. Total energies are obtained, not merely the sum of one-electron eigenvalues. The results show that the (100) split interstitial has the lowest formation energy, not the cubic, hexagonal or bond-centred forms favour previously. The nature of the interstitial does not support the local heating model of enhanced diffusion in the presence of recombination or ionisation. A Bourgoin-Corbett mechanism involving negative hexagonal and neutral split interstitials is possible, but the apparent stability of the negative hexagonal interstitial may be an artefact of the calculation. We suggest a local excitation model is appropriate in fourfold-coordinated semiconductors

Band-edge problem in the theoretical determination of defect energy levels: the O vacancy in ZnO as a benchmark case

Calculations of formation energies and charge transition levels of defects routinely rely on density functional theory (DFT) for describing the electronic structure. Since bulk band gaps of semiconductors and insulators are not well described in semilocal approximations to DFT, band-gap correction schemes or advanced theoretical models which properly describe band gaps need to be employed. However, it has become apparent that different methods that reproduce the experimental band gap can yield substantially different results regarding charge transition levels of point defects. We investigate this problem in the case of the (+2/0) charge transition level of the O vacancy in ZnO, which has attracted considerable attention as a benchmark case. For this purpose, we first perform calculations based on non-screened hybrid density functionals, and then compare our results with those of other methods. While our results agree very well with those obtained with screened hybrid functionals, they are strikingly different compared to those obtained with other band-gap corrected schemes. Nevertheless, we show that all the different methods agree well with each other and with our calculations when a suitable alignment procedure is adopted. The proposed procedure consists in aligning the electron band structure through an external potential, such as the vacuum level. When the electron densities are well reproduced, this procedure is equivalent to an alignment through the average electrostatic potential in a calculation subject to periodic boundary conditions. We stress that, in order to give accurate defect levels, a theoretical scheme is required to yield not only band gaps in agreement with experiment, but also band edges correctly positioned with respect to such a reference potential