The challenges of nanostructures for theory

It is tempting to believe that modelling in nanotechnology is much the same as that for conventional solid-state physics. However, important areas of nanotechnology address different systems. The mechanics of DNA (for instance) resembles spaghetti more than silicon, the statistical physics needed is often not carrier statistics, and the role of viscosity (the low Reynolds number limit) is not always the familiar one. The idea of equilibrium may be irrelevant, as the kinetics of nonequilibrium (perhaps quasi-steady state) can be crucial. Even when the issues are limited to nanoscale structures (rather than functions), there is a complex range of ideas. Some features, like elasticity and electrostatic energies, have clear macroscopic analogies, but different questions emerge, such as the accuracy of self-organisation. Others concepts like epitaxy and templating are usually micro- or mesostructural. Some of the ideas, which emerge in modelling for the nanoscale, suggest parallels between molecular motors and recombination enhanced diffusion in semiconductors. (C) 2002 Elsevier Science B.V. All rights reserved

ELECTRONIC-STRUCTURE OF SELF-TRAPPED EXCITON IN SODIUM-CHLORIDE

The electronic structure of the relaxed exciton is investigated using a Hartree-Fock method. The model concentrates on a cluster of two sodium and two chlorine ions surrounded by an array of point charges. The excited states of the exciton and the associated transitions are discussed. The results are compared with experiment and with the phenomenological models proposed earlier workers. A number of features are confirmed, giving a general picture close to that of Kabler and his co-workers. One problem emerges, since it proves possible to understand the sigma luminescence or the pi luminescence, but not both at present. Possible reasons are discussed. In other respects, there is good agreement with experiment, especially for the optical transitions starting from the lowest triplet state of the exciton

DEFECT MIGRATION IN SOLIDS - MICROSCOPIC CALCULATION OF DIFFUSION RATES

Diffusion includes some of the commonest processes on an atomic scale, in which uncorrelated atomic jumps bring about changes in solids. The many predictions of activation energies characterising the temperature dependence of diffusion have high accuracy when good interatomic potentials are known, and a continually improving accuracy from direct quantal calculations. Much more is needed to find the absolute rate at some temperatures. Recent work for both classical and quantum diffusion shows that rates too can be estimated with respectable accuracy. Such calculations highlight basic problems of solid-state defect physics. They alsow offer predictions of techonogical value for diffusion rates in cases where the timescale or physical conditions are too hard for direct experiment. This talk will discuss (a) various ways of obtaining absolute rates, (b) rates of cation diffusion in oxides and their for the so-called Compensation rule (c) issues of charge state in oxides and semiconductors or the motion of shortlived species, and (e) quantum diffusion of hydrogen in metals

The electronic structure of the tetrachedral N2 centre and of the neutral vacancy in diamond

The N2 centre, a tetrachedral array of four F centres, and the neutral vacancy in diamond present similar theoretical problems. Their electronic structure has been calculated by a molecular orbital approach using one-electron wave functions centred on the geometric centre of the defect. The results are compared with those based on functions which are linear combinations of atomic orbitals (LCAO). We conclude that in LiF and KC1 the ground state of the N2 centre is 5A2 and the centre is not responsible for the N2 band, although it should give rise to spin resonance. For the diamond vacancy the ground state is 3T1 or possibly 1E. Previous calculations using LCAO wave functions give a 1E ground state for diamond. Possible ways of observing these centres are discussed

VOLUME CHANGES AND DIPOLE TENSORS FOR POINT-DEFECTS IN CRYSTALS

Results for the volume change and dipole tensors for point defects are derived using a generalisation of the Betti reciprocity theorem. These results incorporate the correction terms derived by Flynn (1971) by Lidiard (1981) and by Gillan (1983) in a simpler way, and allow some more general results to be obtained straightforwardly

DIMENSION CHANGES IN A SOLID CONTAINING ANISOTROPIC DEFECTS

Aligned anisotropic defects in a solid cause different changes in macroscopic dimensions and in lattice parameters along the different crystal axes. A simple general method is given for relating these differences to the microscopic forces associated with the individual defects. Explicit results are given for isotropic, cubic and hexagonal host lattices

NONRADIATIVE-TRANSITIONS IN SEMICONDUCTORS

Non-radiative transitions affect many aspects of semiconductor performance. Normally they reduce device efficiency by suppressing luminescence, creating defects, reducing carrier lifetimes, or enhancing diffusion during operation. The present review surveys both the theoretical and practical understanding of non-radiative transitions. It includes general theoretical results and the associated ideas, with the emphasis on phonon-induced and defect Auger processes. Most of the purely formal aspects are omitted, but the points of principle where uncertainties remain are discussed. The review also covers the relation between basic theoretical studies and practical applied work on device degradation. This includes a description of the atomic processes involved in the more important mechanism of device deterioration and the theoretical understanding of the mechanism of these underlying processes. Finally, there is a survey of models proposed for 'killer' centres

IMPURITY TRAPPING EFFECTS IN THE LOCALIZATION OF MUONS IN SOLIDS

Muon spin rotation ( mu SR) experiments are now regularly used to study solids and solid-state processes. The interpretation of mu SR data is usually based on a 'standard' picture in which the muons localise randomly in the solid, and then diffuse, possibly encountering impurities. There remain some important cases where no satisfactory interpretation results. For some of these anomalous systems the authors propose an alternative picture in which the two different factors are the importance of metastable (free muon) excited states, and the role of impurities in causing localisation. They show this allows a possible explanation of results for Al:Mn and demonstrate that elastic strain fields of defects may be a major factor in influencing localisation. They also propose a new mechanism for delayed self-trapping

QUANTUM-THEORY OF DIFFUSION - TEMPERATURE DEPENDENCE OF DIFFUSION OF LIGHT INTERSTITIALS IN DEBYE SOLIDS

The temperature dependence of the motion of light interstitials is calculated using the quantum theory of Flynn and Stoneham (1970). The results are compared with the asymptotic expressions given previously. They show that the high temperature asymptotic is accurate over a wider range of temperature than expected, whereas the low temperature form is of very restricted application. Deviations from the asymptote depend both on the temperature and on the ratio of the activation energy to the Debye energy. Results for protons in Ta agree remarkably well with the theory for an activation energy 0.188 eV and for a hopping integral J of 29% of the Debye energy