1,721,163 research outputs found
STABILITY OF A SELF-TRAPPING HOLE IN ALPHA-QUARTZ
Full text linkPrevious calculations of self-trapping in quartz adopt quantum chemical methods. However, for certain purposes, for example, when more than a few atoms are involved in a defect process, it would be helpful to use instead the shell model methods which work well for halides. We present the first calculation of the self-trapped hole (STH) in alpha-quartz and other forms of silicon dioxide using the classic defect simulation technique. The calculation suggests that the hole can be self-trapped on oxygen atom with a binding energy of 0.41 eV. The self-trapping is accompanied by a large network distortion, in which the O- ion on which the hole is self-trapped shifts 0.14 angstrom and the nearest-neighbour silicon atoms move 0.4-0.6 angstrom away from the O- ion. These results are similar to those obtained from the ab initio HF Calculation of STH in amorphous SiO2. We have also estimated the effective activation energy of a STH to be 0.12 eV at 180 K though there will also be a significant component of conduction from excitation of the small polaron to the delocalized large-polaron state
LINEWIDTHS WITH GAUSSIAN AND LORENTZIAN BROADENING
No full textSpin resonance lines occur which are broadened both by mechanisms giving a lorentzian shape and by mechanisms giving a gaussian shape It is often important to separate these contributions to the peak-to-peak separation of the derivative of the lineshape A simple and accurate expression is obtained to achieve this..
NONRADIATIVE-TRANSITIONS IN SEMICONDUCTORS
No full textNon-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
INTERSTITIAL MUONS AND HYDROGEN IN DIAMOND AND SILICON
No full textThe 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
CALCULATION OF ABSOLUTE DIFFUSION RATES IN OXIDES
No full textThe authors have calculated the absolute rate of diffusion for Mg2+ and Fe3+ ions in MgO using atomistic modelling. The calculations use a shell model, incorporating tested interatomic potentials, and exploit recent advances in computer codes. The agreement is extremely good where experimental data are available for comparison. For Mg2+ in MgO at 1400 degrees C they predict a pre-exponential factor of 32.9 THz using simple Vineyard theory (experiment 18+or-7 THz after correction for the important volume dependence of the activation energy) and an activation energy of 2.26 eV (experiment 2.3+or-0.2 eV). Close inspection of the energy changes for displacements from the saddle point normal to the jump path shows that within kT of the saddle point energy there are significant departures from the harmonic dependence required for validity of Vineyard theory. Corrections by both analytical methods and numerical integration improve agreement with experiment, predicting 23-25 THz overall. For Fe3+ much slower diffusion is predicted even though the jump path bifurcates to give two saddle points. The authors do not predict the rapid Fe3+ motion reported in aggregation experiments and conclude that other mechanisms are involved. They have also used the dynamical theory of Rice and Slater which gives similar, but by no means identical, predictions for the diffusion rates
INTRINSIC DEFECT ENERGIES OF LITHIUM HYDRIDE AND LITHIUM DEUTERIDE CRYSTALS
No full textA theoretical study has been made of the defect structure of lithium hydride and lithium deuteride. A potential model is obtained describing the statics and dynamics of these crystals. Intrinsic defect energies are calculated using the Harwell HADES program which is based on a generalised Mott-Littleton method. The results are in good agreement with the experimental data, and suggest that the vacancy and interstitial migration mechanisms of anions and cations are all comparable in their contribution to ionic conduction
ELECTRONIC STRUCTURE OF GE IN SIO2
No full textIt is argued that one-electron theory is insufficient to account for the origin of the observed spectra of Ge in SiO2 ( alpha -quartz) crystals. A simple model is employed to show that impurity states responsible for ESR spectra of SiO2:Ge are stabilised by many-electron polarisation effects associated with the Ge atom itself and its immediate oxygen neighbour
A COMPARISON OF DEFECT ENERGIES IN MGO USING MOTT-LITTLETON AND QUANTUM-MECHANICAL PROCEDURES
No full textThe authors compare the predictions of Mott-Littleton calculations, based on empirical interatomic potentials, with predictions based on self-consistent solutions of the Schrodinger equation for embedded clusters. Simple vacancy and substitutional defects in MgO are modelled using both the classical Mott-Littleton and quantum mechanical methods. Particular attention is paid to the size of the quantum mechanical cluster, the different ways that polarisation is taken into account and the choice of basis set. Results are presented for closed-shell systems only, namely V"Mg and Vo vacancies and for Li'Mg, Na'Mg, AlMg, Fo and Clo substitutional impurities. They find a respectable level of agreement between the quite distinct approaches. This both validates the classical calculations and indicates useful generalisations combining the two approache
CATION DIFFUSION IN ALKALINE-EARTH OXIDES
No full textAbsolute jump rates for cation-vacancy interchanges in MgO, CaO, SrO and BaO are calculated from a set of model inter-ionic potentials. Internal energies and vibrational entropies over a wide range of temperatures (i.e. at expansions which within the models correspond to these temperatures in the quasi-harmonic approximation) are evaluated and, from these, migration enthalpies and pre-exponential frequency factors are deduced. Correlations between these two diffusion parameters for the family of oxides are investigated
MEASUREMENT OF SURFACE-TENSION BY LATTICE-PARAMETER CHANGES - THEORY FOR FACETTED MICROCRYSTALS
Full text linkA number of workers have estimated surface tensions of solids from measurements of the differences in the lattice parameters of bulk crystals and microcrystals. The data are then analysed by an expression valid only for spherical, isotropic microcrystals. The author discusses problems arising from the fact that real microcrystals are generally faceted. A general expression is given for the change in volume of a microcrystal for an arbitrarily anisotropic surface tension and arbitrary crystal habit; common special cases are also given. The discussion of the relation between the volume change and the change in X-ray lattice parameter includes the effects of the dispersions in crystalline sizes and shapes, and the possible non-uniformity in internal strain produced. It is concluded that any errors will be small
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