95 research outputs found

    SELF-CONSISTENT CALCULATIONS FOR SHALLOW DEFECTS IN SEMICONDUCTORS .1.

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    Shallow defects in semiconductors are of major importance, both scientifically and technologically. These include donors, acceptors, isoelectronic impurities and various clusters, and may involve trapped carriers or bound excitons. Hopfield provided the key to understanding the more complicated defects by stressing their relation to atomic and molecular systems. The authors have developed a general computer program, SEMELE to exploit this relation further. SEMELE provides a systematic and straightforward way of performing self-consistent calculations on shallow defects and is particularly suited to donor-acceptor pair and related systems. The main assumptions are discussed, together with the various ways of treating central-cell corrections. Results are given for excitons bound to neutral donors in GaP

    INITIAL PRODUCTION OF DEFECTS IN ALKALI-HALIDES - F AND H CENTER PRODUCTION BY NON-RADIATIVE DECAY OF SELF-TRAPPED EXCITON

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    Radiation damage in KCl can be produced by the decay of a self-trapped exciton into an F centre and an H centre. The authors present calculations of the energies of the states involved for various stages in the evolution of the damage. These lead to important conclusions about the very rapid damage process, and support strongly Itoh and Saidoh's suggestion (1973) that damage proceeds through an excited hole state. The results also help in understanding the prompt decay of F and H pairs at low temperatures, the thermal annihilation of F and H centres, the effects of optical excitation of the self-trapped exciton, and some of the trends within the alkali halides. The calculations use a self-consistent semi-empirical molecular-orbital method. A large cluster of ions is used (either 42 or 57 ions) plus long-range Madelung terms. The ion positions were obtained from separate lattice-relaxation calculations with the HADES code. The choice of CNDO parameters and the adequacy of the method were checked by a number of separate predictions

    Field emission theory for an enhanced surface potential: a model for carbon field emitters

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    We propose a non-JWKB-based theory of electron field emission for carbon field emitters in which, for electrons with energy in the vicinity of the order of v to the Fermi level, the effective (1/x) surface potential is strongly enhanced. The model grossly violates the WKB validity criteria and necessitates an analytic treatment of the one-dimensional Schrodinger equation, which we first obtain. We determine v (which is field-dependent) from the wavefunction matching point close to the surface. For reasonable values of the surface parameters-work function phi approximate to 2-5 eV, electron affinity chi approximate to 2phi and an empirical electron loss factor sigma approximate to 10(-3) (and with no other adjustable parameters)-the theory provides an intriguing agreement with experimental data from carbon epoxy graphite composite (PFE) and certain graphitized carbon nanotube field emitters. We speculate on the surface potential enhancement, which can be interpreted as a massive (field-induced) dielectric effect of dynamic origin. This can be related via time-dependent perturbation theory to second-order non-linear polarizability enhancements at ultraviolet similar to3000 Angstrom wavelengths near the tunnelling region. Finally some exact mathematical results are included in the appendix for future reference

    Effect of Electrode Geometry on Sensor Response

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    We have demonstrated the feasibility of using a finite-element program to study the effect of electrode geometry on the performance of an oxide gas sensor. The gas concentration is varied within the sensor. The program is general enough to be extended to complex three-dimensional geometries. For the simpler cases we consider, we find that at a given temperature the back contact sensor will be more selective for moderately reactive gases than for less or more reactive ones, which is in general agreement with the observations made on Figaro gas sensors. We also find that the performance of the sensor improves by using interlacing electrodes, as expected

    Spin-Orbit Effects in Non-Central-Force Systems - Host-Lattice Effects in F-Centers

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    Two new trends associated with spin-orbit effects in a non-central-force field are evident in recent data for F centers: (1) an inverse power-law dependence of spin-orbit splitting on the lattice parameter of the host crystal and (2) a direct dependence of the splitting on the size of the host-lattice ions. These features may be summarized by a simple semiempirical formula closely related to the Mollwo-Ivey relation for the absorption energy

    Purely elastic contributions to friction

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    Self-consistent calculations for shallow defects in semiconductors: II. Donor-acceptor pairs

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    For pt.I see ibid., vol.8, 1102 (1975). A general method has been developed for performing self-consistent calculations for shallow defects or defect complexes in semiconductors. Results are given here for donor-acceptor pairs in GaP and a comparison is made of the various treatments of central-cell corrections and the effects arising from the overlap of the donor and acceptor wavefunctions. Whilst no detailed refitting of host parameters is attempted, it seems likely that the current estimate of the band gap (2.339 eV) is too large, and the latest estimates of the dielectric constant may also be inaccurate

    Bistable collective behavior of polymers tethered in a nanopore.

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    Polymer-coated pores play a crucial role in nucleo-cytoplasmic transport and in a number of biomimetic and nanotechnological applications. Here we present Monte Carlo and Density Functional Theory approaches to identify different collective phases of end-grafted polymers in a nanopore and to study their relative stability as a function of intermolecular interactions. Over a range of system parameters that is relevant for nuclear pore complexes, we observe two distinct phases: one with the bulk of the polymers condensed at the wall of the pore, and the other with the polymers condensed along its central axis. The relative stability of these two phases depends on the interpolymer interactions. The existence the two phases suggests a mechanism in which marginal changes in these interactions, possibly induced by nuclear transport receptors, cause the pore to transform between open and closed configurations, which will influence transport through the pore

    Luminescence and electronic structure of the self-trapped exciton in alkali fluorides and chlorides

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    Luminescence of the self-trapped exciton in alkali halides is analysed on the basis of recent theoretical works. It is shown that the short-lived σ-band originates from an orbital state which is distinct from that of the much studied triplet state. Luminescence from the lowest orbital state consists of two components and this gives rise to the peculiar behaviour of the π-band, as has been reported recently by Purdy and Murray for KCl
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