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Suche nach „[P.] [Harrison]“ hat 51 Publikationen gefunden
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    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    Thomas Stirner, P. Harrison, W. Hagston, J.H.C. Hogg, J. Fatah

    Double crystal x-ray diffraction simulations of diffusion in semiconductor microstructures

    Journal of Applied Physics, vol. 83, pp. 4037-4041

    1998

    DOI: 10.1063/1.367159

    Abstract anzeigen

    Diffusion in group IV, III-V and II-VI semiconductors is an interesting problem not only from a fundamental physics viewpoint but also in practical terms, since it could determine the useful lifetime of a device. Any attempt to control the amount of diffusion in a semiconductor device, whether it be a quantum well structure or not, requires an accurate determination of the diffusion coefficient. The present theoretical study shows that this could be achieved via x-ray diffraction studies in quantum well structures. It is demonstrated that the rocking curves of single quantum wells are not sensitive to diffusion. However the intensity of the first order satellite, which is characteristic of superlattice rocking curves, is strongly dependent upon diffusion and it is proposed that this technique could be used to measure the diffusion coefficient D.

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    Zeitschriftenartikel

    F. Long, W. Hagston, Thomas Stirner, P. Harrison

    The structural dependence of the effective mass and Luttinger parameters in semiconductor quantum wells

    Journal of Applied Physics, vol. 82, pp. 3414-3421

    1997

    Abstract anzeigen

    A detailed comparison of the empirical pseudopotential method with single and multiple band calculations based on the envelope function and effective mass approximations are presented. It is shown that, in order to give agreement with the more rigorous microscopic approach of the pseudopotential method, structural dependent effective masses and Luttinger parameters must be invoked. The CdTe/Cd1−xMnxTe system has been employed as an example, and the first pseudopotential calculations of quantum wells and superlattices in this material are presented. It is shown that the electron, light- and heavy-hole effective masses tend towards twice their bulk values in the limit of narrow quantum wells.

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    Zeitschriftenartikel

    W. Hagston, Thomas Stirner, P. Harrison

    Effects of interface imperfections on the Zeeman splitting of excitons in diluted magnetic semiconductor quantum wells

    Philosophical Magazine Part B, vol. 75, no. 3, pp. 349-361

    1997

    Abstract anzeigen

    Theoretical estimates are given of the influence that imperfect interfaces have on the Zeeman splittings of excitons in diluted magnetic semiconductor quantum well structures in which one or both of the barriers are magnetic. A first-principles theoretical treatment of the microscopic magnetic response of the interfaces has been utilized in the calculations. The results show how sensitive these splittings can be to even the smallest degree of imperfection at the interface region provided that both barriers are magnetic and the initial potential profile is symmetric with respect to the centre of the well. A comparison with experimental data on the CdTe/Cd1- x Mn x Te system shows that all such symmetric quantum well structures reported to date have imperfections at the interface region extending over at least one or two monolayers. The calculations also show that asymmetric quantum well structures, in which one of the barriers is non-magnetic, can be a far less sensitive probe of interface imperfections.

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    Zeitschriftenartikel

    T. Piorek, P. Harrison, W. Hagston, Thomas Stirner

    Magnetic field induced transitions in diluted magnetic semiconductor quantum wells

    Journal of Crystal Growth, vol. 159, no. 1-4, pp. 1037-1040

    1996

    DOI: 10.1016/0022-0248(95)00693-1

    Abstract anzeigen

    The purpose of this paper is to present a detailed theoretical study of the behaviour of excitonic transitions which are forbidden in a zero magnetic field but whose oscillator strength increases with the applied field. The calculation utilizes a two parameter excitonic wave function which permits the relative motion term to assume the symmetry of either a prolate or an oblate spheroid should it be energetically favourable to do so. In addition the effect of the coulombic interaction of the electron and hole is treated in a self-consistent manner and the calculation includes enhanced paramagnetic effects at the interface. A detailed study is given of experimental observations [Ribayrol et al., Phys. Rev. B 51 (1995) 7882] in which the behaviour of the e1-hh3 transition is explained in terms of a magnetic field induced type-I–type-II transition in a diluted magnetic semiconductor quantum well system.

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    Zeitschriftenartikel

    P. Harrison, W. Hagston, Thomas Stirner, J. Fatah

    Alloy nonrandomness in diluted magnetic semiconductors

    Journal of Applied Physics, vol. 79, pp. 1684-1688

    1996

    DOI: 10.1063/1.360954

    Abstract anzeigen

    A simple model of alloy nonrandomness is introduced within a framework where the effective concentration of spin singlets as a function of the nominal concentration of magnetic ions in a nonrandom alloy can be obtained by transformations of the corresponding function in random alloys. The theory shows that a given system that is appreciably nonrandom can have a magnetic response almost identical with that of a random distribution. Possible ways of identifying alloy nonrandomness in diluted magnetic semiconductor structures are described.

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    Zeitschriftenartikel

    Thomas Stirner, P. Harrison, W. Hagston, J. Goodwin

    Reply to "Comment on ‘Theoretical investigation of observed magnetic-polaron energies in quantum wells’"

    Physical Review B - condensed matter and materials physics, vol. 53, pp. 2143-2144

    1996

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    Zeitschriftenartikel

    Thomas Stirner, P. Harrison, W. Hagston, T. Piorek

    Optical properties of magnetic sawtooth superlattices

    Physical Review B - condensed matter and materials physics, vol. 53, pp. 11060-11063

    1996

    DOI: 10.1103/PhysRevB.53.11060

    Abstract anzeigen

    Self-consistent calculations of the optical properties of magnetic sawtooth superlattices are presented. Sawtooth modulation of the graded gap superlattice as produced by linear variations of the magnetic Mn2+ ion in the Cd1-xMnxTe alloy is described. The Brillouin function response of the band gap together with the varying manganese concentration produces unusual potential profiles in an external magnetic field. The calculations show that the low-temperature magnetic field tunability of the exciton emission is greater in sawtooth superlattices than in an equivalent rectangular superlattice structure. A means of utilizing this for probing alloy nonrandomness is described. © 1996 The American Physical Society.

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    Zeitschriftenartikel

    Thomas Stirner, P. Harrison, W. Hagston, M. O’Neill

    The symmetry of the relative motion of excitons in semiconductor heterostructures

    Superlattices and Microstructures, vol. 20, no. 1, pp. 45-47

    1996

    DOI: 10.1006/spmi.1996.0048

    Abstract anzeigen

    A theoretical model of excitonic states in semiconductor heterostructures is presented. The approach employs the envelope function approximation, and involves a two parameter variational calculation in which the symmetry of the component of the wave function representing the relative motion is allowed to vary between the two- and three-dimensional limits. Detailed calculations are described for a variety of single quantum wells and superlattices. The results show that the excitons are neither 2D nor 3D like, but are intermediate in character. Furthermore, in the main, they assume the symmetry of a prolate spheroid. An exception to this occurs in the special case of an asymmetric double quantum well close to resonance, where two stable exciton states are found for the same one-particle states. One of these ‘twin’ exciton states is an oblate spheroid. The results illustrate the need for accurate determination of excitonic properties if the dynamical evaluation of exciton states, in for example, quantum well lasers, is to be readily determined.

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    Vortrag

    T. Piorek, P. Harrison, W. Hagston, Thomas Stirner

    Magnetic field induced transitions in diluted magnetic semiconductor quantum wells

    7th International Conference on II-VI Compounds and Devices, Edinburgh, Großbritannien

    1995

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    Zeitschriftenartikel

    P. Harrison, W. Hagston, Thomas Stirner

    Optical methods for determining diffusion in magnetic quantum well structures

    Journal of Applied Physics, vol. 77, no. 3, pp. 1314-1316

    1995

    DOI: 10.1063/1.358935

    Abstract anzeigen

    It is shown that the occurrence of magnetic polaron formation can have an appreciable influence on the values of the diffusion constants deduced from observations of the photoluminescence in diffused magnetic quantum‐well structures. Arguments are presented that show that photoluminescence excitation spectroscopy provides several independent, more accurate measurements from which the diffusion constants can be more reliably estimated. © 1995 American Institute of Physics.

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    Zeitschriftenartikel

    Thomas Stirner, S. Weston, P. Harrison, S. Bardorf, S. Jackson, W. Hagston, J.H.C. Hogg, J. Nicholls, M. O’Neill

    Experimental determination of the effect of alloy composition on the band alignment of the CdTe-Cd1-xMnxTe heterojunction

    Physical Review B - condensed matter and materials physics, vol. 51, no. 8, pp. 5477-5479

    1995

    Abstract anzeigen

    A technique based on photoluminescence excitation measurements is described for the direct determination of the band alignment in a quantum well formed from nonmagnetic and semimagnetic materials. Results are presented for a series of CdTe-Cd1-xMnxTe quantum well samples. These show that the valence band offset, when expressed as a fraction of the total band discontinuity, is a function of the alloy concentration x in the barrier.

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    Zeitschriftenartikel

    P. Harrison, W. Hagston, Thomas Stirner

    The magnitude and sign of the carrier‐magnetic-ion exchange-interaction term in the monolayer regions next to the interface between a magnetic and nonmagnetic semiconductor

    Physical Review B - condensed matter and materials physics, vol. 52

    1995

    DOI: 10.1103/PhysRevB.52.R5519

    Abstract anzeigen

    The magnitude and/or the sign of the carrier–magnetic-ion exchange interaction J could be different from that of bulk material in the monolayer regions next to an interface between a magnetic and a nonmagnetic semiconductor. A theoretical investigation of the influence this would have on the Zeeman splittings of both light and heavy holes is evaluated as a function of the well width. Comparison with experimental results for a range of quantum wells of varying width, and differing concentrations of the magnetic ion in the barrier, enables interesting conclusions to be drawn with respect to the variation in this parameter. In particular, the experimental data are shown to be consistent with a variation in J for the electron that is markedly different from that for the hole.

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    Vortrag

    J. Fatah, I. Karla, J.H.C. Hogg, P. Harrison, W. Hagston, Thomas Stirner

    Defect induced diffusion mechanisms in ion implanted quantum well structures

    Posterpräsentation

    22nd International Conference on the Physics of Semiconductors (ICPS22), Vancouver, Kanada

    1994

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    Zeitschriftenartikel

    W. Hagston, S. Weston, M. O’Neill, J.H.C. Hogg, D. Ashenford, B. Lunn, Thomas Stirner, P. Harrison

    Characteristic Zeeman patterns in novel graded gap II-VI quantum well structures

    Journal of Vacuum Science & Technology B, vol. 12, pp. 1146-1149

    1994

    DOI: 10.1116/1.587069

    Abstract anzeigen

    Novel methods of growing graded gap structures are described. In order to provide a check that the structures are indeed graded in the correct manner, magnetic ions in the structure have been employed. The good agreement between the predicted and observed Zeeman splitting of the corresponding light‐ and heavy‐hole exciton transitions, together with the magnetic field dependence of the Zeeman splittings and intensity of a forbidden heavy‐hole exciton transition, is taken as direct evidence that graded gap structures having the correct composition have indeed been fabricated.

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    Zeitschriftenartikel

    P. Harrison, W. Hagston, Thomas Stirner, J. Nicholls

    Excitonic relaxation channels in double quantum wells

    Semiconductor Science and Technology, vol. 9, no. 5S, pp. 743-745

    1994

    Abstract anzeigen

    The purpose of this paper is to give a theoretical interpretation of the experimental results of Goede et al. (Superlatt. Microstruct. 12 363 (1992)) describing the relaxation of heavy-hole excitons in asymmetric double quantum well systems based on the semimagnetic CdTe-CdMnTe system. In order to explain certain spectroscopic data, Goede et al. had to invoke excitonic tunnelling between the wells for systems with narrow (25 AA) inner barriers. In this work it is shown that the experimental results imply tunnelling via a 'crossed' excitonic state with the electron and hole localized in different wells. The experimental observations are interpreted in terms of a model involving phonon scattering and calculations of intersubband relaxation rates via confined phonon modes are shown to be in agreement with experimental observations.

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    Zeitschriftenartikel

    S. Jackson, J. Nicholls, P. Harrison, W. Hagston, Thomas Stirner, S. Bardorf

    Zeeman studies of CdTe-Cd1-xMnxTe multiquantum wells

    Solid-State Electronics, vol. 37, no. 4-6, pp. 1129-1132

    1994

    DOI: 10.1016/0038-1101(94)90371-9

    Abstract anzeigen

    Various novel features of the magnetic field splitting associated with the photoluminescence excitation spectra (PLE) and the photoluminescence spectra (PL) for CdTe/Cd1 − xMnxTe quantum well structures are described. The unusual Zeeman splitting pattern of the heavy-hole barrier exciton state, together with that of the light-hole well exciton state, is shown to be consistent with the magnetic response of the monolayers adjacent to an interface being different from that of the bulk. In particular it is shown that for the carrier in the conduction band, the magnitude of the exchange integral with the magnetic ions in the first two monolayers is of approximately the same magnitude but of opposite sign to that occurring in the bulk.

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    Beitrag (Sammelband oder Tagungsband)

    J. Fatah, I. Karla, J.H.C. Hogg, P. Harrison, W. Hagston, Thomas Stirner

    Defect induced diffusion mechanisms in ion implanted quantum well structures

    Proceedings of the 22nd International Conference on the Physics of Semiconductors (ICPS22)

    1994

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    Zeitschriftenartikel

    J. Goodwin, P. Harrison, W. Hagston, Thomas Stirner

    Dynamical aspects of free exciton magnetic polaron formation in CdTe-Cd1-xMnxTe quantum wells

    Advanced Materials for Optics and Electronics, vol. 3, no. 1-6, pp. 117-125

    1994

    DOI: 10.1002/amo.860030117

    Abstract anzeigen

    Calculations of the energies of magnetic polarons formed by free excitons localised in non-magnetic CdTe wells next to magnetic Cd1 − xMnxTe barriers are presented. A comparison with recent time-resolved spectroscopy results allows insight to the physical aspects governing the dynamics of the formation of the polarons. It is shown that the experimentally measured energy shift is not the polaron energy itself but the difference between this and the change in the exciton binding energy. The latter is calculated within the envelope function approximation and by employing a variational technique. The polaron energy calculation uses a modified version of an approach described by Wolff. The results show that static polaron calculations are not generally reliable and that the exciton–magnetic polaron has to be viewed as a dynamically evolving complex. It is initially energetically favourable for the exciton–magnetic polaron complex to increase its spatial localisation, since the gains in polarisation energy exceed the loss in exciton binding energy. However, thermodynamic considerations suggest that in general saturation of the magnetic ions will not occur.

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    Zeitschriftenartikel

    W. Hagston, Thomas Stirner, P. Harrison

    Origin of the Stokes’ shift in semiconductor quantum wells

    Advanced Materials for Optics and Electronics, vol. 3, no. 1-6, pp. 95-101

    1994

    Abstract anzeigen

    Excitonic energy levels have been calculated in CdTe quantum wells with Cdl - xMn,Te barriers. It has been shown that the introduction of small-scale (relative to the exciton Bohr radius) interface disorder, which either preserves or breaks the inversion symmetry, can produce appreciable Stokes shifts between optical absorption and emission and that the associated iine-widths can remain narrow. A criterion for the existence of high-quality interfaces, based on optical spectra, is described.

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    Zeitschriftenartikel

    Thomas Stirner, M. O’Neill, P. Harrison, W. Hagston

    Exciton dynamics in multiquantum well CdTe-Cd1-xMnxTe systems

    Journal of Vacuum Science & Technology B, vol. 12, no. 2, pp. 1150-1152

    1994

    DOI: 10.1116/1.587070

    Abstract anzeigen

    The exciton dynamics associated with the transfer of energy from the light‐hole state to both the heavy‐hole state and to a donor bound exciton (D0X) are investigated utilizing time‐resolved spectroscopy, in a multiquantum well sample of CdTe–Cd1-xMnxTe containing 75 Å wells and 150 Å barriers. The results show that the relaxation time of the light‐hole to the heavy‐hole state occurs, at low temperature, in a time ≤40 ps whereas the combined drift/diffusion of the excitons in the plane of the quantum well and their subsequent trapping at donor states occurs in a time ∼80 ps. Theoretical calculations show that the light‐hole–heavy‐hole carrier relaxation time is consistent with confined (acoustic) phonon relaxation and that the equivalent in‐plane mobility is consistent with interface roughness scattering.