Publikationen


Suche nach „[Stirner] [Thomas]“ hat 141 Publikationen gefunden
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    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    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

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    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.

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    Vortrag

    Thomas Stirner

    Alloy clustering and alloy randomness in DMS structures

    National II-VI Semiconductor Interaction Meeting, Hull, Großbritannien

    1994

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    Zeitschriftenartikel

    P. Harrison, W. Hagston, B. Lunn, D. Ashenford, S. Jackson, J.H.C. Hogg, J. Nicholls, Thomas Stirner

    Magneto-optical study of exciton binding energies, band offsets and the role of interface potentials in CdTe/Cd1-xMnxTe multiple quantum wells

    Physical Review B - condensed matter and materials physics, vol. 50, no. 8, pp. 5392-5403

    1994

    DOI: 10.1103/PhysRevB.50.5392

    Abstract anzeigen

    Magneto-optical studies on a series of CdTe/Cd1-xMnxTe multiple-quantum-well structures with x~=0.08 have identified the 1S and 2S states of both the light- and heavy-hole n=1 excitons. This has allowed changes of the exciton binding energies to be studied as a function of the depth of the confining potentials, which were tuned through the sp3-d exchange interaction in the barrier layers by application of a magnetic field. Calculations of these binding energies by a variational technique are in general agreement with the observations. The exchange-induced splitting of the heavy-hole exciton is found to be consistent with between 0.35 and 0.45 of the band offset being in the valence band, which accounts for the absence of any evidence that the valence-band structure changes from a type-I structure to a type-II structure above a certain value of a magnetic field. However, this offset is found to be too large to account for the exchange-induced splitting of the light-hole exciton, which appears to be anomalously large. Calculations have shown that this anomaly cannot be explained in terms of the diffusion of Mn ions from the barrier regions into the wells. An alternative explanation is given in terms of field-dependent interface potentials wherein the sp3-d exchange interaction is considered to be different in the interface regions of the multiple quantum wells to that in the barrier regions.

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    Zeitschriftenartikel

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

    Effect of interface disorder on the optical properties of CdTe-Cd1-xMnxTe microstructures

    Surface Science, vol. 313, no. 3, pp. 417-426

    1994

    DOI: 10.1016/0039-6028(94)90061-2

    Abstract anzeigen

    High quality CdTe/Cd1−xMnxTe quantum well structures can be routinely grown in the authors' laboratory by molecular beam epitaxy (MBE). At temperatures of ∼ 2 K these structures have optical linewidths ≲ 3 meV for quantum well widths ≲ 20 Å. The present paper shows how information on the nature and extent of the interface roughness in such systems can be deduced from studies of the optical linewidths and Stokes' shifts.

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    Zeitschriftenartikel

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

    Magnetic field effects in diluted magnetic semiconductors

    Physical Review B - condensed matter and materials physics, vol. 50, pp. 5255-5263

    1994

    DOI: 10.1103/PhysRevB.50.5255

    Abstract anzeigen

    A general formalism for evaluating the effects of the large magnetic exchange interaction between single carriers and the magnetic ions in a diluted magnetic semiconductor, in the presence of an external magnetic field, is developed. In particular, a justification is provided for incorporating a modified Brillouin function into the description of the resulting magnetic effects.

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    Zeitschriftenartikel

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

    Theoretical investigation of observed magnetic polaron energies in quantum wells

    Physical Review B - condensed matter and materials physics, vol. 50, pp. 5713-5716

    1994

    DOI: 10.1103/PhysRevB.50.5713

    Abstract anzeigen

    Theoretical calculations of free-exciton magnetic-polaron energies in semimagnetic quantum-well structures have been performed. The dependence of the polaron energy on well width, magnetic field, and temperature has been calculated and compared with the experimentally determined values. In this manner conclusions have been drawn with regard to the structural characteristics of the quantum-well systems.

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    Zeitschriftenartikel

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

    Exciton magnetic polarons in quantum wells

    Journal of Applied Physics, vol. 75, no. 7, pp. 3466-3471

    1994

    DOI: 10.1063/1.356107

    Abstract anzeigen

    Theoretical calculations of free‐exciton magnetic polaron energies in semimagnetic quantum‐well structures have been performed. The dependence of the polaron energy on well width, magnetic field, and temperature has been calculated. Polaron energy calculations in diffused semimagnetic quantum wells show a strong dependence of the polaron energy on the amount of diffusion in the quantum well. Consequently, magnetic polaron energies could be used as a means of measuring the diffusion coefficient.

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    Zeitschriftenartikel

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

    Numerical simulation of antiferromagnetic spin-pairing effects in diluted magnetic semiconductors and enhanced paramagnetism at interfaces

    Physical Review B - condensed matter and materials physics, vol. 49, pp. 10341-10344

    1994

    DOI: 10.1103/PhysRevB.49.10341

    Abstract anzeigen

    A numerical simulation of the antiferromagnetic spin pairing of neighboring magnetic ions within a diluted magnetic semiconductor is presented. Utilizing a random distribution of the magnetic ions requires the inclusion of nearest-neighbor interactions only in order to give agreement with the low-field bulk-magnetization properties observed experimentally. However, the latter are shown to be a relatively insensitive measure of the degree of ordering in the alloy system, and the experimental results could equally well be accounted for by assuming appreciable alloy clustering. The simulations also show that there is an enhancement in the paramagnetism associated with the interface layers in a quantum-well structure.

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    Zeitschriftenartikel

    P. Harrison, W. Hagston, Thomas Stirner

    Neutral donors and spin-flip Raman spectra in dilute-magnetic-semiconductor microstructures

    Physical Review B - condensed matter and materials physics, vol. 49, pp. 8242-8248

    1994

    DOI: 10.1103/PhysRevB.49.8242

    Abstract anzeigen

    An approach is presented for calculating the binding energy of neutral donors in semiconductor heterostructures. The technique avoids the need for analytical solutions. A generalization of the method to include central cell corrections, effective mass, and dielectric mismatch at heterojunctions, together with band nonparabolicity, is discussed. The utility of the method is demonstrated via a study of the spin-flip Raman spectrum of double quantum wells. Finally it is shown how spin-flip Raman spectroscopy could be used, in conjunction with δ doping of donors, as a probe of the possible enhancements to the paramagnetism of a dilute magnetic semiconductor at a heterojunction with a nonmagnetic material.

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    Zeitschriftenartikel

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

    Impurity bound magnetic polarons in diluted magnetic semiconductors

    Physical Review B - condensed matter and materials physics, vol. 50, pp. 5264-5271

    1994

    DOI: 10.1103/PhysRevB.50.5264

    Abstract anzeigen

    Theoretical calculations of magnetic polaron energies resulting from charge carriers bound to donor or acceptor ion sites in diluted magnetic semiconductors have been performed. The dependence of the polaron energy on the magnetic field, the temperature, and the concentration of the magnetic ion has been evaluated. Similarly the effects of central cell corrections on polaron energies have been determined. The calculated values of the polaron energies are compared with experimental results and the implications of the comparison are discussed.

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    Zeitschriftenartikel

    P. Harrison, W. Hagston, T. Piorek, Thomas Stirner, J. Fatah, R. Roberts, D. Ortega, A. Chesworth

    Monte Carlo simulations of carrier transport and relaxation in superlattices

    Superlattices and Microstructures, vol. 15, no. 2, pp. 209-213

    1994

    DOI: 10.1006/spmi.1994.1042

    Abstract anzeigen

    Monte Carlo simulations of carrier transport through an undoped superlattice into an extended well are described. The results illustrate clearly the important role played by LO phonons in relaxing the energy and the desirability of designing structures which minimize the need for acoustic phonon participation. Attention is drawn to the fact that although, for the standard structures investigated, the LO phonon relaxation rate is relatively independent of the miniband and/or the quantum well eigenstates, the precise shape of the form-factor for an allowed LO transition could play an important role in deciding the relative number of 'slow' carriers in an actual system.

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    Zeitschriftenartikel

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

    Boundary conditions on current carrying states and the implications for observations of Bloch oscillations

    Superlattices and Microstructures, vol. 15, no. 2, pp. 199-202

    1994

    Abstract anzeigen

    The problem of the precise form (together with the associated boundary conditions) of a one-dimensional hamiltonian describing a particle of variable mass is addressed. It is shown that although hermiticity may be a necessary condition, it is not sufficient to specify uniquely the hamiltonian. In particular one form of the latter that is widely employed in the literature is shown to lead to a violation of the Heisenberg Uncertainty Principle. However imposition of the additional demand that any singular terms arising from the kinetic energy can be transformed away does lead to a unique specification of the hamiltonian. Finally the implications of the latter with regard to the standard probability interpretation of the wavefunction and for the observation of Bloch oscillations in quantum well structures is described.

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    Zeitschriftenartikel

    P. Harrison, W. Hagston, T. Piorek, Thomas Stirner, J. Fatah, R. Roberts

    Effects of interfaces and interface roughness in diluted magnetic semiconductor microstructures

    Superlattices and Microstructures, vol. 16, no. 1, pp. 11-15

    1994

    DOI: 10.1006/spmi.1994.1100

    Abstract anzeigen

    In this work a numerical model of anti-ferromagnetic spin-pairing in diluted magnetic semiconductors is used to determine the paramagnetic behaviour of both abrupt and rough interfaces. A detailed study of the paramagnetic behaviour of the individual monolayers adjacent to an abrupt interface with a non-magnetic material has not only predicted quantitatively the enhanced paramagnetism in the ultimate monolayer, it has also predicted a depressed paramagnetism in the penultimate monolayer. Furthermore, the results of the theoretical model have been compared with experimental observations in order to gain insight into intrinsic and structural properties of heterojunctions in diluted magnetic semiconductors.

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    Vortrag

    R. Roberts, P. Harrison, W. Hagston, Thomas Stirner

    Stark ladders in strongly coupled finite superlattices

    Posterpräsentation

    3rd International Conference on Optics of Excitons in Confined Systems, Montpellier, Frankreich

    1993