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Suche nach „[D.] [Ashenford]“ hat 14 Publikationen gefunden
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    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    P. Chen, J. Nicholls, M. O’Neill, Thomas Stirner, J.H.C. Hogg, B. Lunn, D. Ashenford

    Spectroscopic evidence for different laser gain mechanisms in optically pumped ZnCdS/ZnS quantum well structures

    Journal of Applied Physics, vol. 84, pp. 5621-5625

    1998

    DOI: 10.1063/1.368609

    Abstract anzeigen

    The role of biexcitons has been studied in two optically pumped quantum well(QW) structures of Zn1−xCdxS/ZnS, one with 18% Cd concentration and the other with 3% in the wells. For the x=18%QW structure, high excitation photoluminescence and stimulated emission indicate that the laser gain mechanism involves biexcitons. For the x=3%QWs, even though biexcitons are clearly observed in the spontaneous emission, they are not responsible for laser gain in this structure. Instead exciton–exciton scattering may be the more likely mechanism responsible for laser gain close to threshold, while at higher densities an estimate of the carrier density indicates an electron–hole plasma as the likely source of optical gain. The different mechanisms in the two cases can, very likely, be attributed to one of differing degrees of localization both within the QW and at alloy fluctuations.

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    Zeitschriftenartikel

    Thomas Stirner, M. O’Neill, D. Ashenford, N. Cain, W. Hagston, J. Nicholls

    Photoluminescence of CdTe/CdMnTe multiple quantum wells excited near the Mott transition

    Journal of Luminescence, vol. 75, no. 4, pp. 269-275

    1997

    DOI: 10.1016/S0022-2313(97)00135-X

    Abstract anzeigen

    Using pulses from a 6 ns dye laser, the photoluminescence from CdTe/CdMnTe quantum wells is investigated as a function of carrier density. The spectra broaden with density but many sharp features remain. A biexciton is identified with a transition energy of 3.8meV less than that of the free exciton. The Mott transition for the el-hl bandedge is approached at an excitation density of 2.2 × 1012 cm−2. Photoluminescence from the el-h3 exciton is observed as a sharp peak at high densities. Its transition energy does not change and its intensity increases linearly with carrier density.

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    Vortrag

    N. Cain, M. O’Neill, J. Nicholls, W. Hagston, D. Ashenford, Thomas Stirner

    Photoluminescence of highly excited CdTe/CdMnTe quantum wells

    European Quantum Electronics Conference (EQEC), Hamburg

    1996

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Beitrag (Sammelband oder Tagungsband)

    N. Cain, M. O’Neill, J. Nicholls, W. Hagston, D. Ashenford, Thomas Stirner

    Photoluminescence of highly excited CdTe/CdMnTe quantum wells

    Proceedings of the 1996 European Quantum Electronics Conference, EQEC'96, Hamburg, 08.-13.09.1996

    1996

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

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

    Determination of the band offset of CdTe/Cd1-xMnxTe multiple quantum wells with very low x values

    Physical Review B - condensed matter and materials physics, vol. 52, no. 7, pp. 4732-4735

    1995

    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, 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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    Vortrag

    S. Weston, P. Harrison, W. Hagston, J. Nicholls, D. Ashenford, Thomas Stirner

    Optical properties of single magnetic quantum wells in an external magnetic field

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

    1993

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

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

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

    13th North American Conference on Molecular Beam Epitaxy, Stanford, CA, USA

    1993

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

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

    Comparative studies of excitons in magnetic asymmetric double quantum well structures

    Posterpräsentation

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

    1993

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    Zeitschriftenartikel

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

    Molecular-beam epitaxy investigation of interface disorder effects in magnetic II-VI quantum wells

    Journal of Vacuum Science & Technology B, vol. 11, no. 3, pp. 881-884

    1993

    Abstract anzeigen

    The present article describes the results of a combined theoretical and experimental investigation into the quality of the interfaces of II–VI quantum well structures grown by molecular‐beam epitaxy (MBE). Detailed information is presented on the dilute magnetic semiconductor system CdTe/Cd1-xMnxTe in which the CdTe forms the well and the Cd1-xMnxTe forms the barrier. Structures are grown routinely in which both the photoluminescence (PL) and photoluminescence excitation (PLE) linewidths are narrow (∼1–2 meV). This is indicative of high quality material, a feature which is confirmed by the x‐ray data in which Pendellösung fringes can be seen. However, in spite of this, it is found that although the same growth conditions are nominally employed quantum well structures are obtained which show either (i) different discernible structures in the PL and the PLE spectra, or (ii) marked differences in the Stokes’ shift between the PLE and PL from one sample to the next. A related feature concerns observations of an asymmetrical magnetic field splitting of the heavy‐hole exciton states in the barriers. The results of theoretical calculations of the exciton energy levels and their associated linewidths are presented. A comparison of theory with the experimental observations shows that the above effects can be accounted for in terms of interface disorder and magnetic field dependent interface potentials. Thus in case (i) above, the relative intensity of the components in the PLE is different from that in the PL. This is consistent with large island growth in the plane of the well, large here meaning that the island diameters exceed those of the exciton diameter (∼140–150 Å). Observations of a Stokes’ shift, even though the absorption and emission lines are narrow, can be accounted for theoretically if the concept of smaller scale disorder is i- - ntroduced, i.e., island sizes that are small compared with the diameter of the exciton. Finally, results are presented which show a remarkable asymmetry in the Zeeman splitting of the heavy‐hole exciton lines. It is shown that this can be accounted for by introducing deep, short‐range interface potentials (∼1 or 2 monolayers). The latter are magnetic field dependent, the source of the asymmetry being attributed to band‐gap renormalization effects. The implications of all these results for MBE growth conditions are described.

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    Zeitschriftenartikel

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

    Comparative studies of excitons in magnetic asymmetric double quantum well structures

    Journal de Physique IV, vol. 3, no. C5, pp. 401-404

    1993

    DOI: 10.1051/jp4:1993585

    Abstract anzeigen

    Four different asymmetric double quantum well structures in the dilute magnetic semiconductor system Cd1-xMnx Te have been investigated. Theoretical calculations are presented which show that the observed photoluminescence (PL) and photoluminescence excitation (PLE) energies are consistent with an anticrossing of the lowest two hole states in one of the double quantum well structures when subjected to an external magnetic field.

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    Zeitschriftenartikel

    S. Weston, P. Harrison, W. Hagston, J. Nicholls, D. Ashenford, Thomas Stirner

    Optical properties of single magnetic quantum wells in an external magnetic field

    Journal de Physique IV, vol. 3, no. C5, pp. 397-400

    1993

    DOI: 10.1051/jp4:1993584

    Abstract anzeigen

    Photoluminescence measurement performed on magnetic Cd1-xMnxTe quantum wells surrounded by Cd1-yMny Te barriers ([MATH]), grown by M. B. E. and subjected to an external magnetic field, are described. The observed excitation spectra are shown to be in agreement with calculations of exciton energies based on the envelope function approximation. Characteristic features of the samples are either the large ([MATH]18meV) Stokes' shift and/or the width of the P. L. lines ([MATH]11meV). These are to be contrasted with the values in comparable non-magnetic wells of CdTe where the Stokes' shift is typically less than 1 meV and the halfwidth of the P. L. lines is less than 1 or 2 meV. Arguments are presented which show that these observations, together with their magnetic field dependence, are consistent with the formation of magnetic polarons. The photoluminescence emission also contains a band attributed to excitons bound to donors. It is observed experimentally that this donor-bound exciton emission disappears with increasing magnetic field. A theoretical interpretation of the observed properties of this band is given in terms of excitons bound to donors which are situated at different positions in the well and barrier region.

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    Vortrag

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

    Molecular-beam epitaxy investigation of interface disorder effects in magnetic II-VI quantum wells

    Posterpräsentation

    12th North American Conference on Molecular Beam Epitaxy, Ottawa, Kanada

    1992