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Suche nach „[P.] [Jackson]“ hat 9 Publikationen gefunden
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    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    G. Bergmann, P. Jackson, J.H.C. Hogg, Thomas Stirner, M. O’Neill, W. Duffy, S. Kelly, G. Clark

    Photoinduced changes of surface order in coumarin side-chain polymer films used for liquid crystal photoalignment

    Applied Physics Letters, vol. 87

    2005

    DOI: 10.1063/1.2009070

    Abstract anzeigen

    Specular x-rayreflectivity probes morphological changes in a crosslinkable coumarin photoalignment polymer film resulting from ultraviolet irradiation. An ordered surface layer with density oscillations compatible with planar side-chain alignment is obtained before irradiation. The ordering is enhanced in the early stages of crosslinking. This is attributed to the photoinduced increase of mobility of the side-chains resulting from the creation of free volume by the crosslinking process. The expansion of the thin film confirms that free volume is created. The surface ordering decreases with prolonged ultraviolet irradiation because of increased material viscosity resulting from a high crosslinked density. The implications of surface ordering on liquid crystal photoalignment are discussed.

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    Vortrag

    G. Bergmann, P. Jackson, J.H.C. Hogg, Thomas Stirner, W. Duffy, S. Kelly, G. Clark, M. O’Neill

    In-situ study of the ultraviolet irradiation of side-chain coumarin polymers using specular x-ray reflectivity

    Synchrotron Radiation in Polymer Science II Conference, Sheffield, Großbritannien

    2002

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    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

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

    Zeeman studies of CdTe-Cd1-xMnxTe multiquantum wells

    Posterpräsentation

    6th International Conference on Modulated Semiconductor Structures, Garmisch-Partenkirchen

    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

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

    Interface disorder and the inhomogeneous broadening of optical spectra in semiconductor quantum wells

    Superlattices and Microstructures, vol. 13, no. 4, pp. 431-435

    1993

    Abstract anzeigen

    The photoluminescence linewidth of a CdTe/Cd1-xMnxTe structure, grown by molecular beam epitaxy, containing quantum wells of width 26, 37 and 45Å separated by large (250Å) barriers, was investigated. The high quality of the quantum well structure was confirmed by X-ray topography, X-ray rocking curves and narrow photoluminescence lines. It is shown that these results are consistent with small scale interface roughness resulting from clusters involving pairs or triplets of Mn2+ ions forming into small islands.

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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