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Suche nach „[J.H.C.] [Hogg]“ hat 18 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

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    Zeitschriftenartikel

    Thomas Stirner, N. Kirkman, L. May, C. Ellis, J. Nicholls, S. Kelly, M. O’Neill, J.H.C. Hogg

    CdTe nanocrystals: synthesis, optical characterisation and pseudopotential calculation of the bandgap

    Journal of Nanoscience and Nanotechnology, vol. 1, no. 4, pp. 451-455

    2001

    DOI: 10.1166/jnn.2001.053

    Abstract anzeigen

    CdTe nanocrystals were synthesized in aqueous solution using 1-thioglycerol and 2-mercaptoethanol as surface stabilizers. The nanocrystals were characterized by means of X-ray powder diffraction and UV-vis absorption measurements. The UV-vis absorption spectra exhibit two distinct transition lines. Comparison of the experimental measurements with the results of the empirical pseudopotential calculations of the CdTe nanocrystals showed that the lower energy absorption line can be assigned to the heavy-hole exciton transition, whereas the higher energy absorption line can be attributed to the light-hole exciton transition.

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    Zeitschriftenartikel

    Thomas Stirner, W. Hagston, J.H.C. Hogg, J. Nicholls, H. Howari, D. Sands

    Excimer laser induced diffusion in magnetic semiconductor quantum wells

    Journal of Applied Physics, vol. 88, pp. 1373-1379

    2000

    DOI: 10.1063/1.373826

    Abstract anzeigen

    Studies of pulsed laser annealing (PLA) of CdTe/CdMnTe quantum well structures are made in order to examine depth dependent effects in laser irradiated semiconductors. Since diffusion coefficients are strongly dependent on the temperature, depth resolution is achieved because the diffusion of Mn from the barriers into the quantum wells is depth dependent. Multiple quantum well(MQW) structures of CdTe/CdMnTe were annealed with single pulses from an XeCl laser at 308 nm. At a threshold of 90 mJ cm−2 two new emission bands are observed that are attributed to the diffusion of Mn from barrier layers to QWs. The diffusion associated with these bands, measured as the integrated product of the diffusion constant and time, is found to be 300 and 30 Å2. Calculations of the temperature, reached within the surface following PLA, using an analytical solution of the heat diffusion equation coupled with known high temperature diffusion coefficients predict the diffusion to decrease by one order of magnitude within one period at the top of the MQW stack. It is suggested that at the threshold surface melting occurs and that these emission bands arise from the QWs immediately beneath the melt front. The diffusion of Mn ions into the QWs is confirmed by magneto-optical data. A further emission band occurs at this same threshold with a Mn concentration above that of the concentration in the barrier layers of the MQW stack. This emission is attributed tentatively to the segregation of the Mn ion within the molten region following recrystallization.

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    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, 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

    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

    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

    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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    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

    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

    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

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    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

    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