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Suche nach „[J.] [Nicholls]“ hat 22 Publikationen gefunden
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    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    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, W. Hagston, J. Nicholls, M. O’Neill, S. Weston, J. Miao

    Mn ion clustering in II-VI semimagnetic semiconductor heterostructures

    Physical Review B - condensed matter and materials physics, vol. 58, pp. 7040-7045

    1998

    DOI: 10.1103/PhysRevB.58.7040

    Abstract anzeigen

    The results of photoluminescence excitation experiments in Cd1-xMnxTe and Zn1-xMnxSe diluted magnetic semiconductor quantum-well structures are shown to be consistent with the occurrence of a clustering of the magnetic Mn ions. Possible clustering mechanisms are discussed briefly together with alternative explanations of the data.

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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

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    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

    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

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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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    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

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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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    Vortrag

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

    Excitonic relaxation channels in double quantum wells

    8th International Conference on Hot Carriers in Semiconductors, Oxford, Großbritannien

    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

    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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    Zeitschriftenartikel

    P. Harrison, W. Hagston, J. Nicholls, M. O’Neill, T. Piorek, Thomas Stirner, S. Weston

    Investigation of the effect of quantum well width on the binding energy of excitons to neutral donors

    Superlattices and Microstructures, vol. 14, no. 4, pp. 249-252

    1993

    DOI: 10.1006/spmi.1993.1133

    Abstract anzeigen

    Observations of bound exciton states in some bulk semiconductor materials has shown the validity of Haynes' rule, namely that the binding energy of the exciton to a donor is a multiple 1/ξ (say) of the binding energy of the electron to the donor (i.e., the neutral donor binding energy E D ), although in other bulk materials more general linear dependencies are required. Quantum well structures (QWS) typically exhibit such donor hound exciton complexes. There are several points of difference however, E D is a function of donor position, the donor distribution is unknown and interface roughness could also influence the donor bound exciton emission. Hence in order to determine whether a generalised form of Haynes' rule can be applied to excitons bound to donors in quantum wells of varying width requires a careful combination of theory with experiment. The binding energies of the donors at various positions in each well region must be calculated with due allowance made for the effects of interface roughness. A presumed distribution of the donor concentration is then made and, on the assumed validity of Haynes' rule, the lineshape of the bound exciton emission calculated. Comparison with the observed emission spectra will then give insight into whether Haynes' rule is satisfied as a function of position and well width in QWS. This analysis has been carried out for a series of single quantum wells, all of different widths and all grown in the same sample by the technique of molecular beam epitaxy. A careful analysis of all the data showed that Haynes' rule is not applicable--i.e., the observed exciton energies are not a constant multiple of the donor binding energy for QWS of different well width. PACS: 71.55.Gs; 78.66.Hf.