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Suche nach „[Stirner] [Thomas]“ hat 139 Publikationen gefunden
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    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

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

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    W. Hagston, Thomas Stirner, M. Farrow

    Semimagnetic semiconductor quantum wells: magnetic polarons and paramagnetic effects

    Journal of Physics: Condensed Matter, vol. 12, no. 5, pp. 701-708

    2000

    DOI: 10.1088/0953-8984/12/5/316

    Abstract anzeigen

    Photoluminescence experiments were carried out for Cd1-y Mny Te-Cd1-x Mnx Te-Cd1-y Mny Te quantum wells with y >x . The magnetic field dependence of the exciton emission lines reveals paramagnetic effects which are markedly different from those of bulk CdMnTe. It is shown that these effects are consistent with a clustering of the Mn ions. Furthermore, at low magnetic fields an additional energy shift in the transition energy is observed. This energy shift can be explained by the formation of exciton magnetic polarons in the quantum wells. The polaron energy is calculated as a function of magnetic field strength and an estimate of the localization radius of the magnetic polaron is made.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    Thomas Stirner

    Computer simulation of alumina (lecture)

    2000

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    Thomas Stirner

    Computer simulation of alumina (lecture)

    2000

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    J. Miao, W. Hagston, Thomas Stirner, S. Takeyama

    Asymmetric two-dimensional exciton magnetic polarons

    Posterpräsentation

    Autumn Meeting, Iwate, Japan

    1999

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Miao, T. Wojtowicz, W. Hagston, Thomas Stirner, J. Kossut, G. Karczewski, S. Takeyama

    Exciton magnetic polarons in asymmetric diluted magnetic semiconductor quantum wells

    Physical Review B - condensed matter and materials physics, vol. 60, no. 16, pp. 11545-11549

    1999

    DOI: 10.1103/PhysRevB.60.11545

    Abstract anzeigen

    Photoluminescence experiments were carried out for a 3.6-nm Cd0.78Mg0.22Te-CdTe-Cd0.86Mn0.14Te asymmetric quantum well and show the occurrence of two lines, both associated with excitons confined in the quantum-well region. The magnetic-field dependence of the transition energy revealed the formation of magnetic polarons associated only with the excitonic line at the lower energy. Calculations are presented that are consistent with the experimental observations and in particular show that the lower energy line has a component associated with alloy fluctuations at the well-barrier interface in addition to a component associated with polaron bifurcation.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Miao, W. Hagston, Thomas Stirner

    Faraday rotation and interface enhancement effects in diluted-magnetic-semiconductor superlattices

    Physical Review B - condensed matter and materials physics, vol. 59, pp. 5784-5790

    1999

    Abstract anzeigen

    A two-level system model is extended to analyze the Faraday rotation effect in CdTe/Cd1-xMnxTe superlattice structures and to explain experimental Faraday rotation measurements. Furthermore the interface paramagnetism enhancement effect on the Faraday rotation is investigated.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    Thomas Stirner

    Diluted Magnetic Semiconductors (lecture)

    1999

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

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

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

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

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Fatah, P. Harrison, W. Hagston, Thomas Stirner, J.H.C. Hogg

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    Thomas Stirner

    Free and localized magnetic polarons in diluted magnetic semiconductors

    National II-VI Semiconductor Interaction Meeting, Norwich, Großbritannien

    1997

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

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

    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.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Miao, W. Hagston, Thomas Stirner

    Localized magnetic polarons in diluted magnetic semiconductors

    Journal of Applied Physics, vol. 82, pp. 5653-5657

    1997

    DOI: 10.1063/1.366426

    Abstract anzeigen

    A theory of localized magnetic polarons in diluted magnetic semiconductors is developed. The theory involves below band gap states and alloy fluctuations utilizing the optimal fluctuation method. A comparison of the theoretical calculations with published experimental results is performed. The implications of the comparison are discussed and the need for extending the theory to include an additional impurity/native lattice defect potential described. The magnetic field and temperature dependence of the polaron energies are also evaluated.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    P. Harrison, W. Hagston, F. Long, Thomas Stirner

    The structural dependence of the effective mass and Luttinger parameters in semiconductor quantum wells

    Journal of Applied Physics, vol. 82, pp. 3414-3421

    1997

    Abstract anzeigen

    A detailed comparison of the empirical pseudopotential method with single and multiple band calculations based on the envelope function and effective mass approximations are presented. It is shown that, in order to give agreement with the more rigorous microscopic approach of the pseudopotential method, structural dependent effective masses and Luttinger parameters must be invoked. The CdTe/Cd1−xMnxTe system has been employed as an example, and the first pseudopotential calculations of quantum wells and superlattices in this material are presented. It is shown that the electron, light- and heavy-hole effective masses tend towards twice their bulk values in the limit of narrow quantum wells.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Miao, W. Hagston, Thomas Stirner

    Spin-fluctuation effects in magnetic-polaron formation

    Philosophical Magazine Part B, vol. 75, no. 6, pp. 897-908

    1997

    DOI: 10.1080/13642819708205715

    Abstract anzeigen

    Magnetic-polaron energies in various systems (e.g. donor- or acceptor-bound magnetic polarons and free magnetic polarons) are calculated and the relative importance of the spin-fluctuation term is investigated. It is found that the spin fluctuations of the magnetic ions play a different role in the various kinds of magnetic-polaron complex. Futhermore, it is demonstrated how experimental measurements, for example the magnetic field or temperature dependence of the polaron energy, can be utilized to determine the spin state of the magnetic ions within the polaron. A clear picture of the relationship between the Mn2+ spin fluctuations and magnetic-polaron formation is thus revealed.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    P. Harrison, W. Hagston, Thomas Stirner

    Effects of interface imperfections on the Zeeman splitting of excitons in diluted magnetic semiconductor quantum wells

    Philosophical Magazine Part B, vol. 75, no. 3, pp. 349-361

    1997

    Abstract anzeigen

    Theoretical estimates are given of the influence that imperfect interfaces have on the Zeeman splittings of excitons in diluted magnetic semiconductor quantum well structures in which one or both of the barriers are magnetic. A first-principles theoretical treatment of the microscopic magnetic response of the interfaces has been utilized in the calculations. The results show how sensitive these splittings can be to even the smallest degree of imperfection at the interface region provided that both barriers are magnetic and the initial potential profile is symmetric with respect to the centre of the well. A comparison with experimental data on the CdTe/Cd1- x Mn x Te system shows that all such symmetric quantum well structures reported to date have imperfections at the interface region extending over at least one or two monolayers. The calculations also show that asymmetric quantum well structures, in which one of the barriers is non-magnetic, can be a far less sensitive probe of interface imperfections.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Zeitschriftenartikel

    J. Miao, W. Hagston, Thomas Stirner

    The magnetic localization of free exciton magnetic polarons in diluted magnetic semiconductors

    Journal of Applied Physics, vol. 81, no. 9, pp. 6297-6302

    1997

    DOI: 10.1063/1.364364

    Abstract anzeigen

    A model of free exciton magnetic polarons (EMPs) in diluted magnetic semiconductors is proposed on the basis of the localization of the hole in the magnetic potential well created by its own exchange field. The corresponding calculations of the free EMP binding energies, as a function of Mn2+ ion concentration and temperature, are performed. The results are in good agreement with recent experimental measurements. It is found that free EMPs are stable up to relatively high temperatures (T>~30 K) over a wide range of Mn concentrations x without any other primary localization, e.g., alloy potential fluctuations. The dependence of the critical temperature of free EMP formation on the Mn concentration is determined for various diluted magnetic semiconductor materials, and is in good agreement with the experimental results for Cd1-xMnxTe. The suppression of free EMPs under the application of an external magnetic field is also considered. © 1997 American Institute of Physics.

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

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

    Photoluminescence of highly excited CdTe/CdMnTe quantum wells

    European Quantum Electronics Conference (EQEC), Hamburg

    1996

    Angewandte Naturwissenschaften und Wirtschaftsingenieurwesen

    Vortrag

    Thomas Stirner

    Magnetic polarons in diluted magnetic semiconductors

    Highlights of UK Research and R&D in Physics and Physics-based Technologies by Young Physicists, London, Großbritannien

    1996