J. D. Thomson

Cardiff University, Cardiff, Wales, United Kingdom

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Publications (32)37.61 Total impact

  • J.D. Thomson, P.M. Smowton, P. Blood, J.F. Klem
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    ABSTRACT: The modal gain, modal loss and spontaneous emission of a GaAsSb-based type-II quantum-well (QW) laser structure emitting at 1.3 mum have been experimentally determined as a function of current injection and temperature. The system is able to provide a maximum of 900 cm<sup>-1</sup> of material gain from the n = 1 transition despite an electron-hole overlap of 32%, however, the gain from the n = 2 transition becomes dominant before this value can be achieved. The presence of the n = 2 transition has a detrimental effect on device performance, limiting the usable gain from the first transition and increasing the total radiative recombination current. Energy level calculations show that reducing the hole QW to 4 nm would increase the separation of the n = 1 and n = 2 transition by a further 45 meV, reducing the limiting effect of the transition. Carrier distribution spectra show the carriers are in thermal equilibrium for the temperatures and injection currents studied. A low radiative efficiency for this structure is measured due to a very large nonradiative current. We believe a combination of different mechanisms contribute to the nonradiative current.
    IEEE Journal of Quantum Electronics 08/2007; · 2.11 Impact Factor
  • J.D. Thomson, P.M. Smowton, P. Blood, J.F. Klem
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    ABSTRACT: Experimental results of the modal gain and recombination currents of a GaAsSb/InGaAs type-II quantum well laser structure emitting at 1.3 mum as a function of current injection and temperature are presented. The radiative efficiency versus injection level at different temperatures is analyzed.
    Lasers and Electro-Optics, 2007 and the International Quantum Electronics Conference. CLEOE-IQEC 2007. European Conference on; 01/2007
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    ABSTRACT: We have measured the time response of the emission spectra of In <sub>0.07</sub>Ga<sub>0.93</sub>N quantum wells with widths of 2, 3, and 4nm in GaN following pulsed optical excitation. We observe a blue shift of the emission peak during the excitation and a subsequent red shift as the carriers recombine in the 3- and 4-nm wells, and a negligible shift for the 2-nm well. Using a comprehensive theory we are able to fit both the time evolution of the peak emission energy and the integrated emission intensity. The shift of the emission peak (by about 17 meV) arises from the balancing of the change in screening of the internal piezoelectric field as the carrier density changes and bandgap renormalization. We have projected the calculations to quantify the degree of screening at typical threshold carrier densities. At transparency we estimate carrier densities of 4.3times10<sup>16</sup> m<sup>-2</sup> and 4.8times10<sup>16</sup> m<sup>-2</sup> for the 4- and 3-nm wells, respectively, which reduce the internal piezoelectric field in the well to 0.97times10<sup>8</sup> (4 nm) and 1.03times10 <sup>8</sup> (3 nm) Vmiddotm<sup>-1</sup> compared with the unscreened value of about 1.23times10<sup>8</sup> Vmiddotm<sup>-1</sup>. Thus, a substantial field remains in these wells under laser conditions. We find that this partially screened field is beneficial in reducing the threshold current compared with that of a square well for modal gains up to about 150 cm<sup>-</sup>1
    IEEE Journal of Quantum Electronics 01/2007; · 2.11 Impact Factor
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    ABSTRACT: Temperature-dependent measurements of the pulsed light-current characteristics of InGaN light-emitting diodes that were thermally annealed at different temperatures have been investigated. A distinct light output, at a fixed current density, with operating temperature arises where the light output increases as the operating temperature is reduced from 300 K, reaches a maximum, and then decreases with subsequent reductions of the operating temperature. We observe that light-emitting diodes thermally annealed at higher temperatures, which is believed to increase the number of electrically activated acceptors in the p layers, have a lower light output below 300 K and the maximum light output shifts to higher operating temperatures. Measured absorption and emission spectra show that the thermal anneal process has not affected the structure of the quantum wells within these samples. The light output, for a fixed current density, has been simulated as a function of operating temperature, and we find that by changing the concentration of acceptor atoms, compensating donor atoms, and the hole mobility in the p layers, the trends observed experimentally can be reproduced. On the basis of the simulations we find that the distinct behavior of the light output with operating temperature is due to the combination of Shockley-Reed-Hall recombination, at operating temperatures around 300 K, and electron drift leakage, at operating temperature below 300 K, and the increase of the acceptor concentration results in an increased electron drift leakage due to the change of the concomitant hole mobility. The simulations support the view that the experimental observations can be explained through changes of the acceptor concentration in the p layers when the thermal anneal temperature is increased.
    Journal of Applied Physics 01/2006; 99(2):024507-024507-7. · 2.21 Impact Factor
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    ABSTRACT: By modelling time resolved emission spectra of 3 and 4 nm In0.07Ga0.93N quantum wells we show that injected carriers reduce the internal field to only 70% of its unscreened value at typical laser thresholds
    01/2006;
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    ABSTRACT: The authors measure the temperature dependence of the components of threshold current of 1300 nm undoped and p-doped quantum dot lasers and show that the temperature dependence of the injection level necessary to achieve the required gain is the largest factor in producing the observed negative T0 in p-doped quantum dot lasers.
    Applied Physics Letters 01/2006; 89(15):151118-151118-3. · 3.52 Impact Factor
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    ABSTRACT: We measure the temperature dependence of the components of threshold current of 1300 nm undoped and p-doped quantum-dot-lasers and show that the temperature dependence of gain is the largest factor in producing the observed negative T0
    01/2006;
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    ABSTRACT: Not Available
    Lasers and Electro-Optics Europe, 2005. CLEO/Europe. 2005 Conference on; 07/2005
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    ABSTRACT: We have measured the pulsed light-current characteristics of a series of InGaN/GaN quantum well light-emitting diodes which were annealed post-growth at different temperatures as a function of their operating temperature. The light output at a fixed current density increases with the temperature of measurement, reaches a maximum and then decreases for all the diodes. The measurement temperature at which the maximum light output occurs and the magnitude of the light output depend on the post-growth thermal anneal temperature. The thermal anneal temperature is thought to affect the acceptor concentration in the p-doped cap layer, which also changes the carrier mobility. A simulation, incorporating carrier leakage, is used to reproduce the experimental behavior where the acceptor concentration is changed to represent the effects of the different anneal temperatures.
    Proc SPIE 04/2005;
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    ABSTRACT: In many studies, the value of the experimentally determined internal piezoelectric field has been reported to be significantly smaller than theoretical values. We believe this is due to an inappropriate approximation for the electric field within the depletion region, which is used in the analysis of experimental data, and we propose an alternative method. Using this alternative, we have measured the strength of the internal field of InGaN p-i-n structures, using reverse bias photocurrent absorption spectroscopy and by fitting the bias dependent peak energy using microscopic theory based on the screened Hartree-Fock approximation. The results agree with those using material constants interpolated from binary values.
    Applied Physics Letters 03/2005; 86(13). · 3.52 Impact Factor
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    ABSTRACT: Not Available
    Lasers and Electro-Optics, 2003. CLEO '03. Conference on; 07/2003
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    ABSTRACT: Pulsed light–current characteristics of InGaN/GaN quantum well light-emitting diodes have been measured as a function of temperature, with sublinear behavior observed over the whole temperature range, 130–330 K. A distinctive temperature dependence is also noted where the light output, at a fixed current, initially increases with temperature, before reaching a maximum at 250 K and then decreases with subsequent increases in temperature. On the basis of a drift diffusion model, we can explain the sublinear light–current characteristics and the temperature dependence by the influence of the large acceptor ionization energy in Mg-doped GaN together with a triangular density of states function characteristic of localized states. Without the incorporation of localization effects, we are unable to reproduce the temperature dependence whilst maintaining emission at the observed wavelength. This highlights the importance of localization effects on device performance. © 2003 American Institute of Physics.
    Applied Physics Letters 04/2003; 82(17):2755-2757. · 3.52 Impact Factor
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    ABSTRACT: We have measured piezoelectric fields in p-i-n LED structures using the quantum confined Stark effect and photocurrent absorption. The results agree with calculations of the absorption where material parameters are interpolated from the binaries.
    Conference Proceedings - Lasers and Electro-Optics Society Annual Meeting-LEOS 01/2003; 2.
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    ABSTRACT: The reduction in penetration of the optical mode into the cladding layers in large optical cavity (LOC) laser structures offers the possibility of reducing the cladding-layer thickness. This could be particularly beneficial in GaInP-AlGaInP high-power devices by reducing the thermal impedance and the electrical series resistance. We have designed and characterized 650-nm LOC lasers by modeling the optical loss due to incomplete confinement of the optical mode by the cladding layers and calculating the thermally activated leakage current. This indicated that the cladding thickness could be reduced to 0.5 μm without adversely affecting performance. We investigated devices with 0.3-, 0.5-, and 1-μm-wide cladding layers. The measured optical mode loss of the 0.3-μm-wide cladding device was 36.2 cm<sup>-1</sup> compared with 12.4 and 11.3 cm<sup>-1</sup> for the 0.5- and 1-μm-wide cladding samples, respectively. The threshold current densities of the 0.5- and 1.0-μm devices were similar over the temperature range investigated (120-320 K), whereas the 0.3-μm devices had significantly higher threshold current density. We show that this can be attributed to the higher optical loss and increased leakage current through the thin cladding layer. The intrinsic gain characteristics were the same in all the devices, irrespective of the cladding-layer thickness. The measured thermal impedance of 2-mm-long devices was reduced from 30.7 to 22.3 K/W by reducing the cladding thickness from 1 to 0.5 μm. Our results show that this can be achieved without detriment to the threshold characteristics
    IEEE Journal of Quantum Electronics 04/2002; · 2.11 Impact Factor
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    ABSTRACT: Measurements of the spontaneous recombination current, quasi-fermi level separation and gain spectra as a function of current density provide a complete characterisation of the semiconductor laser gain medium. This is particularly useful for analysing the benefits of strain in, for example, GaInP short wavelength laser diodes. In this work we present a novel technique that enables us to measure both the TE and TM polarised true spontaneous emission spectra of GaInP tensile strained devices as a function of quasi-fermi level separation. Using these results we are able to assess the affect of strain on all the significant recombination pathways within the device for a given value of gain requirement.
    Lasers and Electro-Optics, 2002. CLEO '02. Technical Digest. Summaries of Papers Presented at the; 02/2002
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    ABSTRACT: The radiative efficiency is critical in applications and it is therefore necessary to have a good understanding of the factors which determine this in GaN based LEDs. To this end we have studied light output versus current characteristics as a function of temperature to separate out the relevant physical processes. We have measured the pulsed light-current characteristics of GaN/(x5)InGaN quantum well (QW) LEDs as a function of temperature using a low duty cycle to avoid self-heating.
    Lasers and Electro-Optics Society, 2002. LEOS 2002. The 15th Annual Meeting of the IEEE; 02/2002
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    ABSTRACT: Measurement of the spontaneous emission and gain spectra provides a complete characterization of a semiconductor gain medium, however, this requires the observation of emission in two directions to avoid amplification of the spontaneous emission spectrum. We show that both the gain spectrum and the true spontaneous emission spectrum can be obtained from amplified spontaneous emission (ASE) spectra measured from the end of a segmented-contact device. The spontaneous emission spectra agree with spectra measured through a top contact window. If the carrier populations are fully inverted at low photon energy, it is possible to convert the ASE-derived spontaneous emission into real units. © 2002 American Institute of Physics.
    Applied Physics Letters 01/2002; 80(1):1-3. · 3.52 Impact Factor
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    ABSTRACT: We observe that the emission wavelength of edge-emitting InGaAs quantum dot lasers has a much weaker temperature dependence (0.6 Å K−1) than equivalent quantum well devices (3 Å K−1). Measured gain and absorption spectra show that the gain peak wavelength due to dot states is almost independent of temperature for a given value of peak gain whereas the absorption edge shifts at a rate of about 2 Å K−1. Above 100 K the occupancy of dot states can be described by Fermi functions and on this basis we find that the measured gain and absorption spectra are in excellent quantitative agreement. Although the band edge energy reduces with increasing temperature, this analysis shows that the energy distribution of dot states matches the evolution of the Fermi functions such as to leave the quasi Fermi level separation and the wavelength of the gain peak unchanged as a function of temperature for a given value of peak gain. This energy distribution is a consequence of the dot size distribution so the match to the Fermi functions is probably fortuitous. © 2001 American Institute of Physics.
    Journal of Applied Physics 10/2001; 90(9):4859-4861. · 2.21 Impact Factor
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    ABSTRACT: The spontaneous emission and optical gain spectra from an InGaAs quantum dot laser have been independently measured under the same operating conditions. Using these spectra a combined probability-distribution function describing the electron occupancy in the conduction and valence bands has been experimentally determined. Comparison of this function with theoretical curves based on Fermi-Dirac statistics shows that for temperatures down to 100 K the carrier occupancy statistics are accurately described by thermal distributions. Measurements at 70 K show a breakdown of thermodynamic equilibrium indicated by non-thermal carrier distributions.
    Semiconductor Science and Technology 03/2001; · 1.92 Impact Factor
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    ABSTRACT: The unamplified spontaneous emission spectrum emitted under high injection conditions provides a valuable insight into the behaviour of semiconductor laser structures. To determine this spectrum directly, amplification or absorption of the emission must be avoided, for example by observation through a top-contact window. While it is possible, under favourable circumstances, to transform the emission spectrum to a gain spectrum, it is preferable to measure optical gain by more direct methods, usually by analysis of the amplified spontaneous emission from the end of the device, for example by a single-pass stripe length method. A complete characterisation of the semiconductor gain medium therefore requires a combination of techniques for measurement of spontaneous emission and of optical gain as has been done by fabricating a segmented-contact structure for a single-pass gain measurement with a window in the top contact for observation of spontaneous emission. However this is a complex device and it is necessary to observe emission from the device in two different directions. Furthermore the gain and emission spectra are obtained from different regions of the sample. In this paper we show that the gain and true spontaneous emission spectrum can be obtained from amplified spontaneous emission spectra alone. We confirm the validity of the process by comparison with the true spontaneous emission spectrum observed through a top-contact window and show how the emission spectrum may be expressed in real units. This approach provides considerable simplification of the technique and enables both the TE and TM emission spectra to be measured, which is not possible with window methods
    Lasers and Electro-Optics Society, 2001. LEOS 2001. The 14th Annual Meeting of the IEEE; 02/2001