Browsing by Author "Sopanen, M."

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  • Carrier capture processes in strain-induced InxGa1−xAs/GaAs quantum dot structures
    (2000-11-15) Lingk, C.; Helfer, W.; von Plessen, G.; Feldmann, J.; Stock, K.; Feise, W.M.; Citrin, D.S.; Lipsanen, H.; Sopanen, M.; Virkkala, R.; Tulkki, J.; Ahopelto, J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We investigate carrier capture processes in strain-induced quantum dot structures. The quantum dots consist of a near-surface InGaAs/GaAs quantum well in which a lateral confining potential is generated by the strain from InP stressor islands grown on the sample surface. Using photoluminescence spectroscopy, we show that the rate of carrier capture into the quantum dots increases dramatically when the energetic depth of the confinement potential is reduced by enlarging the quantum well/surface separation D. While carriers in the quantum well region between the quantum dots are found to experience D-dependent nonradiative surface recombination, this process seems to be negligible for carriers in the quantum dots, presumably due to the protecting InP islands.
  • Carrier relaxation dynamics in quantum dots: scattering mechanisms and state-filling effects
    (1997-02-15) Grosse, S.; Sandman, J.; von Plessen, G.; Feldman, J.; Lipsanen, H.; Sopanen, M.; Tulkki, J.; Ahopelto, J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Stressor-induced InxGa1−xAs quantum dot structures of high structural quality allow a detailed experimental investigation of carrier relaxation between distinct zero-dimensional quantized states. Time-resolved photoluminescence studies combined with appropriate model calculations show that state filling effects, Coulomb scattering, and acoustic phonon scattering determine the relaxation scenario in a way characteristic for a zero-dimensional electronic system. These investigations allow a quantitative estimation of the inter-dot-level relaxation rates mediated by (i) Coulomb scattering and (ii) acoustic phonon scattering.
  • Electron-hole correlation in quantum dots under a high magnetic field (up to 45T)
    (1999-12-06) Cingolani, R.; Rinaldi, R.; Lipsanen, H.; Sopanen, M.; Virkkala, R.; Maijala, K.; Tulkki, Jukka; Ahopelto, J.; Uchida, K.; Miura, N.; Arakawa, Y.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The influence of the direct and exchange Coulomb interaction on Landau level formation in strain induced quantum dots has been studied by high-field (45 T) magnetoluminescence and by many-electron many-hole Hartree-Fock calculations. The Darwin Fock states of the dots are found to merge into a unique Landau level at very high fields with a considerable reduction in the total diamagnetic shift due to the enhanced electron-hole correlation caused by the increased degeneracy of the state. We calculate a 50% reduction of the diamagnetic shift as a result of direct and exchange Coulomb interaction in the squeezed carrier states, in excellent agreement with the experimental findings.
  • Enhanced light extraction from InGaN/GaN quantum wells with silver gratings
    (2013) Homeyer, E.; Mattila, P.; Oksanen, J.; Sadi, T.; Nykänen, H.; Suihkonen, S.; Symonds, C.; Tulkki, J.; Tuomisto, Filip; Sopanen, M.; Bellessa, J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We demonstrate that an extraction enhancement by a factor of 2.8 can be obtained for a GaN quantum well structure using metallic nanostructures, compared to a flat semiconductor. The InGaN/GaN quantum well is inserted into a dielectric waveguide, naturally formed in the structure, and a silver grating is deposited on the surface and covered with a polymer film. The polymer layer greatly improves the extraction compared to a single metallic grating. The comparison of the experiments with simulations gives strong indications on the key role of weakly guided modes in the polymer layer diffracted by the grating.
  • Experimental investigation towards a periodically pumped single-photon source
    (2006-07-15) Bödefeld, C.; Ebbecke, J.; Toivonen, Juha; Sopanen, M.; Lipsanen, H.; Wixforth, A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Experiments towards a periodically pumped single-photon source are presented. The lateral piezoelectric field of a surface acoustic wave dissociates laser-generated two-dimensional excitons into electrons and holes. These carriers are separated by the wave potential and are transported over macroscopic length scales without recombining. When reaching a stress-induced quantum dot in the quantum well they periodically populate the zero-dimensional states and recombine, emitting single photons periodically in time according to the surface acoustic-wave frequency. We have successfully reduced the number of pumped quantum dots down to 100 and have detected a strong blinking photoluminescence signal. By further reducing the number of quantum dots down to 1 a periodically pumped single photon source could be realized.
  • Fabrication of InGaAs quantum disks using self-organized InP islands as a mask in wet chemical etching
    (1996) Sopanen, M.; Lipsanen, H.; Ahopelto, J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    GaInAs quantum disks are fabricated by wet chemical etching from a GaInAs/GaAs near‐surface quantum well using self‐organized InP islands as an etch mask. InP islands are formed in coherent Stranski–Krastanow growth mode by metalorganic vapor phase epitaxy. The free‐standing GaInAs/GaAs columns, produced by a three‐step etching process, are overgrown at 550 °C. The luminescence efficiency per emitting area from the regrown quantum disks is one order of magnitude larger than that from a regrown reference quantum well.
  • Investigation of significantly high barrier height in Cu/GaN Schottky diode
    (2016-01-01) Garg, Manjari; Kumar, Ashutosh; S., Nagarajan; Sopanen, M.; Singh, R.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Current-voltage (I-V) measurements combined with analytical calculations have been used to explain mechanisms for forward-bias current flow in Copper (Cu) Schottky diodes fabricated on Gallium Nitride (GaN) epitaxial films. An ideality factor of 1.7 was found at room temperature (RT), which indicated deviation from thermionic emission (TE) mechanism for current flow in the Schottky diode. Instead the current transport was better explained using the thermionic field-emission (TFE) mechanism. A high barrier height of 1.19 eV was obtained at room temperature. X-ray photoelectron spectroscopy (XPS) was used to investigate the plausible reason for observing Schottky barrier height (SBH) that is significantly higher than as predicted by the Schottky-Mott model for Cu/GaN diodes. XPS measurements revealed the presence of an ultrathin cuprous oxide (Cu2O) layer at the interface between Cu and GaN. With Cu2O acting as a degenerate p-type semiconductor with high work function of 5.36 eV, a high barrier height of 1.19 eV is obtained for the Cu/Cu2O/GaN Schottky diode. Moreover, the ideality factor and barrier height were found to be temperature dependent, implying spatial inhomogeneity of barrier height at the metal semiconductor interface.
  • Low energy electron beam induced vacancy activation in GaN
    (2012) Nykänen, H.; Suihkonen, S.; Kilanski, L.; Sopanen, M.; Tuomisto, Filip
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Experimental evidence on low energy electron beam induced point defect activation in GaN grown by metal-organic vapor phase epitaxy(MOVPE) is presented. The GaN samples are irradiated with a 5–20 keV electron beam of a scanning electron microscope and investigated by photoluminescence and positron annihilation spectroscopy measurements. The degradation of the band-to-band luminescence of the irradiatedGaN films is associated with the activation of point defects. The activated defects were identified as in-grown Ga-vacancies. We propose that MOVPE-GaN contains a significant concentration of passive VGa-H n complexes that can be activated by H removal during low energy electron irradiation.
  • Luminescence from excited states in strain-induced In_(x)Ga_(1-x)As quantum dots
    (1995-05-15) Lipsanen, H.; Sopanen, M.; Ahopelto, J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have fabricated quantum dots by locally straining InxGa1−xAs quantum wells with self-organized growth of nanometer-scale InP stressors on the sample surface. The structure is completed in a single growth run using metalorganic vapor-phase epitaxy. Photoluminescence from the dots is redshifted by up to 105 meV from the quantum-well peak due to the lateral confinement of excitons. Clearly resolved luminescence peaks from three excited states separated by 16–20 meV are observed when the quantum well is placed at the depth of 1–10 nm from the surface of the sample. The observed redshift and peak separation are in agreement with simple calculations using a finite-element method and two-dimensional parabolic potential model. This structure is easily fabricated and offers a great potential for the optical study of relaxation and recombination phenomena.
  • Temperature dependence of carrier relaxation in strain-induced quantum dots
    (1998-12-15) Brasken, M.; Lindberg, M.; Sopanen, M.; Lipsanen, H.; Tulkki, J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report experimental observation and theoretical interpretation of temperature-dependent, time-resolved luminescence from strain-induced quantum dots. The experimental results are well described by a master equation model for the electrons. The intraband relaxation in the conduction band and the radiative recombination rate are governed by the hole populations resulting in prominent temperature dependence of the relaxation process. Even when only a few electrons and holes are confined in a single quantum dot the Auger-like process provides a rapid intraband relaxation channel for electrons that can replace the phonon scattering as the dominant relaxation mechanism.
  • Zeeman effect in parabolic quantum dots
    (1996-07-08) Rinaldi, R.; Guigno, P.; Cingolani, R.; Lipsanen, Harri; Sopanen, M.; Tulkki, J.; Ahopelto, J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    An unprecedentedly well resolved Zeeman effect has been observed when confined carriers moving along a closed mesoscopic path experience an external magnetic field orthogonal to the orbit plane. Large Zeeman splitting of excited higher angular momentum states is observed in the magnetoluminescence spectrum of quantum dots induced by self-organized InP islands on InGaAs/GaAs. The measured effect is quantitatively reproduced by calculations including the vertical quantum well confinement and strain induced, nearly parabolic, lateral confinement, together with the magnetic interaction.