### Browsing by Author "Tulkki, J."

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Item Carrier capture processes in strain-induced InxGa1-xAs/GaAs quantum dot structures(American Physical Society (APS), 2000) Lingk, C.; Helfer, W.; von Plessen, G.; Feldmann, J.; Stock, K.; Feise, M. W.; Citrin, D. S.; Lipsanen, Harri; Sopanen, Markku; Virkkala, R.; Tulkki, J.; Ahopelto, J.; Department of Micro and Nanosciences; Mikro- ja nanotekniikan laitos; Sähkötekniikan korkeakoulu; School of Electrical EngineeringWe 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.Item 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.; Department of Micro and Nanosciences; Department of Neuroscience and Biomedical EngineeringWe 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.Item 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.; Department of Micro and NanosciencesStressor-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.Item Carrier relaxation dynamics in quantum dots: Scattering mechanisms and state-filling effects(American Physical Society (APS), 1997) Grosse, S.; Sandmann, J. H. H.; von Plessen, G.; Feldmann, J.; Lipsanen, Harri; Sopanen, Markku; Tulkki, J.; Ahopelto, J.; Department of Micro and Nanosciences; Mikro- ja nanotekniikan laitos; Sähkötekniikan korkeakoulu; School of Electrical EngineeringStressor-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.Item Effects of a few-particle interaction on the atomiclike levels of a single strain-induced quantum dot(2000-07-15) Rinaldi, R.; Antonaci, S.; De Vittorio, M.; Cingolani, R.; Hohenester, U.; Molinari, E.; Lipsanen, H.; Tulkki, J.; Department of Micro and Nanosciences; Department of Neuroscience and Biomedical EngineeringWe investigate the effects of few-particle population of a single strain-induced quantum dot by optical excitation. The low-power photoluminescence spectra consist of sharp lines with energy separation of a few meV, associated to the formation of excitonic molecules in the single dot. With increasing photoexcitation intensity, the population of higher states is observed; however, we also observe a clear intensity dependence of the transition energies, inconsistent with a simple filling of exciton levels. Based on a theoretical model for interacting electron-hole pairs in the dot, we obtain good agreement with experiment and show that exciton-exciton interactions control the spectral changes as the number of pairs is increased.Item Effects of lateral current injection in GaN multi-quantum well light-emitting diodes(American Institute of Physics, 2012) Kivisaari, P.; Oksanen, J.; Tulkki, J.Item Electron-Hole Correlation in Quantum Dots under a High Magnetic Field (up to 45 T)(American Physical Society (APS), 1999) Cingolani, R.; Rinaldi, R.; Lipsanen, Harri; Sopanen, Markku; Virkkala, R.; Maijala, K.; Tulkki, J.; Ahopelto, J.; Uchida, K.; Miura, N.; Arakawa, Y.; Department of Micro and Nanosciences; Mikro- ja nanotekniikan laitos; Sähkötekniikan korkeakoulu; School of Electrical EngineeringThe 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.Item Enhanced light extraction from InGaN/GaN quantum wells with silver gratings(AIP Publishing, 2013) Homeyer, E.; Mattila, P.; Oksanen, J.; Sadi, T.; Nykänen, H.; Suihkonen, S.; Symonds, C.; Tulkki, J.; Tuomisto, Filip; Sopanen, M.; Bellessa, J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe 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.Item Enhanced optical properties of in situ passivated near-surface AlxGa1-xAs/GaAs quantum wells(AIP Publishing, 1996) Lipsanen, Harri; Sopanen, Markku; Taskinen, M.; Tulkki, J.; Ahopelto, J.; Department of Micro and Nanosciences; Mikro- ja nanotekniikan laitos; Sähkötekniikan korkeakoulu; School of Electrical EngineeringAn epitaxial method for in situpassivation of epitaxial Al x Ga1−x As/GaAs surfaces is reported. The deposition of an ultrathin InP layer (about one monolayer) on the surface of Al x Ga1−x As/GaAs structures by metalorganic vapor phase epitaxy results in drastically reduced surface recombination. The effect is studied by low‐temperature photoluminescence of near‐surface Al0.22Ga0.78As/GaAs quantum wells where the top barrier thickness is varied from 0 to 50 nm. At the thicknesses of ≤5 nm, the intensity from passivated samples is more than four orders of magnitude larger than that obtained from unpassivated structures. For a passivated surfacequantum well where InP is deposited directly onto the GaAsquantum well, we observe a blueshift of 15 meV and an intensity reduction of only a factor of 10 as compared to the luminescence from a quantum well placed at a depth of 50 nm from the surface.Item Magneto-optical properties of strain-induced InxGa1-xAs parabolic quantum dots(American Physical Society (APS), 1998) Rinaldi, R.; Mangino, R.; Cingolani, R.; Lipsanen, Harri; Sopanen, Markku; Tulkki, J.; Braskén, M.; Ahopelto, J.; Department of Micro and Nanosciences; Mikro- ja nanotekniikan laitos; Sähkötekniikan korkeakoulu; School of Electrical EngineeringWe have investigated the Zeeman splitting in strained InxGa1-xAs quantum dots with different quantization energies by means of magnetoluminescence. A multifold splitting of Π and Δ states is observed due to lifted degeneracy of m>0 states. The experimental data were systematically compared to the diamagnetic and Zeeman shift of the single-particle states, taking into account the details of the valence-band structure and the spin splitting. Our data indicate that excitonic effects are negligible and that the magneto-optical properties of these strongly confined dots can be described rather accurately within the single-particle model.Item Magneto-optical properties of strain-induced InxGa1−xAs parabolic quantum dots(1998-04-15) Rinaldi, R.; Mangino, R.; Cingolani, R.; Lipsanen, H.; Sopanen, Markku; Tulkki, J.; Brasken, M.; Ahopelto, J.; Department of Micro and NanosciencesWe have investigated the Zeeman splitting in strained InxGa1−xAs quantum dots with different quantization energies by means of magnetoluminescence. A multifold splitting of Π and Δ states is observed due to lifted degeneracy of m>0 states. The experimental data were systematically compared to the diamagnetic and Zeeman shift of the single-particle states, taking into account the details of the valence-band structure and the spin splitting. Our data indicate that excitonic effects are negligible and that the magneto-optical properties of these strongly confined dots can be described rather accurately within the single-particle model.Item Spectral mapping of heat transfer mechanisms at liquid-solid interfaces(2016-05-24) Sääskilahti, K.; Oksanen, Jani; Tulkki, J.; Volz, S.; Department of Neuroscience and Biomedical EngineeringThermal transport through liquid-solid interfaces plays an important role in many chemical and biological processes, and better understanding of liquid-solid energy transfer is expected to enable improving the efficiency of thermally driven applications. We determine the spectral distribution of thermal current at liquid-solid interfaces from nonequilibrium molecular dynamics, delivering a detailed picture of the contributions of different vibrational modes to liquid-solid energy transfer. Our results show that surface modes located at the Brillouin zone edge and polarized along the liquid-solid surface normal play a crucial role in liquid-solid energy transfer. Strong liquid-solid adhesion allows also for the coupling of in-plane polarized modes in the solid with the liquid, enhancing the heat-transfer rate and enabling efficient energy transfer up to the cutoff frequency of the solid. Our results provide fundamental understanding of the energy-transfer mechanisms in liquid-solid systems and enable detailed investigations of energy transfer between, e.g., water and organic molecules.Item Temperature dependence of carrier relaxation in strain-induced quantum dots(1998-12-15) Brasken, M.; Lindberg, M.; Sopanen, M.; Lipsanen, H.; Tulkki, J.; Department of Micro and Nanosciences; Department of Neuroscience and Biomedical EngineeringWe 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.Item Temperature dependence of carrier relaxation in strain-induced quantum dots(American Physical Society (APS), 1998) Braskén, M.; Lindberg, M.; Sopanen, Markku; Lipsanen, Harri; Tulkki, J.; Department of Micro and Nanosciences; Mikro- ja nanotekniikan laitos; Sähkötekniikan korkeakoulu; School of Electrical EngineeringWe 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.Item Vibrational mean free paths and thermal conductivity of amorphous silicon from non-equilibrium molecular dynamics simulations(2016-12-01) Sääskilahti, K.; Oksanen, Jani; Tulkki, J.; McGaughey, A. J H; Volz, S.; Department of Neuroscience and Biomedical Engineering; Carnegie Mellon University; Centre National de la Recherche Scientifique (CNRS)The frequency-dependent mean free paths (MFPs) of vibrational heat carriers in amorphous silicon are predicted from the length dependence of the spectrally decomposed heat current (SDHC) obtained from non-equilibrium molecular dynamics simulations. The results suggest a (frequency)−2 scaling of the room-temperature MFPs below 5 THz. The MFPs exhibit a local maximum at a frequency of 8 THz and fall below 1 nm at frequencies greater than 10 THz, indicating localized vibrations. The MFPs extracted from sub-10 nm system-size simulations are used to predict the length-dependence of thermal conductivity up to system sizes of 100 nm and good agreement is found with independent molecular dynamics simulations. Weighting the SDHC by the frequency-dependent quantum occupation function provides a simple and convenient method to account for quantum statistics and provides reasonable agreement with the experimentally-measured trend and magnitude.Item Zeeman Effect in Parabolic Quantum Dots(American Physical Society (APS), 1996) Rinaldi, R.; Giugno, P. V.; Cingolani, R.; Lipsanen, Harri; Sopanen, Markku; Tulkki, J.; Ahopelto, J.; Department of Micro and Nanosciences; Mikro- ja nanotekniikan laitos; Sähkötekniikan korkeakoulu; School of Electrical EngineeringAn 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.Item Zeeman effect in parabolic quantum dots(1996-07-08) Rinaldi, R.; Guigno, P.; Cingolani, R.; Lipsanen, Harri; Sopanen, M.; Tulkki, J.; Ahopelto, J.; Department of Micro and NanosciencesAn 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.