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  • 1.
    Aggerstam, Thomas
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Pinos, Andrea
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Linnarsson, Margareta
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Lourdudoss, Sebastian
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Electron and hole capture cross-sections of Fe acceptors in GaN:Fe epitaxially grown on sapphire2007In: Journal of Electronic Materials, ISSN 0361-5235, E-ISSN 1543-186X, Vol. 36, no 12, p. 1621-1624Article in journal (Refereed)
    Abstract [en]

    Carrier trapping of Fe (3+)/Fe2+ deep acceptors in epitaxially grown GaN:Fe on sapphire was studied by time-resolved photoluminescence. For the investigated Fe doping levels on the order of 10(18) cm(-3), the luminescence decay times are strongly dependent on the Fe concentration, indicating that Fe centers act as predominant nonradiative recombination channels. Linear dependence of the decay time on the iron concentration allows estimation of the electron capture cross-section for the Fe3+ ions, which is equal to 1.9 x 10(-15) cm(2). The upper bound for the cross-section of the hole capture of Fe2+ was evaluated as 10 x 10(-15) cm

  • 2.
    Dong, Lin
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Pinos, Andrea
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Sugunan, Abhilash
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Li, Shanghua
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Toprak, Muhammet
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP. Helsinki University of Technology, Finland.
    Muhammed, Mamoun
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Measurement of radiative lifetime in CdSe/CdS core/shell structured quantum dots2009In: 2009 Asia Communications and Photonics Conference and Exhibition, ACP 2009, 2009, p. 5377385-Conference paper (Refereed)
    Abstract [en]

    Radiative lifetime of chemically synthesized colloidal CdSe/CdS core/shell quantum dots is measured. Influence of the core size on the electron-hole pair separation is analyzed. A long radiative lifetime and the existence of electron-hole pair separation suggest high potential of these dots as gain material to achieve lasing under continuous-wave excitation.

  • 3. Gautier, S.
    et al.
    Aggerstam, Thomas
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Pinos, Andrea
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Liu, K.
    Shur, M.
    O'Malley, S. M.
    Sirenko, A. A.
    Djebbour, Z.
    Migan-Dubois, A.
    Moudakir, T.
    OLIgazzaden, A.
    AlGaN/AlN multiple quantum wells grown by MOVPE on AlN templates using nitrogen as a carrier gas2008In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 310, no 23, p. 4927-4931Article in journal (Refereed)
    Abstract [en]

    AlxGa1-xN/AlN multiple quantum wells (MQWs) structures were grown by metalorganic vapour phase epitaxy (MOVPE) on pseudo AlN substrates using nitrogen as a carrier gas. Results of X-ray diffraction (XRD) and reciprocal space mapping (RSM) indicated no sign of strain relaxation in the quantum wells with respect to the AlN substrate. The MQW parameters such as thicknesses, growth rates and material compositions were extracted from XRD measurements and demonstrated an agreement with our growth conditions. No indication of parasitic reactions between ammonia and trimethyl-aluminium (TMAI) was detected in our growth process. Optical measurements revealed well-defined photoluminescence peaks at 288 and 280 nm, which are in a good agreement with the transmission experimental data. The piezoelectric field value in the Studied structures was estimated to be 900kV/cm.

  • 4.
    Liuolia, Vytautas
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Pinos, Andrea
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Gaska, R.
    Shur, M. S.
    Dynamics of carrier recombination and localization in AlGaN quantum wells studied by time-resolved transmission spectroscopy2009In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 95, no 9Article in journal (Refereed)
    Abstract [en]

    Time-resolved transmission and photoluminescence measurements were performed on Al0.35Ga0.65N/Al0.49Ga0.51N quantum well structures with different well widths. Comparison of transmission and luminescence data shows that dynamics of electrons and holes excited into extended quantum well states are governed by nonradiative recombination. For excitation into potential minima formed by band gap fluctuations, localization of electrons was observed. Excitation energy dependence of the pump-probe transient shape allows estimating localization potential, which is about 80 meV independently of the well width, and is probably caused by fluctuations of AlN molar fraction.

  • 5.
    Liuolia, Vytautas
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Pinos, Andrea
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Lin, Y. D.
    Ohta, H.
    DenBaars, S. P.
    Nakamura, S.
    Carrier localization in m-plane InGaN/GaN quantum wells probed by scanning near field optical spectroscopy2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 97, no 15, p. 151106-Article in journal (Refereed)
    Abstract [en]

    Scanning near field optical microscopy (SNOM) was applied to study the carrier localization in single InGaN/GaN quantum well structures grown on nonpolar m-plane GaN substrates. Dual localization potential consisting of hundreds of nanometers- to micrometer-size areas as well as smaller localization centers were identified from the SNOM scans and near field photoluminescence spectral widths. The localization areas were found to align along the [0001] direction, which was attributed to partial strain relaxation at the monolayer steps. (C) 2010 American Institute of Physics. [doi:10.1063/1.3502482]

  • 6.
    Marcinkevicius, Saulius
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Pinos, Andrea
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Liu, K.
    Veksler, D.
    Shur, M. S.
    Zhang, J.
    Gaska, R.
    Intrinsic electric fields in AlGaN quantum wells2007In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 90, no 8Article in journal (Refereed)
    Abstract [en]

    Intrinsic electric fields in AlxGa1-xN/AlyGa1-yN quantum wells embedded into p-i-n structures are studied using photoluminescence experiments. Spectral shifts induced by external bias and screening by photoexcited carriers allow evaluating the intrinsic fields caused by piezoelectric and spontaneous polarizations. In quantum wells with low Al content, the field is about 1 MV/cm, which is in agreement with theoretical estimations. For high Al molar fractions (35% well, 50% barrier), the extracted intrinsic field is lower and, most importantly, has the opposite sign to that predicted by the theory.

  • 7.
    Pinos, Andrea
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Photonics.
    Optical properties and degradation of deep ultraviolet AIGaN-based light-emitting diodes2011Doctoral thesis, comprehensive summary (Other academic)
  • 8.
    Pinos, Andrea
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Liuolia, Vytautas
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Yang, J
    Gaska, R
    Shur, M S
    Localization potentials in AlGaN epitaxial films studied by scanning near-field optical spectroscopy2011In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 109, no 11Article in journal (Refereed)
    Abstract [en]

    Scanning near-field photoluminescence spectroscopy has been applied to evaluate bandgap fluctuations in epitaxial AlGaN films with the AlN molar fraction varying from 0.30 to 0.50. A dual localization pattern has been observed. The potential of the small-scale (<100 nm) localization, evaluated from the width of the photoluminescence spectra, is between 0 and 51 meV and increases with increased Al content. These potential variations have been assigned to small-scale compositional fluctuations occurring due to stress variations, dislocations, and formation of Al-rich grains during growth. Larger area potential variations of 25-40 meV, most clearly observed in the lower Al-content samples, have been attributed to Ga-rich regions close to grain boundaries or atomic layer steps. The density, size, and bandgap energy of these domains were found to be composition dependent. The lower bandgap domains were found to be strongly correlated with the regions with efficient nonradiative recombination.

  • 9.
    Pinos, Andrea
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Scanning near-field optical spectroscopy of AlGaN-based light emitting diodes2010In: GALLIUM NITRIDE MATERIALS AND DEVICES V / [ed] Chyi JI; Nanishi Y; Morkoc H; Litton CW; Piprek J; Yoon E, 2010, Vol. 7602Conference paper (Refereed)
    Abstract [en]

    Electroluminescence of 285 and 340 nm AlGaN quantum well light emitting diodes (LEDs) has been studied by scanning near-field optical spectroscopy. In the 285 nm devices, the near-field scans revealed hexagonal cross hatch microcracks that can be related to strain relaxation. Besides, mu m size areas emitting with a higher intensity and at a longer wavelength, presumably, due to lower AlN molar fraction, have been observed. Near-field scans performed during subsequent days revealed that with time, intensity from these spots increases and emission wavelength shifts to the red, indicating further change in the quantum well alloy composition. This has allowed distinguishing a novel LED aging mechanism that involves locally increased current, heating and Al atom migration. For the 340 nm emitting device with lower Al content in the active region, no such features have been observed.

  • 10.
    Pinos, Andrea
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Liu, K.
    Shur, M. S.
    Kuokstis, E.
    Tamulaitis, G.
    Gaska, R.
    Yang, J.
    Sun, W.
    Screening dynamics of intrinsic electric field in AlGaN quantum wells2008In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 92, no 6, p. 061907-Article in journal (Refereed)
    Abstract [en]

    Shift of the transition energy after pulsed optical excitation in Al0.35Ga0.65N/Al0.49Ga0.51N quantum well (QW) structures with varying well width has been studied by time-resolved photoluminescence. The shift dynamics, which is due to descreening of the intrinsic electric field, has characteristic times similar to carrier lifetimes revealing negligible influence of trapped carriers on screening. Comparison of the experimental spectral shifts with the calculations has shown that the intrinsic field in our AlGaN QWs is about 0.4-0.5 MV/cm, which is about a factor of two smaller than the value calculated using the theoretical polarization constants.

  • 11.
    Pinos, Andrea
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics.
    Liu, K.
    Shur, M. S.
    Yang, J.
    Shatalov, M.
    Gaska, R.
    Carrier lifetimes in AlGaN quantum wells: electric field and excitonic effects2008In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 41, no 15Article in journal (Refereed)
    Abstract [en]

    Photoexcited carrier dynamics in a 280 nm AlGaN quantum well (QW) light emitting diode has been studied by time-resolved photoluminescence at forward and reverse bias. Long ( for AlGaN QWs with high Al content) room temperature carrier lifetimes of about 600 ps were measured with only a slight dependence on bias. These lifetimes are much longer than calculated free carrier tunnelling and thermionic emission times, pointing out the importance of excitonic effects for carrier dynamics in AlGaN QWs.

  • 12.
    Pinos, Andrea
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Liuolia, Vytautas
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Yang, Jinwei
    Gaska, Remis
    Shur, Michael S.
    Scanning near-field optical spectroscopy of AlGaN epitaxial layers2012In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 9, no 7, p. 1617-1620Article in journal (Refereed)
    Abstract [en]

    Band gap fluctuations and carrier localization in AlxGa1-xN films with x values varying from 0.30 to 0.50 has been studied by scanning near-field optical microscopy (SNOM) by measuring photoluminescence. The measurements have revealed a dual localization potential. Microscopic scale potential variations, detected by the SNOM, were most pronounced in the lower Al content samples. The nanoscopic carrier localization potentials, evaluated from the width of the photoluminescence spectra, were largest in layers with the largest AlN molar fraction. The large scale potential fluctuations were attributed to Ga rich regions close to grain boundaries or atomic layer steps. The density, size and band gap energy of these domains were found to be composition dependent. The nanoscopic potential variations have been assigned to small-scale compositional fluctuations, possibly, occurring due to formation of Al rich grains during growth.

  • 13.
    Pinos, Andrea
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Shur, M. S.
    High current-induced degradation of AlGaN ultraviolet light emitting diodes2011In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 109, no 10, p. 103108-Article in journal (Refereed)
    Abstract [en]

    Degradation under high current stress of AlGaN quantum well based light emitting diodes emitting at 285 and 310 nm has been studied using electroluminescence, time-resolved photoluminescence and current-voltage experimental techniques. The measurements have revealed that during aging decrease of the emission intensity is accompanied by increase of the tunneling current, increase of the nitrogen vacancy concentration and partial compensation of the p-doping. The main role in the device degradation has been ascribed to formation of tunneling conductivity channels, probably, via activation of the closed core screw dislocations with the help of nitrogen vacancies. Carrier lifetimes in the quantum wells and the p-cladding were found to be unaffected by the aging process, suggesting that the nonradiative recombination has a lesser influence on the device degradation.

  • 14.
    Pinos, Andrea
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Usman, Muhammad
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Hallén, Anders
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Time-resolved luminescence studies of proton-implanted GaN2009In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 95, no 11Article in journal (Refereed)
    Abstract [en]

    Time-resolved photoluminescence measurements performed on proton implanted and annealed GaN layers have shown that carrier lifetime can be tuned over two orders of magnitude and, at implantation dose of 1 x 10(15) cm(-2), decreases down to a few picoseconds. With annealing at temperatures between 250 and 750 degrees C, carrier lifetime, contrary to electrical characteristics, is only slightly restored, indicating that electrical compensation and carrier dynamics are governed by different defects. Ga vacancies, free and bound at threading dislocations, are suggested as the most probable defects, responsible for electrical compensation and carrier lifetime quenching.

  • 15.
    Pinos, Andrea
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Yang, J.
    Bilenko, Y.
    Shatalov, M.
    Gaska, R.
    Shur, M. S.
    Aging of AlGaN quantum well light emitting diode studied by scanning near-field optical spectroscopy2009In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 95, no 18Article in journal (Refereed)
    Abstract [en]

    Emission from a 285 nm AlGaN quantum well light emitting diode has been studied by scanning near-field optical spectroscopy. The scans revealed micrometer-size domainlike areas emitting with a higher intensity and at a longer wavelength; presumably, because of a lower AlN molar fraction in these regions. Experiments performed on different days have shown that with time, intensity from these spots increases and emission wavelength shifts to the red, indicating a further change in the quantum well alloy composition. This has allowed distinguishing an aging mechanism that involves locally increased current, heating, and atom migration.

  • 16.
    Pinos, Andrea
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Yang, J.
    Gaska, R.
    Shatalov, M.
    Shur, M. S.
    Optical studies of degradation of AlGaN quantum well based deep ultraviolet light emitting diodes2010In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 108, no 9, p. 093113-Article in journal (Refereed)
    Abstract [en]

    Aging under high current stress of AlGaN quantum well based light emitting diodes with high and low Al content in the wells emitting at 270 nm and 335 nm, respectively, has been studied by scanning near field optical spectroscopy and far field electroluminescence, photoluminescence and time-resolved photoluminescence. In the high Al content devices emission band related to optical transitions in the cladding involving nitrogen vacancies has been found. Evolution of this band during aging suggests that the role of N vacancies is crucial in the aging process by aiding defect generation and formation of high conductivity channels.

1 - 16 of 16
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