J. Linnros

KTH Royal Institute of Technology, Stockholm, Stockholm, Sweden

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Publications (97)160.52 Total impact

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    ABSTRACT: The results of low-temperature photoluminescence characterization of single silicon nanocrystals prepared from highly doped silicon-on-insulator wafers are presented. The effect of B, P, As, and Sb impurities on ensemble as well as individual emission spectra are determined by comparison with the line shapes of undoped nanocrystals. From the statistical analysis of the luminescence spectra, the donor ionization energies for nanocrystals emitting in the range of 1.5–2.0 eV are estimated to be 140–200 meV, while the exciton-impurity binding energy for As- and Sb-doped nanocrystals is found to be about 40–45 meV.
    Physical review. B, Condensed matter 08/2012; 86(7). · 3.77 Impact Factor
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    ABSTRACT: Optical transitions in silicon nanocrystals with different surface passivations were probed at low temperatures on a single-particle level. A type of quasidirect recombination process, different from the quantum-confined exciton transition, is identified. The luminescence spectra have different emission energies, but the contribution of a no-phonon transition is significantly higher than expected from the quantum-confinement model. Its relative strength was found to be temperature dependent, suggesting spatial localization of excitons as a possible origin.
    Physical review. B, Condensed matter 01/2011; 84(12). · 3.77 Impact Factor
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    ABSTRACT: Residual stress and carrier lifetime variation have been measured in free-standing n-type 3C-SiC wafer grown on undulated Si substrate. We identify extended regions of residual stress that lie parallel to epilayer surfaces. The opposite polarity of stress is identified toward the interface and toward the top surface. Integrated carrier lifetime has been determined by random defect density distribution which is enhanced in the areas of double-positioning boundary defects. It is shown that carrier lifetimes are severely reduced by the presence of residual stress towards epilayer surfaces. In this way lifetime depth-distribution can be mistakenly attributed to enhanced surface recombination.
    11/2010;
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    ABSTRACT: A 3D X-ray detector for imaging of 30–200 keV photons is described. It comprises a stack of semitransparent structured scintillators, where each scintillator is a regular array of waveguides in silicon, and with pores filled with CsI. The performance of the detector is described theoretically and explored in detail through simulations. The resolution of a single screen is shown to be determined only by the pitch, at least up to 100 keV. In comparison to conventional homogenous screens an improvement in efficiency by a factor 5–15 is obtainable. The cross-talk between screens in the 3D detector is shown to be negligible. The concept of such a 3D detector enables ray tracing and super resolution algorithms to be applied. Realized pore geometries have a lower aspect ratio than used in simulations and the roughness of the pore walls gives a 13% decrease in waveguide efficiency. Compared to currently used regular scintillators with similar resolution an efficiency increase by a factor 4 has been found for the structured scintillator.
    Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 01/2009;
  • N Elfström, J Linnros
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    ABSTRACT: A 2-dimensional simulation tool was designed to investigate the threshold voltage behaviour for a silicon nanowire constructed in a top down approach on silicon on insulator (SOI) material. The simulation shows, assuming a positive charge of +11011 cm-2 between the silicon/silicon dioxide interface and negatively charged surface states on top of the nanowire that the threshold voltage increases with decreasing height of the nanowire.
    Journal of Physics Conference Series 03/2008; 100(5):052042.
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    ABSTRACT: Periodic deflections of infrared probe beam in TlGaSe2 crystal after its lateral excitation by laser pulse with photon energy near the band gap are observed. Such deflections arise from travelling of the acoustic pulse within sample which, in turn, is produced by optical pump pulse through photoacoustic effect. The photoacoustic pulse is generated within thin region near the crystal excited face, much shorter then the light penetration depth. In the case of volume excitation photoacoustic pulse is also generated in the region near back face as well. Tentative explanation of the generation mechanism is discussed.
    Journal of Physics Conference Series 03/2008; 100(4):042007.
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    ABSTRACT: Two-photon absorption (TPA) in GaSe is detected in the narrow (about kT) spectral range above the indirect exciton energy for E⊥c light polarization. The appearance of TPA enhancement within narrow spectral region links to the resonant Raman scattering and anti-Stockes photoluminescence previously identified in GaSe. Extracted TPA coefficient is of the order of 10 cm/W and 5–10 times exceeds ones obtained previously for lower quanta energies.
    Journal of Physics Conference Series 03/2008; 100(4):042008.
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    ABSTRACT: Fundamental band edge absorption is investigated in nominally undoped (n<1014 cm−3) and heavily doped (n ∼ 8×1018 cm−3) 4H‐SiC by a spectroscopy technique based on spatially and time-resolved free-carrier absorption. The spectra are extracted over a wide absorption range (0.02–500 cm−1) at temperatures from 75 to 450 K. The experimental results are supported by an indirect transition theory with a unique set of dominating momentum-conserving phonons, showing good correlation with earlier findings of differential absorption measurements at 2 K. Exciton binding energy of 30±10 meV is derived from fitting the data at 75 K. The detected polarization anisotropy of absorption with respect to c axis is shown to be consistent with the selection rules for the corresponding phonon branches. An analytical model related to constant degree of involved phonons describes well the obtained energy gap variation with temperature. Finally, doping induced band gap narrowing is characterized above the impurity-Mott transition and compared with theoretical calculations in the random phase approximation. The shape of the fundamental absorption edge at high carrier concentrations is discussed in terms of excitonic enhancement above the Mott transition, as recently detected in Si.
    Journal of Applied Physics 06/2007; 101(12):123521-123521-10. · 2.21 Impact Factor
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    ABSTRACT: A newly developed fabrication method makes the formation of deep structured scintillator screens possible. We demonstrate that electrochemical etching in silicon can be used to produce regular arrays of 120 μm deep pores with a 4 μm pitch. A layer of SiO2 is grown on the pore walls and CsI:Tl is melted into the pores, resulting in a structure with a high refractive index core surrounded by a quartz cladding, providing efficient light guiding. The efficiency and radiation hardness of the scintillator is evaluated in realistic environment at beamline ID15 at the ESRF synchrotron. The efficiency is measured to be a factor two higher than a planar YAG: Ce scintillator of equal thickness, while radiation damage is found to be neglectable for doses up to at least 2×104 Gy.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2007; 576(1):52-55. · 1.14 Impact Factor
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    ABSTRACT: Cited By (since 1996): 23, Export Date: 8 February 2013, Source: Scopus, Art. No.: 233203
    Physical Review B 12/2006; 74(23):233203. · 3.66 Impact Factor
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    ABSTRACT: An experimental technique using optical excitation by a YAG laser pulse for studying avalanche injection in power devices is demonstrated. This technique enables the creation of high uniform excess carrier concentrations in an optically defined device volume, involving very little heating. A method for determining the onset of avalanche multiplication, by studying the time integral of the reverse recovery current, is proposed. A PiN diode is observed to turn off from avalanching at a dissipated power density of more than 200 kW/cm2.
    Physica Scripta 12/2006; 1997(T69):134. · 1.03 Impact Factor
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    ABSTRACT: For determination of the effective surface recombination velocity (SRV) at the Si–SiO2 interface, a new method based on depth- and time-resolved free-carrier absorption measurements is put forward. The proposed technique is more advantageous than conventionally used methods and makes it possible to obtain the SRV and its injection-level dependence on a particular sample surface without additional knowledge of other silicon parameters. The experimental measurements have been performed on n–Si with polished facets and a cleaved edge. All surfaces were covered by thermally grown SiO2. The result is that the SRV increases with the injection level followed by saturation. The computer simulation of the excess carrier decay has shown that, for polished surfaces, the value of the SRV should be reduced in the high injection region and not only does depend on the injected level, but also on the initial excitation. It is proposed that temporal charging of the Si–SiO2 interface can be responsible for such behavior.
    Physica Scripta 11/2006; 1999(T79):322. · 1.03 Impact Factor
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    ABSTRACT: Scintillating films are usually used to improve the sensitivity of CCD-based X-ray imaging detectors. For an optimal spatial resolution and detection efficiency, a tradeoff has to be made on the film thickness. However, these scintillating layers can also be structured to provide a pixellated screen. In this paper, the study of CsI(Tl)-filled pore arrays is reported. The pores are first etched in silicon, then oxidized and finally filled with CsI(Tl) to form scintillating waveguides. The dependence of the detector sensitivity on pore depth, varied from 40 to 400 μm here, follows rather well theoretical predictions. Most of the detectors produced in this work have a detective quantum efficiency of the incoming X-ray photons of about 25%. However, one detector shows that higher efficiency can be achieved approaching almost the theoretical limit set by Poisson statistics of the incoming X-rays. Thus, we conclude that it is possible to fabricate scintillating waveguides with almost ideal performance. Imaging capabilities of the detectors are demonstrated.
    IEEE Transactions on Nuclear Science 03/2006; · 1.22 Impact Factor
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    A Galeckas, J Linnros, P Pirouz
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    ABSTRACT: We report on optically induced nucleation and expansion of stacking faults in hexagonal SiC structures. The activation energy for partial dislocation glide under optical excitation is found to reduce to 0.25 +/- 0.05 eV, which is about 2 eV lower than for pure thermal activation. From the measurements of thermal activation and below-gap excitation spectroscopy of dislocation glide, we conclude that the elementary process controlling expansion of stacking faults is kink pair nucleation aided by the phonon-kick mechanism. We propose that solitons on 30 degrees Si(g) partials with a silicon core act as deep 2.4 eV + Ev trap sites, readily providing electron-hole recombination energy to enhance the motion of dislocations. Our results suggest that this is a general mechanism of structural degradation in hexagonal SiC.
    Physical Review Letters 02/2006; 96(2):025502. · 7.73 Impact Factor
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    Advanced Materials 01/2006; 18(3):270 - 274. · 14.83 Impact Factor
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    ABSTRACT: Analysis of low-temperature photoluminescence measurements performed on single silicon nanocrystals is presented. The luminescence emission linewidth of Si nanocrystals is found to be less than thermal broadening at low temperature, confirming the atomic-like nature of their energetic states. Beside the main peak the low-temperature spectra reveal a ∼6 meV replica, the origin of which is discussed. For some of the investigated dots, we also observe a ∼60 meV transverse optical (TO) phonon replica. The regular arrangement of individual nanocrystals used in this work enables combined high-resolution transmission electron microscopy (TEM) and low-temperature photoluminescence characterization of the same single quantum dot.
    Applied Surface Science. 01/2006;
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    ABSTRACT: Cited By (since 1996): 2, Export Date: 8 February 2013, Source: Scopus
    01/2006: pages 395-398;
  • Journal of Luminescence 01/2006; 121(2). · 2.14 Impact Factor
  • I. Sychugov, J. Lu, N. Elfstrom, J. Linnros
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    ABSTRACT: Individual silicon quantum dots were fabricated by electron-beam lithography, plasma etching and a two-step oxidation process. This enables photoluminescence (PL) from individual dots at various temperatures to be detected and spectrally resolved using a sensitive charge-coupled device camera-imaging system, as reported previously. The regular array-like arrangement of oxidized pillars containing individual nanocrystals, in principle, enables combined transmission electron microscopy (TEM) and low-temperature PL characterization of the same Si quantum dot. To this end, a technique employing focused ion beam was developed for preparation of the pillar/nanocrystal of interest for TEM. It is shown that silicon quantum dots of several nanometers in size can be characterized using such a method.
    Journal of Luminescence 01/2006; 121(2):353-355. · 2.14 Impact Factor
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    ABSTRACT: This paper demonstrates the fabrication of a membrane permeated by a silicon dioxide nanocapillary array for manipulating highly charged ions at the nanoscale. The fabrication method involves (i) the formation of pores at the nanoscale on lithographically patterned, n-type silicon using photo-assisted electrochemical etching, followed by (ii) thermal oxidation, (iii) bulk silicon back etching and (iv) oxide etching using silicon micromachining technology. The electrochemical etching parameters were chosen to form arrays of straight pores with a diameter of about 250 nm, a length of 30 µm and interpore distances of 1 and 1.4 µm. The back side of the pore arrays was etched in potassium hydroxide and tetramethyl ammonium hydroxide. Finally, the inside of the pores was thermally oxidized to yield SiO2 capillary arrays. The present method could allow the fabrication of capillaries with further smaller dimensions by increasing the thermally grown oxide thickness.
    Nanotechnology 07/2005; 16(9):1697. · 3.84 Impact Factor

Publication Stats

713 Citations
160.52 Total Impact Points

Institutions

  • 1995–2012
    • KTH Royal Institute of Technology
      • • Department of Material- and Nano Physics
      • • Department of Microelectronics and Applied Physics (MAP)
      • • Department of Microelectronics and Information Technology
      Stockholm, Stockholm, Sweden
  • 2006
    • Case Western Reserve University
      • Department of Materials Science and Engineering
      Cleveland, OH, United States
  • 2005
    • Claude Bernard University Lyon 1
      Villeurbanne, Rhône-Alpes, France
  • 1999
    • Stockholm Environment Institute
      Tukholma, Stockholm, Sweden
    • Vilnius University
      Vil'nyus, Vilniaus Apskritis, Lithuania