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  • 1.
    Elfving, Anders
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Karim, Amir
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Hansson, Göran V.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Ni, Wei-Xin
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Three-terminal Ge dot/SiGe quantum-well photodetectors for near-infrared light detection2006In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 89, p. 083510-083513Article in journal (Refereed)
    Abstract [en]

    A three-terminal metal-oxide-semiconductor field-effect transistor type of photodetector has been fabricated with a multiple stack of Ge dot/SiGe quantum-well heterostructures as the active region for light detection at 1.3–1.55  µm. Gate-dependent edge incidence photoconductivity measurements at room temperature revealed a strong dependence of the photoresponse on the gate voltage. At positive gate bias, the hole transport from the dots into the wells was improved, resulting in a faster response. The high photoresponsivity at negative VG, measured to be 350  mA  W–1 at 1.31  µm and 30  mA  W–1 at 1.55  µm, was ascribed to the photoconductive gain.

  • 2.
    Karim, Amir
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Si-based structures for light emission and detection2008Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Efforts to improve the optical performance of the indirect bandgap semiconductor silicon (Si) has been a major subject of research in the field of Si photonics due to the promising applications of Si based light emitters and detectors for optical communication. With that motivation three different Si based material systems were investigated; Si:Er/O layered structures, SiGe quantum dots and SiSn nano structures, all grown using the technique of molecular beam epitaxy (MBE). The main focus of this work has been on Si:Er/O layers, which lead to fabrication of Si-based light emitting diodes (LED) emitting at 1.54 mm wavelength. The work on SiGe structures lead to the fabrication of near-infrared light detectors, whereas the SiSn structures have not shown any strong optical character.

    Studies include epitaxial growth, structural characterization, device processing, electrical and optical characterizations. Material characterization of Si:Er/O structures using analytical electron microscopy (AEM) revealed interesting results with identification of two different type of microstructures in these layers depending on the Er and O concentrations. Several Si:Er/O LEDs were fabricated with different Er and O concentrations and the optical characteristics were investigated in order to find the best doping levels of Er and O for efficient light emission. The electroluminescence measurements revealed a strong 1.54 mm emission from these devices due to the intra 4f shell transition of Er3+ from the excited state (4I13/2) to the ground state (4I15/2). Si:Er/O waveguide LEDs have also been grown on SOI wafers using the optimized structure parameters obtained from mode confinement simulations as well as the microstructure investigations. The Si:Er/O waveguide LEDs are aimed at fabricating a planar Si cavity with Bragg mirrors on both sides to obtain light amplification and realise an electrically pumped Si laser. A focused ion beam (FIB) instrument was used to fabricate the Bragg mirrors but initial attempts did not result in light amplification in our Si:Er/O waveguide cavities.

    SiGe quantum dots are well-known quantum structures which are formed in a selfassembled fashion from Si/SiGe layer structures with a variety of shapes, sizes and compositions depending mainly on parameters like growth temperature and layer thicknesses. Optical properties of SiGe quantum structures have been studied while there has been little knowledge about their composition. A detailed compositional investigation of different SiGe dots on a nanometer scale was performed using AEM. The results showed a large degree of interdiffusion in large quantum dots, which was consistent with the optical properties of these dots. Using a multiple stack of Ge quantum dots and SiGe quantum wells, MOSFET type photodetectors working at 1.3 – 1.55 mm wavelength have also been fabricated and characterized.

    Research on the SiSn system was mainly motivated by the possibility to obtain a direct bandgap transition in Si based material as it was predicted theoretically and experimentally observed in the related GeSn material system by other researchers. Structural and optical characterizations of SiSn nano structures were performed. Although the same SiSn nano structures exhibit a weak signature of optical absorption, low temperature photoluminescence measurements did not reveal any emission peaks related to the SiSn dots.

  • 3.
    Karim, Amir
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Du, Chun-Xia
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Hansson , Göran
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Influence of exposure to 980 nm laser radiation on the luminescence of Si: Er/O light-emitting diodes2008In: JOURNAL OF APPLIED PHYSICS, ISSN 0021-8979 , Vol. 104, no 12, p. 123110-Article in journal (Refereed)
    Abstract [en]

    Erbium (Er) codoping with oxygen (O) in Si is a well-known method for producing electroluminescent material radiating at 1.54 mu m through a 4f shell transition of Er3+ ions. In this work the influence of exposure to 980 nm radiation on the electroluminescence (EL) of reverse biased Si:Er/O light-emitting diodes (LEDs), which give a strong room temperature 1.54 mu m intensity, is presented and discussed. All the device layers, including Er/O doped Si sandwiched between two Si0.82Ge0.18 layers, have been grown on silicon on insulator substrates using molecular beam epitaxy and processed to fabricate edge emitting Si:Er/O waveguide LEDs. Electromagnetic mode confinement simulations have been performed to optimize the layer parameters for waveguiding. The temperature dependence of the 1.54 mu m EL intensity exhibits an abnormal temperature quenching with a peak near -30 degrees C, and at -160 degrees C it has decreased by a factor of 5. However, irradiating the devices with a 980 nm laser gives an enhancement of the 1.54 mu m EL intensity, which is more dramatic at low temperatures (e.g., -200 degrees C) where the quenched EL signal is increased up to almost the same level as at room temperature. The enhancement of the EL intensity is attributed to the photocurrent generated by the 980 nm laser, reducing the detrimental avalanche current.

  • 4.
    Karim, Amir
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Elfving, Anders
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Larsson, Mats
    Linköping University, Department of Physics, Chemistry and Biology, Materials Science . Linköping University, The Institute of Technology.
    Ni, Wei-Xin
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Hansson, G. V.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Compositional analysis of Si/SiGe quantum dots using STEM and EDX2006In: Volume 6129 - Quantum Dots, Particles, and Nanoclusters III, Proceedings of SPIE, 2006Conference paper (Refereed)
    Abstract [en]

    Ge islands fabricated on Si(100) by molecular beam epitaxy at different growth temperatures, were studied using crosssectional scanning transmission electron microscopy and energy-dispersive X-ray spectrometry combined with electron energy loss spectrometry experiments. The island size, shape, strain, and material composition define the dot-related optical transition energies, but they are all strongly dependent on the growth temperature. We have performed quantitative investigations of the material composition of Ge/Si(001) quantum dots. The samples were grown at temperatures ranging from 430 to 730 °C, with one buried and one uncapped layer of Ge islands separated by 140 nm intrinsic Si. The measurements showed a Ge concentration very close to 100 % in the islands of samples grown at 430 °C. With a growth temperature of 530 °C, a ~20 % reduction of the Ge fraction was observed, which is due to intermixing of Si and Ge. This is consistent with our previous photoluminescence results, which revealed a significant blue shift of the Ge dot-related emission peak in this growth temperature range. The Ge concentration decreases more slowly when the growth temperature is increased above 600 °C, which can be explained by geometrical arguments. The longer distance between the interface and the core of these larger sized dome-shaped islands implies that less Si atoms reach the dot center. In general, the uncapped Ge dots have similar widths as the embedded islands, but the height is almost exclusively larger. Furthermore, the Ge concentration is slightly lower for the overgrown dots.

  • 5.
    Karim, Amir
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Hansson, G. V.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Ni, Wei-Xin
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Holtz, Per-Olof
    Linköping University, Department of Physics, Chemistry and Biology, Materials Science . Linköping University, The Institute of Technology.
    Larsson, Mats
    Linköping University, Department of Physics, Chemistry and Biology, Materials Science . Linköping University, The Institute of Technology.
    Atwater, H.A.
    Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA, USA.
    Photoluminescence Studies of Sn Quantum Dots in Si Grown by MBE2005In: Optical materials (Amsterdam), ISSN 0925-3467, E-ISSN 1873-1252, Vol. 27, no 5, p. 836-840Article in journal (Refereed)
    Abstract [en]

    A few nanometer thick SnxSi1−x layers with x 0.1 grown on silicon (1 0 0) surfaces can be used to form tin (α-Sn) quantum dots as a result of heat treatment of such structures up to 800 °C. These quantum dots with a well-defined shape are expected to be a candidate for obtaining a low-energy direct band gap structure in Si. Absorption measurements reported by Ragan et al. have shown the onset of absorption at 0.27 eV indicating that the MBE-grown α-Sn quantum dots could be used, e.g. in infrared detectors or emitters. We have performed low temperature photoluminescence (PL) studies of some of the structures produced in this first study and observed no emission peak near 0.27 eV. The PL spectra are instead characterised by a broadband emission in the range 0.7–1 eV. Furthermore there are narrow features that have previously been described as the 789 meV C–O band and 1018 meV W or I1 band. The broad emission at 0.7–1 eV is attributed to the presence of defects introduced by the grown layers, which have suppressed the emission peaks related to the substrate as well. We have also grown α-Sn quantum dot samples on Si (1 0 0) substrates with very low doping concentrations. These samples show PL spectra with Si-substrate related peaks and a relatively lower broad feature at 0.7–1 eV. However, no emission was observed near 0.27 eV.

  • 6.
    Karim, Amir
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Hansson, Göran V.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Linnarsson, M. K.
    Lab of Materials and Semiconductor Physics, Royal Institute of Technology, Stockholm, Sweden.
    Influence of Er and O concentrations on the microstructure and luminescence of Si:Er/O LEDs2008In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 100, no 042010Article in journal (Refereed)
    Abstract [en]

    Erbium(Er)/Oxygen(O) doped Silicon (Si) layers grown by molecular beam epitaxy (MBE), can be used for fabricating Si-based light emitting diodes. The electroluminescence intensity from these layers depends sensitively on the formation of specific types of Er/O precipitates inside the Si host. We have performed a detailed microstructure analysis of MBE-grown Er/O doped Si layers using electron microscopy and combined it with secondary ion mass spectrometry (SIMS) measurements as well as electroluminescence studies. Two types of microstructures are observed in different samples with specific Er and O concentrations and grown using Er and Si co-evaporation in O ambient. The first type of microstructure consists of planar precipitates along (311) planes mostly initiated at the onset of the growth of the Si:Er/O layer. The second characteristic type of microstructure observed contain round precipitates of Er/O. Using analytical microscopy techniques it was revealed that the round precipitates contain a higher ratio of Er to O as compared to the planar precipitates of the first type. The planar precipitates normally result in structures with high electroluminescence intensity while the structures with round precipitates have low intensity.

  • 7.
    Karim, Amir
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Ni, Wei-Xin
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Elfving, Anders
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Persson, Per O.Å
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Hansson, Göran
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Characterization of Er/O-doped Si-LEDs with low thermal quenching2005In: Material Research Society Symposium Proceedings, 2005, p. 117-124Conference paper (Refereed)
    Abstract [en]

    Electroluminescence studies of MBE-grown Er/O-doped Si-diodes at reverse bias have been done. For some devices there is much reduced thermal quenching of the emission at 1.54 µm. There are examples where the temperature dependence is abnormal in that the intensity for a constant current even increases with temperature up to e.g. 80 oC. These devices have been studied with cross-sectional transmission electron microscopy to see the microstructure of the Er/O-doped layers as well as the B-doped SiGe-layers that are used as electron emitters during reverse bias. Although there are defects in the layers there is no evidence for large thick precipitates of SiO2. While reduced thermal quenching often is attributed to having the Er-ions within SiO2 layers, this is not the case for our structures as evidenced by our TEM-studies. The origin of the abnormal temperature dependence is attributed to the two mechanisms of breakdown in the reverse-biased diodes. At low temperature the breakdown current is mainly due to avalanche resulting in low-energy electrons and holes that quenches the intensity by Auger de-excitation of the Er-ions. At higher temperature the breakdown current is mainly phonon-assisted tunnelling which results in a more efficient pumping with less de-excitation of the Er-ions. Finally at the highest temperatures the thermal quenching sets in corresponding to an activation energy of 125 meV, which is slightly lower than 150 meV that has been reported in other studies.

  • 8.
    Zhao, Ming
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics .
    Karim, Amir
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Hansson, Göran
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics .
    Ni, Wei-Xin
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics .
    Townsend, P
    University of Cambridge.
    Lynch, S A
    University of Cambridge.
    Paul , D J
    University of Cambridge.
    Molecular beam epitaxy growth of Si/SiGe bound-to-continuum quantum cascade structures for THz emission2008In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 517, no 1, p. 34-37Article in journal (Refereed)
    Abstract [en]

    A Si/SiGe bound-to-continuum quantum cascade design for THz emission was grown using solid-source molecular beam epitaxy on Si0.8Ge0.2 virtual substrates. The growth parameters were carefully studied and several samples with different boron doping concentrations were grown at optimized conditions. Extensive material characterizations revealed a high crystalline quality of the grown structures with an excellent growth control. Layer undulations resulting from a nonuniform strain field, introduced by high doping concentration, were observed. The device characterizations suggested that a modification on the design was needed in order to enhance the THz emission.

  • 9.
    Zhao, Ming
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Karim, Amir
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Ni, Wei-Xin
    Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
    Pidgeon, C. R.
    Department of Physics, Heriot-Watt University, Riccarton, Edinburgh, UK.
    Phillips, P. J.
    Department of Physics, Heriot-Watt University, Riccarton, Edinburgh, UK.
    Carder, D.
    Department of Physics, Heriot-Watt University, Riccarton, Edinburgh, UK.
    Murdin, B. N.
    Department of Physics, University of Surrey, UK.
    Fromherz, T.
    Institut für Halbleiter- und Festkörperphysik, Johannes Kepler Universität, Linz, Austria.
    Paul, D. J.
    Cavendish Laboratory, University of Cambridge, UK.
    Strain-symmetrized Si/SiGe multi-quantum well structures grown by molecular beam epitaxy for intersubband engineering2006In: Journal of luminescence, ISSN 0022-2313, Vol. 121, no 2, p. 403-408Article in journal (Refereed)
    Abstract [en]

    Three strain-symmetrized Si/SiGe multi-quantum well structures, designed for probing the carrier lifetime of intrawell intersubband transitions between heavy hole 1 (HH1) and light hole 1 (LH1) states with transition energies below the optical phonon energy, were grown by molecular beam epitaxy at low temperature on fully relaxed SiGe virtual substrates. The grown structures were characterized by using various experimental techniques, showing a high crystalline quality and very precise growth control. The lifetime of the LH1 excited state was determined directly with pump-probe spectroscopy. The measurements indicated an increase of the lifetime by a factor of 2 due to the increasingly unconfined LH1 state, which agreed very well with the design. It also showed a very long lifetime of several hundred picoseconds for the holes excited out of the well to transit back to the well through a diagonal process.

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