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  • 1.
    Gogova, Daniela
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Gesheva, K
    Bulgarian Acad Sci, Cent Lab Solar Energy & New Energy Sources, Sofia 1784, Bulgaria Linkoping Univ, Dept Phys & Measurement Technol, S-58183 Linkoping, Sweden Univ Sofia, Dept Phys, BU-1126 Sofia, Bulgaria.
    Kakanakova-Georgieva, Anelia
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Surtchev, M
    Bulgarian Acad Sci, Cent Lab Solar Energy & New Energy Sources, Sofia 1784, Bulgaria Linkoping Univ, Dept Phys & Measurement Technol, S-58183 Linkoping, Sweden Univ Sofia, Dept Phys, BU-1126 Sofia, Bulgaria.
    Investigation of the structure of tungsten oxide films obtained by chemical vapor deposition2000In: European Physical Journal: Applied physics, ISSN 1286-0042, E-ISSN 1286-0050, Vol. 11, no 3, p. 167-174Article in journal (Refereed)
    Abstract [en]

    Thin amorphous and polycrystalline tungsten oxide films have been prepared by Chemical Vapor Deposition (CVD) from metallorganic precursor - tungsten hexacarbonyl - at atmospheric pressure. The dependence of the composition and the structure of tungsten oxide films on the technological conditions has been investigated by XPS, XRD, DTA-TGA and Raman spectroscopy. As a result it has been established that: at high values of the flow-rates of the reaction gases amorphous films of very low density have been obtained, in the XPS spectra of the understoichiometric WO3-y (0 < y < 0.3) films besides W6+, also W5+ and W4+ states have been observed. First to observe in the Raman spectra of amorphous CVD-WO3 films is the band at similar to 950 cm(-1), characteristic for terminal W6+=O bonds in result of the presence of structural water. The existence of structural water in the amorphous material has been established by thermal analyze, also.

  • 2. Gogova, Daniela
    et al.
    Kanev, S.
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Talik, E.
    Growth and characterization of free-standing HVPE GaN on two-step epitaxial lateral overgrown GaN template2005Conference paper (Refereed)
  • 3.
    Gogova, Daniela
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Kasic, A.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Larsson, Henrik
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Hemmingsson, Carl
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Tuomisto, F.
    Laboratory of Physics, Helsinki University of Technology, Finland .
    Saarinen, K.
    Laboratory of Physics, Helsinki University of Technology, Finland .
    Dobos, L.
    Research Institute for Technical Physics and Materials Science of the Hungarian Academy of Science, Hungary .
    Pécz, B.
    Research Institute for Technical Physics and Materials Science of the Hungarian Academy of Science, Hungary .
    Gibart, P.
    LUMILOG, 2720, Chemin de Saint Bernard, France .
    Beaumont, B.
    LUMILOG, 2720, Chemin de Saint Bernard, France .
    Strain-free bulk-like GaN grown by hydride-vapor-phase-epitaxy on two-step epitaxial lateral overgrown GaN template2004In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 96, no 1, p. 799-806Article in journal (Refereed)
    Abstract [en]

    Crack-free bulk-like GaN with high crystalline quality has been obtained by hydride-vapor-phase-epitaxy (HVPE) growth on a two-step epitaxial lateral overgrown GaN template on sapphire. During the cooling down stage, the as-grown 270-μm-thick GaN layer was self-separated from the sapphire substrate. Plan-view transmission electron microscopy images show the dislocation density of the free-standing HVPE-GaN to be ∼2.5×107 cm−2 on the Ga-polar face. A low Ga vacancy related defect concentration of about 8×1015 cm−3 is extracted from positron annihilation spectroscopy data. The residual stress and the crystalline quality of the material are studied by two complementary techniques. Low-temperature photoluminescence spectra show the main neutral donor bound exciton line to be composed of a doublet structure at 3.4715 (3.4712) eV and 3.4721 (3.4718) eV for the Ga- (N-) polar face with the higher-energy component dominating. These line positions suggest virtually strain-free material on both surfaces with high crystalline quality as indicated by the small full width at half maximum values of the donor bound exciton lines. The E1(TO) phonon mode position measured at 558.52 cm−1 (Ga face) by infrared spectroscopic ellipsometry confirms the small residual stress in the material, which is hence well suited to act as a lattice-constant and thermal-expansion-coefficient matched substrate for further homoepitaxy, as needed for high-quality III-nitride device applications. © 2004 American Institute of Physics.

  • 4.
    Gogova, Daniela
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Kasic, A
    Larsson, Henrik
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Pecz, B
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Magnusson, Björn
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Tuomisto, F
    Saarinen, K
    Miskys, C
    Stutzmann, M
    Bundesmann, C
    Schubert, M
    Optical and structural characteristics of virtually unstrained bulk-like GaN2004In: Japanese Journal of Applied Physics, ISSN 0021-4922, E-ISSN 1347-4065, Vol. 43, no 4A, p. 1264-1268Article in journal (Refereed)
    Abstract [en]

    Bulk-like GaN with high structural and optical quality has been attained by hydride vapor-phase exitapy (HVPE). The as-grown 330 mum-thick GaN layer was separated from the sapphire substrate by a laser-induced lift-off process. The full width at half maximum values of the X-ray diffraction (XRD) omega-scans of the free-standing material are 96 and 129 arcsec for the (1 0 -1 4) and (0 0 0 2) reflection, respectively, which rank among the smallest values published so far for free-standing HVPE-GaN. The dislocation density determined by plan-view TEM images is 1-2 x 10(7) cm(-2). Positron annihilation spectroscopy studies show that the concentration of Ga vacancy related defects is about 1.5 x 10(16) cm(-3). The high-resolution XRD, photoluminescence, mu-Raman, and infrared spectroscopic ellipsometry measurements consistently prove that the free-standing material is of high crystalline quality and virtually strain-free. Therefore it is suitable to serve as a substrate for stress-free growth of high-quality III-nitrides based device heterostructures.

  • 5.
    Gogova, Daniela
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Kasic, A.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Larsson, Henrik
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Pécz, B.
    Research Institute for Technical Physics and Materials Science of the Hungarian Academy of Sci., Budapest, Hungary .
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Characterization of high-quality free-standing GaN grown by HVPE2004In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T114, p. 18-21Article in journal (Refereed)
    Abstract [en]

    Single-crystalline 330µm thick GaN has been grown on 2'' Al2O3 (0 0 0 1) by hydride vapour phase epitaxy (HVPE). Upon laser-induced lift-off the GaN was delaminated from the sapphire substrate, and bulk-like free-standing GaN was achieved. Various characterization methods were utilized to assess the structural and optical quality of the freestanding material. The X-ray rocking curves of the (1 0–1 4) and (0 0 0 2) diffraction peaks revealed full width at half maximum (FWHM) values of 96 and 129arcsec, respectively. These data compare well with the smallest corresponding values published so far for bulk-like HVPE-GaN. The dislocation density determined by plan-view transmission electron microscopy studies is 1–2 × 107cm–2. The low-temperature near-band-gap photoluminescence spectrum shows the main donor bound exciton (DBE) peak at 3.4718eV with a FWHM of 1.4meV, verifying the high crystalline quality of the bulk-like GaN. The DBE peak position suggests complete stress relief. The phonon spectra measured by infrared spectroscopic ellipsometry confirm as well, that the free-standing material is of high crystalline quality and virtually stress-free.

  • 6.
    Gogova, Daniela
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Larsson, Henrik
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Kasic, A.
    Yazdi, Gholam Reza
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Aujol, E.
    Frayssinet, E.
    Faurie, J-P.
    Beaumont, B.
    Gibart, P.
    High-Quality 2'' Bulk-Like Free-Standing GaN Grown by HydrideVapour Phase Epitaxy on a Si-doped Metal Organic Vapour Phase Epitaxial GaN Template with an Ultra Low Dislocation Density2005In: Japanese Journal of Applied Physics, ISSN 0021-4922, E-ISSN 1347-4065, Vol. 44, p. 1181-1185Article in journal (Refereed)
  • 7.
    Gogova, Daniela
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Larsson, Henrik
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Zolnai, Z.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Fast growth of high quality GaN2003In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 200, no 1, p. 13-17Article in journal (Refereed)
    Abstract [en]

    We have grown bulk-like GaN with a thickness up to 335 μm on 2″ sapphire substrates in a vertical HVPE reactor with a bottom-fed design. A very high growth rate of 250 μm/h is reached with high crystalline quality of the grown material. The low temperature PL spectra show the free A-exciton line at 3.483 eV and rather narrow I2 lines with FWHM of 1–2 meV indicating high crystalline quality and low doping concentration. This HVPE-GaN has the potential to provide lattice-matched and thermally-matched substrates for further epitaxial growth of high quality GaN with a low dislocation density for advanced heterostructure devices.

  • 8.
    Gogova, Daniela
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Talik, E.
    Institute of Physics, University of Silesia, Universytecka 4, 40007 Katowice, Poland.
    Ivanov, Ivan Gueorguiev
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Large-area free-standing GaN substrate grown by hydride vapor phase epitaxy on epitaxial lateral overgrown GaN template2006In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 371, no 1, p. 133-139Article in journal (Refereed)
    Abstract [en]

    In this paper, the potential of the high growth rate hydride vapor phase epitaxy technique and laser lift-off for the fabrication of large-area (2?) free-standing GaN substrates is revealed. Structural and optical properties of 250-µm-thick GaN layer grown on a MOVPE epitaxial lateral overgrown GaN template have been investigated employing different analytical experimental techniques. A low value of dislocation density of ~1×107 cm-2 on the Ga-terminated face of the free-standing material was determined from AFM images. X-ray diffraction (XRD), Raman scattering measurements, and low-temperature photoluminescence (PL) were exploited to assess the structural and optical quality of the GaN. The full-width at half-maximum value of XRD ?-scans of the free-standing GaN material was determined to be 264 arcsec for the (101¯4) reflection. The XRD and low-temperature PL mapping measurements consistently proved the good crystalline quality and lateral homogeneity and small residual stress inside the material. Hence, the free-standing GaN achieved is highly advantageous for a lattice-constant and thermal-expansion-coefficient matched substrate for additional strain-free homoepitaxy of III-nitrides-based device heterostructures. © 2005 Elsevier B.V. All rights reserved.

  • 9.
    Kasic, A.
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Gogova, Daniela
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Larsson, Henrik
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Hemmingsson, Carl
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Bundesmann, C.
    Institut für Experimentelle Physik II, Universität Leipzig, Leipzig, Germany.
    Schubert, M.
    Institut für Experimentelle Physik II, Universität Leipzig, Leipzig, Germany.
    Micro-Raman scattering profiling studies on HVPE-grown free-standing GaN2004In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 201, no 12, p. 2773-2776Article in journal (Refereed)
    Abstract [en]

    Free-standing GaN of ∼330 μm thickness with low defect density was prepared by hydride vapor-phase epitaxy (HVPE) on sapphire in a vertical atmospheric-pressure reactor and a subsequent laser-induced lift-off process. The structural and optical properties of the material were assessed by various characterization techniques, like X-ray diffraction, photo- and cathodoluminescence, spectroscopic ellipsometry, positron annihilation spectroscopy, and transmission electron microscopy. Here, we focus on μ-Raman scattering profiling studies providing the vertical strain distribution and the evolution of the crystalline quality with increasing layer thickness. Profiles of the free-carrier concentration are obtained from monitoring the LO-phonon plasmon coupled mode. Comparative investigations are performed on the material before and after separation of the sapphire substrate. The GaN material presented here is well capable of serving as a substrate for further homoepitaxial strain-relaxed and crack-free growth needed for fabrication of high-quality III-nitride device heterostructures.

  • 10.
    Kasic, A.
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Gogova, Daniela
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Larsson, Henrik
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Hemmingsson, Carl
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Heuken, M.
    Aixtron AG, Germany .
    Characterization of crack-free relaxed GaN grown on 2″ sapphire2005In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 98, no 7, p. 73525-Article in journal (Refereed)
    Abstract [en]

    We demonstrate the growth of high-quality and virtually strain-free bulklike GaN by hydride vapor-phase epitaxy in a vertical atmospheric-pressure reactor with a bottom-fed design. The 300‐μm-thick GaN layer was grown on a 2″ (0 0 0 1) sapphire substrate buffered with a ∼ 2‐μm-thick GaN layer grown by metal-organic chemical-vapor deposition. During the cool down process to room temperature, cracking was induced in the sapphire substrate, thereby allowing the bulklike GaN layer to relax without provoking cracking of itself. The crystalline quality and the residual strain in the 2″ GaN wafer were investigated by various characterization techniques. The lateral homogeneity of the wafer was monitored by low-temperature photoluminescence mapping. High-resolution x-ray diffraction and photoluminescence measurements proved the high crystalline quality of the material grown. The position of the main near-band-gap photoluminescence line and the phonon spectra obtained from infrared spectroscopic ellipsometry show consistently that the 2″ crack-free GaN is virtually strain-free over a diameter of approximately 4 cm.

  • 11.
    Larsson, Henrik
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Gogova, Daniela
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Kasic, A.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Miskys, C. R.
    Walter Schottky Institut, Technische Universität Munchen, Germany.
    Stutzmann, M.
    Walter Schottky Institut, Technische Universität Munchen, Germany.
    Free-standing HVPE-GaN Layers2003In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 0, no 7, p. 1985-1988Article in journal (Refereed)
    Abstract [en]

    We have grown GaN layers with a thickness up to 340 μm in an rf-heated vertical HVPE reactor with a bottom-fed design. The GaN layers were separated from the sapphire substrate by a LLO process. The free-standing GaN was investigated by HRXRD, AFM and low temperature CL. The FWHM values of the ω-scans are 96 and 129 arcsec for the (104) and (002) reflection, respectively, which indicates high crystalline quality. The c and a lattice parameters are determined as c = 0.51850 ± 0.00004 nm and a = 0.31890 ± 0.00004 nm, indicating stress free material. The etch pit density was estimated to be 1 × 107 cm−2. The used HVPE growth procedure together with the subsequent LLO are obviously capable to provide high-quality free-standing GaN material for further epitaxial overgrowth.

  • 12. Malinauskas, T.
    et al.
    Aleksiejunas, R.
    Jarasiunas, K.
    Beaumont, B.
    Gibart, P.
    Kakanakova-Georgieva, Anelia
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Janzén, Erik
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Gogova, Daniela
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials.
    Heuken, M.
    All-optical characterization of carrier lifetimes and diffusion lengths in MOCVD-, ELO-, and HVPE- grown GaN2007In: Journal of Crystal Growth, Vol. 300, 2007, Vol. 300, no 1, p. 223-227Conference paper (Refereed)
    Abstract [en]

    The metrological capability of the picosecond four-wave mixing (FWM) technique for evaluation of the photoelectrical properties of GaN heterostructures grown on sapphire, silicon carbide, and silicon substrates as well as of free-standing GaN films is demonstrated. Carrier recombination and transport features have been studied in a wide excitation, temperature, and dislocation density (from ∼1010 to 106 cm-2) range, exploring non-resonant refractive index modulation by a free carrier plasma. The studies allowed to establish the correlations between the dislocation density and the carrier lifetime, diffusion length, and stimulated emission threshold, to reveal a competition between the bimolecular and nonradiative recombination, and to verify the temperature dependence of bimolecular recombination coefficient in the 10-300 K temperature range. It was shown that the FWM technique is more advantageous than the time-resolved photoluminescence technique for determination of carrier lifetimes in high quality thick III-nitride layers. © 2006 Elsevier B.V. All rights reserved.

  • 13. Malinauskas, T
    et al.
    Jarasiunas, K
    Aleksiejunas, R
    Gogova, Daniela
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology.
    Monemar, Bo
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Beaumont, B
    Gibart, P
    Contribution of dislocations to carrier recombination and transport in highly excited ELO and HYPE GaN layers2006In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 243, no 7, p. 1426-1430Article in journal (Refereed)
    Abstract [en]

    Nonequilibrium carrier dynamics has been investigated in ELO and HYPE grown GaN layers in a wide temperature and excitation range by using the time-resolved picosecond FWM technique. Carrier lifetime in the samples at 300 K increased up to 2.8-5.1 ns in accordance with the decreasing threading dislocation density from 4 x 10(7) cm(-2) (ELO) to mid 106 cm(-2) in HYPE layers. At T < 100 K, the hyperbolic shape of FWM kinetics indicated carrier density dependent radiative lifetimes, which gradually decreased at lower temperatures to a few hundreds of ps. The dominance of bimolecular recombination in HVPE layers at 10-40 K was demonstrated by the exposure characteristic of FWM, that has shown a sublinear growth of carrier density with excitation, N proportional to I-1/2. Numerical fitting of the set of FWM kinetics at various T confirmed the temperature dependence of bimolecular recombination coefficient B proportional to T-1/5 and provided its value B = 2 x 10(-11) cm(3)/s at 300 K and 3.2 x 10(-9) cm(3)/s at 9 K. The measured bipolar diffusion coefficients allowed determination of carrier diffusion length of 0.8-1 mu m at 300 K and its dependence on dislocation density and temperature. (c) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  • 14.
    Misheva, M.
    et al.
    Department of Physics, Sofia University, Sofia, Bulgaria.
    Larsson, Henrik
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Gogova, Daniela
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Monemar, Bo
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Positron annihilation study of HVPE grown thick GaN layers2005In: Physica Status Solidi (a) applications and materials science, ISSN 1862-6300, E-ISSN 1862-6319, Vol. 202, no 5, p. 713-717Article in journal (Refereed)
    Abstract [en]

    Single-crystalline GaN layers with a thickness up to 330 µm were grown by hydride vapor phase epitaxy on basal plane sapphire at gallium stable conditions in a bottom-fed vertical reactor at atmospheric pressure. Positron annihilation spectroscopy experiments were implemented in order to identify native point defects in the as-grown non-intentionally doped n-type GaN. Comparatively low concentrations of Ga vacancy related defects in the order of 1016 to 1017 cm–3 were extracted from the positron annihilation spectroscopy data. The Ga vacancy defect concentration was related to the intensity of the yellow photoluminescence band centered at 2.2 eV. The influence of the growth rate on the Ga vacancy related defect concentration was investigated. A trend of decreasing of the defect concentration with increasing of layer thickness is observed, which correlates with improving crystalline quality with the thickness.

  • 15.
    Monemar, Bo
    et al.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Larsson, Henrik
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Hemmingsson, Carl
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Gogova, Daniela
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Growth of thick GaN layers with hydride vapour phase epitaxy2005In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 281, no 1, p. 17-31Article in journal (Refereed)
    Abstract [en]

    In this paper we describe recent experimental efforts to produce high quality thick (⩾300 μm) GaN layers on sapphire, the removal of such a layer from the sapphire substrate, and the properties of the so obtained free-standing GaN material. The growth process is described in some detail in the vertical reactor geometry used in this work. Defects like dislocations, micro-cracks and pits produced during growth are discussed, along with procedures to minimize their concentration on the growing surface. The laser lift-off technique is shown to be a feasible technology, in particular if a powerful laser with a large spot size can be used. A major problem with the free-standing material is the typically large bowing of such a wafer, due to the built in defect concentrations near the former GaN-sapphire interface. This bowing typically causes a rather large width of the XRD rocking curve of the free-standing material, while optical data confirm virtually strain free material of excellent quality at the top surface.

  • 16.
    Yakimova, Rositsa
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Virojanadara, Chariya
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Gogova, Daniela
    Leibniz Institute for Crystal Growth, Berlin, Germany.
    Syväjärvi, Mikael
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Siche, D.
    Leibniz Institute for Crystal Growth, Berlin, Germany.
    Larsson, Krister
    Department of Materials Chemistry, Uppsala University, Uppsala, Sweden.
    Johansson, Leif
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Analysis of the Formation Conditions for Large Area Epitaxial Graphene on SiC Substrates2010In: SILICON CARBIDE AND RELATED MATERIALS 2009, PTS 1 AND 2 / [ed] Bauer, AJ; Friedrichs, P; Krieger, M; Pensl, G; Rupp, R; Seyller, T, Trans Tech Publications Inc., 2010, Vol. 645-648, p. 565-568Conference paper (Refereed)
    Abstract [en]

    We are aiming at understanding the graphene formation mechanism on different SiC polytypes (6H, 4H and 3C) and orientations with the ultimate goal to fabricate large area graphene (up to 2 inch) with controlled number of monolayers and spatial uniformity. To reach the objectives we are using high-temperature atmospheric pressure sublimation process in an inductively heated furnace. The epitaxial graphene is characterized by ARPES, LEEM and Raman spectroscopy. Theoretical studies are employed to get better insight of graphene patterns and stability. Reproducible results of single layer graphene on the Si-face of 6H and 4H-SiC polytypes have been attained. It is demonstrated that thickness uniformity of graphene is very sensitive to the substrate miscut.

  • 17.
    Yazdi, Gholamreza R.
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Materials Science . 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.
    Gogova, Daniela
    Leibniz Institute for Crystal Growth, 12 489 Berlin, Germany.
    Hultman, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Syväjärvi, Mikael
    Linköping University, Department of Physics, Chemistry and Biology, Materials Science . Linköping University, The Institute of Technology.
    Yakimova, Rosita
    Linköping University, Department of Physics, Chemistry and Biology, Materials Science . Linköping University, The Institute of Technology.
    Defect-free Single Crystal AlN Nanowires by Physical Vapor Transport GrowthManuscript (Other academic)
    Abstract [en]

    Growth by vapor-solid mechanism of AlN nanowires with a diameter in the range of 40-500nm and a length reaching 100 μm, resulting in a max aspect ratio of 600, is reported. Theobjects are obtained at 1750 oC and 850 mbar nitrogen pressure on 4H-SiC patternedsubstrates by sublimation epitaxy, which is a version of the physical vapor transport techniqueand provides a high growth rate. The nanowires are hexagonally shaped and perfectly alignedalong the 0001 direction with a small tilt given by the substrate vicinality. It is observed thatunder nitrogen excess a preferential growth along the c-axis of the wurtzite structure takesplace, and switches to lateral growth below some critical value of nitrogen pressure.Investigations by SEM, TEM, CL and Raman spectroscopy measurements were carried out. Itis shown that the nanowires consist of wurtzitic AlN with defect free crystal structure.Possible applications have been depicted.

  • 18.
    Yazdi, Gholamreza
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Vassilevski, K.
    Newcastle University.
    Cordoba Gallego, Jose Manuel
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials . Linköping University, The Institute of Technology.
    Gogova, Daniela
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Nikitina, I. P.
    Newcastle University.
    Syväjärvi, Mikael
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Odén, Magnus
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials . Linköping University, The Institute of Technology.
    Wright, N.G.
    Newcastle University.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Free standing AIN single crystal grown on pre-patterned and in situ patterned 4H-SiC substrates2010In: Materials Science Forum, Vols. 645-648, Transtec Publications; 1999 , 2010, Vol. 645-648, p. 1187-1190Conference paper (Refereed)
    Abstract [en]

    Free standing AIN wafers were grown on pre-patterned and in situ patterned 4H-SiC substrates by a physical vapor transport method. It is based on the coalescence of AIN microrods, which evolve from the apex of SiC pyramids grown on the SIC substrate during a temperature ramp up for in situ patterned substrate and SiC pyramids formed by reactive ion etching (RIE). This process yields stress-free (according XRD and Raman results) AIN single crystals with a thickness up to 400 mu m and low dislocation density.

1 - 18 of 18
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