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  • Thonke, Klaus  (26)
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  • 1
    Language: English
    In: Journal of Applied Physics, 28 October 2014, Vol.116(16)
    Description: Spatially resolved luminescence and Raman spectroscopy investigations are applied to a series of ( 11 2 ¯ 2 ) -GaN samples grown by hydride vapor phase epitaxy (HVPE) grown over an initial layer deposited by metal organic vapor phase epitaxy on patterned sapphire substrates. Whereas these two differently grown GaN layers are crystallographically homogeneous, they differ largely in their doping level due to high unintentional oxygen uptake in the HVPE layer. This high doping shows up in luminescence spectra, which can be explained by a free-electron recombination band for which an analytical model considering the Burstein-Moss shift, conduction band tailing, and the bandgap renormalization is included. Secondary ion mass spectrometry, Raman spectroscopy, and Hall measurements concordantly determine the electron density to be above 10 19  cm −3 . In addition, the strain state is assessed by Raman spectroscopy and compared to a finite element analysis.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 2
    Language: English
    In: Journal of Applied Physics, 15 May 2010, Vol.107(10)
    Description: Piezoelectric polarization in GaInN/GaN quantum well (QW) structures is not only dependent on the strain in the pseudomorphically grown QWs but also severely influenced by the strain already present in the underlying template. Here, we investigate the influence of template strain on the piezoelectric polarization of QWs grown in the polar [0001] and in semipolar off-[0001] directions. We find that an efficient strain-engineering can be used to decrease polarization fields or even cancel them out completely. Even a change of sign of the built-in polarization fields within QWs can be achieved in certain strain-situations. Our results shed light on recent ambiguous results concerning the piezoelectric tensor component e 15 and are able to explain unexpected differences in device performance of polar and semipolar QW devices.
    Keywords: Articles
    ISSN: 0021-8979
    E-ISSN: 1089-7550
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  • 3
    Language: English
    In: Physical Review B, 1/2011, Vol.83(3)
    ISSN: 1098-0121
    ISSN: 24699950
    E-ISSN: 1550-235X
    Source: American Physical Society (APS) (via CrossRef)
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  • 4
    Language: English
    In: Japanese Journal of Applied Physics, 2013, Vol.52(8S)
    Description: We have decreased the dislocation density in Al x Ga 1- x N epitaxial layers grown on sapphire wafers by introducing an in-situ deposited SiN nano-mask layer. Taking together results obtained by transmission electron microscopy, photoluminescence, cathodoluminescence, and X-ray diffraction, we were able to derive a schematic model about the AlGaN growth on the SiN nanomask: On the open pores of the nano-mask, Ga-rich AlGaN hillocks develop, whereas on the SiN layer Al-rich AlGaN nucleates owing to the reduced selectivity of Al-containing material. Once the hillocks are formed, Ga-rich material is more efficiently incorporated on the inclined side-facets leading to an Al-rich coverage of the central c -plane part of the hillocks. We observed a bending of the dislocations towards the side-facets of the hillocks, which eventually leads to dislocation bundles with increased probability of dislocation annihilation, separated by fairly defect-free regions. Thus, we could achieve a significant reduction of the edge-type dislocation density in these epitaxial layers.
    Keywords: Aluminum Gallium Nitrides ; Reduction ; Bundles ; Dislocations ; Epitaxial Layers ; Aluminum ; Dislocation Density ; Nanostructure ; Materials Selection ; Defects and Transport (Wc) ; Applied Physics (General) (So) ; Microstructure (EC) ; Constitution/Crystal Structure/Microstructure (AI) ; Physics (General) (Ah);
    ISSN: 00214922
    E-ISSN: 13474065
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  • 5
    In: Journal of Applied Crystallography, 01 October 2013, Vol.46(5), pp.1425-1433
    Description: Three‐dimensional reciprocal space mapping of semipolar () GaN grown on stripe‐patterned ‐plane () sapphire substrates is found to be a powerful and crucial method for the analysis of diffuse scattering originating from stacking faults that are diffracting in a noncoplanar geometry. Additionally, by measuring three‐dimensional reciprocal space maps (3D‐RSMs) of several reflections, the transmission electron microscopy visibility criteria could be confirmed. Furthermore, similar to cathodoluminescence, the 3D‐RSM method could be used in future as a reliable tool to distinguish clearly between the diffuse scattering signals coming from prismatic and from basal plane stacking faults and from partial dislocations in semipolar () GaN. The fitting of the diffuse scattering intensity profile along the stacking fault streaks with a simulation based on the Monte Carlo approach has delivered an accurate determination of the basal plane stacking fault density. A reduction of the stacking fault density due to the intercalation of an SiN interlayer in the GaN layer deposited on the sidewall of the pre‐patterned sapphire substrate has led to an improvement of the optoelectronic properties, influenced by the crystal quality, as has been demonstrated by a locally resolved cathodoluminescence investigation.
    Keywords: Three‐Dimensional Reciprocal Space Mapping ; Diffuse Scattering ; Stacking Faults
    ISSN: 0021-8898
    E-ISSN: 1600-5767
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  • 6
    Language: English
    In: physica status solidi (c), July 2011, Vol.8(7‐8), pp.2063-2065
    Description: We report the growth of high crystal quality AlGaN directly on sapphire substrates with metalorganic vapour phase epitaxy. We studied the improvements in crystal quality by introducing an deposited SiN interlayer. It acts as a nanomask which results in termination of the dislocations near the interface between the nanomask and epilayer. The epilayers with no SiN interlayer have very low density of screw type dislocations evident from transmission electron microscopy (TEM) investigations confirmed by a very narrow X‐ray diffraction symmetric reflection (60”). This is a result of our oxygen doped AlN nucleation layer used to grow such epilayers. On the other hand, such epilayers have very broad XRD asymmetric reflections (typically a few thousand arcsec) indicating a high density of edge type dislocations. We could decrease the latter which are the main existing dislocations in our AlGaN epilayers. In TEM micrographs, we observed the formation of dislocation “bundles” when introducing SiN interlayers. This phenomenon promotes the dislocation free areas on the surface of the wafer. We carried out accurate optimizations of the SiN deposition. Consequently, we could grow high quality AlGaN epilayers with much narrower XRD asymmetric peaks–782” with total sample thickness of 1.4 µm (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
    Keywords: Algan ; Sinx Nanomask ; Movpe ; Dislocation Reduction
    ISSN: 1862-6351
    E-ISSN: 1610-1642
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  • 7
    Language: English
    In: physica status solidi (b), March 2011, Vol.248(3), pp.611-615
    Description: We report about defect‐related emission bands in GaN. We establish a direct correlation between results from spatially and spectrally resolved cathodoluminescence bands at 3.32, 3.23, 3.20, 3.16, and 3.07 eV with findings obtained by transmission electron microscopy. The band around 3.32 eV was unambiguously assigned to basal plane stacking faults (BSFs). The bands at 3.23, 3.20, 3.16, and 3.07 eV were detected in sample regions, where BSFs exist, but were not assigned to specific defect types, except for the 3.07 eV line which is a phonon replica of the transition at 3.16 eV.
    Keywords: Cathodoluminescence ; Gan ; Stacking Faults ; Tem
    ISSN: 0370-1972
    E-ISSN: 1521-3951
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  • 8
    Language: English
    In: Applied Physics Letters, 24 July 2006, Vol.89(4)
    Description: The authors demonstrate the fabrication and evaluation of bright semipolar Ga In N ∕ Ga N blue light emitting diodes (LEDs). The structures are realized by growing five Ga In N ∕ Ga N quantum wells on the { 1 1 ¯ 01 } side facets of selectively grown n - Ga N stripes with triangular shape running along the ⟨ 11 2 ¯ 0 ⟩ direction covered with a Mg-doped GaN top layer. The growth was done by metal organic vapor phase epitaxy using a conventional [0001] sapphire substrate. The devices have circular mesa structures with diameters between 70 and 140 μ m . Continuous wave on-wafer optical output powers as high as 700 μ W and 3 mW could be achieved under dc conditions for 20 and 110 mA , respectively. The current dependent blueshift of the peak emission wavelength caused by screening effects of the piezoelectric field was only 1.5 nm for currents between 1 and 50 mA . This is less than half the value measured on c -plane LEDs and confirms the reduced piezoelectric field in our LED structures.
    Keywords: Lasers, Optics, And Optoelectronics
    ISSN: 0003-6951
    E-ISSN: 1077-3118
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  • 9
    In: physica status solidi (b), January 2016, Vol.253(1), pp.13-22
    Description: In order to solve issues related to huge internal electrical fields in GaN‐based optoelectronic device structures, less polar structures show very promising properties. This Feature Article reviews the authors' recent activities on this field concentrating on two different approaches: On the one hand, triangular stripes with semipolar side‐facets can be formed by selective area growth on conventional ‐plane sapphire wafers. On the other hand, high‐quality planar semipolar structures are deposited on stripe‐patterned sapphire wafers.
    Keywords: Semipolar Gan ; Gainn Quantum Wells ; Patterned Substrate ; Led
    ISSN: 0370-1972
    E-ISSN: 1521-3951
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  • 10
    Language: English
    In: physica status solidi b - basic solid state physics, 2016, Vol.253(1), p.13(10)
    Description: Byline: Ferdinand Scholz, Ulrich T. Schwarz, Ferdinand Scholz, Marian Caliebe, Gulnaz Gahramanova, Dominik Heinz, Martin Klein, Robert A. R. Leute, Tobias Meisch, Junjun Wang, Matthias Hocker, Klaus Thonke Keywords: semipolar GaN; GaInN quantum wells; Patterned substrate; LED Abstract This paper reviews our recent investigations about semipolar GaN-based optoelectronic heterostructures grown on foreign substrates. Two basically different approaches are discussed, both making use of epitaxial growth in the polar c-direction to minimize any crystalline defects. By selective area growth, stripes with triangular cross-section have been formed with semipolar side-facets, on which quantum well and electroluminescence test structures have been deposited. By careful optimisation of many growth parameters, we could drastically increase the growth temperature of GaInN quantum wells emitting beyond 500nm. In the second approach, the GaN growth starts on inclined sapphire c-planes, which form the side facets of trenches etched into the substrates. After coalescence, planar semipolar GaN layers can be achieved. We investigated various sapphire wafer orientations leading to {112a3/42}, {101a3/41}, and {202a3/41} layers. After careful optimisation with a major focus on the decrease of the stacking fault density, we have also investigated the doping behaviour of such semipolar structures. Eventually, full electroluminescence test structures could be grown.
    Keywords: Epitaxy ; Quantum Wells ; Liquor
    ISSN: 0370-1972
    E-ISSN: 15213951
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