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Author: Weyers, M. × Subject: Konferenzschrift ×

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Now showing 1 - 4 of 4
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    Advances in electron channelling contrast imaging and electron backscatter diffraction for imaging and analysis of structural defects in the scanning electron microscope
    (London [u.a.] : Institute of Physics, 2020) Trager-Cowan, C.; Alasmari, A.; Avis, W.; Bruckbauer, J.; Edwards, P.R.; Hourahine, B.; Kraeusel, S.; Kusch, G.; Jablon, B.M.; Johnston, R.; Martin, R.W.; Mcdermott, R.; Naresh-Kumar, G.; Nouf-Allehiani, M.; Pascal, E.; Thomson, D.; Vespucci, S.; Mingard, K.; Parbrook, P.J.; Smith, M.D.; Enslin, J.; Mehnke, F.; Kneissl, M.; Kuhn, C.; Wernicke, T.; Knauer, A.; Hagedorn, S.; Walde, S.; Weyers, M.; Coulon, P.-M.; Shields, P.A.; Zhang, Y.; Jiu, L.; Gong, Y.; Smith, R.M.; Wang, T.; Winkelmann, A.
    In this article we describe the scanning electron microscopy (SEM) techniques of electron channelling contrast imaging and electron backscatter diffraction. These techniques provide information on crystal structure, crystal misorientation, grain boundaries, strain and structural defects on length scales from tens of nanometres to tens of micrometres. Here we report on the imaging and analysis of dislocations and sub-grains in nitride semiconductor thin films (GaN and AlN) and tungsten carbide-cobalt (WC-Co) hard metals. Our aim is to illustrate the capability of these techniques for investigating structural defects in the SEM and the benefits of combining these diffraction-based imaging techniques.
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    Cathodoluminescence and TEM investigations of structural and optical properties of AlGaN on epitaxial laterally overgrown AlN/sapphire templates
    (Milton Park : Taylor & Francis, 2013) Zeimer, U.; Mogilatenko, A.; Kueller, V.; Knauer, A.; Weyers, M.
    Surface steps as high as 15 nm on up to 10 μm thick AlN layers grown on patterned AlN/sapphire templates play a major role for the structural and optical properties of AlxGa1−xN layers with x ≥ 0.5 grown subsequently by metalorganic vapour phase epitaxy. The higher the Ga content in these layers is, the stronger is the influence of the surface morphology on their properties. For x = 0.5 not only periodic inhomogeneities in the Al content due to growth of Ga-rich facets are observed by cathodoluminescence, but these facets give rise to additional dislocation formation as discovered by annular dark-field scanning transmission electron microscopy. For AlxGa1−xN layers with x = 0.8 the difference in Al content between facets and surrounding material is much smaller. Therefore, the threading dislocation density (TDD) is only defined by the TDD in the underlying epitaxially laterally overgrown (ELO) AlN layer. This way high quality Al0.8Ga0.2N with a thickness up to 1.5 μm and a TDD ≤ 5x108 cm−2 was obtained.
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    Origin of a-plane (Al,Ga)N formation on patterned c-plane AIN/sapphire templates
    (Milton Park : Taylor & Francis, 2013) Mogilatenko, A.; Kirmse, H.; Hagedorn, S.; Richter, E.; Zeimer, U.; Weyers, M.; Tränkle, G.
    a-plane (Al,Ga)N layers can be grown on patterned c-plane AlN/sapphire templates with a ridge direction along [1bar 100]Al2O3. Scanning nanobeam diffraction reveals that the formation of a-plane layers can be explained by nucleation of c-plane (Al,Ga)N with [11bar 20](Al,Ga)N
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    Optimization of the Epitaxial Growth of Undoped GaN Waveguides in GaN-Based Laser Diodes Evaluated by Photoluminescence
    (Warrendale, Pa : TMS, 2020) Netzel, C.; Hoffmann, V.; Einfeldt, S.; Weyers, M.
    Non-intentionally doped c-plane GaN layers are generally employed as p-side waveguide layers in violet/blue-emitting laser diodes. The recombination and diffusion of charge carriers in the p-side GaN waveguide influence the injection efficiency of holes into the InGaN quantum wells of these devices. In this study, the non-radiative recombination and the diffusivity in the [000-1] direction for charge carriers in such GaN layers are investigated by the photoluminescence of buried InGaN quantum wells, in addition to the GaN photoluminescence. The vertical charge carrier diffusion length and the diffusion constant in GaN were determined by evaluating the intensity from InGaN quantum wells in different depths below a top GaN layer. Additionally, the intensity from the buried InGaN quantum wells was found to be more sensitive to variations in the non-radiative recombination rate in the GaN layer than the intensity from the GaN itself. The study enables conclusions to be drawn on how the growth of a p-side GaN waveguide layer has to be optimized: (1) The charge carrier diffusivity in the [000-1] direction at device operation temperature is limited by phonon scattering and can be only slightly improved by material quality. (2) The use of TMGa (trimethylgallium) instead of TEGa (triethylgallium) as a precursor for the growth of GaN lowers the background silicon doping level and is advantageous for a large hole diffusion length. (3) Small growth rates below 0.5 μm/h when using TMGa or below 0.12 μm/h when using TEGa enhance non-radiative recombination. (4) A V/III gas ratio of 2200 or more is needed for low non-radiative recombination rates in GaN.