Filters

2016 - 20194

More filters

2019 - 201922020 - 20212
Reset filters

Settings

Author: Riechert, Henning × DDC: 620 ×

Search Results

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Electroluminescence and current-voltage measurements of single-(In,Ga)N/GaN-nanowire light-emitting diodes in a nanowire ensemble
    (Frankfurt, M. : Beilstein-Institut zur Förderung der Chemischen Wissenschaften, 2019) van Treeck, David; Ledig, Johannes; Scholz, Gregor; Lähnemann, Jonas; Musolino, Mattia; Tahraoui, Abbes; Brandt, Oliver; Waag, Andreas; Riechert, Henning; Geelhaar, Lutz
    We present the combined analysis of electroluminescence (EL) and current-voltage (I-V) behavior of single, freestanding (In,Ga)N/GaN nanowire (NW) light-emitting diodes (LEDs) in an unprocessed, self-assembled ensemble grown by molecular beam epitaxy. The data were acquired in a scanning electron microscope equipped with a micromanipulator and a luminescence detection system. Single NW spectra consist of emission lines originating from different quantum wells, and the width of the spectra increases with decreasing peak emission energy. The corresponding I-V characteristics are described well by a modified Shockley equation. The key advantage of this measurement approach is the possibility to correlate the EL intensity of a single-NW LED with the actual current density in this NW. This way, the external quantum efficiency (EQE) can be investigated as a function of the current in a single-NW LED. The comparison of the EQE characteristic of single NWs and the ensemble device allows for a quite accurate determination of the actual number of emitting NWs in the working ensemble LED and the respective current densities in its individual NWs. This information is decisive for a meaningful and comprehensive characterization of a NW ensemble device, rendering the measurement approach employed here a very powerful analysis tool. © 2019 van Treeck et al.
  • Thumbnail Image
    Item
    Formation of resonant bonding during growth of ultrathin GeTe films
    (London : Nature Publishing Group, 2017) Wang, Ruining; Zhang, Wei; Momand, Jamo; Ronneberger, Ider; Boschker, Jos E.; Mazzarello, Riccardo; Kooi, Bart J.; Riechert, Henning; Wuttig, Matthias; Calarco, Raffaella
    A highly unconventional growth scenario is reported upon deposition of GeTe films on the hydrogen passivated Si(111) surface. Initially, an amorphous film forms for growth parameters that should yield a crystalline material. The entire amorphous film then crystallizes once a critical thickness of four GeTe bilayers is reached, subsequently following the GeTe(111) 
  • Thumbnail Image
    Item
    Light coupling between vertical III-As nanowires and planar Si photonic waveguides for the monolithic integration of active optoelectronic devices on a Si platform
    (Washington, DC : Optical Society of America, 2016) Giuntoni, Ivano; Geelhaar, Lutz; Bruns, Jürgen; Riechert, Henning
    We present a new concept for the optical interfacing between vertical III-As nanowires and planar Si waveguides. The nanowires are arranged in a two-dimensional array which forms a grating structure on top of the waveguide. This grating enables light coupling in both directions between the components made from the two different material classes. Numerical simulations show that this concept permits a light extraction efficiency from the waveguide larger than 45% and a light insertion efficiency larger than 35%. This new approach would allow the monolithic integration of nanowire-based active optoelectronics devices, like photodetectors and light sources, on the Si photonics platform.
  • Thumbnail Image
    Item
    Sub-nanometre resolution of atomic motion during electronic excitation in phase-change materials
    (London : Nature Publishing, 2016) Mitrofanov, Kirill V.; Fons, Paul; Makino, Kotaro; Terashima, Ryo; Shimada, Toru; Kolobov, Alexander, V.; Tominaga, Junji; Bragaglia, Valeria; Giussani, Alessandro; Calarco, Raffaella; Riechert, Henning; Sato, Takahiro; Katayama, Tetsuo; Ogawa, Kanade; Togashi, Tadashi; Yabashi, Makina; Wall, Simon; Brewe, Dale; Hase, Muneaki
    Phase-change materials based on Ge-Sb-Te alloys are widely used in industrial applications such as nonvolatile memories, but reaction pathways for crystalline-to-amorphous phase-change on picosecond timescales remain unknown. Femtosecond laser excitation and an ultrashort x-ray probe is used to show the temporal separation of electronic and thermal effects in a long-lived (>100 ps) transient metastable state of Ge2Sb2Te5 with muted interatomic interaction induced by a weakening of resonant bonding. Due to a specific electronic state, the lattice undergoes a reversible nondestructive modification over a nanoscale region, remaining cold for 4 ps. An independent time-resolved x-ray absorption fine structure experiment confirms the existence of an intermediate state with disordered bonds. This newly unveiled effect allows the utilization of non-thermal ultra-fast pathways enabling artificial manipulation of the switching process, ultimately leading to a redefined speed limit and improved energy efficiency and reliability of phase-change memory technologies.