Growth of axial and core-shell (In,Ga)N/GaN heterostructures on GaN nanowires on TiN

In dieser Arbeit werden das Wachstum und die optischen Eigenschaften von selbstorganisierten GaN Nanodrähten auf TiN und nanodrahtbasierten (In,Ga)N/GaN Heterostrukturen für LED Anwendungen untersucht. Zu diesem Zweck wird das selbstorganisierte Wachstum von langen, dünnen und nicht koaleszierten GaN Nanodrähten auf TiN mittels Molekularstrahlepitaxie demonstriert. In weiteren Untersuchungen werden diese gut separierten und nicht koaleszierten GaN Nanodrähte auf TiN als Basis für die Herstellung von axialen und radialen Heterostrukturen verwendet. Trotz der definierten Morphologie der aktiven Zonen ist die Lichtausbeute der axialen (In,Ga)N Quantentöpfen eher gering. Um das Potenzial der Molekularstrahlepitaxie für das Wachstum von Kern-Hüllen-Strukturen im Allgemeinen besser zu verstehen, wird der Aspekt, dass die Seitenfacetten der Nanodrähte nur sequentiell den verschiedenen Materialstrahlen ausgesetzt werden, durch Modellierung des Wachstums von GaN Hüllen auf GaN Nanodrähten untersucht. Es wird gezeigt, dass Ga Adatomdiffusionsprozesse zwischen verschiedenen Facetten das Wachstum auf den Seitenfacetten stark beeinflussen. Neben der Untersuchung von radialsymmetrischen (In,Ga)N Hüllen wird ein neuer Wachstumsansatz vorgestellt, der die kontrollierte Abscheidung von III-Nitridhüllen auf verschiedenen Seiten des Nanodrahtes ermöglicht. Unter Ausnutzung der Richtungsabhängigkeit der Materialstrahlen in einer Molekularstrahlepitaxieanlage ermöglicht der neuartige Ansatz die sequentielle Abscheidung verschiedener Verbundstoffmaterialien auf einer bestimmten Seite der Nanodrähte, um eine einseitige Schale zu wachsen. Diese sequentielle gerichtete Abscheidungsmethode ermöglicht prinzipiell die Kombination mehrerer aktiver Zonen mit unterschiedlichen Eigenschaften auf verschiedenen lateralen Seiten ein und derselben Nano- oder Mikrostruktur. Solche Architekturen könnten beispielsweise für die Realisierung von mehrfarbigen Pixeln für Mikro-LED-Displays interessant sein.In this thesis, the growth and the optical characteristics of self-assembled GaN nanowires on TiN and nanowire-based (In,Ga)N/GaN heterostructures for LED applications is investigated. To this end, the self-assembled growth of long, thin and uncoalesced GaN nanowires on TiN by molecular beam epitaxy is demonstrated. Subsequently, these well-separated and uncoalesced GaN nanowires on TiN are used as a basis for the fabrication of axial and radial heterostructures. Despite the well-defined morphology of the active regions, the luminous efficiency of axial (In,Ga)N quantum wells is found to be rather low. To better understand the potential of molecular beam epitaxy for the growth of core-shell structures in general, the aspect of the side facets of the nanowires being only sequentially exposed to the different material beams is studied by modeling the shell growth of GaN shells on GaN nanowires. It is shown that Ga adatom diffusion processes between different facets strongly affect the growth on the side facets. Besides the fundamental investigation of the growth of radially symmetric (In,Ga)N shells, a new growth approach which allows the controlled deposition of III-nitride shells on different sides of the nanowire is presented. Using the directionality of the material beams in an molecular beam epitaxy system, the novel approach facilitates the sequential deposition of different compound materials on a specific side of the nanowires to grow a one-sided shell. This sequential directional deposition method may in principle allow the combination of multiple active regions with different properties on different lateral sides of one and the same nano- or microstructure. Such architectures, for instance, might be interesting for the realization of multi-color pixels for micro-LED displays

Growth mechanisms in molecular beam epitaxy for GaN-(In,Ga)N core-shell nanowires emitting in the green spectral range

Using molecular beam epitaxy, we demonstrate the growth of (In,Ga)N shells emitting in the green spectral range around very thin (35 nm diameter) GaN core nanowires. These GaN nanowires are obtained by self-assembled growth on TiN. We present a qualitative shell growth model accounting for both the three-dimensional nature of the nanostructures as well as the directionality of the atomic fluxes. This model allows us, on the one hand, to optimise the conditions for high and homogeneous In incorporation and, on the other hand, to explain the influence of changes in the growth conditions on the sample morphology and In content. Specifically, the impact of the V/III and In/Ga flux ratios, the rotation speed and the rotation direction are investigated. Notably, with In acting as surfactant, the ternary (In,Ga)N shells are much more homogeneous in thickness along the NW length than their binary GaN counterparts

Editorial: Bildung gemeinsam verändern: Diskussionsbeiträge und Impulse aus Forschung und Praxis

Das vorliegende Themenheft 28 der Zeitschrift MedienPädagogik ist dem breit angelegten Motto «Bildung gemeinsam verändern» gewidmet. Dabei mag es einiger erläuternder Worte der Kontextualisierung bedürfen, um diese grosse Klammer verständlich zu machen. «Bildung gemeinsam verändern» lautete das Thema des vierten Jungen Forums für Medien und Hochschulentwicklung (JFMH), welches am 8. und 9. Juni 2015 an der Heinrich-Heine-Universität Düsseldorf stattfand (www.hhu.de/jfmh15). Mit diesem Heft liegen nun die Proceedings zur Tagung vor. Das JFMH ist eine Tagung(sreihe), deren Schwerpunkt auf Beiträgen von Young Researchers und Young Professionals im Feld von Medienpädagogik, Medien- und Hochschuldidaktik, E-Learning an Hochschulen, Schulen, (Aus-/Weiter-)Bildungsanbietern und allen weiteren Lernorten liegt. Das Forum wird seit 2012 unter der Schirmherrschaft der Deutschen Gesellschaft für Hochschuldidaktik (dghd), der Gesellschaft für Medien in der Wissenschaft (GMW), der Gesellschaft für Informatik (GI, Fachgruppe E-Learning) sowie der Deutschen Gesellschaft für Erziehungswissenschaft (DGfE, Junges Netzwerk Medienpädagogik der Sektion Medienpädagogik) ausgerichtet und rückt den Austausch zwischen den Fachbereichen wie auch zwischen forschungs- und anwendungsorientierten Perspektiven in den Vordergrund. Entsprechend den genannten Ansprüchen greift dieses Themenheft der Zeitschrift MedienPädagogik verschiedene Problemstellungen und Entwicklungsbedarfe auf, von denen wir drei konkreter benennen und ausführen möchten: a) Bildung interdisziplinär: der Gegenstand, der aus verschiedenen Perspektiven bearbeitet wird; b) Austausch zwischen Forschung und Praxis: die Vernetzung von Forschung und Praxis; c) Förderung von Young Researchers und Young Professionals: die Förderung von Praktikerinnen und Praktikern in frühen Karrierephasen

Sequential directional deposition of one-sided (In,Ga)N shells on GaN nanowires by molecular beam epitaxy

Capitalizing on the directed nature of the atomic fluxes in molecular beam epitaxy, we propose and demonstrate the sequential directional deposition of lateral (In,Ga)N shells on GaN nanowires. In this approach, a sub-monolayer thickness of each constituent atomic species, i.e. Ga, In, and N, is deposited subsequently from the same direction by rotating the sample and operating the shutters accordingly. Using multiple iterations of this process, we achieve the growth of homogeneous shells on a single side facet of the nanowires. For higher In content and thus lattice mismatch, we observe a strain-induced bending of the nanowire heterostructures. The incorporation of In and the resulting emission spectra are systematically investigated as a function of both the growth temperature and the In/Ga flux ratio

X-ray scattering study of GaN nanowires grown on Ti/Al2_{2}O3_{3} by molecular beam epitaxy

GaN nanowires (NWs) grown by molecular beam epitaxy on Ti films sputtered on Al$_{2}$O$_{3}$ are studied by X-ray diffraction (XRD) and grazing incidence small-angle X-ray scattering (GISAXS). XRD, performed both in symmetric Bragg reflection and at grazing incidence, reveals Ti, Ti$_{3}$O, Ti$_{3}$Al, and TiO$_x$N$_y$ crystallites with in-plane and out-of-plane lattice parameters intermediate between those of Al$_{2}$O$_{3}$ and GaN. These topotaxial crystallites in Ti film, formed due to interfacial reactions and N exposure, possess fairly little misorientation with respect to Al$_{2}$O$_{3}$. As a result, GaN NWs grow on the top TiN layer possessing a high degree of epitaxial orientation with respect to the substrate. The measured GISAXS intensity distributions are modeled by the Monte Carlo method taking into account the orientational distributions of NWs, a variety of their cross-sectional shapes and sizes, and roughness of their side facets. The cross-sectional size distributions of the NWs and the relative fractions of $(1\bar{1}00)$ and $(11\bar{2}0)$ side facets are determined

Electroluminescence and current-voltage measurements of single (In,Ga)N/GaN nanowire light-emitting diodes in the nanowire ensemble

We present the combined analysis of the electroluminescence (EL) as well as the 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 the 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 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

Phosphoenolpyruvate carboxylase dentified as a key enzyme in erythrocytic Plasmodium falciparum carbon metabolism

Phospoenolpyruvate carboxylase (PEPC) is absent from humans but encoded in thePlasmodium falciparum genome, suggesting that PEPC has a parasite-specific function. To investigate its importance in P. falciparum, we generated a pepc null mutant (D10Δpepc), which was only achievable when malate, a reduction product of oxaloacetate, was added to the growth medium. D10Δpepc had a severe growth defect in vitro, which was partially reversed by addition of malate or fumarate, suggesting that pepc may be essential in vivo. Targeted metabolomics using 13C-U-D-glucose and 13C-bicarbonate showed that the conversion of glycolytically-derived PEP into malate, fumarate, aspartate and citrate was abolished in D10Δpepc and that pentose phosphate pathway metabolites and glycerol 3-phosphate were present at increased levels. In contrast, metabolism of the carbon skeleton of 13C,15N-U-glutamine was similar in both parasite lines, although the flux was lower in D10Δpepc; it also confirmed the operation of a complete forward TCA cycle in the wild type parasite. Overall, these data confirm the CO2 fixing activity of PEPC and suggest that it provides metabolites essential for TCA cycle anaplerosis and the maintenance of cytosolic and mitochondrial redox balance. Moreover, these findings imply that PEPC may be an exploitable target for future drug discovery