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- ItemZeitaufgelöste PIV-Untersuchungen zur Strömungskontrolle mittels elektromagnetischer Kräfte in schwach leitfähigen Fluiden(Dresden : Forschungszentrum Dresden-Rossendorf, 2009) Cierpka, Christian[no abstract available]
- ItemAn investigation of target poisoning during reactive magnetron sputtering(Dresden : Forschungszentrum Dresden-Rossendorf, 2009) Güttler, Dominik[no abstract available]
- ItemIntegration der Positronen-Emissions-Tomographie in die Strahlentherapie mit hochenergetischen Photonen(Dresden : Forschungszentrum Dresden-Rossendorf, 2009) Kunath, Daniela[no abstract available]
- ItemEchtzeit-in-situ-Messung der Oberflächenbelegung einer Magnetron-Kathode bei der reaktiven Sputter-Abscheidung(Dresden : Forschungszentrum Dresden-Rossendorf, 2004) Güttler, Dominik[no abstract available]
- ItemStudy of [Lambda] hyperon production in C+C collisions at 2 AGeV beam energy with the HADES spectrometer(Dresden : Forschungszentrum Dresden-Rossendorf, 2007) Kanaki, Kalliopi[no abstract available]
- ItemMessung des Wirkungsquerschnitts astrophysikalisch relevanter Kernreaktionen(Dresden : Forschungszentrum Dresden-Rossendorf, 2009) Trompler, Erik[no abstract available]
- ItemInvestigation and comparison of GaN nanowire nucleation and growth by the catalyst-assisted and self-induced approaches(Berlin : Humboldt-Universität zu Berlin, 2010) Chèze, CarolineThis work focuses on the nucleation and growth mechanisms of GaN nanowires (NWs) by molecular beam epitaxy (MBE). The main novelties of this study are the intensive employment of in-situ techniques and the direct comparison of self-induced and catalyst-induced NWs. On silicon substrates, GaN NWs form in MBE without the use of any external catalyst seed. On sapphire, in contrast, NWs grow under identical conditions only in the presence of Ni seeds. The processes leading to NW nucleation are fundamentally different for both approaches. In the catalyst-assisted approach, Ga strongly reacts with the catalyst Ni particles whose crystal structure and phases are decisive for the NW growth, while in the catalyst-free approach, N forms an interfacial layer with Si before the intense nucleation of GaN starts. Both approaches yield monocrystalline wurtzite GaN NWs, which grow in the Ga-polar direction. However, the catalyst-assisted NWs are longer than the catalyst-free ones after growth under identical conditions, and they contain many stacking faults. By comparison the catalyst-free NWs are largely free of defects and their photoluminescence is much more intense than the one of the catalyst-assisted NWs. All of these differences can be explained as effects of the catalyst. The seed captures Ga atoms arriving at the NW tip more efficiently than the bare top facet in the catalyst-free approach. In addition, stacking faults could result from both the presence of the additional solid phase constituted by the catalyst-particles and the contamination of the NWs by the catalyst material. Finally, such contamination would generate non-radiative recombination centers. Thus, the use of catalyst seeds may offer an additional way to control the growth of NWs, but both the structural and the optical material quality of catalyst-free NWs are superior.
- ItemMolecular beam epitaxy of GaAs nanowires and their suitability for optoelectronic applications – comparing Au- and self-assisted growth methods(Berlin : Humboldt-Universität zu Berlin, 2011) Breuer, SteffenIn this work the synthesis of GaAs nanowires by molecular beam epitaxy (MBE) using the vapour-liquid-solid (VLS) mechanism is investigated. A comparison between Au- and self-assisted VLS growth is at the centre of this thesis. While the Au-assisted method is established as a versatile tool for nanowire growth, the recently developed self-assisted variation results from the exchange of Au by Ga droplets and thus eliminates any possibility of Au incorporation. By both methods, we achieve nanowires with epitaxial alignment to the Si(111) substrates. Caused by differences during nanowire nucleation, a parasitic planar layer grows between the nanowires by the Au-assisted method, but can be avoided by the self-assisted method. Au-assisted nanowires grow predominantly in the metastable wurtzite crystal structure, while their self-assisted counterparts have the zincblende structure. All GaAs nanowires are fully relaxed and the strain arising from the lattice mismatch between GaAs and Si of 4.1\% is accommodated by misfit dislocations at the interface. Self-assisted GaAs nanowires are generally found to have vertical and non-polar side facets, while tilted and polar nanofacets were described for Au-assisted GaAs nanowires. We employ VLS nucleation theory to understand the effect of the droplet material on the lateral facets. Optoelectronic applications require long minority carrier lifetimes at room temperature. We fabricate GaAs/(Al,Ga)As core-shell nanowires and analyse them by transient photoluminescence (PL) spectroscopy. The results are 2.5 ns for the self-assisted nanowires as well as 9 ps for the Au-assisted nanowires. By temperature-dependent PL measurements we find a characteristic activation energy of 77 meV that is present only in the Au-assisted nanowires. We conclude that most likely Au is incorporated from the droplets into the GaAs nanowires and acts as a deep, non-radiative recombination centre.
- ItemOptical properties of single semiconductor nanowires and nanowire ensembles – probing surface physics by photoluminescence spectroscopy(Berlin : Humboldt-Universität zu Berlin, 2011) Pfüller, CarstenThis thesis presents a detailed investigation of the optical properties of semiconductor nanowires (NWs) in general and single GaN NWs and GaN NW ensembles in particular by photoluminescence (PL) spectroscopy. NWs are often considered as potential building blocks for future nanometer-scaled devices. This vision is based on several attractive features that are generally ascribed to NWs. In the first part of the thesis, some of these features are examined using semiconductor NWs of different materials. On the basis of the temperature-dependent PL of Au- and self-assisted GaAs/(Al,Ga)As core-shell NWs, the influence of foreign catalyst particles on the optical properties of NWs is investigated. The effect of the substrate choice is studied by comparing the PL of ZnO NWs grown on Si, Sapphire, and ZnO substrates. The major part of this thesis discusses the optical properties of GaN NWs. The investigation of the PL of single GaN NWs and GaN NW ensembles reveals the significance of their large surface-to-volume ratio and that each NW exhibits its own individual recombination behavior. An unexpected broadening of the donor-bound exciton transition is explained by the abundant presence of surface donors in NWs. The existence and statistical relevance of these surface donors is confirmed by PL experiments of single GaN NWs which are either dispersed or free-standing. Furthermore, the influence of electric fields on the optical properties of GaN NWs is investigated and the coupling of light with GaN NWs is studied by reflectance and Raman measurements. The central results of this thesis motivate the introduction of a model that explains the typically observed nonexponential recombination dynamics in NW ensembles. It is based on a distribution of recombination rates. Preliminary simulations using this model describe the nonexponential decay of GaN NW ensembles satisfactorily and allow for an estimation of their internal quantum efficiency.
- ItemThe sorption of uranium(VI) and neptunium(V) onto surfaces of selected metal oxides and alumosilicates studied by in situ vibrational spectroscopy(Dresden : Forschungszentrum Dresden-Rossendorf, 2010) Müller, Katharina[no abstract available]
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