Raman Spectroscopy of GaAs Nanowires:Doping Mechanisms and Fundamental Properties

Semiconductor nanowires offer a wide range of opportunities for newgenerations of nanoscale electronic and optic devices. For these applications to become reality, deeper understanding of the fundamental properties of the nanowires is required. In this thesis, Raman spectroscopy has been applied to examine the characteristics of GaAs nanowires facing the following challenges in current nanowire research: (i) understanding of the doping mechanisms in catalyst-free GaAs nanowires grown byMBE, (ii) examination of the electronic band structure of the wurtzite polytype of GaAs, and (iii) probing the properties of free carrier systems in nanowire based quantum-heterostructures. The Si-doping of GaAs nanowires was studied in the first part of the thesis. The investigation of the local vibrational modes of the silicon dopants in the GaAs host lattice allowed to identify the incorporation pathways the dopants take during the nanowire growth and to determine the limitations of the doping process due to compensation effects. Important information on the concentration and mobility of the free carriers in the doped material has been obtained by analyzing the coupled longitudinal optical phonon-plasmon modes. The second part of the thesis focused on the electronic band structure in wurtzite GaAs. Here, resonant Raman scattering from first and second order longitudinal optical phonons was used in oder to determine the band-gap, the position of the light-hole band as well as the temperature dependence of the crystal-field split-off band. As important parameters the spin-orbit coupling and crystal-field splitting have been obtained. The photoexcited electron-hole plasma and the confined optical phonons in nanowire-based GaAs/AlAs multi-quantum-well structures were studied in the last part of the thesis. Structural parameters could be deduced from the position of the confined phononmodes. The coupling of the plasmon to the phonon gives important information on the system of photogenerated carriers within the quantum structure

Phonon confinement and plasmon-phonon interaction in nanowire based quantum wells

Resonant Raman spectroscopy is realized on closely spaced nanowire based quantum wells. Phonon quantization consistent with 2.4 nm thick quantum wells is observed, in agreement with cross-section transmission electron microscopy measurements and photoluminescence experiments. The creation of a high density plasma within the quantized structures is demonstrated by the observation of coupled plasmon-phonon modes. The density of the plasma and thereby the plasmon-phonon interaction is controlled with the excitation power. This work represents a base for further studies on confined high density charge systems in nanowires

Compensation mechanism in silicon-doped gallium arsenide nanowires

P-type gallium arsenide nanowires were grown with different silicon doping concentrations. The incorporation is monitored by Raman spectroscopy of the local vibrational modes. For Si-concentrations up to 1.4 x 10(18) cm(-3), silicon incorporates mainly in arsenic sites. For higher concentrations, we observe the formation of silicon pairs. This is related to the Coulomb interaction between charged defects during growth. An electrical deactivation of more than 85% of the silicon acceptors is deduced for nominal silicon concentration of 4 x 10(19) cm(-3). This work is important to understand the limiting mechanisms of doping in compound semiconductor nanowires

Determination of the bandgap and split-off band of wurtzite GaAs

GaAs nanowires with a 100% wurtzite structure are synthesized by the vapor-liquid-solid method in a molecular beam epitaxy system, using gold as a catalyst. We use resonant Raman spectroscopy and photoluminescence to determine the position of the crystal-field split-off band of hexagonal wurtzite GaAs. The temperature dependence of this transition enables us to extract the value at 0 K, which is 1.982 eV. Our photoluminescence excitation spectroscopy measurements are consistent with a band gap of GaAs wurtzite below 1.523 eV

In(Ga)As quantum dot formation on group-III assisted catalyst-free InGaAs nanowires

Growth of GaAs and InxGa1-xAs nanowires by the group-III assisted molecular beam epitaxy growth method on (001)GaAs/SiO2 substrates is studied in dependence on growth temperature, with the objective of maximizing the indium incorporation. Nanowire growth was achieved for growth temperatures as low as 550 degrees C. The incorporation of indium was studied by low temperature micro-photoluminescence spectroscopy, Raman spectroscopy and electron energy loss spectroscopy. The results show that the incorporation of indium achieved by lowering the growth temperature does not have the effect of increasing the indium concentration in the bulk of the nanowire, which is limited to 3-5%. For growth temperatures below 575 degrees C, indium rich regions form at the surface of the nanowires as a consequence of the radial growth. This results in the formation of quantum dots, which exhibit spectrally narrow luminescence

Untangling the Electronic Band Structure of Wurtzite GaAs Nanowires by Resonant Raman Spectroscopy

In semiconductor nanowires, the coexistence of wurtzite and zinc-blende phases enables the engineering of the electronic structure within a single material. This presupposes an exact knowledge of the band structure in the wurtzite phase. We demonstrate that resonant Raman scattering is a important tool to probe the electronic structure of novel materials. Exemplarily, we use this technique to elucidate the band structure of wurtzite GaAs at the Gamma point. Within the experimental uncertainty we find that the free excitons at the edge of the wurtzite and the zinc-blende band gap exhibit equal energies. For the first time we show that the conduction band minimum in wurtzite GaAs is of Gamma(7) symmetry, meaning a small effective mass. We further find evidence for a light-hole-heavy-hole splitting of 103 meV at 10 K

Low-Temperature Preparation of Tailored Carbon Nanostructures in Water

The development of low-temperature carbonization procedures promises to provide novel nanostructured carbon materials that are of high current interest in materials science and technology. Here, we report a "wet-chemical" carbonization method that utilizes hexayne amphiphiles as metastable carbon precursors. Nearly perfect control of the nanoscopic morphology was achieved by self-assembly of the precursors into colloidal aggregates with tailored diameter in water. Subsequent carbonization furnished carbon nanocapsules with a carbon microstructure resembling graphite-like amorphous carbon materials