Growth and characterisation of group-III nitride-based nanowires for devices

One of the main goals of this thesis was to get more insight into the mechanisms driving the growth of nitride nanowires by plasma-assisted molecular beam epitaxy (PA-MBE). The influence of the group-III and group-V flux as well as the substrate temperature Tsub has been studied leading to the conclusion that the III-V ratio determines the growth mode. N-rich conditions lead to nanowire growth and Tsub has an important influence. For GaN an increase of Tsub enhances the Ga desorption, thus lowering the III-V ratio, whereas for InN higher temperatures lead to a higher N evaporation and increases the III-V ratio. Ga desorption limits the temperature range to grow GaN nanowires and dissociation of InN is the limiting factor for InN nanowire growth. A reduction of the surface diffusivity on polar surfaces under N-rich conditions explains the anisotropic growth. Growth kinetics of the nanowires show that there are two important contributions to the growth. The first is growth by direct impingement and its contribution is independent of the nanowire diameter. The second contribution comes from atoms, which absorb on the substrate or wire sidewalls and diffuse along the sidewalls to the top of the wire, which acts as an effective sink for the adatoms due to a reduced surface mobility on the polar top of the wires. This diffusion channel, which is enhanced at higher Tsub, becomes more significant for smaller wire diameters, because its contribution scales like 1/d. Experiments with an interruption of the growth and sharp interfaces in TEM images of heterostructures show that the suggestion in literature of a droplet-mediated PA-MBE nitride growth has to be discarded. An accumulation or depletion of group-III adatoms during growth can lead to an increase or a reduction of the wire diameter. This can be compensated by changing one of the fluxes or the substrate temperature to produce wires with a uniform diameter. Despite a thin amorphous silicon nitride wetting layer on the substrate surface, both GaN and InN nanowires grow in the wurtzite structure and epitaxially in a one-to-one relation to the Si(111) substrate surface. There is no evidence for cubic phases. TEM images and optical studies display a high crystalline and optical quality of GaN and InN nanowires. Lattice constants determined from TEM images agree well with literature values for strain-free GaN and high-quality inn. The substrate induces some strain in the bottom part of the nanowires, especially in InN due to the lower Tsub than for GaN, which is released without the formation of dislocations. Only some stacking faults sometimes form at the base of the wires. Optical spectra also exhibit a band gap consistent with strain-free GaN in the upper part of the wires. For InN a band gap value of 0.7-0.8 eV has been determined at low temperatures in agreement with recent literature reports for InN of high crystalline quality. The doping concentration and the position of the Fermi-level in InN depend very much on the crystalline quality. In general the luminescence of GaN and InN improves with increased Tsub. For GaN the defect-related peaks decrease and in InN the carrier concentration is reduced, which changes the shape of the PL spectrum. Si- and Mg-doping can change the morphology of GaN nanowires, but the growth parameters can be chosen to produce wires with uniform diameter. Optical spectra confirm the effective incorporation of the dopant species in the nanowires despite a large nanowire surface and unfavourable growth conditions for doping, which are required for nitride nanowire growth. GaN and InN nanowires display a high mechanical and chemical resistance against common processing steps in spite of a high aspect ratio and large free surface. Producing contacts to the wires by e-beam lithography allows to determine the electrical behaviour of GaN, InN and Si-doped GaN nanowires. Fermi-level pinning due to surface states has a major influence on all nanowires especially for diameters below 100 nm. For GaN the pinning induces a depletion region, which depends on the doping concentration. The conductivity for small diameters is low for undoped GaN, but can be significantly enhanced by UV illumination or Si-doping. The diameter dependence of the photocurrent displays a rapid drop below a critical radius equal to the extension of the depletion layer and the wires show a fast photoresponse in this region caused by a lowering of the barrier for surface recombination. The carrier concentration of GaN nanowires can be determined from the critical radius and is enhanced by Si-doping. In InN nanowires the Fermi-level pinning in the conduction band induces a highly conductive surface accumulation layer and as a result the nanowires display very high currents several orders of magnitude higher than the GaN nanowires, independent of illumination. There is no pronounced diameter dependence