In this work the electrical transport properties of individual gallium nitride (GaN) nanowires are investigated. For the first time, the effect of germanium as a shallow donor in such nanostructures was investigated.The growth of GaN nanowires on silicon (111) substrates is performed by molecular beam epitaxy in a self-assembled process without catalyst. Depending on doping concentration and growth time, the nanowires have a diameter between 30 nm and 120 nm and a length of 1 micron to 2 microns.For the electrical characterization of individual nanowires a process of micro- and nano-patterning, based on a combination of photo- and electron-beam lithography, was developed to realize individually designed, few nanometers wide and precisely positioned metal contacts. Individual nanowires could be contacted in various geometries to address the requirements of specific measurements. For example, nanowires with only 1 micron length were provided with four and 2 microns long wires even with six individual contacts. Very high overlay accuracy was realized with a deviation between layout and result of less than 10 nm.Individually contacted nanowires were investigated in several ways. By four-wire measurements, which exclude the impact of contact resistance, the electrical conductivity could be determined with the geometry of the specific nanowires being considered after analysis by scanning electron microscopy. The comparison with two-point measurements revealed the impact of the contact resistance. For nominally undoped nanowires both highly resistive and conductive wires were identified. For the latter, the enhanced conductivity at the nanowire base could be assigned to diffusion of silicon from the silicon substrate. For Ge-doped nanowires a continuous increase in conductivity with increasing dopant concentration was observed. Temperature-dependent measurements showed small activation energies and an almost constant conductivity at low temperatures. For the highest conductivity even metallic behavior was observed.An analysis of the temperature-dependent thermoelectric properties of the nanowires allowed the determination of the free carrier concentration. The extracted charge carrier densities of up to 5.5×10^19/cm^3 are well above the metal-insulator-transition for GaN of 2×10^18/cm^3. Photoluminescence spectroscopy of individual nanowires showed a weak increase of the width of the emission but no spectral shift with increasing carrier concentration. By combining the results of conductivity measurements and carrier-density determination, the mobility of the carriers could be estimated. Combination of the different results demonstrates the efficiency of germanium as a shallow donor in GaN nanowires.
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