The effect of Si-doping on the morphology, structure, and transport properties of nanowires was investigated. The nanowires were deposited by selective-area metal organic vapor phase epitaxy in an N 2 ambient. It is observed that doping systematically affects the nanowire morphology but not the structure of the nanowires. However, the transport properties of the wires are greatly affected. Room-temperature four-terminal measurements show that with an increasing dopant supply the conductivity monotonously increases. For the highest doping level the conductivity is higher by a factor of 25 compared to only intrinsically doped reference nanowires. By means of back-gate field-effect transistor measurements it was confirmed that the doping results in an increased carrier concentration. Temperature dependent resistance measurements reveal, for lower doping concentrations, a thermally activated semiconductor-type increase of the conductivity. In contrast, the nanowires with the highest doping concentration show a metal-type decrease of the resistivity with decreasing temperature.

We report on the fabrication and measurements of planar mesoscopic Josephson junctions formed by InAs nanowires coupled to superconducting Nb terminals. The use of Si-doped InAs-nanowires with different bulk carrier concentrations allowed to tune the properties of the junctions. We have studied the junction characteristics as a function of temperature, gate voltage, and magnetic field. In junctions with high doping concentrations in the nanowire Josephson supercurrent values up to 100 nA are found. Owing to the use of Nb as superconductor the Josephson coupling persists at temperatures up to 4 K. In all junctions the critical current monotonously decreased with the magnetic field, which can be explained by a recently developed theoretical model for the proximity effect in ultra-small Josephson junctions. For the low-doped Josephson junctions a control of the critical current by varying the gate voltage has been demonstrated. We have studied conductance fluctuations in nanowires coupled to superconducting and normal metal terminals. The conductance fluctuation amplitude is found to be about 6 times larger in superconducting contacted nanowires. The enhancement of the conductance fluctuations is attributed to phase-coherent Andreev reflection as well as to the large number of phase-coherent channels due to the large superconducting gap of the Nb electrodes.

We report on the conditions necessary for the electrical injection of spin-polarized electrons into indium nitride nanowires synthesized from the bottom up by molecular beam epitaxy. The presented results mark the first unequivocal evidence of spin injection into III-V semiconductor nanowires. Utilizing a newly developed preparation scheme, we are able to surmount shadowing effects during the metal deposition. Thus, we avoid strong local anisotropies that arise if the ferromagnetic leads are wrapping around the nanowire. Using a combination of various complementary techniques, inter alia the local Hall effect, we carried out a comprehensive investigation of the coercive fields and switching behaviors of the cobalt micromagnetic spin probes. This enables the identification of a range of aspect ratios in which the mechanism of magnetization reversal is single domain switching. Lateral nanowire spin valves were prepared. The spin relaxation length is demonstrated to be about 200 nm, which provides an incentive to pursue the route toward nanowire spin logic devices.

Ohmic contacts to GaAs/AlGaAs core/shell nanowires are prepared by using a Ni/AuGe/Ni/Au layer system. In contrast to Ohmic contacts to planar GaAs/AlGaAs layer systems here, relatively low alloying temperatures are used in cylindrical geometry. Lowest resistances are found for annealing temperatures of 320 °C and 340 °C. For annealing temperatures exceeding 360 °C, the nanowires degraded completely. Nanowires annealed under optimized conditions preserved their Ohmic characteristics even down to temperatures of 4 K.

Comprehensive electrical transport studies are performed on InSb nanowires by varying temperature, gate voltage, and magnetic field. The 3-dimensional bulk conduction is found to dominate in the nanowire channel after investigating a large number of nanowires with different diameters, which show approximately a linear relation between the conductance normalized to the length and the wire cross section. At low temperatures, universal conductance fluctuations are observed. From the amplitude and the correlation voltage of the conductance fluctuations, the phase-coherence length in InSb nanowires is determined at various temperatures. V

Back-gated InAs nanowire field-effect transistors are studied focusing on the formation of intrinsic quantum dots, i.e. dots not intentionally defined by electrodes. Such dots have been studied before, but the suggested explanations for their origin leave some open questions, which are addressed here. Stability diagrams of samples with different doping levels are recorded at electron temperatures below 200 mK, allowing us to estimate the number and size of the dots as well as the type of connection, i.e. in series or in parallel. We discuss several potential physical origins of the dots and conclude that they are most probably induced by potential fluctuations at the nanowire surface. Additionally, we show that via gate voltage and doping, the samples can be tuned to different regimes of Coulomb blockade.

We investigated the suitability of lanthanum lutetium oxide (LaLuO 3 ) as a gate dielectric by fabricating InAs nanowire field-effect transistors. The LaLuO 3 layer was deposited by employing pulsed laser deposition. On transistors with a 1.6 μm long gate, a maximum transconductance of 11 μS at a source-drain bias voltage of 0.5 V was measured, while the threshold voltage had a value of −4.5 V. Owing to the complete coverage of the InAs nanowire by the LuLuO 3 layer no significant leakage current was found. On a transistor with a 240 nm long gate short-channel effects were observed. The transfer characteristics showed a hysteretic behavior, which is attributed to charging of states at the InAs/LaLuO 3 interface. We found that the threshold voltage gets reduced considerably when the temperature was decreased to 25 K. At this temperature the hysteresis in the transfer characteristics showed no dependence on the sweep rate.

The conductivity and crystal structure of nominally undoped InAs nanowires deposited by three different methods – 1. selective area metal organic vapor phase epitaxy (SA MOVPE), 2. gold assisted vapor liquid solid (VLS) MOVPE and 3. extrinsic catalyst free VLS molecular beam epitaxy (MBE) – is investigated. The influence on conductivity by stacking faults and different growth conditions is analyzed to determine the main impact. It is found that in terms of crystal structure, nanowires deposited by VLS MOVPE and VLS MBE behave similarly showing a zinc blende (ZB) phase while nanowires deposited by SA MOVPE feature a high density of stacking faults and a tendency to higher amounts of wurtzite (WZ) when grown with a decreased growth rate. However, the conductivity of wires deposited by VLS MOVPE is found to be much higher and statistically less dispersive compared to the other two wire types. An electrical similarity between nominally undoped wires in VLS MOVPE and previously reported intentionally doped wires in SA MOVPE is observed and discussed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)