Electrical transport in semiconductor nanowires

Semiconductor nanowires are promising building blocks for future nanoscale electronic devices. A fundamental control of the impurity and free-carrier concentration as well as the understanding of charge injection and extraction is required. This thesis describes numerical and experimental studies on the electrical transport in semiconductor nanowires. We present a numerical study on geometric scaling of space-charge-limited current, which is often observed in semiconductor nanowires due to carrier depletion and reduced electrostatic screening. The model highlights the effects of the surroundings for nanowires and shows that the dielectric properties of the semiconductor have a negligible effect on the space-charge-limited transport for small dimensions. The results of numerical calculations agree with a simple capacitance formalism which assumes a uniform charge distribution along the nanowire, and experimental measurements for InAs nanowires confirm these results. We discuss the elemental composition and electrical transport characteristics of nominally-undoped and Ga-doped ZnO nanowires, a promising candidate for optoelectronic applications in the UV range. We estimate an upper limit of the Ga impurity concentration with atom-probe tomography and present the electrical transport characteristics measured with a nanoprobe technique and with lithographically-defined contacts allowing back-gated measurements. An increase in apparent resistivity by two orders of magnitude and drop in the effective carrier concentration and mobility was found. Little change in resistivity was observed with Ga doping, which indicates that the concentration of native or background dopants was higher than the Ga doping concentration. We investigate the electrical properties of undoped, Si-doped and Mg-doped InN nanowires directly on degenerate n-type and p-type Si substrates, with a nanoprobe technique. The resulting transport characteristics are weakly rectifying for InN grown on n+-Si with similar ratios for all InN dopant types. On p+-Si, Mg-doped InN nanowires show a strong rectification behaviour with opposite voltage polarity compared to n+-Si, while undoped and Si-doped nanowires show nearly symmetric transport. These characteristics are analyzed in terms of the properties of broken gap band offsets at the Si/InN heterojunction.