Infoscience

Thesis

Synthesis and characterization of group II-Vi semiconductor nanowires

Semiconductor nanowires are an emerging class of nanostructures that represent attractive building blocks for nanoscale electronic and photonic devices. To the present, nanowires are synthesized on a small scale by experimentally demanding gas phase deposition techniques. This thesis describes a simple and cost-efficient solution-based synthesis towards high-quality nanowires of Group II-VI compound semiconductors. The effects of various reaction parameters and of doping on the wires' structural, electrical and magnetic properties are investigated. The first part of this thesis is dedicated to a novel variant of the solvothermal synthesis of CdS nanowires employing a single-source precursor. The wires are of single-crystalline quality and can be indexed to wurtzite-type bulk CdS with the preferential growth direction along the <001> axis. As-grown nanowires have a smooth surface, are up to 40 µm long and exhibit aspect ratios up to 1000. The wires' aspect ratio can be increased by choosing a low precursor concentration. Time-dependent experiments reveal the wires to axially grow at a rate of ≈1µm/h while lateral growth only occurs during the initial reaction phase. As shown by photoluminescence spectroscopy the surface defect density can be significantly lowered by raising the reaction temperature from 180 to 200 °C. Electrically contacted CdS nanowires are highly resistive in the dark, but become more conductive by 4 orders of magnitude upon illumination above the bandgap energy (λ<505 nm). Field effect transistor devices (FET) built from individual CdS nanowires show the formation of Schottky barriers and n-channel enhancement type behavior with poor on/off ratios of ≈15. Spatially resolved photocurrent measurements on single nanowire FETs reveal a strongly localized photocurrent response in the vicinity of the lower biased contact. A qualitative explanation for this effect can be given by assuming a back-to-back arrangement of Schottky diodes and by considering different majority and minority carrier mobilities, such that the total photocurrent is limited by the minority carrier diffusion length. The second part addresses the tailoring of the electrical and magnetic properties of CdS nanowires by doping with indium and manganese, respectively. The addition of small amount of In(III) salts (<0.1 mol%) prior to solvothermal reaction yields single-crystalline and high-aspect ratio nanowires, while for higher In amount the wires' aspect ratio drastically decreases. The conductivity of highly In-doped CdS wires (0.01 mol% In) is increased by one order of magnitude compared to undoped CdS wires, which can be directly attributed to the raised majority carrier concentration of the doped wires. Similarly, single-crystalline wurtzite type Cd1-xMnxS nanowires with x<0.15 can be grown by adding Mn(II) salts. Magnetic studies on Cd1-xMnxS nanowires with x<0.15 show the antiferromagnetic Mn–Mn interactions to be weaker than in the bulk material. The difference can be attributed to an effective reduction of nearest neighbor Mn–Mn interactions on the wire surface, which is caused by the wires' high surface/volume ratio. In the last part of this thesis, the hydrothermal growth of single-crystalline wurtzite type ZnO nanowires and their use in FETs devices is demonstrated. Up to 30 µm long nanowires can be directly grown from alkaline solution on a zinc foil substrate when using the additional oxidant ammonium peroxosulfate. ZnO nanowire FET devices display n-type behavior, a high conductivity and poor gate switching characteristics with an on/off ratio ≈3. Post-growth annealing at 600 °C in air leads to significantly improved on/off ratios of ≈106 and reduces the conductivity by two orders of magnitude. The annealing effect can be explained by a reduction of point defects, namely oxygen vacancies.

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