Intrinsische Quantenpunkte in InAs-Nanodrähten

This work deals with InAs nanowire field effect transistors in back gate configuration. In such devices, quantum dots can form at low temperatures in the order of magnitude of a few Kelvin. These dots are henceforth referred to as intrinsic as they are not intentionally defined by electrodes. For the interpretation of their stability diagrams, a thorough knowledge of the structure and transport properties of the nanowires is required. Therefore, first of all, the influence of growth method and doping on the transport properties is studied at room temperature. The wires are grown by two types of metal-organic vapour phase epitaxy: a selective-area (SA-MOVPE) and an Au-catalyzed vapour-liquid-solid method (VLS-MOVPE). Transport data shows that the background doping of VLS-MOVPE wires is higher than for SA-MOVPE wires, but the variability of the transport properties is lower. The polytypism of the SA-MOVPE wires (they are composed of wurtzite and zinc blende segments) is a possible explanation for the second observation. Furthermore, it is shown that the measured transport properties significantly depend on the dielectric environment of the nanowires and on the way the electrical measurements are done (two- or four-terminal configuration). The conductivity is tunable via doping and the gate voltage. Conductivity measurements in the temperature range from 10 K to 300 K show that different transport regimes can occur (partially metallic behaviour for sufficiently high conductivity, otherwise purely semiconducting behaviour). This is attributed to different positions of the Fermi level and thus, a different effect of potential fluctuations. If conductivity and temperature are sufficiently low, the onset of Coulomb blockade is observed for semiconducting samples. It is even possible to tune the very same sample to different regimes via the gate voltage. The semiconducting behaviour observed in many samples contradicts the Thomas-Fermi theory. This is attributed to the transport not being governed by ionized impurities, but by potential fluctuations at the nanowire surface. Stability diagrams of differently doped SA-MOVPE wires are recorded at bath temperatures below 1.4 K and analyzed. Inter alia, they provide information on the number of quantum dots and if the latter are connected in series or in parallel. The single-particle energy spectrum and the excitation spectrum are examined. Typically, the quantum dot size is in the order of magnitude of 10 nm to 100 nm. Most probably, the barriers are formed by potential fluctuations at the nanowire surface. Finally, it is shown that via gate voltage and doping, the sample can be tuned to different regimes of Coulomb blockade