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978-3-8439-1743-8, Reihe Physik
Thermal and Electronic Transport through Nanosized GaAs Pillars
131 Seiten, Dissertation Universität Hamburg (2014), Softcover, A5
The thermal and electronic transport through a novel type of semiconductor point contacts is studied. The contacts are realized as epitaxial GaAs pillars that connect two epitaxial GaAs layers. They are fabricated with molecular beam epitaxy using a combination of conventional layer deposition, material removal by in situ local droplet etching and a subsequent filling of the droplet etched holes. Because of the epitaxial growth, the pillars connect the two GaAs layers with perfect, defect free interfaces. They have diameters of about 100 nm, as well as lengths of only a few nanometers that are precisely adjustable during the growth. Because of the short length and absence of defects, the pillars are considered as point contacts between 3D reservoirs.
In the first part of this thesis, the thermal transport through the pillars is studied. The pillars opends an air gap between the GaAs layers. Because of the good thermal isolation by the air-gaps, large temperature gradients are realizable along the pillars. The temperature dependence of the thermal conductance is measured in a temperature range from 20-300 K using the so-called 3 $\omega$ method. To explain the data, a simple model is applied, which considers the pillars as ballistic point-contacts between three-dimensional phonon reservoirs. Good agreements between model and experimental data are found. The transport through the pillars is thus assumed to be ballistic nearly up to room temperature. Moreover, the influences of sample geometry, growth conditions, and ambient conditions on the thermal transport were investigated.
The second part of this thesis focuses on the electronic transport through pillars, which are embedded in an AlGaAs barrier along their circumference. Pillars with various lengths and doping concentrations are studied. The results are compared to reference samples containing tunnel barriers without pillars. The current-voltage characteristics of the pillars are tentatively explained with a tunneling transport. Thereby, we assume that the barrier height is determined by the confinement in the pillars. Distinctive asymmetries in the current-voltage characteristic are associated with the conical shape of the pillars. The differential conductivity of doped pillars shows oscillating features that are tentatively explained with zero-dimensional states of the donator atoms. In addition, magneto transport experiments also indicate current carrying states in the pillars.