Thermoelectric measurements on InAs nanowires with a ratchet-barrier

Abstract: Efficient extraction of thermal energy from electrons at the micro- and nanoscale is a long-standing scientific goal that could enable novel types of heat engines, heat management, and solar cell applications. Semiconductor nanowires are a promising system for investigating such devices for several reasons, such as their thermoelectric properties, and the potential to be grown in heterostructures with high flexibility. This provides great potential to tune the nanowire properties to suit specific applications, for example to generate band structures that act as filters to extract charge carriers with specific energies. Such energy filters have previously been investigated in nanowires, including rectangular thermionic barriers and quantum dots, but have been limited in either conversion efficiencies or power output. It is therefore of interest to investigate other systems to function as energy filters. For example, the influence of the shape of a thermionic barrier on thermionic current extraction has not been explored in-depth. In order to expand the understanding of how to most effectively extract energy from energetic electrons, this master thesis aims to investigate whether an asymmetrically shaped thermionic barrier leads to an asymmetric thermoelectric response. For this purpose, thermoelectric measurements have been performed on indium arsenide (InAs) nanowires with a compositionally graded indium arsenide phosphide (InAs(1−x)P(x)) segment, forming an asymmetrically shaped potential barrier, using a top-heater architecture for applying temperature gradients along the nanowire. Thermoelectric currents were successfully generated as a result of a temperature gradient along the nanowire at ambient temperatures of 77 K and 300 K. At an ambient temperature of 77 K, asymmetric thermoelectric behaviour was demonstrated, with a generation of higher open circuit voltage when heating on one side of the barrier compared to heating on the other. This was proposed to be a result of the asymmetry of the barrier, and an indication that the shape of the barrier matters thermionic current extraction. The experimentally obtained results were compared to a theoretical model based on the WKB approximation, which captured some of the key features of the experimental results. At an ambient temperature below 1 K, indications quantum dot formation could be seen, further complicating the thermoelectric characterization and therefore such measurements were not further pursued.