Impact of surface modification on charge carrier transport in axial GaAs nanowire structures

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The preparation and analysis of complex nanowire (NW)-based electronic and optoelectronic devices has gained increasing relevance in recent years. However, preparation and precise determination of abrupt charge separating homo- and heterojunctions in NWs and their characterization is challenging. In contrast to planar structures, there is a lack of suitable methods for high-resolution charge carrier transport measurements. Typically, transfer line measurements are performed, which entail special preparation and possess limited resolution. Also, only NWs which have been exposed to ambient air can be investigated, implying a modified surface state density and hence a different surface potential, which also affects the conductivity. The present proposal aims at high-resolution investigation of charge carrier transport in axial NW structures. Applying an ultrahigh vacuum (UHV)-based multi-tip scanning tunneling microscope (MT-STM) enables the measurement of charge carrier transport and hence doping profiles of individual free-standing NWs with high spatial resolution. In addition, a UHV transfer system enables a contamination-free sample transfer between the established NW preparation apparatus (a metalorganic vapor phase epitaxy system allowing for vapor-liquid-solid (VLS) NW growth) and the MT-STM, such that as-grown NWs can be electrically and optoelectronically characterized in-vacuo.The first work package focuses on the investigation of modified charge carrier transport in NWs with different surface termination. For this purpose, intrinsic gallium arsenide NWs are prepared by established preparation routes. After UHV transfer, resistance profiles of individual NWs are recorded by the MT-STM. Subsequently, their surface is modified in a separate vacuum chamber by applying water vapor, oxygen or ambient air. Subsequent MT-STM measurements of the thus modified NWs will elucidate the impact of the surface modification on charge carrier transport. Additionally, temperature dependent measurements will enable the detailed investigation of the charge transport mechanism.In parallel, supporting simulations of the charge carrier mechanism in NW structures are performed. The simulations aim at identifying the parameters which crucially influence the charge carrier transport, taking into account doping, surface charges, junction properties and charge carrier depletion.In the second work package, the obtained findings are applied to NWs with axial pn-junction. Subsequent to growth and measurement of axial homo junctions, the impact of applying different surface modification is to be investigated. MT-STM measurement in dependence of illumination should qualitatively reveal the influence of surface potentials.