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

近年来，基于复合纳米线（NW）的电子和光电器件的制备和分析获得了越来越多的相关性。然而，制备和精确测定突然的电荷分离的同质和异质结的纳米线及其表征是具有挑战性的。与平面结构相反，缺乏用于高分辨率电荷载流子输运测量的合适方法。通常，执行传输线测量，这需要特殊的准备并且具有有限的分辨率。此外，只有已经暴露于环境空气中的纳米线可以被研究，这意味着修改的表面态密度，因此不同的表面电位，这也影响电导率。本建议的目的是在高分辨率的调查，在轴向NW结构的电荷载流子输运。应用基于超高真空（UHV）的多尖端扫描隧道显微镜（MT-STM）能够测量电荷载流子输运，从而以高空间分辨率测量单个独立纳米线的掺杂分布。此外，一个超高真空传输系统，使无污染的样品之间的转移建立NW制备装置（一个金属有机气相外延系统，允许气-液-固（VLS）NW生长）和MT-STM，这样生长的NWs可以在vacuum.The第一个工作包的特点是改性的电荷载流子输运与不同的表面终止在NWs的调查。为此目的，本征砷化镓纳米线通过已建立的制备路线制备。在特高压传输后，MT-STM记录了单个纳米线的电阻分布。随后，在单独的真空室中通过施加水蒸气、氧气或环境空气对其表面进行改性。随后的MT-STM测量，从而修改的纳米线将阐明的影响的表面改性的电荷载流子输运。此外，温度依赖的测量将使详细的调查电荷传输mechanism.In并行，支持模拟的电荷载流子机制在NW结构进行。模拟的目的是确定的参数，其中至关重要的影响的电荷载流子输运，考虑到掺杂，表面电荷，结的属性和电荷载流子depletion.In第二个工作包，所获得的研究结果被应用到与轴向pn结的纳米线。在轴向同质结的生长和测量之后，将研究应用不同表面改性的影响。依赖于照明的MT-STM测量应定性地揭示表面电位的影响。

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.