Ladungstransport durch Graphenschichten und GaAs-Nanodrähte untersucht mit einem Multispitzen-Rastertunnelmikroskop

This work describes the use of the combination of a scanning electron microscope (SEM) and a multitip scanning tunneling microscope (STM) with four tips as a nanoprober. Electrical measurements on graphene layers and freestanding gallium arsenide (GaAs) nanowires were conducted. Four-probe-measurements are necessary to measure the resisitvity of such one- and two-dimensional conductors. Due to unknown voltage drops at contacts that carry currents, additional contacts have to be employed for current-free potential measurements. Therefore, the multitip scanning tunneling microscope with its four individually controllable tips has been upgraded with extended electronics, enabling us to use it as a flexible nanoprober. Graphene layers on insulating SiO2 and hexagonal boron nitride (h-BN), prepared by mechanical exfoliation, were contacted with the multitip STM. Tunneling current could not be used as feedback when approaching the first tip. Therefore, a contrast change in the SEM image upon contacting a graphene flake with a tip was used. Once contacted, flakes were scanned by RTM and electrical measurements were conducted. Graphene transferred to h-BN showed bubbles, wrinkles and contaminations. Still, STM images of clean areas revealed a moiré pattern, proving that the atomically thin graphene lay flat on the atomically flat h-BN surface. Four point measurements of these samples showed a poor conductivity of 1=s = 16kW= and a low field effect mobility of m = 300cm2=Vs. The reason for this might be the contaminations from the transfer process, as well as effects from prolonged irradiation with electrons from the SEM. Freestanding p-doped GaAs nanowires, grown by metal-organic vapor-phase-epitaxy in the vapor-liquid-solid-growth mode, in a process with two temperature steps, were contacted with the multitip STM. Using three tips as well as the substrate as contacts, four point measurements were performed. It showed that elastic deformation of these flexible nanowires has no significant influence on their conductivity. The high spatial resolution of the combination of a SEM with a multitip STM made it possible to record resistance profiles of freestanding nanowires by performing four point measurements along a nanowire. The main segment of the nanowires, grown at 400C for better crystal quality exhibits a resisitivity of a few kW=m, in agreement with literature values. The nanowire base, grown at 450C to facilitate better nucleation, shows an increased resisitvity of several MW=m. The resistance of the nanowire base is relevant especially for future opto-electronical components based on freestanding nanowires and thus has to be understood. Comparing profiles of nanowires grown by an identical process on different substrates showed that the substrate is not the cause of the increased resistance. From the measured resistivities the dopant concentrations, as well as the thickness of the space charge layer at the surface of the GaAs nanowires were calculated. The nanowire segments grown at 400C have a dopant concentration of roughly 1019cm??3, those grown at 450C about 21017 cm??3. In the base the space charge layer poses a considerable constriction to the conduction. A qualitative explanation for the temperature dependence of the dopant concentration is given.