Scanning tunneling potentiometry at nanoscale defects in thin films

The present thesis deals with electrical transport measurements performed with a four-tip scanning tunneling microscope (STM) setup. Samples are contacted in a four-probe geometry to exclude contact resistances and allow precise measurement of the sample resistance. To realize the four-probe geometry in the experiments, two of the four STM tips are contacted to the sample surface to inject a lateral current into the sample. The resulting voltage drop across the sample is measured with the remaining two tips which operate as voltage probes. The positioning of the individual tips is monitored by use of a scanning electron microscope. Using the scanning tunneling potentiometry (STP) technique the voltage drop across the sample can further be resolved locally with resolution down to Å and µV, respectively. This technique was implemented into the present setup and is based on a voltage probe which is scanned across the sample surface. It measures the sample topography and potential at each scan point quasi-simultaneously and thereby allows a detailed analysis of the local transport properties of the sample with respect to its topographic features. Based on this technique, a further measurement method was developed and implemented into the four-tip STM which allows non-invasive four-probe measurements. A major advantage of the resulting experimental setup is the possibility of combining different surface analysis tools such as electron diffraction and photoelectron spectroscopy with transport measurements on different length scales. The setup was applied to topological insulator thin films, bismuth thin films and different reconstructions of the Si(111) surface. At the surface of a BiSbTe3 topological insulator thin film three different kinds of defects are found to result in a local voltage drop: Step edges, domain boundaries and void defects. Using STP, the resistance of the domain boundaries is determined to be almost four times larger than that of the step edges and thereby the dominant defect induced contribution to the transport. By gate-dependent four-probe measurements, it is shown that the carrier concentration and mobility of the topological surface states can be deduced from the measurements by application of a suitable transport model. Furthermore, the undesired influence of the BiSbTe3 bulk on the conduction through the topological insulator thin-film is found to be small. Another undesired transport channel of BiSbTe3 thin film samples can occur at the substrate interface as a result of the epitaxial growth of the films. An initial Te passivation of the Si(111) substrate is found to result in a Te/Si(111)-(1×1) termination, the conductivity of which is however found to be low compared to the typical conductivity of the dominant transport channels. Application of STP to Bi thin films, transport dipoles are found to be located at voids in the film. The measurement of different sizes of defects in theory allow to characterize the crossover between classical and quantum transport in the system and application of a respective model allows to extract the charge carrier mean free path and the Fermi wave vector from the measurements. For the detailed analysis of STP data, resistor network calculations prove to be an important tool throughout the thesis with the ultimate goal to determine resistivity profiles directly from measurements of the potential