Willke, Philip

English

In this thesis scanning probe techniques are used to study the transport properties of epitaxial-grown graphene on SiC on a local scale. It moreover focusses on the role of one-dimensional growth defects such as steps and interfaces as well as substitutional atomic species introduced by low-energy ion implantation.
In the first result part of this thesis we use scanning tunneling potentiometry (STP) and Kelvin probe force microscopy (KPFM) to investigate the transport properties of localized defects in graphene. We present an STP-study focusing on the local voltage drop at these graphene monolayer-bilayer junctions. Here, we use STP to show that the voltage drop at this particular defect is not located strictly at its topographic position, but extends spatially up to a few nanometers into the bilayer side. Additionally, different scattering centers of the junction can be disentangled. Thus, we can show that the exact location of the voltage drop with respect to the defect gives additional insight into the underlying scattering mechanism.
In macroscopic transport experiments the electrical resistance of a sample is usually measured as a function of an external parameter like the charge carrier concentration, a magnetic field or temperature, since a scattering mechanism often leaves a unique fingerprint as a function of such parameters. Measuring defect resistances locally as a function of temperature or magnetic field is another subject of this thesis. Using KPFM, we study the variation of the local sheet resistance of graphene on SiO2 under ambient conditions and as a function of temperature in a range of 20° C – 100° C. Additionally, we resolve the defect resistance of a folded wrinkle for which a temperature-independent model yields the best fit to the data. Thus, we suggest a scattering mechanism due to the interlayer tunneling between graphene layers, different from transport on the pristine graphene sheets. In addition, we introduce a new magnetic field STP setup. We study the local sheet resistance and defect resistance as a function of magnetic field up to 6 T. We are able to extract the charge carrier concentration locally evaluating the change in electric fields similar to the macroscopic Hall effect. We find the resistance of localized defects such as steps and monolayer-bilayer junctions along with their respective underlying scattering mechanisms to be independent on the magnetic field. 
In the second part of this thesis we investigate the properties of substitutional doping atoms in graphene. Defects can be used for electronic band engineering, in particular via atomic doping. Foreign atoms have been used in the past already to change the charge carrier concentration in graphene. However, while the charge carrier concentration is in this way nicely tunable, the presence of additional scatterers reduces the electric conductivity. In a first study we introduce doping with single nitrogen atoms in graphene. In this structural analysis, we use STM topography measurements to investigate substitutional nitrogen implanted by low-energy ion implantation. We find that only 10% of the ions get implanted, most likely due to adsorbate layers present during the ion bombardment. Moreover, we find that they are not randomly distributed, but observe a short-range ordering triggered by the 6x6-corrugation emerging from the underlying substrate. In a second study we investigate the influence of single boron, nitrogen and carbon atoms in graphene by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and additionally their influence on transport. We find by using STS that incorporation of nitrogen and boron atoms leads to effective doping of the graphene sheet and reduces or raises the position of the Fermi level. Additionally, the influence of foreign atoms and defects is investigated in macroscopic transport experiments. While for all samples the sheet resistance increases compared to pristine graphene, this effect is especially pronounced for samples with lattice defects (e.g. vacancies) and less for dopant atoms only. The positive magnetoresistance of pristine graphene changes to a strong negative one for ion-implanted samples by the effect of weak localization

Georg-August-University Göttingen

Atomic-scale transport in graphene: the role of localized defects and substitutional doping

https://ediss.uni-goettingen.de/bitstream/11858/00-1735-0000-0023-3F90-1/3/Diss_PW.pdf

Atomic-scale transport in graphene: the role of localized defects and substitutional doping

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Georg-August-University Göttingen