Graphen auf Siliziumcarbid: elektronische Eigenschaften und Ladungstransport

Abstract

In this thesis the local electronic as well as charge transport properties down to atomic length scales of epitaxial graphene on the SiC(0001) surface are investigated. To achieve this, next to well known scanning probe techniques, the method of scanning tunneling potentiometry (STP) is applied at 6 K and under UHV-conditions for the first time. For this purpose, epitaxially grown graphene samples were prepared by resistive heating under UHV conditions and subsequently electrically contacted. The so-prepared samples are characterized by atomic force microscopy and low energy electron diffraction, showing a heterogeneous covered surface with mono- and bilayer graphene. In addition to the number of formed graphene layers the surface potential difference between mono- and bilayer graphene is determined under atmospheric conditions with the method of Kelvin probe force microscopy (KPFM). For transport experiments and future applications the contact resistivity between epitaxially grown graphene and the electrical contacts plays a major role. By applying spatially resolved potential measurements at the gold-graphene interface with the KPFM method we, for the first time, succeeded to estimate an upper boundary for the contact resistivity of ρ_c=1×10-6 Ωcm². The investigation of the epitaxially grown graphene samples with the method of scanning tunneling microscopy (STM) allows the unambiguous identification of mono- and bilayer graphene and their hexagonal atomic structure without lattice defects over a few 100 nm². The underlying interface layer shows a strongly disordered quasi-periodic structure with numerous trimer structures, which are also visible at mono- and bilayer graphene coverage. Monolayer graphene is electronically strongly inhomogeneous on atomic length scales. In the energy range of E_F±100 mV a significant number of energetically localised and spatially varying states can be identified. These states even lead to variations in the local density of states at the Fermi energy on areas of 5 nm². In terms of bilayer-graphene, the measured degree of variations turns out to be rather small. To analyse the electronic structure of graphene on energy scales around the Fermi energy, which is relevant for electron transport, the effect of thermovoltage in the tunnelling junction is exploited by the STP method. The spatial variation of the thermovoltage at estimated temperature differences of a few 10 to 100 K between tip and sample is in the order of a few 10 to 100 µV on mono- and bilayer graphene on the atomic scale as well as between mono- and bilayer graphene. Also it depends notably on the atomic properties of the used STM-tip. The high spatial and energetic resolution of this method makes it possible to analyse scattering mechanisms as intra- and intervalley scattering and demonstrates in contrast to previous assumptions, that also bilayer graphene is electronically strongly influenced by the interface layer. The distinct local electronic disorder of the samples at the Fermi energy is also reflected in the transport experiments with the STP method. Significant voltage drops are observed on mono- and bilayer graphene surface and on localised defects like monolayer-monolayer and monolayer-bilayer junctions. The measured spatial potential distribution can be described well by a classical Ohmic transport model with specific resistivities for different defect types. Averaging over different sample areas, mono- and bilayer graphene have a virtually identical mobility of ~1000 cm²/Vs and a corresponding averaged mean free path of ~40 nm at 6 K. These values are well below theoretically expected values for a defect free graphene surface. Furthermore, the influence of thermovoltage is investigated in the course of the transport studies, which is - in this case - an undesired side effect because the thermovoltage is in the same order of magnitude as the measured variations in the local electrochemical potential due to the lateral electron flow through the surface. This can lead to a faulty analysis of voltage drops. Yet, as it is demonstrated in terms of the experimental resolution the thermovoltage can be eliminated for measurements of the electrochemical potentials with opposite macroscopic current directions. Moreover, the distribution of the electrochemical potential in the vicinity of monolayer-bilayer and monolayer-monolayer boundaries is investigated. The potential drops are localized below half of the Fermi wavelength around the boundaries, whereas related to the topography the potential distribution shows a lateral shift in direction to bilayer graphene. A combined scattering mechanism of local changes of electron doping and wave function mismatch at the boundary of a monolayer‐bilayer junction is suggested as the cause for the observed lateral shift.