Mattias Kruskopf

Epitaxial graphene on SiC: modification of structural and electron transport properties by substrate pretreatment

The electrical transport properties of epitaxial graphene layers are correlated with the SiC surface morphology. In this study we show by atomic force microscopy and Raman measurements that the surface morphology and the structure of the epitaxial graphene layers change significantly when different pretreatment procedures are applied to nearly on-axis 6H-SiC(0 0 0 1) substrates. It turns out that the often used hydrogen etching of the substrate is responsible for undesirable high macro-steps evolving during graphene growth. A more advantageous type of sub-nanometer stepped graphene layers is obtained with a new method: a high-temperature conditioning of the SiC surface in argon atmosphere. The results can be explained by the observed graphene buffer layer domains after the conditioning process which suppress giant step bunching and graphene step flow growth. The superior electronic quality is demonstrated by a less extrinsic resistance anisotropy obtained in nano-probe transport experiments and by the excellent quantization of the Hall resistance in low-temperature magneto-transport measurements. The quantum Hall resistance agrees with the nominal value (half of the von Klitzing constant) within a standard deviation of 4.5 × 10(-9) which qualifies this method for the fabrication of electrical quantum standards.

Nonequilibrium mesoscopic conductance fluctuations as the origin of 1/f noise in epitaxial graphene

We investigate the 1/f noise properties of epitaxial graphene devices at low temperatures as a function of temperature, current and magnetic flux density. At low currents, an exponential decay of the 1/f noise power spectral density with increasing temperature is observed that indicates mesoscopic conductance fluctuations as the origin of 1/f noise at temperatures below 50 K. At higher currents, deviations from the typical quadratic current dependence and the exponential temperature dependence occur as a result of nonequilibrium conditions due to current heating. By applying the theory of Kubakaddi [S. S. Kubakaddi, Phys. Rev. B 79, 075417 (2009)], a model describing the 1/f noise power spectral density of nonequilibrium mesoscopic conductance fluctuations in epitaxial graphene is developed and used to determine the energy loss rate per carrier. In the regime of Shubnikov-de Haas oscillations a strong increase of 1/f noise is observed, which we attribute to an additional conductance fluctuation mechanism due to localized states in quantizing magnetic fields. When the device enters the regime of quantized Hall resistance, the 1/f noise vanishes. It reappears if the current is increased and the quantum Hall breakdown sets in.

Minimum Resistance Anisotropy of Epitaxial Graphene on SiC

We report on electronic transport measurements in rotational square probe configuration in combination with scanning tunneling potentiometry of epitaxial graphene monolayers which were fabricated by polymer-assisted sublimation growth on SiC substrates. The absence of bilayer graphene on the ultralow step edges of below 0.75 nm scrutinized by atomic force microscopy and scanning tunneling microscopy result in a not yet observed resistance isotropy of graphene on 4H- and 6H-SiC(0001) substrates as low as 2%. We combine microscopic electronic properties with nanoscale transport experiments and thereby disentangle the underlying microscopic scattering mechanism to explain the remaining resistance anisotropy. Eventually, this can be entirely attributed to the resistance and the number of substrate steps which induce local scattering. Thereby, our data represent the ultimate limit for resistance isotropy of epitaxial graphene on SiC for the given miscut of the substrate.

AC quantum Hall effect in epitaxial graphene

This paper describes measurements of the AC quantum Hall resistance in epitaxial graphene performed with a newly developed digitally assisted impedance bridge. Capacitive losses cause a negative frequency dependence of the quantum Hall resistance, in contrast to the positive frequency dependence observed in GaAs devices. The magnitude of the frequency dependence varies among individual graphene samples and can be significantly larger than in GaAs devices.

A morphology study on the epitaxial growth of graphene and its buffer layer

Abstract We investigate the epitaxial growth of the graphene buffer layer and the involved step bunching behavior of the silicon carbide substrate surface using atomic force microscopy. The results clearly show that the key to controlling step bunching is the spatial distribution of nucleating buffer layer domains during the high-temperature graphene growth process. Undesirably high step edges are the result of local buffer layer formation whereas a smooth SiC surface is maintained in the case of uniform buffer layer nucleation. The presented polymer-assisted sublimation growth method is perfectly suited to obtain homogenous buffer layer nucleation and to conserve ultra-flat surfaces during graphene growth on a large variety of silicon carbide substrate surfaces. The analysis of the experimental results is in excellent agreement with the predictions of a general model of step dynamics. Different growth modes are described which extend the current understanding of epitaxial graphene growth by emphasizing the importance of buffer layer nucleation and critical mass transport processes.

Infrared Nanospectroscopy of Phospholipid and Surfactin Monolayer Domains

A main challenge in understanding the structure of a cell membrane and its interactions with drugs is the ability to chemically study the different molecular species on the nanoscale. We have achieved this for a model system consisting of mixed monolayers (MLs) of the biologically relevant phospholipid 1,2-distearoyl-sn-glycero-phosphatidylcholine and the antibiotic surfactin. By employing nano-infrared (IR) microscopy and spectroscopy in combination with atomic force microscopy imaging, it was possible to identify and chemically detect domain formation of the two constituents as well as to obtain IR spectra of these species with a spatial resolution on the nanoscale. A novel method to enhance the near-field imaging contrast of organic MLs by plasmon interferometry is proposed and demonstrated. In this technique, the organic layer is deposited on gold and ML graphene substrates, the latter of which supports propagating surface plasmons. Plasmon reflections arising from changes in the dielectric environment provided by the organic layer lead to an additional contrast mechanism. Using this approach, the interfacial region between surfactin and the phospholipid has been mapped and a transition region is identified.

Magnetocapacitance and dissipation factor of epitaxial graphene Hall bars

We report measurements of the magneto-capacitance and the associated dissipation factor of graphene-based quantum Hall effect devices at frequencies in the kHz range. The main motivation for this work is yielding information for the modeling and optimization of quantum Hall resistance devices measured with alternating current, but here we focus on the characteristic ac properties of graphene and the relevant loss mechanism.

Dissipation factor and frequency dependence of graphene quantum Hall devices

Quantum Hall resistors made from epitaxial graphene measured with alternating current exhibit an unusual negative frequency dependence. To test the hypothesis that parasitic capacitances between contact pads cause this effect, we studied capacitance and dissipation factor in the quantum Hall regime for various in-plane electrode configurations. The largest dissipation factor is found for configurations where a strong electric field penetrates the bulk of the graphene. The measured effect of different electrode configurations on the frequency dependence of the quantized Hall resistance confirms our hypothesis. The findings are important for modeling and optimization of a future graphene-based quantum impedance standard.