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Register a new userMini array of quantum Hall devices based on epitaxial graphene
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Alexandre Satrapinski Dr.
Publication date
2015
fields
Physics
and research's language is
English
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Series connection of four quantum Hall effect (QHE) devices based on epitaxial graphene films was studied for realization of a quantum resistance standard with an up-scaled value. The tested devices showed quantum Hall plateaux RH,2 at filling factor i = 2 starting from relatively low magnetic field (between 4 T and 5 T) when temperature was 1.5 K. Precision measurements of quantized Hall resistance of four QHE devices connected by triple series connections and external bonding wires were done at B = 7 T and T = 1.5 K using a commercial precision resistance bridge with 50 microA current through the QHE device. The results showed that the deviation of the quantized Hall resistance of the series connection of four graphene-based QHE devices from the expected value of 4*RH,2 = 2h/e^2 was smaller than the relative standard uncertainty of the measurement (< 1*10^-7) limited by the used resistance bridge.
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The observation of the anomalous quantum Hall effect in exfoliated graphene flakes triggered an explosion of interest in graphene. It was however not observed in high quality epitaxial graphene multilayers grown on silicon carbide substrates. The quantum Hall effect is shown on epitaxial graphene monolayers that were deliberately grown over substrate steps and subjected to harsh processing procedures, demonstrating the robustness of the epitaxial graphene monolayers and the immunity of their transport properties to temperature, contamination and substrate imperfections. The mobility of the monolayer C-face sample is 19,000 cm^2/Vs. This is an important step towards the realization of epitaxial graphene based electronics.
Depositing magnetic insulators on graphene has been a promising route to introduce magnetism via exchange proximity interaction in graphene for future spintronics applications. Molecule-based magnets may offer unique opportunities because of their synthesis versatility. Here, we investigated the magnetic proximity effect of epitaxial iron phthalocyanine (FePc) molecules on high-quality monolayer and bilayer graphene devices on hexagonal boron nitride substrate by probing the local and non-local transport. Although the FePc molecules introduce large hole doping effects combined with mobility degradation, the magnetic proximity gives rise to a canted antiferromagnetic state under a magnetic field in the monolayer graphene. On bilayer graphene and FePc heterostructure devices, the non-local transport reveals a pronounced Zeeman spin-Hall effect. Further analysis of the scattering mechanism in the bilayer shows a dominated long-range scattering. Our findings in graphene/organic magnetic insulator heterostructure provide a new insight for the use of molecule-based magnets in two-dimensional spintronic devices.
The quantum Hall effect, with a Berrys phase of $pi$ is demonstrated here on a single graphene layer grown on the C-face of 4H silicon carbide. The mobility is $sim$ 20,000 cm$^2$/V$cdot$s at 4 K and ~15,000 cm$^2$/V$cdot$s at 300 K despite contamination and substrate steps. This is comparable to the best exfoliated graphene flakes on SiO$_2$ and an order of magnitude larger than Si-face epitaxial graphene monolayers. These and other properties indicate that C-face epitaxial graphene is a viable platform for graphene-based electronics.
We discuss transport through double gated single and few layer graphene devices. This kind of device configuration has been used to investigate the modulation of the energy band structure through the application of an external perpendicular electric field, a unique property of few layer graphene systems. Here we discuss technological details that are important for the fabrication of top gated structures, based on electron-gun evaporation of SiO$_2$. We perform a statistical study that demonstrates how --contrary to expectations-- the breakdown field of electron-gun evaporated thin SiO$_2$ films is comparable to that of thermally grown oxide layers. We find that a high breakdown field can be achieved in evaporated SiO$_2$ only if the oxide deposition is directly followed by the metallization of the top electrodes, without exposure to air of the SiO$_2$ layer.
We propose a mechanism for the quenching of the Shubnikov de Haas oscillations and the quantum Hall effect observed in epitaxial graphene. Experimental data show that the scattering time of the conduction electron is magnetic field dependent and of the order of the cyclotron orbit period, textit{i.e.} can be much smaller than the zero field scattering time. Our scenario involves the extraordinary graphene $n=0$ Landau level of the uncharged layers that produces a high density of states at the Fermi level. We find that the coupling between this $n=0$ Landau level and the conducting states of the doped plane leads to a scattering mechanism having the right magnitude to explain the experimental data.
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