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Graphene for quantum capacitance standards

First-time AC precision measurements confirm the material's potential as a standard for the unit of capacitance.

PTB-News 2.2015
Especially interesting for

metrology institutes

calibration laboratories

The particular properties of the two-di-mensional conductor material graphene can also be exploited in the case of the quantum Hall effect on which extremely precise calibrations of the electric DC resistance are based worldwide. Graphene is, in many aspects, superior to all previously used materials. Now PTB has shown that graphene exhibits excellent properties also in the case of AC applications. Developing a quantum standard for AC resistance (impedance) – and, thus, also for capacitance – there-fore seems realistic.

As graphene is highly transparent, apart from the eight gold supply lines, it can hardly be differentiated under an optical microscope from the also transparent SiC substrate. Therefore, the quantum Hall structure is marked with a blue line. Contrary to the previous article, here, only the substrate is visible, the film itself is invisible.

From applications with DC current, it is already known that the quantized resis-tance values of quantum Hall structures made of graphene do not – not even at maximum measurement accuracy – differ from those of conventional semicon-ductor materials (e.g. GaAs heterostruc-tures). Graphene, however, offers further advantages: to realize the quantum Hall effect (QHE), the magnetic fields do not need to be as high and the temperatures do not need to be as low. It is expected that in the next few years, this will make considerably more practicable calibration facilities possible.

Precise AC measurements of the quan-tum Hall effect of graphene have been carried out at PTB for the first time. Hereby, high-quality graphene material was used which had been grown epitaxially on a SiC substrate. Contrary to the DC QHE, in the case of AC measurements, parasitic effects caused by stray capacitance between the electron system of graphene and the ambient environment play a role. Dissipation processes resulting from these effects cause deformations of the ideally flat QHE resistance plateau and, thus, deviations from the quantized resistance values. These effects must therefore be sufficiently reduced or even suppressed to allow applications as a quantum standard for AC resistance. In the case of the usual GaAs-based quantum Hall structures, this has, so far, only been achieved at PTB to such an extent that the AC QHE could be successfully used to derive the unit of capacitance, the farad. The relative uncertainty achieved hereby is 1 · 10–8, which is clearly lower than that of all conventional calibration methods for the electric capacitance.

The AC measurements carried out on graphene at PTB at frequencies in the kHz range have now shown that graphene, compared to GaAs structures, exhibits particularly beneficial properties. Due to the geometry, parasitic dissipation effects are, even without special compensation measures, already so small that the QHE plateaus remain practically flat. Remain-ing stray capacitances between the contacts of the structure cause, at a frequency of 1 kHz, only a very small relative deviation from the quantized resistance value of approx. 1 · 10–7. The researchers are currently working on reducing this influence by optimizing the structure geometry of graphene QHE samples. The results achieved so far allow us to expect that graphene-based impedance standards will soon be widely used by national metrology institutes as well as by industrial calibration laboratories.


Jürgen Schurr
Department 2.6 Electrical Quantum Metrology
Phone: +49 (0)531 592-2114
E-mail: juergen.schurr(at)ptb.de

Scientific publication:

C.-C. Kalmbach, J. Schurr, F. J. Ahlers,  A. Müller, S. Novikov, N. Lebedeva, A. Satrapinski: Towards a graphene-based quantum impedance standard. Applied Physics Letters 105, 073511 (2014)