Resistance measurements using graphene
New developments in the field of quantum Hall standards
Up to now, quantum Hall resistors made of semiconductor heterostructures have been used in electrical quantum metrology. In comparison, quantum Hall resistors made of graphene can be operated with considerably less technical effort. If the material quality is suitable and the charge carrier density is stable, these resistors allow resistance quantization to be realized at lower magnetic fields and at temperatures that are not so low.
Within the scope of an EMPIR project titled “Graphene Impedance Quantum Standard”, which is coordinated by PTB and involves 11 partners from Europe and Asia, graphene quantum Hall resistors were manufactured at PTB’s Clean Room Center using optimized procedures. Measurements performed at the BIPM (the International Bureau of Weights and Measures) have confirmed the high quality of these devices: Their DC resistance is in agreement with the nominal quantized value to a few parts in a billion, even at a relatively high temperature of 4.2 K and a magnetic field of only 5 T. Comparison measurements carried out at the institutes involved in the project have demonstrated the temporal stability of the device properties and have shown that their high quality is hardly affected by long-distance transport. The main prerequisites for the future practical use of graphene-based quantum resistance standards are thus fulfilled.
The envisaged use of such standards in AC operation (i.e., for measuring impedance quantities) places additional requirements on the measuring instruments and the graphene devices themselves. The devices manufactured at PTB were investigated in the laboratories of eight project partners using diverse methods. For this purpose, different types of impedance bridges were optimized during the project. PTB provided a Josephson impedance bridge which uses precise reference voltages generated by modern quantum voltage sources (based on the Josephson effect). This leads to particularly high flexibility as regards the experimental parameters; it also allows automated measurement cycles to be used. In addition, the measuring system is more user-friendly. For impedance and frequency measurements, the accessible measuring ranges have been extended.
All in all, the project results were such that a quantum-based realization of the unit of capacitance, the farad, is now possible with a relative uncertainty of better than 10–7. This was summarized in the form of a good practice guide for the realization of the farad by means of graphene quantum standards (https://www.ptb.de/empir2019/giqs/home/). The new measurement capabilities will be made accessible to interested user groups within the scope of the Quantum Technology Competence Center of PTB.
Contact
Klaus Pierz
Department 2.5
Semiconductor Physics and Magnetism
Phone: +49 531 592-2412
klaus.pierz(at)ptb.de
Scientific publication
D.-H. Chae, M. Kruskopf, J. Kucera, J. Park, N. T. M. Tran, D. B. Kim, K. Pierz, M. Götz, Y. Yin, P. Svoboda: Investigation of the stability of graphene devices for quantum resistance metrology at direct and alternating current. Meas. Sci. Technol. 33, 065012 (2022)