01.12.2021
In a recent study, the Bureau International des Poids et Mesures (BIPM) has investigated a graphene quantum Hall resistor developed and fabricated by PTB. By comparison with conventional quantum resistance standards based on semiconductor heterostructures, the excellent properties of the graphene sample were confirmed, underpinning its suitability as a primary quantum resistance standard.
Graphene-based quantum Hall resistors have already been developed and optimized at PTB for several years. Advances in fabrication technology, achieved in the framework of the ongoing research project "Graphene Impedance Quantum Standard" (
GIQS), now allow operation at relatively low magnetic flux densities below 5 T and at relatively high temperatures of about 4.2 K (liquid helium). Such conditions can be provided by modern cryo-cooler magnet systems, greatly simplifying the operation of graphene-based quantum Hall resistors compared to conventional circuits made of semiconductor heterostructures, which typically require lower temperatures (about 1.5 K) and higher magnetic fields (flux densities of about 10 T and above). This opens the perspective of using primary resistance standards under user-friendly and economic conditions in industrial and calibration laboratories.
The BIPM investigations were carried out on a quantum Hall resistor of PTB made from epitaxially grown graphene. After the growth process, the graphene sample was encapsulated in chemically doped polymer layers. In this way, the charge carrier density and thus the "operating point" of the sample could be specifically adjusted. By means of electrical precision measurements, the equivalence of the PTB graphene sample (operated at a temperature of 4.2 K and at magnetic flux densities between 4 T and 5 T) and the previously used primary standards of the BIPM based on semiconductor heterostructures was verified: The measured (quantized) resistance values of both sample types agreed well within the very small relative measurement uncertainty of 3 nΩ/Ω. This excellent agreement proves the suitability of the PTB graphene sample for its use as a quantum resistance standard. Further investigations on PTB graphene samples including the evaluation of the temporal stability of the charge carrier density are planned.
Figure: Graphene quantum Hall resistor developed and manufactured at PTB, electrically contacted in the sample holder.
Sources:
Department 2.5 “Semiconductor Physics and Magnetism”