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Hans He
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World-leading electrical measurements with graphene and quantum effect
Graphene and quantum mechanical phenomena are enabling even more precise electrical measurements for industry and society. In a cleanroom in Gothenburg, researchers from RISE manufacture world-leading chips that drive this development and are used by countries such as the United States, South Korea, Canada and France.
In the MyFab Chalmers cleanroom, the air is extremely clean and free from particles such as dust, pollen and other contaminants. This is a prerequisite for the sensitive processes carried out here. The microchips are so small that a single dust particle can be larger than the components on the chip. Before entering the facility, a careful multi-step procedure is required to put on full protective clothing.
“It takes some getting used to if you’re not familiar with it, but for those of us who work here and go in and out several times a day, it’s quite straightforward,” says Hans He, researcher at RISE.
He is in the cleanroom together with fellow researcher Naveen Shetty to manufacture so-called quantum Hall elements and quantum Hall arrays in graphene. The chips are produced using lithography, a method in which an electron beam is used to draw extremely small structures onto the material – in this case graphene of the highest quality.
“Right now we are inspecting the graphene samples before continuing with the rest of the production process. They must be completely defect-free for everything to work. From graphene sample to finished chip takes about a week here in the cleanroom,” says Hans He.
With the quantum Hall effect, we obtain resistance values that are extremely stable and well defined, which makes them perfectly suited for the realization of the unit of resistance.
Precision measurements at the National Laboratory for Electrical Quantities
Resistance and voltage are the cornerstones of electrical measurements. To ensure reliable measurement results, calibrated measurement instruments are required, and the quantum Hall chips serve as the reference point at the top of the calibration chain for resistance. Once the chips are completed, they are transported to the National Laboratory for Electrical Quantities at RISE in Borås. There, precision measurements are carried out for several weeks before they are ready to be used.
'“We have worked for several years on the realization of the resistance unit using the quantum mechanical phenomenon of the quantum Hall effect and graphene, and have, among other things, published our successes in Nature Communications,” says Hans He.
The classical Hall effect occurs when an electric current passes through a material in a magnetic field. A voltage is then generated transversely across the material. The stronger the magnetic field, the larger the voltage becomes. In ordinary materials, this relationship is linear. But in extremely thin materials – two-dimensional systems – at very strong magnetic fields and low temperatures, something unexpected happens. For certain magnetic field values, the transverse resistance takes on exactly defined values that depend only on fundamental constants of nature. This quantum mechanical phenomenon is called the quantum Hall effect.
“With the quantum Hall effect, we obtain resistance values that are extremely stable and well defined, which makes them perfectly suited for the realization of the unit of resistance,” says Hans He.
Graphene opens new possibilities
The quantum Hall effect has been used for a long time. What is new is the use of graphene, which makes it possible to create quantum Hall elements that can operate at higher temperatures, higher currents, and lower magnetic fields, together with the ability to connect many quantum Hall elements into so-called arrays.
“With a single quantum Hall element in graphene we obtain an exact resistance level close to 12.9 kΩ. But resistance values used in industry can be several orders of magnitude lower or higher. To reach other resistance levels, several steps of secondary calibrations are required, and each step increases the measurement uncertainty. To get around this, we have developed methods to connect many elements into arrays, making it possible to choose an arbitrary resistance level for the realization,” says Hans He.
Not only affecting electrical measurements
When the SI system was redefined in 2019, the definition of the kilogram was linked to the Planck constant instead of a physical metal cylinder – the International Prototype of the Kilogram. One way to realize the kilogram is to use a so-called Kibble balance, a type of scale that uses electromagnetic force to balance an unknown mass. RISE is working together with NPL in the United Kingdom to develop a Swedish Kibble balance.
“The Kibble balance requires extremely accurate current measurements, and by replacing a traditional resistor with a quantum Hall chip in graphene, the uncertainty in mass measurements can be improved,” says Hans He.
Development in three steps
Hans He describes three steps for the development. The first step is individual quantum Hall elements made from graphene, which are now moving toward international standardization and are already used today in comparison measurements between national metrology institutes. The second step is quantum Hall arrays.
“So far we are almost alone in the world in achieving the stability and quality required for the arrays, so it will take some time before this can become an internationally standardized method,” says Hans He.
One limitation of quantum Hall arrays is that the resistance level is determined during manufacturing (for example fixed to 100 Ω). The third step is therefore programmable arrays, where the user can set the desired resistance level in real time using a single chip.
“This is a long-term goal and requires extensive research, but it has the potential to become the ultimate resistance standard. Voltage standards based on another quantum phenomenon, the Josephson effect, have already completed this journey and are today programmable,” says Hans He.
Strong international interest
The technology is already used internationally. RISE collaborates with and supplies the world-leading chips to national metrology institutes in countries such as the United States, France, Canada and South Korea, and sells chips commercially to the Canadian measurement instrument manufacturer Measurements International Ltd.
“The interest in the technology is growing. We receive many enquiries, we are invited to an increasing number of research collaborations, and the technology we have developed is actively used both in research and in commercial activities,” says Hans He.
What is the quantum Hall effect?
The quantum Hall effect is a quantum mechanical phenomenon that occurs in two-dimensional materials under strong magnetic fields. Electrons can only move in certain discrete paths, which causes the resistance to take on discrete, extremely stable levels (plateaus). These resistance values depend only on fundamental constants of nature, making them ideal as an absolute reference for metrology at the highest level.
Why graphene?
Graphene consists of a single atomic layer of carbon. Its crystal structure gives graphene unique electrical properties that make it ideal for quantum Hall components. The quantum Hall effect in graphene occurs at higher temperatures, lower magnetic fields, and can withstand higher currents compared with the semiconductor materials used previously. This improves performance and makes practical operation easier.
For metrological applications, epitaxially grown graphene on silicon carbide is used, resulting in a monolayer of monocrystalline graphene of the high quality required for precision measurements.