Scientists using lasers at the CLF's Artemis facility believe that they are a step closer to finding a replacement for silicon chips that are faster and use less energy than at present.
The team has tested the behaviour of bilayer graphene to discover whether or not it could be used as a semiconductor. Their results suggest that it could replace silicon transistors in electronic circuits.
Graphene is pure carbon in the form of a very thin, almost transparent sheet, one atom thick. It is known as a ‘miracle material’ because of its remarkable strength and efficiency in conducting heat and electricity.
In its current form graphene is not suitable for transistors, which are the foundation of all modern electronics. For a transistor to be technologically viable, it must be able to ‘switch off’ so that only a small electric current flows through its gate when in standby state. Graphene does not have a band gap so cannot switch off.
The research team, led by Professor Philip Hofmann from Aarhus University in Denmark, used a new material – bilayer graphene – in which two layers of graphene are placed one on top of the other, leaving a small band gap to encourage the transfer of energy between layers.
Using Artemis at STFC’s Central Laser Facility, the researchers fired ultra-short pump laser pulses at the bilayer graphene sample, boosting electrons into the conduction band.
A second short, extreme ultraviolet, wavelength pulse then ejected electrons from the sample. These were collected and analysed to provide a snapshot of the energies and movement of the electrons.
“We took a series of these measurements, varying the time delay between the infrared laser pump and extreme ultraviolet probe, and sequenced them into a movie,“ said STFC’s Dr Cephise Cacho, one of the research team. “To see how the fast-moving electrons behave, each frame of the movie has to be separated by just a fraction of a billionth of a second.”
Professor Hofmann said, “What we’ve shown with this research is that our sample behaves as a semiconductor, and isn’t short-circuited by defects.”
There can be imperfections in bilayer graphene as the layers sometimes become misaligned.
The bilayer graphene team: a collaboration between eight institutes in five European countries. Credit: STFC.
The results of this research, in which the graphene showed no defects, suggest that further technological effort should be carried out to minimise imperfections. Once this is done, there is a chance that the switch-off performance of bilayer graphene can be boosted enough to challenge silicon-based devices.
Graphene transistors could make smaller, faster electronic chips than are achievable with silicon. Eventually more and more transistors could be placed onto a single microchip to produce faster, more powerful processors for use in electronic equipment.
The paper Ultrafast Dynamics of Massive Dirac Fermions in Bilayer Graphene (link opens in a new window)is published in Physical Review Letters 112 27401 (2014).
It is also featured as a spotlight article (link opens in a new window) in the American Physical Society’s Physics Viewpoint.
The research was carried out by a team from Aarhus University in Denmark, École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, Sincrotrone Trieste, University of Trieste and IOM-CNR Laboratorio TASC in Italy, Technische Universitat Chemnitz in Germany, the Central Laser Facility and the University of St Andrews.
