STFC's Dr Emma Springate, one of the research team, with the Artemis laser [Credit: Monty Rakusen]

Graphene bilayer acts as semiconductor

Scientists have used lasers to get a graphene bilayer to act as a semiconductor, an important step towards computer chips made of the wonder material.

The strength and conductive efficiency of the one atom thick carbon-based material makes it an attractive contender for new faster, more efficient microchips, but in its current form graphene is unsuitable 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.

In the latest research a 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 the Artemis laser at the Science and Technology Facilities Counci’s Central Laser Facility, which is based at the Rutherford Appleton Laboratory in Oxfordshire, the researchers fired ultra-short pump laser pulses at the sample, boosting electrons into the conduction band.

A second short, extreme ultraviolet, wavelength pulse then ejected electrons from the sample, which 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.”

There can be imperfections in bilayer graphene as the layers sometimes become misaligned, but the graphene sample used by the researchers showed no sign of such defects.

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.”

The team say the results suggest that further technological effort should be carried out to minimise imperfections in graphene sample, which could allow the switch-off performance of bilayer graphene to 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.

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