Amorphous semiconductors — solids that are not crystalline — are relatively cheap to produce and deposit on to the plastic materials used in flat displays. However, to be useful, the switching has to be extremely fast. Previous attempts at using amorphous semiconductors to make diodes, which are needed to switch electric current to different areas of the display to form an image, have failed to produce the gigahertz switching speeds needed.
The crucial quality of a fast-switching semiconductor, said researcher Jeremy Allam from the university’s Advanced Technology Institute (ATI), is negative resistance: ‘The current passing through the material decreases, rather than increases, as the voltage across it rises.’ This can be achieved only with quantum-mechanical tunnelling, he said, whereby a specific voltage across the material allows electrons to ‘tunnel’ through energy barriers that would otherwise be impassible. Increasing the voltage past this value cuts off the tunnelling, hence the drop in current.
Previous fast semiconductors have been made of layers of high-quality crystalline alloys, Allam said, which was expensive. The team, led by ATI director Ravi Silva, claim to have produced a semiconductor made from a single material, rather than layers of different materials — with carbon as its basis. ‘Because carbon can exist in different forms, with different conducting properties, we can make layers that allow electrons through in different ways,’ Allam said.
Starting off with a layer of doped silicon — a semiconductor with ‘spare’ electrons that can act as a source of current — the team used a laser to deposit carbon atoms on to its surface. ‘We shine a stream of laser light at a lump of carbon to blow atoms off its surface. By changing the intensity of the laser we can determine the structure of the carbon atoms on the deposition surface. Carbon’s main crystalline forms are graphite, which conducts electricity well, and diamond, which conducts very badly,’ Allam said.
‘Amorphous carbon is like a mixture of these two forms, and we can vary the proportion of the graphite-like fraction to determine the electronic properties.’
The team deposited a first layer of diamond-like carbon, which acts as an electron barrier; followed by a graphite-like layer that acts as a ‘well’ where electrons can accumulate; then another diamond-like layer. Each is only nanometres thick. The combination of the thinness and semiconducting properties of the layers allows the tunnelling that should give rise to gigahertz-level oscillation speeds.
Diamond-like carbon has major advantages for electronics, Allam said: it is very resistant to corrosion and chemical attack, is thermally stable, biocompatible and can be deposited over large areas at room temperature. ‘It is also very easy to integrate with other electronic components,’ he said. ‘It means that we’d be able to make very high-frequency RF oscillators. Although display technologies are an obvious application, as they can be deposited over large, cheap, flexible substrates, we can look at mobile comms and other high-speed communications technologies.’