"Until now, researchers have only investigated nanotube transistors with a channel length much longer than state-of-the-art silicon transistors," Franz Kreupl of Infineon told nanotechweb.org. "But the speed and performance of a possible device depends tremendously on the channel length."
According to Kreupl, nanotubes have been shown to be superior at channel lengths above 50 nm. However, examples in the literature show relatively low on/off ratios and so called ambipolar behaviour, mainly due to the use of nanotubes with diameters above 1.4 nm, which results in a low bandgap.
In this study, the team used nanotubes with a diameter of 0.7 - 1.1 nm. These have a much higher bandgap. "Consequently, we could demonstrate transistors with high on/off ratios without ambipolar behaviour, and which deliver enough current to outperform the requirements for future devices by a factor of 10," said Kreupl.
To create the devices, Kreupl, Robert Seidel and colleagues grew single-walled carbon nanotubes by chemical vapour deposition at 700 °C onto a highly doped n-type silicon substrate. They used islands of nickel catalyst, controlling the catalyst thickness to ensure that, on average, only one nanotube bridged the 2 µm gap between catalyst islands.
Next, the team employed electron beam (e-beam) lithography together with hydrogen-silsesquioxane (HSQ) or calix[6]arene resists to create 10-25 nm wide lines between the catalyst pads, defining the channel length. They also used e-beam exposure and a poly(methyl methacrylate) resist to create palladium source and drain contacts on either side of the channel. The highly doped silicon substrate, meanwhile, acted as a back-gate.
"It turned out that we were able to fabricate 10 nm-wide lines with a high aspect ratio of about 6 with HSQ negative resist," said Kreupl. "This allows for a self-aligned fabrication of source drain regions with such a small gap."
The devices had an on/off current ratio of more than 106. And, unlike carbon nanotube field-effect transistors with smaller band-gaps, they didn't show ambipolar behaviour. The devices could deliver currents of more than 15 µA at a supply voltage of 0.4 V, compared to the normal value of 0.7 V, and the current density was more than 10 times that of silicon.
According to the researchers, in many ways the transistors already perform as well as much larger silicon-based devices. What's more, the high on/off current ratio indicates that additional scaling is possible. Infineon says the results demonstrate that nanotube transistors can fulfil the demands of further scaling until 2018.
"Besides the persisting manufacturability problem, which continuously has to be addressed, the next steps will be to investigate the effect doping has in such short channel transistors," said Kreupl. "Logic needs both p- and n-type transistors."
The scientists reported their work in Nano Letters.