by Liz Kalaugher

Scientists in Germany claim to have come up with a new way of growing single-walled carbon nanotubes that are contacted in situ. The work, by Infineon Technologies and the Technical University of Dresden, gave record conductances of up to 16 µS per single nanotube device.

"This work will accelerate understanding of nanotube devices as it enables the mass production of nanotube transistors," Franz Kreupl of Infineon told nanotechweb.org. "It will form the basis for a more statistically based investigation of nanotubes, and in particular enable the study of different gate materials and the influence of the material deposition method."

To make the devices, Kreupl and colleagues grew multilayered metal systems by ion-beam deposition onto silicon wafers covered by a thermal oxide layer and a resist mask patterned by conventional 350 nm photolithography. They created a layer of metal to act as the electrode, a 5-10 nm thick layer of aluminium to separate the electrode material from the catalyst, and a layer of iron, iron/molybdenum or cobalt catalyst. In some cases, depending on the electrode material, they also deposited a layer of titanium or tantalum between the substrate and multilayer system to improve adhesion.

Then the team grew single-walled carbon nanotubes onto the multilayers in a quartz-tube furnace, using methane as the carbon feedstock. The researchers tried electrodes made of molybdenum, tantalum, tungsten, gold, copper, cobalt, tantalum nitride and titanium nitride. The aluminium separation layer enabled them to grow single-walled carbon nanotubes onto all the materials except tungsten.

After nanotube growth, the scientists deposited nickel by an electroless process onto the devices with cobalt electrodes. The devices exhibited conductances of up to 16 µS per single device and current modulation ratios of up to 10,000,000 per nanotube. According to Kreupl, these figures emphasize the high contact quality after electroless encapsulation.

"Only one lithographic step is necessary to grow and obtain a huge number of in situ contacted devices on a wafer," added Kreupl. "The lithographic step defines source and drain contacts, and the proposed multilayer gives in situ contacted nanotube devices after their growth by CVD."

Kreupl also believes that the encapsulation method will enable the devices' scaling behaviour - "one of the hottest topics of nanotube device physics" - to be addressed. "By carefully tuning the electroless encapsulation process, the scaling behaviour of ultrashort (~10 nm) nanotube devices can be investigated without employing challenging electron-beam lithography," he explained.

The researchers reported their work in Nano Letters.

About the author
Liz Kalaugher is editor of nanotechweb.org.