Carbon nanotubes (CNTs) have been a hot research topic since Iijima's seminal paper in 1991. So far they've proven that they can take the heat, exhibiting thermal conductivity about 10 times greater than copper – comparable to diamond! Their electrical properties have placed the material firmly on the ITRS roadmap as a candidate to replace silicon CMOS beyond the 10 nm node. However, these one-dimensional devices inhabit a three-dimensional world, and such small molecular conductors must be measured carefully to overcome variability at the atomic scale.

In detail: carbon nanotube test device

For example, CNT transistors exhibit hysteretic behaviour in direct-current (DC) measurements with respect to gate voltage. In other words, the measured current depends on the voltage sweep direction. The reading also varies depending on sweep rate or environmental conditions. Hysteresis is typically attributed to charge trapping by surrounding water molecules or charge injection into the nearby substrate, making it difficult to extract "true" device properties, such as threshold voltage and electron mobility. A key question remains, how can we probe the intrinsic properties of such tiny electronic materials while limiting the effects of the ambient environment on the measurement?

Suppressing hysteresis

Pulsed characterization methods offer one possible solution and researchers from the University of Illinois, Urbana-Champaign, US, have been busy investigating the idea. Pulsed measurements performed over a wide temperature range (80–450 K) show that hysteresis in the transfer characteristics is reduced by varying the pulse off time. Longer off times allow trapped charge to tunnel out of the dielectric, preventing screening of the gate voltage and the resultant hysteresis between the forward and reverse gate sweeps.

The team has adapted a tunnelling model to correlate the trap relaxation times to the trap depth, which have been found to be in the ranges 0.01–10 s and 4–8 nm, respectively. The effective CNT mobility is extracted from the pulsed data and has been found to be in good agreement with recently published simulations. Mobility extracted from DC measurements on the same device is shown to vary greatly between the forward and reverse gate sweeps. This indicates that pulsed measurements offer a better approach to extracting the "true" electrical properties of nanoscale devices including CNTs, graphene, nanowires and molecular electronics.

Further details can be found in the journal Nanotechnology.

About the author

David Estrada is a PhD candidate working under the direction of Prof. Eric Pop in the Electrical and Computer Engineering department at the University of Illinois, Urbana-Champaign, US.

http://nanotechweb.org/cws/article/tech/41941