As the diameters of carbon nanotubes are reduced, the potential barriers for charge carriers injected from a metal contact increase. This can cause a wide variability in the properties of carbon nanotube transistors and is one of the major issues that have to be overcome for large-scale integration of nanotubes in electronics.

In spite of many theoretical predictions and calculations of Schottky barriers in nanotube–metal contacts, experimental studies of Schottky barriers have been limited to single devices or the barrier height has been inferred indirectly using the on state current of the transistors.

Due to the nanoscale dimensions of the devices, methods commonly used for bulk metal-semiconductor contacts cannot be used to measure the barrier height. Instead, researchers have used temperature-dependent electrical measurements to extract the activation energy needed for charge carriers to pass over the Schottky barrier.

The team uses electron beam lithography to create palladium contacts to individual nanotubes and a cryogenic probe station connected to a semiconductor parameter analyser to perform electrical measurements over a large temperature range. The extracted barrier heights are found to be inversely proportional to the diameter of the nanotubes. The diameter dependence originates from the fact that the bandgap of carbon nanotubes is inversely proportional to their diameter – small tubes will have a larger Schottky barrier.

This relationship implies that large tubes are needed to create devices with negligible barriers. However, as the bandgap is reduced the off state current increases and causes leakage currents to flow through the devices. Therefore it is of great importance to find the optimum combination of carbon nanotube dimension and metal properties to obtain a high-performance device.

The researchers presented their work in Nanotechnology.