Analysis of hopping conduction in semiconducting and metallic carbon nanotube devices

Single-walled carbon nanotube field-effect transistors were irradiated with 20 keV electrons using an e-beam lithography exposure method. Analysis of conduction data in the temperature range from 25 to 300 K indicated the creation of insulating regions containing traps along the nanotube channel. Further analysis of semiconducting and metallic nanotube devices shows dramatic differences in the effect of the electron exposure on the hopping defect barrier heights. Barriers for metallic nanotubes saturate at significantly larger values than semiconducting nanotubes due to shorter localization lengths. The limited and near constant density of states at the Fermi level induces a larger hopping length to localization length ratio, further limiting current and increasing measured trap heights. Poole-Frenkel hopping with an adjustment for electron localization is utilized to explain the inconsistencies. n-type and p-type barriers in the nanotube devices displayed exponential dependence on applied gate voltage bias, with the peak barrier height in the metallic device defining a switch of majority carrier. (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3151916]