Metallization considerations for carbon nanotube device optimization

Abstract

As a one-dimensional structure with atomically thin sidewalls, charge transport in carbon nanotubes can be regarded as a surface phenomenon. As such, perturbations from the surrounding environment can have a dramatic impact on transport and consequently on the device behavior of carbon nanotube field-effect transistors (CNTFETs). Importantly, this includes effects from device fabrication processes like contact metallization. With this as motivation, several aspects of contact metallization are investigated herein. First, it is found that ON current in n-type CNTFETs is enhanced to the level of p-type CNTFETs through the utilization of titanium as an adhesion layer, a result of improved wetting. Effects of different metallization techniques, namely, thermal and electron-beam evaporation, are also explored in p-type devices using titanium/palladium/gold contacts. It is found that thermal metallization consistently produces devices with higher ON current. In-depth analysis of the transfer characteristics reveals that this is due to radiation effects encountered during electron-beam processing, resulting in channel scattering and a decreased transconductance in the devices. This effect is particularly severe upon gold processing, and attempts to heal this device degradation through annealing proved unsuccessful. All studies presented here are conducted through the analysis of a large number of nanotube devices, giving a degree of confidence to the average results. Additionally, only device parameters that can be directly extracted from the transfer characteristics are analyzed. Assumptions including nanotube diameter, capacitance, and nanotube number per device are not made. Results from this study provide recommendations for optimizing CNTFET performance as it relates to contact metallization.