Band Alignment among CNTs with Varying Diameters and Schottky Barrier Fluctuations between CNTs and Metals: A First-Principles Study

In carbon nanotube thin film transistors (CNT-TFTs), the performance is significantly influenced by the fluctuation in the diameter of the CNTs in channel. In this work, first-principles calculations were employed to explore the fluctuations in the valence band maximum and conduction band minimum of CNTs with diameters ranging from 0.78 to 2.07 nm. Band alignment analysis reveals maximum offsets reach 0.54 eV for the valence band and 0.48 eV for the conduction band, with larger CNTs (diameters over 1.8 nm) exhibiting energy band fluctuations of less than 0.06 eV. Further analysis shows that the fluctuation in Schottky barrier height (SBH) is smaller between CNTs and Pd compared to CNTs and Ti. Atomistic simulations of Pd-CNT-Pd devices at −2 V demonstrate that the variation in on-state current between 1.49 and 2.04 nm CNTs is six times smaller than that between 1.02 and 1.49 nm CNTs. This indicates that inter-nanotube fluctuations in larger-diameter CNTs have a smaller impact on device current compared to fluctuations in smaller-diameter CNTs. These findings suggest that larger-diameter CNTs (greater than 1.8 nm) enhance the uniformity of electrical properties, presenting a promising approach to improving the reliability of CNT-based thin-film devices.