Electrical properties and transport characteristics of single-walled carbon nanotube bundles

As silicon approaches its fundamental performance limits, single-walled carbon nanotubes (SWCNTs) are emerging as a promising alternative channel material for transistors. Floating catalyst chemical vapor deposition (FC-CVD) is a highly effective technique for growing highly crystalline, defect-free SWCNTs while enabling their direct deposition from the reactor onto the wafer. During growth in an FC-CVD reactor, carbon nanotubes agglomerate due to Brownian diffusion, forming bundles, as predicted by simple aerosol models. In this dissertation, a fast and clean dry deposition method was developed to fabricate hundreds of functional SWCNT field-effect transistors (FETs), aiming to obtain a large number of single-bundle FETs. This method enabled the characterization of the pristine electrical properties of assynthesized FC-CVD SWCNTs and was also used to investigate the transport characteristic and electrical performance of SWCNT bundles. FETs were fabricated using two sets of FC-CVD-grown SWCNT bundles: Small Bundle Small Diameter (SBSD) SWCNTs and Large Bundle Large Diameter (LBLD) SWCNTs. The mean diameters of SBSD and LBLD SWCNTs were 4.1 ± 2.1 nm and 7.1 ± 3.7 nm, respectively, with nanotubes having mean diameters of 1.4 nm and 1.9 nm, respectively. Electron diffraction (ED) analysis determined that as-synthesized SBSD and LBLD SWCNTs contained metallic fractions of 38% and 46.3%, respectively. Due to the presence of a large fraction of metallic SWCNTs, bundles consist of a mixture of metallic and semiconducting nanotubes, leading to the expectation that FETs would lose their semiconducting switching efficiency. Interestingly, our experimental data show the opposite. The fraction of semiconducting FETs (s-FETs) was higher than the fraction of assynthesized semiconducting SWCNTs. The SBSD SWCNT FETs exhibited a semiconducting fraction of 71.5% out of 1,887 functioning FETs, while the LBLD SWCNT FETs showed a semiconducting fraction of 62% out of a total of 1,839 functioning FETs. The charge carrier mobility of SWCNT s-FETs was extracted using two rigorous methods: the peak transconductance (𝜇𝜇𝑃𝑃𝑃𝑃𝑃𝑃) and the Y-function (𝜇𝜇𝑌𝑌𝑌𝑌) methods. The ohmic-contact SBSD SWCNT s-FET exhibited mean 𝜇𝜇𝑃𝑃𝑃𝑃𝑃𝑃 and 𝜇𝜇𝑌𝑌𝑌𝑌 mobility values of 1,061 cm2V-1S-1 and 2,817 cm2V-1S-1, respectively, while the ohmic-contact LBLD SWCNT s-FET exhibited mean 𝜇𝜇𝑃𝑃𝑃𝑃𝑃𝑃 and 𝜇𝜇𝑌𝑌𝑌𝑌 mobility values of 1,854 cm2V-1S-1 and 5,378 cm2V-1S-1, respectively. These values are several orders of magnitude higher than the performance of SWCNTs grown using other methods. Similarly, the mobility of ohmic-contact single-junction SWCNT s-FETs was extracted for both SBSD and LBLD SWCNTs. Compared to single-bundle FETs, the mean 𝜇𝜇𝑃𝑃𝑃𝑃𝑃𝑃 and 𝜇𝜇𝑌𝑌𝑌𝑌 of single-junction SBSD SWCNT s-FETs decreased about fourfold to 255 cm2V-1S-1 and 737 cm2V-1S-1, respectively, while for single-junction LBLD SWCNT s-FETs, these values decreased about threefold to 856 cm2V-1S-1 and 1,732 cm2V-1S-1, respectively. Furthermore, both SBSD and LBLD SWCNT s-FETs exhibited an on-off ratio of up to 108.