Experimental and Theoretical Studies of Transport through Large Scale, Partially Aligned Arrays of Single-Walled Carbon Nanotubes in Thin Film Type Transistors

Abstract

Gate-modulated transport through partially aligned films of single-walled carbon nanotubes (SWNTs) in thin film type transistor structures are studied experimentally and theoretically. Measurements are reported on SWNTs grown by chemical vapor deposition with systematically varying degrees of alignment and coverage in transistors with a range of channel lengths and orientations perpendicular and parallel to the direction of alignment. A first principles stick-percolation-based transport model provides a simple, yet quantitative framework to interpret the sometimes counterintuitive transport parameters measured in these devices. The results highlight, for example, the dramatic influence of small degrees of SWNT misalignment on transistor performance and imply that coverage and alignment are correlated phenomena and therefore should be simultaneously optimized. The transport characteristics reflect heterogeneity in the underlying anisotropic metal-semiconductor stick-percolating network and cannot be reproduced by classical transport models.

Nanoscale Charge Percolation Analysis in Polymer‐Sorted (7,5) Single‐Walled Carbon Nanotube Networks

“…Using the AFM image in Figure S2 [37] and the histogram of lengths in Figure S3, [37] the effective nanotube length (LS) was estimated to be 680 nm, with a surface density well above the percolation threshold of 12 SWNT µm -2 (ρth = 4.236 2 /πLS 2 ), [25] and thus the carrier mobility can be extracted using the parallel plate capacitor model within the gradual channel approximation. [38] Further details on the CNT length characterization can be found in the Supporting Information.…”

“…This approximation is known as the constant conductivity approximation and the transport properties of the (7,5) SWNT channels can be described by the two-dimensional (2D) homogeneous random-network stick-percolation model. [28] The universal current-scaling equation for ID in the low-bias regime reduces to: [25] …”

“…In the case of a random network with a normalized coverage much higher than the percolation threshold (ρLS 2 ≫ 4.236 2 /π), [25] most of the sticks are connected together forming several conductive paths and therefore the network behaves as a 2D conductor, with m ~ 1. Thus, in the limit of the 2D conductor, the universal current-scaling formula reduces to the well-known…”

“…On the basis of the previous considerations, we used the channel ON and OFF currents of different (7,5) By using previously published experimental results obtained with CVD-grown carbon nanotubes by Kocabas et al [25] and Sun et al, [26] we have plotted the curve of the universal current exponent m by fitting their experimental data, as shown in However, the presence of the polymer in our network certainly affects the value of the parameter k in the current-scaling Equation 1 with respect to the one obtained without any polymer. Moreover, the nanotubes in our network are not rigid sticks but they are flexible, with some defects, they interact with each other and with the substrate, forming bundles and consistently that ρmet is very small compared to the total (7,5) SWNTs surface density.…”

“…For short channel devices (LC ≤ LS) the situation is very different because the presence of even a single metallic nanotube could in principle electrically short the source and drain electrodes and drastically reduce the transistor's current ON/OFF ratio due to their high current transporting capabilities. [25,26] This case is known as the direct conduction regime. Lowering the density of the SWNT network can help to minimize the impact of metallic SWNTs but it has an adverse effect on charge carrier mobility and channel transconductance.…”