Channel length scaling behavior in transistors based on individual versus dense arrays of carbon nanotubes

摘要

Recent advances in the solution-phase sorting and assembly of semiconducting single-walled carbon nanotubes (SWCNTs) have enabled significant gains in the performance of field-effect transistors (FETs) constructed from dense arrays of aligned SWCNTs. However, the channel length (L_(CH)) downscaling behaviors of these arrays, which contain some organizational disorder (i.e., rotational misalignment and non-uniform pitch), have not yet been studied in detail below L_(CH) of 100 nm. This study compares the behaviors of individualized SWCNTs with arrays of aligned, solution-cast SWCNTs in FETs with L_(CH) ranging from 30 to 240 nm. The on-state conductance of both individual and array SWCNTs rises with decreasing L_(CH). Nearly ballistic transport is observed for L_(CH) < 40 nm in both cases, reaching a conductance of 0.82 G_o per SWCNT in arrays, where G_o = 2e~2/h is the quantum conductance. In the off-state, the off-current and subthreshold swing of the individual SWCNTs remain nearly invariant with decreasing L_(CH) whereas array SWCNT FETs suffer from increasing off-state current and deteriorating subthreshold swing for L_(CH) below 100 nm. We analyze array disorder using atomic force microscopy, which shows that crossing SWCNTs that arise from misoriented alignment raise SWCNTs off of the substrate for large portions of the channel when L_(CH) is small. Electrostatics modeling analysis indicates that these raised SWCNTs are a likely contributor to the deteriorating off-current and subthreshold characteristics of arrays. These results demonstrate that improved inter-SWCNT pitch uniformity and alignment with minimal inter-SWCNT interactions will be necessary in order for solution processed SWCNT arrays to reach subthreshold performance on par with isolated SWCNTs. These results are also promising because they show that arrays of solution-processed SWCNTs can nearly reach ballistic conductance in the on-state despite imperfections in pitch and alignment.