Inkjet printed single-walled carbon nanotube field effect transistors
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Inkjet printing technology has the potential to drastically reduce the process time and cost by generating the patterns without physical masks and conventional vacuum processes. In addition, the inkjet printing process can be applied to flexible and large area substrates. Among the printable semiconductors, single walled carbon nanotubes (SWCNTs) have been attracting increasing attention for their high carrier mobility and potential application in transparent, flexible, high- current and high frequency electronics. The effects of fluoropolymer capping onto SWCNT devices are investigated. Remarkable improvements in key device characteristics of SWCNT field-effect transistors (FETs) are achieved by coating of the active semiconductor with a fluoropolymer layer such as poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)). These favorable changes in device characteristics also enhance circuit performance. The origins of these improvements are the dipolar nature of the fluoropolymer and the mechanism is confirm by exposing SWCNT FETs to a number of vapor phase polar molecules which produce similar effects on the FET characteristics as the application of P(VDF-TrFE). High-performance inkjet printed single walled carbon nanotube (SWCNT) field effect transistors (FETs) with channel lengths of 150-250 nm are demonstrated. Optimized electrode geometry has been developed to confine the inkjet droplet to the active channel area. This minimizes waste of material outside of the channel while enabling short channel length devices that exhibit high effective carrier mobilities and transconductances. This novel fabrication approach is compatible with roll-to-roll processing and enables the formation of high-performance short channel device arrays based on inkjet printing with at least a 50-fold reduction in consumption of semiconducting SWCNT ink compared to other solution processing methods. In these short channel SWCNT FETs, the influences of nanotube bending and gate insulator-semiconductor interface modification on the characteristics of inkjet printed short channel length SWCNT are investigated. Employing recessed source and drain (S/D) electrodes to minimize the mechanical distortion of CNTs, high performance short channel ambipolar transistors based on inkjet printed SWCNTs are demonstrated. Mechanical distortion of the nanotubes due to bending near source and drain contacts when they are not recessed is found to suppress electron transport and transform the ambipolar transistors into p-type devices. Inclusion of interfacial polymer layers such as P(VDF-TrFE) between the SWCNTs and Al2O3 top dielectric also results in p-type doping and reductions in electron transport transforming amibipolar transistors into p-type devices.