Conduction mechanisms in on- and off-states of Thin Film Transistors of length-sorted Single-Wall Carbon Nanotubes
The network of SWCNTs has been investigated for the application to thin film transistors (SWCNT-TFTs). In SWCNT-TFTs, both primary and secondary structural properties of SWCNTs, such as their length, tube diameter and tube density, can considerably affect the device performance, because these should dominate the way of forming SWCNT networks. However, the relation between the device performance and structural properties of SWCNTs has not been fully explored yet. Furthermore, optimization of SWCNTs would be important for their practical application as printable electronic materials. In this study, we investigated the detailed analyses on SWCNT-TFTs with various tube diameters and the constant tube length.
For this study, we synthesized four SWCNT samples with different average diameters of ca. ~1.0, 1.3, 1.8, and 2.1 nm by the enhanced direct injection pyrolytic synthesis (eDIPS) method . These SWCNTs were mono-dispersed by using DNA as a dispersant, and length-sorted by a size exclusion chromatography to prepare SWCNTs with 400 nm length for fabricating the SWCNT-TFTs. Resulted four SWCNTs with different diameters and constant length were dropped on the amino-functionalized surface with a 100 nm-thick SiO2 layer to prepare their networks. On the SWCNT networks, Au electrodes were deposited through a shadow mask. The tube length and network density were characterized by AFM. Transport characteristics of SWCNT-TFTs and their temperature dependence were measured in dry air condition .
Clear dependences on the tube diameter were observed in the carrier mobility, on/off ratio and off-current. In particular, in the plots of on- and off-currents against the tube density, the onset density of increasing off-current considerably varied according to the tube diameter due to the non-negligible off-state conductance, while that of the on-current was independent. Furthermore, the observed off-state conductance in the large diameterís SWCNTs (> 1.8 nm) showed the Arrhenius-type temperature dependence in which the activation energy was consistent with the bandgap of semiconducting SWCNTs. This result suggests that the origin of the non-negligible conductance in off-state is the thermal excitation of carriers in semiconducting SWCNTs. As a result, the behavior of off-current showed the deviation from the simple percolation theory. In addition, such diameter dependence stems from the conductive behavior in the off-state affects the suitable process range of tube density in SWCNT-TFTs. These results confirm the superiority of narrow SWCNTs for TFT application in terms of both on/off ratio and carrier mobility, and will provide a solid guideline for the improvement of the device performance in SWCNT-TFTs.
This work has been partially supported by Research Fellow of the Japan Society for the Promotion of Science (JSPS), and New Energy and Industrial Technology Development Organization (NEDO). A part of this work was conducted at the Nano-Processing Facility, supported by IBEC Innovation Platform, AIST and "Nanotechnology Network Japan" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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