A Study on Thin Film Transistor Performance Based on Percolating Carbon Nanotube Networks

• Main Authors: Chen, Ting-Yeh, 陳定業
• Other Authors: Tsui, Bing-Yue
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/80430600713494923130

博士 === 國立交通大學 === 電子研究所 === 100 === In this work, percolating carbon nanotube (CNT) network fabricated by spin-coating method was applied to local bottom gate thin film transistor. Since the percolating CNT networks consist of mixture of both metallic and semiconducting CNTs, there is a trade off of device performance between high on-state current and high on/off ratio. One extreme case is devices with high on-state current of 10^-5 A at low on/off ratio < 10, and another case is devices with low on-state current of 10^-8 A at high on/off ratio > 10^3. A systematical analysis based on percolation theory was applied to determine various effects including CNT density, CNT length, devices dimension, gate dielectric thickness, and different coating surfaces on on-state current, on/off ratio, and field-effect mobility for optimized device performance in this thesis. Increasing CNT density results in the increase of on-state current due to increase of the number of CNT conducting paths in channel. On the other hand, if metallic CNT density exceeds percolation threshold for high CNT coating density, the on/off ratio would dramatically degrade. Increasing channel length decreases on-state current nonlinearly due to non-uniform CNT coverage resulted from geometric rise of bottom gate. In addition, the increase of channel length also improves on/off ratio since metallic conducting paths are hard to form. The optimized CNT coating density for devices with high on-state current at acceptable on/off ratio > 100 is CNT coating density of 30, 40, and 60 cycles for channel length of 1.4, 4, and 7 μm and channel width of 50 μm, respectively. Decreasing channel width decreases on-state current nonlinearly and enhances on/off ratio. It is observed that on/off ratio > 100 remains and is not varied with increasing channel length of L> 4 μm for high CNT coating density. Decreasing Al2O3 gate dielectric from 10 nm to 5 nm further reduces the operate voltage. But the dependence of on-state current and on/off ratio is weak. Increasing CNT length reduces the number of CNT intersections and then increases on-state current significantly. However, the CNT coating density needs to be below 20 cycles to exhibit semiconducting behavior. For different dielectric layers of Al2O3 and HfO2, since poor wet ability of HfO2 film determined by SEM images attributes low on-state current, Al2O3 film is proper for local bottom gate CNTTFTs. Monte Carlo simulations of two-dimensional percolating CNT networks were performed to obtain percolation threshold for channel length varying from 1.4 to 7 μm. Increasing channel length results in the increase of percolation threshold. The simulation results of percolation threshold are lower than CNT density determined by SEM images since counting error and resolution of SEM contribute to the deviation of percolation threshold between simulation and experimental results. The effective field-effect mobility ranging from 0.01 to 2.7 cm2/Vs is superior to mobility of organics. Since CNT network coverage is lower than 1% in SEM images, the normalized field-effective mobility is in a range of 1-270 cm2/Vs, which is limited by the series of Schottky barriers between CNTs. Besides, decreasing channel length and width would increase field-effective mobility. Finally, we also performed adapted electrical breakdown method to enhance on/off ratio. It is noticed that on/off ratio could be improved by larger than two orders of magnitude for devices with channel length L< 4 μm. For L> 4 μm, all devices exhibit semiconducting behavior without the help of this method.