交絡奈米碳管網路薄膜電晶體特性之研究

Abstract

本論文利用旋塗法製作交絡碳管網路於區域性背閘極薄膜電晶體，因金屬性與半導體性碳管同時存在於碳管網路中，元件特性必須在導通電流和電流開關比之間取得平衡。在此元件特性有兩種極端情形，一為元件有大導通電流10^-5安培但低電流開關比小於10，另一類元件特性為低導通電流10^-8安培與高電流開關比大於10^3。 為求最佳化之元件特性，本次研究應用交絡理論於隨機分布之碳管網路，系統性的分析各項參數包含碳管密度、碳管長度、元件通道長度與寬度、不同介電層材料與厚度對元件導通電流、電流開關比與載子遷移率之影響。 研究發現增加碳管密度能增加通道中導通路徑之數目，因而提高導通電流。但當金屬性碳管之密度高於滲濾閾值(percolation threshold)時，元件開關比會大幅衰退。隨著通道長度增加，導通電流呈非線性下降，此因區域性背閘極與基板間之高低差造成碳管在通道上分布不均。同時增加通道長度能降低金屬性導通路徑的形成而改善元件開關比。為使元件有高導通電流之特性下同時保有良好的元件開關比，在通道寬度50微米、通道長度分別為1.4、4、7微米之元件尺寸下，最佳化之碳管溶液旋塗次數分別為30、40、60次。 減少通道寬度同樣造成導通電流非線性減少與電流開關比之改善。在此可發現當通道長度大於4微米時，電流開關比將維持在固定範圍，不隨通道寬度而改變。氧化鋁介電層之厚度由10奈米降至5奈米能有效降低元件操作電壓，但對導通電流與元件開關比之影響甚小。增加碳管之平均長度能減少導通路徑所需之碳管數目與碳管間接觸阻抗，使得導通電流大幅提升。然而使用長碳管同時也增加金屬性導通路徑形成之機率，須將碳管旋塗次數降至20次以下，元件才有可能形成半導體特性。比較分別利用氧化鋁與二氧化鉿介電層製作之元件，可發現因二氧化鉿對碳管之浸潤性較差，造成碳管不易分布在二氧化鉿表面而使導通電流較低，因此氧化鋁較適合製作區域性背閘極元件。 除了元件電性分析外，論文中同時模擬二維交絡碳管網路在通道長度1.4到7微米時所需的滲濾閾值，發現增加元件長度會使得滲濾閾值提升。模擬得出之滲濾閾值略高於利用掃描式電子顯微鏡下觀測之滲濾閾值，此因肉眼觀察碳管數目產生之誤差。 利用電性量測轉換之載子遷移率介於0.01至2.7 cm2/Vs之間，此值優於有機化合物之載子遷移率。當考慮碳管分布只佔通道之1%，歸一化之載子遷移率介於1至270 cm2/Vs之間，其載子遷移率低於單根碳管元件，乃是因為碳管網路中串聯之蕭基位障所限制，減少通道長度與寬度可提高載子遷移率。論文最後利用電壓崩潰法，使元件操作在合適的崩潰電壓區間內，對通道長度小於4微米之元件，電流開關比能提升超過兩個數量級。通道長度大於4微米之元件，不需此法元件即可擁有半導體操作特性。

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.