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標題: 奈米碳管之場發射特性探討
Field Emission Properties of Carbon Nanotubes
作者: 張慶州
Chang, Ching-Chou
關鍵字: Carbon Nanotube;奈米碳管;Field Emission;場發射
出版社: 精密工程研究所
在本論文中,主要利用微波電漿輔助化學氣相沉積系統來成長超微細奈米碳管(CNT, carbon nanotube)及準直排列(well aligned) 之碳管陣列的陰極材料,並利用積體電路製程製作二極場發射元件,探討CNT材料特性對場發射元件(Field Emission Devices)性能的影響。
本研究共分為兩個部份,在第一個部份「區域選擇性成長奈米碳管暨成長機制研究」中,藉由控制鎳催化金屬層厚度可獲得不同密度與大小的催化金屬微粒,高密度且具有方向性的奈米碳管可從較薄的催化金屬層約5 nm厚度來獲得。實驗中基板加溫在300℃可得到較佳之成長速率。隨著甲烷流量比增加,奈米碳管成長速率越快,甲烷流量比為15% 時,其沉積速率約 3µm/min。然而隨著氮氣流量增加,奈米碳管成長速率減小,但其管形也隨之越筆直。
在第二個部份「二極式奈米碳管場發射元件特性研究」中,應用田口實驗設計法於沉積奈米碳管的製程,以電漿功率、前處理時間、甲烷的流量比[CH4 flow ratio,CH4/(H2+CH4)]、及氮氣的流量四個製程參數為控制因子,成長時間固定為18分鐘,來做場發射臨限電壓望小特性與電流密度望大特性的分析。發現電漿功率是影響場發射性能最大的因子,並得到了最佳電性(低臨限電壓、高電流密度)的製程參數條件為:電漿功率1200W、前處理時間6min、甲烷流量比5%、氮氣流量20sccm完成的。所製作出二極式奈米碳管場發射元件,其臨限電場為0.7V/µm (J=10µA/cm2)、在電場E=5V/µm下電流密度為3.9 mA/cm2,而拉曼光譜(Raman spectroscopy)的分析顯示此優異場發射特性可能來自於奈米碳管中碳的鑽石鍵結結構多於石墨鍵結結構。

In this thesis, carbon nanotubes (CNTs) and carbon nanotube emitter arrays are synthesized by microwave plasma-enhanced chemical vapor deposition (MPCVD) method. CNT diode devices are fabricated using conventional integrated circuit (IC) processes. The influences of material properties of CNTs on the field emission characteristics of the device are investigated.
This research includes two major parts: (I) selective deposition of CNTs and the corresponding growth mechanism, and (II) field emission properties of carbon nano-tubes. Various densities and sizes of Ni nano-particles are generated from different thicknesses of catalytic films by hydrogen pretreatment. Dense and well-aligned CNTs are generated on thinner catalytic film of about 5 nm. The CNTs can be coated properly when the temperature of substrate heated at 300℃. The deposition rate of CNTs increases with the methane flow ratio [CH4/(H2+CH4)]. The deposition rate is about 3µm/min at 15% methane flow ratio. The deposition rate decreases with the increase of nitrogen flow, but CNTs are more up-grown and better aligned.
Taguchi method is applied in the study of field emission properties of carbon nanotubes. Four process parameters, source power, pretreatment time, methane flow ratio, and nitrogen flow rate, are selected as control factors. Deposition duration is fixed at 18 minutes. From smaller-the-better analysis of threshold voltage and larger-the-better analysis of emission current density, it is observed that plasma power is the most important factor that influences the performance of the CNT diode device. The condition of power 1200watts, pretreatment 6 min, 5% CH4 flow ratio and 20 sccm N2 is selected for confirmation experiment. Under this fabrication conditions, the CNT device can be performed at low threshold electric field of about 0.7V/µm (J=10µA/cm2) and high current density of about 3.9mA/cm2 at 5V/µm. As revealed by Raman spectroscopy, the CNT structure obtained in this experiment comprises more carbon diamond bonds than graphite bonds. This fact may explain the better field emission characteristics for the CNT device obtained in this research.
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