利用二段式熱化學氣相沉積法成長不同間距高度比之奈米碳管柱列之場發射特性研究

Abstract

此篇論文之實驗當中,成長奈米碳管方面,為了可以減低成本且能夠均勻的將奈米碳管應用於場發射顯示器當中,熱化學氣相沉積法比起其他分法具有簡單且低成本的優勢,因此被認為是最具有潛力的碳管成長發法之一。在催化金屬的使用方面,於多層共鍍催化金屬合成柱狀形式奈米管的奈米探管陣列於場發射的應用,採取了鈷鈦共鍍的合金,此方式不僅得到高度一制性的碳管管徑並且利用了金屬鈦於高溫下與炭的化合結果,能夠有效提升奈米碳管對於基板的附著能力以及展現較佳的場發射特性。 首先,根據以往關於柱狀形式奈米碳管用於場發射方面的論文當中,數種不同高寬比都被提及,例如 R/H = 6，R/H=3，R/H=2等，並沒有真正探討出高寬比與柱狀形式奈米碳管陣列組合上之絕對關係，在這實驗當中，設計了三種不同柱狀奈米碳管之間的間距，且由控制成長時通入碳源的流量與成長的時間可以穩定成長出所需要柱狀奈米碳管的高度，進而討論其R/H值，當中，也發現到當柱狀奈米碳管之間的間距越遠，可以得到其對應最小起始電場的理想R/H值有趨近於2的趨勢，實驗中定義了電流密度為J=10 iv μA/cm2對應到的電場為起始電場，在間距80μm，150μm與250μm最佳R/H值為8，4.7與3.2分別能夠對應到最小起始電場為Eturn-on=1.78V/μm, Eturn-on=1.55V/μm與 Eturn-on=1.53V/μm，並且在最佳R/H情況下可以得到均勻性最佳的場發射效果。 根據先前實驗中奈米碳管成長機制,倘若要得到長度較長的奈米碳管，必須控制其成長時碳源氣體流量與成長時間的搭配，因此，改良了單一階段成長的參數而成功的達到二階段形式的成長方式,此方法不僅能夠成長出較高的柱狀碳管並且在場發射方面可以得到相當優異的起始電場Eturn-on=0.1V/μm,以及飽和電場Ethreshold=0.3 V/μm,實驗中也定義了電流密度為J=10mA/cm2對應到的電場為飽和電場，並且利用了SEM, Raman, TEM 作了材料特性方面上的分析。 由於奈米碳管密度會影響其場發射特性,為了使其均勻特性有效提升,近一步使用電漿來改善柱狀奈米碳管的表面密度分佈是可行的方法之一，本論文中提出了使用純氧電漿或氧氣加氯氣的電漿作為柱狀奈米碳管的後段處理，當中發現氧氣加上氯氣的電漿可以藉由改變轟擊的時間，造成某些奈米碳管被摧毀，而留下一些較高的奈米碳管，而這些較高的奈米碳管表現了較高的場發射電流，當中也明顯發現了，經過電將處理後柱狀奈米碳管的幾何型態有顯著的變化，也因為型態上的改變使得電場遮蔽效應的下降，以致於起始電場的降低與均勻性顯著的提升，並且得到了當氧氣加氯氣電漿處理三分鐘後的結果,電場遮蔽效應能夠最有效的抑至，而起始電場Eturn-on=1.4V/μm相較於未經過電漿後處理的起始電場Eturn-on=2.6V/μm有很明顯的下降。 最後，在奈米碳管場發射的應用當中，低操作電壓是個很重要的因素，因此設計了一種溝槽式的奈米碳管三極結構，利用氧化層來取代傳統的絕緣間隙層，藉由控制奈米碳管的長度可以改變閘極到發射極的間距，實驗中固定陽極電壓為800V，當閘極電壓從0V增加到80V時，場發射結果的亮度和均勻性都會大幅提升，並且論文中也利用了SIMION 3D軟體，模擬了不同閘極到發射極的間距與閘極對於均勻性的影響,因此這結構應用於奈米碳管場發射將是不錯的優勢。

Cost reduction and uniformly distribution of carbon nanotubes are the two main topics in the field emission display application. Thermal chemical vapor deposition (T-CVD) is regarded as one of the best candidates of the growth methods due to the merits of simplicity and cost efficiency in the fabrication and large scalability. Besides, we also find titanium (Ti) layer can improve the uniformity and strengthen the adherence between the CNTs and substrate for CNTs’ growth. Therefore, co-deposited multilayer catalyst composed of Co and Ti on Al (Co-Ti/Al) is used in the T-CVD to grow pillar arrays of CNTs. It has been reported that the field emission can be effectively enhanced for the aligned CNTs as the field emitters when the ratio of distance between neighboring nanotubes (R) to the height of each individual CNT (H) is about 2. Although the pillar arrays of CNTs with different optimum R/H ratios to get best field emission characteristics were investigated by the different results have been reported. In this thesis, three different inter-pillar distances(80μm、150μm and 250μm) are designed to find the optimum R/H ratio of pillar arrays of CNTs by controlling the flow rate of carbon source and growth time precisely. The optimum R/H ratio is approaching to 2 when the inter-pillar distance is as larger as possible.CNT pillars probably behaves like an individual field emitter and has less field screening effect between pillars. The results show that when R/H ratios are 8, 4.7 and 3.2 for inter-pillar distance of 80μm、150μm and 250μm , respectively, ultra low turn-on field(1.78V/μm, 1.55V/μm and 1.53V/μm, respectively) is achieved and fluorescent uniformity can also be improved. Two-step method is used to grow CNT pillar array until with 80 μm in the height successfully. CNT pillar array synthesized with two-step method revealed a good field emission performance of an emission current density of 10mA/cm2 at the electric field of 0.64 V/μm, and field emission characteristics show a very ultra-low turn-on field of 0.1V/μm at an emission current density 10 μA/cm2 .As CNT pillar with one-step skill to two-step technique exhibit the IG/ID increases from 1.5 to 1.8 which confirms that the CNTs for the two-step one have a highly crystalline graphite structure as shown in TEM micrograph SEM show that thinner diameter about 24 nm is observed on the top of CNTs′ pillar. In addition to better crystallizing, the thinner diameter (i.e. high aspect ratio) also facilitate to the field emission characteristic. Plasma post-treatment is one of the best ways to improve the field emission properties and fluorescent uniformity. In this thesis, we propose O2 plus Cl2 plasma post-treatment to the pillar array of CNTs and the experimental results reveal that the improved emission properties can be achieved by optimizing the density of CNTs and the defects on the nanotubes under proper plasma treatment conditions. The turn-on field is reduced from 2.6 V/μm to 1.4 V/μm and the fluorescent uniformity is also improved when the plasma post-treatment time is 3 min for the O2 30sccm plus Cl2 10sccm. Finally, field emission triode is fabricated to reduce the operation voltage and improve emission uniformity by the beam spreading. The optimum parameters of the field emission trides are obtained according to the simulations, the gate-to-emitter length is 1 μm. The experimental results show that the driving voltage is as low as 17 V. In the future, this structure is promising for the applications in a planar backlight because of the large-area uniformity and simple fabrication process with low cost.