藉由多層催化金屬之低溫合成奈米碳管之成長機制研究

Abstract

由於要將奈米碳管應用於玻璃基板之場發射顯示器以降低成本，且達到大面積面板之製作，因此便需要在低於玻璃熔點(~570℃)的溫度環境下，以化學氣相沉積法(chemical vapor deposition)的方式來成長奈米碳管。本論文是利用Thermal CVD於低溫成長奈米碳管，選擇具有適當表面位能(Surface Energy)的中間層及其金屬碳合金吸熱反應(Heat of formation Carbide)降低催化金屬層(catalyst films)的底部溶碳度，間接助益碳管析出。經過許多實驗驗證，多層催化金屬層(multilayer catalyst films)極具有上述優勢，不論在碳管的morphology和 field emission properties上均表現優異。利用多層催化金屬層所成長的奈米碳管具有表面擴散的高活性成長機制，在最佳化條件下利用多層催化金屬所成長的低溫奈米碳管，展現了優異的場發射特性，在550℃部份，在搭配適當中間層厚度下，鉻(Cr)及鈦(Ti) 分別有: 低起始電場(turn on field) 3.71V/um ,3.5V/um及高場發射電流(current density) 18.24mA/cm2,28.6 mA/cm2。除此之外，在經過一小時的可靠度測試後，均能保持在穩定10 mA/cm2電流密度之能力。在金屬閘極控制的三極結構方面，整個結構分別是在低溫製成下完成(550℃及500℃) 。控制金屬閘極側吃的深度和利用成長時間來控制奈米碳管的長度，進而達到增加陽極電流的控制能力並減少閘極的漏電流來提升場發射效率。 最後，我們利用上述最佳化的多層催化金屬層在低溫(550℃及500℃)下於玻璃基板(sodalime glass)上成長奈米碳管，在長時間可靠度性測試得到了相當優異的場發射特性，在經過一小時的可靠度測試後，均能保持在穩定20 mA/cm2電流密度之能力。 經過我們的研究，目前已經可以利用熱化學氣相沉積法(thermal CVD)，並配合多層催化金屬層，在低溫的環境下成長密度均勻的奈米碳管。同時我們也將其應用在玻璃基板上製造場發射顯示器，如能配合適當的三極結構，加強閘極控制能力，相信將對於場發射顯示器有所改善，並期待在未來大尺寸且高解析度奈米碳管場發射顯示器的誕生。

Simple thermal catalytic-CVD method via careful selection of the interlayer is be chosen on which the thin catalyst layer is deposited, it was found that the interlayer can tune the shapes and sizes of the catalyst particles, which are controlled by the relative surface energies of the interlayer. Previously, efforts have been made to optimize CNTs growth process by adding a metal multilayer below the catalyst layer, then, synthesized the carbon nanotubes at low temperature via specified interlayer with suitable surface energy. The mechanism of the metal catalyst in the formation of the nanoutbes at low temperature has been completely clarified and showed the superior field emission characteristics. Under the proofs of many experimental results, multilayer catalyst films possessed the above advantages. Especially the 20A Co/30A Cr/100A Al and 20A Co/30A Ti/100A Al were the best candidates for multilayer catalyst film , first at 550℃ including low turn on field (3.71V/um)/(3.5V/um) and high current density (18.24mA/cm2)/ (28.6mA/cm2) , then, at 500℃ low turn on field (4.03V/um)/(3.875V/um) and high current density (4.16mA/cm2)/ (3.41mA/cm2) by each, Additionally, there were show great performance no matter the reliability and field emission properties, which exceeded 10 mA/cm2 after 1 hrs stress test. For the gated controlled triode structure, the whole structures were fabricated at 550℃ and 500℃. By controlling the depth of side etching of metal gate and the length of CNTs, the ability of increasing anode currents and reducing gate leakage currents could be realized. Besides, the improvement of gate controlled anode current CNT-triodes was proposed and characterized, and the insulated gate structure field emission triodes can avoid the short circuit problem between cathode and gate.

Finally, CNTs were grown on sodalime glass substrate at low temperature by utilizing multilayer catalyst films as described above and the excellent field emission properties could be obtained, which exceeded 20 mA/cm2 after 1 hr’s stress test. The uniform CNT films were grown successfully at low temperature with the multilayer catalyst films by thermal CVD. Simultaneously, CNTs were grown on glass substrate for the application of field emission display. We think that the field emission display will be developed if a proper gate structure is combined with the glass substrate. And we expect that a large size field emission display with higher resolution will be fabricated in the future.