具有鈷/鈦/鋁催化金屬和新穎自動聚焦閘極結構之奈米碳管場發射元件之研究

Abstract

本實驗室已經研究多層催化金屬在低溫下合成奈米碳管，其中以鈷/鈦/鋁和鈷/鉻/鋁表現最好，然而在550℃下，中間層鈦金屬的場發射特性表現又明顯優於鉻，但是，尤其在更低溫500℃下，它們的特性並沒有在我們預期中的好，因此，本篇論文主要在解決奈米碳管在500℃下，有效率與最佳化的合成，最佳化的合成條件是利用乙烯、氫氣和氮氣的流量控制達成的，經由電子顯微鏡、拉曼光譜以及場發射特性的量測可以達到證實，我們甚至在370℃的超低溫下利用熱化學氣相沉積法合成出奈米碳管，未來如更降低此沈積溫度，將可以運用在塑膠軟板上，當作量產化的生化感應晶片使用。 利用熱化學氣相沉積法合成出奈米碳管，多層催化金屬鈷/鈦/鋁有相當高的成長速率，利用最佳化的成長條件與在不同的溫度下合成奈米碳管，求得多層催化金屬的活化能，由實驗結果得知，多層催化金屬的活化能為0.89電子伏特，相對的低於單層催化金屬的1.54電子伏特，活化能的降低是多層催化金屬能夠在低溫下成長出奈米碳管的一個證據之一，利用原子力顯微鏡，可以證實金屬鋁為平均分散奈米顆粒的效果，利用沒有中間層鈦的方式，間接證實鈦的功能是增加鈦原子的析出，此三層金屬在低溫下合成奈米碳管缺一不可，由於它們有各自的功能。 利用上述在低溫合成催化金屬的技巧，使用多層催化金屬，研發出一種以奈米碳管為場發射源的自動聚焦閘極結構，這種三極結構的閘極，可以有擷取出電子束的功能，同時，又可以擁有電子束聚焦的功能，不像其他運用在場發上元件上的結構，需要多一層聚焦層，不但增加成本，且複雜了製程；本發明只需要簡單的結構就可以達到聚焦功能，對稱長條狀的閘極設計，是經由多次的模擬所得到最佳化的設計，經過實驗結果與發光效果驗證它的可行性。在模擬中，傳統的三極結構的亮點長達622um，相反的，新穎自動聚焦閘極結構只有232um，實際成品，也相差不遠；未來在大面積場發射螢幕上的運用，由於此新穎的結構具有製程簡單以及成本便宜的優勢下，非常大有可為。

In order to decrease the cost and improve the uniformity of CNT-FEDs, the CVDs process is necessary for the synthesis of CNTs on glass substrate at low temperatures below the softening point of glass (~550℃). In this thesis, the thermal CVD was employed to synthesize CNTs at temperatures (lower than 500℃). As a multilayer catalyst, Co/Ti/Al, has been successfully utilized to synthesize carbon nanotubes (CNTs) at 550 ℃ by thermal CVD previously. Its morphologies and field emission characteristics are not as good as we thought. Therefore, the objective of this thesis is optimizing and achieving an effective growth of CNTs using Co/Ti/Al catalyst at 500 ℃ or even lower temperatures to improve the characteristics of CNTs by controlling the flow rates of ethylene, hydrogen and nitrogen. According to the experimental results, the optimum flow rates of hydrogen, nitrogen, and ethylene are 10, 100, and 125 sccm, respectively. It is also found that nanotubes grown using this recipe at 500 ℃ exhibited excellent field emission characteristics. Co/Ti/Al is used to synthesize CNTs at atmospheric pressure by thermal CVD. The relative growth rates, calculated on the basis of the average lengths of nanotubes grown at different temperatures, are utilized to estimate an activation energy of 0.89 eV for the multilayer catalyst as compared with 1.54 eV for the single Co catalyst. Low activation energy is the indirect evidence for CNTs grown at low temperature. Such a low activation energy implies that the nucleation and growth of nanotubes could be effectively enhanced via the multilayer catalyst due to the well-distributed small catalytic nanoparticles by Al supporting layer and higher activity from Ti layer. A self-focusing gate structure is fabricated for the CNT devices with growth conditions is as the same as that mentioned above. The symmetric gate electrode can be employed to extract electrons; meanwhile, it can also act as a focus lens. According to the simulation results and luminescent images, this self-focusing gate structure has a well controllability on the trajectory of electrons emitted from CNTs, and therefore represents a smaller luminescent spot size than conventional structure; that is, it shows an excellent focusing effect. The novel gate structure which only adopts a simple fabrication process has the advantages of low-cost manufacturing and scalability, and is promising for the application in field emission diplays.