低溫合成之奈米碳管與薄膜側向場發射子之場發射特性研究

Abstract

在本論文中，我們主要研究標的為場發射材料與場發射元件的低溫製作方法與其場發射特性研究。為了可以均勻且低成本的將奈米碳管應用於場發射顯示器上，熱化學氣相沈積比起其他方法具有簡單且低成本的優勢，因此被認為是最有潛力的碳管成長方法之一。藉由多層結構催化金屬的使用，我們可利用熱化學氣相沈積在低溫下於玻璃基板上合成奈米碳管。多層結構催化金屬的組成包含支撐層、中間層與催化金屬，其中支撐層可以有效幫助催化金屬均勻分散避免其聚合成過大的粒子，而中間層除了幫助催化金屬保持均勻分散外還可以促進碳原子的析出進而形成石墨層結構，由實驗結果可以發現，當中間層材料為鉻與鈦時，奈米碳管的型態是最佳的，同時其表現出絕佳的場發射特性：當所施加的電場強度為6 V/μm時，其場發射電流密度分別可達到18.24與28.60 mA/cm2，遠超過應用上所需的數值。除此之外，碳管的場發射特性與外觀也會受到製程條件的影響，如反應氣體流量。藉由逐步改變與控制製程氣體的流量比例，我們得到一組最佳的成長參數，當製程氣體乙烯、氫氣與氮氣分別為125、10與1000 sccm時，奈米碳管具有最好的電性表現：。實驗結果也說明場發射特性與碳管的結晶性有相當程度的關係，隨碳管結晶性增加其場發射電流的穩定性也跟著增加。同時，碳管成長的活化能圖說明多層結構催化金屬具有較低的成長活化能，因此相較於其他催化金屬可以有效於低溫下催化碳管成長。 接下來，為有效改善電子束發散的現象，我們提出一基於低溫成長奈米碳管的自聚焦場發射元件，此一元件結構不需要額外的聚焦閘極而是採用將電極設計成一雙條狀電極且平行於場發射區域旁，不同於傳統元件結構具有被閘極包圍的場發射區域，自聚焦閘極結構因為條狀電極只鄰近單一邊的碳管，因此會造成異於傳統的非對稱的場發射區，結合此兩個非對稱場發射區域，場發射電子將於陽極板上形成一重疊的顯示區塊。從實驗結果來看，此一新穎閘極結構可以有效控制電子束軌跡且於陽極板上形成較小的發光區。模擬結果說明相較於傳統沒有聚焦結構的元件，當採用自聚焦閘極結構時陽極板上的場發射區域可由622微米縮小至232微米，同時實際螢光版上的發亮區域也顯示出相同的實驗結果，因此證明自對焦閘極結構可簡單且有效地控制電子束於陽極板上的大小。 最後，我們提出兩種形成次微米間距的方法並將其應用於製作具有低超作電壓的薄膜側向場發射子。元件可藉由薄膜沈積與濕式蝕刻完成，且電極間距可由蝕刻時間來控制，次微米間距形成於射極與集極間，當其間距為200奈米時，元件的啟始場發射電壓可以降低到48伏特。此外，為了進一步更可靠地製作此一次微米間距，我們提出一類平面側向場發射元件結構，此元件的電極間距可以藉由薄膜的厚度來調變，不同的膜厚將形成不同的間距大小。另外，經由一形成製程改變場發射子的表面型態，形成較高的表面粗糙度，元件的場發射特性可明顯的改善。當控制電極間距此一膜層的厚度為200奈米時，經過形成製程處理的元件，其場發射啟始電壓值可降低到9伏特。

In this thesis, low-temperature synthesis processes of emitter materials and devices as well as their field emission characteristics were investigated. For their application in field emission displays, carbon nanotubes (CNTs) should be employed uniformly on glass substrates in order to reduce the manufacture cost. Thermal chemical vapor deposition (t-CVD) had the merits of simplicity and cost efficiency in fabrication and large scalability as compared with other techniques. Therefore, it seemed to be a potential method for synthesizing nanotubes on glass substrates. Multilayer catalysts utilized in thermal CVD systems showed a remarkable catalytic ability for growth of CNTs at low temperatures. The multilayer catalysts were composed of supporting layer, interlayer, and catalytic metal. A supporting layer had the functionality in uniformly distribution of catalytic nanoparticles, meanwhile preventing their agglomeration. Besides improving the uniformity of catalytic particles, interlayers were able to enhance the precipitation of carbon atoms, thus resulting in the formation of graphite sheets. According to the results of experiment, while the interlayers were chromium (Cr) and titanium (Ti), carbon nanotubes showed the better morphologies and field emission performances: field emission current density of 18.24 and 28.60 mA/cm2 for Cr and Ti, respectively, at the electric field of 6 V/μm. Moreover, field emission characteristics of nanotubes can be improved by optimizing the synthesis conditions, i.e. reaction gas flow rates. According to the results of experiments, an optimal synthesis condition formed of reaction gases was 125, 10, and 1000 sccm for C2H4, H2, and N2, respectively. The morphology and field emission performance also showed a significant relationship corresponding to the crystallinity of nanotubed analyzed by Raman Spectra. A high stability of emission current was correlative with a better crystallinity. The growth activation energy calculated based on the dependence of nanotube length versus temperature revealed the fact that CNTs synthesized with a multilayer catalyst displayed a lower value than single or binary catalysts. Next, a CNT-based device with a self-focusing gate structure was proposed to obviate the issue of electron beam divergence. Without additional focusing electrodes, the self-focusing gate structure employed a pair of gate electrodes parallel with the vicinity of emitters, which resulted in an asymmetric emission area as compared with the conventional gate structure. Therefore, electrons emitted from the emitters gave rise to an overlapping region on the anode plate so that a reduction of spot size had been achieved. According to the simulation results and luminescent images, this self-focusing gate structure had a well controllability on the trajectory of electrons, and therefore showed a smaller luminescent spot size than the conventional one. Because of the overlapping of electron beams, the luminescent spot sizes could be remarkably reduced to 232 μm in x direction as compared with 622 μm for the conventional gate structure which had a serious issue of beam divergence. As a result, the self-focusing gate structure manufactured with a simple process can produce well-focused electron beams for the application in FEDs. Finally, two simple techniques of creating sub-micron gaps were proposed for thin film edge emitters in order to realize the feasibility in low-voltage operation and simplicity in fabrication. A lateral field emitter was manufactured by thin film deposition and wet etching processes. The spacing was determined by the lateral etching distance formed during etching stage. By controlling the duration of etching, the distances between emitters and collectors were well defined in submicron ranges. Device performance showed a low turn-on voltage of 48 V at an emission current of 100 nA as the emitter-collector spacing was 200 nm. In addition, for creation of submicron gaps in a more robust way, a novel quasi-planar thin film field emitter was proposed and fabricated utilizing the similar idea. The spacing between the emitter and collector could be well controlled via the thickness of Cr layers, which created submicron gap. A forming process caused an increased surface roughness of emitters and resulted in a higher field enhancement factor, which showed better field emission characteristics. The quasi-planar field emission diode with the first Cr layer of 200 showed a low turn-on voltage of 9 V at the current level of 100 nA.