Study on the Field Emission Characteristics of Low-Temperature-Synthesized Carbon Nanotubes and Thin Film Edge Field Emitters
|關鍵字:||場發射;奈米碳管;低溫;側向場發射子;field emission;carbon nanotubes;low temperature;Edge field emitters|
|摘要:||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。
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.
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