藉助催化金屬與元件結構以增進奈米碳管場發射特性之研究

Abstract

在本篇論文當中，我們利用奈米碳管為場發射元件之電子源，並藉由改善催化金屬材料及其元件結構以改善場發射特性。於此研究中，我們運用金屬鈦覆蓋或摻雜於催化金屬鐵以控制催化金屬之活性、大小以及分佈，進而增進場發射電流密度、增強元件可靠度或是改善場發射電流之均勻性。此外，我們亦藉由微影的方式製作柱狀之奈米碳管場發射源，藉由人造結構來得到均勻之場發射電子源分佈，並有效控制柱狀場發射源之間距以降低電場遮蔽效應，而得到最佳化之場發射電流以及啟始電場。最後，於三極場發射元件中加入一層氮化矽以阻絕閘極與陰極間之漏電，進而改善三極場發射元件效率不佳之缺點。 首先，在場發射電流密度方面，對於奈米碳管而言，影響其電流密度之主要原因在於過高的奈米碳管密度所導致的電場屏蔽效應。對此，我們在經過氫氣前處理之鐵奈米顆粒上沈積一層微薄之金屬鈦，藉助鈦阻擋含碳之反應物質進入鐵催化金屬進而抑制奈米碳管之生成以降低奈米碳管的密度。藉由沈積不同厚度之金屬鈦，我們可以有效地控制奈米碳管之密度以增進其場發射電流密度並降低其啟始電場。此外，藉由所沈積之金屬鈦在奈米碳管成長過程中受熱部分融化並包圍鐵催化金屬奈米顆粒，我們發現元件在量測過程所發生之電流崩潰現象有效地被抑制了，而其場發射電流在高電場下之劣化現象亦有效地被改善，針對此一結果，我們認為金屬鈦之包圍使得奈米碳管與基板之間的附著性被增強且降低其接觸電阻，因此改善了元件的可靠度。 然而，雖然電流密度與可靠度被改善了，其均勻性不佳之現象依然無法得到解決，是以我們利用金屬鈦與催化金屬鐵共鍍作為成長奈米碳管之催化金屬層，藉此使催化金屬顆粒之形成更為均勻，並藉由金屬鈦抑制催化金屬顆粒的聚集以得到小尺寸、且尺寸均勻之催化金屬奈米顆粒，最後成長出長度均勻之奈米碳管並在塗佈螢光粉之陽極板上得到均勻之光源。此外，因為鐵催化金屬顆粒於催化金屬層中受熱析出而形成部分被埋於催化金屬層中之結構，其可靠度亦大為改善。 接著，我們利用微影的方式以鐵鈦共鍍為催化金屬製作柱狀奈米碳管之場發射元件。藉由鐵鈦共鍍以得到均勻、筆直且具有穩定場發射電流之奈米探管；利用微影控制其間距，以有效降低電場屏蔽效應並且避免距離過大而減少場發射區域之總面積。是以，得到一個均勻分佈且具有較高場發射電流密度之場發射電子源。 最後，針對三極式場發射電子元件效率不佳之缺點，我們在元件中增加一層氮化矽絕緣層於閘極之上或閘極之下，以阻絕由陰極被閘極電場所吸引出來的電子電流，藉此改善電流效率以及功率效能。 在本論文中，我們提出了簡單、便宜且不會對場發射源奈米碳管造成結構損傷之方式來改善場發射電子元件之場發射特性。是以相當具有應用於場發射平面顯示器或是液晶螢幕背光模組之潛力。

In this dissertation, the carbon nanotubes (CNTs) were utilized as the electron source in field-emission devices. By modifying the metallic catalysts and the device structures, the field-emission characteristics were greatly improved. In this research, Ti capping layer and Ti codeposited with Fe were used to control the activity, size, and distribution of the Fe catalytic nanoparticles for improving the emission current density, reliability, and uniformity of the devices. Moreover, the diameter and position of pillar-like CNTs synthesized from Fe-Ti codeposited catalyst were controlled by lithography. Therefore, a uniform distribution of emitters with suppressed screening effect was obtained for high emission current density and low turn-on field. Finally, a silicon nitride film was added into the triode-type field-emission devices to block the leakage current between the gate and the cathode for improving the power efficiency. For emission current density, the high density of CNTs caused a serious screening effect which could greatly decrease the emission current density. Accordingly, a thin Ti capping layer was deposited on the hydrogen pretreated Fe nanoparticles to resist the diffusion of the carbon radicals and effectively reduce the density of emitters. By altering the thickness of the Ti capping layer, a suitable density of CNTs was obtained with high emission current density and low turn-on field. Moreover, the Ti capped on the Fe nanoparticles held the nanoparticles firmly to provide stronger adhesion and lower contact resistance than those synthesized from the pure Fe. It remarkably suppressed the breakdown of the field-emission devices and diminished the degradation of emission current density at high electric field. It might result from the improvements of the contact properties with the modification of metallic catalyst. However, the problems of uniformity were still not solved by means of the thin Ti capping layer. Therefore, a Fe-Ti codeposited metal layer was utilized as the catalyst of CNTs. During being heated, the nucleation of Fe atoms formed smaller nanoparticles with better uniformity due to the suppression of coalescence between Fe nanoparticles than those synthesized from pure Fe. A homogeneous light emission was therefore observed on the phosphor (P22) coated glasses. Furthermore, the nucleation of Fe nanoparticles resulted in a partially immersed structure of the CNTs which provided better contact properties between the CNTs and the substrates. As previous description, the reliability of the device could be improved. Additionally, the lithography was utilized to form an artificial structure of pillar-like CNTs to control the diameter and distribution of emitters more precisely. The Fe-Ti codeposited catalyst was utilized for uniform CNTs with reliable emission current. A uniform distribution of emitters with low turn-on field was therefore achieved. Finally, a silicon nitride layer was deposited on the poly-gate or under the poly-gate to block the electron emission from the cathode to the gate. Both of them could effectively improve the current efficiency and therefore increase the power efficiency of triode-type field-emission devices. In this dissertation, simple, costless, and harmless methods have been proposed to improve the field-emission characteristics of CNTs. It showed a great potential in the applications of the field-emission displays and the back-light units in near future.