藉由密度控制以提高真空微電子用之奈米碳管之場發射特性

Abstract

利用熱化學氣相沉積系統伴隨乙烯與氫氣混合環境於鍍鐵矽基板下可成長出高密度奈米碳管。由真空量測中可以發現奈米碳管具有優異的場發射特性。奈米碳管在高密度成長下場發射特性呈現衰退現象，然而由於鈦覆蓋層(Ti capping layer)使奈米碳管的密度由1010/cm2 減少至 108/cm2 ，進而改善場遮蔽效應(screening effect)影響。在第二章中，我們提出利用覆蓋層使碳管密度減少而降低起始電壓，且由於管壁缺陷的增加進而提高場發射電流。另一方面，由於碳管與矽基板間的附著性較差，在高電場操作下，碳管易於被拔起而無場發射現象。利用覆蓋層也可使附著性增加，使在高電場操作下達到高電流密度(30 mA/cm2)，進而實現長效穩定場發射電流。由觀測螢光板光源中可以證實，利用覆蓋層在奈米碳管成長中於長時間操作下，可以達到均勻且穩定的電流。本論文對於奈米碳管在覆蓋層效應下提出兩種成長機制：(1)覆蓋層原子在高溫下會因為內聚力移動使鐵催化金屬暴露在碳源氣體中，進而遵循氣相—液相—固相(VLS)成長機制；(2)假設催化金屬在覆蓋層原子移動後無法直接與碳源氣體分子接觸，碳源分子仍可利用高溫穿越覆蓋層原子而擴散進入催化金屬中，進而析出奈米碳管。 在第三章中，我們提出奈米碳管應用於側向場發射元件。利用氮化矽絕緣層結構有利於在真空下封裝且可以避免奈米碳管與電極間的短路產生。氮化矽層不僅可在碳管成長中當作硬遮蔽層，而且提供較佳的物理強度。應用此結構可成功避免短路現象進而改善場發射電性。此外，配合高密度氬電漿後處理可以切斷無秩序的奈米碳管且可使碳管頂部催化金屬被移除。由上述方法製造出側向場發射元件，其起始電壓在電極與碳管間距1□m下可低於2 Volt，且場發射電流密度在10 Volt時可達到0.4 A/cm2。

High density of carbon nanotubes (CNTs) array was synthesized via thermal chemical vapor deposition on iron-coated silicon substrates with an acetylene and hydrogen mixture. The fabricated CNTs emission arrays showed excellent field emission properties. However, the field emission properties of the high density CNTs degraded because of the screening effect of the electric field. The density of CNT decreases from 1010/cm2 to 108/cm2. In chapter 2, by means of capping layer method, the turn-on voltage decreases with the modification of the screening effect while the emission current density increases with the increasing of defects. At high electric field, some of the aligned CNTs were pulled off due to the weak adhesion between the CNTs and the substrate. Capping layer can improve the adhesion problem and the current density can attain to 30 mA/cm2. Furthermore, long-term stability of the emission current at high operating voltage can be realized. The photographs of the light emitted by the phosphor screen clearly show a homogeneous field emission current with capping layer. Here, we suppose two mechanisms to synthesize CNTs with capping layer. First, when Ti atoms assemble together like liquid, the Fe may be exposed and follow the vapor-liquid-solid (VLS) growth model. Second, the Fe may not be exposed but the carbon atoms still can diffuse through the thin Ti capping layer and extract the carbon graphite. In chapter 3, a lateral field emission device which is based on carbon nanotubes is proposed. In order to avoid the short circuit between the CNT emitters and electrode, a nitride-insulated structure was used in lateral field emission device. The nitride layer not only was used as hard mask for CNT synthesizing but also had superior mechanical strength. It successfully avoided the short circuit problem between the electrode and emitters, and therefore, improved the field emission characteristics. Furthermore, argon plasma-post-treatment was introduced on the lateral device to cut off the disordered CNTs and remove the catalyst on the tip of CNTs. The turn-on voltage was 2 V with 1 um interelectrode gap, and the emission current density is as high as 0.4 A/cm at 10 volt. .