表面傳導電子發射元件顯示技術之研究

Abstract

在本論文中，我們提出鈀金屬氫化法(Palladium Hydrogenation)來加以製作奈米級裂隙(Nanogap)，此製程擁有簡易、高操控性與可控制尺寸大小等特色。藉由實驗結果與理論分析來深入研究表面傳導電子發射源(Surface Conduction Electron Emission, SCE)之場發射特性、效率及其穩定性測試。此外，我們更著手利用最佳氫化法製程參數調控、奈米裂隙結構最佳化與氫電漿表面處理等方法來加以提升SCE元件之特性。 首先，我們利用高壓氫氣吸附處理，可在細條狀鈀金屬電極上形成奈米級之裂隙。氫氣吸附處理會使得鈀金屬發生相變化，進而由α相轉變成β相，導致其晶格常數增加，此相轉變會伴隨著12 % 的體積膨脹，進而造成鈀薄膜內部產生一巨大壓應力。在室溫（25 oC）下，藉由適當的SCE鈀金屬電極結構安排，即可製造出單一奈米級裂隙之SCE元件。相變化引致之壓應力集中在鈀金屬條狀電極與Pt/Ti接觸電極的交接階梯處，於是在該處便產生鈀金屬破裂。我們探討了氫化SCE元件之電子傳導特性，裂隙寬度不只隨著鈀氫化條件而改變，也可藉由不同SCE結構尺寸來加以改變。具有25 nm寬度奈米裂隙之SCE電子發射源的啟始電壓約41 V。為了在SCE結構中獲得較小裂隙寬度，我們利用有限元素分析探討在不同Pt/Ti電極厚度條件下，SCE結構內之應力分布情形，根據研究結果顯示，最佳化之SCE結構之Pt/Ti電極厚度為20 nm，其奈米裂隙寬度可小於18 nm，並且具有較佳之場發射特性，如較高的場發射電流與較低的啟始電壓(~30 V)。我們利用聚焦離子束法(Focused Ion Beam, FIB)製作具有平坦的陰極表面與平整的裂隙邊緣，我們將之與氫化SCE電子源進行場發射特性比較，結果顯示氫化SCE電子源擁有高達~4 %之場發射效率，顯著優於傳統平面式SCE電子源(~1.03 %)。我們將其歸因於氫化SCE電子源具有粗糙且傾斜突起之陰極結構表面形貌。此研究結果證明對於表面傳導電子發射顯示器應用而言，鈀氫化製程是一個製作鈀奈米級裂隙SCE電子發射源十分理想的方法。 此外，我們也探討氫電漿處理對於鈀奈米裂隙電子源場發射特性的影響。我們利用FIB在鈀金屬條狀電極上製作出一平整奈米裂隙，此鈀奈米裂隙電子源在氫電漿處理後，其場發特性會大幅度提升，此場發射特性的提升主要歸咎於氫電漿處理過程中所造成的粗糙表面形貌，此粗糙表面形貌提供了更多的電子發射位置，且伴隨著較高的場強因子，此外粗糙表面亦可降低鈀奈米裂隙電子源之功函數。

In this thesis, we present a simple, highly controllable, and scaleable method to produce nanogaps by palladium hydrogenation. Experimental results and theoretical analysis are conducted to investigate the field emission properties of the surface conduction electron emission (SCE) emitters, in terms of the I-V curve、field emission efficiency and field emission stability. We used undertake various approaches to improve field-emission characteristics of the Pd nanogap SCE device, including optimization of hydrogenation conditions, nanogap structure optimization and hydrogen plasma treatment. Nanometer-scale gaps in Pd strips were obtained by hydrogen absorption under high pressure treatment. The resulting lattice constant increase due to the Pd phase transformation from the α-phase to the β-phase after hydrogen absorption is accompanied by a volume expansion of ~ 12 %, resulting in a large compressive stress in the Pd thin film. With proper geometric arrangement of the Pd electrode within the SCE emitter structure, a single nanogap per SCE device was obtained at 25 oC. The large stress induced by phase transformation during the Pd hydrogenation resulted in a nanogap in the Pd line electrode at the step area over the Pt/Ti contact pad in the SCE structure. Electron conduction properties of the hydrogenated SCE device were studied, and a turn-on voltage of 41 V for the SCE emitter with a 25 nm nanogap was obtained. The gap width was a function of not only Pd hydrogenation conditions but also the dimension of the SCE structure. Finite element analysis was use to study the stress distribution in the SCE structure with the Pt/Ti contact pad of various thicknesses so that an SCE structure with a minimized gap width could be obtained. Among the SCE emitters under study, the optimal SCE structure, which was with a Pt/Ti contact pad thickness of 20 nm and had a Pd nanogap width of 18 nm, had the best field emission performance in terms of the field emission current and turn-on voltage (~30 V). For comparison, a focused ion beam (FIB) was used to prepare a single nanogap in a conventional SCE emitter which had smooth gap edges. Compared with the conventional SCE emitter, the hydrogenated SCE emitter demonstrated a much higher emission efficiency (~4%). We ascribed the better electron emission performance of the hydrogenated SCE emitter to that the cathode had a rugged and tilting protruding structure on the nanogap edge. The study demonstrates that the Pd hydrogenation is an ideal method to fabricate Pd nanogaps in SCE emitters for surface conduction electron emission display (SED) applications. To study the effect of the hydrogen plasma treatment on field emission properties of the nanogap emitter, a planar nanogap was also prepared on a Pd line electrode by FIB. After the hydrogen plasma treatment, the field-emission property of the Pd nanogap emitter was significantly enhanced. The improvement in the field-emission property was mainly attributed to formation of a ragged morphology on the nanogap emitter during the hydrogen plasma treatment. The ragged morphology provided more emitting sites with a high field enhancement factor. Aside from creating new emission sites, the increase in film roughness could significantly reduce the work function of Pd nanogap emitter, thereby improving field emission efficiency.