利用金屬薄膜與奈米碳管為場發射材料之側向式場發射元件之研究

Abstract

對於場發射元件而言，為改善場發射特性使其可應用在高效能元件上，降低元件的操作電壓與增加場發射電流是非常必要的。此篇論文中，我們提出兩種側向式場發射元件以達成上述目標。 第一種元件為金屬薄膜邊緣側向場發射器，此種結構可將發射器與收集器之間的距離降至150奈米，因此可降低操作電壓，同時，此距離可藉由非晶質矽的厚度準確控制。在元件製備完成之後，對元件進行氫氣或乙烯之後處理，使其進一步改善場發射特性。對鈷薄膜邊緣場發射器而言，在經過乙烯處理之後，場發射特性有明顯改善，這是由於處理後產生石墨層與碳化物，造成鈷表面的起伏遽增，因而增大了場發射增強因子，使場發射電流達到10-7 安培的起始電壓可降至8伏特，相較之下，經過氫氣處理之後，其起始電壓為12伏特，這是由於經氫氣處理後的表面起伏不如乙烯處理來得明顯；另一方面，鈀薄膜邊緣場發射器在經過乙烯及氫氣個別處理之後，表面起伏無明顯改變，這是由於鈀薄膜與底層的鎢膜表面能差異較大，底層的鎢膜不傾向露出表面，而經過乙烯處理後，其起始電壓為25.5伏特，相較之下，經過氫氣後處理之後，場發射特性具有明顯改善，這是由於鈀經過氫處理後產生PdHX，而PdHX的功函數較純Pd小，於是，此種利用Pd為場發射極的側向式場發射元件可進一步將起始電壓降至6.5伏特。 此外，基於奈米碳管的優異場發射特性，我們提出同平面式與階梯式兩種不同型態的側向式場發射元件。對於同平面式的場發射元件而言，當發射器與收集器之間的距離固定為2 μm時，其起始電壓為9伏特。為進一步改善場發射特性，我們提出另一種階梯式的側向式場發射元件，利用收集器底下的氧化層厚度變化，使得碳管頂端更加接近收集器以增強場發射電流，此種結構可將起始電壓進一步降至1.8伏特。 根據實驗的結果，奈米碳管的場發射特性較金屬薄膜為佳，即使金屬薄膜場發射器可將收集器與發射器之間的距離縮得更短。

For the sake of improving field emission characteristics of field emission devices to apply on high performance devices, lower operation voltage, and higher field emission current are required in field emission devices. In this thesis, two kinds of lateral field emitters are proposed to accomplish such aims. Thin film edge field emitters with small emitter-to-collector gap as small as 150 nm are proposed to reduce the operation voltage. The distance between emitter and collector is defined by the thickness of amorphous silicon and hence could be controlled precisely. After the devices are fabricated, the samples are treated by C2H4 and H2 individually to improve the field emission characteristics further. The cobalt thin film edge field emitters treated with C2H4 gas could enhance field emission current by increasing the roughness of Co surface due to the graphite layers and carbide formation, thus a higher field enhancement factor. For the Co emitter, the turn-on voltage at an emission current of 100 nA could be reduced to 8 V after being treated with C2H4 gas. As compared with treated with C2H4, the improvement of field emission characteristics of Co emitters treated with H2 was not so obvious due to less increase of roughness and the turn-on voltage was 12 V. On the other hand, the change of roughness of the palladium field emitters treated with C2H4 and H2 gas was not apparent due to the large difference of surface energy between Pd and under-layer, W. The turn-on voltage of Pd emitters treated with C2H4 was 25.5V. However, it could be reduced to 6.5 V after the Pd ones being treated with H2 to form the PdHX which owns lower work function than pure Pd. Additionally, two types of emission devices, co-planar and step-type ones, based on carbon nanotubes are proposed. The turn-on voltage of co-planar type lateral field emission devices with emitter-to-collector gap fixed at 2 μm is 9 V. On the other hand, step-type lateral field devices are also proposed to improve the field emission characteristics. In contrast to the co-planar ones, the distance between the tips of CNTs and collector could be shrunk by increasing the thickness of oxide under collector and hence could lower the operation voltage and enhance the field emission current. The turn-on voltage of step-type lateral field emission devices could be further reduced to 1.8 V. According to the experimental results, the field emission characteristic of CNTs is better than the metal thin films even with a shorter emitter-to-collector distance.