具有適當密度與元件結構之奈米碳管之場發射特性最佳化

Abstract

利用熱化學氣相沉積系統成長出來的奈米碳管的密度依舊太高而導致因遮蔽效應而造成場發射電流密度下降和起始電場上升。在此論文中，我們提出了三種新穎的密度控制方法去控制奈米碳管的密度進而改善場發射特性，其中包括了兩種薄的鈦覆蓋層的製程和一個鐵和鈦共鍍的製程。對於沒有前處理而蓋薄的鈦覆蓋層的方法，其場發射特性不好。對於前處理完後蓋薄的鈦覆蓋層的方法，當奈米碳管的密度由2.8x10^8 cm^-2下降到2.8x10^7 cm^-2時，在6 V/μm 的電場下其場發射電流密度從11.12 mA/cm^2增加到97.6 mA/cm^2，起始電場從3.786 V/μm下降到2.5 V/μm。然而，從發光的圖像來看，其均勻度依舊沒有被改善。對於鐵和鈦共鍍的方法，由於在濺鍍的過程中鐵和鈦的原子是均勻分佈在其中，所以可以得到一個比較均勻的催化金屬膜。由此可知，共鍍的技巧將會改善奈米碳管的均勻度並期待在未來大尺寸且高解析度奈米碳管場發射顯示器的誕生。除此之外，我們提出了兩種機制來解釋這三種密度控制的現象。 為了符合場發射顯示器低電壓操作的目的，我們利用一控制閘極來製造奈米碳管之發射三極元件。然而，傳統的三極結構 (二氧化矽-多晶矽) 有一個很嚴重的問題，那就是低發射效率，大概是1.23 % 左右。因此我們在傳統的三極結構中加入了一個氮化矽絕緣層來克服這個問題。從模擬來看，當加入了氮化矽絕緣層之後，閘極附近的電位線有變疏的現象，而且電位線的分佈也因此被改變。另一方面，氮化矽絕緣層將會阻擋部分的電子發射到閘極端因此使閘極的漏電流下降。綜合上述的原因，這將會使得更多的電子穿越過閘極的區域而到達陽極端，因此場發射效率將會被提升。如果氮化矽絕緣層是加在多晶矽的上方的話，其發射效率將會增加到8.56 %，而且也因為維持了閘極的控制力，故使得陽極的場發射電流密度高達5.44 mA/cm^2.

The density of CNTs synthesized by thermal CVD was still too high to reduce the field emission current density and ascend the turn-on field due to the screening effects. In this thesis, three novel methods including two thin Ti capping layer processes and a co-deposition of Ti and Fe process were introduced to improve the field emission properties via controlling the density of CNTs. For the thin Ti capping layer without pre-treatment, the field emission properties were poor. For the thin Ti capping layer with pre-treatment, the field emission current density improved from 11.12 mA/cm^2 to 97.6 mA/cm^2 at the electric field of 6 V/μm and the turn-on field decreased from 3.786 V/μm to 2.5 V/μm when the density of CNTs diminished from 2.8x10^8 emitter/cm^2 to 2.8x10^7 emitter/cm^2. However, the uniformity was not still improved from the luminescent images. For the co-deposition of Fe and Ti, the atoms of Fe and Ti were uniform distribution during the sputtering, so it could obtain a more uniform catalytic metal layer. This showed the co-deposition technique could improve the uniformity of CNTs and expect that a large size field emission display with higher resolution would be fabricated in the future. Besides, two mechanisms were put forward to expound these phenomena of three density control methods. The CNTs triode structures with an extraction gate were proposed to achieve the low voltage modulation. However, the traditional triode structure (oxide-poly) had a serious problem of low efficiency (1.23 %). In order to overcome this problem, the traditional triode structure adding a nitride insulting layer was introduced. From the simulations, the electrostatic potential near the gate electrode was getting sparse and the distribution of electrostatic potential was also changed after adding a nitride insulting layer. On the other hand, the nitride insulting layer would block some electrons emitting to the gate electrode hence cause the reduction of gate leakage current. Sum up these reasons, it would make more electrons to pass through the gate holes and collect by the anode hence cause the increase of efficiency. If the nitride insulting layer was acceding above the poly-Si, the efficiency was added to 8.56 % and the emission anode current density was about 5.44 mA/cm^2 which might result from the keeping of gate controllability.