奈米碳管之合成與場發射特性研究

Abstract

在本論文中，我們利用微波電漿輔助化學氣相沉積系統來成長不同形態的奈米碳管，藉由控制催化層厚度可獲得不同密度與大小的催化金屬微粒，所成長的奈米碳管形態和管徑大小是與金屬微粒的密度和顆粒大小息息相關，高密度且具有方向性的奈米碳管可從較薄的催化金屬層來獲得，而較厚金屬層所成長的奈米碳管就不具有方向性且管徑較大，我們也探討了溫度對奈米碳管成長的影響，此外由場發射的測試中可以發現，兩種奈米碳管都有不錯的場發射特性，但具有方向性的奈米碳管需要較高的外加電場來發射電子，這是因為其密度太高而造成電場的遮蔽效應(screening effect)，而在穩定性的測試中，兩種奈米碳管的發射電流都沒有大幅度的衰減。由此可知利用奈米碳管來製作場發射顯示器將有相當大的潛力。 由於奈米碳管密度會影響其發射特性，為了更近一步改變奈米碳管的表面密度分佈，我們提出了用準分子雷射作處理的新方法，藉由改變雷射的能量密度及掃描次數，在適當的處理下，某些奈米碳管會被破壞，而留下一些較高的奈米碳管，這些較高的奈米碳管展現了較高的發射電流，這是由於其電場遮蔽效應能被有效抑制而具有較高的電場增益係數(field enhancement factor)，然而當掃描次數過多時，大部份的奈米碳管都會被破壞，而其電場增益係數也會變小。 在場發射顯示器的應用中，低操作電壓是一個很重要的因素，對此我們設計了一種溝槽式的奈米碳管三極結構，利用氧化層來取代傳統的絕緣間隙層(spacer)，而藉由控制奈米碳管的長度可以改變閘極(gate)到發射極的間距，由分析不同間距三極結構的場發射特性可知，間距越小的元件所需的操作電壓也越小，代表縮小距離可以有效地提高電場增益係數。而此三極結構可以得到一個相當均勻的電子發射圖樣(emission pattern)，當陽極電壓從600 V增加到1000 V時，發射圖樣的亮度也會大幅提升。此外，這種三極結構的發射電流在長時間的測試下也都沒有很明顯的減少，因此對於應用於場發射顯示器將有不錯的優勢。 為了更近一步的縮短閘極到發射極的間距，我們也提出了另一種平面式奈米碳管的三極結構。利用半導體的蝕刻技術以及奈米碳管可選擇區域成長的特點，可以很容易地製作此種三極結構，不需要很先進的曝光設備就可以將間距縮小到1.5 微米，此元件的啟動電壓可以降低到20 V，而當閘極電壓為35 V時就可得到5微安培的發射電流。而奈米碳管的長度對三極結構場發射特性的影響也有所探討，太長或太短的奈米碳管都會使得場發射電流變小，所以必須適當地控制奈米碳管的長度。另外，此三極結構在閘極電壓30 V時可以獲得亮度值約為1000 cd/m2 的場發射圖樣，而在穩定度方面也有不錯的特性。 雖然奈米碳管具有優越的場發射特性且其發射電流並沒有很明顯的衰減現象，但是仍會有不小的變動(fluctuation)產生，這對應用於場發射顯示器上將有相當程度的影響，像是驅動電路的複雜度提高等。為了改善此一問題，我們結合了薄膜電晶體(TFT)以及奈米碳管，來製作一個主動控制的場發射元件。而奈米碳管是直接整合在電晶體的汲極端(drain)，透過電晶體中受閘極電壓所控制的穩定飽合(saturated) 汲極電流，來提供奈米碳管一個穩定的電子流來源。為了要能夠承受較高汲極電壓，一種具有較長的通道長度之薄膜電晶體首先被設計與製作，其通道長度約為250 微米，當閘極電壓從26 V增加到45 V時可達到開關電流比值為1000，不同大小且非常穩定的發射電流都可藉由閘極電壓來控制，且其電流變動程度小於2%，遠比未受控制的50% 好的很多。 為了要提升薄膜電晶體的特性，我們也設計了一種具有offset結構的電晶體，此種結構的通道長度可以縮小到20 微米，元件面積可大幅縮減，在較低的閘極電壓下(0 V到30 V)就可獲得較高的開關電流比(10000)，而當閘極電壓為15 V時可提供12 微安培的發射電流，展現了低電壓操作的可行性，此外其電流變動率也都可以控制在3%以下。此種主動控制的元件架構不但結構簡單，並提供一個可低電壓操作且非常穩定的場發射電流，相信對奈米碳管應用於場發射顯示器上將有非常大的助益。

In this thesis, various types of carbon nanotubes (CNTs) have been synthesized using microwave plasma-enhanced chemical vapor deposition (MPECVD). By controlling the thickness of the catalyst film, different size and density of catalytic Fe nanoparticles can be obtained. The diameter of as-grown CNTs was found closely correlated with the size of metal particles. Well-aligned CNTs with a small diameter ranging from 20 nm to 60 nm were grown on the highly packed nanoparticles from the thin catalyst film of 3.5 nm in thickness. In contrast, the CNTs grown on the thicker catalyst film of 15 nm in thickness showed no obvious orientation and had a larger diameter ranging from 70 nm to 250 nm. The effect of growth temperature was also characterized. Field emission properties of the aligned and randomly oriented CNTs were investigated in a high-vacuum system. To achieve an emission current density of 10 mA/cm2, the required electric fields were 7.4-8 V/□m and 5.5-5.9 V/□m for the aligned and randomly oriented CNTs, respectively. Higher fields were needed to draw the same current density for the aligned CNTs due to the field-screening effect from the densely packed CNTs. Both of the aligned and randomly oriented CNTs exhibited quite reliable emission currents with some fluctuation. According to the low field and stable emission, the CNTs synthesized by MPECVD are attractive for applications in future field emission display (FED). In order to modify the density and surface morphology of CNTs, a new post-treatment using excimer laser is proposed. Various conditions of excimer laser treatment (ELT) were investigated, including different laser energy densities and laser overlaps. For a small laser overlap, some nanotubes were shortened by the excimer laser. The remains of CNTs were protruded from the surface of CNT films and the length variation of CNTs was increased. In contrast, for a large laser overlap, most of the CNTs were shortened and the density of CNTs was not changed obviously. A high emission current density can be achieved from the CNTs with a proper ELT. It was attributed to that the effective length of CNTs sticking out in the electric field could be increased by the ELT, resulting in a large field enhancement factor (B) of the CNTs. However, after the ELT with 99% overlap, most of CNTs were destroyed by the excimer laser. The average length of CNTs decreased to 2 □m, and the B value decreased to only 137. To achieve a low-voltage FED, the trench-type CNT triodes have been fabricated using oxide as the insulator between the gate and CNTs to replace the conventional spacer. The spacing between the gate and emitter can be easily reduced by controlling the length of CNTs. The anode current of 10 uA can be achieved at the gate voltages of 150 V and 98 V for the gate-to-emitter gaps of 18.5 um and 10.3 um, respectively. The required gate voltage can be significantly reduced by decreasing the gap between the gate and the CNTs. Enhanced luminance was obtained as the anode voltage increased from 600 V to 1000 V. In addition, no apparent degradation of the emission current from the CNT triodes was observed over a period of 10 h. Furthermore, a new fabrication method of planar-type CNT triodes has been demonstrated. The CNT triodes can be easily fabricated by combining semiconductor fabrication technology and selective-area growth of CNTs. No advanced lithographic tool is needed and the gate-to-emitter gap can be reduced to 1.5 um. The device can be turned on at the gate voltage of 20 V. The effect of the length of CNTs was investigated to achieve the low turn-on voltage field emission triodes. A uniform field emission pattern with the luminance of 1000 cd/m2 was achieved at the gate voltage of 30 V. Moreover, the planar-type CNT triodes showed no apparent degradation of the emission current over a period of 1 h. A new field emission device composed of CNTs and a long channel thin-film transistor (LC-TFT) has been successfully demonstrated to significantly improve emission stability. CNTs are directly integrated in the drain region of the LC-TFT and the emission current from the CNTs is controlled via the TFT drain current. An ON/OFF current ratio of 1000 can be achieved with the gate voltage switching from 26 V to 45 V. The fluctuation of the emission current of LC-TFT-controlled CNTs can be suppressed to less than 2%, below the fluctuation of uncontrolled CNTs. Finally, a new TFT design with an offset region has been proposed to improve the characteristics of TFT. The device area of offset-TFT can be significantly reduced. The control gate voltage can be lowered to 15 V to obtain an anode current of 12 uA. An ON/OFF current ratio of 10000 was achieved with the gate voltage switching from 0 V to 30 V. Furthermore, stable emission current can be obtained via the offset-TFT-control over 1 h and the current fluctuation of offset-TFT-controlled CNTs can be reduced to below 3%. This novel field emission device exhibits low-voltage controllability, good emission stability, and structural simplicity, making it promising for application in future FED.