奈米碳管場發射特性之改善與其側向元件之研究

Abstract

本論文主要針對奈米碳管場發射特性之改善及其側向元件之研製為主題作深入之探討。由於奈米碳管具有奈米级的管徑,極大的高寬比,堅強的機械性質及穩定的化學性質,因此,一直是極具潛力的場發射顯示器材料。由場發射的測試中，發現奈米碳管具有非常優異的場發射特性;我們利用微波電漿輔助化學氣相沉積系統及熱化學氣相沉積系統進行奈米碳管的合成,可惜其密度很高(109~1010/cm2), 而密度較高的奈米碳管因為電場之遮蔽效應(screening effect)使得其場發射特性並不因其具有較高密度之場發射源而變好。為了進一步改善奈米碳管之場發射特性,我們提出包括覆蓋二氧化矽於催化金屬層,局部氧化催化金屬,改變氫氣含量及前處理時間,成長柱狀奈米碳管陣列,高密度氧電漿後處理等方法,進行奈米碳管之場發射特性之改善,此外,我們也研製奈米碳管之側向場發射元件。 由實驗結果,我們發現覆蓋二氧化矽於催化金屬層可以有效控制奈米碳管的密度，而實驗結果亦表示優異的場發射特性可藉由調變奈米碳管之密度而得到。此外, 於奈米碳管成長前,先進行催化金屬的局部氧化亦有密度改善的作用, 我們得到兩層不同高度及密度的碳管分佈,其中較低層的碳管是由準直的碳管上覆蓋一層疑是催化金屬氧化物,而較長的碳管其密度更疏,由底部成長並穿出催化金屬氧化物層之上,由實驗結果,我們可以有效的降低起始電場到1.9V/μm。至於改變氫氣含量及前處理時間的方法,我們得到明顯由較長及較短的碳管交錯組成的雙層形式的碳管組態, 由於高密度碳管由此雙層形式的碳管組態瓜分,因此其密度相對降低,但是其場發射源並未減少, 由實驗結果,我們不僅減少其起始電場,同時,於較低電場下,有非常高的電流密度。 有些研究指出,個別碳管間距與個別碳管高度之比值為2時,可以有效減少電場之遮蔽效應, 因此, 我們嘗試成長柱狀型態的碳管陣列, 經由成長條件之控制, 調變陣列間距與碳管長度的比值, 我們發現雖然每個陣列的碳管密度都很高, 但是卻具有極佳的場發射特性, 當陣列間距與碳管長度的比值為1/3時, 不僅有最低的起始電場及超高的電流密度, 其導通電場也比其他研究團隊更低。 我們也對碳管進行了高密度氧電漿後處理, 由實驗結果,適當的電漿後處理條件可以有效的控制碳管密度,其之場發射起始電場可以大幅降低，而場發射電流亦可大幅增加。 最後, 我們也研製奈米碳管之側向場發射元件, 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二極結構，不需要很先進的曝光設備就可以將間距縮小到0.53 □m，此元件的啟動電壓可以降低到0.2 V，而當陽極電壓為10 V時就可得到9.72 mA 的發射電流。而奈米碳管的長度對二極結構場發射特性的影響也有所探討，太短的奈米碳管會使得場發射電流變小，而太長的奈米碳管有短路的問題，所以必須適當地控制奈米碳管的長度。同時此二極元件在短時距1500秒的穩定度電性量測下，其電流波動率小於3.5﹪。

In this dissertation, we study on field emission improvement of carbon nanotubes (CNTs) and the fabrication of lateral field emission devices. Due to CNTs’ high aspect ratio, well chemical stability, high mechanical strength and small radii of curvature, carbon nanotube has become the hot material for field emission display. Microwave plasma-enhanced chemical vapor deposition (MPCVD) and thermal chemical vapor deposition (TCVD) are used to grow nanotubes. The screening of the electric field by the dense arrangement of CNTs has been reported by several groups. The electric field is screened out for the closely spaced CNTs, which results in a reduced effective electric field near the CNT emitters. As a result, turn-on electric field increases and emission current density decreases. However, the density of CNTs synthesized by MPCVD and TCVD is very high and is difficult to control. To obtain better field emission characteristics, the density of CNTs should be optimized. We proposed some novel methods including partial oxidation of catalyst, oxide capping layer, morphology of CNTs with intermix of long and short nanotubes, pillar array of nanotubes and high density plasma post-treatment of nanotubes are used to improve the high density of CNTs and their field emission characteristics are also investigated. It is observed that the density of nanotubes can be reduced by using an oxide capping layer on metal catalyst. The turn-on field can be reduced and the field emission density increases obviously. Partial oxidation of metal catalyst to control the density of CNTs is proposed to improve the field emission characteristics. The results show that this method can produce two sets of CNTs with the structure of intermixture of long and short nanotubes and its field emission can also be enhanced. The turn-on filed can be reduced to 1.9V/μm and high field emission current density is achieved owing to effectively controlling of the density of CNTs. A novel density control with the structure of intermixture of long and short carbon nanotubes are first synthesized by appropriately choosing the pre-treatment time and the contents of hydrogen during CNTs’ growth. The results show that turn-on field can be reduced greatly and ultra high field emission current density can be achieved. Pillar array of CNTs as field emitter by adjusting the growth condition is proposed to improve the field emission characteristics. The results show that when the ratio of distance between pillars to height of pillar is 1/3, ultra low turn-on field, ultra low threshold and ultra high field emission current density can be achieved. The effects of oxygen plasma post-treatment (PPT) on the morphology and field emission properties of carbon nanotube (CNT) arrays grown on silicon substrates are proposed. The experimental results reveal that improved emission properties can be achieved by optimizing the density of CNTs and the defects on nanotubes produced by plasma under proper plasma treatment conditions. Finally, a vertical lateral field emission device (LFED) of CNTs is fabricated. The techniques employed are very simple and allow for good reproducibility in controlling the short distance from the polysilicon anode to the CNTs cathode inter-electrode distance. The experimental results show that low turn-on voltage (~0.2V) and high lateral field emission current (9.72 mA at 10 V) can be achieved when inter-electrode gap is 0.53 um.