Study on the Characteristics and Applications of the Field-emission Devices with an Array of the Carbon Nanotube Pillars
|關鍵字:||柱狀奈米碳管;場發射;氣體解離式感測器;Carbon Nanotube Pillars;Field-emission;Gas Ionization Sensor|
首先，利用微影的方式以鈷鈦共鍍為催化金屬製作柱狀奈米碳管之場發射元件，當柱狀奈米碳管高度為15微米時，其具有較高之場發射增強因子(5384)、較低之開啟電場(0.84 V/m)與臨界值電場(1.51 V/m)；由於鈷鈦共鍍方式能獲得較均勻的碳管長度與較佳之碳管附著力，使元件在電場為0.16 V/m時操作1800秒，擁有只有25 %之場發射電流變動率。同時藉由ISE TCAD模擬軟體驗證，當高度從5微米增加至15微米，其柱狀碳管陣列之最邊緣電場將隨之增加；然而當碳管高度從15微米增加至60微米，其柱狀碳管陣列之最邊緣電場將趨於飽和。另一方面，受到鄰近之柱狀碳管之屏蔽效應，當碳管高度大於10微米時，陣列內的柱狀碳管之邊緣電場(包含中間及中心之柱狀碳管之邊緣)將隨著碳管高度增加而降低。在兩種效應(高度與屏蔽效應)交互作用下，元件於高度15微米時有最佳之場發射特性。
接著，利用塗佈方式使聚二甲基矽氧烷滲透於具有薄膜氧化層沉積之柱狀奈米碳管陣列， 成功製作具有高穿透率、高場發射特性之軟性照明元件。其中，氧化層薄膜可有效保護柱狀碳管之結構，以避免碳管於製程中遭到損壞。並藉由高密度電漿乾式蝕刻機使碳管裸露於元件表面，當蝕刻氧化層時間為20分鐘時，除了可移除非晶態之碳膜，提高碳管之結晶性，並可打開碳管之開口，導致具有15微米高之聚二甲基矽氧烷-柱狀奈米碳管於矽基板上擁有較高之場增強因子、低的開啟電場(0.8 V/m)與臨界值電場(1.43 V/m)，且由於聚二甲基矽氧烷可有效附著於碳管側壁進以提升碳管與基板之附著力，使其在電場為0.16 V/m時操作1800秒，擁有只有15 %之電流變動率。當此聚二甲基矽氧烷-柱狀奈米碳管薄膜剝離矽基板後，其場發射特性與直接成長於矽基板之場發射特性差異甚小，故此薄膜在照明元件應用上應有莫大之潛力。
針對薄膜氣體解離式感測元件，我們利用共鍍催化金屬能有效提升催化金屬活性與大小分佈。在量測十顆相同結構之元件於氮氣環境下，其崩潰電壓相對於非共鍍者之60 %變化僅有25 %之變化。且當元件於0.035 Torr之氮氣環境下操作1000次後，其元件變動量僅有14 %。另外，在量測空氣-氬氣與空氣-二氧化碳之混合氣體時，其線性度為97.8 與98.6 %。因此利用共鍍催化金屬所合成之碳管薄膜氣體解離式感測器能有效提升元件之穩定性、再現性與線性度等特性。
進一步利用共鍍催化金屬成長柱狀奈米碳管陣列來製作氣體解離式感測元件，相對於薄膜氣體解離式感測器進一步優化其氣體解離之特性。當柱狀碳管高度為15微米時，其場發射增強因子為碳管薄膜之20倍，因此該元件於0.035 Torr之氮氣環境下擁有較薄膜結構更小之崩潰電壓(350 V)，且該元件能有效分辨各式不同之氣體。此外，由於利用共鍍催化金屬來成長碳管，故在量測空氣-氬氣與空氣-二氧化碳之混合氣體時，亦顯示優異之線性度特性(98 與98.4 %)。由此得知具有15微米之柱狀奈米碳管陣列比碳管薄膜更適合應用於氣體解離式感測器。|
In this thesis, the field-emission characteristics of the CNT pillars with the different heights were investigated. Various applications, including field-emission display, flexible lighting device, and gas ionization sensor (GIS), were successfully fabricated by using such CNT pillars with the different heights. The co-deposited catalytic layer (Co-Ti) was also utilized for improving the reliability and uniformity of the field-emission characteristics. At first, the CNT pillars with the co-deposited catalytic layer were fabricated by the lithography process. The optimal field-emission properties, including a high field enhancement factor β of 5384, the low turn-on field Eto of 0.84 V/m, and the low threshold field Eth of 1.51 V/m were obtained at the height of 15 m. Furthermore, such a CNT pillar had a better reliability, the fluctuation of 25 % in the current density under an electric field of 1.6 V/m for 1800 s owing to the uniform length of the CNT synthesized using the co-deposited catalyst layer and the better adhesion between CNTs and the substrate. A simulation of ISE-TCAD system has been also performed to verify the experimental results. When the pillar height increased up to 15m, the electrostatic field of the outermost position would be increased. However, the electrostatic field of the outermost position tended to saturation when the pillar height increased from 15 to 60 m. On the other hand, the edge effect was markedly intensified with increasing pillar height above 10 m for the electrostatic fields at the pillar edges, other than those of the outermost positions. Two effects, which were edge and height effect, caused the optimal field enhancement factor at the pillar height of 15 m. Subsequently, a technical with the PDMS solution infiltrated into the SiOx-coated CNT pillars has been utilized to directly transfer the CNT pillars from Si substrate as the flexible, high transparency, and superior field-emission lighting device. In the fabrication process, the oxide coating layer was utilized to protect the morpholgy of as-grown vertical aligmed CNT pillars. To open the CNTs on the surface of the substrate, the HDP-RIE system was utilized. The appropriate etching time of 20 min not only removed some amorphous carbon on the top layers but also opened the tip structure of the individual CNTs. For the PDMS-infilitrated CNTs pillar on Si wafer, the emission current properties were 0.8 V/m and 1.43 V/m, resepctively for the Eto and the Eth, for the CNT pillars with a height of 15 m. That the field enhancement factor would be improved after the dry etching of 20 min and could be attributed to the open-ended structures and better crystalline of CNTs. Moreover, the fluctuation in the current density was improved to 15 % since the PDMS was uniform-covered on the sidewall of CNT pillars to enhance the adhesion between CNTs and Si substrate after curing PDMS solution. When the PDMS-infiltrate CNT pillar with the height of 15 m was peeled off from Si substrate, the field-emission characteristics were changed only a little with respect to those of the as-grown CNTs on Si substrate. The flexible detached film with PDMS-infiltrated CNT pillars was therefore promising for the application in the flexible field-emission lighting device. For the application of GIS, the activation and size distribution of catalytic nanoparticle were improved by using the co-deposited catalytic layer. The variation of the Vbr was less than 25 % for the ten devices of the co-deposited samples since the lengths of the CNT synthesized were uniform and aligned. The fluctuation of the Vbr was about 14 % after 1000 operation times in nitrogen at the pressure of 0.035 Torr due to the better adhesion between CNTs and Si substrate. Moreover, the linearities of the sample with the co-deposited catalytic layer were 97.8 % and 98.6 % for detecting Ar-air and CO2-air gases in mixture, respectively. Therefore, the CNT film with the co-deposited catalytic layer had superior characteristics, including reproducibility, stability, and linearity. Furthermore, the structure of CNT pillars has been also proposed for the first time to improve the gas ionization characteristics since the structure of CNT pillars has higher field enhancement factor. For comparison, the CNT film GIS with the same catalytic layer was fabricated. The optimal ionization characteristics of the CNT pillar GIS were found at the height of 15 m. Such an optimal CNT pillar GIS exhibited the 20 times higher with respect to the CNT thin film ones and the lower Vbr of 350 V for the nitrogen gas at the pressure of 0.035 Torr. Moreover, such an optimum GIS with the CNT pillars not only exhibited the lower Vbr, but also had high sensitivity and selectivity for detecting different kinds of gases. The linearities of such an optimal CNT pillar GIS with the co-deposited catalytic layer were 98 % and 98.4 % for the Ar-air and CO2-air gases in mixture, correspondingly. Therefore, the experimental results showed that the optimum CNT pillars were more effective than the CNT film with the same catalytic layer for the application in the GIS.