以簡易製程合成碳奈米結構在場發射、電晶體與感測元件之應用

Abstract

本研究的目的在開發不同碳奈米結構的簡易製程，以及製造奈米結構輔助元件，如:場發射發射極、電晶體與光及氣體感測器。鑑於碳奈米結構具有高有效比表面積和潛在優異的場發射特性，本論文乃建構於(1)準直性碳奈米纖維束成長於奈米錐頂端(CNF/NCT)，與(2)水平方向碳奈米管網絡(CNT-NW)等之合成方法。在矽晶片上以氧化鋁與二氧化矽為緩衝層，在甲烷與氫氣之反應氣體中，以鐵為觸媒，藉由微波電漿化學氣相沉積法，可以合成出這兩種碳奈米結構。 這兩種碳奈米結構的製備不需使用複雜的蝕刻步驟或微影製程，亦不需使用有圖案的模板(輔助成長)。對於合成CNF/NCT 奈米結構的實驗結果指出，其頂端的碳奈米纖維束乃鯡魚骨狀類型同時包含竹節狀結構，而底部的奈米錐為多晶碳化矽錐包覆單晶矽錐結構。欲得到具備優異場發射性質之碳奈米結構其最理想的製程條件為:較高的基材負偏壓與電漿功率、較低的甲烷/氫氣流量比與工作壓力、以及適當的沉積時間。在此最佳條件下所得之碳奈米結構作為場發射元件時，其起始與開端電場強度可分別下降到0.10 V/μm (於電流密度達10 μA/cm2)與0.26 V/μm (於電流密度達10 mA/cm2)。此外，其發射電流密度在電場強度1.1 V/μm下可上升至660 mA/cm2，且在電場強度1 V/μm條件下，電流密度仍可維持在200 mA/cm2超過265分鐘沒有明顯的衰退。此結果意味著該錐狀奈米結構乃一極佳場發射材料。本論文也對這種碳奈米結構的成長機制進行探討。 對於合成CNT-NW 奈米結構的實驗結果顯示，利用本研究所創新的由一矽晶片覆蓋於鍍有鐵及氧化鋁的二氧化矽/矽基板上所堆疊出的三明治結構，可實現於相對低溫(below 600 °C)與較短的製程時間(4–6 min)下直接在沒有定義圖案的基板上成長水平方向網路狀碳奈米管。此外，在該CNT-NW 結構中，觸媒被石墨層纏繞並藉由水平方向之碳奈米管互相連接形成一網絡結構，且經由拉曼徑向呼吸模式訊號與G譜峰鑑定得知，該碳奈米管屬於高石墨化程度的單壁或雙壁奈米管。達成該” 混合生成”的網絡狀奈米結構之主因可能是具備粗糙表面的氧化鋁緩衝層與矽晶片覆蓋物所塑造的三明治效果所致。起始階段，此三明治效果限制了氣體流動於觸媒顆粒周圍，形成石墨層環繞觸媒顆粒發展。在後半成長階段，三明治結構的空間被石墨層環繞的觸媒顆粒所限制，使得氣體流動成為水平方向流動的奈米流，促使碳奈米管發展成小管徑的碳奈米管網絡。 初成長的CNT-NW試片直接被使用組裝成簡單的CNT-NW輔助元件，並對其電性、光感測及氣體感測性質進行探討。對於CNT-NW輔助電晶體的測試，其電流電壓特性在溫度300到11 K範圍表現出非線性行為，且其電流可受閘極電壓所控制，該元件之遷移率約為14.5 cm2/V•s，電流開關比則達1000。這樣的半導體特性與本研究的拉曼圖譜之表徵相符合。此外，藉由調控製程參數可合成出的不同CNT-NW奈米結構產量/密度，其乃電晶體元件特性由半導體性趨向金屬性變化的決定因素。 至於CNT-NW輔助光感測元件的測試，至於CNT-NW輔助光感測元件的測試，結果顯示在波長1.25–25微米範圍的紅外線光照射下，溫度在300到11 K範圍內施加電壓±2 V於元件，可獲得相對暗電流增加1.1–2.7倍的光電流，意味著該碳奈米結構於光感測器的應用具備高潛力。相反的，當IR光照射在準直性碳奈米管層輔助與觸媒顆粒輔助元件上，於施加電壓控制在﹣2到﹢2 V間，幾乎沒有明顯的光電流變化可被偵測到。換句話說，此結果暗示CNT-NW輔助元件所表現出較高的光電流值可能與其導電本質及較高的有效感測比表面積有關。 對於CNT-NW輔助氣體感測元件的測試，結果顯示當暴露於濃度為6到246 ppm的氨氣/乾空氣及198到12089 ppm的甲醇/乾空氣下，元件的電阻率(也就是□(delta R)/R0)可分別從0.008增加到0.038及從0.0064增加到0.0390。相反的，當暴露於濃度為5到250 ppm的二氧化氮/乾空氣下，元件的電阻率由-0.005下降到-0.531。換句話說，甚至在室溫下該元件便可偵測不同氣體與廣泛的濃度範圍。觸媒輔助元件所產生對任何氣體均無感測反應的結果暗示，以上氣體感測能力的有效性可歸因於碳奈米管網絡的存在。元件的氣體感測機制於本文中亦有討論。

The purposes of this work are to develop simple processes for various carbon nanostructure syntheses and to fabricate nanostructure-assisted devices, such as field emission emitters, transistors, and photo and gas sensors. In view of highly effective specific surface areas and potentially excellent field emission properties of carbon nanostructures, this work is focus on the synthesis methods of the (1) vertically-oriented carbon nanofiber-bundles on nanocone-tip (CNF/NCT) and (2) horizontally-oriented carbon nanotube network (CNT-NW). The two carbon nanostructures are deposited on silicon (Si) wafers by microwave plasma chemical vapour deposition (MPCVD) with methane (CH4) and hydrogen (H2) as source gases, iron (Fe) as catalyst, and aluminum oxide/silicon dioxide (Al2O3/SiO2) as buffer layers. The two kinds of carbon nanostructures can be fabricated without using complicated etching and lithography processing steps, and patterned templates. For CNF/NCT nanostructure synthesis, results indicate that the top CNF-bundles are in the form of herringbone structures with bamboo-like compartment layers, whereas the underneath nanocones are the structures made of a poly-SiC (silicon carbide)-cone with a single-crystalline Si-cone embedded. The optimal processing conditions for the carbon nanostructures with excellent field emission properties are a combination of higher negative substrate bias and plasma power, lower CH4/H2 flow ratio and working pressure, and proper deposition time. Under these optimum conditions, the turn-on and threshold electric fields of the carbon nanostructure-assisted field emission devices can go down to 0.10 V/μm (at 10 μA/cm2), and 0.26 V/μm (at 10 mA/cm2), respectively. Furthermore, the emission current density can go up to 660 mA/cm2 at 1.1 V/μm, and of about 200 mA/cm2 at 1 V/μm condition can be maintained over 265 minutes without significant decay. The results indicate that such cone-shaped nanostructure is a promising field emission material. The growth mechanism of the CNF/NCT nanostructures is discussed in the text as well. For CNT-NW nanostructure synthesis, results show that a sandwiched specimen stack structure comprising an upper Si cover and an Al2O3-supported Fe catalyst layer deposited on a SiO2/Si substrate created in this work can be used for carrying out the directed growth strategy of horizontally-networked CNTs on unpatterned substrates at relatively low temperature (below 600 °C) with short time process (4–6 min). Furthermore, within the structures of CNT-NWs, catalyst particles are wrapped by graphite layers and interconnected by horizontally-oriented CNTs, forming a network architecture. The CNTs are highly-graphitized single- or double-walled nanotubes in view of significant Raman radial breathing mode (RBM) signals and G-band peaks. The main reason to form such “hybridized” networked nanostructures may be due to the sandwich effect of two rougher surfaces of the Al2O3 buffer layer and the covering Si wafer. At the beginning stage, the sandwich effect restricts gases to flow around the catalyst particles, resulting in the development of graphite layers around them. At the latter growth stage, the spaces of the sandwich configuration are greatly limited to cause the gas flow to become nano-streams in the horizontal direction, leading to the CNTs to develop the CNT-NWs with smaller tube thickness. As-grown CNT-NW samples are directly employed to fabricate simple CNT-NW-assisted devices and their electrical, photo-sensing and gas-sensing properties are investigated. For CNT-NW-assisted transistors, their current–voltage (I–V) properties at temperatures ranging from 300 to 11 K show a nonlinear behavior and the current can be modulated by a gate voltage with a device mobility of ~14.5 cm2/V•s and on/off current ratio of ~1000. Such semiconducting properties are conformed to the characterization of the present Raman spectra. Furthermore, the yield/density of the CNT-NWs which can be easily tailored by process parameters determines the performance of a transistor device varying its properties from semiconducting towards metallic ones. For CNT-NW-assisted photo sensing devices, results reveal that 1.1- to 2.7-fold increase in photocurrents, as compared to dark currents, can be obtained at the temperatures ranging from 300 to 11 K with ±2 V applied voltage upon irradiation with infrared (IR) light with 1.25–25 μm in wavelength, indicating that the carbon nanostructures have high potential for photo-sensor applications. In contrast, when the IR light irradiates the devices built with vertical CNT-films and catalyst particles, no significant variations in photocurrents could be detected as the bias voltage is modulated from -2 to +2 V. In other words, the results imply that the higher magnitude in photocurrents for the CNT-NW-assisted devices may relate to their conducting nature and higher effective specific sensing areas. For CNT-NW-assisted gas-sensing devices, results demonstrate that their electrical resistance ratios (i.e. delta R/R0) upon exposure to ammonia (NH3) and methanol (MeOH) balanced with dry air can be increased from 0.008 to 0.038 and from 0.0064 to 0.0390 for the concentrations of 6 to 246 ppm and 198 to 12089 ppm, respectively. In contrast, the □R/R0 upon exposure to nitrogen dioxide (NO2) balanced with dry air can be decreased from -0.005 to -0.531 for the concentration of 5 to 250 ppm. In other words, the devices enable to detect various gases over a wide range of gas concentrations even at room temperature. The results of no response given by the catalyst particle-assisted devices indicate that the above good gas-sensing capabilities might be attributed to the presence of CNT-NWs. The gas-sensing mechanism of the CNT-NW-assisted devices is discussed in the text.