氣體組成和後處理對ECR-CVD觸媒輔助碳奈米管成長及特性之影響

Abstract

本研究利用電子迴旋共振化學汽相沉積(ECR-CVD)觸媒輔助成長碳奈米結構，探討反應氣體組成及後處理對碳奈米結構成長之影響。探討之反應氣體包括甲烷+氫氣、乙炔+氫氣、甲烷+二氧化碳、和甲烷+氨氣、等。所使用之觸媒材料包括Fe、Co 和Ni。另外初成長之碳奈米管(CNTs)分別做如下之後處理包括氫電漿蝕刻、氧電漿蝕刻及無電鍍鎳、等。在各種製程條件與參數之下，包括基材溫度、氣體流量、沉積時間和不同的電漿處理及後處理，碳奈米結構利用下列檢測技術，包括掃描式電子微顯鏡(SEM)、穿透式電子微顯鏡(TEM)、拉曼光譜儀(Raman spectroscopy)及以場效發射I-V量測儀，來探討其特性。從這些研究可以得到下列結論。 氫電漿前處理的效應基本上是在蝕刻觸媒薄膜形成有利於CNTs成長的良好分佈奈米顆粒。結果顯示在前處理時基材溫度需提高至600℃以上方可順利穫得半球型有利於CNTs成長的奈米顆粒。 以碳源氣體而言，奈米結構取決於反應氣體混合的碳濃度。低碳濃度較易形成CNTs但其成長速率較低，而較高碳濃度易形成形成海草狀奈米結構。換句話說在同樣氣體的比例下，乙炔與甲烷相比，因每分子具兩個碳原子所以較易在管周圍有碳薄片沉積。基本上二氧化碳之效應可當碳源並供應具有優先蝕刻非晶質碳之氧電漿。所以碳沉積速率與氫電漿或氧電漿蝕刻速率之相互競爭決定最後奈米結構。因此CH4與C2H2相較之下，在形成CNTs時無碳薄片在管周圍之條件下，CH4具有較高濃度容許度。 氫電漿除了具有蝕刻非晶質碳的效應之外，也具有將觸媒表面維持活性之效果可延長CNTs成長時間。在較高氫濃度時成長之CNTs長度較長，特別在鈷當觸媒的試片。由於氫電漿具蝕刻效應，所以較高氫濃度，在具觸媒圖案的基材成長CNTs時將具較佳選擇性成長。 氨氣效應基本上是提供氮氣，其具有明顯形成氮化物之趨勢，並且比氫氣具較大質量所以產生較大轟擊效應。因此之故，在甲烷氨氣的環境下具較低管數密度及較短CNTs長度。這可能與氮氣形成氮化物，所以具較大毒化效應相一致。形成氮化物之證明可由在氨氣的環境下，氮化物可能增加系統電阻，所以系統電流明顯減少。類似的結論可應用在CO2氣氛下氧化物的形成。 關於奈米結構的場發射特性量測結果指出，在目前成長之CNTs氣氛其FE特性普遍不佳。但以CH4+微量NH3氣氛下稍微較易量測到J-E曲線。並發現在CH4+CO2氣源下拉長沉積時間至30分鐘將量測到較佳的場發射效果。其原因有待進一步研究。 將初成長之CNTs以氫電漿後處理，基本上是優先蝕刻CNTs頂端部分，使得長時間蝕刻後將變成較短並具尖錐外型的CNTs。氫電漿經過長時間蝕刻也會破壞CNTs骨架。相對而言，氧電漿與CNTs管外的碳一層一層反應，因此延長氧電漿後處理時間將使管直徑逐漸減少。其影響造成氧電漿處理後CNTs的深寬比變高，且管與管之間的距離變大可減少場發射的遮蔽效應。 以鎳當觸媒初成長的CNTs經過無電鍍鎳後處理，其結果顯示從垂直排列外觀變成管束聚集狀排列;並改善場發特性。CNTs的聚集似乎是由於鎳觸媒及無電鍍鎳的化學吸附所造成，因此此聚集現象並未在以鈷當觸媒成長出之CNTs鍍鎳時產生。

Effects of gas compositions and post-treatment on the catalyst-assisted growth of carbon nanostructures via ECR-CVD were investigated. The reactant gases include CH4 + H2, C2H2 + H2, CH4 + CO2 and CH4 + NH3, respectively. The catalysts are Co, Ni or Fe. The post-treatments of the as-grown CNTs include H-plasma, O-plasma and electroless Ni plating. The nanostructures and their properties under various process conditions and parameters, including substrate temperature, gas ratio, deposition time, different plasma pretreatment and post-treatments, were characterized by SEM, TEM, Raman spectroscopy and field emission J-E measurements. The following conclusions can be drawn from these studies. Effect of H-plasma pretreatment is essentially to etch the film catalyst to become the well-distributed nano-particles for CNTs growth. The results show that the substrate temperature above ~ 600oC during pretreatment is required to obtain the nano-particles with semi-spherical shape, which are favor for CNTs growth. As to the carbon gas source, the nanostructures are depending on the carbon concentration in the gas mixture. A lower carbon concentration has a greater tendency to grow CNTs but at a lower growth rate; and a higher one is more favor to form rattan-like nanostructures with carbon sheets around the tubes. In other words, at the same gas ratio, the tendency of C2H2 to form the carbon sheets around the tubes is higher than CH4 due to two carbons in a molecule of C2H2. Effects of CO2 as a reactant gas are essentially to act as a carbon source and to supply the oxygen to etch amorphous carbon preferentially. As a consequence, the competition between carbon deposition rate and etching rate of H-plasma or O-plasma determines the final nanostructures. Therefore, CH4 has a higher concentration tolerance than C2H2 to form CNTs without carbon sheets around the tubes. In addition to etching effect of H-plasma on amorphous carbon, hydrogen can keep catalyst surface clean to prolong the CNTs growth. In other words, a higher H concentration generally gives rise to a longer tube length, especially for Co catalyst. Furthermore, due to etching effect of H-plasma, the growth selectivity of CNTs on a substrate with catalyst pattern will be higher under a higher H concentration. Effect of ammonia gas is basically to supply nitrogen, which has a greater tendency to form nitrides and a higher bombardment effect than hydrogen due to a greater mass. Consequently, the tube number density and CNTs length are generally smaller under CH4 + ammonia atmosphere. This may be in agreement with greater poisoning effect of nitrogen due to nitride formation. The evidence of nitride formation may be demonstrated by a tremendous decrease in system current under ammonia atmosphere, where nitrides may cause an increase in system electrical resistance. Similar conclusions can be applied to oxide formation due to presence of CO2. Regarding field emission (FE) properties of the nanostructures, the results indicate that FE properties under the present conditions are generally poor. However, FE properties of the CNTs grown under CH4 + ammonia atmosphere are slightly better. It is found that the CNTs grown under CH4 + CO2 gas sources can be much better by extending the deposition time to 30 min. Further study is required to clarify the reasons. Effect of H-plasma post-treatment on morphology of the as-grown CNTs is essentially to preferentially etch the top parts of CNTs to become a shorter cone-shaped CNTs after a long etching time. The H-plasma may also damage the stems of CNTs at longer etching time. In contrast, effect of O-plasma is basically to react with the carbon on the tube surface layer by layer. Therefore, the tube diameter gradually decreases by prolonging O-plasma post-treatment. Consequently, the aspect ratio of CNTs become higher after O-plasma treatment, and inter-tube distance becomes larger to reduce the screening effect of field emission. After the electroless Ni plating post treatment, the results show that morphologies of the as-grown Ni-assisted CNTs can be changed from the well aligned tube arrays to agglomerated CNTs bundle arrays; and the FE properties are improved. Agglomeration of tubes seems to be due to chemical attraction between Ni catalyst and Ni coating. Therefore, the tube agglomeration phenomenon was not found for the Co-assisted CNTs after Ni plating.