含Si, N之碳基晶體及奈米結構材料之合成及其鑑定

Abstract

碳基底材料的合成與開發，在學術界與工業界，已發展了數十年；例如人工鑽石，超硬材料C3N4，Si-C-N的合成，及碳基奈米材料等。合成這些材料，其實可以藉由同一機台，藉由製程參數的改變來達成。但現今的研究，對於合成條件，成長機制與生成產物間的連結仍嫌缺乏，且尚未成功。因此，本篇論文旨在使用單一微波電漿化學氣相沉積系統，藉由製程參數的改變，來合成各種不同的碳基底材料，包括：Si-C-N晶體薄膜，Si-C-N奈米管，碳奈米管，和尖錐狀碳奈米棒等奈米結構。而製程參數與合成產物間的關係，可以歸納為èSi-C-N晶體薄膜：有/無觸媒輔助，CH4/N2混合氣體與外加固體矽源。Si-C-N奈米管：觸媒輔助，CH4/N2/H2混合氣體與外加固體矽源。碳奈米管與碳奈米棒：觸媒輔助，CH4與H2，N2等混合氣體。以上這些製程條件與合成產物間的關係，可以簡示於一張製程摘要地圖，(頁數: ix)。 在合成Si-C-N晶體部份，比較了三種不同條件對於形成Si-C-N晶體的影響。條件1 (路徑j於摘要地圖) 是探討Si-C-N薄膜沉積於不同緩衝層上，結果顯示薄膜的成核與性質可藉由外加緩衝層來操控改變。條件2與條件3 (路徑k於摘要地圖) 為探討並比較外加固體源 (鍍鈷矽棒) 於不同添加時機，對於Si-C-N薄膜組成，形貌，結構與性質的影響。條件2添加固體源的時機是在通入反應氣體前，即 “沉積前”。相對的，條件3則是在 “沉積前” 與 “沉積中” 都有添加外加固體源。 在Si-C-N奈米管合成部份，比較了Co 觸媒輔助成長Si-C-N奈米管 (路徑l於摘要地圖) 與Si-C-N晶體 (路徑j於摘要地圖)。對於沉積時間 (4小時) 能夠成長管狀形貌結構是相關於反應氣體中有無包含H2氣。H2氣可以延遲 “觸媒毒化” 效應，且可以維持管端在成長過程中開口狀態。 其它的碳奈米結構 (例如，觸媒輔助碳奈米管及碳奈米棒) 藉由製程參數的改變，成功的合成在圖案Si 晶圓與無圖案晶圓上 (路徑m，n，o於摘要地圖)。並探討了觸媒種類，合成氣體的比例與種類，緩衝層，溫度等對碳奈米結構形貌，結構，性質與成長機制的影響。而選擇性碳奈米管則是沉積於 (a) 平行Fe–披覆SiO2線陣列，(b) CoSix–披覆SiO2孔洞陣列。這是一個新奇並可與現有Si電子元件相容的方法。此外，提出了單顆觸媒成長多支多管壁奈米管的機制。是相關於觸媒經氫活化的形貌與碳奈米管的沉積溫度。提供了不同的觀點來解釋觸媒輔助–多管壁碳奈米管的成長機制。 關於不同的碳基底晶體及其奈米結構間的連結可以歸納如下： (1) 外加固體矽源提供了Si-C-N晶體薄膜/奈米管結構中Si的主要來源，雖然少部份的矽會是來自於Si基材，但藉由電漿中解離的矽源，活性較高，有較多機會參與反應。(2) 觸媒的添加輔助了管狀形貌或是尖錐柱狀形貌的生成，但觸媒在Si-C-N晶體及Si-C-N奈米結構的作用是不同的，因為有添加外加矽源的條件下，若無適量的H2氣體環境，管狀的結構並無法藉由觸媒而生成，取而代之的是Si-C-N晶體結構。觸媒對於形成Si-C-N晶體結構提供了成核點，並增加了沉積速率。相對的，形成Si-C-N奈米結構則是類似於vapor-liquid-solid成長模型，觸媒汲取了反應物並提供生成物的析出媒界。 (3) CH4與H2氣體的比例影響管狀或是實心晶體結構的形成，高比例的CH4/H2有助於C-sp2鍵結 (石墨結構) 的形成。相對的，低比例CH4/H2有助於C-sp3鍵結 (鑽石結構)。因此，在有催化劑的情形下與改變CH4/H2比例，碳原子環繞著催化劑析出，形成了中空管狀或是實心棒狀的結構。(4) N2氣體的添加，有助於形成竹節狀奈米結構。N2氣增加了石墨結構的彎曲應力。改變了石墨六角環晶格的晶體結構，部份晶格被五角環或是七角環形式的晶格所取代，造成了竹節狀奈米管的形成。 在性質分析結果顯示，Si-C-N晶體的硬度範圍藉於30 GPa ~ 57 GPa間，能隙藉於3.76 ~ 3.95 eV間。而在場效發射結果顯示，Si，N碳基底晶體及其奈米結構都具有不錯的場效發射性質。但是Si-C-N奈米管與碳奈米管比Si-C-N晶體的場效發射電流大了一個等級。在電場10 V/mm時，奈米管結構的發射電流密度 >0.03 A/cm2，而Si-C-N晶體則為 0.0025 A/cm2。因此碳基底晶體及其奈米結構預期將會是應用於場效發射顯示器的候選者。

Syntheses of carbon-based materials have been developing for many decades in both academic and industrial communities, such as, man-made diamonds, superhard C3N4 materials, Si-C-N crystals and other carbon-based nanostructured materials. However, researches so far have not successfully linked the growth mechanisms of various carbon-based materials deposited by different synthetic conditions and methods. In fact, a single machine could synthesize many of such materials. This dissertation aimed to study the linkages among various carbon-based materials synthesized on Si wafers under the same microwave plasma chemical vapor deposition (MPCVD) system, including Si-C-N crystalline films, Si-C-N nanotubes, carbon nanotubes (CNTs), conical carbon nanorods and other nanostructured materials. The process parameters can be divided into three groups according to the structures of the synthesized materials, i.e. Si-C-N crystalline films, nanotubes and other nanostructures. The main parameters include CH4/N2 gases, buffer layer application, additional Si source and its application timing for Si-C-N crystalline films; CH4/N2/H2 gases, catalyst application, additional Si source for Si-C-N nanotubes; CH4, N2, H2 gases and catalyst application for carbon nanotubes and carbon nanorods, as shown in a process abstract roadmap or figure, (page ix). With regard to the synthesis of Si-C-N film, three conditions, namely conditions 1, 2 and 3 were compared. Under condition 1 (route j in abstract figure), the results reveal that formation and properties of Si-C-N films can be manipulated by applying seven different buffer layers. Conditions 2 and 3 (route k in abstract figure), it depicts that application timing of the additional solid source (Co-coated Si columns) can be used to vary the compositions, morphologies, structures and properties of Si-C-N films. Under condition 2, the solid sources were applied “before” film deposition. This condition applies the solid source to the substrate by H2 pretreatment. Under condition 3, the solid sources were applied both “before and during” film deposition. By comparing the conditions of forming catalyst-assisted Si-C-N nanotubes with forming Si-C-N films (routes l and j in abstract figure), the formation of the tubular structure may be related to introduction of H2 gas during tubular deposition, which may delay the action of the so-called catalyst poisoning and keep the tube end open during growth. Other nanostructured materials, e.g. catalyst-assisted CNTs and carbon nano-rods, were successfully synthesized on patterned and un-patterned Si wafers (routes m, n, o in abstract figure) by varying process parameters including catalyst materials, source gases, gas ratios, interlayers and deposition temperatures. The CNTs could be selectively deposited on the patterned wafers, including: (a) parallel Fe-coated line arrays, and (b) CoSix-coated hole arrays. This is a novel method that is compatible with Si microelectronic device manufacturing. Besides, many of the vertically-grown, dense MWCNTs are found to protrude from a single catalyst particle. This is believed to be associated with the lower temperatures in H2 reduction and CNTs deposition stages. The result also offers a different perspective on growth mechanism of the catalyst-assisted MWCNTs. Regard to the linkages of forming various kinds of carbon-based crystals and nanostructured materials, the following conclusions can be drawn: (1) The additional solid Si sources mainly contribute the Si component of Si-C-N crystals and nanotubes. Although some Si could be derived from the Si substrate, the solid Si columns ionized by plasma are highly active to participate in the reaction. (2) The nano-sized catalysts promote the formations of tubular or rod morphology. The catalytic functions of the process environments without H2 gas differ from those with H2 gas. The catalysts are suggested to provide nucleation sites for Si-C-N crystal nucleation, and effectively reduce the energy of formation in the initial stage. The catalytic function is lost when the growing film covers the catalytic particle. In contrast, the role of the catalyst in forming Si-C-N tubes is similar to that described in the vapor-liquid-solid model. The tube grows by precipitating of graphite sheets from a super-saturated catalytic droplet. The formation of a curved graphite basal plane is energetically favorable, and so the tubular structure is formed. (3) The CH4/H2 ratio influences the formation of tubular and crystalline structures. A high CH4/H2 ratio favors the formation of C-sp2 bonding (graphite structure), whereas, a low CH4/H2 ratio favors the formation of C-sp3 bonding (diamond structure). Therefore, carbon atoms surround and precipitate from catalysts with different CH4/H2 ratios form hollow tubes or solid nano-rods. (4) N2 gas gives rise to bamboo-like CNTs. Introducing N atoms into the carbon nanotube structure may induce distortion; change the bonding to that of pentagonal, heptagonal or other crystal lattices, and increase bending stress. Analysis results indicate that the nanohardness of Si-C-N crystals ranges from 30 GPa ~ 57 GPa; the energy gap ranges from 3.76 eV ~ 3.95 eV. The field emission results show that carbon-based crystals that contain Si and N, and their nanostructured materials exhibit good field emission properties. The emission currents of Si-C-N nanotubes or CNTs are at least one order of magnitude more than those of Si-C-N crystals at specific electric field intensity. At an electric field of 10 V/mm, the emission current of nanostructured materials is > 0.03 A/cm2 and of Si-C-N crystal is 0.0025 A/cm2. The carbon-based materials that contained Si and N, and the corresponding nanostructured materials are promising candidates for field emission applications.