Properties and processes of superhard wide-band -gap carbon-based novel optoelectronic materials containing N and Si
|關鍵字:||氮化碳;矽氮化碳;鑽石;鍍膜;場效發射;能隙;cabon nitride;silicon carbon nitride;diamond;coating;field emission;band gap|
研究大致分為四部份，第一部份是比較三種具代表性商業方法所製作鑽石厚膜的優缺點，結果顯示DC arc法沉積之膜具有最高沉積速率及最高壓應力。MPCVD 及HFCVD法可沉積出透明膜，膜之透明度可藉由消除膜中非鑽石成分含量來改善，HFCVD法之沉積膜具有最低壓應力，MPCVD法之膜則具最低表面粗度。不同沉積法之膜其性質也不同，此與沉積溫度及電漿中成分有關聯。
第二部份是以兩種具有SP3鍵結結構，與氮化碳之預測結構相同之生化材料當靶材，利用離子束濺射方法濺射沉積。此目的想要複製部分靶材鍵結結構到沉積膜，以降低反應所需之活化能。結果顯示此想法是可行的，沉積膜內含有足以讓XRD量測到之大量晶體相 (波峰在d= 0.3276 nm (2q= 27.20°))，以及得到比文獻值(0.2 ~ 0.35)高之成分N/C比例值(= 0.5)。薄膜結構及特性似乎與所用之基材材料(摻硼Si(100)、Si(111)晶圓、AISI 300不鏽鋼、銅、銀、鈷及鎳等)無關。此結果暗示，由改變不同靶材及沉積條件來操控化學鍵結資訊，是可成為一種探索氮化碳晶體形成機制之有效線索。
第三部份是用ECR-PVD法在-50 V基材偏壓及CH4、N2(CH4/N2= 1/4 or 1/8 sccm/sccm) 氣源條件下沉積Si-C-N薄膜，結果顯示沉積膜是不含Si-N鍵結之無晶質結構，其膜中含氧成分大於20%則不具場效發射性質。薄膜奈米硬度可高至39 GPa。從FTIR鍵結分析中，薄膜具有C=N, CºN及 Si-C化學鍵。在較高靶材偏壓、較高沉積溫度、較低背景氣壓及較低含氧量條件下，薄膜具有較高硬度。在起始電流密度門檻1 mA/cm2下，薄膜最低起始電場強度是 12 V/mm，而最大電流密度為2.8 mA/cm2。
第四部份是在有添加固態矽源、八種不同介層及不同前處理條件下，以MPCVD法沉積碳基材料。不同成長階段沉積膜有用TEM檢 測，結果顯示不同成長階段之膜其結構及成分皆不相同。從基材表面算起，沉積層順序為SiO2 (~ 100 nm)/ 多晶 Si-C-N (~ 100 nm) / (a-, b- and t-Si3N4 晶體) (2 mm) / 無晶質碳膜。膜中Si3N4晶體的形成與Si3N4粉末刮搔前處理增加成核密度結果一致，此結果亦解釋了單檢驗表面而不用斷面分析的文獻錯誤結論。氣源中加8 at. % 氫氣之效果能降低晶體尺寸及成長速率，與文獻合成鑽石之結果相同。介層之作用基本上能偏移CL波長到更低方向，也就是能增加薄膜能隙，換句話說，介層能應用在調整薄膜之能隙及場效發射性質，其中以SiC介層具有最大場效發射值(6.3 mA/cm2 在電場為 20 V/mm時)。|
It was proposed that the crystalline carbon-based materials containing N or Si may possess many unique properties, such as, highest hardness, wide-band-gap and chemical inertness, but there are no successful methods to be able to synthesize the bulk new crystalline materials containing binary C、N or ternary Si、C、N to measure their properties. Furthermore, One of the hottest topics at present is on carbon nanotubes (CNTs), which are also one of the carbon-based materials. In this study, it was intended to clarify the main process parameters to fabricate various carbon-based materials including various binary and ternary crystalline materials. This study can be roughly divided into four parts. The first part was to compare the advantages and drawbacks of three typical commercial methods to fabricate the diamond thick films. The results show that the DC arc method possesses highest growth rate and the highest residual compressive stress in the deposited films. The MPCVD and HFCVD methods can deposit the transparent films, where the transparency of the films can be improved by diminishing the non-diamond content in the films. The HFCVD-synthesized films possess the lowest compressive residual stress, and the MPCVD-synthesized films the smallest surface roughness. The differences in properties for different deposition methods are related to deposition temperature and species in plasma. The second part was to use ion beam sputtering method with two different bio-molecular materials as targets, which possess the same sp3 bonding structure as the proposed structure of carbon nitrides. It was intended to partly duplicate the bonding structure from the target material to the deposited films to minimize the required activation energy. The results indicate that the idea is feasible, and the deposited films contain enough amounts of crystalline phases to be detected by XRD (high peak at d= 0.3276 nm (2q= 27.20°)), and the higher N/C ratio (= 0.5) than the reported values (0.2 ~ 0.35) in the literature. The film structures and properties seem to be independent of the substrate materials (B-doped Si(100) wafer, Si(111) wafer, AISI 300 stainless steel, Cu, Ag, Co and Ni). The results imply that manipulation of chemical bonding information by changing different target materials and deposition conditions can be an effective key to explore the formation mechanisms of crystalline carbon nitrides. The third part was to synthesize Si-C-N films by ECR-PVD under -50 V substrate bias and with CH4 and N2 (CH4/N2= 1/4 or 1/8 sccm/sccm) as source gases. The results indicate that the deposited films are amorphous Si-C-N with no Si-N bonding, and the films with O% > 20 at. % have no field emission. The nano-hardness of the films can go up to 39 GPa. Films possess C=N, CºN, and Si-C chemical bonding in FTIR spectrum. Under higher target bias voltage, higher deposition temperature, lower base pressure and lower oxygen %, the films possess higher hardness. The lowest turn-on field intensity is 12 V/mm at threshold field 1 mA/cm2 and maximum current density is 2.8 mA/cm2. The fourth part was to synthesize the carbon-based materials by MPCVD, adding additional Si solid source and using eight different buffer layers and pretreatments. The different stages of the deposited films were examined by TEM. The results show that the structures and compositions are different at different growth stages. The sequence of coating materials on the substrate in order of layers from the substrate surface is SiO2 (~ 100 nm)/ polycrystalline Si-C-N (~ 100 nm) / (a-, b- and t-Si3N4 crystals) (2 mm) / a-C film. The Si3N4 crystal formation in the films is in agreement with the Si3N4 scratching pretreatment to enhance its nucleation density. The results also explain that the false conclusion from merely examining the film surface instead of the cross section is often drawn in the literature. Effect of adding 8 at. % H2 in the source gases can cause a decrease in crystal size and growth rate, as indicated also in the literature for diamond synthesis. Effect of buffer layers is essentially to shift the wave number to the lower side, i.e. to increase the band-gap of the films. In other words, the buffer layer application can be manipulated to tune the band-gap and field emission properties of the films, where the SiC buffer gives the best field emission properties (6.3 mA/cm2 at 20 V/mm).
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