Properties and processes of superhard wide-band -gap carbon-based novel optoelectronic materials containing N and Si

• Main Authors: Jin-Yu Wu, 吳錦裕
• Other Authors: Cheng-Tzu Kuo
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/40355067317113614374

博士 === 國立交通大學 === 材料科學與工程系 === 90 === 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).