Chemical Vapor Growth Of Highly Transparent Diamond Thin Films And Single Crystals

Diamond has the highest hardness, atomic density, sound velocity, thermal conductivity, and elasticity modulus among the known naturally existing materials. In addition, diamond also has very low thermal expansion coefficient, extremely wide band gap, and good visible light transmittance. Diamond has high chemical stability and does not react with common acid even at high temperature. Due to the numerous advantages diamond has important applications for cutting tools, thermal conduction, optical windows, high-power devices, microwave devices, and so on. However, the growth of high transparency free-standing diamond films and epitaxial single crystals growth is still a problem in China.In this thesis, growth of high transparency diamond films and single crystals was investigated by microwave plasma chemical vapor deposition(MPCVD). The research work includes two parts. The first is epitaxial growth of high transparency diamond single crystals on diamond seeds prepared by high temperature and high pressure technology. The second is deposition of highly transparent free-standing diamond films on molybdenum substrates.At low temperature, the effects of the gas flow rate of the different gases on the epitaxial growth of diamond single crystals was investigated. CH4 and H2 were used as the reactive gases during the experiments and the effects of the power, working pressure, and temperature on the growth of the single crystal diamond were investigated, obtaining the optimal process parameters. In a certain range, epitaxial growth rate increases with increasing the power, working pressure, and temperature. It is found that the temperature is an important factor affecting the quality of diamond single crystals and the higher quality was achieved at lower temperature. Therefore, the effect of the gas flow rate was investigated at low temperature. The epitaxial growth rate increases with increasing the methane flow rate. However, too high temperature results in graphitization of the diamond. Oxygen was added during the epitaxial growth of the diamond single crystals due to its etching reactivity on non-diamond phase. It was found that in the oxygen flow rate range from 0.1 to 0.5 sccm the quality of the epitaxial diamond increases with increasing the oxygen flow rate. However, the growth rate decreases correspondingly. Increasing the flow rates of methane and oxygen sychronously with their ratio constant results in the graphitization of the diamond continue to grow single crystal finally.The growth of the highly transparent free-standing diamond films was investigated as a function of temperature, power, and oxygen flow rate. The growth rate increase with the increase of temperature and power, decrease with the increase of oxygen flow rate. In a certain range, the visible light transmittance increase with the increase of oxygen flow rate and power. The visible light transmittance increase with the increase of temperature first and then decrease. When the oxygen flow rate is 1.0 sccm, diamond films has the highest transmittance.