Diamond synthesis on Au-Si liquid substrate by using microwave plasma chemical vapor deposition
Abstract Chemical vapor deposition of diamond films on solid state substrates has been investigated for the past two decades, but the growth of high quality or highly oriented diamond film accepted by applications has not been achieved yet. Many parameters of growth process would affect directly the quality of diamond, such as methane concentration, gas flow rate, working pressure, substrate temperature, and surface conditions of substrate. The selection of substrate materials for diamond film deposition has to consider lattice structure, surface energy, carbon solubility of the substrates, or the affinity with carbon to form carbide phase. Furthermore, elements or compounds attempted to use as substrates for diamond deposition have covered all over the periodic table, but the effect on improvement quality of diamond is not notable. Thus, in this thesis, we used a low melting point alloy as substrate for chemical vapor deposition of diamond, which was very different to the conventional solid-state substrate. The liquid-state would exist at the temperature of chemical vapor deposition and to promote the synthesis of diamond. We used Au-Si alloy as substrate, which has the melting point about 700oC by using Au-Si weight ratio in 1:2. Diamond was synthesized by microwave assisted plasma chemical vapor deposition system, and hydrogen and methane was used as gas source. Then, we investigated the mechanism of the nucleation and growth of diamond synthesis on liquid state substrate under various bias voltage, methane concentration, and nucleation, growth duration, and patterned substrate by X-Ray Diffraction, Raman, Transmission Electron Microscopy, Scanning Electron Microcopy, Energy Dispersive Spectrometry, and Electron Energy Loss Spectrum analysis technology. In this study, we found: 1. The high quality diamond was obtained by using Au-Si liquid substrate. 2. The mechanism of diamond nucleation on liquid substrate was different on convention solid substrate. Substrate would form protrudent single crystal silicon, in cone shapel, and each protrudent single crystal silicon oriented along  crystallographic direction. Then, diamond was deposited on t top of the protrudent single crystal silicon. 3. The planar interface between diamond and silicon was observed, and silicon oxide and silicon carbide was existed at the interface. 4. The size of diamond depended on the size of the protrudent Si, and the thicker protrudent would form larger-size diamond. 5. When diamond deposited on the patterned substrates, diamond shows selective nucleation on the pattern where the liquid alloy had been formed, indicating that diamond nucleation have highly selective between liquid state and solid state substrate. 6. Varying bias voltages affects the diamond nucleation density. The higher nucleation density could be formed, by increasing bias positive voltage. 7. The variation on methane concentration was not found to affect the nucleation densities; however, the shape of diamond depends on the methane concentration. The methane concentration lower than 0.3﹪would result in well-faceted diamond.