Diamond growth by the oxy-acetylene torch and dc glow discharge CVD process has been investigated.
The radial variation of the morphology of the diamond crystals synthesized by the oxy-acetylene torch, within the circular deposition area, has been investigated at various deposition temperatures and nozzle-substrate distances. We show that the radial morphology variation is caused by the composition profile in the flame, not by the substrate temperature profile. The tendency of the morphology variation was compared with previous reports on the effect of oxygen addition on the diamond morphology in the hot filament CVD. It was then interpretated on the basis of previous reports on the analysis of gas species in the flame. We propose that the composition profile of the flame which caused the morphology variation was the in-diffusion of the oxygen from the atmosphere surrounding the flame.
The effect of supersaturation on the step formation on the {100} facet of the diamond crystal grown by oxy-acetylene torch has been investigated. The spiral steps formed at the periphery of deposition area where the supersaturation is the lowest. In contrast, concentric loops of steps formed at the inner region of the deposition area where the supersaturation is higher. The spiral steps formed randomly on the {100} facet, including the central region. In contrast, the concentric loops of steps formed only at the edges or vertice of the {100} facet. Moreover, when the {100} facet was tilted with respect to the substrate, the concetric loops of steps formed only at the edges or vertices at the upper region of the facet. This observations was in accord with the prediction of the crystal growth theory. It was also shown that the twin boundary serves as a step nucleation site for concentric loops of steps.
The instability of the discharge during the diamond synthesis by the dc glow discharge process has been investigated. Elevating the cathode temperature above 2000-2100℃ greatly stabilized the discharge. The high temperature of the cathode prevented the solid carbon formation on the cathode surface which was found to be the major source of the discharge instability. The prevention of the solid carbon formation at high cathode temperature was in accord with the previous thermodynamical analysis. The high cathode temperature greatly reduced the discharge resistance, which was an additional factor stabilizing the discharge. Using the stabilized dc glow discharge proposed in this research, it was possible to deposit diamond film thicker than several hundred micrometers, without employing the previous stabilizing techniques which was problematic in many aspects.
The growth behaviour and properties of diamond film by dc glow discharge process has been investigated. It was possible to deposit the diamond film thicker than several hundred μm on a large area of diameter of 30-50mm. The uniformity of the thickness over the deposition area was improved by lowering the gas pressure. The Raman-quality of the diamond film was strongly dependent on the deposition rate rather than on the current density. White and transluscent film with high quality could be deposited by this process. The minimum value of the FWHM of diamond peak in Raman spectrum of the latter was 2.5-3.4$cm^{-1}$ which was similar to the Type I single crystalline diamond crystal.
A new simple technique to reveal the microstructural features embedded within the diamond thick film was demonstrated. The diamond thick film grown by dc glow discharge was annealed in contact with Fe or Mn at 900-1000℃ for 0.5-20 hours in hydrogen atmosphere. It was cleaned after annealing and was observed by SEM under ordinary secondary electron mode. A characteristic etch pattern was found on the diamond film surface. This etch pattern was closely related to the microstructural features on the as-grown surface and gave informations on the growth history of the diamond film. The secondary electron image contrast was strongly dependent on the local distribution of the lattice defects. The more defective region appeared darker in the secondary electron image. Two factors contributed to the image forming: topographic contrast and non-topographic contrast. The latter was due to the differential modification of the surface state according to the local defect quantity, by the high-temperature reaction with metal.
