Unusual dependence of diamond growth rate on methane concentration in hot filament chemical vapor deposition

Abstract

Diamond synthesized utilizing chemical vapor deposition process has long been studied, in order to exploit superior thermal, mechanical, and optical properties of diamond. Thorough understanding of growth mechanism of CVD diamond is therefore an important task to enhance deposition speed of diamond and to develop new applications of CVD diamond. Considering thermodynamic paradox encountered when explaining the formation of CVD diamond in atomic growth, theory of charged nanoparticles suggested that diamond nanoparticles are generated in the gas phase and become building blocks for diamond film. In chapter 1, an odd phenomenon occurring in CVD diamond deposition process is introduced, which cannot be explained with conventional understanding on chemical vapor deposition. In hot filament chemical vapor deposition, growth rate of diamond film is proportional to the amount of methane injected. However, when the temperature of the filament is low, the growth rate is inversely proportional to methane concentration. To investigate this behavior, nanoparticles existing in the gas phase were directly captured with TEM grid, in order to see if there is any change when the filament temperature is lowered, since they become building blocks for diamond film. Furthermore, surface condition of the filament and its emission of electricity was investigated to understand environmental factors of the formation of charged nanoparticles. As methane concentration increases and filament temperature decreases, the filament gets poor in charging clusters nearby, and therefore the growth rate of diamond plummets. In chapter 2, typical CVD diamond deposited utilizing hot filament chemical vapor deposition is analyzed with transmission electron microscope. Significant difference in crystal structure appeared between inner and outer part of deposited diamond. Diffraction pattern of injected electron beam through whole bulk and TEM image obtained at the edge of the bulk have shown different crystallographic information. Diamond deposited in CVD system was confirmed to have lonsdaleite structure on the outer surface.