Effect of methane flow rate on gas-jet MPCVD diamond synthesis

The paper describes synthesis of diamonds by the method of gas-jet deposition with microwave activation of precursor gases. This method involves the use of a supersonic jet for delivering the components activated in the discharge chamber to the substrate located in the deposition chamber. A series of experiments was carried out with different amounts of methane supplied at a hydrogen flow rate of 8000 sccm. The obtained samples of diamond coatings were studied by scanning electron microscopy and Raman spectroscopy. The temperature of the mixture and the intensities of H, CH, and C-2 lines in the plasma of the discharge chamber were measured by optical emission spectroscopy. The values of pressure and temperature in the discharge chamber were used to estimate the composition of the mixture. Thus, the numerical dependences of the molar concentrations of CH3, CH, C-2 and C2H2 on the initial concentration of methane have been obtained. These dependences are in qualitative agreement with the dependences of the intensities of H, CH, and C-2 lines. The numerical-experimental study performed allows us to conclude that the optimal value of methane concentration in the supplied mixture for the gas-jet deposition method in the considered range of parameters is about 1%.