Spectroscopic analysis of pulsed-mode plasma with argon addition for diamond growth

The advancement of high-speed growth technology for large-scale single-crystal diamonds is desired. In the widely used microwave plasma chemical vapor deposition method, the gas temperature in the plasma atmosphere significantly contributes to the generation of reactive radicals and enhancement of crystal growth. However, the impact of electron-dominated reactions in the plasma on the crystal growth remains unclear. In this study, we actively controlled the plasma environment by adding argon gas and adopting microwave pulse modulation to generate the plasma. We estimated the gas temperature and electron density of the plasma using optical emission spectroscopy. Our results implied that an increase in gas temperature alone hardly explained the enhancement of the growth rate by the argon addition or pulse modulation. In addition to the increase in the electron density due to the argon addition and pulse modulation, gas-phase chemical reaction calculations showed that the radical production enhancement became remarkable under an electron density higher than a threshold (10¹⁷ m⁻³). Therefore, the enhancement of the growth rate mentioned above may be attributed to electron-dominated reactions in the discharge region. These findings suggest that increasing the electron density can further improve the growth rate and potentially enable diamond synthesis at lower temperatures than traditional methods.