Compositional mapping of the argon–methane–hydrogen system for polycrystalline to nanocrystalline diamond film growth in a hot-filament chemical vapor deposition system

The transition in diamond crystal morphology from well-faceted microcrystalline to nanocrystalline phases as a function of increasing argon (Ar) concentration has been studied in a hot-filament chemical vapor deposition (HFCVD) system. The range of Ar concentration in a Ar–methane–hydrogen mixture that permits well-faceted diamond growth is up to 90%. At a concentration of 95.5%, a marked transition into nanocrystalline diamond phase could be observed. Although no graphitic phases were observed by transmission electron microscope and x-ray diffraction up to 95.5% Ar concentration, Raman analyses revealed a proportionate increase in amorphous carbon content with an increase in Ar concentration. The addition of Ar appears to create an electron-rich gas environment because a plasma can be readily discharged by applying a bias to the sample. Analysis of the plasma by optical emission spectroscopy revealed a linear correlation between Ar addition and the occurrence of $C2$ dimers in the plasma. Our results identified the narrow Ar-methane-hydrogen composition window that is effective for the growth of thick and smooth nanocrystalline diamond in HFCVD.