Synthesis of Polycrystalline Diamond Films in Microwave Plasma at Ultrahigh Concentrations of Methane

Polycrystalline diamond (PCD) films are usually grown by chemical vapor deposition (CVD) in hydrogen–methane mixtures. The synthesis conditions determine the structure and quality of the grown material. Here, we report the complex effect of the microwave plasma CVD conditions on the morphology, growth rate and phase composition of the resulting PCD films. Specifically, we focus on the factors of (i) increased methane concentrations (${\nu }_c$) that are varied over a wide range of 4%–100% (i.e., pure methane gas) and (ii) substrate temperatures (${\mathrm{T}}_{\mathrm{s}}$) varied between 700–1050 °C. Using scanning electron microscopy, X-ray diffraction and Raman spectroscopy, we show that diamond growth is possible even at ultrahigh methane concentrations, including ${\nu }_c$ = 100%, which requires relatively low synthesis temperatures of ${\mathrm{T}}_{\mathrm{s}}$ < 800 °C. In general, lower substrate temperatures tend to facilitate the formation of higher-quality PCD films; however, this comes at the cost of lower growth rates. The growth rate of PCD coatings has a non-linear trend: for samples grown at ${\mathrm{T}}_{\mathrm{s}}$ = 800 °C, the growth rate increases from 0.6 µm/h at ${\nu }_c$ = 4% to 3.4 µm/h at ${\nu }_c$ = 20% and then falls to 0.6 µm/h at ${\nu }_c$ = 100%. This research is a step toward control over the nature of the CVD-grown PCD material, which is essential for the precise and flexible production of diamond for various applications.