Preparation of highly preferred （110） optical-grade diamond films under CO₂ atmosphere

Diamond films prepared by conventional microwave plasma chemical vapor deposition （MPCVD） often face issues such as low growth rates，disordered grain orientations，and non-diamond phase defects，which limit further enhancement of their optical properties. This study introduces CO₂ as an auxiliary gas to deposit high-quality diamond films using the MPCVD method. Diamond film deposition is conducted by introducing different flow rates of CO₂， and the optical emission characteristics of the plasma under varying CO₂ flow rates are measured using optical emission spectroscopy （OES）. The crystallinity quality， grain orientation， surface morphology， and growth rate of the diamond films are characterized by XRD， Raman， and SEM. The results indicate that adding a certain amount of CO₂ （flow ratio of CO₂/CH₄=2/6） is conducive to obtaining diamond films with a high （110） preferred orientation （I₂₂₀/I₁₁₁=18.19）. An excessively high CO₂/CH₄ flow ratio intensifies the etching effect of oxygen-containing groups， reducing the degree of preferential grain orientation. The addition of an appropriate amount of CO₂ significantly improves the crystalline quality of the diamond， with the full width at half maximum （FWHM） of the diamond characteristic peak decreasing from 12.68 cm-1 to 8.26 cm-1. Furthermore， the addition of a suitable amount of CO₂ promotes diamond growth to some extent. Under the optimized flow ratio condition of CO₂/CH₄=2/6， this study successfully prepares a high （110） orientation （I₂₂₀/I₁₁₁=18.19） diamond film with a growth rate of 4 μm/h. After polishing， the self-standing diamond film achieves a transmittance of 71% at the 10.6 μm wavelength. Its transmittance in the long-wave infrared band approaches the theoretical value， meeting the requirements for applications such as long-wave infrared optical windows and extreme ultraviolet lithography machine windows.