Accelerating nanocrystalline-diamond nucleation in the low-temperature MW-CVD growth of diamond-like carbon films via reacting-gas pre-heating

The reacting gas pre-heating is presented as a means to enhance the diamond nucleation density in the diamond-like carbon (DLC) films with embedded nanocrystallinity (NCD). Utilizing C₂H₂ as the precursor gas having low ionization potential, allows for faster NCD-embedded DLC growth. The additions of H₂, a conventional dilution gas, along with CO₂, a weak oxidant, increase the chemical activity of the growth precursors and aids in the removal of a-C phase from the growth site. Notably, the bandgap widens from 3.40 to 3.48 eV, while the sp³/sp² ratio increases from 1.9 to 2.38. Raman spectroscopy reveals higher I_Dia/I_G (0.85), I_Dia/I_D (0.94), alongside a lower I_D/I_G ratio (0.90) for the film grown at the optimum gas-pre-heating temperature (T_G = 250 °C) demonstrate the superiority of the less-stressed DLC network, with higher optical transmittance of ∼93% at 600 nm. The DLC matrix developed at a substrate-temperature (T_S) ∼300 °C holds a good quality nano-diamond phase (grain size ∼8–10 nm), containing distinct trans-PA peaks (1171 and 1497 cm⁻¹) and diamond peaks (1333 cm⁻¹) as the signature. The preheating of the source gas mixture elevates the precursor gas molecules’ vibrational energy states, facilitating the conversion of C₂H₂ to CH₃, which is energetically favorable for diamond nucleation.