Enhanced growth rates of N-type phosphorus-doped polycrystalline diamond via in-liquid microwave plasma CVD

Phosphorus-doped diamond (PDD) exhibits excellent properties, making it suitable for a wide range of applications, such as electronic devices and electrodes. Here, we report the first synthesis of PDD by in-liquid microwave plasma CVD (IL-MPCVD) under high-pressure and low-power conditions. A mixture of methanol (MeOH) and ethanol (EtOH) with triethyl phosphate ((C₂H₅)₃PO₄) and (P/C = 1000 ppm) was used for the PDD deposition. Samples were characterized by laser microscopy, Raman spectroscopy, and glow discharge optical emission spectroscopy. Notably, PDD was successfully produced at a growth rate of 280 μm/h, which is two orders of magnitude higher than conventional CVD methods. Additionally, cyclic voltammetry (CV) and impedance spectroscopy (EIS) were used to evaluate the electrochemical properties of PDD. As a result, we confirmed the wide potential window characteristic of conductive diamond and determined that the donor density was [P] = 3.8 × 10¹⁷cm⁻³. Therefore, it is clear that IL-MPCVD is applicable for very high growth rates in the CVD process for PDD synthesis.