High-speed synthesis of heavily boron-doped diamond films by in-liquid microwave plasma CVD

Here, we report the first synthesis of polycrystalline boron-doped diamond (BDD) by an In-liquid microwave plasma CVD (IL-MPCVD) process from a mixture of alcohols and boron trioxide (B2O3). We simultaneously achieved high growth rates (up to 287 μm/h) and high boron concentrations (up to 7 × 1021 cm−3). We examined the growth mechanism and show that water generated by dissolution of B2O3 in the alcohol is key to rapid growth of BDD. Even with the same C:H:O ratio, diamond growth rate and nucleation density drastically decreased by addition of water. Water has a strong etching effect, which shifts diamond to grow in carbon rich conditions. Finally, we evaluated the as-grown BDD film as a diamond electrode. Our IL-MPCVD-grown BDD showed electrochemical properties comparable to those of BDD grown by the conventional CVD process in terms of its potential window (3.2 V) and peak separation (85 mV) in 1 mM K3[Fe(CN)6]. Owing to the high growth rate, IL-MPCVD is an effective technique for synthesizing BDD.