We present an implementation of a liquid-phase boron precursor trimethyl borate for boron-doped diamond films deposition by microwave plasma CVD. Trimethyl borate vapors were used not only as a source of boron for doping but also as the only source of carbon and oxygen, while completely saturating the requirements for the growth of high-quality boron-doped diamond films. The film morphology was controllable from microcrystalline to ultra-nanocrystalline by changing the concentrations of trimethyl borate. Using this unique precursor system, we were able to grow diamond films with a doping level in the range from 10^17 cm to 10^22 per cubic centimeter and low resistivity values. Low activation energies were calculated from the Arrhenius plot and growth rates as high as up to 170 nm/h for the low pressure microwave plasma CVD were reached. Based on the results an EU patent was filled and granted (EP 4 177 372 A1). Trimethyl borate is a low-cost and safe liquid source for large-area MW CVD growth of doped diamond films.
