The thermal CVD process was conducted in a horizontal hot-wall CVD reactor. Triethylboron (TEB) (semiconductor grade, from SAFC Hitech) was used as single source precursor for BxC and palladium membrane purified H2 as a carrier gas and co-reactant. The TEB was kept in a stainless-steel bubbler, in a thermostat bath at 0 °C to maintain a vapor pressure of ~1.65 kPa for a stable precursor delivery achieved by bubbling H2 through the TEB liquid. A co-flow of Xe (99.998 %) was introduced as diffusion additive. Ar (99.997 %) was used in control experiments. Some experiments were conducted by delivering Xe in a pulsed manner, see supplementary information. This was done by pulsing Xe gas for 1 sec after every 30 sec.