Heavily phosphorus-doped NCD diamond films with phosphorus incorporation of 5 × 10²⁰ cm^− 3 were successfully grown by plasma-enhanced chemical vapor deposition for thermionic emission cathode.

Thermionic emission properties were measured in vacuum as a function of temperature from 300 to 600 °C, and its maximum current density was recorded to be 8 mA/cm² at 600 °C, which value is comparable to that of the reported nitrogen-doped NCD film.

From the least-squares fitting to the data using the Richardson–Dushman equation, the work function and Richardson constant of heavily phosphorus-doped diamond are estimated to be 2.3 eV and 15 A/cm²/K², respectively.