Resolved donor-acceptor pair-recombination lines in diamond luminescence

Cathodoluminescence has been investigated in diamond films, prepared by microwave plasma assisted chemical-vapor deposition. Results are presented for various diamond films differing in crystalline structure and doping. In the energy range 2.0-2.7 eV a series of 13 sharp luminescence lines has been observed at temperatures below 200 K. In a tentative interpretation they are attributed to donor-acceptor pair recombination. The Coulomb energy depends on the discrete values for the pair separation in the expected manner including a Bohr radius correction for the three closest pairs. The relative intensities of the lines depend on the growth conditions and on the film texture. Likely candidates for the pairs are the nitrogen donor and the boron acceptor. Both impurities are contained in the present samples. The sum of the binding energies is 3.57 eV, and inserting Esuba = 0.37 eV for boron yields Esubd = 3.2 eV for nitrogen.