Cathodoluminescence of highly and heavily boron doped (100) homoepitaxial diamond films

Homoepitaxial films containing from 2×1016 to 1.75×1021 B−cm−3 are deposited by Microwave Plasma Chemical Vapor Deposition on Ib substrates. From their full continuity for all [B] with those found for the lower [B], the high energy structures in their cathodoluminescence spectra originate from recombination of free and boron bound excitons, with no signal from band to band transitions. Therefore, the metallic conduction takes place on a boron impurity band with a Fermi level outside the valence band in metallic diamond, in agreement with the original Mott model. The energy for the recombination of the boron bound exciton assisted by the TO phonon decreases from 5.211 to 5.036eV when [B] increases from ≈1.5×1019 to 6×1020cm−3, then remains constant. An interpretation of the results somewhat similar to that used for GaP:N might account for the excitons bound to the boron atoms of the impurity band.