Intrinsic and Extrinsic Recombination Radiation from Natural and Synthetic Aluminum-Doped Diamond

The edge-recombination-radiation spectrum from natural semiconducting diamond has been re-examined and compared with spectra obtained for the first time from aluminum and nominally boron-doped General Electric synthetic diamond. The intrinsic components are due to the phonon-assisted decay of free indirect excitons of internal binding energy ∼0.08 eV. Comparison of the phonon energies with recently obtained dispersion curves for the fundamental lattice vibrations shows that the conduction-band minima are located at points $\frac{3}{4}$ of the way from the center to the boundaries of the reduced zone. Substructure has been observed in the intrinsic components due to the ∼7-meV spin-orbit splitting in the valence-band energy states at the zone center. The major extrinsic components are due to the zero-phonon and phonon-assisted decay of excitons bound to a characteristic acceptor center of semiconducting diamond ($E_A=0.36\mathrm{}$ eV). The bound excitons have a thermal and optical ionization energy of ∼50 meV. These extrinsic components exhibit enhanced spin-orbit splitting (∼12 meV). Radiation due to the zero-phonon and phonon-assisted recombination of free electrons at the neutral acceptor center has been detected. Infrared absorption measurements, neutron-activation analysis, and electrical-transport (Hall-effect) measurements have also been made. Intercomparison of these results and the edge-emission data shows that the acceptor center is due to isolated substitutional aluminum impurities. These acceptor centers are considerably more abundant in the synthetic diamonds, but the degree of compensation is generally much higher than in the available natural semiconducting specimens. Nitrogen donors with very deep energy levels apparently play a major role in the compensation.