Bound Excitons and Donor-Acceptor Pairs in Natural and Synthetic Diamond

Abstract

Recent studies on x-ray and electron-excited luminescence (cathodoluminescence) in natural and synthetic diamond provide evidence that the previously reported band-$A$ luminescence is due to the radiative recombination across the indirect energy gap of electrons and holes separately trapped at donor and acceptor centers. Analysis of the temperature dependence of the luminescence from unirradiated natural specimens suggests that the principal acceptor is that responsible for the semiconductivity of type-IIb specimens. The donor and acceptor centers are, respectively, identified with isolated substitutional nitrogen and aluminum. Radiative recombinations apparently occur at both highly associated and diffuse pairs in natural diamond. Diffuse pair transitions predominate for typical General Electric, South African, and Swedish synthetic specimens. A band-$R$ luminescence, produced in the cathodoluminescence spectrum of type-I diamonds by electron-irradiation damage, is tentatively identified in part with relatively close pair recombinations between $\mathrm{GR}1$ (vacancy?) donor centers and $\mathrm{ND}1$ (impurity+interstitial?) acceptor centers. A new M\"ossbauer-type fluorescence system with zero-phonon line at 1.40 eV, which is prominent in the cathodoluminescence spectra of nitrogen-doped synthetic diamond, is identified with the annihilation of indirect gap excitons tightly bound to ionized isolated nitrogen donors. The well-known $N3$ (3.0 eV) and $N9$ (5.26 eV) absorption-luminescence systems of natural diamond are identified with the creation or annihilation of excitons bound to ionized or neutral nitrogen donors which are modified by the presence of an ionized aluminum acceptor center in an adjacent substitutional lattice site.