Extrinsic Recombination Radiation from Natural Diamond: Exciton Luminescence Associated with theN9Center

Abstract

A luminescence system with no-phonon lines at 5.251 and 5.261 eV (at 100\ifmmode^\circ\else\textdegree\fi{}K), observed in the edge luminescence spectra of natural diamonds containing relatively low concentrations of nitrogen in "platelet" form, has been identified with the so-called $N9$ center which is responsible for a well-known impurity-absorption feature. Evidence is presented supporting a recent identification of this absorption system with the creation of an indirect exciton bound to nearest-neighbor donor-acceptor pairs involving substitutional nitrogen donors and aluminium acceptors. The donor-acceptor pair behaves like a modified donor with reduced ionization energy, since the ratio of the ionization energies of the isolated donor and acceptor is approximately 10:1 and the donor binding energy is larger than the donor-acceptor interaction energy. The $N9$ absorption/luminescence system is only observed when the modified donor is neutral in the unexcited crystal. Comparison of the $N9$ absorption and luminescence spectra provides an unambiguous identification of the phonon replicas and of the principal no-phonon excited electronic states in the absorption spectrum. The one-phonon replicas have been studied in detail and are compared with those observed in a system involving excitons which are relatively weakly bound at neutral isolated aluminium acceptors in $p$-type semiconducting diamond. For both systems the exciton-phonon coupling is a maximum for the optical phonons of wave vector ${\mathrm{k}}_{c}$ which conserve momentum in the intrinsic indirect transitions. The breadth and shape of the $N9$ optical-phonon replicas indicate, however, that coupling occurs for phonons with wave vectors distributed throughout at least the outer half of the reduced zone, whereas the replicas for the less tightly bound acceptor-exciton complex indicate that the coupling is negligible other than to phonons with wave vector relatively close to ${\mathrm{k}}_{c}$.