Optical studies of the 5RL centre in diamond

Abstract

The 5RL centre has been produced in natural semiconducting diamond by radiation damage and annealing to about 300 degrees C. The 5RL system is observed in both absorption and cathodoluminescence, and is of interest because the zero-phonon transition couples to local-mode phonons with energies well above those of the lattice phonons. The form of the cathodoluminescence band can be understood in terms of coupling of the zero-phonon transition to one dominant and two secondary local-mode phonons, as well as the band of lattice phonons. Because of mode-softening effects, only one of the secondary local-mode phonons participates in absorption; the energy of the other lies within the phonon density of states and decays rapidly through lattice mode coupling. In the cathodoluminescence spectrum the energy spacing between the phonon replicas progressively decreases, and the widths of the lines progressively increase with the number of phonons involved. These effects are shown to be as expected for a centre that possesses intrinsic anharmonicity and for which the coupling of the localised modes to the band modes is anharmonic. Uniaxial-stress measurements show that the centre has rhombic I symmetry. This information alone does not enable one to propose an atomic model for the 5RL centre. However, the annealing data, together with the continued production of 5RL centres observed at high levels of radiation damage, make it very unlikely that the boron-vacancy model proposed previously is correct.