Spectroscopic study of cobalt-related optical centers in synthetic diamond

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Abstract
This article presents evidence that cobalt forms a series of optically active defect centers in diamond grown by high‐temperature, high‐pressure synthesis. Photoluminescence (PL) studies reveal that the newly observed vibronic systems with zero‐phonon energies at 1.989, 2.135, 2.207, 2.277, 2.367, and 2.590 eV appear only in samples grown using a cobalt‐containing solvent–catalyst. Results of an annealing study, carried out in the temperature range 1500 to 1800 °C, establish that many of the new bands appear during the temperature regime of nitrogen aggregation. It is therefore proposed that nitrogen forms complexes with cobalt to produce optically active centers, in a manner analogous to that of nickel point defects in diamond. Detailed radiative decay time measurements and temperature dependence measurements show that all but one of the bands which are here associated with nitrogen–cobalt complexes have long radiative decay times (∼100 μs), and this again is a characteristic of the PL centers arising from nickel–nitrogen complexes. All of the vibronic bands observed by PL may also be produced by electron‐beam excitation (cathodoluminescence). In this case it is necessary to use a low beam current density (≤10 mA cm−2), otherwise the spectra are dominated by emission from optical centers with much shorter decay times (∼20 ns). Only one vibronic band, with a zero‐phonon line at 1.852 eV, has been detected in absorption measurements, and the center responsible for this system does not give rise to luminescence.

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