It was shown that Co formed a series of optically active defect centers in samples which had been grown by means of high-temperature, high-pressure synthesis. The use of photoluminescence methods revealed that newly observed vibronic systems, with zero-phonon energies at 1.989, 2.135, 2.207, 2.277, 2.367 and 2.590eV, appeared only in samples that had been grown by using a Co-containing solvent-catalyst. Annealing at temperatures ranging from 1500 to 1800C established that many of the new bands appeared during the temperature regime of N aggregation. It was proposed that N formed complexes with Co so as to produce optically active centers; similar to the case of Ni point defects. Radiative decay-time measurements and temperature dependence measurements showed that all but one of the bands which were associated here with N-Co complexes had radiative decay times that were of the order of 100s. This was again characteristic of the photoluminescence centers that arose from Ni-N complexes. All of the vibronic bands which were observed by means of photoluminescence could also be monitored by means of cathodoluminescence. In this case, it was necessary to use a beam current density of less than 10mA/cm2, otherwise the spectra were dominated by emission from optical centers with decay times that were of the order of 20ns. Only one vibronic band, with a zero-phonon line at 1.852eV, was detected by means of absorption measurements. The center which was responsible for this system did not give rise to luminescence.

S.C.Lawson, H.Kanda, K.Watanabe, I.Kiflawi, Y.Sato, A.T.Collins: Journal of Applied Physics, 1996, 79[8], 4348-57