A study was made of Ho centers in mixed-polytype crystals, that were co-doped with Ag, by using polarized site-selective photoluminescence and photoluminescence excitation spectroscopy and electron paramagnetic resonance techniques. The optical data revealed evidence for the presence of Ho with a 3+ charge state (4f10). The electron paramagnetic resonance measurements, which were performed under illumination in the inter-band range, detected centers which were in the 2+ charge state (4d11). On the basis of a crystal field analysis, it was concluded that the predominant centers which were seen in the 2 types of experiment were identical with respect to their atomic configuration; apart from small nearest-neighbor relaxation effects which were associated with the change in charge state. The crystal field was found to have approximately cubic symmetry, with a small trigonal field component whose axis was parallel to the [111] direction. The data indicated that the Ho occupied zincblende-type interstitial sites with 4 metallic ions as nearest neighbors. It was thought to be most probable that the Ag substituted for Zn ions. The Ho2+ ground state which was involved in the charge transfer process was suggested to lie close to the bottom of the conduction band, and thus explain the optical behavior which was observed in the experiments.

R.Boyn, S.Müller, J.Dziesiaty, H.Zimmermann: Journal of Physics - Condensed Matter, 1995, 7[47], 9061-73