Site selective laser spectroscopy of the insulator-to-semiconductor transition in CdF2:Er3+

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Abstract
The individual Er3+ sites in CdF2:Er3+ are determined by site selective laser spectroscopy and are monitored as the crystal is changed from the insulating to the semiconducting state. The site distribution is controlled by clustering of the Er3+ dopant ions and their charge compensating fluoride interstitials (F′i). Simple, single pair sites where an Er3+ ion is locally compensated by a Fi are found to have minor importance in contrast with other fluorites. The conversion to a semiconductor state results from oxidation of the F′i that locally compensate the Er3+ clusters. No new sites are observed that arise from local compensation by electrons. Differences in the oxidation potential between the various clusters suggest differences in the numbers of Fi associated with each cluster. It is suggested that the changes in the site distribution that occur for dopants with different ionic radii account for the inability of some dopants like La3+, Ce3+, and Pr3+ to produce the semiconducting state. The observation of the dominance of clustering in CdF2 lends support to previous interpretations of the activation energies for electron transport.