Electrical study of Pr3+ ion environment in Ca1-xPrxF2+x thin films

The environment of Pr3+ ions and their spatial distribution in Ca1-xPrxF2+x thin films epitaxially grown on silicon substrates were studied by means o f complex admittance spectroscopy (CAS) and thermally stimulated depolariza tion (TSD) studies. Contrary to the case of bulk single crystals, these tec hniques reveal only one type of praseodymium-fluoride compensating ion cent ers in the lavers. whatever x varying from 0.001 to 0.10. These centers cor respond to isolated substituted Pr3+ ions compensated by interstitial F-i(- ) ions in nearest-neighbor (nn) sites of C-4v symmetry. Moreover, it is dem onstrated that for these praseodymium concentrations? the F-i(-) ions can b e transferred between two neighboring centers. The corresponding dipolar ch aracteristics show that the nn pairs are homogeneously distributed into the bulk of the layers. These results are of great interest. They indicate tha t energy transfers between excited rare-earth ions, which arise in bulk sin gle crystals for high concentrations, must be considerably reduced in thin films. Consequently the self-quenching phenomena, which rapidly annihilate the luminescence, must be minimized.