Electrical characterization of Ca1−xErxF2+x luminescent thin films

Abstract

Ca 1−x Er x F 2+x thin films, epitaxially grown on silicon substrates, present a 1.53 μm infrared luminescence line, which gives them an evident interest for optical communications. At room temperature, using an argon laser as the excitation source, the maximum of this emission is obtained in thin films for x=0.16, when it is observed for very low erbium contents in bulk CaF2:Er3+ single crystals. Thus, we can think that the Er3+ ion environment, which governs the self-quenching phenomena, differs in thin films from that in bulk single crystals. In this paper, the nature of the erbium doping centers and their spatial distribution into the host material are studied versus x by using complex admittance and thermally stimulated depolarization techniques. In thin films, it is shown that for x⩽0.01, the luminescent centers correspond to isolated Er3+–Fi− ion pairs in nn sites, which behave like dipoles, D1. Their activation energy is 0.68 eV. From x=0.01 to x=0.05 clustering begins to be observed and leads when x>0.05, to the formation of extended clusters, usually called D2a in the literature. So, the clustering process in thin films appears only for concentrations two orders of magnitude higher than in single crystals. Moreover, it is shown that, whatever x varying from 0.001 to 0.20, charges can be transferred between pairs or clusters. Then, the activation energy depends on the mean distance between these defects, and thus, on the erbium content: the dielectric response corresponds to a homogeneous distribution of the pairs or clusters into the bulk of the host matrix.