Electronic structure of Ce³⁺ in yttrium and lutetium orthoaluminate crystals and single crystal layers

Low temperature infrared absorption spectra of yttrium orthoaluminate and lutetium orthoaluminate bulk crystals and liquid phase epitaxy grown single crystalline films doped with Ce³⁺ are presented. In the region of intra-configurational 4f – 4f transitions the spectra indicate the existence of at least three different Ce³⁺ related centers. The dominant center is associated with Ce substituting yttrium or lutetium. The two additional centers are most probably related to so called antisite positions of rare-earth ions in orthoaluminates, i.e., ions on Al sites. The concentration of antisite defects is lower in epitaxially grown layers of yttrium orthoaluminate than in the bulk crystals, in spite of the higher doping concentration. In contrast, in lutetium orthoaluminate additional Ce³⁺ centers are also present in epitaxial layers at quite high concentration. Crystal field analysis based on the exchange charge model exhibits excellent agreement of the calculated transition energies with experimental data for the dominant Ce³⁺ center. The linewidths of the 4f-4f transitions in both types of crystals are different depending on the final level of transition. This is explained as a result of electron-phonon coupling, enhanced by the matching inter-level spacing of the ²F_7/2 state with phonon energies of the examined materials.