Creation of trapped electrons and holes in alkaline-earth fluoride crystals doped by rare-earth ions

Defects in Ce 3+ - and Eu 2+ -doped alkaline-earth fluorides, created by vacuum ultraviolet (UV) photons at energies lower than at the bandgap, were investigated by various methods: thermostimulated luminescence, photostimulated luminescence and optical absorption. The CaF 2 :Eu 2+ thermoluminescence curves in the range of 60-330 K due to various types of trapped holes were the same after vacuum UV illumination as after x-irradiation. Thermoluminescence curves of Ce 3+ -doped alkaline-earth fluorides created by vacuum UV illumination or x-irradiation were generally similar. However, V k thermoluminescence peaks were absent in vacuum UV illuminated CaF 2 :Ce 3+ and SrF 2 :Ce 3+ crystals. This fact is obviously associated with the presence of charge-compensating fluorine interstitials in Ce 3+ -doped crystals. The creation of Ce 2+ characteristic bands was observed in photostimulated luminescence spectra as well as in optical absorption spectra of vacuum UV illuminated or x-irradiated Ce 3+ -doped crystals. The suppression of hole thermoluminescence peaks in CaF 2 :Eu 2+ crystals by blue light is due to the photoionization of Eu + ions. The proposed mechanism for the creation of trapped hole and trapped-electron defects by vacuum UV illumination involves charge-transfer-type transitions, in which the electron transfers from the valence band to an impurity level lying in the bandgap. Comparison of all energies involved of transitions in the crystals investigated shows that the sum of all the transition energies is less than that of the bandgap by 1.5-3.5 eV. This energy difference can be considered to be the energy of lattice relaxation around the created Ce 2+ or Eu + ions.