LUMINESCENCE AND PHOTOCONDUCTIVITY OF CERIUM COMPOUNDS

The study of the luminescence properties of rare earth doped systems h istorically focuses on systems which exhibit strong luminescence. More recently, extensive studies on materials with high quantum efficiency are in part motivated by the search for new phosphor and scintillator materials. However, a thorough study of certain systems which show ve ry low quantum yield will certainly lead to a better understanding of phosphor materials and rare earth systems in general. As an example of recent studies which address both the fundamental question of relaxat ion processes in rare earth doped systems and phosphor applications we present studies on cerium-doped lutetium oxide crystals which are cha racterized by a complete quenching of the 5d-4f luminescence and compa re its optical properties to that of very efficient cerium doped phosp hor material, lutetium oxyorthosilicate. To find the mechanisms which lead to the different quantum efficiency in these systems, extensive a bsorption, photoexcitation and photoconductivity studies were performe d on single crystals. We demonstrate that the radically different emis sion properties of the investigated systems originate in small but cru cial differences in the location of the emitting 5d level of the ceriu m ion with respect to the conduction band of the host - a general resu lt which can be applied to a broad range of materials.