Thermally stimulated luminescence as a tool for the investigation of point defects in luminescent materials

Point defects can give rise to localized levels in a material forbidden gap; according to their position with respect to the conduction and valence bands, they can trap electrons or holes during irradiation with ionizing beams. A significant influence of such trapping levels in luminescence processes is often observed. In general, carrier trapping at defects is a competitive process with respect to their prompt radiative or non radiative recombination. In scintillators, the presence of traps is harmful since they can give rise to slow tails in the scintillation time decay. Conversely, traps are profitably exploited in persistent phosphors and in dosimeters. Thermally stimulated luminescence (TSL) studies are frequently employed for the investigation of point defects; wavelength resolved measurements, where the amplitude of the TSL emitted light is measured both as a function of temperature and emission wavelength, are particularly useful for a deep understanding of traps and recombination centres. The seminar is devoted to a description of the investigation of point defects by wavelength resolved TSL with emphasis on the kind of information that can be found, like the energy position of the trapping levels, their thermal stability, and the kind of recombination mechanism. Experimental procedures and methods of data analysis are presented and compared, in order to give a picture of the potential of this high sensitivity technique. In addition, the capability to evidence less common features will also be highlighted, like the presence of local disorder, the existence of microscopic defect aggregates, and the occurrence of athermal or thermally assisted tunneling recombinations. Finally, it will be shown that the technique can also be exploited as a spectroscopic tool to investigate the position of activators energy levels. The discussion is grounded on several examples concerning TSL studies in selected materials featuring different trapping-recombination mechanisms, including crystalline and amorphous systems as well as optical ceramics.