Localized excitons and defects in PbWO₄ single crystals: a luminescence and photo‐thermally stimulated disintegration study

Abstract

PACS 71. 78.55.Hx, 78.60.Kn Luminescence from the exciton-and defect-related states and photo-thermally stimulated disintegration of these states under selective UV irradiation were studied in the PbWO 4 : Mo, Ce crystal and compared with the characteristics of other undoped and doped PbWO 4 crystals of different origin. For the first time, various localized exciton states were identified and their decay into stable defects was found. Different positions, halfwidths and temperature dependences were observed for the blue emission spectrum in different PbWO 4 crystals and explained by the presence of strongly overlapping emission bands of the WO 4 2--type self-trapped and localized excitons. Binding energies of the self-trapped and localized exciton states were calculated. The dominant thermally stimulated luminescence peaks, created in the PbWO 4 : Mo, Ce crystal by the UV radiation, are located at 110, 202 and 247 K. The related trap depths were calculated using the initial rise method. It was concluded that under irradiation in the exciton absorption region, defects are mainly created due to the disintegration of the localized exciton states, which occurs without release of free charge carriers.

Complex oxide scintillators: Material defects and scintillation performance

“…Scintillation decay time is reported of 12-15 µs at RT, quite similar in the value to the photoluminescence one (20 µs at 65 K [19]) and no slower component in the scintillation decay have been reported so far.…”

“…5. Migrating holes coming to vicinity of self-trapped electrons will give rise to self-trapped excitons, which will be, however, thermally disintegrated and localized excitons will come into play as described in [19,20]. Due to several kinds of shallow electron and hole traps discussed above in an undoped crystal, see also Fig.…”

“…Even if the nature of excitonic emission is similar in all three materials, there is considerable difference in thermal stability and onset of thermal disintegration/quenching of the exciton state. While both CaWO 4 and CdWO 4 show temperature independent self-trapped exciton (STE) emission intensity practically up to RT, STE in PbWO 4 thermally disintegrates already around 150 K and the blue emission component at higher temperatures is actually based on various localized exciton emissions which become seriously thermally quenched at RT [19]. Different degree of exciton emission thermal quenching/disintegration is thus responsible for different RT decay times observed in CaWO 4 or CdWO 4 (several microseconds) and in PbWO 4 (few nanoseconds) and much lower luminescence and scintillation efficiency of the latter.…”

“…Different degree of exciton emission thermal quenching/disintegration is thus responsible for different RT decay times observed in CaWO 4 or CdWO 4 (several microseconds) and in PbWO 4 (few nanoseconds) and much lower luminescence and scintillation efficiency of the latter. Mechanism of creation of localized excitons and their thermal stability in PbWO 4 has been recently reported in great details [19,20].…”

Tunneling recombination processes in PbWO4 crystals

“…In PbWO 4 , the blue (B) emission, two types of green emission, G(I) and G(II), and two types of red emission were detected. The B and G(I) emissions of undoped PbWO 4 crystals are suggested to be of exciton-like origin and arise from the WO 4 2-groups located in the regular and in the lead-deficient crystal regions, respectively, and perturbed by various defects [3,4]. In Mo 6+ -doped crystals, the G(I) emission arises from the MoO 4 2-groups [1,2].…”