Electron traps related to oxygen vacancies inPbWO4

Abstract

Four types of electron traps in ${\mathrm{PbWO}}_{4},$ based on regular W sites perturbed by oxygen vacancies, are identified by electron spin resonance (ESR). The analysis of the ESR spectra parameters $(g$-factor values and principal-axes orientations) shows that the revealed centers are $({\mathrm{WO}}_{3}{)}^{\ensuremath{-}}$ vacancy-containing complex anions associated with a defect in the Pb sublattice: $({\mathrm{WO}}_{3}{)}^{\ensuremath{-}}\ensuremath{-}{A}_{\mathrm{Pb}}$ complexes. Two of the centers $({\mathrm{W}}_{1}$ and ${\mathrm{W}}_{2})$ are thermally stable up to 350--370 K, while the other two $({\mathrm{W}}_{3}$ and ${\mathrm{W}}_{4})$ are stable only up to 270--290 K. Above these temperatures the trapped electrons become free and recombine with localized holes, giving rise to a composite TSL glow peak at $T\ensuremath{\approx}323\mathrm{K}$ (characterized by a trap depth of 0.9 eV evaluated by the initial rise method) and to a weaker TSL structure at $T\ensuremath{\approx}365\mathrm{K}.$ The correspondence between $({\mathrm{WO}}_{3}{)}^{\ensuremath{-}}\ensuremath{-}{A}_{\mathrm{Pb}}$ centers and TSL peaks is supported by their similar room temperature time decays, and by the similar dependences of ESR and TSL signals upon annealing treatments. A scheme of local electronic levels in the band gap of ${\mathrm{PbWO}}_{4}$ is proposed.