Electronic Defect Structure of Single-Crystal ThO2by Thermoluminescence

Abstract

Some electronic defects and the associated photoelectronic processes in Th${\mathrm{O}}_{2}$ are analyzed by utilizing the data from thermoluminescence (TL) and EPR measurements on a number of rare-earth-doped and undoped single crystals from various sources. (The EPR measurements were made by others.) Some fluorescence and absorption measurements are also ultilized. Some TL glow peaks in the undoped Th${\mathrm{O}}_{2}$ correlate with the annealing of EPR spectra which are associated with trapped electrons. The similarity of glow curves for different crystals, the lack of hyperfine EPR structure, and the dependence on rare-earth doping suggests that some of the major electron traps are associated with oxygen vacancies, which may be complexed. The TL and EPR were induced by uv excitation, which created electrons and holes which are trapped. Some of the hole traps are identified as rare-earth ions in cubic sites. The rare earths provide all of the TL and fluorescence observed in ${\mathrm{Li}}_{2}$O \ifmmode\cdot\else\textperiodcentered\fi{} 2W${\mathrm{O}}_{3}$ flux-grown Th${\mathrm{O}}_{2}$, and the total TL at saturation depends on the doping level. The TL excitation spectra and optical absorption measurements on ${\mathrm{Li}}_{2}$O \ifmmode\cdot\else\textperiodcentered\fi{} 2W${\mathrm{O}}_{3}$ flux-grown undoped thoria indicate a band gap of 5.75 eV which is larger than previously reported. The thermal activation energies are given for electron traps, and some indications of relative cross sections for electron or hole trapping or recombination processes are reported.