Cathodoluminescence (CL) techniques are used to investigate the defect structures of pure synthetic silicon dioxide (SiO_2) polymorphs. Pure, synthetic Types I, II, III and IV amorphous SiO_2 polymorphs, pure, synthetic crystal -SiO_2 and pure, synthetic amorphized crystal -SiO_2 have been investigated and their characteristic defects have been determined and compared. The CL emission from pure SiO_2 polymorphs is generally related to local point defects in the tetrahedrally coordinated SiO_2 host lattice. A range of CL emissions associated with non bridging oxygen defects, oxygen deficient defects and the radiative recombination of self trapped excitons are observed from both the pure synthetic crystal and amorphous SiO_2 polymorphs. In addition CL emissions associated with residual concentrations of Aluminium impurities are also observed from -SiO_2 (quartz) and Type I and II a-SiO_2 (fused quartz). Localised amorphous micro-volumes may exist within natural -SiO_2 due to the presence of a high concentration of pre-existing or induced defects. Amorphization of -SiO_2 diminishes the difference between the defect structures and associated CL from -SiO_2 and a-SiO_2. Thus CL investigation of the defect structure of a-SiO_2 polymorphs provides useful insight into the microstructure of amorphized -SiO_2.
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