Detection of Defects in Electron-irradiated Synthetic Silica Quartz Probed by Positron Annihilation

Abstract

Defects in quartz glasses introduce absorption bands and as a result, the transmission efficiency of light decreases in long-path-length optical systems. Defects in amorphous SiO₂ (a-SiO₂) films, formed on MOS (metal/oxide/semiconductor) devices as gates, perturb its operation. The positron annihilation techniques, which has developed as a powerful method for detecting vacancy-type defects in materials, were applied to the study of the annealing behavior of the defects, introduced in the high purity synthetic quartz glass by the irradiation of 3-MeV electrons up to the 1 × 10¹⁸ e^- /cm² dosage. It was proved that the positron annihilation techniques were sufficiently sensitive to detect the defects in the electronirradiated silica glasses (Figs.3, 4, 5). Three types of open-space defects were detected by the positron lifetime measurements. These can be attributed to monovacancy or divacancy type defects, vacancy clusters, and open-volume defects (∼0.07 nm³). A high formation probability (∼90%) of positroniums (Ps) was found in unirradiated specimens. These Ps were considered to be formed in openvolume defects. The formation probability of Ps was drastically decreased by the electron irradiation. But the size of open-volume defects was kept unchanged by the irradiation. These facts suggest that vacancy-type defects such as monovacancies or divacancies were introduced by the electron irradiation and that positrons were trapped in these defects. By the isochronal annealing in nitrogen atmosphere, the lifetime component (τ₂) and its relative intensity (I₂), attributed to positrons trapped in monovacancy or divacancy type defects and annihilated there, changed remarkably (Figs.9, 10). τ₂ was constant in the temperature range up to 300°C, getting slightly shorter between 300°C and 700°C, and constant above 700°C.I₂ decreased gradually up to 300°C, constant between 300°C and 550°C, decreased above 550°C, and constant above 700°C. This revealed that the behavior of the defects, in which positrons were trapped, change by the elevation of the annealing temperature.