Vacancy-type defects in crystalline and amorphous SiO2

Positron lifetime spectroscopy and two-dimensional angular correlation of annihilation radiation have been used to investigate grown-in vacancy structures in synthetic crystalline α-SiO2, synthetic fused quartz, and in a 60-μm-thick chemical-vapor-deposited amorphous SiO2 film. For α-SiO2 a ∼300 ps lifetime component suggests trapping by either silicon monovacancies or by oxygen divacancies (or both). The vacancies are neutral and present at a concentration level of 1017/cm3. The positron bulk lifetime for α-SiO2 is estimated to be ∼238 ps in good agreement with semiempirical predictions. In the fused quartz significant positronium formation is found (80%) and the remaining positrons annihilate in voids yielding a lifetime of ∼500 ps. The amorphous SiO2 film contains a mixture of small vacancy clusters and voids and ∼30% of the positrons form positronium. Heat treatment above 950 °C results in a substantial reduction in defect concentration, but up to 1100 °C a small vacancy cluster contribution persists. The positron data indicate that positronium formation in the fused quartz and in the amorphous film takes place in the voids.