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 alpha-SiO2, synthetic fused quartz, and in a 60-mum-thick chemical-vapor-deposited amorphous SiO2 film. For alpha-SiO2 a approximately 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 10(17)/cm3. The positron bulk lifetime for alpha-SiO2 is estimated to be approximately 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 approximately 500 ps. The amorphous SiO2 film contains a mixture of small vacancy clusters and voids and approximately 30% of the positrons form positronium. Heat treatment above 950-degrees-C results in a substantial reduction in defect concentration, but up to 1100-degrees-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.