Positron beam study of defects in SiO₂

Defects in crystalline and amorphous SiO₂ were studied by variable-energy positron annihilation spectroscopy in an attempt to clarify what defects in SiO₂ are observable in positron annihilation studies. The level of the S parameter in amorphous SiO₂ films grown by plasma chemical vapor deposition (CVD) in a tetraethylorthosilicate (TEOS) and O₂ atmosphere was found to be correlated to the concentration of Si-OH in the silicon dioxide layer. The difference in S parameter between crystalline and amorphous SiO₂ is assumed to be due to the effects of para-Ps self-annihilation in free volumes in the amorphous sample. The S parameters in crystalline and thermally grown silicon dioxide were both found to decrease after C ion implantation (1×10¹⁴ cm^-2, 140 keV) or alternatively, ArF excimer laser (6.4 eV) irradiation. Both methods are expected to produce numerous types of Frenkel defects of which, it is most likely that negatively charged species such as ≡Si-O^- are the trapping sites. In the case of ion implantation into amorphous SiO₂ a greatly lowered S parameter is observed, this is explained as a combination of defect trapping and reduction in para Ps production after ion implantation.