Effects of coexistent pores and paramagnetic defects on positron annihilation in silicon oxide thin films

We measured the Doppler broadening of positron 2γ annihilation and the probability of positron 3γ annihilation (I3γ) for paramagnetic defects (E′ and Pb) containing SiOx (1.5<x<2) films with different porosities, which were prepared by radio-frequency cosputtering of Si and SiO2 at different Ar pressures. The observed increase of I3γ with increasing Ar pressure was attributed to the increased open porosity of the film. Under a condition of constant paramagnetic defect concentration, Doppler broadening S parameter increases with increasing I3γ and open porosity of the film. This was caused by enhancement of a relative contribution from p-Ps to the 511 keV 2γ annihilation peak as a result of reduced probability of o-Ps pickoff annihilation. The relationship between the S parameter and I3γ can be affected strongly by the paramagnetic defects in the sense that film with a higher open porosity does not necessarily present a higher S parameter. A low porosity film with higher concentration Pb centers exhibited a higher S parameter than did more porous films containing lower concentration Pb centers because in the former film a larger number of positrons annihilate from p-Ps as a consequence of the Ps spin exchange caused by the paramagnetic defects.