Abstract
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.
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