D. Pliszka

Positron annihilation study of vacancy-like defects related to oxygen precipitates in Czochralski-type Si

We report the direct measurement of vacancy-like defects related to oxygen in the oxygen precipitation process in Czochralski Si. The vacancy-like defects were detected by measuring the positron lifetime and narrowing of the positron–electron annihilation momentum distribution. Oxygen atoms surrounding the vacancy-like defects were detected by analyzing the high-momentum part of the positron–electron momentum distribution measured by a Doppler broadening coincidence technique. It was found that the majority of the defects associated with oxygen have an effective open volume smaller than that of a silicon monovacancy.

Energy scale determination and energy resolution in positron total cross section measurements

We compare the recent values of positron total cross sections in benzene, cyclohexane and aniline by Zecca et al. with our previous measurements [Karwasz et al. Acta Phys. Pol. A, 107 (2005) 666] on the same apparatus. An apparent discrepancy disappears if Zeccas data are shifted by +0.2 eV; then their data can be approximated by the modified effective range theory, as well. This observation induced us to compare recent Trento data with other experiments and theories for polar targets, H2O and HCOOH. The evidence points out towards the need of a careful energy calibration in positron experiments.

Low-energy positron scattering on atoms and molecules

We report total cross sections (TCS) measurements for positron scattering on N2, Ar, He and three cyclic hydrocarbons. The data have been obtained with a new linear low-energy spectrometer at Trento University. Both nitrogen and argon total cross sections shows a flat part in TCS from few eV up to the positronium formation thresholds. For cyclocarbons he has found very high values of the TCS in low energy range. Modeling using effective range theory is performed for benzene and cyclohexane. In He results we found some resonant structures indicating formation of virtual positronium.

Positron annihilation and optical spectroscopy of silicon-related materials

Optical (VIS photoluminescence, photoluminescence decay, IR absorption) measurements on Czochralski-grown silicon, subjected to several thermal treatments are presented. These data are supplemented by positron annihiliation techniques (lifetime Doppler broadening). We report also some preliminary positron-annihilation data on these films of Si-related, low-ε materials.