Surface Effects on Positron Annihilation in Silicon Powders

Positron lifetime measurements were made in silicon powder samples with different grain size and doping. Two lifetime components were detected in low-resistivity $n$ samples and in high-resistivity $p$ samples having grain sizes smaller than about 50 μ. The faster component [${\tau }_1=(2.59\mathrm{}\pm 0.10)\times {10}^{-10\mathrm{}}$ sec] was easily ascribed to annihilations in the bulk; the slower one [${\tau }_2=(5.20\mathrm{}\pm 0.20)\times {10}^{-10\mathrm{}}$ sec] was ascribed to annihilation processes taking place in the oxide surface layer.