Positron/positronium annihilation in low dimensional silicon materials

Positron and positronium annihilation investigations were applied to porous silicon made by anodization of crystal silicon. While three lifetime components (ca. 0.2, ca. 0.5 and ca. 30 ns) were found for porous silicon made from high resistivity wafer, only two components (ca. 0.2 and ca. 0.5 ns which increased above 200 K) were obtained for porous silicon made from low resistivity wafer. The lifetime of ortho-positronium becomes short, and the full width at half maximum of the Doppler-broadened annihilation-radiation decreases in oxygen atmosphere. Positron/positronium states are affected by surface states and by gas adsorbents in the nanoscale cavities of the porous silicon. We deduce that a fraction of the ortho-positronium is converted to para-positronium through spin conversion, under interaction with unpaired electrons located near pore surfaces.