Absence of positronium formation in clean buried nanocavities inp-type silicon

Buried nanocavities at about 350 nm depth in Si were produced by thermal treatment of He implanted $p$-type (100) Si. The internal surfaces of the nanocavities were found free of impurity decorations by examining the high-momentum part of the Doppler-broadened positron annihilation spectra. Positron lifetime measurements with a pulsed slow positron beam show neither a short lifetime (125–150 ps) ascribable to parapositronium nor a longer lifetime (2–4 ns) ascribable to pick-off annihilation of orthopositronium. The lifetime of positrons trapped into nanocavities was found to be about 500 ps. The absence of positronium formation could be explained by an insufficient electron density and a lack of electron states in the band gap at the nanocavities internal surfaces produced in the $p$-type silicon.