A POSITRON-ANNIHILATION INVESTIGATION OF POROUS SILICON

Positron lifetime experiments have been conducted on 13 different poro us silicon layers formed from p(+), p, and n(+) substrates. Four disti nct positron lifetimes could be associated with the porous layers. One lifetime was close to 0.5 ns and is claimed to arise from positrons t rapped by small vacancy clusters. Positronium is formed either on the surface with a lifetime of similar to 5 ns, or in the pores, from whic h exceptionally long lifetimes of up to 90 ns originate. Both of these lifetimes arise from the pickoff annihilation of ortho-positronium; t he fourth lifetime is the usual 125 ps component associated with para- positronium. The long lifetime component gives rise to 3-gamma annihil ations which are readily detectable in the energy spectrum of the anni hilation quanta. In situ heat treatments of two of the samples provide evidence that gases play a very important role in the passivation of defects, both in the native oxide layer and on the surface, albeit on very different time scales. (C) 1996 American Institute of Physics.