A positron annihilation investigation of porous silicon

Positron lifetime experiments have been conducted on 13 different porous silicon layers formed from p(+), p, and n(+) substrates. Four distinct positron lifetimes could be associated with the porous layers. One lifetime was close to 0.5 ns and is claimed to arise from positrons trapped by small vacancy clusters. Positronium is formed either on the surface with a lifetime of similar to 5 ns, or in the pores, from which exceptionally long lifetimes of up to 90 ns originate. Both of these lifetimes arise from the pickoff annihilation of ortho-positronium; the fourth lifetime is the usual 125 ps component associated with para-positronium. The long lifetime component gives rise to 3-gamma annihilations which are readily detectable in the energy spectrum of the annihilation 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.