Positron lifetime studies of defects in MBE-grown silicon

A timed positron beam has been used to study defects in MBE-grown silicon layers previously studied by conventional slow-positron techniques and electron microscopy. An expected void-related positron lifetime component has been observed, but at a much lower intensity than expected from the Doppler-broadening results. The implication is therefore that the momentum of the annihilation positron-electron pair at the void is considerably lower than previously assumed. In the lifetime spectra, there is evidence of a trapped positron state in the overlayers with a lifetime similar to that of a positron freely diffusing in silicon. The nature of this defect is, at present, unclear.