Indium Phosphide layers grown by gas source Molecular Beam Epitaxy, (MBE) have been studied by positron lifetime spectroscopy using the recently modified pulsed positron beam in Munich. The as-grown samples are known to be phosphorous rich and contain a high concentration of vacancy-type defects. On annealing, phosphorous precipitates are formed and the concentration of free volume defects increases. Positron lifetime spectroscopy has identified the grown in defects to be indium vacancies at a c... »