Vacancy accumulation in pure float-zone crystals, covered with thin Si3N4 films, was monitored during irradiation in a high-voltage electron microscope at temperatures of between 20 and 250C. This led to the suppression of the formation of (113) interstitial-type defects in the central part of the irradiated area. Very small secondary clusters of interstitial type, with a density of about 1011/cm2, appeared after prolonged irradiation. Small stacking-fault tetrahedra and Frank loops of vacancy type, connected with interstitial clusters, were found in the thinnest part of areas which had been irradiated in a 400keV electron microscope. It was suggested that vacancy clusters might act as nucleation centers for secondary defect formation.

L.Fedina, J.Van Landuyt, J.Vanhellemont, A.L.Aseev: Nuclear Instruments and Methods in Physics Research B, 1996, 112[1-4], 133-8