The formation of extended defects in Si below 25K was observed by means of in situ high-resolution transmission electron microscopy. The defects were distorted spheres, occasionally truncated by facets, less than 5nm in diameter. These defects were stable up to 773K, and gradually shrank during annealing at 773 to 973K. From an analysis of high-resolution transmission electron microscopic images of the defects, it was suggested that the defects were voids which formed via the athermal migration of vacancies under electron irradiation.

Extended Vacancy-Type Defects in Silicon Induced at Low Temperatures by Electron Irradiation. J.Yamasaki, Y.Ohno, S.Takeda, Y.Kimura: Philosophical Magazine, 2003, 83[2], 151-63