The depth distribution of open‐volume defects has been studied in Si(100) crystals grown by molecular beam epitaxy at 300 °C by the variable‐energy monoenergetic positron beam technique combined with well‐controlled chemical etching. This procedure gave a 10 nm depth resolution which is a significant improvement over the inherent depth resolving power of the positron beam technique. The epitaxial layer was found to grow defect‐free up to 80 nm, from the interface, where small vacancy clusters, larger than divacancies, appear. The defect density then sharply increases toward the film surface. The result clearly shows that the nucleation of small open‐volume defects is a precursor state to the breakdown of epitaxy and to the evolution of an amorphous film. ©1995 American Institute of Physics.