We report a study aimed at highlighting the mechanism of a new amorphous silicon crystallization phenomenon that originates from the perimeter of a germanium layer during low-temperature annealing (500 degrees C). Results are reported on doped and undoped amorphous silicon films, with thicknesses in the range 40-200 nm, annealed at a temperature of 500 or 550 degrees C. A comparison is made of crystallization arising from Ge and SiGe layers and the role of damage from a high-dose fluorine implant is investigated. Plan-view scanning electron microscope images show that perimeter crystallization is only present in amorphous silicon films with thicknesses <= 100 nm, and that the crystallization width increases with decreasing film thickness and increasing doping level. Cross-sectional scanning electron microscope images show that the perimeter crystallization originates from grains at the bottom of the amorphous silicon film. The perimeter crystallization phenomenon disappears when the amorphous silicon is implanted with fluorine and when an Si80%Ge20% layer is employed instead of germanium. The perimeter crystallization is due to the formation of large grains as a result of an increased growth rate of pre-existing grains and this is attributed to the strain generated by the thermal expansion of the germanium layer during anneal. (c) 2007 The Electrochemical Society.