The accumulation of defects in the surface region of Si bombarded with 0.5-MeV Bi ions at a temperature of −196°C is considered. It is shown that the buildup of disorder in the surface region as the ion dose increases manifests itself in the planar growth of an amorphous layer starting from the Si-SiO2 interface, and this growth sets in after a threshold implantation dose is attained. The results obtained can be described adequately in the context of a model based on the migration of mobile point defects generated by fast ions to the surface, subsequent processes of segregation of these defects at the interphase boundary, and the presence of saturable sinks in the initial samples.