Thin films of single crystals of both n-type and p-type silicon contain disordered zones ∼ 50 Å in diameter, discernible in the electron microscope after bombardment with ∼ 1013Ne+ions cm−2. As the dose increases these zones become more numerous until eventually (≳ 1014ions cm−2) they overlap, creating a continuous surface layer which electron diffraction shows to be amorphous silicon. The individual zones disappear on annealing between 400 and 500°C, and at ∼630°C the amorphous layer recrystallizes epitaxially upon the underlying silicon, leaving an array of dislocation loops and dipoles.