Specular scattering of 3 keV He ions was observed for incidence angles of 2°–6° from GaAs(001). During molecular beam epitaxy, the scattered ion current dropped rapidly upon opening the Ga shutter, showed damped oscillations, and then increased gradually upon closing the shutter. The oscillation periods corresponded to monolayer growth times. Oscillation amplitudes decreased with increasing substrate temperature, indicating a transition to step-flow growth. The oscillations were not a diffraction effect, allowing a quantitative interpretation based on scattering by adatoms and step edges. © 1997 American Institute of Physics.