One-monolayer deposition characteristics have been investigated for Ge on the Si(1 0 0) surface and Si on the Ge(1 0 0) surface by using a co-axial impact collision ion scattering spectroscopy. A subtracted spectrum method has been proposed for the detailed analysis of the interface structure. Incident angle dependence of the signal intensity shows that both Ge/Si and Si/Ge hetero-interfaces are atomically abrupt without intermixing and surface segregation.