The detailed structure of erbium (Er) sheet‐doped GaAs grown by molecular‐beam epitaxy is directly determined by analysis of the ion‐channeling spectra in the [100], [110], and [111] directions with 2.0 MeV He+beams followed by a recently developed Monte Carlo simulation. It is shown that Er atoms form NaCl‐type crystalline ErAs clusters in the GaAs epitaxial layer at 500 and 580 °C. The clusters grow in the [100] direction with the common three principal axes of the cubic unit cell; the lattice constant of the clusters coincides exactly with that of the zinc‐blende‐type crystalline GaAs epitaxial layer. The shape and size of the clusters can be roughly deduced in the simulation from the dependence of the visible Er fraction on the cluster size for various cluster shapes. These results are consistent with those obtained by cross‐sectional transmission electron microscopy. Almost all Er atoms are located precisely in tetrahedral interstitial sites with 0.1 A˚ standard deviation; only 2% of the Er atoms occupy random sites. These results mean that the lattice constant of the crystalline ErAs clusters is compressed to that for the crystalline GaAs host during the cluster formation. ©1995 American Institute of Physics.