We investigate the effect of Si ion implantation and subsequent rapid thermal annealing (RTA) on the homogeneity of the crystal quality of metalorganic chemical‐vapor‐deposited GaAs (150 A˚)/InGaP (100 A˚)/n+InGaAs:Si (150–200 A˚)/GaAs (800 A˚) epilayers, which is the structure for heterostructure metal–semiconductor field‐effect transistors. It is found that ion implantation to a dose higher than 3×1013cm−2at energies between 30 and 90 keV and RTA causes the appearance of dark regions in and near the InGaP layers and the disappearance of the InGaP ordered structure in the lattice images taken with a transmission electron microscope. In the dark regions, the interfaces between the InGaP layer and the GaAs and InGaAs layer become indistinct. An energy‐dispersive x‐ray analysis shows that compositional atoms intermix through these interfaces. The intermixing seems to be enhanced by implantation damage rather than by the interactions of Si impurities. On the other hand, 1×1013cm−2implantation causes no dark regions. ©1995 American Institute of Physics.