Lattice‐matched heterostructures of GaAs/Sc0.3Er0.7As/GaAs have been grown on GaAs substrates with various orientations by molecular beam epitaxy and characterized by transmission electron microscopy. The Sc0.3Er0.7As layer usually has good epitaxy on GaAs; the purpose of this study is to investigate the overgrowth of GaAs on Sc0.3Er0.7As. In comparison with the samples grown on nominal (100) substrates, the epitactic growth of GaAs on Sc0.3Er0.7As is slightly improved on a vicinal substrate oriented (100) 6° off towards (111)A. {311} and {211} are shown to be preferred orientations for the epitactic growth of GaAs/Sc0.3Er0.7As/GaAs heterostructures. The epilayers grown on {311} and {211} substrates are epitactically well aligned, and the density of planar defects (stacking faults and microtwins) in the overgrown GaAs layer is significantly reduced. It is suggested that the mixed {111} and {100} character surfaces assist the nucleation of GaAs on Sc0.3Er0.7As. Stacking faults and microtwins are the major defects for the epilayers grown on (110) substrates. For samples grown on (111)B‐oriented substrates, twinning at the interfaces occurs frequently even when selftwinning inside each material is eliminated. ©1995 American Institute of Physics.