We have studied the temperature dependence of migration of atoms on growth-interrupted and uninterrupted GaAs and AlAs growth front in molecular beam epitaxy. The optimum growth condition for achieving truly-smooth surface on an atomic scale is found to be quite different between GaAs and AlAs. On this basis, we have developed a novel temperature-switched technique by which both top (AlAs-on-GaAs) and bottom (GaAs-on-AlAs) interfaces in GaAsAlAs quantum wells can be prepared truly-smooth.