This letter deals with the atomically flat interfaces at GaAs/AlGaAs heterojunctions grown by a modified metalorganic chemical vapor deposition, flow‐rate modulation epitaxy. Single quantum wells show low‐temperature photoluminescence with narrower linewidths than those grown by conventional metalorganic chemical vapor deposition. An x‐ray diffraction spectrum of (GaAs)2(AlAs)2superlattices exhibits no forbidden (003) and (001) diffractions, suggesting that the interfaces are fairly flat.