A simultaneous reduction of interface roughness and of impurity and trap incorporation in GaAs/AlGaAs quantum wells is observed for a decrease of the molecular beam epitaxy growth rate below its standard value 1 &mgr;m/h, down to 0.1 &mgr;m/h, at a substrate temperature of 620 °C. Thus, layer quality is drastically improved at low, nonstandard growth rates. Incorporation of impurities from the background is observed to induce a transition from two‐ to three‐dimensional growth. The conclusions are based on a detailed study and line shape analysis of quantum well luminescence.