We have studied the correlation length (&Lgr;) of interface roughness in GaAs/AlAs quantum wells (QWs) prepared by molecular beam epitaxy (MBE). It is found that the mobility of two‐dimensional electrons in very thin selectively doped GaAs/AlAs QW structures is substantially enhanced when the bottom AlAs barrier layer is prepared by alternate beam MBE and/or by the use of superlattice buffer layer below the QW. By measuring the electron concentration dependence of mobility and comparing with the theory of interface roughness scattering, we have found that &Lgr; of the bottom (GaAs‐on‐AlAs) interface of the QW gets as large as 200–300 A˚, when prepared by the modified growth technique, which is about three times as large as that (∼70 A˚) by conventional MBE.