It was found that a thick AlGaAs layer grown on a GaAs–AlGaAs superlattice buffer layer does not affect the optical quality of single quantum well structure successibly grown on the AlGaAs layer; that is, background impurities such as carbon and oxygen, which would come from residual gases, do not accumulate on the AlGaAs surface during molecular beam epitaxy (MBE) growth. Extremely high‐quality GaAs–AlGaAs GRIN‐SCH lasers were obtained by introducing a superlattice buffer layer. The minimum threshold current density of 175 A/cm2was achieved for a broad‐area Fabry–Perot laser with a cavity length of 450 μm at room temperature. This is the lowest threshold current density of semiconductor lasers with a similar cavity length.