We report a study of the effect of source switching sequences on the interface abruptness of InGaAs/InGaAsP quantum wells (QWs) grown by gas-source molecular beam epitaxy. Four methods were investigated : i.e., at each interface during growth interruption, (1) introducing a residual group-V source evacuation time (RSE) t where all sources are shut off; (2) no RSE; (3) no RSE but adding a group-III In + Ga prelayer of m monolayers; or (4) using RSE plus the In + Ga prelayer. InGaAs/InGaAsP QWs grown by the different methods were characterized by low-temperature photoluminescence (PL). The results show that by optimizing t or m, both RSE and prelayer methods can improve the heterostructure interface abruptness, which is evidenced by the stronger PL intensity and narrower PL linewidth, and that combining the two with optimized t and m gives the best result. This is due to the minimization of the memory effect by using RSE and of the As/P exchange by using the group-III prelayer.