In this work, impurity-free interdiffusion of GaAs/AlGaAs quantum wells was investigated as a function of the quality of SiOx capping layers grown by plasma-enhanced chemical vapor deposition. The quality of dielectric layers was varied by changing the nitrous oxide flow rate N (30 sccm less than or equal to N less than or equal to 710 sccm), while maintaining a fixed silane flow rate. The pressure, substrate temperature, and rf power were also kept constant during depositions. Deposited films were characterized by Rutherford backscattering spectroscopy, spectroscopic ellipsometry, and Fourier transform infrared spectroscopy. Following rapid thermal annealing, we have observed an increase in the energy shift of quantum wells with increasing N with a maximum in the range 100 sccm < N < 200 sccm. Any further increase in N resulted in a lowering in blueshift, and reached an almost constant value for N > 350 sccm. The variation of energy shift with N cannot be explained by considering only the oxygen content of the SiOx layer. It is shown that the deposition rate of the capping layer plays an important role in determining the extent of quantum well intermixing. Effects related to the stress imposed by the capping layer on the GaAs/AlGaAs heterostructure and the presence of hydrogen in the capping layer are also discussed.