Impurity-free interdiffusion of GaAs/AlGaAs quantum wells was investigated for SiOx capping layers grown by plasma-enhanced chemical vapor deposition. Dielectric layers were deposited for various nitrous oxide flow rates, N(30 sccm less than or equal to N less than or equal to 710 sccm), while maintaining a fixed silane flow rate. The oxygen content, x, of the dielectric increased monotonically with N and stoichiometric oxides were deposited above N = 350 sccm. However, the blue shift in quantum wells did not follow a similar trend as the variation of x. Following rapid thermal annealing, the blue shift increased with the increasing N to exhibit a maximum in the range 100 sccm < N < 200 sccm. Any further increase in N resulted in a decrease in blue shift, which reached an almost constant value for N. 350 sccm. It is shown that this maximum in the blue shift is due to the stress imposed by the SiOx layer on the GaAs/AlGaAs heterostructure.