Impurity-free intermixing of GaAs/Al0.54Ga0.46As quantum wells was carried out using SiOx capping layers grown by plasma enhanced chemical vapor deposition followed by rapid thermal annealing at 950 degrees C. A dependence of quantum well intermixing on the quality of dielectric layer was observed, which was varied by changing the silane flow rate, M (40 sccm less than or equal to M less than or equal to 480 sccm). The quality of capping layers were determined using Rutherford backscattering spectroscopy, spectroscopic ellipsometry, secondary ion mass spectrometry, Fourier transform infrared spectroscopy, and P-etch measurements. The porosity and oxygen content of capping layers, and the incorporation of nitrogen during deposition were the main factors influencing the extent of blue shifts achieved in intermixed quantum wells. Substantially higher energy shifts were obtained for capping layers deposited using M < 160 sccm. Slightly overstoichiometric layers which were relatively more porous but containing less nitrogen were obtained for M < 160 sccm. For higher silane flow rates, substoichiometric oxide layers with increased hydrogen and nitrogen contents were obtained. Our results suggested that the mobility of out-diffusing Ga atoms during annealing was enhanced by film porosity but was suppressed with increasing nitrogen content in the capping layer.