We have investigated the effects of the stoichiometry of SiOx and SiNx capping layers on the band gap energy shift induced by impurity-free vacancy disordering of the In0.2Ga0.8As/GaAs multiple quantum well structures. The stoichiometry of the SiOx and SiNx capping layers was changed by varying the flow rate of silane (SiH4) gas, and argon gas was employed as the carrier gas of the diluted SiH4 gas to eliminate any possible incorporation of nitrogen into the deposited film when nitrogen gas is employed as the carrier gas. A blueshift of photoluminescence peak of up to 112 meV is observed after rapid thermal annealing at 950 °C for 50 s from the sample capped with SiOx (provided with a SiH4 flow rate of 20 sccm). It is observed that the magnitude of the blueshift increases with the decrease of SiH4 flow rate for the SiOx and SiNx capping layer because of the increased porosity of dielectric capping layers. The insertion of intermediate GaAs cap layer reduces the band gap energy shift irrespective of the SiOx or SiNx capping layer. © 2002 American Institute of Physics.