The authors report the investigation of impurity-induced disordering (IID) in strained In_xGa_1-xAs/GaAs (x=0.21-0.24) single quantum well (QW) heterostructures using a two-stage Zn diffusion and thermal annealing process. The samples were grown by MOVPE, and different shallow Zn diffusion depths were allowed in the GaAs cap layer in order to vary the Zn concentration in the QW. Thermal annealing of the samples at 785 degrees C for 10 min under an AsH₃/H₂ atmosphere then created IID. Both partially disordered and completely disordered QW heterostructures were studied. The In-Ga interdiffusion was monitored by the photoluminescence PL spectroscopy of the ground state emission from the QW. The interdiffusion coefficients were determined by adjusting the data calculated by using the envelope function approximation with the shifts of the PL peak position after the annealing. The interdiffusion coefficient is found to be very strongly dependent on the Zn diffusion depth, and consequently on the Zn concentration in the QW layer. Finally, they propose a model that involves an interstitial migration process in order to explain the enhancement of In-Ga interdiffusion by Zn diffusion.