We have studied the influence of impurities on the dislocation behavior in heteroepitaxial layers of ZnSe on GaAs, grown by photoassisted organometallic vapor phase epitaxy. In undoped ZnSe layers, the dislocation densities are similar to those obtained by [S. Akram, H. Eshani, and I. B. Bhat, J. Cryst. Growth124, 628 (1992)] whose data show that there is an inverse relationship between layer thickness and dislocation density. Incorporation of the electronically active impurity Cl increases the dislocation densities relative to undoped layers of the same thickness. Also, there is a correlation between the normalized dislocation density and the concentration of incorporated Cl. The isoelectronic impurity Cd has a similar effect on the dislocation density. Incorporation of Cd to a concentration of ∼1020/cm3increases the dislocation density by a factor of about 3 compared to undoped layers of equal thickness. We also observed that doping of both Cd and Cl together resulted in low dislocation densities similar to the undoped case. Based on our results, we believe that the controlling factor for this phenomenon is the absolute value of the tetrahedral misfit for the impurity, rather than its sign, or the electronic activity of the impurity, or the sublattice on which the impurity resides. We propose an ‘‘impurity hardening’’ model to explain these results. According to this model the glide of dislocations is inhibited by the addition of impurities due to the local strain fields around the substitutional sites. The higher dislocation densities observed here with doping are an indirect result of impurity hardening. However, for single crystals or pseudomorphic heteroepitaxial layers, impurity hardening can inhibit the introduction of dislocations. ©1996 American Institute of Physics.