That defects have deleterious effects on the performance and reliability of minority carrier devices, such as light emitters, photodetectors, solar cells, etc., is well recognized. Since there is one to one replication of dislocations from substrates into multilayer structures required for various devices, we need substrates with low dislocation densities. An approach to lower dislocation densities by the addition of isoelectronic impurities in III-V crystals will be presented and discussed.Light emitters degrade by the multiplication of dislocations in active layers. This multiplication occurs by non-radiative recombination enhanced glide and climb of dislocations existing in the active layers. We will argue that the multiplication process is retarded in mixed III-V active layers containing atomic species differing in their covalent tetrahedral radii. This retardation occurs because phase separation and atomic ordering coexist in these layers. We will develop arguments for the evolution of these microstructural features and their effects on glide and climb.