InGaAs/GaAs multiple quantum well structures were grown by molecular beam epitaxy with a variation in deposition temperature among the samples to change crystal and physical properties. High resolution x-ray diffraction and transmission electron microscopy were utilized to probe crystal properties, whereas photoluminescence spectroscopy evaluated optical response. An optimal growth temperature Tdep = 505°C was found for 20% In composition. The density of 60° dislocation loops increased continuously at lower growth temperatures and reduced crystal perfection. Elevated deposition temperatures led to In decay in the structures and manifested in different crystalline defects with a rather isotropic distribution and no lateral ordering, as well as a growth surface instability against perturbations.