Transmission and High Resolution Electron Microscopy have been used to study the dramatic changes in crystalline quality which occur in As-rich In0.52Al0.48As/InP layers when the growth temperature is lowered. We have found that for temperatures as low as 200°C and for a flux ratio of 20, the layers can be of high quality. For the lowest growth temperature of 150°C, pyramidal defects as well as hexagonal As grains are found which are characteristic of the breakdown of the monocrystalline growth in these layers. A mechanism for the formation of these defects is proposed based on their crystallographic structure. The fast photo response previously observed in these layers must be related to the presence of these defects.