We describe herein a phase separation of InAlAs on an InP substrate grown at a low temperature in terms of carbon doping. Phase separation of the InAlAs bulk material appeared below the growth temperature of 580degreesC and this was enhanced with decrease in the growth temperature. At 550degreesC, the phase separation drastically increased and was accompanied with surface corrugations. However, when the growth temperature was lower than 550degreesC, the surface corrugation was again suppressed and flatness was recovered. An accumulation of In atoms was observed in the valleys between the columns in corrugations by using a scanning transmission electron microscope with high-angle annular dark field and energy dispersive X-ray analysis. Photo-luminescence revealed an InAlGaAs/InAlGaAS multi-quantum well sample with corrugations showing an abnormal red-shift compared to that found with higher growth temperature. This result can be well explained if the changes in the well thickness are attributed to In accumulation. In conclusion, carbon doping at higher temperatures where phase separation does not occur, is the most suitable for fabricating devices. (C) 2004 Elsevier B.V. All rights reserved.