Mechanism of the phase separation observed in molecular beam epitaxial (MBE) grown InAsSb is investigated. We have reported that the alloys grown at or below similar to400degreesC show phase separation whereas the expected critical temperature (T-c) from the DLP model is 303degreesC. The phase-separated alloys consist of two alternating plates with different alloy compositions lying approximately parallel to the layer surface. The structural studies by transmission electron microscopy (TEM) showed that the alternating plates are similar to50nm thick relatively uniform InAs-rich plates and similar to20nm thick InSb-rich plates containing dense misfit dislocations. We believe that the spontaneous formation of such platelet Structures at such temperatures is not associated with the spinodal decomposition but is induced by the Sb surface segregation. To confirm this model, InAsSb are grown below the critical temperatures with a periodic annealing during the growth to enhance desorption of the excess Sb accumulated on the growth surface due to the Sb surface segregation. With sufficient annealing time, no platelet structures are observed, i.e., homogeneous InAsSb form inside the empirical miscibility gaps. (C) 2002 Elsevier Science B.V. All rights reserved.