CuPt-ordering and phase separation were directly investigated in In1-xGaxAsyP1-y with a low arsenic content grown by organometallic vapor phase epitaxy on GaAs substrates. CuPt-ordering and phase separation in samples grown at the substrate temperatures of 630 and 690 degrees C were characterized by transmission electron diffraction and transmission electron microscopy. Although the immiscibility of InGaAsP was enhanced at the lower substrate temperature, the sample grown at 630 degrees C showed less phase separation than the 690 degrees C-grown sample. The degree of CuPt-ordering was significantly enhanced in the sample grown at 630 degrees C. The results demonstrated that the CuPt-ordering originating from surface reconstruction of P(2 x 4) suppressed the phase separation even in the miscibility gap. The detailed characterization of the phase separation clearly revealed a vertical composition modulation (VCM) in InGaAsP for the first time. The mechanism of the VCM formation is discussed based on the modulated-strain field on the surface. (C) 2010 Elsevier B.V. All rights reserved.