In conventional growth models of MOVPE, it was assumed that the group-III elements adsorbed on the surface are directly converted into a crystal layer. However, while investigating the depth profile of the In1-xGaxP film, it was found that an In-rich region exists within similar to10 nm from the top of the film. Conventional growth models cannot explain this phenomenon because the atoms cannot move once they are incorporated from gas phase to a crystal. In order to explain this variation of the In content, we propose a novel crystal growth model considering the surface adsorbed layer, in which the atoms can easily interchange each other. We call this layer a 'subsurface'. The In surface segregation can be explained by assuming that the reaction rate to incorporate atoms from gas phase to the subsurface and the rate to convert the subsurface into crystal layers have a significant difference. We have performed a quantitative analysis on the existence of the subsurface using the flow modulation growth, in which a group-III source was supplied for a certain period followed by a purge period while a group-V source was continuously supplied. By this analysis, we evaluated the rate constants of the generation of a subsurface from gas phase and the conversion from the subsurface to a crystal layer. The steady-state thickness of the subsurface was estimated to be 2.5 nm. (C) 2004 Elsevier B.V. All rights reserved.