The molecular-beam epitaxial growth processes of GaAs on Si(001) were investigated using in situ synchrotron X-ray diffraction. Three-dimensional X-ray intensity distributions around the Si and GaAs 022 Bragg points in reciprocal space were measured during growth using combination of an area detector and one-axis scans. During the initial stage of growth, the average radius of GaAs islands was found to follow a second power law function of the growth time, in accordance with the growth being limited by the binding of Ga with As at step edges. With increasing GaAs thickness, streaky scattering extending from the GaAs 022 peak in the < 111 > directions was observed, indicating the development of plane defects. (c) 2013 Elsevier B.V. All rights reserved.