The structural quality of GaAs/Si layers deposited directly at the normal growth temperatures (650°C,750°C) is compared with that of layers grown by the standard two-step growth process using Metalorganic Chemical Vapor Deposition (MOCVD). The directly grown layers are highly twinned and polycrystalline in nature whereas the two step growth process produces single crystal layers even at growth temperatures as high as 800°C. The origin of these effects is related to the initial growth of the GaAs, which is in the form of highly defective separate islands, for the case of direct growth, and connected single crystal islands for the case of two-step growth. The directly grown islands exhibit a “ripening” type behavior, with a strong dependence of the Si substrate surface coverage by the GaAs layer, on the growth temperature. The generation of structural defects such as stacking faults and microtwins, which destroy the epitaxial relationship could be triggered by the presence of surface imperfections on the Si substrate.