We demonstrate the influence of the arsine partial pressure (p(AsH3)) on the quality of a GaAs layer grown on Ge substrate by metal organic chemical vapor deposition. The GaAs quality improves with p (AsH3) used during the 100 nm thick GaAs buffer layer. By growing a GaAs buffer layer at 630 degrees C with p (AsH3) of 5 mbar, we obtain a smooth GaAs layer with a root mean square roughness of 4.7 angstrom. This GaAs layer does not contain anti phase boundaries. With these optimized growth parameters, we fabricate a virtual GaAs substrate on a 200 mm silicon wafer as a first step towards the integration of III-V devices on silicon. (C) 2015 Elsevier B.V. All rights reserved.