Selective epitaxial growth of GaAs on Ge is a prerequisite for the integration of GaAs and Ge in the sub-22 nm CMOS nodes. The problems encountered for epitaxial growth of GaAs on Ge are described and illustrated. Mainly the problem of anti-phase boundary (APB) formation is addressed. Selective epitaxial growth of GaAs on Ge with a SiO(2) mask is discussed and selectively grown layers are characterized by X-ray diffraction, electron microscopy, defect etching and photoluminescence spectroscopy. An optimized growth procedure is presented, which simultaneously reduces loading effects and APB creation. Low temperature photoluminescence measurements show the good quality of the selectively grown GaAs on Ge. (C) 2008 Elsevier B.V. All rights reserved.