We describe efforts to epitaxially integrate GaAs with Si, using thin, relaxed Ge layers. The Ge films are deposited by molecular beam epitaxy using a self-assembled, selective-area growth technique, where atomic Ge etches an SiO2 mask layer and then grows from pores extending to the Si substrate. The resulting Ge film coalesces over the SiO2 mask and is planarized, using H2O2-based chemical-mechanical polishing. We subsequently deposit a GaAs/AlAs heterostructure on the polished Ge on Si substrate by metal-organic vapor phase epitaxy. While the initial Ge films were completely relaxed and dislocation-free, they contain a high density of stacking faults that propagate through the GaAs/AlAs heterostructure. These stacking faults create phase domains that appear as non-radiative recombination centers in cathodoluminescence images. Further development of two-step Ge epitaxy with an anneal near the Ge melting point eliminates stacking faults in the Ge, but decomposes the SiO2 mask allowing threading dislocations to form and propagate through the GaAs/AlAs heterostructure. We discuss our strategy to prevent the loss of the SiO2 mask and thus reduce threading dislocations. (C) 2009 Elsevier B.V. All rights reserved.