The growth of high quality GaAs layers on single crystal Ge substrates has attracted a great deal of interest in recent years. Currently under development at NREL is the GaAs/GalnP-on-Ge high efficiency multi-junction solar cell. Unlike other heteroepitaxial systems such as GaAs-on-Si, physical properties such as the lattice parameter and thermal expansion coefficient of the Ge and GaAs are much closer. Further, Ge has a bandgap of 0.7 eV that makes it a suitable bottom cell in a multi-junction stack. However, the growth of GaAs on Ge is not a straightforward process, and little is known about the Ge surface and the nucleation of GaAs on Ge in the MOCVD environment. In this study, TEM results show that a number microstructural defects are associated with the Ge/GaAs interface and the initial stages of growth.