GaAs/Ge heterostructures were grown under different growth conditions by low-pressure metal organic vapor phase epitaxy (LP-MOVPE) and investigated by transmission electron microscopy (TEM). Abrupt heterointerface and antiphase domain (APD)-free single domain GaAs epilayers on Ge substrates were achieved under specific growth conditions. The lattice indexing of high-resolution transmission electron microscopy (HRTEM) exhibited excellent lattice line matching between the GaAs epilayer and the Ge substrate. These results led us to conclude that the optimal growth parameters for achieving high-quality GaAs/Ge heterostructure are As/Ga ratio of similar to 88:1, growth rate of similar to 3 mu m/h, and growth temperature of 675 degrees C,