High quality Ge epitaxy on a GaAs (100) substrate was grown by metal-organic chemical vapor deposition using germane. The effects of growth temperature and deposition rate on the quality of the Ge epitaxy are investigated. Significant improvement in surface root-mean-square roughness is observed with increasing Ge growth temperature or deposition rate, while keeping all other deposition parameters unchanged. Investigation of the Ge material quality grown after different GaAs surface preparation conditions shows that Ga rich surfaces are beneficial for smooth Ge surfaces, which is attributed to the formation of Ge-Ga bonds at the initiation of the Ge deposition. The good crystalline quality of grown Ge film was further confirmed by high-resolution X-ray diffraction and secondary ion mass spectrometry characterization. (C) 2012 Elsevier B.V. All rights reserved.