Germanium (Ge) homoepitaxy is still a big challenge due to the immature surface cleaning of Ge, which often results in imperfect Ge epitaxial growth in terms of surface roughness, pits and islands. In this work, high crystal quality Ge and GeSi with very smooth surface were epitaxially grown on Ge (0 0 1) substrates, which were thermally cleaned at 650 degrees C in an ultrahigh vacuum chemical vapor deposition system after ozone oxidation. The ozone oxidation of Ge substrates at optimal duration could strongly diminish Ge suboxides which should be responsible for the formation of square concave or islands in Ge epilayers. The complete removal of fully stoichiometric GeO2 on Ge substrates by thermal annealing provided a clean surface for almost perfect Ge homoepitaxy or GeSi heteroepitaxy with a low temperature Ge epilayer. This technology would solve the dilemma of epitaxy on Ge for micro- and opto-electronic device applications.