Low-energy (300 eV) hydrogen-ion bombardment is used to clean the surfaces of GaAs at 150 degrees C and of Ge at 300 degrees C. The H-ion beam produces contamination-free surfaces without changes in surface composition (stoichiometry) and surface roughness. The wafers were brought into contact at room temperature after cleaning under ultrahigh vacuum and bonded over the whole area without application of external mechanical pressure. High-resolution transmission-electron microscopy imaues reveal that the wafers were directly bonded without damage of the crystal lattice or intermediate layer and the interface is smooth. Current-voltage characterization shows near-ideal forward characteristics and the recombination in p-n heterojunction of the Ge/GaAs space charge region. (c) 2007 American Vacuum Society.