Ge-MOS capacitors were fabricated by a novel method of ultra-thin SiO2/GeO2 bi-layer passivation (BLP) for Ge surface combined with the subsequent SiO2-depositions using magnetron sputtering. For the Ge-MOS capacitors fabricated by BLP with O-2, to decrease oxygen content in the subsequent SiO2 deposition is helpful for improving interface quality. By optimizing process parameters of the Ge surface thermal cleaning, the BLP, and the subsequent SiO2 deposition, interface states density of 4 x 10(11) cm(-2) eV(-1) at around mid-gap was achieved, which is approximately three times smaller than that of non-passavited Ge-MOS capacitors. On the contrary, for the Ge-MOS capacitors fabricated by BLP without O-2, interface quality could be improved by an increase in oxygen contents during the subsequent SiO2 deposition, but the interface quality was worse compared with BLP with O-2. (C) 2011 Elsevier Ltd. All rights reserved.