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A novel method of electrical passivation of a Ge surface by an ultrathin SiO2/GeO2bilayer is proposed as an effective method for fabricating metal--oxide--semiconductor (MOS) structures, which can be processed through the thermal etching of GeO2by vacuum annealing and subsequent SiO2deposition. We demonstrated the feasibility of this passivation technique by performing interface state density ($D_{\text{it}}$) measurements of MOS capacitors, which were fabricated using several surface preparations and subsequent gate insulating film deposition. A $D_{\text{it}}$ of $4\times 10^{11}$ cm-2eV-1was obtained at around midgap. We also investigated the effect of postmetallization annealing after Al deposition (Al-PMA). Al-PMA was found to be very effective for decreasing $D_{\text{it}}$, which was $9.4\times 10^{10}$ cm-2eV-1at around midgap for a capacitor with PMA at 400 °C. The role of Al as a defect terminator was discussed.