Ge-MOS capacitors were fabricated by a novel method of ultra-thin SiO 2 /GeO 2 bilayer passivation (BLP) for Ge surface combined with the subsequent SiO 2-depositions using magnetron sputtering. For the Ge-MOS capacitors fabricated by BLP with O 2 , to decrease oxygen content in the subsequent SiO 2 deposition is helpful for improving interface quality. By optimizing process parameters of the Ge surface thermal cleaning, the BLP, and the subsequent SiO 2 deposition, interface states density of 4 10 11 cm-2 eV-1 at around midgap 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 SiO 2 deposition, but the interface quality was worse compared with BLP with O 2 .

A novel method of electrical passivation of a Ge surface by an ultrathin SiO 2 /GeO 2 bilayer is proposed as an effective method for fabricating metal-oxide-semiconductor (MOS) structures, which can be processed through the thermal etching of GeO 2 by vacuum annealing and subsequent SiO 2 deposition. We demonstrated the feasibility of this passivation technique by performing interface state density (D it ) measurements of MOS capacitors, which were fabricated using several surface preparations and subsequent gate insulating film deposition. A D it of 4 Â 10 11 cm À2 eV À1 was 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 it , which was 9:4 Â 10 10 cm À2 eV À1 at around midgap for a capacitor with PMA at 400 C. The role of Al as a defect terminator was discussed. #

The postmetallization annealing (PMA) effect was investigated for a TiN-gate Ge metal-oxide-semiconductor capacitor with an ultrathin SiO2/GeO2 bilayer passivation. PMA at 450 °C led to the incorporation of nitrogen atoms into the gate stack. Consequently, the flat band voltage shifted from −0.79 to +0.23 V, resulting from a decrease in the dipole at the SiO2/GeO2 interface and the accompanying creation of a negative charge. The hysteresis decreased from 98 to 27 mV and the interface state density decreased from 6×1011 to 2.5×1011 cm−2 eV−1, as results of the nitrogen termination of defects at the SiO2/GeO2 interface and/or in the GeO2 interlayer.

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