5 mol% Y-doped HfO₂ (Y:HfO₂) and CeO_x capping layers of metal-ferroelectric-metal (MFM) capacitors were deposited by atomic-layer-deposition (ALD) with W bottom and top electrodes by sputtering. Post metallization annealing (PMA) was performed at 450 and 500^oC for the various duration. Figure 1 shows hysteresis loops of the capacitors with and without CeO_x capping annealed at 450^oC for 20 minutes. Large hysteresis on CeO_x capping is observed even same annealing temperature and duration. Enhancement of transformation to ferroelectric phase can be explained as tensile stress to Y:HfO₂ due to the difference of coefficient of thermal expansion (CTE) between CeO_x and Y:HfO₂. Variation of switching polarization (P_SW), the difference between positive and negative P_r, as a function of the number of switching cycles is also investigated and shown in figure 2. Both capacitors are annealed at 500^oC for 100 minutes to crystallize. In the case of without CeO_x capping, continuous wake-up effect and breakdown over 3×10⁷ times switching are observed. On the other hand, the capacitor with CeO_x capping shows a slight wake-up effect and fatigue after 10⁶ times switching, additionally, endurance is improved to 3×10⁹ times switching. The reason why fatigue and prolonging endurance occur is suspected that oxygen ions in the CeO_x layer pin polarization domains and compensate V_O in Y:HfO₂. To suppress fatigue, we propose the recovery process by positive DC voltage application to top electrodes. In figure 3, hysteresis loops of the capacitor with CeO_x capping before and after recovery voltage application. One can observe the larger hysteresis after recovery.

This work was supported by JST-COI (JPMJCE1309). One of the authors, Kazuto Mizutani, was supported by The FUTABA Foundation.

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