Nonvolatile flash memory devices using CeO₂ nanocrystal trapping layer for two-bit per cell applications

In this study, we demonstrated the characteristics of nonvolatile silicon oxide nitride oxide silicon (SONOS)-type memories using cerium oxide (CeO ₂) nanocrystals as a charge storage agent. We observed that the shape of the formed CeO₂ nanocrystals is nearly spherical and that their size is almost similar identical to their high density of 5 × 10 ¹¹ cm^-2. Such CeO₂ nanocrystals were formed by depositing a thin CeO₂ film of ca. 2-3 nm thickness using an evaporater gun system and then rapid thermal annealing (RTA) in O₂ ambient at 900°C for 1 min. The fabricated memory devices show good electrical properties in terms of a sufficiently large memory window (>2 V), program/erase (P/E) speed (0.1/1 ms), retention time up to 10⁴ s with only 5% charge loss, and endurance after 10⁵ cycles with small memory window narrowing and two-bit operation. These properties suggest that the nonvolatile SONOS-type memories with the CeO₂ nanocrystal trapping agent can be applied in future flash memories.