Nonvolatile Flash Memory Devices Using CeO₂ Nanocrystal Trapping Layer for Two-Bit per Cell Applications

Abstract

In this study, we demonstrated the characteristics of nonvolatile silicon oxide nitride oxide silicon (SONOS)-type memories using cerium oxide (CeO2) 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 5×1011 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.