Proton Radiation Effects on Nanocrystal Non-Volatile Memories

We report on proton radiation effects on Si-nanocrystal (Si-NCs) MOS capacitors and nMOS transistors aiming at non-volatile memory applications. Irradiation experiments were conducted on NC MOS capacitors using protons of 1.5 MeV and 6.5 MeV and on NC nMOS transistors using protons of 1.5 MeV. The range of doses investigated was ~ 1 to ~ 100 Mrad (SiO₂). A 2-D layer of Si NCs with ~ 3 nm mean diameter and 10¹² cm^-2 surface density was successfully achieved by low-energy (1 keV) ion-beam-synthesis in thin SiO₂ layers. After irradiation, programmed capacitors are found to undergo bit flip while programmed transistors are not. Charge retention measurements at room temperature for the write and erase states of irradiated and non-irradiated samples reveal that a significant memory window exists at an extrapolated time of ten years even after an irradiation dose as high as 120 Mrad (SiO₂). The flat-band decay rate of the erase state in NC MOS capacitors does not depend on the irradiation-dose while the opposite occurs for the write state which is found to be directly dependent on D_it values after irradiation. These results clearly indicate that NC non-volatile memories (NVM) are promising radiation tolerant devices.