Proton radiation tolerance of nanocrystal memories

We report on proton radiation tolerance of Si-nanocrystal (Si-NCs) MOS structures aiming at non-volatile memory applications. Si-NCs were formed by low-energy (1 keV) ion-beam-synthesis within a 9 nm thick SiO₂ layer. A 2-D layer of Si-NCs with 3 nm mean diameter and 10¹² cm⁻² surface density was successfully achieved. After fabrication of Al capacitors, samples with and without Si-NCs were 1.5 and 6.5 MeV proton were irradiated at doses ranging from 1 Mrad (SiO₂) to 120 Mrad (SiO₂). Significant irradiation-dose-dependent shifts are detected in the C–V curves of the NC-MOS cells and programmed cells are found to undergo bit flip. Despite the above, the attainable memory windows after write/erase operations remain unchanged. Retention time characteristics at room temperature for the write and erase states of irradiated and non-irradiated samples reveal that even after an irradiation dose as high as 120 Mrad (SiO₂) the devices still exhibit long time charge storage behavior. We observe that the erase state flat-band voltage decay rate does not depend on the irradiation-dose while the opposite happens for the write state flat-band voltage decay rate which is found to be directly dependent on D_it values giving insight to the physics of the discharging mechanisms.