Effects of ionizing radiation on ion-bombarded MNOS

The radiation hardness for charge retention (memory) in MNOS capacitor structures under⁶⁰Co γ-ray irradiation is increased ∼25 percent by an ion-bombardment thermal-anneal (700°C) process. Ohmic transport in MNOS is observed for carriers produced by 5.4-eV photons, and a previously proposed model based upon ohmic transport adequately describes the decay rate for accumulated charge in both bombarded and nonbombarded structures under⁶⁰Co γ-ray irradiation for zero applied bias. The model does not, however, adequately describe the effect of applied bias during ionizing radiation of bombarded structures, possibly because an assumption of planar accumulated charge is invalid. Applying the model to the decay rate obtained with zero bias gives a µr product of 1.5 to 2 × 10^-13cm²/V, and an inferred 10-20-Å mean transport distance for ionization produced charge carriers at low fields (∼10⁶V/cm). This result suggests that ion-bombardment-produced gradients in transport properties are too broad to be resolved in low-field experiments. Furthermore, if hot carrier transport dominates, as previously suggested, then neither the decay rate nor the applied bias effects will be strongly dependent upon trap densities within Si3N4.