Charge centroid in MIOS nonvolatile memory structures

Abstract

The experimental charge-centroid relations in MNOS, MAOS (A = Al 2 O 3 ), and MANOS nonvolatile memory structures were analyzed and compared. New data on trap density, trapping length, and capture cross section for Al 2 O 3 and Si 3 N 4 were derived from these measurements. The trapping length in Al 2 O 3 is about 5 times larger than in Si 3 N 4 . The charge-detrapping rate in Al 2 O 3 is smaller than in Si 3 N 4 . The experimental charge-centroid relation during charge trapping and charge detrapping is described by two expressions which include the capture cross section as the only physical parameter. The calculated capture cross section is constant for MNOS and also during charge trapping in MAOS, but decreases linearly with propagating centroid position after onset of charge detrapping within Al 2 O 3 . Trap-assisted tunneling over multiple deep trap levels in Al 2 O 3 is proposed to interpret the results for MAOS. Oxygen annealing of Al 2 O 3 enhances storage capability of injected negative charge within the lower bandgap Si 3 N 4 region of the MANOS structure. This behavior is related to blocking of detrapped nitride charge at the oxygen densified energy barrier created at the Si 3 N 4 - Al 2 O 3 boundary.