Modelling and Simulation of Charging and Discharging Processes in Nanocrystal Flash Memories During Program and Erase Operations

TL;DR

This paper uses time-dependent numerical simulations to analyze charging and discharging in silicon nanocrystal flash memories, revealing electron trapping mechanisms near the silicon valence band edge.

Contribution

It introduces a master equation-based simulation approach combined with Poisson-Schroedinger calculations to study memory dynamics during program and erase operations.

Findings

Electrons are trapped in localized states near the silicon valence band edge.

Simulations match experimental data, validating the proposed trapping mechanism.

Electrons are not stored in the conduction band of nanocrystals.

Abstract

This work is focused on the understanding of charging and discharging processes in silicon nanocrystal flash memories during program and erase operations through time-dependent numerical simulations. Time dependent simulations of the program and erase operations are based on a description of the nanocrystal memory dynamics in terms of a master equation. The related transition rates are computed with a one dimensional Poisson-Schroedinger solver which allows the computation of the tunnelling currents and of generation and recombination rates between the outer reservoir and localized states in the dielectric layer. Comparison between simulations and experiments available in the literature provides useful insights of the storing mechanisms. In particular, simulations allow us to rule out that electrons are stored in confined states in the conduction band of silicon nanocrystals, whereas they suggest that electrons are actually trapped in localized states in the silicon gap at an energy close to the silicon valence band edge, and located at the interface between the nanocrystals and the surrounding silicon oxide.