Wigner crystallization of localized electrons in a flash memory

Abstract

The operating principle of modern flash memory is based on the localization of electrons at dielectric traps and a subsequent change in two-dimensional semiconductor channel resistance. Amorphous silicon nitride (Si3N4) serves as a charge storage medium in a flash memory. In previous studies, Wigner crystallization driven by Coulomb interactions among free electrons was extensively investigated. For localized electrons, Coulomb repulsion limits the maximum charge density reachable in the dielectric. This study focuses on investigating the structural ordering, specifically Wigner crystallization, of localized electrons at deep traps (≈1.5 eV) in Si3N4 at room temperature. Coulomb repulsion of electrons localized at traps was experimentally observed. Experimentally, it is proven that the concentration of localized electrons is two orders of magnitude lower than that of neutral traps, indicating Coulomb repulsion between localized electrons. In this work, we investigate the structural ordering in a one-dimensional cluster of localized electrons using numerical simulations. The correlation functions of localized electrons demonstrate their Wigner crystallization. The study of Wigner ordering of localized electrons reveals that the memory properties of flash devices are governed by Coulomb interactions among these electrons rather than by the concentration of neutral traps.