A six-level ferroelectric storage cell based on a bidirectional imprint field

Abstract

The need for novel memory devices with low energy usage, strong reliability, and multi-level capacity is growing significantly nowadays. Among one of the promising candidates, hafnia (HfO₂)-based ferroelectric devices with a coercive field (E_c) designed for a multi-peak profile are known to provide stable multi-level capabilities in terms of suppressed device-to-device variations. In this study, a novel approach was demonstrated using a fixed charge method to realize a six-level ferroelectric cell. Using the Landau–Khalatnikov model, it is verified that the fixed charge in the ferroelectric device generated a bidirectional imprint field leading to separate E_c peaks. For experimental demonstration, tantalum oxide (TaO) and hafnium zirconium oxide (HZO) were employed as a fixed charge source and ferroelectric layer, respectively, to fabricate a HZO/TaO/HZO/TaO/HZO device. The imprint field created by positively charged oxygen vacancies at TaO/HZO interfaces shifted the switching properties of HZO layers, allowing the device to exhibit three distinct switching behaviors from the HZO layers. Therefore, the overall device showed a triple-peak E_c profile and corresponding six polarization states. Moreover, because of the preferential polarization switching within the shifted HZO layers, polarization states were well maintained over time. The findings of this work may provide a hint toward a scalable path for future memory solutions.