Ferroelectric random-access memories (FeRAM) switching is achieved by ferroelectric switching of dipoles. FeRAMs offer low-energy and faster switching as compared to conventional memory circuitry. They excel in terms of power consumption and low voltage operation, when stacked against current-driven contenders. Unfortunately, FeRAMs have been restricted to niche markets due to their limited CMOS-compatibility and severe scaling issues of the complex ferroelectric perovskite systems. However, the discovery of ferroelectricity in binary oxides gave an impetus for development of universal memory concept, which may lead to a significant breakthrough in the development of memory devices. Binary oxides generally do not suffer from a “dead layer effect”, which makes non-binary oxides, such as perovskites, ineffective for thin film technology. Moreover, high coercive fields inside binary oxides give them a considerable resilience toward internal depolarization of the ferroelectricity, crucial to achieve scalability as well as overcome the widespread reliability disadvantages of FE material. The underlying reasons for the stable ferroelectricity and distinct switching of FE domains inside binary oxides at an atomic level are poorly understood. Moreover, non-idealities seen upon continuous electronic switching cycles like wake-up and fatigue introduce uncertainties in device performance and endurance. In this work, we present the first proof of the underlying reasons for these non-idealities with cycle-to-cycle tracking of morphology changes in few-nm thick binary oxide ferroelectric ultra-thin films.

This work presents the first proof of the underlying reasons for these non-idealities with cycle-to-cycle tracking of morphology changes in few-nm thick binary oxide ferroelectric ultra-thin films. With our Nanoparticle-on-Mirror (NPoM) geometry, we capture for the first time both migration of <1% oxygen ions and material phase change in just 5nm-thick binary oxide ferroelectric films when under continuous electronic switching, and therefore track in real-time and in-operando the nanoscale kinetics of wake-up and fatigue in ferroelectric ultrathin memories. We use in-situ electrical and optical characterizations like darkfield scattering, photoluminescence and Raman spectroscopy to understand the nano-kinetics of the atomic level switching. The tracking of vacancy migration and phase change with the above-mentioned techniques combined with density functional theory (DFT) and finite-difference time-domain (FDTD) simulations provide the first insights into the morphological changes in ultra-thin binary oxide films.

Figure caption: Simultaneous electrical and optical capture of ferroelectric memory's polarization evolution reveals pre-wake-up, wake-up, and fatigue stages. The distribution of vacancies in the film (represented by red circles at top) is captured optically as dark-field scattering data. The migration of vacancies is represented by the change in intensities and location of the optical modes.