Unveiling the Origin of Robust Ferroelectricity in Sub-2 nm Hafnium Zirconium Oxide Films

Recent progress on defect-engineering in ferroelectric HfO2: The next step forward via multiscale structural optimization

The discovery of unconventional scale-free ferroelectricity in HfO2-based fluorite thin films has attracted great attention in recent years for their promising applications in low-power logic and nonvolatile memories. The ferroelectricity of HfO2 is intrinsically originated from the widely accepted ferroelectric metastable orthorhombic Pca21 phase. In the last decade, defect-doping/solid solution has shown excellent prospects in enhancing and stabilizing the ferroelectricity via isovalent or aliovalent defect-engineering. Here, the recent advances in defect-engineered HfO2-based ferroelectrics are first reviewed, including progress in mono-ionic doping and mixed ion-doping. Then, the defect-lattice correlation, the point-defect promoted phase transition kinetics, and the interface-engineered dynamic behaviour of oxygen vacancy are summarized. In addition, thin film preparation and ion bombardment doping are summarized. Finally, the outlook and challenges are discussed. A multiscale structural optimization approach is suggested for further property optimization. This article not only covers an overview of the state-of-art advances of defects in fluorite ferroelectrics, but also future prospects that may inspire their further property-optimization via defect-engineering.

Revival of Ferroelectric Memories Based on Emerging Fluorite-Structured Ferroelectrics

Over the last few decades, the research on ferroelectric memories has been limited due to their dimensional scalability and incompatibility with complementary metal-oxide-semiconductor (CMOS) technology. The discovery of ferroelectricity in fluorite-structured oxides revived interest in the research on ferroelectric memories, by inducing nanoscale nonvolatility in state-of-the-art gate insulators by minute doping and thermal treatment. The potential of this approach has been demonstrated by the fabrication of sub-30 nm electronic devices. Nonetheless, to realize practical applications, various technical limitations, such as insufficient reliability including endurance, retention, and imprint, as well as large device-to-device-variation, require urgent solutions. Furthermore, such limitations should be considered based on targeting devices as well as applications. Various types of ferroelectric memories including ferroelectric random-access-memory, ferroelectric field-effect-transistor, and ferroelectric tunnel junction should be considered for classical nonvolatile memories as well as emerging neuromorphic computing and processing-in-memory. Therefore, from the viewpoint of materials science, this review covers the recent research focusing on ferroelectric memories from the history of conventional approaches to future prospects.

From Ferroelectric Material Optimization to Neuromorphic Devices

Due to the voltage driven switching at low voltages combined with nonvolatility of the achieved polarization state, ferroelectric materials have a unique potential for low power nonvolatile electronic devices. The competitivity of such devices is hindered by compatibility issues of well-known ferroelectrics with established semiconductor technology. The discovery of ferroelectricity in hafnium oxide changed this situation. The natural application of nonvolatile devices is as a memory cell. Nonvolatile memory devices also built the basis for other applications like in-memory or neuromorphic computing. Three different basic ferroelectric devices can be constructed: ferroelectric capacitors, ferroelectric field effect transistors and ferroelectric tunneling junctions. In this article first the material science of the ferroelectricity in hafnium oxide will be summarized with a special focus on tailoring the switching characteristics towards different applications.The current status of nonvolatile ferroelectric memories then lays the ground for looking into applications like in-memory computing. Finally, a special focus will be given to showcase how the basic building blocks of spiking neural networks, the neuron and the synapse, can be realized and how they can be combined to realize neuromorphic computing systems. A summary, comparison with other technologies like resistive switching devices and an outlook completes the paper.

A perspective on the physical scaling down of hafnia-based ferroelectrics

HfO₂-based ferroelectric thin films have attracted significant interest for semiconductor device applications due to their compatibility with complementary metal oxide semiconductor (CMOS) technology. One of the benefits of HfO₂-based ferroelectric thin films is their ability to be scaled to thicknesses as low as 10 nm while retaining their ferroelectric properties; a feat that has been difficult to accomplish with conventional perovskite-based ferroelectrics using CMOS-compatible processes. However, reducing the thickness limit of HfO₂-based ferroelectric thin films below the sub 5 nm thickness regime while preserving their ferroelectric property remains a formidable challenge. This is because both the structural factors of HfO₂, including polymorphism and orientation, and the electrical factors of HfO₂-based devices, such as the depolarization field, are known to be highly dependent on the HfO₂thickness. Accordingly, when the thickness of HfO₂drops below 5 nm, these factors will become even more crucial. In this regard, the size effect of HfO₂-based ferroelectric thin films is thoroughly discussed in the present review. The impact of thickness on the ferroelectric property of HfO₂-based thin films and the electrical performance of HfO₂-based ferroelectric semiconductor devices, such as ferroelectric random-access-memory, ferroelectric field-effect-transistor, and ferroelectric tunnel junction, is extensively discussed from the perspective of fundamental theory and experimental results. Finally, recent developments and reports on achieving ferroelectric HfO₂at sub-5 nm thickness regime and their applications are discussed.

Low Operating Voltage, Improved Breakdown Tolerance, and High Endurance in Hf0.5Zr0.5O2 Ferroelectric Capacitors Achieved by Thickness Scaling Down to 4 nm for Embedded Ferroelectric Memory

The comparatively high coercive field in Hf_0.5Zr_0.5O₂ (HZO) and other HfO₂-based ferroelectric thin films leads to two critical challenges for their application in embedded ferroelectric memory: high operating voltage due to a large thickness-field product and poor endurance due to the high operating field close to the breakdown field. In this study, we demonstrate that the thickness scaling of ferroelectric HZO down to 4 nm is a promising approach to overcome these challenges. As the coercive voltage scales down almost linearly with the film thickness, the operating voltage of 4 nm HZO is reduced to 0.6 V for one-shot operation and 1.2 V for stable memory operation, which is in the voltage range compatible with scaled silicon technologies. Furthermore, it is found that the breakdown field is substantially improved in thinner HZO since the breakdown mechanism is dominated by the stress voltage, not the stress field, resulting in improved cycle-to-breakdown by more than 4 orders of magnitude when thinning from 9.5 to 4 nm. We identify two concerns accompanying thickness scaling: the increase in crystallization temperature and the pinched hysteresis behavior, which can be addressed by carefully preparing temperature-thickness mapping and applying strong-field wake-up cycling, respectively. Our optimal 4 nm-thick HZO ferroelectric capacitor exhibits an operating voltage of 1.2 V with over 10 year data retention and 10¹² endurance cycles at 100 kHz, which can be further improved to more than 10¹⁴ with a smaller capacitor size and higher operating frequency.

Emergent ferroelectricity in subnanometer binary oxide films on silicon

The critical size limit of voltage-switchable electric dipoles has extensive implications for energy-efficient electronics, underlying the importance of ferroelectric order stabilized at reduced dimensionality. We report on the thickness-dependent antiferroelectric-to-ferroelectric phase transition in zirconium dioxide (ZrO2) thin films on silicon. The emergent ferroelectricity and hysteretic polarization switching in ultrathin ZrO2, conventionally a paraelectric material, notably persists down to a film thickness of 5 angstroms, the fluorite-structure unit-cell size. This approach to exploit three-dimensional centrosymmetric materials deposited down to the two-dimensional thickness limit, particularly within this model fluorite-structure system that possesses unconventional ferroelectric size effects, offers substantial promise for electronics, demonstrated by proof-of-principle atomic-scale nonvolatile ferroelectric memory on silicon. Additionally, it is also indicative of hidden electronic phenomena that are achievable across a wide class of simple binary materials.