IN two previous notes1, Prof. Max Born and I have shown that one can obtain a theory of superconductivity by taking account of the fact that the interaction of the electrons with the ionic lattice is appreciable only near the boundaries of Brillouin zones, and particularly strong near the corners of these. This leads to the criterion that the metal should be superconductive if a set of corners of a Brillouin zone is lying very near the Fermi surface, considered as a sphere, which limits the region in the momentum space completely filled with electrons.

On the theory of superconductivity

The recent progress in ferroelectricity and antiferroelectricity in HfO2-based thin films is reported. Most ferroelectric thin film research focuses on perovskite structure materials, such as Pb(Zr,Ti)O3, BaTiO3, and SrBi2Ta2O9, which are considered to be feasible candidate materials for non-volatile semiconductor memory devices. However, these conventional ferroelectrics suffer from various problems including poor Si-compatibility, environmental issues related to Pb, large physical thickness, low resistance to hydrogen, and small bandgap. In 2011, ferroelectricity in Si-doped HfO2 thin films was first reported. Various dopants, such as Si, Zr, Al, Y, Gd, Sr, and La can induce ferro-electricity or antiferroelectricity in thin HfO2 films. They have large remanent polarization of up to 45 μC cm(-2), and their coercive field (≈1-2 MV cm(-1)) is larger than conventional ferroelectric films by approximately one order of magnitude. Furthermore, they can be extremely thin ( 5 eV). These differences are believed to overcome the barriers of conventional ferroelectrics in memory applications, including ferroelectric field-effect-transistors and three-dimensional capacitors. Moreover, the coupling of electric and thermal properties of the antiferroelectric thin films is expected to be useful for various applications, including energy harvesting/storage, solid-state-cooling, and infrared sensors.

Ferroelectricity and Antiferroelectricity of Doped Thin HfO2‐Based Films

The rapid increase in information in the big-data era calls for changes to information-processing paradigms, which, in turn, demand new circuit-building blocks to overcome the decreasing cost-effectiveness of transistor scaling and the intrinsic inefficiency of using transistors in non-von Neumann computing architectures. Accordingly, resistive switching materials (RSMs) based on different physical principles have emerged for memories that could enable energy-efficient and area-efficient in-memory computing. In this Review, we survey the four physical mechanisms that lead to such resistive switching: redox reactions, phase transitions, spin-polarized tunnelling and ferroelectric polarization. We discuss how these mechanisms equip RSMs with desirable properties for representation capability, switching speed and energy, reliability and device density. These properties are the key enablers of processing-in-memory platforms, with applications ranging from neuromorphic computing and general-purpose memcomputing to cybersecurity. Finally, we examine the device requirements for such systems based on RSMs and provide suggestions to address challenges in materials engineering, device optimization, system integration and algorithm design. Resistive switching materials enable novel, in-memory information processing, which may resolve the von Neumann bottleneck. This Review focuses on how the switching mechanisms and the resultant electrical properties lead to various computing applications.

Resistive switching materials for information processing

Dielectric capacitors with ultrahigh power densities are fundamental energy storage components in electrical and electronic systems. However, a long-standing challenge is improving their energy densities. We report dielectrics with ultrahigh energy densities designed with polymorphic nanodomains. Guided by phase-field simulations, we conceived and synthesized lead-free BiFeO3-BaTiO3-SrTiO3 solid-solution films to realize the coexistence of rhombohedral and tetragonal nanodomains embedded in a cubic matrix. We obtained minimized hysteresis while maintaining high polarization and achieved a high energy density of 112 joules per cubic centimeter with a high energy efficiency of ~80%. This approach should be generalizable for designing high-performance dielectrics and other functional materials that benefit from nanoscale domain structure manipulation.

Ultrahigh–energy density lead-free dielectric films via polymorphic nanodomain design

Novel hafnium oxide (HfO2)-based ferroelectrics reveal full scalability and complementary metal oxide semiconductor integratability compared to perovskite-based ferroelectrics that are currently used in nonvolatile ferroelectric random access memories (FeRAMs). Within the lifetime of the device, two main regimes of wake-up and fatigue can be identified. Up to now, the mechanisms behind these two device stages have not been revealed. Thus, the main scope of this study is an identification of the root cause for the increase of the remnant polarization during the wake-up phase and subsequent polarization degradation with further cycling. Combining the comprehensive ferroelectric switching current experiments, Preisach density analysis, and transmission electron microscopy (TEM) study with compact and Technology Computer Aided Design (TCAD) modeling, it has been found out that during the wake-up of the device no new defects are generated but the existing defects redistribute within the device. Furthermore, vacancy diffusion has been identified as the main cause for the phase transformation and consequent increase of the remnant polarization. Utilizing trap density spectroscopy for examining defect evolution with cycling of the device together with modeling of the degradation results in an understanding of the main mechanisms behind the evolution of the ferroelectric response.

Physical Mechanisms behind the Field-Cycling Behavior of HfO2-Based Ferroelectric Capacitors

The effects of annealing temperature (Tanneal) and film thickness (tf) on the crystal structure and ferroelectric properties of Hf0.5Zr0.5O2 films were examined. The Hf0.5Zr0.5O2 films consist of tetragonal, orthorhombic, and monoclinic phases. The orthorhombic phase content, which is responsible for the ferroelectricity in this material, is almost independent of Tanneal, but decreases with increasing tf. In contrast, increasing Tanneal and tf monotonically increases (decreases) the amount of monoclinic (tetragonal) phase, which coincides with the variations in the dielectric constant. The remanant polarization was determined by the content of orthorhombic phase as well as the spatial distribution of other phases.

Evolution of phases and ferroelectric properties of thin Hf0.5Zr0.5O2 films according to the thickness and annealing temperature

The ferroelectricity in fluorite-structure oxides such as hafnia and zirconia has attracted increasing interest since 2011 They have various advantages such as Si-based complementary metal oxide semiconductor-compatibility, matured deposition techniques, a low dielectric constant and the resulting decreased depolarization field, and stronger resistance to hydrogen annealing However, the wake-up effect, imprint, and insufficient endurance are remaining reliability issues Therefore, this paper reviews two major aspects: the advantages of fluorite-structure ferroelectrics for memory applications are reviewed from a material’s point of view, and the critical issues of wake-up effect and insufficient endurance are examined, and potential solutions are subsequently discussed

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/3DB31D1931EA9955AF5E300BC33891D6/S2159685918001751a.pdf/div-class-title-review-and-perspective-on-ferroelectric-hfo-span-class-sub-2-span-based-thin-films-for-memory-applications-div.pdf

Review and perspective on ferroelectric HfO2-based thin films for memory applications

To elucidate the origin of the formation of the ferroelectric phase in Hf0.5Zr0.5O2 films, the effects of film strain and crystallographic orientation on the properties were examined. Using a (111)-textured Pt bottom electrode, Hf0.5Zr0.5O2 films with a (111)-preferred texture inappropriate for transforming their phase from non-ferroelectric tetragonal to ferroelectric orthorhombic phase were deposited. In contrast, randomly oriented Hf0.5Zr0.5O2 films, grown on the TiN electrode, showed feasible ferroelectric properties due to their transformation to the ferroelectric orthorhombic phase. The origin of such transformation is the large in-plane tensile strain for the elongation of the c-axis of the tetragonal phase.

Min Hyuk Park

Papers

Ferroelectricity and Antiferroelectricity of Doped Thin HfO2‐Based Films

Evolution of phases and ferroelectric properties of thin Hf0.5Zr0.5O2 films according to the thickness and annealing temperature

Review and perspective on ferroelectric HfO2-based thin films for memory applications

The effects of crystallographic orientation and strain of thin Hf0.5Zr0.5O2 film on its ferroelectricity

Thin HfxZr1‐xO2 Films: A New Lead‐Free System for Electrostatic Supercapacitors with Large Energy Storage Density and Robust Thermal Stability