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HfO2-based dielectrics are promising for nanoscale ferroelectric applications, and the most favorable material within the family is Zr-substituted hafnia, i.e., Hf1−xZrxO2 (HZO). The extent of Zr substitution can be great, and x is commonly set to 0.5. However, the bandgap of ZrO2 is lower than HfO2, thus it is uncertain how the Zr content should influence the electronic band structure of HZO. A reduced bandgap is detrimental to the cycling endurance as charge injection and dielectric breakdown would become easier. Another issue is regarding the comparison on the bandgaps between HfO2/ZrO2 superlattices and HZO solid-state solutions. In this work, we systematically investigated the electronic structures of HfO2, ZrO2, and HZO using self-energy corrected density functional theory. In particular, the conduction band minimum of Pca21-HfO2 is found to lie at an ordinary k-point on the Brillouin zone border, not related to any interlines between high-symmetry k-points. Moreover, the rule of HZO bandgap variation with respect to x has been extracted. The physical mechanisms for the exponential reduction regime and linear decay regime have been revealed. The bandgaps of HfO2/ZrO2 ferroelectric superlattices are investigated in a systematic manner, and the reason why the superlattice could possess a bandgap lower than that of ZrO2 is revealed through comprehensive analysis. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT)-1/2 is an efficient bandgap rectification method for DFT under local density approximation (LDA) or generalized gradient approximation. It was suggested that non-self-consistent DFT-1/2 should be used for highly ionic insulators like LiF, whereas self-consistent DFT-1/2 should still be used for other compounds. Nevertheless, there is no quantitative criterion prescribed for which implementation should work for an arbitrary insulator, which leads to severe ambiguity in this method. In this work, we analyze the impact of self-consistency in DFT-1/2 and shell DFT-1/2 calculations in insulators or semiconductors with ionic bonds, covalent bonds, and intermediate cases and show that self-consistency is required even for highly ionic insulators for globally better electronic structure details. The self-energy correction renders electrons more localized around the anions in self-consistent LDA-1/2. The well-known delocalization error of LDA is rectified, but with strong overcorrection, due to the presence of additional self-energy potential. However, in non-self-consistent LDA-1/2 calculations, the electron wave functions indicate that such localization is much more severe and beyond a reasonable range because the strong Coulomb repulsion is not counted in the Hamiltonian. Another common drawback of non-self-consistent LDA-1/2 is that the ionicity of the bonding gets substantially enhanced, and the bandgap can be enormously high in mixed ionic-covalent compounds like TiO2. [ABSTRACT FROM AUTHOR]

Conductive bridge random access memories (CBRAMs) usually involve active Ag or Cu metals, where the formation of metal filaments accounts for the low resistance state. For the application of neuromorphic computation, it is highly desirable to develop artificial neurons and synapses, which utilize the complicated volatile or nonvolatile resistive switching phenomena, respectively. This can be achieved by controlling the morphology and stability of the filaments, which requires a deep understanding of the filament formation and disruption mechanisms. Using ab initio calculations, we explored the physical mechanism behind various Ag filament morphologies and growth modes, using GeSe, ZrO2, SiO2, and a-Si as the examples. The roles of Ag and Ag+ stability inside the dielectric, the migration barrier of Ag+, and the Ag+ solvation effect have been investigated in detail. A comprehensive model has been proposed, which in particular could explain the diverse Ag filament morphology experimentally observed in sputtered SiO2 and PECVD SiO2. Our theoretical approach can serve as a pre-screening method in designing new solid-state electrolyte materials of CBRAM, aiming at new functionalities in neuromorphic computation or in-memory logic computing. [ABSTRACT FROM AUTHOR]

Ultra-thin ferroelectric hafnia-based thin films are very promising candidates for nanoscale ferroelectric random access memories. However, dielectric breakdown is a main failure mechanism during repeated polarization switching. Generalizing Lou et al.'s local phase decomposition model, originally for ferroelectric fatigue, we propose a dielectric breakdown model for ferroelectric hafnia. While charging injection during the polarization reversal is regarded as a key step, eventual phase separation of the Hf cluster accounts for the dielectric breakdown. Using this model, we explain why TaN/HfO2/TaN ferroelectric capacitors are more prone to dielectric breakdown than TiN/HfO2/TiN, and conclude that the lower Schottky barrier for the TaN/Pca21-HfO2 interface stabilizes neutral oxygen vacancies within the dielectric. On the other hand, when TiN electrodes are employed, oxygen vacancies tend to be positively charged. They can further pin the domain walls, resulting in ferroelectric fatigue. The relationship between the conductive filament formation, dielectric breakdown, wake up, and fatigue in ferroelectric HfO2 is discussed within the framework of our model. [ABSTRACT FROM AUTHOR]

The stability and aggregation mechanisms of oxygen vacancy chains are studied for hafnia using self-energy corrected density functional theory. While oxygen vacancies tend not to align along the c-axis of monoclinic HfO2, oxygen vacancy chains along a-axis and b-axis are energetically favorable, with cohesive energies of 0.05 eV and 0.03 eV per vacancy, respectively. Nevertheless, with an increase of the cross section area, intensive oxygen vacancy chains become much more stable in hafnia, which yields phase separation into Hf-clusters and HfO2. Compared with disperse single vacancy chains, intensive oxygen vacancy chains made of 4, 6, and 8 single vacancy chains are energetically more favorable by 0.17, 0.20, and 0.30 eV per oxygen vacancy, respectively. On the other hand, while a single oxygen vacancy chain exhibits a tiny electronic energy gap of around 0.5 eV, metallic conduction emerges for the intensive vacancy chain made of 8 single vacancy chains, which possesses a filament cross section area of ∼0.4 nm2. This sets a lower area limit for Hf-cluster filaments from metallic conduction point of view, but in real hafnia resistive RAM devices the cross section area of the filaments can generally be much larger (>5 nm2) for the sake of energy minimization. Our work sets up a bridge between oxygen vacancy ordering and phase separation in hafnia, and shows a clear trend of filament stabilization with larger dimensions. The results could explain the threshold switching phenomenon in hafnia when a small AFM tip was used as the top electrode, as well as the undesired multimode operation in resistive RAM cells with 3 nm-thick hafnia. [ABSTRACT FROM AUTHOR]

While the stability of tellurene is generally unsatisfactory among 2D materials, we show that elements from group-VIA may constitute stable 2D materials through the design principle of ionic bonding. In particular, a 2D wide-bandgap semiconductor, TeO3 monolayer with the CrI3-type crystal structure, has been predicted. High kinetic, thermodynamic, and mechanical stabilities have been confirmed by means of first-principles calculations. Remarkably, due to the van Hove singularities in the valence band electronic structures, TeO3 monolayer possesses tunable magnetism and half-metallicity with hole doping or elemental substitution. The corresponding Curie temperatures is above the room temperature according to the results of mean field approximation calculations. Furthermore, the carrier mobilities, exfoliation features, origin of high stability, and possible applications are discussed in detail. The TeO3 monolayer can serve as promising candidates for future nanoelectronics and spintronics applications. SeO3 and SO3 in the same 2D structure are also investigated. [ABSTRACT FROM AUTHOR]

HfOx memristor is one of the most promising candidates for nonvolatile memory and neuromorphic computing applications, but for the latter, its gradual conduction modulation and switching linearity still need improvement. In this work, uniform and multilevel resistance switching (RS) of HfOx memristors have been achieved through Mg doping, which is a distinct dopant other than previously used elements like Al and Gd. Moreover, the Mg-doped HfOx memristor exhibits excellent continuous conductance modulation characteristics under pulse voltage. First-principles calculations and conductive mechanism analysis show that Mg can migrate into the conductive filaments (CFs) by adjusting oxygen vacancy migration and defect states in the CFs, resulting in the improved RS performance of HfOx memristors. The reason why Mg doping rather than Ca doping could yield improved RS performances is clarified. Our work shows the unique functionality of Mg dopants in HfOx and provides a feasible way to improve RS performance of transition metal oxide memristors and its guiding principles. [ABSTRACT FROM AUTHOR]

The fast development of high-accuracy neuromorphic computing requires stable analog memristors. While filamentary memory switching is very common in binary oxides, their resistive switching usually involves abrupt changes due to the rupture or reformation of metallic filaments. In this work, we designed a memristor consisting of dual-layer HfOy/HfOx, with different concentrations of oxygen vacancies (y > x). During the electroforming process, both the migration of existing oxygen vacancies in HfOx and the generation of new oxygen vacancies in HfOy occur simultaneously, leaving a semiconducting part close to the HfOy/HfOx interface. The resulting filament is not metallic as a whole, as revealed by first principles calculations. Such a device demonstrates excellent switching uniformity as well as highly gradual resistance change, ideal for neuromorphic computing. Through fine tuning of the filament structure, the device achieves low variation, high speed, gradual SET and RESET processes, and hundreds of stable multi-level state behaviors. The long-term synaptic plasticity was further achieved, showing good linearity and large analog switching window (ΔG as high as 487.5 μS). This works affords a route toward a gradual resistance change in oxide-based memristors. [ABSTRACT FROM AUTHOR]

In this work, we enhance the DC/AC resistive switching performance in AlOx memristors by using a two-technique bilayer approach. Compared to the single-layer memristors (W/AlOx or Al2O3/Pt), the dual-layer memristor (W/AlOx/AlOy/Pt) shows high uniformity in DC cycling (σ/μ < 0.12), large memory window in AC cycling (>100), fast switching speed (20 ns), high endurance (107 cycles), and high-temperature stability (104 s at 125 °C). These performance enhancements are attributed to the localization of the conductive region after using a dual layer with different defect concentrations. Moreover, the W/AlOx/AlOy/Pt memristor exhibits stable III-bit multilevel storage capability by varying the amplitude of negative pulses. Our results provide an effective strategy to develop high-performance memristors for future memory and computing applications. [ABSTRACT FROM AUTHOR]

We systematically investigated the modulation effects of substrates on the ferromagnetism properties of Fe3GeTe2 experimentally and theoretically. The polar refractive magnetic circular dichroism results demonstrate that the ferromagnetism could be modulated significantly by selecting different substrates. We explain this phenomenon using the first-principles calculation, revealing that it was induced by the lattice distortion and charge redistribution between the interfaces. Our work is useful to understand the fundamental mechanism of tunable ferromagnetism and paves the way for exploring interface physics and practical spintronic applications of two-dimensional ferromagnetic van der Waals crystals. [ABSTRACT FROM AUTHOR]

The exact composition and structure of conductive filaments in hafnia-based memristors are still not fully understood, but recent theoretical investigations reveal that hexagonal HfOx phases close to the h.c.p. Hf structure are probable filament candidates. In this work we list h.c.p. Hf, Hf6O, Hf3O and Hf2O as possible phases for the filament in hafnia memristors. Their differences in lattice parameters, electronic structures and O charge states are studied in details. Migration of O ions for both in-plane and out-of-plane directions in these phases is investigated using first-principles calculations. Both single-phase supercells and filament-in-dielectric models are used for migration barrier calculations, while the latter is proven to be more accurate for the c-direction. The migration of O ions is fastest in metal Hf, while slowest in Hf2O. The existence of O interstitials in Hf tends to hinder the transport of O. [ABSTRACT FROM AUTHOR]

The crystal orientations and electrical properties of Bi4Ti3O12 (BIT) and Bi3.25La0.75Ti3O12 (BLT) ferroelectric thin films were studied and compared. Stoichiometric BIT and BLT samples were deposited on Pt(111) substrates and crystallized at 750 °C to get mixed orientations. The BIT sample exhibited a/b axes orientation with (117) component, while the BLT sample was more c-axis oriented. The 2Pr values of such BIT and BLT were 34.3 μC/cm2 and 25.7 μC/cm2, respectively. Nevertheless, BLT has much better leakage current and polarization saturation properties. In order to obtain c-axis oriented BIT thin films, excess bismuth was used. BIT with 15% excess bismuth processed at 750 °C was purely c-axis oriented and only possessed a 2Pr value of 4.9 μC/cm2, while that of 15% excess bismuth BLT sample was 18.3 μC/cm2. This can be attributed to the large polarization anisotropy in BIT, and this anisotropy is reduced by lanthanum doping. [ABSTRACT FROM AUTHOR]