Abstract
The resistive switching effect in memristors typically stems from the formation and rupture of localized conductive filament paths, and HfO
2
has been accepted as one of the most promising ...resistive switching materials. However, the dynamic changes in the resistive switching process, including the composition and structure of conductive filaments, and especially the evolution of conductive filament surroundings, remain controversial in HfO
2
-based memristors. Here, the conductive filament system in the amorphous HfO
2
-based memristors with various top electrodes is revealed to be with a quasi-core-shell structure consisting of metallic hexagonal-Hf
6
O and its crystalline surroundings (monoclinic or tetragonal HfO
x
). The phase of the HfO
x
shell varies with the oxygen reservation capability of the top electrode. According to extensive high-resolution transmission electron microscopy observations and ab initio calculations, the phase transition of the conductive filament shell between monoclinic and tetragonal HfO
2
is proposed to depend on the comprehensive effects of Joule heat from the conductive filament current and the concentration of oxygen vacancies. The quasi-core-shell conductive filament system with an intrinsic barrier, which prohibits conductive filament oxidation, ensures the extreme scalability of resistive switching memristors. This study renovates the understanding of the conductive filament evolution in HfO
2
-based memristors and provides potential inspirations to improve oxide memristors for nonvolatile storage-class memory applications.
Two-dimensional (2D) materials with both ferroelasticity and negative Poisson's ratios have attracted intensive interest, but it is very rare to have both ferroelasticity and negative Poisson's ...ratios in a single material. Directional positive and negative Poisson's ratios in a switchable ferroelastic dielectric may enable non-destructive readout in ferroelastic data storage. Herein, we propose 14 kinds of stable 2D semiconductors: AB monolayers (A = Sc, Y, La; B = N, P, As, Sb, Bi) based on first-principles calculations. The band gaps of AB monolayers cover a wide range from 0.69 eV to 2.15 eV. Mechanical analysis reveals that these materials are flexible and 12 of 14 are predicted to possess an in-plane negative Poisson's ratio (NPR). Moreover, 10 of these 14 systems possess an out-of-plane NPR. More encouragingly, all AB monolayers are identified as 2D ferroelastic materials with reversible strains of around 5.94% to 20.30%. The ferroelastic switching barriers, mechanical properties and electronic structures of these materials are discussed in detail. Such outstanding properties make the AB monolayers very promising as switchable anisotropic 2D materials for nanoelectronics and micromechanical applications.
Two-dimensional (2D) materials with both ferroelasticity and negative Poisson's ratios have attracted intensive interest, but it is very rare to have both ferroelasticity and negative Poisson's ratios in a single material.
Hafnia-based ferroelectrics have greatly revived the field of ferroelectric memory (FeRAM), but certain reliability issues must be satisfactorily resolved before they can be widely applied in ...commercial memories. In particular, the imprint phenomenon severely jeopardizes the read-out reliability in hafnia-based ferroelectric capacitors, but its origin remains unclear, which hinders the development of its recovery schemes. In this work, we have systematically investigated the imprint mechanism in TiN/Hf
0.5
Zr
0.5
O
2
(HZO)/TiN ferroelectric capacitors using experiments and first-principles calculations. It is shown that carrier injection-induced charged oxygen vacancies are at the heart of imprint in HZO, where other mechanisms such as domain pinning and dead layer are less important. An imprint model based on electron de-trapping from oxygen vacancy sites has been proposed that can satisfactorily explain several experimental facts such as the strong asymmetric imprint, leakage current variation, and so forth. Based on this model, an effective imprint recovery method has been proposed, which utilizes unipolar rather than bipolar voltage inputs. The remarkable recovery performances demonstrate the prospect of improved device reliability in hafnia-based FeRAM devices.
The adjustable conductance of a two‐terminal memristor in a crossbar array can facilitate vector‐matrix multiplication in one step, making the memristor a promising synapse for efficiently ...implementing neuromorphic computing. To achieve controllable and gradual switching of multi‐level conductance, important for neuromorphic computing, a theoretical design of a superlattice‐like (SLL) structure switching layer for the multi‐level memristor is proposed and validated, refining the growth of conductive filaments (CFs) and preventing CFs from the abrupt formation and rupture. Ti/(HfOx/AlOy)SLL/TiN memristors are shown with transmission electron microscopy , X‐ray photoelectron spectroscopy , and ab initio calculation findings corroborate the SLL structure of HfOx/AlOy film. The optimized SLL memristor achieves outstanding conductance modulation performance with linearly synaptic weight update (nonlinear factor α = 1.06), and the convolutional neural network based on the SLL memristive synapse improves the handwritten digit recognition accuracy to 94.95%. Meanwhile, this improved synaptic device has a fast operating speed (30 ns), a long data retention time (≥ 104 s at 85 ℃), scalability, and CMOS process compatibility. Finally, its physical nature is explored and the CF evolution process is characterized using nudged elastic band calculations and the conduction mechanism fitting. In this work, as an example the HfOx/AlOy SLL memristor provides a design viewpoint and optimization strategy for neuromorphic computing.
A superlattice‐like (SLL) switching layer design is presented to achieve a high‐performance analog‐type memristive synapse for neuromorphic computing. HfOx/AlOy SLL memristors exhibit high conductance modulation linearity, fast operation speed, and long‐term data retention. AlOy layers are set as migration barriers of oxygen vacancy periodically to enhance the controllability of oxygen vacancy and avoid the abrupt formation/rupture of conductive filaments.
Abstract
It is known that the Kohn–Sham eigenvalues do not characterize experimental excitation energies directly, and the band gap of a semiconductor is typically underestimated by local density ...approximation (LDA) of density functional theory (DFT). An embarrassing situation is that one usually uses LDA+
U
for strongly correlated materials with rectified band gaps, but for non-strongly-correlated semiconductors one has to resort to expensive methods like hybrid functionals or
GW
. In spite of the state-of-the-art meta-generalized gradient approximation functionals like TB-mBJ and SCAN, methods with LDA-level complexity to rectify the semiconductor band gaps are in high demand. DFT-1/2 stands as a feasible approach and has been more widely used in recent years. In this work we give a detailed derivation of the Slater half occupation technique, and review the assumptions made by DFT-1/2 in semiconductor band structure calculations. In particular, the self-energy potential approach is verified through mathematical derivations. The aims, features and principles of shell DFT-1/2 for covalent semiconductors are also accounted for in great detail. Other developments of DFT-1/2 including conduction band correction, DFT+
A
-1/2, empirical formula for the self-energy potential cutoff radius, etc, are further reviewed. The relations of DFT-1/2 to hybrid functional, sX-LDA,
GW
, self-interaction correction, scissor’s operator as well as DFT+
U
are explained. Applications, issues and limitations of DFT-1/2 are comprehensively included in this review.
Ferroelectricity observed in thin-film HfO2, either doped with Si, Al, and so forth or in the Hf0.5Zr0.5O2 form, has gained great technical significance. While a trilinear coupling between phonon ...modes could explain its ferroelectric distortion, from a practical perspective, one may be concerned with a theory that is more straightforward to predict similar ferroelectric candidates through some apparent features of HfO2 and ZrO2. In this work, we propose that the 7 cation coordination number of HfO2/ZrO2 lies at the heart of this ferroelectricity, which stems from the proper ionic radii of Hf/Zr compared with O. Among the numerous compounds with a non-centrosymmetric nature, for example, the mm2 point group, HfO2 and ZrO2 are special in that they are close to the border of 7 and 8 cation coordination, such that the 8-coordination tetragonal intermediate phase could greatly reduce the switching barrier. Other 7-coordination candidates, including SrI2, TaON, YSBr, and YOF, are also studied in comparison to HfO2/ZrO2, and six switching paths are analyzed in detail for the Pca21 phase. A rule of the preferred switching path in terms of the ionic radii ratio and coordination number has been established. We also show the possible route from the ferroelectric Pca21 phase to the monoclinic P21/c phase in HfO2, which is relevant to the fatigue phenomenon.
The wake‐up phenomenon widely exists in hafnia‐based ferroelectric capacitors, which causes device parameter variation over time. Crystallization at a higher temperature has been reported to be ...effective in eliminating wake‐up, but high temperature may yield the monoclinic phase or generate more oxygen vacancies. In this work, a unidirectional annealing method is proposed for the crystallization of Hf0.5Zr0.5O2 (HZO) superlattice ferroelectrics, which involves heating from the Pt/ZrO2 interface side. It is demonstrated that 600 °C annealing only leads to a moderate content of monoclinic phase in HZO, and the TiN/HZO/Pt capacitor exhibits wake‐up free nature and a switchable remnant polarization value of 27.4 µC cm−2. On the other hand, heating from the TiN/HfO2 side, or using 500 °C annealing temperature, could yield ferroelectric devices that require a wake‐up process. The special configuration of Pt/ZrO2 is verified by comparative studies with several other superlattice structures and HZO solid‐state solutions. It is discovered that heating from the Pt/HfO2 side at 600 °C leads to high leakage current and a memristor behavior. The mechanisms of ferroelectric phase stabilization and memristor formation have been discussed. The unidirectional heating method can also be useful for other hafnia‐based ferroelectric devices.
Wake‐up free Hf0.5Zr0.5O2 (HZO) device is achieved through a delicate capacitor design together with a new annealing method. A unidirectional annealing method is proposed for the crystallization of HZO superlattice ferroelectrics, which involves heating from the Pt/ZrO2 interface side. The unidirectional heating is of potential value to optimize the ferroelectric crystallization in hafnia‐based ferroelectric capacitors.
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.
In recent years, hafnia-based ferroelectrics have attracted enormous attention due to their capability of maintaining ferroelectricity below 10 nm thickness and excellent compatibility with ...microelectronics flow lines. However, the physical origin of their ferroelectricity is still not fully clear, although it is commonly attributed to a polar Pca21 orthorhombic phase. The high-temperature paraelectric phases (the tetragonal phase or the cubic phase) do not possess a soft mode at the Brillouin zone center, thus the ferroelectric distortion has to be explained in terms of trilinear coupling among three phonon modes in the tetragonal phase. It is necessary to explore new materials with possible ferroelectricity due to the polar Pca21 phase, which in turn should be very helpful in evaluating the microscopic theory for ferroelectric hafnia. In this work, based on the idea of the Materials Genome Engineering, a series of hafnia-like ferroelectrics have been found, exemplified by LaSeCl, LaSeBr, LuOF and YOF, which possess adequate spontaneous polarization values and also relatively favorable free energies for the polar phase. Their common features and individual differences are discussed in detail. In particular, a promising potential ferroelectric material, Pca21 phase LuOF, is predicted and recommended for further experimental synthesis and investigation.
PDF
