Ferroelectricity and Large Piezoelectric Response of AlN/ScN Superlattice

Excellent Hole Mobility and Out-of-Plane Piezoelectricity in X-Penta-Graphene (X = Si or Ge) with Poisson's Ratio Inversion

Recently, the application of two-dimensional (2D) piezoelectric materials has been seriously hindered because most of them possess only in-plane piezoelectricity but lack out-of-plane piezoelectricity. In this work, using first-principles calculation, by atomic substitution of penta-graphene (PG) with tiny out-of-plane piezoelectricity, we design and predict stable 2D X-PG (X = Si or Ge) semiconductors with excellent in-plane and out-of-plane piezoelectricity and extremely high in-plane hole mobility. Among them, Ge-PG exhibits better performance in all aspects with an in-plane strain piezoelectric coefficient d11 = 8.43 pm/V, an out-of-plane strain piezoelectric coefficient d33 = -3.63 pm/V, and in-plane hole mobility μh = 57.33 × 103 cm2 V-1 s-1. By doping Si and Ge atoms, the negative Poisson's ratio of PG approaches zero and reaches a positive value, which is due to the gradual weakening of the structure's mechanical strength. The bandgaps of Si-PG (0.78 eV) and Ge-PG (0.89 eV) are much smaller than that of PG (2.20 eV), by 2.82 and 2.47 times, respectively. This indicates that the substitution of X atoms can regulate the bandgap of PG. Importantly, the physical mechanism of the out-of-plane piezoelectricity of these monolayers is revealed. The super-dipole-moment effect proposed in the previous work is proved to exist in PG and X-PG, i.e., it is proved that their out-of-plane piezoelectric stress coefficient e33 increases with the super-dipole-moment. The e33-induced polarization direction is also consistent with the super-dipole-moment direction. X-PG is predicted to have prominent potential for nanodevices applied as electromechanical coupling systems: wearable, ultra-thin devices; high-speed electronic transmission devices; and so on.

Perspectives of Ferroelectric Wurtzite AlScN: Material Characteristics, Preparation, and Applications in Advanced Memory Devices

Ferroelectric, phase-change, and magnetic materials are considered promising candidates for advanced memory devices. Under the development dilemma of traditional silicon-based memory devices, ferroelectric materials stand out due to their unique polarization properties and diverse manufacturing techniques. On the occasion of the 100th anniversary of the birth of ferroelectricity, scandium-doped aluminum nitride, which is a different wurtzite structure, was reported to be ferroelectric with a larger coercive, remanent polarization, curie temperature, and a more stable ferroelectric phase. The inherent advantages have attracted widespread attention, promising better performance when used as data storage materials and better meeting the needs of the development of the information age. In this paper, we start from the characteristics and development history of ferroelectric materials, mainly focusing on the characteristics, preparation, and applications in memory devices of ferroelectric wurtzite AlScN. It compares and analyzes the unique advantages of AlScN-based memory devices, aiming to lay a theoretical foundation for the development of advanced memory devices in the future.

Strain-Controlled Formation Energy and Migration of Nitrogen Vacancy in Al1-xScxN: A First-Principles Study

The impact of strain on the formation energy and migration behavior of nitrogen vacancies (VNs) in Al1-xScxN has been investigated by first-principles calculations. The formation energy of VNs is obtained by total energy calculations. The migration barrier calculation utilizes the climbing nudged elastic band method. It is found that the formation energy of VNs is highly tunable with respect to the strain. The formation energy of VNs increases with the tensile strain increasing to +4% and decreases with the increasing compressive strain to -4%. A minimum formation energy of 4.11 eV is obtained when -4% strain is applied. Furthermore, the migration behavior of VNs is studied by calculating the migration barriers. Calculation results show that the migration barrier is strongly affected by strain. When the strain is -4%, the barrier is 2.46 eV while the barrier is increased to 2.71 eV under +4% strain. Therefore, a tensile strain can prevent the formation and migration of VNs. These findings suggest that strain engineering may serve as a tool for regulating VNs behavior in Al1-xScxN, potentially alleviating the ferroelectric degradations associated with VNs.

Engineering Ferroelectricity and Large Piezoelectricity in h-BN

Hexagonal boron nitride (h-BN) is a well-known layered van der Waals (vdW) material that exhibits no spontaneous electric polarization due to its centrosymmetric structure. Extensive density functional theory (DFT) calculations are used to demonstrate that doping through the substitution of B by isovalent Al and Ga breaks the inversion symmetry and induces local dipole moments along the c-axis, which promotes a ferroelectric (FE) alignment over antiferroelectric. For doping concentrations below 25%, a "protruded layered" structure in which the dopant atoms protrude out of the planar h-BN layers is energetically more stable than the flat layered structure of pristine h-BN or a wurtzite structure similar to w-AlN. The computed polarization, between 7.227 and 21.117 μC/cm2, depending on dopant concentration and the switching barrier (16.684 and 45.838 meV/atom) for the FE polarization reversal are comparable to that of other well-known FEs. Interestingly, doping of h-BN also induces a large negative piezoelectric response in otherwise nonpiezoelectric h-BN. For example, we compute d33 of -24.214 pC/N for Ga0.125B0.875N, which is about 5 times larger than that of pure w-AlN (5 pC/N), although the computed e33 (-1.164 C/m2) is about 1.6 times lower than that of pure w-AlN (1.462 C/m2). Because of the layered structure, the rather small elastic constant C33 provides the origin of the large d33. Moreover, doping makes h-BN an electric auxetic piezoelectric. We also show that ferroelectricity in doped h-BN may persist down to its trilayer, which indicates high potential for applications in FE nonvolatile memories.

Atomic-scale polarization switching in wurtzite ferroelectrics

Ferroelectric wurtzites have the potential to revolutionize modern microelectronics because they are easily integrated with multiple mainstream semiconductor platforms. However, the electric fields required to reverse their polarization direction and unlock electronic and optical functions need substantial reduction for operational compatibility with complementary metal-oxide semiconductor (CMOS) electronics. To understand this process, we observed and quantified real-time polarization switching of a representative ferroelectric wurtzite (Al_0.94B_0.06N) at the atomic scale with scanning transmission electron microscopy. The analysis revealed a polarization reversal model in which puckered aluminum/boron nitride rings in the wurtzite basal planes gradually flatten and adopt a transient nonpolar geometry. Independent first-principles simulations reveal the details and energetics of the reversal process through an antipolar phase. This model and local mechanistic understanding are a critical initial step for property engineering efforts in this emerging material class.

Large piezoelectric response in ferroelectric/multiferroelectric metal oxyhalide MOX2 (M = Ti, V and X = F, Cl and Br) monolayers

Flexible two-dimensional (2D) piezoelectric materials are promising for applications in wearable electromechanical nano-devices such as sensors, energy harvesters, and actuators. A large piezo-response is required for any practical applications. Based on first-principles calculations, we report that ferroelectric TiOX₂ and multiferroelectric VOX₂ (X = F, Cl, and Br) monolayers exhibit large in-plane stress (e₁₁) and strain (d₁₁) piezoelectric coefficients. For example, the in-plane piezo-response of TiOBr₂ (both e₁₁ = 28.793 × 10^-10 C m^-1 and d₁₁ = 37.758 pm V^-1) is about an order of magnitude larger than that of the widely studied 1H-MoS₂ monolayer, and also quite comparable to the giant piezoelectricity of group-IV monochalcogenide monolayers, e.g., SnS. Moreover, the d₁₁ of MOX₂ monolayers - ranging from 29.028 pm V^-1 to 37.758 pm V^-1 - are significantly higher than the d₁₁ or d₃₃ of commonly used 3D piezoelectrics such as w-AlN (d₃₃ = 5.1 pm V^-1) and α-quartz (d₁₁ = 2.3 pm V^-1). Such a large d₁₁ of MOX₂ monolayers originates from low in-plane elastic constants with large e₁₁ due to large Born effective charges (Z_ij) and atomic sensitivity to an applied strain. Moreover, we show the possibility of opening a new way of controlling piezoelectricity by applying a magnetic field.

Negative Piezoelectric Coefficient in Ferromagnetic 1H-LaBr2 Monolayer

The discovery of two-dimensional (2D) magnetic materials that have excellent piezoelectric response is promising for nanoscale multifunctional piezoelectric or spintronic devices. Piezoelectricity requires a noncentrosymmetric structure with an electronic band gap, whereas magnetism demands broken time-reversal symmetry. Most of the well-known 2D piezoelectrics, e.g., 1H-MoS₂ monolayer, are not magnetic. Being intrinsically magnetic, semiconducting 1H-LaBr₂ and 1H-VS₂ monolayers can combine magnetism and piezoelectricity. We compare piezoelectric properties of 1H-MoS₂, 1H-VS₂, and 1H-LaBr₂ using density functional theory. The ferromagnetic 1H-LaBr₂ and 1H-VS₂ monolayers display larger piezoelectric strain coefficients, namely, d ₁₁ = -4.527 pm/V for 1H-LaBr₂ and d ₁₁ = 4.104 pm/V for 1H-VS₂, compared to 1H-MoS₂ (d ₁₁ = 3.706 pm/V). 1H-MoS₂ has a larger piezoelectric stress coefficient (e ₁₁ = 370.675 pC/m) than 1H-LaBr₂ (e ₁₁ = -94.175 pC/m) and 1H-VS₂ (e ₁₁ = 298.100 pC/m). The large d ₁₁ for 1H-LaBr₂ originates from the low elastic constants, C ₁₁ = 30.338 N/m and C ₁₂ = 9.534 N/m. The sign of the piezoelectric coefficients for 1H-LaBr₂ is negative, and this arises from the negative ionic contribution of e ₁₁, which dominates in 1H-LaBr₂, whereas the electronic part of e ₁₁ dominates in 1H-MoS₂ and 1H-VS₂. We explain the origin of this large ionic contribution of e ₁₁ for 1H-LaBr₂ through Born effective charges (Z ₁₁) and the sensitivity of the atomic positions to the strain (du/dη). We observe a sign reversal in the Z ₁₁ values of Mo and S compared to the nominal oxidation states, which makes both the electronic and ionic parts of e ₁₁ positive and results in the high value of e ₁₁. We also show that a change in magnetic order can enhance (reduce) the piezoresponse of 1H-LaBr₂ (1H-VS₂).

Coexistence of intrinsic room-temperature ferromagnetism and piezoelectricity in monolayer BiCrX3 (X = S, Se, and Te)

Two-dimensional (2D) materials with intrinsic ferromagnetism and piezoelectricity have received growing attention due to their potential applications in nanoscale spintronic devices. However, their applications are highly limited by the low Curie temperatures (T_C) and small piezoelectric coefficients. Here, using first-principles calculations, we have successfully predicted that BiCrX₃ (X = S, Se, and Te) monolayers simultaneously possess ferromagnetism and piezoelectricity by replacing one layer of Bi atoms with Cr atoms in Bi₂X₃ monolayers. Our results demonstrate that BiCrX₃ monolayers are not only intrinsic ferromagnetic semiconductors with indirect band gaps, adequate T_C values of higher than 300 K, and significant out-of-plane magnetic anisotropic energies, but also exhibit appreciable in-plane and out-of-plane piezoelectricity. In particular, the in-plane piezoelectric coefficients of BiCrX₃ monolayers with ABCAB configuration are up to 15.16 pm V^-1, which is higher than those of traditional three-dimensional piezoelectric materials such as α-quartz. The coexistence of ferromagnetism and piezoelectricity in BiCrX₃ monolayers gives them promising applications in spintronics and nano-sized sensors.

Enhanced in-plane ferroelectricity, antiferroelectricity, and unconventional 2D emergent fermions in quadruple-layer XSbO2 (X = Li, Na)

Low-dimensional ferroelectricity and Dirac materials with protected band crossings are fascinating research subjects. Based on first-principles calculations, we predict the coexistence of spontaneous in-plane polarization and novel 2D emergent fermions in dynamically stable quadruple-layer (QL) XSbO₂ (X = Li, Na). Depending on the different polarization configurations, QL-XSbO₂ can exhibit unconventional inner-QL ferroelectricity and antiferroelectricity. Both ground states harbor robust ferroelectricity with enhanced spontaneous polarization of 0.56 nC m^-1 and 0.39 nC m^-1 for QL-LiSbO₂ and QL-NaSbO₂, respectively. Interestingly, the QL-LiSbO₂ possesses two other metastable ferroelectric (FE) phases. The ground FE phase can be flexibly driven into one of the two metastable FE phases and then into the antiferroelectric (AFE) phase. During this phase transition, several types of 2D fermions emerge, for instance, hourglass hybrid and type-II Weyl loops in the ground FE phase, type-II Weyl fermionsin the metastable FE phase, and type-II Dirac fermions in the AFE phase. These 2D fermions are robust under spin-orbit coupling. Notably, two of these fermions, e.g., an hourglass hybrid or type-II Weyl loop, have not been observed before. Our findings identify QL-XSbO₂ as a unique platform for studying 2D ferroelectricity relating to 2D emergent fermions.

Periodic nanostructures: preparation, properties and applications

Periodic nanostructures, a group of nanomaterials consisting of single or multiple nano units/components periodically arranged into ordered patterns (e.g., vertical and lateral superlattices), have attracted tremendous attention in recent years due to their extraordinary physical and chemical properties that offer a huge potential for a multitude of applications in energy conversion, electronic and optoelectronic applications. Recent advances in the preparation strategies of periodic nanostructures, including self-assembly, epitaxy, and exfoliation, have paved the way to rationally modulate their ferroelectricity, superconductivity, band gap and many other physical and chemical properties. For example, the recent discovery of superconductivity observed in "magic-angle" graphene superlattices has sparked intensive studies in new ways, creating superlattices in twisted 2D materials. Recent development in the various state-of-the-art preparations of periodic nanostructures has created many new ideas and findings, warranting a timely review. In this review, we discuss the current advances of periodic nanostructures, including their preparation strategies, property modulations and various applications.

Large Piezoelectric Response and Ferroelectricity in Li and V/Nb/Ta Co-Doped w-AlN

Enhancement of piezoelectricity in w-AlN is desired for many devices including resonators for next-generation wireless communication systems, sensors, and vibrational energy harvesters. Based on density functional theory, we show that Li and X (X = V, Nb, and Ta) co-doping in 1Li:1X ratio transforms brittle w-AlN crystal to ductile, along with broadening the compositional freedom for significantly enhanced piezoelectric response, promising them to be good alternatives to expensive Sc. Interestingly, these co-doped w-AlN also show quite large spontaneous electric polarization (e.g., about 1 C/m² for Li_0.125X_0.125Al_0.75N) with the possibility of ferroelectric polarization switching, opening new possibilities in wurtzite nitrides. An increase in piezoelectric stress constant (e₃₃) with a decrease in elastic constant (C₃₃) results in an enhancement of piezoelectric strain constant (d₃₃), which is desired for improving the performance of bulk acoustic wave (BAW) resonators for high-frequency radio frequency (RF) signals. Also, these co-doped w-AlN are potential lead-free piezoelectric materials for energy harvesting and sensors as they improve the longitudinal electromechanical coupling constant (K₃₃²), transverse piezoelectric strain constant (d₃₁), and figure of merit (FOM) for power generation. However, the enhancement in K₃₃² is not as pronounced as that in d₃₃ because co-doping increases dielectric constant. The longitudinal acoustic wave velocity (7.09 km/s) of Li_0.1875Ta_0.1875Al_0.625N is quite comparable to that of commercially used piezoelectric LiNbO₃ or LiTaO₃ in special cuts (about 5-7 km/s) despite the fact that the acoustic wave velocities, important parameters for designing resonators or sensors, decrease with co-doping or Sc concentration.

Huge Piezoelectric Response of LaN-based Superlattices

We construct LaN-based artificial superlattices to investigate the ferroelectricity and piezoelectricity using the volume matching conditions of the parent components that soften the elastic constant C₃₃ and increase the piezoelectric modulus d₃₃. The proposed superlattice consists of LaN and YN (or LaN and ScN) buckled monolayers alternately arranged along the crystallographic c-direction. The structure of polar wurtzite (w-LaYN/w-LaScN) is both mechanically and dynamically stable, and the computed energy barrier makes the ferroelectric polarization switching possible. We show that the epitaxial strain can modify the spontaneous ferroelectric polarization as well as d₃₃. The LaN/YN superlattice exhibits a huge piezoelectric response in the unstrained state, due to their small c/a value and extremely soft C₃₃. In addition, the epitaxial strain is revealed as effective control of the nature (indirect and direct) and value of the electronic band gap.

Optimizing the Piezoelectric Strain in ZrO2- and HfO2-Based Incipient Ferroelectrics for Thin-Film Applications: An Ab Initio Dopant Screening Study

HfO₂ and ZrO₂ have increasingly drawn the interest of researchers as lead-free and silicon technology-compatible materials for ferroelectric, pyroelectric, and piezoelectric applications in thin films such as ferroelectric field-effect transistors, ferroelectric random access memories, nanoscale sensors, and energy harvesters. Owing to the environmental regulations against lead-containing electronic components, HfO₂ and ZrO₂ offer, along with AlN, (K,Na)NbO₃- and (Bi_0.5Na_0.5)TiO₃-based materials, an alternative to Pb(Zr_xTi_1-x)O₃-based materials, which are the overwhelmingly used ceramics in industry. HfO₂ and ZrO₂ thin films may show field-induced phase transformation from the paraelectric tetragonal to the ferroelectric orthorhombic phase, leading to a change in crystal volume and thus strain. These field-induced strains have already been measured experimentally in pure and doped systems; however, no systematic optimization of the piezoelectric activity was performed, either experimentally or theoretically. In this screening study, we calculate the ultimate size of this effect for 58 dopants depending on the oxygen supply and the defect incorporation type: substitutional or interstitial. The largest piezoelectric strain values are achieved with Yb, Li, and Na in ZrO₂ and exceed 40 pm V^-1 or 0.8% maximal strain, which exceeds the best experimental findings by a factor of 2. Furthermore, we discovered that Mo, W, and Hg make the polar-orthorhombic phase in the ZrO₂ bulk stable under certain circumstances, which would count in favor of these systems for the ceramic crystallization process. Our work guides the development of the performance of a promising material system by rational design of the essential mechanisms so as to apply it to unforeseen applications.

Designing Multifunctionality via Assembling Dissimilar Materials: Epitaxial AlN/ScN Superlattices

First-principles calculations are performed to investigate the effect of epitaxial strain on energetic, structural, electrical, electronic, and optical properties of 1×1 AlN/ScN superlattices. This system is predicted to adopt four different strain regions exhibiting different properties, including optimization of various physical responses such as piezoelectricity, electro-optic and elasto-optic coefficients, and elasticity. Varying the strain between these four different regions also allows the creation of an electrical polarization in a nominally paraelectric material, as a result of a softening of the lowest optical mode, and even the control of its magnitude up to a giant value. Furthermore, it results in an electronic band gap that cannot only change its nature (direct vs indirect), but also cover a wide range of the electromagnetic spectrum from the blue, through the violet and near ultraviolet, to the middle ultraviolet. These findings thus point out the potential of assembling two different materials inside the same heterostructure to design multifunctionality and striking phenomena.