Lattice Mismatch Strain Research Articles

Strain and piezoelectric control of electronic and photonic properties of p − n diodes

Piezoelectricity is a well-known effect in a vast number of technologically important insulators and semiconductors and exists in 20 out of the 32 three-dimensional crystal classes. The piezoelectric effect is the driving mechanism behind several classical sensors and transmitters, and also most recently, in many nanodevices. Zhong Lin Wang coined the fields piezotronics and piezo-phototronics where the piezoelectric effect plays a dominant role. Piezoelectricity couples in a linear fashion mechanical strain to electrical fields and vice versa. In solids, there is another linear coupling between strain and the electric potential, known as the deformation potential effect. While linear in its coupling nature, this effect does not require the solid to be non-centrosymmetric in contrast to the piezoelectric effect. Moreover, the deformation potential effect is quantitatively huge and leads to changes in the conduction and valence band edges of III–V and II–VI materials of, typically, 50–100 meV in the presence of 1 % strain. Therefore, the deformation potential effect is essential to determine the electronic and photonic properties of bulk and nanostructure semiconductors in the presence of strain. In this work, we compute the relative importance of piezoelectricity and the deformation potential effect in the presence of lattice mismatch and external strain. We choose p − n junctions of ZnO/GaN structures but anticipate that the general conclusions can be carried over to other material structures. The main result of the present work is that both the inclusion of the deformation potential effect and piezoelectricity is crucial to correctly compute the effect of strain on p − n junction current–voltage curves and photonic properties. In our analysis of wurtzite heterostructures, the spontaneous polarization effect is also included but this effect appears to play a minor role for electronic and photonic properties.

Antiferroelectric Ceramics for Energy-Efficient Capacitors by Theory-Guided Discovery.

Antiferroelectric ceramics, via the electric-field-induced antiferroelectric (AFE)-ferroelectric (FE) phase transitions, show great promise for high-energy-density capacitors. Yet, currently, only 70-80% energy release is found during a charge-discharge cycle. Here, for PbZrO3-based oxides, geometric nonlinear theory of martensitic phase transitions is applied (first used to guide supercompatible shape-memory alloys) to predict the reversibility of the AFE-FE transition by using density-functional theory to assess AFE/FE interfacial lattice-mismatch strain that assures ultralow electric hysteresis and extended fatigue lifetime. A good correlation of mismatch strain with electric hysteresis, hence, with energy efficiency of AFE capacitors is observed. Guided by theory, high-throughput material search is conducted and AFE compositions with a near-perfect charge-discharge energy efficiency (98.2%), i.e., near-zero hysteresis are discovered. And the fatigue life of the capacitor reaches 79.5 million charge-discharge cycles, a factor of 80 enhancement over AFE ceramics with large electric hysteresis.

Interfacial strain induced giant magnetoresistance and magnetodielectric effects in multiferroic BCZT/LSMO thin film heterostructures

Layered thin films of the ferroelectric perovskite Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) and the ferromagnetic half-metal La0.80Sr0.20MnO3 (LSMO) are well-known multiferroic systems that show promise for spintronic applications. In this work, the structure–property relationships are explored in novel BCZT/LSMO thin film heterostructures with optimized ferroic properties. Epitaxial BCZT/LSMO thin film heterostructures are grown by varying the lattice mismatch strains on single crystal LaAlO3 (LAO) (100) and MgO (100) substrates using the pulsed laser deposition technique. The epitaxial strain in the films gives rise to a tetragonal distortion of the BCZT and LSMO unit cells and significantly affects their magnetotransport and magnetodielectric properties. The BCZT/LSMO/LAO heterostructure exhibits a colossal magnetoresistance effect due to a large out-of-plane tensile strain, which induces enhanced carrier hopping in the LSMO layer as compared to the BCZT/LSMO/MgO film. The larger tetragonal distortion of the BCZT unit cell in BCZT/LSMO/MgO contributes to higher dielectric permittivity, with a greater dielectric maxima temperature and freezing temperature. Magnetodielectric measurements reveal a hitherto unobserved giant magnetodielectric effect in the BCZT/LSMO/MgO film, attributed to a large in-plane strain, which induces interfacial polarization distortion at the interfacial layer. Overall, this work elucidates the unique strain and charge-mediated cross-coupled phenomena of magnetic and electric orders in multiferroic thin film heterostructures, which are critical for their technological applications.

Effect of thermal mismatch strain on the self-assembly of nanorods in YBCO/NRs nanocomposite films

This study presents a thermoelastic model to investigate the impact of thermal mismatch strain, resulting from distinct thermal expansion coefficients between the film matrix and nanorods, on the self-assembly of nanorods in YBCO/NRs nanocomposite films. An accurate understanding of thermal mismatch strain generated during nanocomposite film preparation is vital for controlling the self-assembly of artificial nanorods in such films. In this model, we calculated the total elastic energy density of the film matrix and nanorods, considering both lattice and thermal mismatch strains, to analyze the effect of thermal strain on the diameter of c-oriented nanorods in YBCO nanocomposite films. The calculated nanorod diameter values match experimentally measured values for BaZrO3 and BaSnO3 nanorods. Furthermore, compared to results that only consider lattice mismatch strain, the proposed model yields closer calculation values to experimental measurements by incorporating the impact of thermal mismatch strain. The thermal mismatch strain also affects the spacing between two adjacent nanorods, having the most profound effect on BSO nanorods compared to BZO and BHO nanorods. These findings contribute to understanding the mechanisms involved in nanorod self-assembly and provide insights to optimize the design of YBCO/NRs nanocomposite films.

Energetics of nanoscale films with self–assembled oxide/metal pillars in nitride matrix

Vertically aligned nanocomposite (VAN) thin films consisting of NiO and Au nanopillars in a TiN matrix exhibits excellent optical anisotropy and tunability along with strong magneto–optical coupling properties. A recent experimental study showed that NiO deposited on a template of TiN matrix containing Au nanopillars nucleates NiO pillars on the already existing Au pillars. Due to the oxygen–deficient NiO, Au and Ni interdiffusion develops a unique Au shell/NiO core structure in the top region and segregated agglomerates of Ni in the lower Au pillars. As a step toward understanding the growth of such a multi–phase film configuration, we report a comparative energetic investigation confirming that configurations with interdiffusion of Au and Ni is energetically favorable compared to other possible configurations without the interdiffusion of Au and Ni. In this work, a thermodynamic model accounting for elastic energy due to lattice mismatch strain, capillarity effects, and interfacial and chemical energies is utilized to compare various film configurations energetically. Among all the configurations examined, the ones that closely resemble the experimental observations possessed the least energy. It was also found that Au–Ni interdiffusion is immediate upon the deposition of oxygen–deficient NiO on TiN–Au template, and a critical ratio of Au and NiO pillar heights favors core–shell structure. These findings are vital for understanding the formation mechanisms of complex VANs and future designs of new three–phase VAN structures with complex morphologies.

Improved polarization retention in epitaxial BiFeO3 thin films induced by strain relaxation

This study investigated the impact of lattice mismatch strain (LMS) on the polarization retention of epitaxial ferroelectric BiFeO3 (BFO) thin films. The application of varying degrees of LMS on the BFO films was achieved by tuning the film thickness, which was verified through X-ray diffraction and reciprocal space mapping. As the strain was gradually eliminated, the film demonstrated a more saturated hysteresis loop with a reduced negative coercive electric field. Additionally, a significant increase in polarization retention was observed with increasing film thickness, strongly indicating the crucial role of LMS in driving the formation of preferred polarization, which is intimately connected with the built-in field induced by oxygen vacancies-relevant space charge alignment. Moreover, the anisotropic LMS state of the BFO films was utilized to horizontally reverse local polarization in a sub-micrometer scale, revealing that strain accelerates the back-switching of polarization to eliminate the artificially created ferroelastic domain walls. To counteract the strain effect, charge injection of electrons was proposed to enhance the retention of switched polarization by fully suppressing the built-in field and pinning the artificially created domain walls. The findings provide valuable insights into the impact of strain on polarization retention for the development of ferroelectric memories.

Interfacial strain evolution and abnormal Poisson's ratios in Al nuclei for the Al/TiB2 heterostructure

The interfacial lattice mismatch and local elastic strains of Al/TiB2 heterostructures are quantitatively investigated by aberration-corrected transmission electron microscopy. Both in-plane atomic expansion and out-of-plane compression of Al layers stacked on (0001)TiB2 are uncovered and favor the further extension of deformed Al layers for realizing a successfully heterogeneous nucleation. These lattice strains can persist up to several Al atomic layers from the topmost (0001) surface of TiB2. Based on the compressive and expansive elastic strain relation, we estimate the Poisson's ratio in Al nuclei and find that large mismatch strains lead to abnormal changes in the Poisson's ratio. This study reveals the effect of lattice mismatch on the heterogeneous nucleation of Al on TiB2 and may have important implications in understanding the heterogeneous nucleation of Al.

Quasi-two-dimensional magnetic properties in atomic layer resolved La0.7Sr0.3MnO3/LaAlO3 superlattices

Materials in low dimensions show interesting physics and excellent potential in practical applications. The atomic-level control of two-dimensional (2D) behavior in a traditional three-dimensional (3D) system is quite hard, and the ${({\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3})}_{n}/{(\mathrm{LaAl}{\mathrm{O}}_{3})}_{n}$ (LMSO/LAO) superlattice on a (001) $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrate was investigated to explore its quasi-2D behavior. The number of magnetic phase transitions has a close relationship with the number of unit cells ($n$): only one phase transition for $n=2$ and four phase transitions for $n=8$. It was found that the local structure of each ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ atomic layer is responsible for the multiple phase transitions: the gradual change of the tetragonal ratio and the in-plane tensile strain-induced $3d$ electronic orbital occupancy control the magnetic properties of 2D-LSMO atomic layers. For the outmost 2D-LSMO atomic layer contacting $\mathrm{LaAl}{\mathrm{O}}_{3}$ directly, interface-related factors reduce the intensity of magnetic interaction significantly. With the increase of $n$, the properties around the LSMO/LAO interface changes, which demonstrates that the superlattice configuration and the lattice-mismatch strain could be employed to explore the quasi-2D behavior of a traditional 3D system.

Direct observation of large-area strain propagation on free-standing nanomembranes

Investigations on epitaxial nanostructures with size of tens of nanometers have been a challenging issue for techniques that present high strain sensitivity but restricted spatial resolution. This is the case of recently developed x-ray nanoprobe techniques. Despite its inherent nondestructive character, submicron x-ray spots have only been successfully applied to the study of individual nanostructures which are either strain free or present extremely mild spatial lattice parameter gradients. Such limitation, with an uttermost barrier given by the diffraction limit, leads to voxel or pixel sizes between 5 and 10 nm obtained in coherent diffraction imaging or ptychographic reconstructions of real-space objects. Whenever the strain field of a nanostructure is successfully reconstructed from reciprocal space measurements, it cannot vary considerably in short distances since this would induce diffraction peak broadening and cause abrupt phase variations, leading to convergence issues on reconstruction algorithms. Here we show how epitaxial systems with large lattice mismatch and appreciable interfacial strain can be identified and directly analyzed throughout their strain field propagation in nanometer-thin crystalline membrane platforms, using the InGaAs/GaAs Stranski-Krastanov system as a model. The strain-induced footprint becomes observable along a few microns if the membrane thickness is comparable to the nanostructure size. It is possible to retrieve both interfacial strain and nanostructure size by probing individual objects.

Influence of substrate on interfacial Dzyaloshinskii-Moriya interaction in epitaxial Tm3Fe5O12 films

The interfacial Dzyaloshinskii-Moriya interaction (iDMI) promotes homochiral magnetic structures such as N\'eel-type domain walls (DWs) and skyrmions. iDMI as well as chiral magnetic structures have been demonstrated in metal/garnet heterostructures offering desirable properties for spintronic applications compared to their metallic counterparts. By measuring the motion of DWs as a function of current and in-plane magnetic field, we show that for 6.6 nm thick epitaxial thulium iron garnet films with perpendicular magnetic anisotropy grown on gallium garnet substrates, the iDMI and the DW width depend on the substrate but do not scale linearly with lattice mismatch strain. The largest iDMI was $0.007 \mathrm{mJ} {\mathrm{m}}^{\ensuremath{-}2}$, obtained for three substrates of different compositions. The total anisotropy, however, does increase linearly with lattice parameter, indicating a dominant magnetoelastic anisotropy contribution. The sensitivity of iDMI and anisotropy to substrate composition provides opportunities for engineering homochiral magnetic structures and their dynamics.

Mechanical Control of Nonlinearity in Doubly Clamped MEMS Beam Resonators Using Preloaded Lattice-Mismatch Strain

We theoretically clarify the mechanism of the strain-tuning effect for control of the mechanical nonlinearity in doubly clamped MEMS beam resonators and experimentally demonstrate that nonlinearity can be effectively suppressed with a preloaded lattice-mismatch strain in the MEMS beam. Mechanical nonlinearity arises from the hardening- and softening-nonlinearity terms in the Duffing motion equation of the MEMS beam. By approaching the buckling condition of the MEMS beam, the substantially increased softening nonlinearity greatly compensates for the hardening nonlinearity, resulting in suppression of the total nonlinearity. Utilizing this knowledge, we fabricate ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ MEMS beams with a preloaded lattice-mismatch strain, which is achieved by adding a small amount (x = \ensuremath{\sim}0.4%) of indium to the $\mathrm{Ga}\mathrm{As}$ MEMS beam during wafer growth. The buckling condition in the experiment is achieved by carefully modulating the length of the ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ MEMS beams. As a result, mechanical nonlinearity is largely modulated from hardening to softening and reaches a quasizero value near the buckling condition, demonstrating the effectiveness of using lattice mismatch for controlling the mechanical nonlinearity of MEMS resonators.

(G03 - Best Student Presentation Award) Lateral Selective SiGe Growth for Dislocation-Free Virtual Substrate Fabrication

SiGe virtual substrates (VS) having low-surface roughness with less defects are essential for fabricating emerging quantum devices such as qubits. Various techniques to grow VS are reported e.g. conventional graded buffers (1,2) and reverse graded buffers (3). However, these methods require several micrometer-thick buffer layers, having threading dislocations (TDs) network and develop crosshatch patterns resulting in high surface roughness, needing chemical mechanical polishing for planarization. In our previous work, lateral-selective Ge growth (4) was performed in a cavity formed by selective vapor phase etching (VPE) of Si from side of mesa-patterned SiO2 /Si structures fabricated on buried oxide (BOX) to create smooth and dislocation-free local Ge by horizontal aspect ratio trapping (ART). In this work, we demonstrate lateral-selective SiGe growth to realize smooth and dislocation-free local SiGe VS by lateral ART and discuss its surface morphology, strain and dislocations.Lateral-selective SiGe growth is done in a reduced-pressure chemical vapor deposition (RPCVD) system. Thermally oxidized silicon-on-insulator (SOI) wafers with 300 nm SiO2/300 nm Si/2 μm BOX are used. For the sample preparation, 6.3 μm square checkerboard SiO2/Si mesa structure with [110] oriented sidewalls is fabricated by photolithography patterning and reactive ion etching. After standard RCA cleaning followed by HF dip, the wafer is loaded into RPCVD reactor and pre-baked at 850oC in H2 to remove residual oxide. Thereafter, lateral HCl VPE of Si is performed at 850oC from side of mesa. Si pillar remains at center of the mesa. Then, SiGe is selectively deposited in the cavity around the Si pillar using a SiH2Cl2-GeH4-HCl gas mixture at 750oC. Finally, the top SiO2 is removed by HF. Atomic force microscopy (AFM) is used to measure the surface roughness. Energy dispersive X-Ray spectroscopy (EDX) is used for measuring Ge content distribution in SiGe. Micro-Raman spectroscopy is used to analyze strain distribution in Si and SiGe. Nano-beam diffraction (NBD) is employed for estimating the relative change of in- and out-of-plane SiGe lattice parameters. Plan-view transmission electron microscopy (TEM) is used for imaging dislocations.Fig. 1 shows the AFM image for surface morphology of lateral-selective Si0.6Ge0.4 around the Si pillar. SiGe has root mean square roughness of ~0.16 nm, which is similar to blanket Si(001) (~0.14 nm), because the SiGe surface roughness is determined by interface roughness of top SiO2 and sacrificially etched Si. Uniform Ge content of ~40% in SiGe layer is observed by EDX analysis (Fig. 2). In the Si pillar part of Fig. 3, relative uniform tensile-strain of ~0.4% is observed. Since Si is negligibly strained in unpatterned area of SOI (taken as reference), hence during HCl VPE at 850oC, Si pillar/SiO2 interface could slip by higher thermal expansion of Si as interface decreases. Tensile-strain in Si pillar could be formed during cooling process. Ideally, ~1.68% lattice mismatch strain should exist in fully-relaxed ~40% SiGe on Si. However, higher tensile-strain (~2.5%) in SiGe is observed at corners along [010]. To clarify the reason of higher strain at corners, in- and out-of-plane SiGe lattice parameters near Si pillar (as marked in Fig 4(a)) are investigated by NBD. Higher in-plane lattice parameter in [110] is observed in the edge part of the SiGe lamella as compared to the center part (Fig. 4(b)). However, out-of-plane lattice parameter in [001] is homogeneous across entire SiGe (Fig. 4(c)). Because SiGe has uniform Ge concentration as shown in Fig. 2, higher lattice parameter in the edge part of Fig. 4(b) is caused by strain difference. This is because SiGe lamella edge (away from Si pillar) is more relaxed as compared to SiGe lamella center (beside Si pillar) which is confirmed by vertically grown ~700 nm 40% SiGe on blanket Si(110). Symmetric strain distribution does also exist in 90° rotated direction. Therefore, the tensile-strain in [010] is induced from both [110] and [-110]. In Fig. 5, stacking faults of SiGe on (111) plane are observed along [110] and [1-10]. TDs are present along [111], terminated by top SiO2/BOX and located up to first ~400 nm from Si while misfit dislocations are at Si/SiGe interface. Wide dislocation-free area in SiGe is present along [010].This technique enables dislocation-free local SiGe-on-insulator substrate fabrication which can be applied for thin and high quality VS for the quantum devices. References E. A. Fitzgerald et al., J. Appl. Phys., 63, 693 (1988).E. A. Fitzgerald et al., Appl. Phys. Lett., 59, 811 (1991).V. A. Shah et al., Appl. Phys. Lett., 93, 192103 (2008).Y. Yamamoto et al., ECS J. of Solid State Sci. and Technol., 3, 353 (2014). Figure 1

Strain-Driven Mixed-Phase Domain Architectures and Topological Transitions in Pb1- x Srx TiO3 Thin Films.

The potential for creating hierarchical domain structures, or mixtures of energetically degenerate phases with distinct patterns that can be modified continually, in ferroelectric thin films offers a pathway to control their mesoscale structure beyond lattice-mismatch strain with a substrate. Here, it is demonstrated that varying the strontium content provides deterministic strain-driven control of hierarchical domain structures in Pb1- x Srx TiO3 solid-solution thin films wherein two types, c/a and a1 /a2 , of nanodomains can coexist. Combining phase-field simulations, epitaxial thin-film growth, detailed structural, domain, and physical-property characterization, it is observed that the system undergoes a gradual transformation (with increasing strontium content) from droplet-like a1 /a2 domains in a c/a domain matrix, to a connected-labyrinth geometry of c/a domains, to a disconnected labyrinth structure of the same, and, finally, to droplet-like c/a domains in an a1 /a2 domain matrix. A relationship between the different mixed-phase modulation patterns and its topological nature is established. Annealing the connected-labyrinth structure leads to domain coarsening forming distinctive regions of parallel c/a and a1 /a2 domain stripes, offering additional design flexibility. Finally, it is found that the connected-labyrinth domain patterns exhibit the highest dielectric permittivity.

Flexoelectric-induced photovoltaic effects and tunable photocurrents in flexible LaFeO3 epitaxial heterostructures

By engineering strain gradients in dielectrics, the flexoelectric effect can be created, which yields interesting physical properties via electromechanical coupling. Here, we report flexoelectric-induced photovoltaic effects in centrosymmetric LaFeO3 thin-film heterostructures grown on flexible mica substrates, in which partial relaxation of lattice-mismatch strain against LaAlO3 stretching layers results in giant strain gradients and pronounced electrical polarizations. The flexoelectric polarization modulates band alignment and leads to significant photovoltaic effects with a short-circuit current density of ∼0.4 mA/cm2 and an open circuit voltage of ∼ -0.45 V in Pt/LaFeO3/LaNiO3 devices. In addition, by concavely/convexly bending the mica substrate, mechanical strain gradients give rise to bi-directionally tunable photocurrents, in which continuously change of short-circuit current density with a magnitude of ∼100% and good reproducibility in repetitive bending operations are observed in the Pt/LaFeO3/LaNiO3 devices. The present work demonstrates an approach to design self-powered photoelectric devices with an electromechanical degree of freedom based on the flexoelectric effect in flexible thin-film heterostructures.

Crystal plasticity phase-field simulation of rafting and creep properties with a continuous change of the mismatch strain in a Co-Al-W superalloy

The lattice mismatch strain between the precipitates and matrix has an essential effect on the creep morphology and properties of Co-based superalloys. The rafting morphology with a continuous change of the mismatch strain is clearly presented in a Co-10Al-8W at.% alloy by utilizing crystal plasticity phase-field simulation. The large mismatch strain accelerates the rafting of γ′-Co3(Al, W) phases, the hardening effect caused by eigenstrain elevates the creep properties in the initial creep stage, while the accumulation of damage variable induced by large eigenstrain reduces the creep life in tertiary creep stage. These results suggest that the suitable lattice mismatch is beneficial for the creep properties of superalloys.

Heat transport through propagon-phonon interaction in epitaxial amorphous-crystalline multilayers

Managing heat dissipation is a necessity for nanoscale electronic devices with high-density interfaces, but despite considerable effort, it has been difficult to establish the phonon transport physics at the interface due to a “complex” interface layer. In contrast, the amorphous/epitaxial interface is expected to have almost no “complex” interface layer due to the lack of lattice mismatch strain and less associated defects. Here, we experimentally observe the extremely-small interface thermal resistance per unit area at the interface of the amorphous-germanium sulfide/epitaxial-lead telluride superlattice (~0.8 ± 4.0 × 10‒9 m2KW−1). Ab initio lattice dynamics calculations demonstrate that high phonon transmission through this interface can be predicted, like electron transport physics, from large vibron-phonon density-of-states overlapping and phonon group velocity similarity between propagon in amorphous layer and “conventional” phonon in crystal. This indicates that controlling phonon (or vibron) density-of-states and phonon group velocity similarity can be a comprehensive guideline to manage heat conduction in nanoscale systems.

Heteroepitaxy of Dirac semimetal Cd3As2 by metal-organic chemical-vapor deposition

We present progress on the synthesis of semimetal Cd3As2 by metal–organic chemical-vapor deposition (MOCVD). Specifically, we have optimized the growth conditions needed to obtain technologically useful growth rates and acceptable thin-film microstructures, with our studies evaluating the effects of varying the temperature, pressure, and carrier-gas type for MOCVD of Cd3As2 when performed using dimethylcadmium and tertiarybutylarsine precursors. In the course of the optimization studies, exploratory Cd3As2 growths are attempted on GaSb substrates, strain-relaxed InAs buffer layers grown on GaSb substrates, and InAs substrates. Notably, only the InAs-terminated substrate surfaces yield desirable results. Extensive microstructural studies of Cd3As2 thin films on InAs are performed by using multiple advanced imaging microscopies and x-ray diffraction modalities. The studied films are 5–75 nm in thickness and consist of oriented, coalesced polycrystals with lateral domain widths of 30–80 nm. The most optimized films are smooth and specular, exhibiting a surface roughness as low as 1.0 nm rms. Under cross-sectional imaging, the Cd3As2-InAs heterointerface appears smooth and abrupt at a lower film thickness, ~30 nm, but becomes quite irregular as the average thickness increases to ~55 nm. The films are strain-relaxed with a residual biaxial tensile strain (εxx = +0.0010) that opposes the initially compressive lattice-mismatch strain of Cd3As2 coherent on InAs (εxx = −0.042). Importantly, phase-identification studies find a thin-film crystal structure consistent with the P42/nbc space group, placing MOCVD-grown Cd3As2 among the Dirac semimetals of substantial interest for topological quantum materials studies.

Properties and precipitates of the high strength and electrical conductivity Cu-Ni-Co-Si-Cr alloy

• The (Ni, Co) 2 Si and Cr nano precipitates uniformly distributed in the alloy provide good strength and conductivity. • Excellent ductility is provided because the small lattice mismatch strain between the precipitates and the Cu matrix. • The introduction of deformation twins improves the strength and ductility of the alloy. In this paper, a novel Cu-1.5Ni-1.1Co-0.6Si-0.1Cr (wt.%) alloy with high strength and electrical conductivity was designed. After aging, excellent properties of 857±12 MPa yield strength, 300±8 HV microhardness, 42.8 ± 2.5% IACS conductivity, and 7 ± 0.5% elongation were obtained. According to the atomic structure, part of Ni atoms in Ni 2 Si can be replaced by Co atoms to form nano-precipitates (Ni, Co) 2 Si. The alloy's high strength and conductivity are mainly attributed to the fine and uniformly distributed (Ni, Co) 2 Si and Cr nano precipitates. The alloy strength was also enhanced by twins, dislocations, and grain refining strengthening. Based on the investigations of deformation microstructure and the orientation relationship between the (Ni, Co) 2 Si precipitates and the Cu matrix, the main reason for elongation increase is attributed to the formation of deformation twins and the small lattice mismatch strain at the coherent interfaces of precipitates and the Cu matrix.

Ferroelectric Domain Structures in Monoclinic (K0.5Na0.5)NbO3 Epitaxial Thin Films

Ferroelectric domain walls (DWs) are an important feature of ferroelectric materials and play a vital role in the ferroelectric materials, where charged DWs are a focus of extensive research due to their unique interfacial properties. Herein, epitaxial (K,Na)NbO3 (KNN)‐based thin films are prepared by a sol–gel method, and the monoclinic MC structure of the thin film is confirmed. Stripe‐shaped domain patterns with head‐to‐head and tail‐to‐tail charged DWs in the (110)pc and (1¯10)pc planes are observed in the KNN‐based epitaxial thin film. Although the screening of the bound charges in the charged DWs inevitably increases the energy of the system, the charged DWs in the KNN‐based epitaxial thin film are stabilized by the lattice mismatch strain between the thin film and the substrate. These interesting findings should shed light on the exploration of the functional charged DWs by strain engineering.

Defect structures in (001) zincblende GaN/3C-SiC nucleation layers

The defect structure of zincblende GaN nucleation layers grown by metalorganic vapor-phase epitaxy on 3C-SiC/Si (001) was investigated by high-resolution scanning transmission electron microscopy. Perfect dislocations, partial dislocations, and stacking faults are present in the layers. Perfect dislocations are identified as 60° mixed-type and act as misfit dislocations to relieve the compressive lattice mismatch strain in GaN. Stacking faults are mainly bounded by 30° Shockley partial dislocations and rarely by Lomer–Cottrell partial dislocations, both of which are able to relieve the compressive lattice mismatch strain in the layer. We propose that the stacking faults and their partial dislocations originate from the dissociation of perfect dislocations present in the zincblende GaN layer and by direct nucleation of partial dislocations loops from the surface. These are the two main mechanisms that lead to the final defect structure of the zincblende GaN nucleation layers.