The 50 most-cited articles in Journal of Applied Physics, 2024

Molecular dynamics (MD) simulations play an important role in understanding and engineering heat transport properties of complex materials. An essential requirement for reliably predicting heat transport properties is the use of accurate and efficient interatomic potentials. Recently, machine-learned potentials (MLPs) have shown great promise in providing the required accuracy for a broad range of materials. In this mini-review and tutorial, we delve into the fundamentals of heat transport, explore pertinent MD simulation methods, and survey the applications of MLPs in MD simulations of heat transport. Furthermore, we provide a step-by-step tutorial on developing MLPs for highly efficient and predictive heat transport simulations, utilizing the neuroevolution potentials as implemented in the GPUMD package. Our aim with this mini-review and tutorial is to empower researchers with valuable insights into cutting-edge methodologies that can significantly enhance the accuracy and efficiency of MD simulations for heat transport studies.

Interconnect resistance and reliability have emerged as critical factors limiting the performance of advanced CMOS circuits. With the slowdown of transistor scaling, interconnect scaling has become the primary driver of continued circuit miniaturization. The associated scaling challenges for interconnects are expected to further intensify in future CMOS technology nodes. As interconnect dimensions approach the 10 nm scale, the limitations of conventional Cu dual-damascene metallization are becoming increasingly difficult to overcome, spurring over a decade of focused research into alternative metallization schemes. The selection of alternative metals is a highly complex process, requiring consideration of multiple criteria, including resistivity at reduced dimensions, reliability, thermal performance, process technology readiness, and sustainability. This Tutorial introduces the fundamental criteria for benchmarking and selecting alternative metals and reviews the current state of the art in this field. It covers materials nearing adoption in high-volume manufacturing, materials…

Magnetic characteristics, magnetocaloric effect, and critical behavior of Nd1−xSrxMnO3 compounds by Sr doping (x = 0.2, 0.3, 0.4, 0.5) were studied. All samples maintained orthorhombic structures, but the space group changed from Pnma (No. 62) for x = 0.2, 0.3 to Imma (No. 74) for x = 0.4, 0.5. As Sr doping increased, the Curie temperature (TC), Curie–Weiss temperature (TCW), and magnetization increased, attributed to the double exchange (DE) interaction. A discrepancy between TCW and TC was observed due to the competition between polarons and DE interaction. The critical behavior was investigated systematically using the self-consistent (modified Arrott plots, MAP) method and the Kouvel–Fisher (KF) relation. The KF relation was suitable for the samples with x = 0.2 and 0.5, while the MAP method was suitable for the samples with x = 0.3 and 0.4. Among the Ising, XY, Heisenberg, and mean-field models, the samples with x = 0.2, 0.3, and 0.4 aligned more closely with the mean-field model, except for the x = 0.5 sample. Entropy change (−ΔSM) of Nd1−xSrxMnO3 (0.2 ≤ x ≤ 0.5) increased…

Based on the transport and polarization relaxation theories, the effects of hydrogen, fluorine, and chlorine atom passivation on the conductivity and dielectric properties of silicon carbide nanowires (SiCNWs) were numerically simulated. The results show that passivation can decrease the dark conductivity of SiCNWs and increase its ultraviolet photoconductivity. Among them, the photoconductivity of univalent (H) passivated SiCNWs is better than that of seven-valent (Cl, F) passivated SiCNWs. In terms of dielectric properties, the passivated SiCNWs exhibit a strong dielectric response in both deep ultraviolet and microwave regions. Hydrogen passivation SiCNWs produce the strongest dielectric response in deep ultraviolet, while fluorine passivation SiCNWs produce the strongest dielectric relaxation in the microwave band, which indicates that atomic passivation SiCNWs have a wide range of applications in ultraviolet optoelectronic devices and microwave absorption and shielding.

We present an atomistic simulation of thermoelectric properties in cove-edged graphene nanoribbons (CGNRs) via the nonequilibrium Green's function. Different from gapless zigzag graphene nanoribbons (ZGNRs), CGNRs exhibit a noticeable bandgap. Such a bandgap can be modulated by varying three structural parameters (namely, the width N, the distance between adjacent coves m, as well as the shortest offset n) of CGNRs, which can give rise to the transition from semiconducting to semi-metallic. Due to the less dispersive phonon bands and the decrease in the number of phonon channels of CGNRs, they are found to have the lower phonon thermal conductance than ZGNRs. Modulation of CGNRs can produce over tenfold improvement of the maximum of ZT compared to ZGNRs. This improvement is due to the promotion of the Seebeck coefficient together with the degradation of the phonon thermal conductance of CGNRs compared to ZGNRs.

Temporal coupled-mode theory (CMT) is an acclaimed and widely used theoretical framework for modeling the continuous-wave response and temporal dynamics of any integrated or free-space photonic resonant structure. It was initially employed to understand how energy is coupled into and out of a cavity and how it is exchanged between different resonant modes. In the 30 years that followed its establishment, CMT has been expanded to describe a broad range of nonlinear interactions as well (self- and cross-phase modulation, saturable absorption, frequency generation, gain, etc.). In this Tutorial, we thoroughly present the basic principles and the evolution of CMT throughout the years, showcasing its immense capabilities for the analysis and design of linear and nonlinear resonant photonic systems. Importantly, we focus on the examples of modern, open nanophotonic resonators incorporating contemporary bulk or sheet (2D) materials that may be lossy and dispersive. For each linear/nonlinear effect under study, we follow a meticulous, step-by-step approach, starting from an accurate model…

As investigations in the biomedical applications of plasma advance, a demand for describing safe and efficacious delivery of plasma is emerging. It is quite clear that not all plasmas are “equal” for all applications. This Perspective discusses limitations of the existing parameters used to define plasma in context of the need for the “right plasma” at the “right dose” for each “disease system.” The validity of results extrapolated from in vitro studies to preclinical and clinical applications is discussed. We make a case for studying the whole system as a single unit, in situ. Furthermore, we argue that while plasma-generated chemical species are the proposed key effectors in biological systems, the contribution of physical effectors (electric fields, surface charging, dielectric properties of target, changes in gap electric fields, etc.) must not be ignored.

Epoxy dielectrics with high through-plane thermal conductivity (λ) hold great promise for applications in the thermal management of advanced power electronics. Intensive attempts have been made to improve the λ of epoxy by filling with boron nitride nanosheets (BNNSs). However, it remains a great challenge to achieve a satisfactory increased λ by a small amount of BNNS loading. Herein, we reported a new strategy to prepare epoxy dielectrics with internal three-dimensional phonon transport channels by vacuum freeze-drying and vacuum impregnation. Aramid nanofibers (ANFs) and BNNSs were used for the collaborative construction of a vertical interconnected thermal framework. The resultant ANF-BNNS/epoxy achieved a high through-plane λ of 0.87 W m−1 K−1 at only 1.43 vol. % BNNS, which is ∼17.1% higher than that of the BNNS/epoxy counterpart with even 18.34 vol. % randomly distributed BNNS. The increasing efficiency of λ of epoxy by ANF-BNNS is tenfold more than that of the conventional blending methods. In addition, the ANF-BNNS/epoxy composite also exhibits a low dielectric constant and…

Two-dimensional (2D) transition metal dichalcogenides (TMDs) hold immense promise as ultrathin-body semiconductors for cutting-edge electronics and optoelectronics. In particular, their sustained charge mobility even at atomic-level thickness as well as their absence of surface dangling bonds, versatile band structures, and silicon-compatibility integration make them a prime candidate for device applications in both academic and industrial domains. Despite such high expectations, group-VI TMDs reportedly exhibit a range of enigmatic properties, such as substantial contact resistance, Fermi level pinning, and limited unipolar charge transport, which are all rooted in their inherent defects. In other words, intrinsic physical properties resulting from their native defects extend their influence beyond the material level. Bridging point-defect-induced material properties and their behavior at the device level, this Perspective sheds light on the significance of crystalline defects beyond a rather simple defect–property relationship. As a distinctive approach, we briefly review the…

The main target of this article is to analyze the role of activation energy and thermal radiation effects on the bioconvective peristaltic transport of Sutterby nanofluid in a two-dimensional flexible porous channel with heat and mass transfer. Also, the consequences of Hall current, heat source, and complaint wall properties along with an inclined magnetic field are taken into consideration. The proposed system of governing equations is simplified by using lubrication approximation and solved numerically using MATLAB's bvp5c solver. Further, numerical observations are analyzed to figure out the consequence of different physical parameters on the flow characteristics. According to the observations, it is identified that the Sutterby nanofluid velocity declines with the climb in the damping force parameter, while it enhances with the upsurge in the Darcy number. The Sutterby fluid temperature profile strengthens when the influence of the heat generation and Brinkman number increase, while it depicts the reverse effect with the elevation in the fluid parameter and radiation parameter.…

C4F7N [2,3,3,3-tetrafluoro-2-(trifluoromethyl)propanenitrile]/CO2 gas mixtures are being developed as an eco-friendly electrical insulator to replace SF6, the most potent greenhouse industrial gaseous dielectric. However, recent studies have reported complicated and often conflicting decomposition pathways for C4F7N/CO2 gas mixtures, which has raised concerns. In this work, the decomposition characteristics of C4F7N/CO2 gas mixtures were studied comprehensively by both designed computations and experiments. Computations were performed starting from fundamental propositions of C4F7N/CO2 decompositions, which were further experimentally verified by pyrolysis, long-term thermal aging with/without catalytic materials (industrial-grade molecular sieves 4A), and electrical decomposition by spark discharge. The results of both computations and experiments suggest that in an ideal thermal decomposition, C4F7N is likely to decompose into C2F6 and small fluoronitriles first at high temperatures. The generation of C3F6 and C2N2 from C4F7N thermal decomposition at lower temperatures appears…

The ultra-wide bandgap of gallium oxide provides a rich plethora of electrically active defects. Understanding and controlling such defects is of crucial importance in mature device processing. Deep-level transient spectroscopy is one of the most sensitive techniques for measuring electrically active defects in semiconductors and, hence, a key technique for progress toward gallium oxide-based components, including Schottky barrier diodes and field-effect transistors. However, deep-level transient spectroscopy does not provide chemical or configurational information about the defect signature and must, therefore, be combined with other experimental techniques or theoretical modeling to gain a deeper understanding of the defect physics. Here, we discuss the current status regarding the identification of electrically active defects in beta-phase gallium oxide, as observed by deep-level transient spectroscopy and supported by first-principles defect calculations based on the density functional theory. We also discuss the coordinated use of the experiment and theory as a powerful…

Sc0.18Al0.82N/GaN with state-of-the-art x-ray diffraction figures of merit grown by metal modulated epitaxy under metal-rich conditions and a low substrate temperature of 400 °C is demonstrated to have improved crystalline order [250 arc sec for the (0002) reflection and 469 arc sec for the (101¯5)] compared to a previous state-of-the-art sample grown at a more conventional temperature of 650 °C. While both samples show a columnar structure, the higher substrate temperature sample has a good symmetric rocking curve (RC) of 229 arc sec, but unlike the lower temperature sample, the RC of the (101¯5) asymmetric reflection could not be measured, indicating a more columnar structure common among ScAlN films. Local lattice constant maps (LLCMs) from 4D-STEM depict abrupt strain relaxation within ∼2 nm from the ScAlN/GaN interface for the sample grown at Tsub = 400 °C. Since these LLCMs suggest a lattice mismatch in the a-lattice constant, and since the films show a sudden roughening, the composition for lattice match to GaN may be less than the accepted 18%–20% Sc, consistent with the…

In this work, we demonstrate an in situ etch technique for β-Ga2O3 inside a metalorganic chemical vapor deposition (MOCVD) reactor using triethylgallium (TEGa) as the etching agent. At sufficiently high substrate temperatures (Tsub), TEGa is introduced into the MOCVD reactor which undergoes pyrolysis, resulting in the deposition of Ga on the β-Ga2O3 surface. These Ga adatoms react with Ga2O3 to form gallium suboxide (Ga2O), which desorbs from the β-Ga2O3 surface resulting in the etching of the epilayer. MOCVD chamber parameters such as TEGa molar flow rate, substrate temperature, and chamber pressure were shown to be key in controlling the etch rate and surface morphology. A wide range of etch rates from ∼0.3 to 8.5 μm/h is demonstrated by varying the etch parameters. In addition, smooth surface morphology on (010) and (001) β-Ga2O3 substrates is also demonstrated. This new etch technique could enable damage free fabrication of 3D structures like fins and trenches, which are key components in many β-Ga2O3 device structures.

This Perspective focuses on experimental and theoretical aspects of positron annihilation spectroscopy. This set of methods is highly suitable for identifying and quantifying vacancy-type defects in semiconductors and also allows for analyzing their physics characteristics. We present selected examples from the past decade, where the methods have been used for obtaining timely and useful insights into the defect-controlled phenomenon in narrow-gap (Ge, GaSb) and wide-gap (III-nitride, oxide) semiconductors. We also discuss possible future developments that may allow more detailed studies in novel semiconductor materials and devices with ever more complex lattice structures.

A systematic investigation of the electrical characteristics of β-Ga2O3 Schottky barrier diodes (SBDs) has been conducted under high-dose 60Co gamma radiation, with total cumulative doses reaching up to 5 Mrad (Si). Initial exposure of the diodes to 1 Mrad resulted in a significant decrease in on-current and an increase in on-resistance compared to the pre-radiation condition, likely due to the generation of radiation-induced deep-level acceptor traps. However, upon exposure to higher gamma radiation doses of 3 and 5 Mrad, a partial recovery of the device performance occurred, attributed to a radiation annealing effect. Capacitance–voltage (C–V) measurements showed a decrease in net carrier concentration in the β-Ga2O3 drift layer, from ∼3.20 × 1016 to ∼3.05 × 1016 cm−3, after 5 Mrad irradiation. Temperature-dependent I–V characteristics showed that 5 Mrad irradiation leads to a reduction in both forward and reverse currents across all investigated temperatures ranging from 25 to 250 °C, accompanied by slight increases in on-resistance, ideality factors, and Schottky barrier…

This paper investigates wake-up and imprint in ferroelectric Al0.70Sc0.30N films. The study employs a series of electrical measurements with varying field amplitudes and waveforms to understand the origin and underlying principle of wake-up and imprint as well as their relation. It is shown that the material can be considered wake-up free. However, inherent imprint and imprint variation of the polarization-electric field hysteresis in combination with minor loops result in a wake-up like effect. This effect is most likely related to the formation of persistent inversion domain nuclei—a mechanism that is discussed in detail and compared with alternative explanations based on charged defects. Investigations using unipolar fields are conducted to explore the reversibility of imprint and ways to program it, while partial switching is applied to investigate domain propagation and support the aforementioned explanation for the evolution of the polarization-electric field hysteresis. It is concluded that after an energetically more demanding domain nucleation, domain wall motion can switch…

We report the electrical properties, deep trap spectra, and diffusion lengths of non-equilibrium carriers in Ni Schottky diodes and NiO/Ga2O3 heterojunctions (HJs) prepared on the same n−/n+ β-Ga2O3 epi structures. The heterojunctions decrease the reverse current of Ga2O3 high-power rectifiers. In HJs, in contrast to Schottky diodes, the capacitance and AC conductance show a prominent frequency and temperature dependence, suggesting the presence of two temperature activation processes with activation energies of 0.17 and 0.1 eV. The deep trap spectra of the Schottky diodes and HJs differ by the absence in the HJ of deep electron traps E2* with level near Ec − 0.7 eV considered to be an important center of non-radiative recombination. This correlates with the observed increase in the diffusion length of non-equilibrium charge carriers in the HJs to 370 nm compared to 240 nm in the Schottky diodes. The diffusion length of charge carriers in p-NiO was found to be quite short, 30 nm. Possible reasons for the observed differences and possible origin of the minority-trap-like feature…

Magnetoelectric (ME) effects in a ferromagnetic and piezoelectric composite are the changes in the polarization caused by a magnetic field or the changes in the magnetization caused by an electric field. These effects are aided by the mechanical deformation in the ferroic phases caused by the combination of magnetostriction and piezoelectricity. Interest in ME effects is due to a variety of physical phenomena they exhibit, as well as their potential applications in the creation of highly sensitive magnetic field sensors and other electronic devices. Linear ME effects in structures with layers of different ferroic materials have been studied extensively. However, nonlinear ME effects, which are caused by the nonlinearity of the magnetic, dielectric, and acoustic properties of ferromagnets and piezoelectrics, are less well understood. The purpose of this review is to summarize the current state of knowledge on nonlinear ME (NLME) effects in composite heterostructures and to discuss their potential applications. The review begins by discussing the characteristics of materials that are…

Compared with solid solution (SS) Hf0.5Zr0.5O2, HfO2–ZrO2 superlattice (SL) ferroelectric films exhibit enhanced endurance and reduced leakage. However, so far, the underlying physical mechanism is still missing. In this work, first-principle calculations reveal that the superior reliability arises from the precise control in the spatial profile of oxygen vacancy with the stacked structure in SL in which oxygen vacancies (Vo) exhibit different formation energies and different energy profiles in Zr- and Hf-layers. However, Vo have the tendency to migrate and its suppression become critical to maintain the superiority of SL. A low annealing temperature is, therefore, suggested and validated with our fabricated SL- and SS-ferroelectric films with different annealing temperatures. This understanding can pave ways for the optimization in SL ferroelectric films.

Optimizing thermal anneals of Si-implanted β-Ga2O3 is critical for low resistance contacts and selective area doping. We report the impact of annealing ambient, temperature, and time on the activation of room temperature ion-implanted Si in β-Ga2O3 at concentrations from 5 × 1018 to 1 × 1020 cm−3, demonstrating full activation (>80% activation, mobilities >70 cm2/V s) with contact resistances below 0.29 Ω mm. Homoepitaxial β-Ga2O3 films, grown by plasma-assisted molecular beam epitaxy on Fe-doped (010) substrates, were implanted at multiple energies to yield 100 nm box profiles of 5 × 1018, 5 × 1019, and 1 × 1020 cm−3. Anneals were performed in an ultra-high vacuum-compatible quartz furnace at 1 bar with well-controlled gas compositions. To maintain β-Ga2O3 stability, pO2 must be greater than 10−9 bar. Anneals up to pO2 = 1 bar achieve full activation at 5 × 1018 cm−3, while 5 × 1019 cm−3 must be annealed with pO2 ≤ 10−4 bar, and 1 × 1020 cm−3 requires pO2 < 10−6 bar. Water vapor prevents activation and must be maintained below 10−8 bar. Activation is achieved for anneal…

The present study probes the effect of cation distributions in the structural, optical, electronic, and magnetic properties of mixed-valent inverse-spinel NiCo2O4 (NCO) nanoparticles (NPs). NCO NPs were prepared using the sol–gel combustion method and the grain size was obtained in the magnetic exchange length range assumed to be from single-ion anisotropy. The Raman and photoluminescence spectroscopies confirm the presence of an inverse-spinel structure with different oxidation states, and vibrating sample magnetometry clarifies the existence of ferromagnetism with the presence of magnetic anisotropy among the cations. These NPs annealed at a higher grain-growth temperature accumulate ferrimagnetic properties and produce magneto-crystalline anisotropy making NCO an assuring material for spintronic applications. A detailed x-ray absorption spectroscopy and x-ray magnetic circular dichroism studies reveal an indestructible correlation between the distribution of the present cations and the element-specific origin of ferrimagnetic behavior. Ni is found to be accountable for the…

This paper presents BubbleID, a sophisticated deep learning architecture designed to comprehensively identify both static and dynamic attributes of bubbles within sequences of boiling images. By amalgamating segmentation powered by Mask R-CNN with SORT-based tracking techniques, the framework is capable of analyzing each bubble's location, dimensions, interface shape, and velocity over its lifetime and capturing dynamic events such as bubble departure. BubbleID is trained and tested on boiling images across diverse heater surfaces and operational settings. This paper also offers a comparative analysis of bubble interface dynamics prior to and post-critical heat flux conditions.

The Rashba spin–orbit coupling effect, primarily arising from structural-inversion asymmetry in periodic crystals, has garnered considerable attention due to its tunability and potential applications in spintronics. Its capability to manipulate electron spin without an external magnetic field opens new avenues for spintronic device design, particularly in semiconductor technology. Within this framework, 2D Rashba materials hold special interest due to their inherent characteristics, which facilitate miniaturization and engineering capabilities. In this Perspective article, we provide an overview of recent advancements in the research of 2D Rashba materials, aiming to offer a comprehensive understanding of the diverse manifestations and multifaceted implications of the Rashba effect in material science. Rather than merely presenting a list of materials, our approach involves synthesizing various viewpoints, assessing current trends, and addressing challenges within the field. Our objective is to bridge the gap between fundamental research and practical applications by correlating…

Five high entropy carbide ceramics, (Hf0.2,Nb0.2,Ta0.2,Ti0.2,Zr0.2)C, (Cr0.2,Hf0.2,Ta0.2,Ti0.2,Zr0.2)C, (Hf0.2,Mo0.2,Ta0.2,Ti0.2,Zr0.2)C, (Hf0.2,Ta0.2,Ti0.2,W0.2,Zr0.2)C, and (Hf0.2,Mo0.2,Ti0.2,W0.2,Zr0.2)C, were synthesized by carbothermal reduction of oxides and direct current sintering. The five high entropy carbide ceramics were determined to be nominally phase-pure with relative densities of more than 98.9% and mean grain sizes of less than 5 μm. Average Vickers hardness values ranged from 19.2 ± 0.4 GPa for (Hf0.2,Nb0.2,Ta0.2,Ti0.2,Zr0.2)C at a load of 2 kgf to 43.5 ± 0.4 GPa for (Hf0.2,Mo0.2,Ti0.2,W0.2,Zr0.2)C at a load of 0.05 kgf. Hardness generally increased with increasing the valence electron concentration and strain as measured by the Williamson–Hall analysis. However, neither correlation was conclusive enough to be a clear indicator of hardness. Instead, it was determined that a combination of effects that includes the valence electron concentration, lattice strain, and grain size all contribute to the hardness of high entropy carbide ceramics.

We demonstrate magnetocaloric entropy change and compensation temperatures in ferrimagnetic Gdx(Fe10Co90)100−x amorphous thin films with transition metal-rich and rare earth-rich configurations. Thin films are sputtered with same Gd/FeCo elemental ratio at different thicknesses and of various Gd/FeCo ratios at a constant thickness to understand the effect of these two parameters on an antiferromagnetically coupled magnetic sub-lattice system. Temperature- and field-dependent magnetic measurements [M(H,T)] and magnetocaloric studies are performed over a broad range of temperature (70–600 K) by applying a magnetic field of ±15 kOe on sputter deposited 90 nm thin films of Gdx(Fe10Co90)1−x(x = 30,40,50,55,70). The compensation temperature is found to increase with increasing Gd concentration for thin films of the same thickness. A high magnetocaloric entropy change around 0.97 J/kg K (ΔH = ± 15 kOe) is observed for thin films having the same Gd/FeCo elemental ratio. Furthermore, we observed a “table-like” magnetocaloric entropy change in GdFeCo thin film stacks with a high operational…

Many researchers observed enhanced water flow through carbon nanotubes (CNTs) and attributed the reason to large slips. Even after taking significant slip effects into account, there remain unaddressed observations of significant improvements in flow rates. As CNTS are deformable, we represent nanotubes with a deformable-wall using a linear pressure–area relationship. We assume lubrication assumption, and using the properties of nanoconfined water, we derive the model for deformable-nanotubes. We validated our derived model in its limiting cases with the previously reported results in the literature. We compare the predictions by our deformable-wall and rigid-wall model with the experimental results and the MD-simulation predictions by multiple literature studies. Many studies were well-predicted by the rigid-wall model with slips. However, we find that there are many studies with high porosity and thin wall tubes, where elasticity or deformability of the tube is essential in modeling, which is well-predicted by our deformable-wall model with slips. In our study, we focus on…

Although K0.5Na0.5NbO3 (KNN) possesses large maximum polarization and relatively high breakdown strength, the large remnant polarization constrains their practical applications as energy storage materials. In this work, through multi-element doping, (K0.5−0.5xNa0.5−0.5xBix)(Nb1−xSn0.2xZn0.6xTa0.2x)O3 relaxor ferroelectrics were prepared. As the superparaelectric states (SPE) were adjusted to room temperature, orthorhombic, tetragonal, and cubic phases coexisted, accompanied by the highly dynamic polar nanoregions (PNRs). In particular, a high recoverable energy storage density of 4.5 J/cm3 and an energy storage efficiency of 83% were achieved for the x = 0.125 ceramic, with the variations less than 11% and 4%, respectively, in the wide temperature range of 20–180 °C. These results demonstrate that the multiphase PNRs in the SPE state is an effective strategy for improving the energy storage performance of KNN-based ceramics.

Over 60 years of studying morphological stability under fundamental ideas of William Wilson Mullins and Robert Floyd Sekerka [J. Appl. Phys. 34, 323 (1963) and J. Appl. Phys. 35, 444 (1964)] it has become possible to explain the origin and selection of surface structures from planar to cellular, dendritic, and fractal patterns. The Mullins–Sekerka (MS) morphological instability theory provides a condition for stability or reconstruction of interfaces, which separates the phases during phase transformation. The MS-theory has come a long way in the conceptual understanding of the incipience of morphological instability and the formation of structures, although today, certain aspects of this theory continue to be discussed at the fundamental and quantitative level of its interpretation. In the sixtieth anniversary of this theory, we re-examine the MS-analysis under boundary conditions satisfying the smooth existence of temperature and its gradients in directional crystallization of a binary melt. These boundary conditions are dependent on the finite distance from the solidification…

We performed Ge doping of α-Ga2O3 thin films grown on m-plane sapphire substrates using mist chemical vapor deposition. Although the typical growth rate was high at 4 μm/h, the resultant α-Ga2O3 thin films exhibited high crystallinity. We controlled the carrier density in the range of 8.2 × 1016–1.6 × 1019 cm−3 using bis[2-carboxyethylgermanium(IV)]sesquioxide as the Ge source. The highest mobility achieved was 66 cm2 V−1 s−1 at a carrier concentration of 6.3 × 1017 cm−3. Through secondary ion mass spectrometry analysis, a linear relationship between the Ge concentration in the α-Ga2O3 thin films and the molar ratio of Ge to Ga in the source solution was established. The quasi-vertical Schottky barrier diode fabricated using the Ge-doped α-Ga2O3 thin films exhibited an on-resistance of 7.6 mΩ cm2 and a rectification ratio of 1010. These results highlight the good performance of the fabricated device and the significant potential of Ge-doped α-Ga2O3 for power-device applications.

In this work, the value and the polarity of the spontaneous and piezoelectric polarization have been investigated, as the use of two different reference structures for wurtzite-type group-III nitrides, namely, the zinc-blende and the layered-hexagonal crystal lattice, have resulted in different predictions. It was found that although the differences in value and polarity of the polarization for heterostructures such as wurtzite Al1−xScxN/GaN lead to similar interface sheet charges, a significant mismatch is observed when polarization reversal is considered. The interface sheet charge predicted before and after the polarization reversal in the wurtzite Al1−xScxN layer on GaN using the zinc-blende lattice as a reference predominantly shows a change in sign. When using the layered-hexagonal lattice as a reference, not only is the same polarity of the interface sheet charge maintained after polarization reversal, but it is even 30 times larger. In this case, the giant and positive spontaneous polarization values for metal-polar Al1−xScxN extracted from the ferroelectric switching, as…

It is a novel-effective process for realizing high-efficiency sensing and continuous gas monitoring by introducing precious metals into metal–oxide–semiconductors (MOSs). In this study, Ag is exploited to prepare surface functionalized SnO2 nanoparticles (NPs) and innovative xAg@SnO2/CsPbBr3, activating and catalyzing the gas sensing reactions on semiconductors. The results show that the precious metal Ag NPs promote the directional transport of carriers, thus improving the gas sensing performances. In addition, innovative xAg@SnO2/CsPbBr3 composites originated from Ag@SnO2 NPs and 3-mercaptopropionic acid treated all-inorganic perovskite CsPbBr3 are constructed to further accelerate electron transfer on heterointerfaces, enabling continuous and efficient monitoring of ethanolamine (EA) at room temperature. The sensing properties of Ag@SnO2/CsPbBr3 on various volatile organic compounds are investigated. Compared with pure CsPbBr3, the EA response of as-prepared 2Ag@SnO2/CsPbBr3 is obviously improved by about sevenfold. The response/recovery time is greatly shortened, besides the…

In this work, we explore the physics and evaluate the merits of strain engineering in two-dimensional van der Waals semiconductor-based FETs (field-effect-transistors) using DFT (density functional theory) to determine the modulation of the channel material properties under strain, and subsequently, their effect on carrier transport properties, i.e., scattering rates, mobility, and then finally simulate and analyze dissipative current transport with a non-equilibrium Green's function–Poisson's equation self-consistent solver. The scattering model includes the effects of charged impurities, intrinsic phonons, and remote phonons as well as the screening effect due to charged carriers. Impact of strain engineering on contact resistance is also incorporated into the transport simulations to determine the potential performance enhancements using strain in practical devices. Based on the comprehensive simulation results, we identify the materials and strain configuration that provide the best improvement in performance. We demonstrate an ON-current gain of 43.3% in a biaxially…

[001]-textured 0.4P(Mg1/3Nb2/3)O3-0.25PbZrO3-0.35PbTiO3-0.5%MnO2 (Mn-PMN-PZT) ceramics were fabricated by templated grain growth using 2 vol. % BaTiO3 in this paper. Full matrices of dielectric (ɛij), elastic (sij, cij), and piezoelectric (dij) parameters were obtained by the resonance–antiresonance method. The dielectric constant ɛ33T of textured ceramics reaches 2600, which is four times that of random ceramics. Textured Mn-PMN-PZT ceramics exhibit high d33 = 984 pC/N and high k33 = 0.89, which is much larger than d33 = 223 pC/N and k33 = 0.70 of random ceramics. However, ɛ11T of ceramics decreases by about 30% after texturing, and the corresponding shear coupling coefficient k15 also decreases from 0.66 to 0.44, which may be due to the reduction in the angle between spontaneous polarization and transverse direction. Furthermore, the temperature stability of the textured ceramics was evaluated as well. The phase transition temperature TR−T was determined by the impedance method to be 120 °C. The textured Mn-PMN-PZT ceramic shows high temperature stability, which is better than…

In this research, we utilized density functional theory (DFT) computations to perform ab initio molecular dynamics simulations and static calculations on graphene, hexagonal boron nitride, and their heterostructures, subjecting them to strains, perturbations, twist angles, and defects. The gathered energy, force, and virial information informed the creation of a training set comprising 1253 structures. Employing the Neural Evolutionary Potential framework integrated into Graphics Processing Units Molecular Dynamics, we fitted a machine learning potential (MLP) that closely mirrored the DFT potential energy surface. Rigorous validation of lattice constants and phonon dispersion relations confirmed the precision and dependability of the MLP, establishing a solid foundation for subsequent thermal transport investigations. A further analysis of the impact of twist angles uncovered a significant reduction in thermal conductivity, particularly notable in heterostructures with a decline exceeding 35%. The reduction in thermal conductivity primarily stems from the twist angle-induced…

Electron emission and transport through and over potential barriers is an essential process requiring modeling and simulation to meet the design needs and characterization of an exceedingly broad range of technologically important devices and processes. The simulation and description of thermal, field, and photoemission, and the related concerns of space–charge affected electron flow, often make use of specialized formulations developed in the early days of quantum mechanics. Advancements in the utilization of electron sources and particularly the simulation of devices and applications using advanced particle-in-cell and trajectory methods for beam optics codes create a strong need for a pedagogical account of the emission models to ensure correct numerical evaluation of their equations. This Tutorial starts from simple phenomenological accounts and progressively builds to comprehensive models emphasizing straightforward and often rapid calculation. It recommends formulations to supplant the canonical Richardson–Laue–Dushman (thermal), Fowler–Nordheim (field), Fowler–DuBridge…

Ultrawide bandgap ZnGa2O4 (ZGO) thin films were grown on sapphire (0001) substrates at various growth temperatures with a perspective to investigate the electrical and optical characteristics required for high-power electronic applications. Due to the variation in the vapor pressure of Zn and Ga, severe loss of Zn was observed during pulsed laser deposition, which was solved by using a zinc-rich Zn0.98Ga0.02O target. A pure phase single-crystalline ZGO thin film was obtained at a deposition temperature of 750 °C and an oxygen pressure of 1 × 10−2 Torr. The out-of-plane epitaxial relationship between the sapphire and ZGO thin film was obtained from φ-scan. The x-ray rocking curve of the ZGO thin film grown at 750 °C exhibits a full width at half maximum of ∼0.098°, which indicates a good crystalline phase and quality of the thin film. Core-level x-ray photoelectron spectroscopy of ZGO grown at 750 °C indicated that Zn and Ga were in the 2+ and 3+ oxidation states, respectively, and the atomic ratio of Zn/Ga was estimated to be ∼0.48 from the fitted values of Zn-2p3/2 and Ga-2p3/2.…

Temperature-dependent Raman spectral studies of BiFeO3 (BFO) films coated on three different substrates, viz., conducting Si (BFO-Si), sapphire (BFO-SAP), and fluorine-doped tin oxide (BFO-FTO), are reported between 123 and 773 K. The activity of Bi–O and Fe–O modes in these samples as a function of temperature shows different spectral features despite having synthesized from the same precursor. To understand the source of these variations, the spectra obtained on the above films were compared with those of bulk BiFeO3 (BFO-bulk) prepared via spark-plasma sintering. As the temperature increases, modes corresponding to the Bi–O activity at low frequency (120–180 cm−1) exhibit a redshift in their positions in all the samples. Between 350 and 550 K, BFO-SAP and BFO-Si samples show discernible anomalies in the positions of modes corresponding to the Fe–O activity (200–500 cm−1), which is not observed in the BFO-bulk and BFO-FTO samples. These anomalies are more pronounced for the modes between 350 and 500 cm−1, suggesting alterations in the Néel transition temperature (∼643 K for…

The experimental and theoretical bandgap reported for stoichiometric V2O5, a layered semiconductor of great technological importance, spans a wide range of 1.7–4.8 eV. Using combined photoemission, absorption, and photoluminescence measurements, we show that the fundamental electronic gap of V2O5 is 1.85 eV, which is in close agreement with the value of 1.8 eV predicted by the first density functional theory studies, but is lower than the value of 2.2–2.8 eV obtained from optical absorption and photoemission studies. It is shown that this difference between the fundamental and optical gaps is due to the presence of a surface electron accumulation layer, which results in a Burstein–Moss shift of the Fermi level well into the conduction band. The underlying cause of degenerate electron doping is due to “surface transfer doping” by ambient water molecules as a result of the high electron affinity of the semiconductor.

In terahertz (THz) wireless communication systems, non-diffraction vortex beams carrying an orbital angular momentum (OAM) have attracted extensive attention due to their ability to transmit information over long distances with high capacity. However, existing metasurfaces can only generate a single OAM mode non-diffracting vortex beam at reflection space for circular polarization (CP) incidence, limiting practical applications. To address this issue, we propose and design a polarization-frequency multiplexing transmissive THz metasurface to realize multi-mode non-diffracting vortex beams at linear polarization (LP) incidence. The meta-atom of this metasurface is composed of three anisotropic rectangular metallic structures embedded in vanadium dioxide (VO2) square rings, two circular aperture metallic grid layers, and four dielectric layers. By reasonably designing the size of the metal patch and the state of VO2, the designed metasurface can achieve polarization multiplexing and frequency multiplexing for LP incidence. Based on the phase response of the proposed meta-atoms, the…

The integration of a composite of porous medium with phase change material (PCM) offers significant advantages in thermal management systems, enhancing heat transfer efficiency and addressing various thermal regulation challenges. This approach utilizes the PCM's latent heat absorption and the enhanced thermal conductivity provided by the porous medium, resulting in optimized system performance. Its applicability spans across electronics cooling and building insulation systems. However, predicting the thermal behavior of this composite material is challenging, necessitating computational tools to anticipate its response under different conditions and evaluate its influence on cooling strategies. The objective of this study is to create a computational tool specifically tailored to evaluate constitutive parameters of this composite material, thereby providing a comprehensive description of its thermal behavior. To achieve this goal, the multiscale homogenization principle is employed to assess the composite's effective thermophysical material properties using the representative…

Understanding the mechanical and thermodynamic properties of transition-metal dichalcogenides (TMDs) and their heterostructures is pivotal for advancing the development of flexible semiconductor devices, and molecular dynamics (MD) simulation is widely applied to study these properties. However, current uncertainties persist regarding the efficacy of empirical potentials in MD simulations to accurately describe the intricate performance of complex interfaces within heterostructures. This study addresses these challenges by developing an interatomic potential based on deep neural networks and first-principles calculations. Specifically focusing on MoS2/WS2 heterostructures, our approach aims to predict Young's modulus and thermal conductivities. The potential's effectiveness is demonstrated through the validation of structural features, mechanical properties, and thermodynamic characteristics, revealing close alignment with values derived from first-principles calculations. A noteworthy finding is the substantial influence of the load direction on Young's modulus of heterostructures.…

The power-law fluid flow in tree-like self similar branching networks prevalent throughout the natural world, but also finds numerous applications in technology. We investigate an analysis of optimal power-law fluid flow conditions and the optimal structures within tree-like branching networks, focusing on maximizing flow conductance under the constraint of the network tube’s volume and the surface-area. The study considered fully developed laminar power-law fluid flow regimes without considering any losses in the network system. A key observation was the sensitivity of the dimensionless effective flow conductance to the network's geometrical parameters. We found that the maximum flow conductance occurs when a dimensionless radius ratio $\beta^*$ satisfies the equation $\beta^* = N^{-1/3}$, and $\beta^* = N^{-(n+1)/(3n+2)}$ under constrained tube-volume and surface-area, respectively. Here, $N$ represents the bifurcation number of branches splitting at each junction and $n$ is the fluid power-law index. We further find that this optimal condition occur at the equipartition of…

The 2D Bi2Te3 narrow bandgap semiconductor is an outstanding applicant for optoelectronics and thermoelectric devices. The doping of Se into Bi2Te3 makes metal-double chalcogenide more important. In the current investigation, the Se diffusion into the Bi2Te3 film by thermal annealing at different temperatures is probed through a temperature-dependent Raman study along with other characterizations. Upon annealing, the Se/Bi2Te3 films of ∼810 nm thickness resulted in significant changes to their structural, electronic, and optical behavior. The existence of a rhombohedral Bi2Te3 phase was confirmed by structural investigation. The improvement in crystallinity and decrease in lattice strain modified the optical behavior of the films. The morphology analysis showed a slight aggregation at the higher annealed stage. The uniform and homogeneous dispersal and the composition of elements in the film were verified through surface mapping and compositional analysis. The optical investigation revealed a drop in absorbance with increased transmittance. The direct optical bandgap increased from…

In this work, we present a simple design of a compact and miniaturized wideband bandpass filter (BPF) based on substrate integrated waveguide (SIW) and microstrip line. Through the order extension, the filtering effect and the order of the Chebyshev response of the designed BPF can be indirectly controlled by preserving square notches (SNs) on the SIW. By introducing one or two SNs in the SIW structure, the resonance frequencies of different modes can be controlled, thus achieving dual- and tri-mode BPFs with wideband and low insert loss. The working principle of the designed BPFs was illustrated by modal analysis, field distribution, and equivalent circuit theory. Based on this, the dual- and tri-mode BPFs with center frequencies of 6.8 and 7 GHz were designed and demonstrated numerically. Further experimental results indicate that the proposed tri-mode BPF has low insertion loss (0.25 dB), compact size (0.27λg2), and wideband bandwidth (60%). In addition, the tri-mode BPF achieves a broadband out-of-band rejection of 1.5f0 (f0 is the center resonance frequency) below the −10 dB…

This study presents a novel implementation of vanadium dioxide (VO2) phase change material in an electromagnetic (EM) surface with beam steering capabilities. For the first time, we present the design, fabrication, and measurement of a globally actuated, VO2-based beamforming reflective EM surface. We propose a design featuring complementary unit cells that enable beam steering with a single excitation, marking a pioneering use of low-loss VO2 in creating a complementary EM surface for the mmWave band. The unit cells incorporating VO2 exhibit low-loss characteristics, with −0.92 and −1.9 dB in the hot state and −0.49 and −0.25 dB in the cold state, respectively. We incorporate both positive and negative phase variants alongside two temperature-independent phase unit cells. The complementary design enables us to utilize two beam operating states, effectively exploiting the otherwise unused cold state. We experimentally demonstrated beamforming at ±5° and ±10° from the broadside. In the case of ±10°, the measured gains at 35 GHz in the cold and hot states were 19.4 and 20.3 dB,…

Lennard-Jones potential parameters have been determined for the interlayered interaction in 12 elemental layered crystals. The layered crystals include corrugated borophene, buckled honeycomb structures with elements from the group IV (Si, Ge, and Sn) and group V (P, As, Sb, and Bi), as well as four puckered structures in the aw-phase (Sb and Bi) and sw-phase (P and As). We first calculated the minimal interfacial energy and the interlayer equilibrium distance for two typical shifted stackings, AA and AB, for every bilayer crystal using the density functional theory with a long-range dispersion correction. Then, we use the Lennard-Jones potential and the corresponding derivative to graphically match the calculated adhesion energy and the zero resultant force along the out-of-plane direction, determining the energy parameter and the size parameter for both stackings. The Lennard-Jones model with the obtained parameters can be in good agreement with the first-principles calculations for the AA and AB stackings in describing the relationship between the interfacial energy and the…

III-nitrides have fomented a revolution in the lighting industry and are poised to make a huge impact in the field of power electronics. In the III-nitride family, the crystal growth and use of hexagonal BN (h-BN) as an ultrawide bandgap (UWBG) semiconductor are much less developed. Bulk crystals of h-BN produced by the high-temperature/high-pressure and the metal flux solution methods possess very high crystalline and optical qualities but are impractical to serve as substrates or for device implementation as their sizes are typically in millimeters. The development of crystal growth technologies for producing thick epitaxial films (or quasi-bulk or semi-bulk crystals) in large wafer sizes with high crystalline quality is a prerequisite for utilizing h-BN as an UWBG electronic material. Compared to traditional III-nitrides, BN has another unique application as solid-state neutron detectors, which however, also require the development of quasi-bulk crystals to provide high detection efficiencies because the theoretical efficiency (ηi) relates to the detector thickness (d) by…

A three-temperature collisional-radiative model for shock-heated nitrogen–argon mixtures is developed to facilitate the study of nonequilibrium electronic excitation and ionization behind strong shock waves. Model predictions accurately reproduce measurements of N2 dissociation for mixtures of 2%–10% N2 in argon, with some discrepancies observed for 20% N2 mixtures. Potential causes of the discrepancies are discussed. Net dissociation in mixtures containing 20% N2 is significantly impacted by the dissociation of N2(A), the first excited electronic state of N2, indicating that molecular electronic excitation can affect net dissociation in shock-heated nitrogen flows. The collisional-radiative model successfully predicts the three-stage behavior and induction time observed in concentration measurements of atomic nitrogen in its fourth excited state, the 3s4P level, behind reflected shocks. Mechanisms for the observed behavior are discussed, which deviate from those inferred using a simpler kinetic model. Excited state number density predictions are strongly influenced by the modeling…

The lattice thermal conductivity (LTC) of Ga2O3 is an important property due to the challenge in the thermal management of high-power devices. In this work, we develop machine-learned neuroevolution potentials (NEPs) for single-crystalline β−Ga2O3 and κ−Ga2O3 and demonstrate their accuracy in modeling thermal transport properties. Combining NEP-driven homogeneous non-equilibrium molecular dynamics simulations with tensor analysis, we determine the spatial distributions of LTCs for two Ga2O3 crystals, showing dissimilar thermal behaviors. Specifically, β−Ga2O3 shows isotropic thermal transport properties, with the LTCs along [100], [010], and [001] directions being predicted to be 10.3±0.2, 19.9±0.2, and 12.6±0.2 W/(m K), respectively, consistent with previous experimental measurements. For κ−Ga2O3, our predictions suggest nearly isotropic thermal transport properties, with the LTCs along [100], [010], and [001] being estimated to be 4.5±0.1, 3.9±0.1, and 4.0±0.1 W/(m K). The reduced LTC of κ−Ga2O3 vs β−Ga2O3 stems from its restricted low-frequency phonons up to 5 THz. Furthermore,…