Polarization Charge Density Research Articles

A novel microwave assisted multi-material 3D printing strategy to architect lamellar piezoelectric generators for intelligent sensing

While the advances in thermoplastic additive manufacturing (3D printing) cover the free-form fabrication of piezoelectric materials, the chief obstacles to the use of 3D printed piezoelectric generator (PEG) are the weak weld within the part and imperfect polarization. Herein, a microwave assisted multi-material 3D printing strategy is proposed to fabricate multiple PEGs consisting of periodically intercalated piezoelectric & conductive layers. The interfacial regulation performed between piezoelectric ceramics and matrix leads to sufficient polarization under the electric field, thus generating the substantially enhanced piezoelectricity. Benefiting from the unique design of embedded electrode with 3D graphene nanoplatelets (GNPs) network, the polarization charge density greatly increases. Then the microwave irradiation technology is employed to realize the intense selective heating of the GNPs to weld polymer interfaces, further improving the piezoelectric output. As a result, the output current of the 3D lamellar PEG is nearly 18 times higher than that of conventional sandwich structure PEG. With significant advantages of the enhanced electromechanical coupling performance, the fabrication strategy is expected to create multifunctional sensors with complex geometries. Based on the above method, the 3D printed biomimetic lamellar denture is developed for health monitoring and Morse code transmission, demonstrating its enormous potential in real-life applications.

Polarization effect induced by strain in hexagonal boron nitride nanoribbons

As a member of the hexagonal boron nitride (h-BN) system, boron nitride nanoribbons (BNNRs) have a BN polar covalent bonding infrastructure, and there are small-scale effects with strong correlation with piezoelectric effects as well as edge-strengthening quantum effects. However, realizing the polarization effect on BNNRs through experiments remains challenging. Here, we verify the strain-induced polarization effect on BNNRs through computational simulations and experiments. Using the ten-point iterative method, a computational model that can be used for the discrete difference of the polarization charge density and energy density of BNNRs is developed, and the correctness of the model for the polarization effect of Gaussian strain-induced BNNRs is verified by the computational programming in Python language. The polarization effects of strain-induced zigzag-edge BNNRs (ZBNNRs) for sawtooth strain, parabolic strain and oblique sawtooth strain are also investigated separately. In addition, the results of the computational simulations are experimentally verified to be consistent with the theoretical calculations. And the piezoelectric constant of − 276.88 pm∙V−1 for strained ZBNNRs is found to be four times higher than that of unstrained ZBNNRs. This study provides a relevant reference for the study of realizing the high integration of nano-scale h-BN based piezoelectricity for piezoelectricity.

Theoretical Study of Carrier Transport in Metal–Ferroelectric–Insulator–Semiconductor Ferroelectric Tunnel Junction Memristor

Ferroelectric tunnel junction (FTJ) based on the metal–ferroelectric–insulator–semiconductor (MFIS) stacks shows great potential in neuromorphic and in-memory computing. Tunneling current in the MFIS-FTJ can be calculated by the Wentzel–Kramers–Brillouin (WKB) method with the band profile solved from Poisson’s equation or by self-consistently solving Poisson’s equation and the drift-diffusion transport equations with a tunneling-induced carrier generation rate. The carrier redistribution in the semiconductor is neglected in the former method, which reduces the complexity and saves the computational cost but may or may not lead to significant errors. In this article, a comprehensive study of the two simulation methods mentioned above is performed to investigate their applicable conditions and balance the accuracy and computational cost. The current densities of the MFIS-FTJs with different material parameters, including the barrier width, barrier height, carrier mobility, and polarization charge density simulated by the two methods, are compared and analyzed. As the voltage drop across the semiconductor cannot be neglected, method II is required to take the carrier transport in the semiconductor into account and method I is not accurate; otherwise, method I is more suitable due to its low computational cost without loss of accuracy.

Electrical properties of Cu/Pd2Si Schottky contacts to AlGaN/GaN-on-Si HEMT heterostructures

In this study, we analyze gold-free Cu/Pd2Si Schottky contacts to AlGaN/GaN-on-Si HEMT (high electron mobility transistor) heterostructures. The silicide layers are fabricated through magnetron sputtering deposition using a stoichiometric Pd2Si target and are subjected to post-gate annealing at different temperatures. After annealing at 500 °C the leakage currents of the planar diodes are reduced below 3.2 mA/cm2. The microstructures of the diodes are analyzed through transmission electron microscopy (TEM). The post-gate annealing increases lateral dimensions of silicide grains. The electrical properties of the contacts are determined by performing temperature-dependent current-voltage (I-V-T) measurements. The forward current exhibits a specific voltage dependence, which is interpreted based on the two-diode model, reflecting the properties of the Pd2Si/heterostructure interface along with those of the heterostructure itself. The model is fitted to extract the parameters of the Schottky contacts. The barrier height of 0.91 eV and ideality factor n = 1.5 are obtained after post-gate annealing at 500 °C. The effects of post-gate annealing on the forward conduction and leakage mechanisms are then discussed. A simple method is proposed to estimate the polarization charge density, which is required to calculate the electric fields in the heterostructure in the reverse bias. It is observed that post-gate annealing affects the I-V characteristics, likely by reducing the density of the unintentional donors and trap states, leading to an increase in the apparent barrier height along with a reduction in the density of defects contributing to the leakage current.

Influence of the ZrO2 gate dielectric layer on polarization coulomb field scattering in InAlN/GaN metal–insulator–semiconductor high-electron -mobility transistors

An InAlN/GaN MIS-HEMT with ZrO2 gate dielectric layer and a Schottky-gate InAlN/GaN HEMT were fabricated. Subsequently, the influence of ZrO2 gate dielectric layer on polarization Coulomb field (PCF) scattering in InAlN/GaN MIS-HEMT was studied. Compared with InAlN/GaN HEMT, the ZrO2 gate dielectric layer in InAlN/GaN MIS-HEMT results in fewer additional polarization charge and higher two-dimensional electron gas density at the same gate-source voltage (VGS) conditions. Also, we established that both of these factors weaken PCF scattering intensity. However, the InAlN/GaN MIS-HEMT has a larger gate swing. Therefore, there is a smaller VGS was applied during regular operation of device. When the VGS is small, PCF scattering is the strongest scattering and play non-negligible impact in the size of total electron mobility. This study provides a new theoretical foundation for further enhancing the performance of InAlN/GaN MIS-HEMTs.

Transesterification reactive extractive distillation process using ionic liquids as entrainers: From molecular insights to process integration

In this work, the transesterification reactive distillation coupled with extractive distillation process for n-propanol (PROH) and methyl acetate (MEAC) was developed based on pseudo homogeneous kinetic parameter model using ionic liquids (ILs) as entrainers. The COSMO-RS model was used for screening ILs entrainers. Here, 1-ethyl-3-methylimidazolium acetate [EMIM][AC] was selected as a potential entrainer due to its excellent solvent capacity and selectivity. Further, the COSMO-RS model was performed to calculate the polarization charge density (σ-profile) of methanol (MEOH), methyl acetate and [EMIM][AC], and to explore the molecular polarity. Based on quantum chemistry (QC) theory, the interaction energy between different molecular pairs is calculated. Independent gradient model (IGM) analysis identifies the interaction types of molecular pairs from the perspective of visualization. Owing to the strong hydrogen bond between anion [AC]− and MEOH, the extractive distillation process performance was significantly improved. Based on Aspen Plus platform, the effects of the theoretical stage number, reflux ratio, feed position and entrainer dosage on the cost and energy consumption of the new process was investigated. Based on the sensitivity analysis method, the operating conditions were optimized. Compared to the conventional process, the new reactive distillation coupled with extraction distillation process has higher energy efficiency and lower economic cost. The new process can save the total annual cost of about 3.42 × 105 $/year.

On the thickness dependence of the polarization switching kinetics in HfO2-based ferroelectric

HfO2-based ferroelectric (FE–HfO2) is a promising material for low-power and high-capacity memory technology. Since thinner ferroelectric films are required for low voltage operation, the impact of film thickness on the switching kinetics of FE–HfO2 needs to be studied in detail. In this paper, metal/ferroelectric/metal capacitors are fabricated with several thicknesses of HfZrO2 (HZO) films and characterized to study the switching kinetics based on the nucleation limited switching (NLS) model. Thinner HZO capacitors show slower polarization switching and asymmetry in program and erase operation, although low-frequency polarization charge density is nearly the same for all thicknesses. Slow switching is due to the large variability of the activation field of FE domains and grain boundaries among small FE grains. The asymmetry is caused by the asymmetric interface property at the top and bottom interfaces. High-resolution TEM and electron diffraction mapping methods provide physical evidence for the above discussions.

Conservation of currents in reduced full-F electromagnetic kinetic and fluid models

In this paper, we present an analysis of the conservation of currents in a full-F electromagnetic gyro-kinetic model in the long-wavelength limit. This equation corresponds to what is usually called the ‘vorticity equation’, which is not strictly correct as it cannot be formulated as the curl of a velocity equation. In the paper, we will therefore use the term ‘current conservation equation’ instead. Our results are relevant to reduced plasma descriptions like gyro-kinetic, drift-kinetic, gyro-fluid and drift-fluid models for tokamaks and stellarators. The equation describes the change of the polarization charge density (often called ‘vorticity’) in terms of the polarization stress due to the flow, external sources and three currents: the parallel current, the curvature current and a current related to the magnetic field fluctuations. We compare this equation with previous drift- and gyro-fluid equations and find general agreement, except in the vorticity source terms where previous drift-fluid models fail to capture the heating and density sources. We discuss the role of currents in the dynamics of diamagnetic and flow shear. The possible connection between these currents with phenomena observed in experiments that influence the radial electric field in the edge of tokamak plasmas, like resonant magnetic perturbations, and different magnetic field configurations and shapes, is presented.

Temperature-dependent source access resistance in AlGaN/AlN/GaN HFETs with different ratios of gate-source distance to gate-drain distance

An investigation about the temperature dependence of the source access resistance (RGS) has been focused on the two prepared AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) with different ratios of gate-source distance (LGS) to gate-drain distance (LGD). It is found that the different ratio of LGS to LGD can lead to a difference in the interaction between the negative additional polarization charge density nearby the gate region (ΔσNG) and the positive additional polarization charge density under the gate (ΔσUG) at each testing temperature, which influence the strength of polarization Coulomb field (PCF) scattering, resulting in the difference in RGS. Furthermore, the influence of the ratio of LGS to LGD on ΔσNG is verified by this study. These results show that it is an optional method to promote the high-temperature performances of AlGaN/AlN/GaN HFETs based on PCF scattering through adopting the appropriate ratio of LGS to LGD.

A 6.5 nm thick anti-ferroelectric HfAlO x film for energy storage devices with a high density of 63.7 J cm−3

In this work, we experimentally demonstrate comprehensively optimized anti-ferroelectric HfAlO x films, achieving high saturated polarization charge density and doping concentration in doped-HfO2 films. This allowed us to produce an ultrathin anti-ferroelectric energy storage device with high energy storage density (ESD). With the optimized deposition temperature of 300 °C, Hf:Al ratio of 18:1 and an electrode of tungsten, a 6.5 nm thick anti-ferroelectric HfAlO x film is realized with a high ESD of 63.7 J cm−3, which is the thinnest anti-ferroelectric film among all the reported works, associated with such a high ESD. This not only provides an effective way to improve the scaling ability of anti-ferroelectric HfAlO x films, but also demonstrates a new approach to strengthen the control of the phase transition.

Insights into unconventional behaviour of negative capacitance transistor through a physics-based analytical model

The present work investigates key attributes of metal–ferroelectric–metal–insulator–semiconductor (MFMIS) cylindrical nanowire (NW) transistor through a physics-based analytical model. The proposed NW MFMIS negative capacitance model is developed by solving the baseline short channel NW model coupled with one-dimensional Landau’s equation while considering the radial dependency of electric field in the ferroelectric, a key feature for NW architecture. Contrary to the expected behaviour of a short channel MOSFET, the analytical framework successfully captures the unconventional effects such as an increase in the threshold voltage, lowering of subthreshold swing, and a negative value of drain-induced barrier lowering with gate length downscaling in MFMIS cylindrical NW devices. Also, the impact of ferroelectric thickness, spacer induced polarization charge density, remnant polarization and coercive field on the unconventional short channel effects have been examined. The influence of different ferroelectric materials (Al–HfO2, Gd–HfO2, Y–HfO2, and HZO) on the extent of internal amplification has also been investigated. The developed model provides new insights into the functioning and optimization of NW MFMIS architecture for facilitating hysteresis-free high internal voltage amplification with sub-60 mV/decade swing.

Investigation on threshold voltage of p-channel GaN MOSFETs based on p-GaN/AlGaN/GaN heterostructure* *Project supported by the Key-Area Research and Development Program of Guangdong Province, China (Grant Nos. 2020B010174001 and 2020B010171002), the Ningbo Science and Technology Innovation Program 2025 (Grant No. 2019B10123), and the National Natural Science Foundation of China (Grant No. 62074122).

The threshold voltage (V th) of the p-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) is investigated via Silvaco-Atlas simulations. The main factors which influence the threshold voltage of p-channel GaN MOSFETs are barrier height Φ 1,p, polarization charge density σ b, and equivalent unite capacitance C oc. It is found that the thinner thickness of p-GaN layer and oxide layer will acquire the more negative threshold voltage V th, and threshold voltage |V th| increases with the reduction in p-GaN doping concentration and the work-function of gate metal. Meanwhile, the increase in gate dielectric relative permittivity may cause the increase in threshold voltage |V th|. Additionally, the parameter influencing output current most is the p-GaN doping concentration, and the maximum current density is 9.5 mA/mm with p-type doping concentration of 9.5 × 1016 cm−3 at V GS = –12 V and V DS = –10 V.

Reducing the polarization mismatch between the last quantum barrier and p-EBL to enhance the carrier injection for AlGaN-based DUV LEDs

In this work, we report an AlGaN-based ∼275 nm deep ultraviolet light-emitting diode (DUV LED) that has AlGaN based quantum barriers with a properly large Al composition. It is known that the increased conduction band barrier height helps to enhance the electron concentration in the active region. However, we find that the promoted hole injection efficiency is also enabled for the proposed DUV LED when the Al composition increases. This is attributed to the reduced positive polarization charge density at the last quantum barrier (LQB) and p-type electron blocking layer (p-EBL) interface, which can suppress the hole depletion effect in the p-EBL. Thus, the hole concentration in the p-EBL gets promoted, which is very helpful to reduce the hole blocking effect caused by the p-EBL. Therefore, thanks to the improved carrier injection, the proposed DUV LED increases the optical power and reduces the forward voltage when compared with the conventional DUV LED.

Correlation Between Corrugation-Induced Flexoelectric Polarization and Conductivity of Low-Dimensional Transition Metal Dichalcogenides

Tunability of polar and semiconducting properties of low-dimensional transition metal dichalcogenides (TMDs) have propelled them to the forefront of fundamental and applied physical research. These materials can vary from non-polar to ferroelectric, and from direct-band semiconductor to metallic. However, in addition to classical controls such as composition, doping, and field effect in TMDs the additional degrees of freedom emerge due to the curvature-induced electron redistribution and associated changes in electronic properties. Here we numerically explore the elastic and electric fields, flexoelectric polarization and free charge density for a TMD nanoflake placed on a rough substrate with a sinusoidal profile of the corrugation using finite element modelling. Numerical results for different flake thickness and corrugation depth yield insight into the flexoelectric nature of the out-of-plane electric polarization and establish the unambiguous correlation between the polarization and static conductivity modulation caused by inhomogeneous elastic strains coupled with deformation potential and strain gradients, which evolve in TMD nanoflake due to the adhesion between the flake surface and corrugated substrate. We revealed a pronounced maximum at the thickness dependences of the electron and hole conductivity of MoS2 and MoTe2 nanoflakes placed on a metallic substrate, which opens the way for their geometry optimization towards significant improvement their polar and electronic properties, necessary for their advanced applications in nanoelectronics and memory devices.Specifically, obtained results can be useful for elaboration of nanoscale straintronic devices based on the bended MoS2, MoTe2 and MoSTe nanoflakes, such as diodes and bipolar transistors with a bending-controllable sharpness of p-n junctions.

Electron transport of perovskite oxide BaSnO3 on (110) DyScO3 substrate with channel-recess for ferroelectric field effect transistors

We report an electron transport study of an La-doped perovskite oxide BaSnO3 thin film grown by molecular beam epitaxy on (110) DyScO3 as a function of electron concentration, by etching the film step-by-step with nanometer precision. Inductively coupled plasma-reactive ion etching with BCl3/Ar plasma is used for etching depth control. The local doping and electron density are experimentally determined after each etching step. The results show that the electron mobility is dominated by threading dislocations if the electron concentration is below 7.8 × 1019 cm−3, while ionized impurities and phonon scattering become more dominant at electron concentrations greater than 1.2 × 1020 cm−3. The charging state of thread dislocations is estimated to be 6.2. Furthermore, using the etch process to control the electron concentration and channel thickness, a gate-recessed ferroelectric field effect transistor is fabricated with 10 nm HfO2 as a gate dielectric. The device exhibits a saturation current of 29.9 mA/mm with a current on/off ratio of Ion/Ioff = 8.3 × 108 and a ferroelectric polarization charge density of 1.9 × 1013 cm−2. Under the forward gate bias sweep, the device operates in the enhancement mode with a threshold voltage of 3 V. Under the reverse gate sweeping bias, the device operates in the depletion mode with a threshold voltage of –1.5 V.

Vacuum polarization near boundaries

We study the effect of boundary conditions on vacuum polarization for charged scalar fields in two space-time dimensions. We find that both Dirichlet and Neumann boundary conditions lead to screening. In the Dirichlet case, the vacuum polarization charge density vanishes at the boundary, whereas it attains its maximum there for Neumann boundary conditions. At a critical field strength, the vacuum polarization diverges for Neumann boundary conditions, an effect due to the instability of the lowest energy mode in the presence of the external field.

Efficient Electrocatalytic CO2 Reduction to C2+ Alcohols at Defect-Site-Rich Cu Surface

Summary Electrochemical CO2 reduction is a promising approach for upgrading excessive CO2 into value-added chemicals, while the exquisite control of the catalyst atomic structures to obtain high C2+ alcohol selectivity has remained challenging due to the intrinsically favored ethylene pathways at Cu surface. Herein, we demonstrate a rational strategy to achieve ∼70% faradaic efficiency toward C2+ alcohols. We utilized a CO-rich environment to construct Cu catalysts with stepped sites that enabled high surface coverages of ∗CO intermediates and the bridge-bound ∗CO adsorption, which allowed to trigger CO2 reduction pathways toward the formation alcohols. Using this defect-site-rich Cu catalyst, we achieved C2+ alcohols with partial current densities of > 100 mA·cm−2 in both a flow-cell electrolyzer and a membrane electrode assembly (MEA) electrolyzer. A stable alcohol faradaic efficiency of ∼60% was also obtained, with ∼500 mg C2+ alcohol production per cm2 catalyst during a continuous 30-h operation.

Numerical simulation of the sensor for toxic nanoparticles based on the heterostructure field effect transistor

A significant rise in the mass production of products that contain nanoparticles is of growing concern due to the detection of their toxic effects on living organisms. The standard method for analyzing the toxicity of substances, including nanomaterials, is toxicological testing, which requires the substantial consumption of time and material resources. An alternative approach is to develop models that predict the effect of nanomaterials on biological systems. In both cases, for the detection of nanoparticles an effective electronic complex consisting of a sensor with high sensitivity and a data reception/processing/transmission system is necessary. In recent times, fundamental and applied research activities aimed at the application of heterostructure field-effect transistors – high electron mobility transistors–as a base for such sensors have been undertaken. The purpose of this work is to develop a technique for modeling a sensor for toxic nanoparticles based on the heterostructure field-effect transistor. The object of the research is a gallium nitride high electron mobility transistor device structure. The subject of the research is the electrical characteristics of the transistor obtained in static mode. The calculation results show that the dependence between the concentration of the toxic nanoparticles in the test medium and the polarization charge surface density could serve as a base for modeling the sensor for toxic nanoparticles based on the heterostructure field-effect transistor. The primary advantage of the proposed technique is the use of the scaling parameter intended directly for calibrating the polarization charge density in accordance with the two-dimensional electron gas concentration. The obtained results can be utilized by the electronics industry of the Republic of Belarus for developing the hardware components of gallium nitride high-frequency electronics.

Reduction of blob-filament radial propagation by parallel variation of flows: Analysis of a gyrokinetic simulation

Data from the XGC1 gyrokinetic simulation are analyzed to understand the three-dimensional spatial structure and the radial propagation of blob-filaments generated by quasi-steady turbulence in the tokamak edge pedestal and scrape-off layer plasma. Spontaneous toroidal flows vary in the poloidal direction and shear the filaments within a flux surface, resulting in a structure that varies in the parallel direction. This parallel structure allows the curvature and grad-B induced polarization charge density to be shorted out via parallel electron motion. As a result, it is found that the blob-filament radial velocity is significantly reduced from estimates that neglect parallel electron kinetics, broadly consistent with experimental observations. Conditions for when this charge shorting effect tends to dominate blob dynamics are derived and compared with the simulation.

Investigations on deep ultraviolet light-emitting diodes with quaternary AlInGaN streamlined quantum barriers for reducing polarization effect

The characteristics of AlGaN-based deep ultraviolet light-emitting diodes (UV LEDs) with quaternary AlInGaN streamlined quantum barriers (QBs) are investigated in this paper. The simulated results show that the light output power and the internal quantum efficiency (IQE) of deep UV LEDs with AlInGaN streamlined QBs have been improved obviously, and the efficiency droop effect has been significantly alleviated. After in-depth study, it is shown that quaternary AlInGaN streamlined QBs is designed to reduce the polarization charge density between quantum wells (QWs) and QBs, and relieve band bending. In addition, the streamline barrier can reduce the kinetic energy of injected carriers, and improve the probability of the carriers being bounced back to the quantum wells, so as to enhance the capture of carriers in the quantum wells. As a result, the recombination rate of electron and hole in the QWs is enhanced, the light output power and the IQE are improved, and the efficiency droop effect is alleviated.