Barrier Thickness Research Articles

Electron conduction mechanisms in magnetic tunnel junctions fabricated using amorphous Si-Zn-Sn-O as a low-resistive semiconducting barrier

CoFeB/ Si-Zn-Sn-O /CoFeB magnetic tunnel junctions (MTJs) have been fabricated using amorphous Si-Zn-Sn-O as a low-resistive semiconducting barrier. In the low bias voltage range (up to ∼0.2 V), direct tunneling is found to be the dominant transport mechanism in MTJs. Tunneling conduction is further verified by simulation of tunnel current density and differential conductance using Simmon’s and Brinkmann model, respectively. Simulated results provided valuable insights into the barrier properties, including interfacial barrier height, thickness, and barrier asymmetry. Above the direct tunneling regime, electron transport in MTJs is governed by Pool Frenkel emission, which possibly arises due to the presence of high-density localized tail states below the conduction band of amorphous Si-Zn-Sn-O. Tunnelling magnetoresistance value of MTJs is found to be very low, which is attributed to the presence of various inelastic conduction channels. The results of this study might be useful to explore the potential of amorphous Si-Zn-Sn-O for fabricating low-resistive MTJ based spintronic devices.

An ionic liquid-assisted strategy for enhanced anticorrosion of low-energy PEO coatings on magnesium–lithium alloy

A low-energy plasma electrolytic oxidation (LePEO) technique is developed to simultaneously improve energy efficiency and anti-corrosion. Ionic liquids (1‑butyl‑3-methylimidazole tetrafluoroborate (BmimBF4)) as sustainable corrosion inhibitors are chosen to investigate the corrosion inhibition behavior of ionic liquid (ILs) during the LePEO process for LA91 magnesium–lithium (Mg–Li) alloy. Results show that the ionic liquid BmimBF4 participates in the LePEO coating formation process, causing an increment in coating thickness and surface roughness. The low conductivity of the ionic liquid is responsible for the voltage and breakdown voltage increases during the LePEO with IL process (LePEO-IL). After adding BmimBF4, corrosion current density decreases from 1.159 × 10−4 A·cm−2 to 8.143 × 10−6 A·cm−2. The impedance modulus increases to 1.048 × 104 Ω·cm−2 and neutral salt spray remains intact for 24 h. The superior corrosion resistance of the LePEO coating assisted by ionic liquid could be mainly attributed to its compact and thick barrier layer and physical absorption of ionic liquid. The ionic liquid-assisted LePEO technique provides a promising approach to reducing energy consumption and improving film performance.

Fabrication of Highly Oriented Ultrathin Zirconium Metal‐Organic Framework Membrane from Nanosheets towards Unprecedented Gas Separation

AbstractConcurrent regulation of crystallographic orientation and thickness of zirconium metal–organic framework (Zr‐MOF) membranes is challenging but promising for their performance enhancement. In this study, we pioneered the fabrication of uniform triangular‐shaped, 40 nm thick UiO‐66 nanosheet (NS) seeds by employing an anisotropic etching strategy. Through innovating confined counter‐diffusion‐assisted epitaxial growth, highly (111)‐oriented 165 nm‐thick UiO‐66 membrane was prepared. The significant reduction in thickness and diffusion barrier in the framework endowed the membrane with unprecedented CO2 permeance (2070 GPU) as well as high CO2/N2 selectivity (35.4), which surpassed the performance limits of state‐of‐the‐art polycrystalline MOF membranes. In addition, highly (111)‐oriented 180 nm‐thick NH2‐UiO‐66 membrane showing superb H2/CO2 separation performance with H2 permeance of 1230 GPU and H2/CO2 selectivity of 41.3, was prepared with the above synthetic procedure.

Fabrication of Highly Oriented Ultrathin Zirconium Metal-Organic Framework Membrane from Nanosheets towards Unprecedented Gas Separation.

Concurrent regulation of crystallographic orientation and thickness of zirconium metal-organic framework (Zr-MOF) membranes is challenging but promising for their performance enhancement. In this study, we pioneered the fabrication of uniform triangular-shaped, 40 nm thick UiO-66 nanosheet (NS) seeds by employing an anisotropic etching strategy. Through innovating confined counter-diffusion-assisted epitaxial growth, highly (111)-oriented 165 nm-thick UiO-66 membrane was prepared. The significant reduction in thickness and diffusion barrier in the framework endowed the membrane with unprecedented CO2 permeance (2070 GPU) as well as high CO2 /N2 selectivity (35.4), which surpassed the performance limits of state-of-the-art polycrystalline MOF membranes. In addition, highly (111)-oriented 180 nm-thick NH2 -UiO-66 membrane showing superb H2 /CO2 separation performance with H2 permeance of 1230 GPU and H2 /CO2 selectivity of 41.3, was prepared with the above synthetic procedure.

Tunable Quasistationary States in a One-dimensional Quantum Heterostructure

In this work, we focus on the quasistationary states, lifetime, and transmittance in opened quantum wells with biased and unbiased. In order to solve the quasibound states, the complex eigenenergies are solved in our calculation model by adaptive finite element method. We have demonstrated the accuracy to exam the numerical convergence. In this case, the 1D quantum heterostructure is commonly composed of GaAs and AlxGa1-xAs. With the different applied bias, the resonant tunneling and transmittance profiles could be changed, respectively. Increasing the thickness of the outermost barrier can be prevented an electron penetrated through the barrier from the quasistationary state. This is a useful way to design easily the high-speed switch for semiconductor devices. Our results of numerical calculations are good agreement with the argument principle method approach. These results are useful and helped us to design quantum devices and quantum computations.

Tunnel magneto-Seebeck effect in magnetic tunnel junctions with a single-crystal barrier

We have studied the tunnel magneto-Seebeck (TMS) effect in magnetic tunnel junctions with a single-crystal barrier using an extending spintronic theory developed previously by us. The theory is founded on optical methods, and thus, compared to the conventional theories, it can better take into account the periodicity of the single-crystal barrier and the coherence of the tunneling process. The results show that, the Seebeck coefficients can oscillate with the barrier thickness. Physically, the oscillations come from the diffraction of the tunneling electrons by the periodic potential of the single-crystal barrier. Owing to the oscillations, there appear a series of peaks of the TMS ratio for some barrier thicknesses which correspond to the zero points of the Seebeck coefficients. In particular, the magnitudes of the calculated Seebeck coefficients are in accordance with the experimental values of the intrinsic Joule heating. Furthermore, the influences of the ferromagnetic electrodes and barrier on the oscillations are investigated. It is found that, the ferromagnetic electrodes only have a remarkable influence on the amplitudes of the oscillations, whereas the barrier has significant influences on both the amplitudes and periods.

Ventilated acoustic meta-barrier based on layered Helmholtz resonators

The performance of a multilayer sound attenuating metamaterial with ventilation capacity based on arrays of wall-embedded Helmholtz resonators is investigated analytically, numerically and experimentally. Each cell of the barrier front wall is a Euclidian polygon, such as square or hexagon. One thickness layer of the barrier cell contains a parallel array of 4 (for square cell) or 6 (for hexagon) Helmholtz resonators connected via the axial ventilation duct element. Multiple layers of resonators can be connected in sequence with the extension of the ventilation channel. The sound attenuation performance of the barriers is investigated first using the lumped parameter theory and transfer matrix method (TMM), and numerically using the finite element (FE) simulations solving the Helmholtz equations in frequency domain. These results indicate that the sound attenuation in audible range (300–2000 Hz) can be meaningfully improved using several layers with a unique peak resonance frequency assigned to each corresponding layer, with each additional layer improving the total attenuation. However, it is acknowledged that the previously studied fundamental trade-offs between the total barrier thickness, ventilation capacity, operational frequency range and integral sound attenuation do apply to the proposed design as well. Nevertheless, it is suggested that the simplicity of both structural design and theoretical methods of performance estimations make the suggested barrier a good candidate for applications with a well known desired attenuation spectrum. The impedance tube experiments validate the sound blocking performance and show a reasonable agreement with the numerical predictions.

A dose calculation algorithm for radiosurgery treatment room shielding optimization.

Methods described in the literature for designing shielding for treatment rooms for radiotherapy systems often involve assumptions that lead to overestimations of the wall thicknesses required to meet dose rate constraints outside the room. The Leksell Gamma Knife (LGK) has built-in shielding that results in primarily scattered photons leaking into the room. The field of leakage radiation, therefore, has a wide spectrum of energies, up to the primary energies of cobalt-60, and is highly anisotropic, making standard site planning methods difficult to adapt to the LGK. Monte Carlo (MC) simulations are an alternative for dose calculations but are computationallyexpensive. The aim was to develop a dose calculation algorithm for fast estimations of dose rates outside the barriers of a treatment room. The algorithm could then be employed in iterative methods to optimize treatment room parameters such as wall thicknesses and position of the LKGunit. The algorithm uses pre-calculated MC simulation data in two steps. First, it uses phase spaces that describe single photons in the radiation field around the LGK. Using each photon's position and direction, they are raytraced to the outside of the room. Based on their energy, angle of incidence to the barrier, and the barrier's thickness, a depth-dose profile is sampled from a pre-calculated library of profiles. The contribution from each photon is added to the space outside the barriers, giving the total dose distribution. The barrier thicknesses are optimized by iteratively running the algorithm and finding the minimum thickness required to keep the resulting maximum dose rate outside below a given upperlimit. Comparisons between dose distributions from the dose algorithm and full MC simulations of two typical rooms show good agreement, with the maximum dose rate outside each barrier being estimated within what represents a 2 cm error in concrete barrier thickness. The algorithm is successfully used in optimizing the barrier thicknesses and the results show potential decreases in most barriers' thickness, compared to a conventionalmethod. The algorithm is a promising alternative to conventional barrier design methods, eliminating several shortcomings of the latter whilst being significantly faster than a full MC simulation. The flexibility of the algorithm would further enable solving more complex cases, such as optimizing the LGK position or barrier material to further reduce material use andcost.

A Method to Probe the Interfaces in La2−xSrxCuO4-LaSrAlO4-La2−xSrxCuO4 Trilayer Junctions

C-axis trilayer cuprate Josephson junctions are essential for basic science and digital circuit applications of high-temperature superconductors. We present a method for probing the interface perfection in La2−xSrxCuO4 (LSCO)-LaSrAlO4 (LSAO)-La2−xSrxCuO4 trilayer junctions. A series of LSCO-LSAO superlattices with atomically smooth surfaces and sharp interfaces were grown by the atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) technique. We have systematically varied the thickness of LSCO and LSAO layers with monolayer precision. By studying the mutual inductance and electrical transport in these superlattices, we detect the non-superconducting (“dead”) layers at the interfaces and quantify their thicknesses. Our results indicate that two optimally doped LSCO monolayers just above and below the one monolayer LSAO barrier are no longer superconducting, rendering the actual barrier thickness of five monolayers. Next, we have shown that introducing a protective highly-overdoped LSCO layer reduces the thickness of dead layers by one or two monolayers.

Improving the collection efficiency in the hot carrier tunneling device by optimizing the thickness of tunneling barrier for balancing the acceleration and scattering processes

Improving the collection efficiency is one of the major challenges in fabricating high performance gated hot carrier tunneling devices that have important applications in electronics and optoelectronics. In this work, a general model for a gated hot carrier tunneling device has been established to figure out the optimized thickness of a tunneling barrier for maximizing the collection efficiency. Simulation indicates that the maximum efficiency increases with the field F, the tunneling barrier height Φ0, and the mean free path λ but decreases with the threshold energy Et. In addition, the optimal thickness of the tunneling barrier decreases as F and Φ0 increase or λ and Et decrease, which varies from ∼6 to ∼9 nm depending on the above-mentioned parameters. To verify the model, electron emission characteristics of a few layers graphene (FLG)/h-BN/FLG heterostructure with different thickness of h-BN have been measured. A similar dependence of the averaged emission efficiency on the h-BN thickness has been obtained, which can be fitted by an extended model for the case of FLG/h-BN/FLG with consideration of a hot hole-induced Auger process. All the results are useful for designing a high performance hot carrier tunneling device.

Vertically aligned ZnO nanoarray directly orientated on Cu paper by h-BN monolayer for flexible and transparent piezoelectric nanogenerator

Piezoelectric nanogenerators (PENGs) harvest various types of mechanical energy and exhibit remarkable potential for powering portable devices and wearable electronic systems. The insulating layer in the device can enhance the output voltage, however, people are still curious on the extreme effect when the dielectric thickness reaches the limit of atomic monolayer. Here we demonstrate vertically aligned growth of zinc oxide (ZnO) nanorods array directly on polycrystalline copper paper through the pre-orientation with two-dimensional (2D) hexagonal boron nitride (h-BN) monolayer. The h-BN layer effectively releases the misfit strain in ZnO nanoarray and leads to highly aligned and ultralong nanorods over 75 µm through chemical vapor deposition method. A flexible and transparent PENG based on h-BN/ZnO nanoarray/h-BN sandwiched structure was designed and fabricated. Ultrathin thickness and high barrier of h-BN strongly suppresses the movement of free-carriers in PENG device and improves the output voltage. It is found that the capacitance of PENG increases exponentially inversely proportional to the dielectric layer thickness and reaches 0.01 mF with 2D h-BN monolayer. The multilayer h-BN based PENG can generate open-voltage of 5 V and short-current of ∼ 18 μA, and achieve a maximum output power density up to 169 mW/cm2. A prototype transparent PENG has been achieved for harvesting walking energy and exhibits the practical potential for self-powered wearable devices in the future.

Architecture characteristics of hyperpycnal deposits: Insights from numerical modeling with numerical simulation platform grade

It is challenging to study hyperpycnal flow deposit characteristics, distribution, and internal architecture in detail using seismic or flume experiments. Therefore, a rule-based model numerical simulation platform (NSP) grade has been developed to simulate various gravity-driven sediment transport processes and fan depositions in three dimensions. Five model scenarios are designed for processing and architecture studies of hyperpycnal flow deposits. Furthermore, the impact of a single flow event’s duration and the flow occurrence also are discussed. The results find that, in all scenarios, complex hyperpycnal flow strata stacked by channels, fans, and background shale according to specific rules were generated, and the progradation, retrogradation, and compensational stacking of hyperpycnal flow sediments were observed. Three facies can be identified in hyperpycnal flow depositions from proximal to the distal deposits, comprising channel complex, channel-fan transition deposits, and fan complex. Due to hyperpycnal deposit development, each event’s channel gradually becomes increasingly complex, and the channel extension distance decreases. The retrogradation characteristics of sedimentary bodies become more apparent. Shortening a flood event’s duration will lead to stronger deposition heterogeneity. Here, multiple depositional centers will be formed. The distal center will form first, and sediments will gradually retrograde to form the next one. The reduced frequency of events results in less sediment supply, thinner thickness, and thicker mudstone barriers. NSP grade reveals that numerical simulation could provide more realistic results than traditional interpretation models. Further studies on NSP grade modeling will couple more processes and improve model accuracy to guide the understanding of hyperpycnal flow depositions.

Ab initio comparison of spin-transport properties in MgO-spaced ferrimagnetic tunnel junctions based on Mn3Ga and Mn3Al

We report on first-principles spin-polarised quantum transport calculations (from NEGF + DFT) in MgO-spaced magnetic tunnel junctions (MTJs) based on two different Mn-based Heusler ferrimagnetic metals, namely Mn3Al and Mn3Ga in their tetragonal DO22 phase. The former is a fully compensated half-metallic ferrimagnet, while the latter is a low-moment high-spin-polarisation ferrimagnet, both with a small lattice mismatch from MgO. In identical symmetric and asymmetric interface reconstructions across a 3-monolayer thick MgO barrier for both ferrimagets, the linear response (low-voltage) spin-transfer torque (STT) and tunneling magneto-resistance (TMR) effects are evaluated. A larger staggered in-plane STT is found in the Mn3Ga case, while the STT in Mn3Al vanishes quickly away from the interface (similarly to STT in ferromagnetic MTJs). The roles are reversed for the TMR, which is practically 100% in the half-metallic Mn3Al-based MTJs (using the conservative definition) as opposed to 60% in the Mn3Ga case. The weak dependence on the exact interface reconstruction would suggest Mn3Ga–Mn3Al solid solutions as a possible route towards optimal trade-off of STT and TMR in the low-bias, low-temperature transport regime.

Lateral Transport During Membrane Permeation in Diffusion Cell: In Silico Study on Edge Effect and Membrane Blocking

Membrane transport in diffusion cell studies is not one-dimensional from the donor to the receptor. Lateral diffusion within the membrane into the surrounding clamped region can lead to edge effect. Lateral diffusion can also affect the impact of an object blocking the membrane in a diffusion cell. The effects of lateral transport on permeation across a two-layer membrane in diffusion cells were investigated in this study under edge effect and membrane blocking conditions that could be encountered in previous gingiva and hypothetical skin permeation studies. Model simulations of time-dependent and steady-state transport were performed using COMSOL Multiphysics. The simulations indicated edge effect could increase the steady-state flux across the membrane up to 35% with a relatively thick membrane and small diffusion cell opening (e.g., gingiva study). The edge effect decreased when the relative thickness and permeability of the major barrier (top layer in the two-layer membrane) decreased. When the membrane was partially blocked by an object, lateral diffusion within the membrane could mitigate its impact: e.g., when the object was in the receptor, the impact caused by membrane blocking was reduced more than half. Therefore, membrane lateral transport should be considered under certain circumstances in permeation studies using diffusion cells.

Seasonal Diversity of El Niño‐Induced Marine Heatwave Increases in the Bay of Bengal

AbstractThis study explored the characteristics of marine heatwaves (MHWs) and the related dynamics in Bay of Bengal (BOB) associated with El Niño events during 1982–2020. This showed that El Niño can induce more MHW days, which displays remarkable seasonality. The strong influences of El Niño were found in the southern BOB at its peak phase in winter and the following spring, characterized by an increase in MHW days by 106% in winter and 83% in spring. In the proceeding summer, its influences weakened and shifted to the central bay. The more MHW days during winter and spring are primarily caused by the enhanced solar radiation associated with El Niño and partly contributed by the thick barrier layer in winter. In contrast, besides atmospheric heating, the deepening thermocline in the central bay induced by equatorial and local winds also plays an important role in the increase in MHWs during summer.

Influence of top-coat thickness of thermal barrier coating on time–frequency domain and dispersion characteristics of laser-induced surface acoustic wave

Thermal barrier coating is commonly used to insulate the hot end components of aero engine and gas turbine in order to meet extreme high temperature service conditions. The performance of the thermal barrier coating will be significantly impacted by thinning or even spalling during the service process. Given the uniqueness of thermal barrier coating in structure and composition materials, and the fact that laser-induced surface acoustic waves are extremely sensitive to material surface characteristic parameters, laser-induced ultrasonic technology is used to detect the thermal barrier coating in this paper. To characterize the size and defect of thermal barrier coating, the excitation mechanism and propagation law of surface acoustic waves are very crucial. Firstly, the surface acoustic wave propagation law is investigated using spectral analysis and wavelet transform. Then, to study the influence of top-coat thickness on laser-induced surface acoustic wave, the surface acoustic wave velocity dispersion curves of thermal barrier coating with different top-coat thicknesses are extracted using wavelet analysis. The simulative and experimental results indicate that the time–frequency domain and dispersion characteristics of laser-induced surface acoustic wave in multilayered heterostructure are strongly affected by the top-coat thickness. By the establishment of the relationship between phase velocity of laser-induced surface acoustic wave and top-coat thickness, a theoretical basis is provided to characterize the top-coat thickness of multilayered heterostructure.

Band-to-Band Transitions in InAs/GaSb Multi-Quantum-Well Structures Using k.p Theory: Effects of Well/Barrier Width and Temperature

We investigated the conduction- and valence-confined energy levels and first band-to-band transition energies of a type-II InAs/GaSb multi-quantum-well at 77 K and room temperature for various well and barrier thicknesses. We calculated the electron and hole confined energies based on Kane’s eight-band k.p formalism. We also explored the effect of the barrier width on the wells’ interactions, which was negligible for wells with a width wider than 30 nm. Moreover, we proposed a single exponential function to predict the first transition energies without considering the complex approach of k.p theory. Then, we measured the photoluminescence spectra of the manufactured samples, including thin wells (1, 2, and 3 monolayers) and wide barriers (50 nm). Finally, we made comparisons between the theoretical band-to-band transition energies for kz=0 and experimental results from the photoluminescence spectra for different well thicknesses at 77 and 300 K.

Barrier thickness dependence of the built-in electric field in pseudomorphic ZnO/Zn0.55Mg0.45O multi-quantum wells

The electric polarization of (Zn, Mg)O alloys, known as stable wide-bandgap semiconductors, is an intrinsic property that has a significant impact on the properties and performance of polar ZnO-based optoelectronic devices, such in the case of the quantum-confined Stark effect. Because Wurtzite (WZ)-ZnMgO exhibits a high potential for epitaxial growth with tunable bandgap energy, we fabricated high-quality pseudomorphic ZnMgO thin films on O-polar ZnO substrates and achieved a maximum Mg solubility of 45%. In addition, the high-quality texture and accurate design of ZnO/ZnMgO multi-quantum wells (MQWs) was confirmed. The electric-polarization-induced built-in electric field (Ein) of polar ZnO/ZnMgO MQWs was examined by using a combination of cryogenic photoluminescence measurements and self-consistent Poisson–Schrödinger method. An electric field of 1.9 MV/cm was obtained for a barrier thickness of 13.6 nm. The barrier-thickness-dependent Ein was fitted with an electrostatic model, and a maximum electric field (Einmax) of 2.4 MV/cm was obtained for a 3.4-nm-thick quantum well. This value was perfectly reproduced by using the crystal parameters of the pseudomorphic ZnMgO epilayer. These results are expected to enhance our understanding of WZ-ZnMgO crystallography and provide a method for the systematic design of polar-ZnO-based optoelectronic devices.

Influence of AlGaN n-type doping and AlN thickness on the two-dimensional electron gas density (ns) and resistance (R2DEG)

In this paper, electrical characterizations by C(VG) and ID(VG) and comparison with 1D Schrödinger-Poisson simulations are carried out to investigate the effects of AlN layer thickness and n-type AlGaN barrier doping on the two-dimensional electron gas resistance (R2DEG). Specifically, these effects are related to either the electron sheet density (ns) or the mobility (µ). We show that varying the AlN thickness from 0.7 nm (ns = 7.8 × 1012 cm−2) to 1.5 nm (ns = 9.0 × 1012 cm−2) leads to a linear increase of ns. However, simultaneous degradation of the transport properties probably due to intensifying roughness mechanism at high ns tends to limit the improvement of the device properties. Furthermore, we show that the heavily doped AlGaN barrier slightly increases ns and reduces the 2DEG mobility to ∼ 1770 cm2.V−1.s−1 leading to a degraded 2DEG resistance. However, the overall resistance is improved (R = 323 Ω/□) compared to the undoped case (R = 380 Ω/□) due to the simultaneous contribution of two conducting channels at VG = 0 V. To go further, the polarization charges at the AlGaN/AlN and AlN/GaN interfaces are computed in the 1D Schrödinger-Poisson simulations to account for the presence of the AlN layer. A reduced polarization for very thin AlN layers is considered to account for the experimental results. Finally, a very simple empirical model is proposed that predicts the enhancement of polarization charges (σ) as a function of AlN thickness.

Effects of electric field and hydrostatic pressure on the exciton states in strained zinc-blende In[formula omitted]Ga[formula omitted]N-GaN coupled double quantum wells

Based on the effective mass approximation, the variational theory study of the exciton properties in a strained zinc blende GaN/InxGa1−xN/GaN/InyGa1−yN/GaN coupled double quantum well is presented considering the combined effects of hydrostatic pressure, strain, and external electric field. The results show that the exciton binding energy first increases, it achieves a maximum value, and then decreases with the reduction of the well width. Contrarily, the exciton binding energy decreases slightly first, it reaches a minimum value, and then increase gradually until a constant with the enhancement of the middle barrier thickness. Meantime, the pronounced dependencies of the exciton binding energy, interband transition energy, emission wavelength, oscillator strength, and radiation decay time of the coupled double quantum well on hydrostatic pressure are analyzed systematically. As the hydrostatic pressure increases, the exciton binding energy increases linearly, and the interband transition energy (emission wavelength) is proportional (inversely) to hydrostatic pressure. In addition, the variation of the exciton binding energy and Stark shift with the external electric field is explored detailedly.