Increasing Barrier Thickness Research Articles

Optimizing breakdown voltage in rod-plane gaps with barrier design for positive DC voltage: an experimental study

Traditional air insulation in high-voltage equipment becomes bulky for high voltage withstand. The research so far conducted is related to electrode shape optimization, without fully exploiting the potential of strategically placed dielectric barriers. This paper introduces a new concept toward miniaturized high-voltage equipment by proposing glass barriers within rod-gap electrodes insulated with air and subjected to positive DC voltage. Barrier thickness and its positioning with regard to withstand characteristics are studied in order to optimize the design of such equipment. Three regimes of barrier thickness have been studied, and how that influences breakdown voltage and path type is presented. It was seen that with increasing barrier thickness, breakdown voltage improves more considerably. Interestingly, while the vertical position influences the breakdown path type, breakdown voltages remain similar for different paths at the same location. This paper, therefore, reveals that strategically placed glass barriers can enhance the dielectric strength of air-insulated systems. The present study leads to the optimization of the thickness and placement of barriers in future work by understanding the interplay with breakdown path to maximize the dielectric strength in miniaturized high-voltage equipment.

Tunable long-range spin transport in a van der Waals Fe3GeTe2/WSe2/Fe3GeTe2 spin valve.

The seamless integration of two-dimensional (2D) ferromagnetic materials with similar or dissimilar materials can widen the scope of low-power spintronics. In this regard, a vertical van der Waals (vdW) heterostructure of 2D ferromagnets with semiconducting transition metal dichalcogenides (TMDCs) forms magnetic junctions with exceptional stability and electrical control. Interestingly, 2D metallic Fe3GeTe2 (FGT) reveals above room temperature Curie temperatures and has large magneto anisotropy due to spin-orbit coupling. In addition, it also possesses topological states and a large Berry curvature. Herein, we designed the FGT/WSe2/FGT vdW heterostructure with a uniform and sharp interface so that FGT could maintain its inherent electronic properties. Also, the uniform thickness of the barrier provides a smooth flow of spins through the junctions as tunneling exponentially decays with an increasing barrier thickness. However, strong energy-dependent spin polarization is crucial for achieving optimum spin valve properties, such as large tunneling magnetoresistance (TMR) along with the manipulation of the magnitude and sign reversal. We have observed a shifting of high-energy localized minority spin states toward low-energy regions, which causes spin polarization fluctuation between -42.5% and 41% over a wide range of bias voltage. This leads to a negative TMR% of ∼-100% at 0.1 V Å-1 and also a large positive TMR% at 0.2 V Å-1 and -0.4 V Å-1. Besides, the system exhibits a highly tunable large anomalous Hall conductivity (AHC) of 626 S cm-1. Interestingly, such unprecedented electronic behaviour with large and switchable spin polarization, anomalous Hall conductivity and TMR can be incorporated into MTJ devices, which provide electrical control and long-range spin transport. Additionally, the system emerges as a standout candidate in low-power spintronic devices (e.g., MRAM and magnetic sensors) owing to its distinctive energy-dependent electronic structure with a wide range of external bias.

Transmission coefficient in double barrier quantum tunnelling effect

At the turn of the twentieth century, the establishment of quantum theory propelled rapid advancements, particularly in the understanding of quantum tunnellinga fundamental phenomenon in quantum mechanics crucial for various physical processes. The quantum phenomenon of particle passes through potential barriers is of great importance. In classical physics, when the energy of a particle is less than the height of a double barrier structure, it is impossible for it to pass through. However, quantum mechanics allows a particle to penetrate the barrier and emerge on the other side. This paper explores the quantum tunnelling effect, focusing on the single potential barrier model in one dimension and subsequently extending to the double potential barrier model. The Schrdinger equation provides the foundational framework for elucidating the motion of microscopic particles, emphasizing wave-particle duality inherent in quantum mechanics. The analysis of the single potential barrier model involves solving the Schrdinger equation in different regions, determining wave functions and coefficients through boundary conditions. The transmission coefficient is derived, representing the probability of a particle passing through a barrier. In the case of a thick barrier, an approximate form for transmission coefficient is provided, demonstrating the exponential decrease in transmission probability with increasing barrier thickness.

Bias dependence of the interlayer conductance in moiré tunnel junctions

We have investigated the bias effect in moiré tunnel junctions using an extending 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 superposition of periods and the corresponding coherence in moiré tunnel junctions. Here, the bias effect on the interlayer conductance is studied under two cases: (1) The thickness of the upper layer is much less that of the lower layer. (2) The thickness of the upper layer is equal to that of the lower layer. In the former case, the bias can significantly depress the intensity of the peak. Physically, the attenuation of the intensity of the peak originates from the breaking of the periodicity of the potential by the bias. In the latter case, the intensity of the peak is less attenuated by the bias, while the position of the peak shifts to smaller angle. These phenomena are due to the significant alteration of the wave vectors at the interface caused by the bias. In addition, we have investigated the influences of barrier thickness on the interlayer conductance in the latter case. It is found that, with increasing barrier thickness, the intensity of the peak will decrease significantly, and be depressed by the bias more significantly. Finally, we discuss how to design the device to decrease the negative impact of the bias on the performance and how to utilize the bias to tune the parameters of the device.

Linac primary barrier transmission: Flattening filter free and field size dependence.

There is widespread consensus in the literature that flattening filter free (FFF) beams have a lower primary barrier transmission than flattened beams. Measurements presented here, however, show that for energy compensated FFF beams, the barrier transmission can be as much as 70% higher than for flattened beams. The ratio of the FFF barrier transmission to the flattened beam barrier transmission increases with increasing barrier thickness. The use of published FFF TVL data for energy compensated FFF beams could lead to an order of magnitude underestimate of the air kerma rate. There are little data in the literature on the field size dependence of the barrier transmission for flattened beams. Barrier transmission depends on the field size at the barrier, not at isocenter Measurements are presented showing the relative dependence of barrier transmission on the field size, measured at the barrier, for 6MV and 10MV beams. An analytical fitting formula is provided for the field size dependence. For field sizes greater than about 150cm in side length, the field size dependence is minimal. For field sizes less than about 100cm, the transmission declines rapidly as the field size decreases.

Linac primary barrier transmission for concrete: Monte Carlo calculations.

Recent publications have called into question the accuracy of reference tenth-value layer (TVL) data cited in official reports for linac primary concrete barriers. Doubts have arisen based on both experimental and theoretical evidence. Most of the standard reference TVL values trace back to a publication that appeared in 1984 that used beam spectra that are not representative of modern linacs. This study reports a new set of TVL data for concrete based on modern linac beam spectra and a definition of the barrier transmission that is consistent with its use in shielding calculations. TVL values have been computed for concrete using Monte Carlo simulation for beam energies of 4, 6, 10, 15, and 18MV. The barrier transmission depends on the field size at the barrier and the distance from the distal surface of the barrier to the point of observation. The TVL values reported here lead to barrier transmission values that are up to a factor of 4 larger than those in official reports. The air kerma rate beyond the barrier does not obey an inverse square law as the barrier now acts like a new (non-point) source of radiation. For distance greater than 0.3m from the distal side of the barrier, inverse square predictions of the air kerma rate are low by up to a factor of 2. The average energy of the transmitted photons declines rapidly for all beam energies with increasing barrier thickness up to a thickness of about 50cm and then slowly increases with increasing thickness.

Uncertainties in linac primary barrier transmission values

Primary barrier design for linac shielding depends very sensitively on tenth value layer (TVL) data. Inaccuracies can lead to large discrepancies between measured and calculated values of the barrier transmission. Values of the TVL for concrete quoted in several widely used standard references are substantially different than those calculated more recently. The older standard TVL data predict significantly lower radiation levels outside primary barriers than the more recently calculated values under some circumstances. The difference increases with increasing barrier thickness and energy, and it can be as large as a factor of 4 for 18 MV and concrete thickness of 200 cm. This may be due to significant differences in the beam spectra between the earlier and the more recent calculations. Measured instantaneous air kerma rates sometimes show large variations for the same energy and thickness. This may be due to confounding factors such as extra material on, or inside the barrier, variable field size at the barrier, density of concrete, and distal distance from the barrier surface. In some cases, the older TVL data significantly underestimate measured instantaneous air kerma rates, by up to a factor of 3, even when confounding factors are taken into account. This could lead to the necessity for expensive remediation. The more recent TVL values tend to overestimate the measured instantaneous dose rates. Reference TVL data should be computed in a manner that is mathematically consistent with their use in the calculation of air kerma rate outside barriers directly from the linac “dose” rate in MU/min.

Josephson junctions of Weyl and multi-Weyl semimetals

We study a Josephson junction involving a Weyl and a multi-Weyl semimetal separated by a barrier region of width $d$ created by putting a gate voltage $U_0$ over the Weyl semimetal. The topological winding number of such a junction changes across the barrier. We show that $I_c R_N$ for such junctions, where $I_c$ is the critical current and $R_N$ the normal state resistance, in the thin barrier limit, has a universal value independent of the barrier potential. We provide an analytical expression of the Andreev bound states and use it to demonstrate that the universal value of $I_c R_N$ is a consequence of change in topological winding number across the junction. We also study AC Josephson effect in such a junction in the presence of an external microwave radiation, chart out its current-voltage characteristics, and show that the change in the winding number across the junction shapes the properties of its Shapiro steps. We discuss the effect of increasing barrier thickness $d$ on the above-mentioned properties and chart out experiments which may test our theory.

The polarization field in Al-rich AlGaN multiple quantum wells

This paper investigates the quantum confined Stark effect in AlGaN multiple quantum well structures with a high Al content grown on single-crystalline AlN substrates. The quantitative relationship between the quantum well structure parameters, photogenerated carrier density, built-in electric field and ground-level emission is discussed. It is found that the electric field strength increases from 0.5 MV cm−1 to almost 3 MV cm−1 when the Al content in the quantum well barriers is increased from 65% to 100%, which is consistent with the theory of spontaneous and piezoelectric polarization in III-nitrides. In addition, the built-in electric field increases significantly with increasing barrier thickness. Based on these results, the electric field in an Al0.55Ga0.45N single quantum well with AlN cladding is predicted to be around 5 MV cm−1.

Reactive diffusion in the presence of a diffusion barrier: Experiment and model

Reactions in thin films and diffusion barriers are important for applications such as protective coatings, electrical contact, and interconnections. In this work, the effect of a barrier on the kinetics of the formation for a single phase by reactive diffusion is investigated from both experimental and modeling point of views. Two types of diffusion barriers are studied: (i) a thin layer of W deposited between a Ni film and Si substrate and (ii) Ni alloy films, Ni(1%W) and Ni(5%Pt), that form a diffusion barrier during the reaction with the Si substrate. The effect of the barriers on the kinetics of δ-Ni2Si formation is determined by in situ X ray diffraction and compared to models that explain the kinetic slowdown induced by both types of barrier. A linear parabolic growth is found for the deposited barrier with an increasing linear contribution for increasing barrier thickness. On the contrary, the growth is mainly parabolic for the barrier formed by the reaction between an alloy film and the substrate. The permeability of the two types of barrier is determined and discussed. The developed models fit well with the dedicated model experiments, leading to a better understanding of the barrier effect on the reactive diffusion and allowing us to predict the barrier behaviour in various applications.

Deviations from Electroneutrality in Membrane Barrier Layers: A Possible Mechanism Underlying High Salt Rejections.

Reverse osmosis and nanofiltration (NF) employ composite membranes whose ultrathin barrier layers are significantly more permeable to water than to salts. Although solution-diffusion models of salt transport through barrier layers typically assume ubiquitous electroneutrality, in the case of ultrathin selective skins and low ion partition coefficients, space-charge regions may occupy a significant fraction of the membrane barrier layer. This work investigates the implications of these deviations from electroneutrality on salt transport. Both immobile external surface charge and unequal cation and anion solvation energies in the barrier layer lead to regions with excess mobile charge, and the size of these regions increases with decreasing values of either feed concentrations or ion partition coefficients. Moreover, the low concentration of the more excluded ion in the space-charge region can greatly increase resistance to salt transport to enhance salt rejection during NF. These effects are especially pronounced for membranes with a fixed external surface charge density whose sign is the same as that of the more excluded ion in a salt. Because of the space-charge regions, the barrier-layer resistance to salt transport initially rises rapidly with increasing barrier thickness and then plateaus or even declines within a certain thickness range. This trend in resistance implies that thin, defect-free barrier layers will exhibit higher salt rejections than thicker layers during NF at a fixed transmembrane pressure. Deviations from electroneutrality are consistent with both changes in NF salt rejections that occur upon changing the sign of the membrane fixed external surface charge, and CaCl2 rejections that in some cases may first decrease, then increase and then decrease again with increasing CaCl2 concentrations in NF feed solutions.

Dependence of output emission wavelength and LD performance on barriers material and thickness

The output emission wavelength of MQW LDs depends on several structural and operating parameters such as the number of QWs, QW and barrier thickness, and material compositions which constitute the active region, operating current, and others. However, output emission wavelength mostly depends on the active region parameters like the thickness and In composition in wells and barriers. In this paper, effect of barriers material and thickness on the performance characteristics of deep violet InGaN DQW LDs were numerically studied. The simulation results demonstrate that output emission wavelength of a particular InGaN LD at different In compositions in the QWs (0.08 to 0.10) and barriers (0.0 (GaN), 0.01, and 0.02) is limited between 388nm and 398nm when the In mole fraction of the barriers is 0.0 (GaN). Likewise, output emission wavelength is limited between 391.7nm and 421.0nm, and 394.2nm and 428.2nm when the In mole fraction of the barriers is 0.01 (In0.01Ga0.99N) and 0.02 (In0.02Ga0.98N), respectively. Furthermore, the results indicated that by increasing barrier thickness, slope efficiency, output power, and DQE decrease, while threshold current increases. Increasing barrier thickness increased strain and piezoelectric fields in wells, thus resulted in an increasing electric field in the active region which helped increase nonradiative recombination and decrease radiative recombination in wells. The results also showed that optical characteristics decline with decreasing barrier thickness less than the critical thickness. Induced quantum-confined Stark effect (QCSE) in this situation resulting from the piezoelectric effect causes threshold current to increase, and output power, slope efficiency, and DQE to decrease.

Magneto-Transport and Nanomagnet Dynamics in MgO-Based Magnetic Tunnel Junctions (MTJs)

We investigate bias and different barrier thicknesses effects on quantities related to spin and charge currents in MgO-based magnetic tunnel junctions. Using the non-Equilibrium Green's function formalism, we demonstrate that the in-plane and out-of-plane components of the spin-transfer torque have asymmetric and symmetric behaviors respectively. Magneto-resistance also decreases with increasing barrier thickness. The Landau–Lifshits–Gilbert equation describes the dynamics of the magnetization made by spin transfer torque. Increasing in spin current above its critical value or smaller the magnet reduces the switching time which is major result for making of new memory devices.

Barrier and well thickness designing of InGaN/GaN multiple quantum well for better performances of GaN based laser diode

The effects of barrier and well thickness in InGaN/GaN (with in content of 15%) multiple quantum well (MQW) on the performances of GaN based laser diode (LD) are investigated by using LASTIP software, and the relevant physical mechanisms are discussed. It is found that when the barrier-thickness in InGaN/GaN MQW is fixed to be 7 nm, for the well thickness values of 3.0, 3.5, 4.0, 4.5, and 5.0 nm, the threshold currents of LD are 76.31, 67.96, 57.60, 64.62, and 74.59 mA, and the output light powers of LD are 12.05, 15.64, 24.70, 18.21, and 11.35 mW under an injection current of 100 mA, respectively. It indicates that too thick or too thin well may lead to a higher threshold current and a lower output power of GaN based LD. A high performance device can be obtained by using an optimized well thickness of around 4.0 nm. It is found that the LD performance is degraded by using too thin well in the device structure mainly due to the high leakage current, while strong polarization will lead to the decrease of overlap integral and luminescence intensity if the well layer is too thick, and thus a poor performance is obtained. It is found that the LD performance can be improved obviously by appropriately increasing barrier thickness from 7 nm to 15 nm. When the barrier thickness in InGaN/GaN MQW is fixed at 15 nm and the well thickness values are 3.0, 3.5, 4.0, 4.5 and 5.0 nm, the threshold currents of LD are 59.54, 52.42, 52.17, 51.38, and 58.99 mA, and the output light powers of LD are 36.12, 39.69, 40.79, 40.27, and 33.19 mW under an injection current of 100 mA, respectively, i.e., LD device parameters are improved. It suggests that the higher performances of GaN based laser diode can be realized by appropriately increasing the thickness of barrier when the thickness of well is optimized to be around 4 nm.

Numerical and physical modeling of geofoam barriers as protection against effects of surface blast on underground tunnels

An explosion on the ground surface may cause significant damage to an underground structure, such as a tunnel or a pipeline. The extent of damage would depend on the intensity of blast, the material and configuration of the structure, as well as the nature and geometry of the intervening material.An underground structure may be protected by means of a protective barrier, installed directly above the structure. The effectiveness of using a compressible barrier, made of polyurethane geofoam, to mitigate the effects of surface explosion was investigated.The effects of a surface explosion were studied through a combination of physical model tests and numerical modeling. Reduced-scale (1:70 scale) physical model tests were conducted using a geotechnical centrifuge, where the scaling law for explosions was utilized to model the effects of a large explosion using a relatively small mass of explosives under a high gravitational field (70 g, in this case). The results of the physical model tests were used to calibrate a three-dimensional numerical model in which a fully-coupled Euler–Lagrange solver was utilized to model the explosions.Tunnel configurations with and without protective barriers were studied to assess the mitigation provided by protective barriers. Material properties for polyurethane geofoam barriers were evaluated from laboratory tests. The influence of barrier thickness in reducing the strains, stresses, and pressures on the tunnel induced by an explosion was studied. The beneficial effects of a protective geofoam barrier were found to increase with increasing barrier thickness only up to a certain thickness, beyond which, further increase in thickness did not result in additional reductions. The results will help in design optimization, while planning protection systems for new tunnels, as well as for retrofitting existing tunnels.

Thickness dependent barrier performance of permeation barriers made from atomic layer deposited alumina for organic devices

Organic devices like organic light emitting diodes (OLEDs) or organic solar cells degrade fast when exposed to ambient air. Hence, thin-films acting as permeation barriers are needed for their protection. Atomic layer deposition (ALD) is known to be one of the best technologies to reach barriers with a low defect density at gentle process conditions. As well, ALD is reported to be one of the thinnest barrier layers, with a critical thickness – defining a continuous barrier film – as low as 5–10nm for ALD processed Al2O3. In this work, we investigate the barrier performance of Al2O3 films processed by ALD at 80°C with trimethylaluminum and ozone as precursors. The coverage of defects in such films is investigated on a 5nm thick Al2O3 film, i.e. below the critical thickness, on calcium using atomic force microscopy (AFM). We find for this sub-critical thickness regime that all spots giving raise to water ingress on the 20×20μm2 scan range are positioned on nearly flat surface sites without the presence of particles or large substrate features. Hence below the critical thickness, ALD leaves open or at least weakly covered spots even on feature-free surface sites. The thickness dependent performance of these barrier films is investigated for thicknesses ranging from 15 to 100nm, i.e. above the assumed critical film thickness of this system. To measure the barrier performance, electrical calcium corrosion tests are used in order to measure the water vapor transmission rate (WVTR), electrodeposition is used in order to decorate and count defects, and dark spot growth on OLEDs is used in order to confirm the results for real devices. For 15–25nm barrier thickness, we observe an exponential decrease in defect density with barrier thickness which explains the likewise observed exponential decrease in WVTR and OLED degradation rate. Above 25nm, a further increase in barrier thickness leads to a further exponential decrease in defect density, but an only sub-exponential decrease in WVTR and OLED degradation rate. In conclusion, the performance of the thin Al2O3 permeation barrier is dominated by its defect density. This defect density is reduced exponentially with increasing barrier thickness for alumina thicknesses of up to at least 25nm.

Influence of AlGaN barrier thickness on electrical and device properties in Al0.14Ga0.86N/GaN high electron mobility transistor structures

We investigate the influence of the AlGaN barrier thickness in Al0.14Ga0.86N/GaN heterostructures on both the electrical properties of the heterostructure itself as well as on high electron mobility transistors fabricated on these structures. With increasing barrier thickness, we observe decreasing sheet resistances, transconductances, and threshold voltages. The observed changes are well-described by modelling. We demonstrate that an increase in the barrier thickness in AlGaN/GaN heterostructures results in an increase in available high-frequency input power swing before turn-on of the Schottky gate. The device long-term stability under direct current stress is not affected by the increase in barrier thickness as shown by on-wafer reliability tests at 150 °C base plate temperature. These results pave the way towards AlGaN/GaN transistors offering high robustness under extreme mismatch conditions as well as excellent high-frequency power performance.

Influence of barrier thickness on the performance of InGaN/GaN multiple quantum well solar cells

The performance of InGaN/GaN multiple quantum well (MQW) solar cells containing 15 periods of 2.7 nm thick In0.21Ga0.79N wells and three different GaN barriers thicknesses of 3.0 nm, 6.3 nm, and 10.0 nm is investigated. Increasing barrier thickness results in absorption at lower energies, consistent with piezoelectric polarization induced electric fields tilting the energy bands of the MQW and changing the transition energy of well states. The internal quantum efficiency and leakage currents are additionally affected by GaN barrier thickness, resulting in the 6.3 nm barrier structure achieving the highest power conversion efficiency (1.66%, 1 sun AM1.5G).

Simulation of electrical properties of InxAl1—xN/AlN/GaN high electron mobility transistor structure

Electrical properties of InxAl1—xN/AlN/GaN structure are investigated by solving coupled Schrödinger and Poisson equations self-consistently. The variations in internal polarizations in InxAl1—xN with indium contents are studied and the total polarization is zero when the indium content is 0.41. Our calculations show that the two-dimensional electron gas (2DEG) sheet density will decrease with increasing indium content. There is a critical thickness for AlN. The 2DEG sheet density will increase with InxAl1—xN thickness when the AlN thickness is less than the critical value. However, once the AlN thickness becomes greater than the critical value, the 2DEG sheet density will decrease with increasing barrier thickness. The critical value of AlN is 2.8 nm for the lattice-matched In0.18Al0.82N/AlN/GaN structure. Our calculations also show that the critical value decreases with increasing indium content.

Elastic and inelastic conductance in Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions

A systematic analysis of the bias voltage and temperature dependence of the tunneling magnetoresistance (TMR) in Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions with barrier thickness ${t}_{B}$ between 1.8 and 4.0 nm has been performed. The resistance measured at low temperature in the parallel state shows the expected exponential increase with increasing barrier thickness. The low-temperature TMR amplitude of about 300% is quite similar for all MgO thicknesses. This is in accordance with microstructural investigations by transmission electron microscopy, which do not give hints to a reduction in the barrier quality with increasing MgO thickness. Both the junction resistance and TMR decrease with increasing temperature and bias voltage. In general, the decrease is much stronger for thicker barriers, e.g., a decrease in the TMR by a factor of 13.4 from 293% at 15 K to 21.9% at 300 K was observed for ${t}_{B}=4.0\text{ }\text{nm}$ compared to a reduction by only a factor of 1.6 for ${t}_{B}=1.8\text{ }\text{nm}$. This behavior can be described self-consistently for all barrier thicknesses within a model that extends the magnon-assisted tunneling model by adding an inelastic, unpolarized tunneling contribution. Furthermore we discuss our results in the framework of a recent model by Lu et al. [Phys. Rev. Lett. 102, 176801 (2009)] claiming that polarized hopping conductance becomes important for larger MgO thickness.