High Potential Barrier Research Articles

Comparative Study of Heterostructure Barrier Diodes in the GaAs/AlGaAs System

A comparative study of the electron transport property and operation of the Potential Well Barrier (PWB) diode and Planar-doped Potential-well Barrier (PPB) diode has been carried out in this study. Both diodes are heterostructures in GaAs/AlGaAs system with similarities in layer design though, with a sheet charge inserted close to the well of the PPB diode. A drift-diffusion and Monte Carlo simulation models were used throughout the study to examine the behavior of electrons in terms of the electric field distribution across the diodes, electron velocities, electron energy and densities. Results of simulation has shown how the electric field varies in the left and right intrinsic regions of the device and the effect of the field on velocity. The I-V characteristics of the experimental and simulation results have shown a good agreement in the two diodes though, with little adjustment of about ± 2.5% to design parameters in order to obtain a good fit with experimental results. The I-V characteristics of the diodes reveal that the PPB diode turns on at a higher voltage than the PWB diode though, with a better asymmetry in the reverse bias operation. This is because the sheet charge in the PPB diode produces additional charge and together with the charge in the well, presents a higher potential barrier than the PWB diode whose barrier is determined by the charge in the well only. The diodes demonstrate promising RF behavior with voltage responsivity of 10900V/W and 6400V/W at 10GH for the PPB and PWB diodes respectively.

Features of Charging–Discharging of Supercapacitors

We present the results of experimental investigations of supercapacitors produced by Panasonic using several methods, namely, measurements of temporal dependences of charging–discharging currents, cyclic dc charging, and cyclic voltammetry. The values of the internal resistance, static and dynamic capacitance, as well as their dependences on the bias voltage of the capacitors, have been determined. Measurements have shown that the initial stage of relaxation of current is well approximated by the exponential time dependence, and then transition to a powerlike dependence occurs. This well-known effect is explained on the basis of the allowance for specific transport properties of a porous fractal-type medium. These processes are adequately described by the fractional-differential model of anomalous diffusion. A weak dependence of relaxation curves on the voltage at low values of the latter ( 3 V) is explained by the appearance of new percolation paths blocked at low charging voltages due to the presence of high-potential barriers. The internal resistance and the static capacitance have been determined by measuring the voltage across the supercapacitor in the mode of dc charging. These parameters have been shown to depend on the voltage applied to the capacitor. The dependence of the dynamic capacitance on the voltage has been determined using cyclic voltammetry. It has been shown that the capacitance depends not only on the voltage, but also on the prehistory of charging and discharging of the capacitor. Comparison of the experimental results and the published data on the models and equivalent circuits with passive R, L, and C elements allows one to conclude that such models and equivalent circuits can be applied only when explaining a limited number of phenomena, in particular, behavior at small relaxation times.

A TD-DFT investigation of photoinduced excited-state intermolecular multi-proton transfer dynamics of a 3-hydroxy-2-(thiophen-2-yl) chromen-4-one in methanol

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods have been performed to investigate the ground and exited states intermolecular multiple protons transfer process of 3-hydroxy-2-(thiophen-2-yl)chromen-4-one (3-HTC) in methanol solvent. We demonstrated that the intermolecular hydrogen bonds formed between 3-HTC and CH3OH are strengthened at the first single excited state by comparing the bond parameters and infrared vibrational spectra with those in S0 state, which provides the possibility for exited state intermolecular multiple protons transfer reaction. Based on the potential energy surface/curve at S0 state and S1 state, a CH3OH-assist exited state simultaneous double proton transfer mechanism has been proposed, whose reaction barrier is slightly lower than the case of intramolecular single proton transfer reaction reported previously (RSC advances, 2016, 6, 96147) and the case of two CH3OH assisted exited state multi-proton transfer reaction; in comparison, the high potential barrier heights among the local minima on the S0 surface imply that the ground state double proton transfer reaction is significantly inhibited. Moreover, the absorption and fluorescence spectra of 3-HTC in CH3OH solvent were simulated for the first time, which haven't been reported in the previous experimental and theoretical studies. Our calculations predicted the dual fluorescence behavior, which corresponds to Stokes shifted emissions of 3-HTC-CH3OH complex and its double protons transfer phototautomer, respectively.

Tunneling Control of Chemical Reactions: The Third Reactivity Paradigm.

This Perspective describes the emergence of tunneling control as a new reactivity paradigm in chemistry. The term denotes a tunneling reaction that passes through a high but narrow potential energy barrier, leading to formation of a product that would be disfavored if the reaction proceeded by passage over kinetic barriers rather than through them. This reactivity paradigm should be considered along with thermodynamic and kinetic control as a factor that can determine which of two or more possible products is likely to be the one obtained. Tunneling control is a concept that can provide a deep and detailed understanding of a variety of reactions that undergo facile (and possibly unrecognized) tunneling.

Properties of perovskite ferroelectrics deposited on F doped SnO2 electrodes and the prospect of their integration into perovskite solar cells

The integration of ferroelectrics in perovskite solar cells is proposed as possible way to enhance charge collection efficiency. First results on solar cell manufactured with PbTiO3 (PTO) instead of TiO2 have shown negligible values for the power conversion efficiency (PCE). This is explained by the high serial resistance of sol-gel deposited PTO on F:SnO2 electrodes (FTO). Although PTO layer has remnant polarization of 22μC/cm2, the high potential barrier (0.25±0.05eV) at the FTO/PTO interface and low carrier mobility (10−8cm2V−1s−1) compared to TiO2 leads to high serial resistance. Better results were obtained with thinner PTO layers grown by pulsed laser deposition, with PCE values up to 0.6%. Further enhancement was obtained by replacing PTO with BaTiO3 (BTO), with PCE value reaching about 0.8% after poling the cell with +3V. The most important finding was that the magnitude of the short circuit current increases with the amplitude of the poling voltage while the value of the open-circuit voltage remains about the same, around 0.9V. This is explained through more efficient collection of the charges generated under illumination in the absorber layer due to the polarization that is present in the ferroelectric film.

Hydrogen-abstraction reactions of fully hydrogenated fullerene cages with the amino radical: a density functional study

ABSTRACTWe have applied density functional theory calculations to study the reactions of NH2 + CnHn (n = 20, 40, 50, 60, 70 and 80). Due to the hard curvature in C20 cage, the NH2• + C20H20 → NH3 + C20H19• reaction is nearly thermoneutral with a high potential barrier height. For the CnHn fulleranes with n > 20 the transition states appear earlier on the reaction paths, as can be anticipated for exothermic reactions. Using the spherical excess parameter, we distinguished different curvatures on the surfaces of fullerane cages. The reaction enthalpies ΔH°298 and potential barrier heights ΔETS of the considered reactions indicate good correlation with the values of ϕi parameter, showing an upward trend with the curvature increasing at carbon sites. We have also investigated the H-abstraction of the chemical derivatives of the C20H20 cage (C20H19–CH3, C20H19–CH2CH3 and C20H19–CH2CH2CH3) in comparison to the corresponding isolated alkanes (CH4, C2H6 and C3H8). Overall, it could be inferred that the H-abstraction from the primary and secondary C–H bonds of isolated alkanes could occur more easily than fullarane derivatives.

Molecular dynamics simulations of ether- and ester-linked phospholipids

Dissimilarities in the bulk structure of bilayers composed of ether- vs ester-linked lipids are well-established; however, the atomistic interactions responsible for these differences are not well known. These differences are important in understanding of why archaea have a different bilayer composition than the other domains of life and why humans have larger concentrations of plasmalogens in specialized membranes? In this paper, we simulate two lipid bilayers, the ester linked dipalmitoylphosphatidylcholine (DPPC) and the ether lined dihexadecylphosphatidylcholine (DHPC), to study these variations. The structural analysis of the bilayers reveals that DPPC is more compressible than DHPC. A closer examination of dipole potential shows DHPC, despite having a smaller dipole potential of the bilayer, has a higher potential barrier than DPPC at the surface. Analysis of water order and dynamics suggests DHPC has a more ordered, less mobile layer of water in the headgroup. These results seem to resolve the issue as to whether the decrease in permeability of DHPC is due to of differences in minimum area per lipid (A0) or diffusion coefficient of water in the headgroup region (Dhead) (Guler et al., 2009) since we have shown significant changes in the order and mobility of water in that region.

Formation of correlated states and tunneling for a low energy and controlled pulsed action on particles

We consider a method for optimizing the tunnel effect for low-energy particles by using coherent correlated states formed under controllable pulsed action on these particles. Typical examples of such actions are the effect of a pulsed magnetic field on charged particles in a gas or plasma. Coherent correlated states are characterized most comprehensively by the correlation coefficient r(t); an increase of this factor elevates the probability of particle tunneling through a high potential barrier by several orders of magnitude without an appreciable increase in their energy. It is shown for the first time that the formation of coherent correlated states, as well as maximal |r(t)|max and time-averaged 〈|r(t)|〉 amplitudes of the correlation coefficient and the corresponding tunneling probability are characterized by a nonmonotonic (oscillating) dependence on the forming pulse duration and amplitude. This result makes it possible to optimize experiments on the realization of low-energy nuclear fusion and demonstrates the incorrectness of the intuitive idea that the tunneling probability always increases with the amplitude of an external action on a particle. Our conclusions can be used, in particular, for explaining random (unpredictable and low-repeatability) experimental results on optimization of energy release from nuclear reactions occurring under a pulsed action with fluctuations of the amplitude and duration. We also consider physical premises for the observed dependences and obtain optimal relations between the aforementioned parameters, which ensure the formation of an optimal coherent correlated state and optimal low-energy tunneling in various physical systems with allowance for the dephasing action of a random force. The results of theoretical analysis are compared with the data of successful experiments on the generation of neutrons and alpha particles in an electric discharge in air and gaseous deuterium.

Enhancing the Ultraviolet/Visible Rejection Ratio of MgxZn1−xO Metal-Semiconductor-Metal Photodetectors Using Oxygen-Plasma Treatment

Oxygen-plasma treatment was employed on Mg x Zn1− x O surface with various treatment times (0–15 min) and metal-semiconductor-metal photodetectors (MSM-PDs) were fabricated. The leakage current of the MSM-PDs greatly decreased with treatment times. Before oxygen-plasma treatment, the MSM-PDs shows a less dependence of ultraviolet (UV) response with bias voltages; however, the visible response, originated from the absorption of defects, increases with bias voltages. After treatment, conversely, the MSM-PDs demonstrate a sharply increasing UV response with bias voltages; whereas, the visible response is independent with bias voltages. Such leads to 320/500-nm rejection ratio increases with bias voltages and a very high value of 168 at −80 V for the 10-min treated MSM-PDs, which is six times larger than that of MSM-PDs with as-deposited Mg x Zn1− x O film. X-ray photoelectron spectroscopy shows that with increasing treatment time, the number of oxygen atoms increase and compensate the oxygen vacancies on the Mg x Zn1− x O surface. Most of these oxygen atoms react with Mg atoms to build a highly stable passivation MgO film on Mg x Zn1− x O surface, the insulating MgO film forms a higher potential barrier between Au and Mg x Zn1− x O layer, hence greatly reducing the leakage current and enhancing the UV/visible rejection ratio in the oxygen-plasma treated MSM-PDs.

Illumination intensity dependence of the performance of interdigitated back contact heterojunction silicon solar cells and the effects of front surface field

The performance of interdigitated back contact silicon heterojunction solar cells as a function of illumination intensity has been studied by two-dimensional simulation. A doped front surface field layer is effective to suppress the reduction of conversion efficiency at the illumination below 0.01 sun even with good front passivation and at 1 sun with poor passivation. The experimental devices showed the almost consistent results. For the illuminations above approximately 1 sun, the fill factor and efficiency decreased with illumination intensity, likely due to the increase in series resistance with the increase of “hole” current passing through the high potential barrier at the p-type heterojunction.

Kinetic model of the bichromatic dark trap for atoms

A kinetic model of atom confinement in a bichromatic dark trap (BDT) is developed with the goal of describing its dissipative properties. The operating principle of the deep BDT is based on using the combination of multiple bichromatic cosine–Gaussian optical beams (CGBs) for creating high-potential barriers, which is described in our previous work (Krasnov 2016 Laser Phys. 26 105501). In the indicated work, particle motion in the BDT is described in terms of classical trajectories. In the present study, particle motion is analyzed by means of the Wigner function (phase-space distribution function (DF)), which allows one to properly take into account the quantum fluctuations of optical forces. Besides, we consider an improved scheme of the BDT, where CGBs create, apart from plane potential barriers, a narrow cooling layer. We find an asymptotic solution of the Fokker–Planck equation for the DF and show that the DF of particles deeply trapped in a BDT with a cooling layer is the Tsallis distribution with the effective temperature, which can be considerably lower than in a BDT without a cooling layer. Moreover, it can be adjusted by slightly changing the CGBs’ radii. We also study the effect of particle escape from the trap due to the scattering of resonant photons and show that the particle lifetime in a BDT can exceed several tens of hours when it is limited by photon scattering.

Parallel tempering algorithm for integration over Lefschetz thimbles

The algorithm based on integration over Lefschetz thimbles is a promising method to resolve the sign problem for complex actions. However, this algorithm often meets a difficulty in actual Monte Carlo calculations because the configuration space is not easily explored due to the infinitely high potential barriers between different thimbles. In this paper, we propose to use the flow time of the antiholomorphic gradient flow as an auxiliary variable for the highly multimodal distribution. To illustrate this, we implement the parallel tempering method by taking the flow time as a tempering parameter. In this algorithm, we can take the maximum flow time to be sufficiently large such that the sign problem disappears there, and two separate modes are connected through configurations at small flow times. To exemplify that this algorithm does work, we investigate the (0+1)-dimensional massive Thirring model at finite density and show that our algorithm correctly reproduces the analytic results for large flow times such as T=2.

Polarization compensation at low p-GaN doping density in InGaN/GaN p-i-n solar cells: Effect of InGaN interlayers

The effectiveness of polarization matching layer (PML) between i-InGaN/p-GaN is studied numerically for Ga-face InGaN/GaN p-i-n solar cell at low p-GaN doping (∼5e17 cm−3). The simulations are performed for four InxGa1-xN/GaN heterostructures (x = 10%, 15%, 20% and 25%), thus investigating the impact of PML for low as well as high indium containing absorber regions. Use of PML presents a suitable alternative to counter the effects of polarization-induced electric fields arising at low p-GaN doping density especially for absorber regions with high indium (>10%). It is seen that it not only mitigates the negative effects of polarization-induced electric fields but also reduces the high potential barriers existing at i-InGaN/p-GaN heterojunction. The improvement in photovoltaic properties of the heterostructures even at low p-GaN doping validates this claim.

Enhancing chaotic behavior at room temperature in GaAs/(Al,Ga)As superlattices

Previous theoretical and experimental work has put forward 50-period semiconductor superlattices as fast, true random number generators at room temperature. Their randomness stems from feedback between nonlinear electronic dynamics and stochastic processes that are intrinsic to quantum transitions. This work theoretically demonstrates that shorter superlattices with higher potential barriers contain fully chaotic dynamics over several intervals of the applied bias voltage compared to the 50-periods device which presented a much weaker chaotic behavior. The chaos arises from deterministic dynamics, hence it persists even in the absence of additional stochastic processes. Moreover, the frequency of the chaotic current oscillations is higher for shorter superlattices. These features should allow for faster and more robust generation of true random numbers.

Investigation on the formation of lonsdaleite from graphite

Structural stability and the possible pathways to experimental formation of lonsdaleite—a hexagonal 2H polytype of diamond—have been studied in the framework of the density functional theory (DFT). It is established that the structural transformation of orthorhombic Cmmm graphite to 2H polytype of diamond must take place at a pressure of 61 GPa, while the formation of lonsdaleite from hexagonal P6/mmm graphite must take place at 56 GPa. The minimum potential barrier height separating the 2H polytype state from graphite is only 0.003 eV/atom smaller than that for the cubic diamond. The high potential barrier is indicative of the possibility of stable existence of the hexagonal diamond under normal conditions. In this work, we have also analyzed the X-ray diffraction and electron-microscopic data available for nanodiamonds found in meteorite impact craters in search for the presence of hexagonal diamond. Results of this analysis showed that pure 3C and 2H polytypes are not contained in the carbon materials of impact origin, the structure of nanocrystals found representing diamonds with randomly packed layers. The term “lonsdaleite,” used to denote carbon materials found in meteorite impact craters and diamond crystals with 2H polytype structure, is rather ambiguous, since no pure hexagonal diamond has been identified in carbon phases found at meteorite fall sites.

Fundamental Mechanisms of Solvent Decomposition Involved in Solid-Electrolyte Interphase Formation in Sodium Ion Batteries

Prolonged decomposition of electrolytes forming a thick and unstable solid-electrolyte interphase (SEI) continues to be a major bottleneck in designing sodium-ion batteries (SIBs). We have carried out quantum chemistry simulations to investigate the fundamental mechanisms of reduction-induced decomposition of electrolyte solvents in the vicinity of a sodium ion. Kinetics and thermodynamics of several reaction pathways for one- and two-electron reduction of ethylene carbonate (EC) have been examined. Our calculations indicate that the high reduction potential and low barrier for the ring opening of EC is the main cause for the continuous growth of SEI observed in SIBs. The impact of two well-known electrolyte additives, vinyl carbonate (VC) and fluoroethylene carbonate (FEC), on SEI composition was evaluated by studying decomposition pathways of (1) VC and FEC molecules in the bulk EC solvent and (2) an EC molecule in a supermolecular cluster comprising an EC and the additive molecule. The additive molecul...

Klein tunneling in Weyl semimetals under the influence of magnetic field.

Klein tunneling refers to the absence of normal backscattering of electrons even under the case of high potential barriers. At the barrier interface, the perfect matching of electron and hole wavefunctions enables a unit transmission probability for normally incident electrons. It is theoretically and experimentally well understood in two-dimensional relativistic materials such as graphene. Here we investigate the Klein tunneling effect in Weyl semimetals under the influence of magnetic field induced by ferromagnetic stripes placed at barrier boundaries. Our results show that the resonance of Fermi wave vector at specific barrier lengths gives rise to perfect transmission rings, i.e., three-dimensional analogue of the so-called magic transmission angles in two-dimensional Dirac semimetals. Besides, the transmission profile can be shifted by application of magnetic field in the central region, a property which may be utilized in electro-optic applications. When the applied potential is close to the Fermi level, a particular incident vector can be selected by tuning the magnetic field, thus enabling highly selective transmission of electrons in the bulk of Weyl semimetals. Our analytical and numerical calculations obtained by considering Dirac electrons in three regions and using experimentally feasible parameters can pave the way for relativistic tunneling applications in Weyl semimetals.

Breakdown of 1D water wires inside charged carbon nanotubes

Using molecular dynamics approach we investigated the structure and dynamics of water confined inside pristine and charged 6,6 carbon nanotubes (CNTs). This study reports the breakdown of 1D water wires and the emergence of triangular faced water on incorporating charges in 6,6 CNTs. Incorporation of charges results in high potential barriers to flipping of water molecules due to the formation of large number of hydrogen bonds. The PMF analyses show the presence of ∼2kcal/mol barrier for the movement of water inside pristine CNT and almost negligible barrier in charged CNTs.

Secondary electron emission of graphene-coated copper

The secondary electron emission of graphene-coated copper has very interesting properties. In this work, we prepared graphene-coated copper foil and measured both the secondary electron emission yield (SEY) and the secondary electron energy spectra. Compared with the copper base, there is a significantly reduction for the SEY. For the copper base without coating, the max value of SEY is beyond 2.1 for the primary electron energy of 300eV. However, the corresponding value for the graphene-coated copper is about 1.5, reduced about 25%. For the secondary electron energy distribution, the elastically backscattered peak becomes higher and peak of the true secondary electrons is shifted from about 1.5eV to 4eV after coating the copper with graphene. The full width at half maximum of the true secondary electrons peak is also increased. Since the position of the peak is related to the surface potential barrier of the material, we believe that the graphene coating could increase the surface potential barrier of the material. This could also be the reason of SEY reduction because a higher potential barrier can reduce the electron emission from the material. A preliminary theoretical model of the surface potential distribution for copper surface with graphene coating was built and used to analyze the secondary electron emission of metal surface with graphene coating. The graphene coating is found to be an effective method to suppress the secondary electron emission.

Favorable Combination of Schottky Barrier and Junctionless Properties in Field-Effect Transistors for High Temperature Applications

High-temperature transistors are of great interest for industrial applications, for instance, oil&gas and geothermal exploration, avionic, space, automotive and energy conversion. Especially, energy conversion electronics using wide-bandgap power transistors, i.e. SiCFET, in combination with high-temperature silicon gate-driver circuits can significantly increase system efficiency and reduce costs [1]. Our improved planar virtually dopant-free silicon-based double gate SOI FET with Ni and WTiN front-gate electrodes demonstrates ultra-low drain leakage currents at high-temperatures opening a path to record operating temperatures for silicon-based FET devices. Device Structure The long-channel device fabrication relies on a standard CMOS-compatible prototyping process using SOITEC Smart-Cut™ SOI-substrates with the lowest commercially available boron background doping of 1E15cm-3 enabling high carrier mobilities. Mid-gap Schottky barrier (SB) contacts are fabricated via nickel silicidation. The metal front-gate (FG) electrode consist of reactively sputtered WTiN for NMOS or Nickel for PMOS operation on a 8nm thick SiO2 gate dielectric. Channel height (CH) is approximated to 15nm. The back-gate (BG) formed by the silicon handle wafer is insulated by 145nm buried SiO2 (BOX) (Fig.1). Device Operation The device is modeled as a combination of two interacting FETs, one fully ambipolar Schottky barrier backside FET and a second unipolar junctionless (JL) topside FET [2,3]. The BG influences the whole body layer including the S/D Schottky barriers forming an ambipolar enhancement mode SBFET. Therefore, the dominant charge carrier type, i.e. electrons (VBG»0V) or holes (VBG«0V) in the channel region is defined by the BG polarity. In this configuration, the channel is located close to the body silicon to BOX interface (Fig.3). The charge carriers originate from the mid-gap NiSi SB contacts and enter the body layer at room temperature mainly by thermionic field emission (TFE) and at elevated temperatures by thermionic emission (TE) (Fig.2). In contrast, the depletion mode JLFET formed by the FG electrode only locally affects the charge carrier density in the middle of the channel and controls the current flow between source and drain with excellent electrostatic control as illustrated by the carrier density simulation in Fig.3. The combination of the enhancement and depletion mode operation in one device can appositely be summarized as enhancement suppression or dehancement mode operation (DeFET). Results & Discussion P- and NMOS transistor behavior is realized by applying the appropriate BG polarity to the very same device enabling transistor level reconfigurability (Fig.4). Maximum on-state current and threshold voltage are proportional to the magnitude of VBG (Fig.4). The work function difference between Ni and WTiN metal front-gate devices as well as process variations result in a significant threshold shift (Fig.4). The channel height (Fig.1) is a crucial parameter in device fabrication as it directly affects the off-state leakage current and threshold voltage [7]. Regarding high-temperature performance, the use of the SOI substrate eliminates bulk drain leakage while the introduction of SB contacts minimizes the band-to-band tunneling effect in comparison to classic steep S/D pn-junctions. For rising operating temperatures up to 470K, the on-state current increases from 8 to 46µA as TFE and TE increase allowing an overcompensation of the temperature dependent carrier mobility degradation effect (Fig.5,right). At the same time the drain leakage current is only increasing from <100fA to 2nA from 300 to 470K due to the high potential barrier induced by the FG (Fig.5,left). Gate leakage currents are <6pA during all measurements. Furthermore, PMOS behavior with nickel metal front-gate follows the same trends. Device simulations predict that reducing the channel height and introducing mid-κ gate dielectrics, i.e. Al2O3, renders record operating temperatures possible even for scaled devices limited only by reliability. Comparing the here presented DeFET generation with previous generations and other long-channel Si-based high-temperature FET structures illustrates the clear advantage of our device concept in suppressing drain leakage currents at high temperatures (Fig.6) [4-7]. Conclusion In this contribution, we demonstrate by means of experimental measurement data and device simulations the significantly improved drain leakage suppression at high operating temperatures of the planar dehancement mode FET CMOS technology in comparison to our previously published results in [7]. The combination of SB and JLFET properties opens a possible path for silicon based FET operation up to temperatures of 700K for various industrial applications.