High Potential Barrier Research Articles

Remarkable charge-trapping performance based in Zr0.5Hf0.5O2 with nanocrystal Ba0.6Sr0.4TiO3 blocking layer for nonvolatile memory device

Two kinds of charge trapping memory device with Au/Zr0.5Hf0.5O2(ZHO)/SiO2/p-Si and Au/Ba0.6Sr0.4TiO3(BST)/Zr0.5Hf0.5O2/SiO2/p-Si structure were fabricated and investigated. The double BST/ZHO films exhibit a larger memory window of 7.36 V under ±14 V sweeping voltages in its C–V curve and the device has good charge retention properties with only small charge loss of ∼5% after more than 104 s. The good characteristics are attributed to the inter-diffusion between BST and ZHO where more deep defect sites were created after RTA treatment, which provides high potential barriers for the trapped charges to tunnel back to the silicon substrate. Furthermore, the nanocrystal in the BST layer increases the tunneling barrier of tunneling current into the gate and effectively restrains the leakage of storage charge from blocking layer, which improves the charge retention characteristic.

Theoretical study on the mechanism of reactions of CX3[rad] radicals (X = H, F, Cl and Br) with C20H20 and C20F20 fullerenes

We have applied density functional calculations to study the reactions CX3 + C20H20, and CX3 + C20F20 (X = H, F, Cl and Br) based on two pathways (H- or F-displacement and H- or F-abstraction from C20H20 and C20F20). Our results show that CH3 radical prefers H- or F- abstraction from C20H20 and C20F20 while F-displacement is favorable for the reaction of CF3 with the fullerenes. Exothermic characters of H- or F-abstraction with the lower potential barrier indicate that the H-abstraction will dominate the reaction CCl3 + C20H20 while endothermic characters with high potential barrier heights for two reaction pathways of C20H20 and C20F20 with CBr3 indicate unfavorable thermodynamically and kinetically pathways for these reactions. The exothermic reactions are proceed via reactant-like transition state while endothermic reactions will proceed via “late” and alkyl radical-like transition state, consistent with Hammond's postulate. These results are in line with the distribution of natural charges and electrostatic surface potentials (ESP) of reactants. Among these reactions, the reaction of CF3 with C20F20 is most favorable both thermodynamically and kinetically which indicate that reaction of CF3 with C20F20 and C20H20 is more favorable than CCl3 and CBr3 radicals.

Effects of mid-gap defects and barrier interface reactions on tunneling behaviors of ZnO-i-Si heterojunctions

Low-leakage pin diodes based on ZnO-i-Si are realized by redox reaction of aluminum with the native oxide SiOx into AlOx and by proper selection of annealing conditions. The main sources of electric leakage was found to arise from charge carrier tunneling via mid-gap states in the semiconductors or lowered tunneling barriers. Less mid-gap states in n-ZnO and high tunneling barrier of the i-layer are key to lowering the leakage. Proper post-annealing of pin diodes effectively heal the mid-gap defects, while maintaining the integrity of the tunneling layers, thus lowering the leakage currents to reach a rectification ratio of 2400, surpassing most similarly benchmarked devices reported in literature. Excessive annealing causes some part of the i-layer to transform into to ZnAl2O4 and Al:ZnO. High Al-doping and lowered potential barrier provided by ZnAl2O4 are responsible for high leakage currents in devices so fabricated.

Features of high-temperature electroluminescence in an LED n-GaSb/n-InGaAsSb/p-AlGaAsSb heterostructure with high potential barriers

The electroluminescent properties of a light-emitting diode n-GaSb/n-InGaAsSb/p-AlGaAsSb heterostructure with high potential barriers are studied in the temperature range of 290–470 K. An atypical temperature increase in the power of the long-wavelength luminescence band with an energy of 0.3 eV is experimentally observed. As the temperature increases to 470 K, the optical radiation power increases by a factor of 1.5–2. To explain the extraordinary temperature dependence of the radiation power, the recombination and carrier transport processes are theoretically analyzed in the heterostructure under study.

DSSCs with ZnO@TiO2 core–shell photoanodes showing improved Voc: Modification of energy gradients and potential barriers with Cd and Mg ion dopants

ZnO@TiO2 core–shell nanostructures have been manipulated with metal ion dopants to create DSSC photoanodes with reduced charge transfer recombination. ZnO and TiO2 were modified by doping with Cd and Mg, respectively, resulting in a reduction in the band-gap for Cd:ZnO and an increase in the band-gap for Mg:TiO2. A series of core–shell nanostructures based on a ZnO or a Cd:ZnO core and TiO2 or a Mg:TiO2 shell have been investigated as DSSC photoanodes. In another series, the core–shell materials were reversed for complete evaluation of our hypothesis. The structures of the core–shell nanoparticles, optoelectronic and interfacial charge transfer of the DSSCs were analyzed by XRD, SEM, TEM, XPS, Mott-Schottky experiment, UV–vis DRS, J–V and EIS measurements. The power conversion efficiencies (PCEs) of DSSCs based on these core–shell structures were observed to depend on the flat band of the shell oxide relative to that of the core oxide. DSSCs with Cd:ZnO@Mg:TiO2 and Cd:ZnO@TiO2 photoanodes achieved Voc and FF values comparable to those of DSSCs with TiO2 anodes. A moderate energy gradient and high potential barrier in these core–shell structures resulted in longer effective electron lifetimes and lower electron recombination in the corresponding DSSCs. The low PCE registered for DSSCs with anodes of Cd:ZnO@Mg:TiO2 and Cd:ZnO@TiO2 were attributed to the presence of a high proportion of rutile phase, caused by sintering at 650°C and lower dye adsorption as a result larger particle sizes caused by Cd doping.

Features of barrier crossing event for Lévy flights

The steady-state time characteristics of anomalous diffusion in the form of Lévy flights in a symmetric bistable quartic potential are examined. Based on a previously developed technique, we obtain the exact formula for the correlation time of particle displacement in the case of Cauchy stable noise. The dependence of the correlation time on the height of the potential barrier separating the two stable states and the position of the potential wells is analyzed in detail. As shown, for a sufficiently high potential barrier the correlation time does not depend on its height that can be interpreted as a revision of Kramers' law for Lévy flight superdiffusion.

Extremely Thin Al2O3 Surface‐Passivated Nanocrystalline ZnO Thin‐Film Transistors Designed for Low Process Temperature

Nanocrystalline ZnO (nc‐ZnO) thin‐film transistors (TFTs) exhibit inherent instability under bias/photo stresses, which originates from the oxygen molecules adsorbed on the surface of the crystal grains. The space charge region at nanocrystal surfaces that is induced by adsorbed oxygen molecules produces a high electrical potential barrier and significantly interrupts charge transport between the source and drain in nc‐ZnO TFTs. In this article, we developed high‐performance TFTs via the continuous deposition of an extremely thin Al2O3 layer on a nc‐ZnO channel. These devices were fabricated by atomic layer deposition at an extremely low process temperature of 150°C, including both the deposition and postannealing temperatures. The nc‐ZnO TFT with an extremely thin Al2O3 layer (1.8 nm) showed a significantly higher mobility (25 cm2/Vs) compared to devices without an Al2O3 layer (3.6 cm2/Vs). This dramatic difference was ascribed to the suppression of the chemisorption of oxygen molecules at the nanocrystal surface during thermal annealing (reducing the potential barrier width/height between adjacent nanocrystals). Furthermore, ultrathin Al2O3‐covered nc‐ZnO TFTs exhibited considerably enhanced electrical/photo stability due to the reduction in adsorption/desorption events of oxygen molecules on the nanocrystal surfaces (with no change in the depletion width after illumination) under gate bias or illumination stress.

Novel p-Type Conductive Semiconductor Nanocrystalline Film as the Back Electrode for High-Performance Thin Film Solar Cells.

Thin film solar cells, due to the low cost, high efficiency, long-term stability, and consumer applications, have been widely applied for harvesting green energy. All of these thin film solar cells generally adopt various metal thin films as the back electrode, like Mo, Au, Ni, Ag, Al, graphite, and so forth. When they contact with p-type layer, it always produces a Schottky contact with a high contact potential barrier, which greatly affects the cell performance. In this work, we report for the first time to find an appropriate p-type conductive semiconductor film, digenite Cu9S5 nanocrystalline film, as the back electrode for CdTe solar cells as the model device. Its low sheet resistance (16.6 Ω/sq) could compare to that of the commercial TCO films (6-30 Ω/sq), like FTO, ITO, and AZO. Different from the traditonal metal back electrode, it produces a successive gradient-doping region by the controllable Cu diffusion, which greatly reduces the contact potential barrier. Remarkably, it achieved a comparable power conversion efficiency (PCE, 11.3%) with the traditional metal back electrode (Cu/Au thin films, 11.4%) in CdTe cells and a higher PCE (13.8%) with the help of the Au assistant film. We believe it could also act as the back electrode for other thin film solar cells (α-Si, CuInS2, CIGSe, CZTS, etc.), for their performance improvement.

Tunneling condition at high Schottky barrier and ambipolar transfer characteristics in zinc oxide semiconductor thin film transistor

This study investigates the behavior of SiOC as gate dielectric materials for zinc-based oxide semiconductor thin film transistors (TFTs). The SiOC with a high potential barrier due to a low ionic energy at a Poole–Frenkel (PF) contact was suitable for use as a gate dielectric material to support tunneling in ZnO/SiOC TFTs. The performance of the ZnO TFTs for the low-polarization SiOC improved by introducing a direct tunneling phenomenon in the minority carriers of the SiOC depletion layer due to the decrease in the activation energy and the very high Schottky barrier (SB) of the SiOC material. PF emission was achieved at a non-polar SiOC with a high SB, and the mobility-stability of the TFTs with the PF contact dramatically improved. The TFT of the PF contact with a high SB was free from a shift in the threshold voltage with a decrease of the drain voltage.

Cosmic censorship and stationary states of half-spin particles in the field of Reissner-Nordström naked singularity

The paper explores quantum mechanics of half-spin particle motion in the field of Reissner-Nordström (RN) naked singularity. It is shown that for any quantum mechanical Dirac particle, irrespective of availability and sign of its electrical charge, the RN naked singularity is separated by an infinitely high positive potential barrier. With like charges of a particle and the source of the RN naked singularity, near the origin there exists the second completely impenetrable potential barrier. It has been proved that in the field of the RN naked singularity, bound states of half-spin particles can exist. The conditions for appearance of such states were revealed and computations were performed to find energy eigenvalues and eigenfunctions.

Variability study of Si nanowire FETs with different junction gradients

Random dopant fluctuation effects of gate-all-around Si nanowire field-effect transistors (FETs) are investigated in terms of different diameters and junction gradients. The nanowire FETs with smaller diameters or shorter junction gradients increase relative variations of the drain currents and the mismatch of the drain currents between source-drain and drain-source bias change in the saturation regime. Smaller diameters decreased current drivability critically compared to standard deviations of the drain currents, thus inducing greater relative variations of the drain currents. Shorter junction gradients form high potential barriers in the source-side lightly-doped extension regions at on-state, which determines the magnitude of the drain currents and fluctuates the drain currents greatly under thermionic-emission mechanism. On the other hand, longer junction gradients affect lateral field to fluctuate the drain currents greatly. These physical phenomena coincide with correlations of the variations betwe...

Encapsulation of ferrocene by self-assembled rosette nanotubes: An investigation using statistical mechanical theory of molecular liquids

Encapsulation thermodynamics of a guest molecule by self-assembled rosette nanotubes (RNTs) was investigated using the statistical mechanical theory of molecular liquids also known as 3D-RISM theory. Ferrocene was chosen as a guest molecule because it is a good mimic for a small drug molecule in terms of logP value. The present computational study predicts that ferrocene can penetrate the channel of all three RNTs examined, however, the potential of mean force profile along the RNT channel suggested that RNT that has the smallest channel diameter has relatively high potential barrier for uptake and release of ferrocene, and therefore it was suggested that this could be a suitable mechanism to trap (during self-assembly) and release (during disassembly) a drug candidate. This work suggests also that tuning the RNT channel dimensions and chemistry should allow us to develop a palette of RNT delivery systems for a variety of drugs.

Comparison of Electron and Hole Mobilities in Multiple-Quantum-Well Solar Cells Using a Time-of-Flight Technique

Understanding of transport dynamics of both electrons and holes in quantum-structure solar cells is essential for their structure design and performance enhancement. By applying our proposed carrier time-of-flight technique on p-on-n and n-on-p configurations, we can separately evaluate electron and hole transport across quantum structures inserted in the i-region of solar cells. Electron and hole behaviors in two sets of InGaAs/GaAsP multiple-quantum-well (MQW) solar cells with different potential barrier heights are investigated in this study. Both types of carriers in In0.21Ga0.79As/GaAs0.75P0.25 MQWs show faster averaged velocities by an order of magnitude than those in In0.21Ga0.79As/GaAs0.59P0.41 MQWs, which have higher potential barriers. Within the same MQW structure, the measured values of electron and hole averaged velocities are very close to each other. This results in the same order of effective mobilities of electrons and holes and the similar tendency of cell performance in p-on-n and n-on-p MQW solar cells. Holes in high-barrier InGaAs/GaAsP MQWs show nonlinear mobility, suggesting that a careful design of high-barrier MQWs is required to avoid the hole bottleneck and enhance charge carrier collection.

Dynamics and Kinetics Study of "In-Water" Chemical Reactions by Enhanced Sampling of Reactive Trajectories.

High potential energy barriers and engagement of solvent coordinates set challenges for in silico studies of chemical reactions, and one is quite commonly limited to study reactions along predefined reaction coordinate(s). A systematic protocol, QM/MM MD simulations using enhanced sampling of reactive trajectories (ESoRT), is established to quantitatively study chemical transitions in complex systems. A number of trajectories for Claisen rearrangement in water and toluene were collected and analyzed, respectively. Evidence was found that the bond making and breaking during this reaction are concerted processes in solutions, preferentially through a chairlike configuration. Water plays an important dynamic role that helps stabilize the transition sate, and the dipole-dipole interaction between water and the solute also lowers the transition barrier. The calculated rate coefficient is consistent with the experimental measurement. Compared with water, the reaction pathway in toluene is "narrower" and the reaction rate is slower by almost three orders of magnitude due to the absence of proper interactions to stabilize the transition state. This study suggests that the "in-water" nature of the Claisen rearrangement in aqueous solution influences its thermodynamics, kinetics, as well as dynamics.

CONTROL OF CURRENT TRANSPORT IN SCHOTTKY DIODE USING THE QUANTUM DOTS

SummaryPurpose. In the framework of the drift-deformation-diffusion approximation considering the self-consistent boundary conditions, investigate the current-voltage characteristic (CVC) of surface-barrier structure of type Schottky with built layer of quantum dots and establish the criteria for the technological parameters of surface-barrier structure of type Schottky with built layer of quantum dots at which current-voltage characteristic has S-shaped character. Research methods. Method of self-consistent boundary conditions. Research results. As seen from the numerical calculations, the current-voltage characteristic (CVC) of low barrier (åV5.0≤Δ) Schottky diode with built layer of quantum dots in the space charge region is S-shaped nature. With increasing barrier height Δ to 0.9 eV the area of negative differential resistance on CVC is disappears, and in the interval of voltages 0.5V - 1V has exponential character. Variation of thickness i- layer (0d0RLL−=) of  toA150 leads to changing the character of CVC, i.e. the instability current of S-type is disappears. In this work the results of numerical calculations of CVC for the direct direction of displacement at two values of temperature (T = 77 K, 300K) is given. As seen from the graphs CVC in the range of the voltages Â52.0VÂ24.0<< the value of the current density of voltage at T=77 K is smaller than at T=300K.Conclusions1. It is established that current-voltage characteristic (CVC) of low barrier (åÂ5.0≤Δ) Schottky diode with built layer of quantum dots in the space charge region is S-shaped nature, when the layer of quantum dots is located at distances from the boundary of the contact metal-semiconductor of the order of (10-30)0a, where 0a is the period lattice of the semiconductor.2. It is shown that both the increasing barrier height Δof 0.5 eV to 0.9eV, and the distance from the metal to layer of QD (dL) of A50 to A150, the area of negative differential resistance on CVC is disappears.3. At high potential barrier (0.9eV) the contact effects is dominate and the site of the exponential dependence on voltage (0.5V-1V) on CVC is present, and the lower potential barrier the dependence of current on voltage is approximates to a quadratic (V> 0.6B).

Introgression between invasive and native blue mussels (genus Mytilus) in the central California hybrid zone.

The ecological and genetic factors determining the extent of introgression between species in secondary contact zones remain poorly understood. Here, we investigate the relative importance of isolating barriers and the demographic expansion of invasive Mytilus galloprovincialis on the magnitude and the direction of introgression with the native Mytilus trossulus in a hybrid zone in central California. We use double-digest restriction-site-associated DNA sequencing (ddRADseq) to genotype 1337 randomly selected single nucleotide polymorphisms and accurately distinguish early and advanced generation hybrids for the first time in the central California Mytilus spp. hybrid zone. Weak levels of introgression were observed in both directions but were slightly more prevalent from the native M.trossulus into the invasive M.galloprovincialis. Few early and advanced backcrossed individuals were observed across the hybrid zone confirming the presence of strong barriers to interbreeding. Heterogeneous patterns of admixture across the zone of contact were consistent with the colonization history of M.galloprovincialis with more extensive introgression in northern localities furthest away from the putative site of introduction in southern California. These observations reinforce the importance of dynamic spatial and demographic expansions in determining patterns of introgression between close congeners, even in those with high dispersal potential and well-developed reproductive barriers. Our results suggest that the threat posed by invasive M.galloprovincialis is more ecological than genetic as it has displaced, and continues to displace the native M.trossulus from much of central and southern California.

V-pits as Barriers to Diffusion of Carriers in InGaN/GaN Quantum Wells

The luminescence characteristics of V-pits in InGaN/GaN quantum wells (QW) correlated directly with the microstructure of the V-pits, as studied by use of transmission electron microscopy with cathodoluminescence. {10-11}-Faceted V-pits, formed in the QW, produce more intense blue-shifted emission than {0001}-plane QW. A dead emission center seems to be present at the corner of the V-pit which connects the R-plane and C-plane QW. High-resolution transmission electron microscopy revealed formation of indium-deficient QW at the corners of the V-pits. High potential barriers occur because of the lack of indium around the hexagonal V-pit; this effectively blocks diffusion of carriers into the threading dislocations known to be non-radiative recombination centers. V-pits thus have promise for improving the internal quantum efficiency of light-emitting diodes.

Quantum-mechanical diffraction theory of light from a small hole: Extinction-theorem approach

In a recent paper [Phys. Rev. A 90, 043830 (2014)] it was shown that the so-called aperture response tensor is the central concept in the microscopic quantum theory of light diffraction from a small hole in a flat screen. It was further shown that the quantum mechanical theory of diffraction only requires a preknowledge of the incident field plus the electronic properties of identical screens with and without a hole. Starting from the quantum mechanical expression for the linear conductivity tensor, we study the related causal conductivity tensor paying particular attention to diamagnetic electron dynamics. Using a nonlocal-potential separation assumption, we present a calculation of the diamagnetic causal surface conductivity for a jellium quantum-well screen using a two-dimensional Hartree-Fock model. In the diamagnetic case the difference between the light-unperturbed electron densities for screens with $({n}^{0})$ and without $({n}_{\ensuremath{\infty}}^{0})$ holes are the primary quantities for the diffraction theory. In a central part (Sec. IV) of this article we determine ${n}^{0}$ via a quantum-mechanical two-dimensional extinction-theorem approach related to elastic electron scattering from a hole with an electronic selvedge. For heuristic purposes we illustrate aspects of the extinction-theorem theory by applying the approach for an infinitely high potential barrier to the vacuum hole. Finally, we calculate and discuss the aperture response tensor in the small hole limit and in the zeroth-order Born approximation. Our final result for the aperture response tensor establishes the bridge to the anisotropic electric dipole polarizability tensor of the hole. It turns out that the effective optical aperture (hole) size relates closely to the extension of the relevant electronic wave functions scattered from the hole.

Metal-atom-induced charge redistributions and their effects on the electrical contacts to WS2 monolayers

The structural and electronic properties of high work function metal atoms (Au and Pd) and low work function metal atoms (Ti and In) adsorbed WS2 monolayers are systematically studied using first-principles calculations. Due to the weak interaction between Au and S atoms, high Schottky barriers may be formed at the interface. Using the metal Pd contact, p-type ohmic contact may be fabricated, but the localized and sparse partial charge densities decrease the electronic transparency and increase the contact resistance. For the low work function metal In, the delocalized n-type conduction interface states form in the bandgap; however, the wide and high electrostatic potential barrier suppresses the electron-transition probability. Strikingly different from Au, Pd, and In, the strongest interaction between Ti and S atoms induces the delocalized n-type conduction interface states with the highest electronic states near the Fermi level. The delocalized and high partial charge densities, as well as the narrowest and lowest electrostatic potential barrier, would enhance the electronic transparency and form the n-type ohmic contact. Furthermore, the results of the multilayer metal contacts are consistent with those of the atom-adsorbed systems. These investigations offer valuable resources for the design and fabrication of two-dimensional nanodevices.

Characteristics of piezoelectric ZnO/AlN−stacked flexible nanogenerators for energy harvesting applications

Flexible piezoelectric zinc oxide (ZnO)-based nanogenerators (NGs) using an aluminum nitride (AlN) interlayer are proposed for high-efficiency energy harvesting applications. The effects of the AlN interlayer on device performance are studied. Use of the AlN interlayer in ZnO-based vertically integrated NGs (VINGs) results in a significant improvement in terms of the magnitude of the output voltages of up to 200 times when compared with a ZnO-based VING without any AlN interlayer. The improved device energy conversion efficiency is mainly attributed to a high contact potential barrier that the AlN interlayer provides in VINGs, along with the relatively high dielectric constant and large Young's modulus of the AlN material. In addition, the effects of AlN thickness on the electric potential and device performance of the VINGs are investigated through observation of the output voltages of ZnO-based VINGs with thickness/position-controlled AlN interlayers. Our findings in this work are expected to provide effective and useful approaches for realizing highly energy-efficient ZnO-based NGs and their extended applications, including self-power sources and sensor devices.