Search

The existence of low-frequency disturbance can affect the discharge stability of hollow cathode, and may even lead to the extinction of discharge. In this paper, a semi-transient algorithm based on classical 0-D theoretical model of orificed hollow cathode was developed to study the discharge stability of hollow cathode under a continuous low-frequency electric field disturbance. A laboratory experiment using an orificed BaO-W cathode was carried out to confirm the validity of transient analysis results and to determine an appropriate time step in the analysis. The theoretical evolution trend of discharge current is in good agreement with experiment for a time step value of 12.5 μs although there is a 7% error in discharge duration prediction. This study has identified the role of two emission enhancement mechanisms in the failure process of self-sustained discharge, viz. Schottky effect and sheath funneling effect. The presence of sheath funneling effect induces larger amplitude oscillations and a sharp drop in discharge parameters when the effect ceases to exist, and this significantly weakens the discharge stability. The influence of operating conditions and disturbance parameters on transient discharge characteristics was examined for the NASA NSTAR cathode. Within the parameter range that was studied, findings show that higher xenon flow rates or lower discharge currents provide optimal duration as a result of higher electron temperatures and denser plasma at the initial state. For a fixed operation point, the discharge duration was found to decline exponentially with disturbance amplitude and quadratically with disturbance frequency.

Gallium nitride (GaN) all-around (wrap) gate vertical nanowire (V-NW) field-effect transistors (FETs) are favorable for enhanced electrostatic control of the gate and selectivity for normally on/off operation. In this work, GaN V-NW FETs with a Schottky barrier gate (V-NW MESFETs), were fabricated for the first time. A nanofabrication process with comprehensive description of all processing steps is reported. It was validated with the demonstration of GaN V-NW MESFETs consisting of an array of 900 (30 × 30) GaN NWs with the narrowest until now reported diameter of 100 nm and all-around gate length of 250 nm. The GaN NWs were formed by a top-down approach, which combines conventional nanopatterning techniques and anisotropic wet etching of an initial GaN epilayer, grown by plasma assisted molecular beam epitaxy on a sapphire (0001) substrate. DC I-V characteristics exhibited normally-off operation and threshold voltage of +0.4 V, due to electron depletion region from the all-around Schottky barrier. A maximum drain-source current density (J ds) of 330 A cm-2 and maximum transconductance (g m) of 285 S cm-2 were obtained from I-V measurements. The results and directions for further optimization were discussed.

Rutile TiOlsubg2l/subg, a high temperature phase, has attracted interests as a capacitor dielectric in dynamic random access memories (DRAMs). Despite its high dielectric constant of g 80, large leakage currents caused by a low Schottky barrier height at the TiOlsubg2l/subg/electrode interface have hindered the use of rutile TiOlsubg2l/subg as a commercial DRAM capacitor. Here, we propose a new Ru-Pt alloy electrode to increase the height of the Schottky barrier. The Ru-Pt mixed layer was grown by atomic layer deposition. The atomic ratio of Ru/Pt varied in the entire range from 100 at.% Ru to 100 at.% Pt. Rutile TiOlsubg2l/subg films were inductively formed only on the Ru-Pt layer containing 43 at.% Pt, while anatase TiOlsubg2l/subg films with a relatively low dielectric constant (~ 40) were formed at Pt compositions g 63 at.%. The Ru-Pt (40 - 50 at.%) layer also attained an increase in work function of ~ 0.3 - 0.4 eV, leading to improvement in the leakage currents of the TiOlsubg2l/subg/Ru-Pt capacitor. These findings suggested that Ru-Pt layer could serve as a promising electrode for next-generation DRAM capacitors.

Nanoscale Schottky barrier metal oxide semiconductor field-effect transistors (MOSFETs) are explored by using quantum mechanism effects for thin-body devices. The results suggest that for small nonnegative Schottky barrier heights, even for zero barrier height, the tunnelling current also plays a role in the total on-state current. Owing to the thin body of device, quantum confinement raises the electron energy levels in the silicon, and the tradeoff takes place between the quantum confinement energy and Schottky barrier lowering (SBL). It is concluded that the inclusion of the quantum mechanism effect in this model, which considers an infinite rectangular well with a first-order perturbation in the channel, can lead to the good agreement with numerical result for thin silicon film. The error increases with silicon thickness increasing.

A novel electron emitter with the characteristics of ferroelectric ceramic materials has been developed. The electron-emitting portion of the electron emitter has vacuum gaps between the upper electrode and the surface of the ferroelectric layer. We confirmed that there are two types of electron emission from the electron emitter. One is considered to be the emission of the electron charge from the ferroelectric surface during a positive polarization reversal. In this paper, the characteristics and mechanism of the other type of electron emission are discussed which is expected to have high efficiency. We consider that field emission occurs as induced by the electric field concentrated in the vacuum gap transiently during a negative polarization reversal. A suitable electric conductivity of the ferroelectric surface is considered to be required for electron emission.

The reverse current–voltage (I–V) characteristics of the Schottky barrier radiation detector structures prepared on semi-insulating gallium arsenide (SI GaAs) have been simulated using a modified thermionic field emission (TFE) model. In order to explain the charge–current transport, the effect of tunnelling and thermionic field emission together with the Schottky barrier lowering were considered taking into account the voltage drop on the quasi-neutral bulk region. The modified TFE model describes reverse I–V characteristics in the temperature range between 300 K and 360 K. An observed agreement between measured and simulated characteristics enabled us to determine the Schottky barrier height and specific resistance of the SI GaAs base from the theoretical simulation. The results of calculated resistivities are compared with resistivities determined by the van der Pauw method. The value of the Schottky barrier height is in good agreement with the previously published data. A much higher value of the calculated ohmic current compared to the current corresponding to the diode structure at a low bias/current region is revealed. A detected difference increases with increasing temperature. This fact led us to conclude that the current in the linear part of the I–V characteristics in the low bias region does not correspond to the ohmic transport, being linear; that is, in contradiction with the more generally considered view.

The effects of oxygen plasma treatment on epitaxial ZnO thin films grown by molecular beam epitaxy were studied. The Au–ZnO–In junction exhibiting ohmic behaviour before the treatment gradually changes to a Schottky junction with the increase in oxygen plasma treatment time. The crystallinity and the surface microstructure did not change to any great extent after the treatment. However, the x-ray photoelectron spectroscopy studies show the removal of conductive OH layer from the surface of ZnO films and the current–voltage characteristics of Au–ZnO–In junction exhibit the rectifying behaviour after oxygen plasma treatment. The fabricated Au–ZnO–Au ultraviolet (UV) detector was successfully tested and was observed to be sensitive to the two UV sources used. The photoresponsivities of the UV detector for the irradiation of two different power densities 350 (λ = 356 nm) and 420 µW cm−2 (λ = 254 nm) are 13.5 A W−1, 15.3 A W−1 at 5 V and 128.9 A W−1, 160 A W−1 at 10 V, respectively.

Most models of solid-state thermionic devices assume that all electrons with energy in the direction of transport greater than the barrier height are transmitted and utilize the Richardson equation. Here we consider a number of thermionic systems where the electron energy spectrum differs from the Richardson model. The electron energy spectra for maximum refrigeration coefficient of performance and maximum power are presented. We then consider multilayer solid-state nanostructures with currents not given by the Richardson equation and discuss the optimization of their energy spectrum. Nanometre gap vacuum thermionic refrigerators are also treated, where significant current is provided by below the barrier tunnelling. Finally, equations are developed for devices that select electrons for emission according to their total momentum, rather than simply the value in the direction of transport as is the case with conventional devices.

Gate leakage currents in high performance InSb/AlInSb quantum well field effect transistors are presented. Thermionic emission theory for Schottky contacts, including the Schottky effect (image force lowering), is successfully used to explain the form of the gate leakage current as a function of gate voltage (I(Vg)) at room temperature. A value for the unlowered Schottky barrier height is extracted from this analysis (~210 meV). The measurement of barrier height is seen to be in good agreement with the assertion that the Al0.15In0.85Sb material system exhibits mid-bandgap pinning at the metal–semiconductor interface. Gate current as a function of temperature is also used to extract barrier heights, using Arrhenius style plots, and they are shown to be consistent with thermionic emission over an image force lowered barrier. Low temperature I(Vg) data are also presented whereby the gate current is seen to increase below ~30 K. This effect is unexpected and is attributed to possible movement of the pinning level at the metal–semiconductor interface.

The nanometric localization of current transport in heterogeneous Schottky barriers was obtained by the combination of the electric field localization at the apex of a biased conductive atomic force microscopy (c-AFM) tip and of the metal films high-resistivity properties. An abrupt increase of the resistivity, modeled by a quantum-mechanical approach, was measured in Au thin films with a thickness below 10 nm. For Au ultrathin film resistivity, exceeding by two orders of magnitude the bulk value, the nanometric localization of the current transport occurs. This physical effect represents the basic principle of a microscopy approch for two-dimensional Schottky barrier height mapping, which is alternative to conventional ballistic electron emission microscopy ( BEEM). A spatial resolution in the order of the tip diameter ( 10 - 20 nm) is demonstrated by considering the realistic description of the system ( physical and geometrical). Schottky barrier inhomogeneities in a Au/4H-SiC system were imaged with an energy resolution better than 0.1 eV.

Effects of a thin native oxide layer on Au/Ni/n-type GaN Schottky diodes were investigated in this study. The tunneling current was induced in the presence of native oxides on the GaN surface, making the thermionic emission (TE) theory inapplicable in this case. We find that the value of the barrier height (BH) calculated using the thermionic field emission (TFE) model is similar to that obtained by capacitance–voltage characteristics. This suggested that the discrepancy between BH according to the TFE and TE model for Au/Ni/n-type GaN Schottky diodes could be attributed to the presence of a native oxide layer at the Ni/n-type GaN interface and oxygen-induced and nitrogen-vacancy-related states on the GaN surfaces. Further, the characteristics of Schottky diodes improved when the n-type GaN was treated with (NH4)2Sx solution, an effective agent for removing native oxides and reducing surface states.

Zirconium oxide (ZrO2) films have been deposited on Ge-rich SiGe heterolayers at 150 ?C by the microwave plasma enhanced chemical vapour deposition (PECVD) technique using zirconium tetra-tert-butoxide. The possible conduction mechanisms in deposited ZrO2 films have been investigated at both room and high temperature. It is found that the conduction mechanism is dominated by Schottky emission at a low electric field (E 1.2 MV cm?1). The extracted trap energy is about 0.78 eV from the conduction band of ZrO2. It is shown that the current in ZrO2 films exhibits strong temperature dependence at a low electric field. The trapping behaviour of the charge carriers in thin ZrO2 gate dielectric stacks during constant gate voltage stress of metal?oxide?semiconductor capacitors has also been investigated.

Undoped and Ce-doped Ba0.5Sr0.5TiO3 (BST) thin films were prepared by pulsed-laser deposition onto a Nb-doped SrTiO3 (STON) substrate. The Ce concentration, ranging from 0.5 to 1.0 at.%, was found to have a strong influence on the electric properties of films at room temperature. We find that, with a positively biased Pt electrode, the leakage current controlled by BST/STON interface can be described by a space-charge-limited-current model. When the Pt electrode is negatively biased, the leakage current controlled by the BST/Pt interface can be explained by the Schottky emission mechanism. In both cases the Ce-doped BST thin films exhibited a lower leakage current (1.2 × 10 −4 and 5.0 × 10 −5 versus 3.4 × 10 −2 Ac m −2 at 450 kV cm −1 ;4 .0 × 10 −4 and 4.0 × 10 −5 versus 6.2 × 10 −3 Ac m −2 at −450 kV cm −1 ) than undoped BST films. The reduction of the leakage current is ascribed to the effect of acceptor Ce 3+ doping, determined by x-ray photoelectron spectroscopy measurement. (Some figures in this article are in colour only in the electronic version)

The introduction of cable joints and optical repeaters into submarine telecommunication cable inevitably results in amalgamation regions in the polyethylene insulation. These amalgamation regions are known to increase the probability of high voltage breakdown, when compared with the bulk material. This increased risk has been attributed to the accumulation of space charge in the amalgamation regions. Measurement of the space charge accumulation, (using the PEA technique), and calculation of the resultant electric field distributions of both press moulded LDPE and injection moulded LDPE containing an amalgamation zone were conducted. The space charge dynamics are discussed, and the role of interfacial stress and Schottky injection on the space charge accumulation are detailed.

We measured the precise energy distributions of Schottky emitters, Zr/O/W, Sc/O/W, and Ba/O/W at low operation currents. The obtained distributions were narrower than those of conventional thermionic emission (TE) models, especially in energy regions higher than the peaks of the distributions. To explain the measurement results, a new model that uses effective mass approximation is introduced. Total reflection of the electrons at the interface, where the effective mass region borders on a vacuum, is considered. The calculated results agreed well with the measurements.

The structural evolution and temperature dependence of the Schottky barrier heights of Pt contacts on n-GaN epilayer at various annealing temperatures were investigated extensively by Rutherford backscattering spectrometry, x-ray diffraction measurements, Auger electron spectroscopy, scanning electron microscopy and current–voltage measurements. The temperature dependence of the Schottky barrier heights may be attributed to changes of surface morphology of Pt films on the surface and variation of nonstoichiometric defects at the interface vicinity. Experimental results indicated the degradation of Pt contacts on n-GaN above 600uC.

A Mg photocathode for use in a radio-frequency gun was manufactured by a technique of hot isostatic pressing with diamond polishing and tested under a peak electric field of 57 MV/m. A laser cleaning process for the Mg cathode was developed. The quantum efficiency (QE) of the Mg cathode before and after the laser cleaning was measured under illumination with 262 nm and 349 nm laser light. A high QE of up to 1.0×10-3 was achieved under the 262 nm laser-light illumination. The QE of the Mg cathode under the 349 nm laser-light illumination was measured to be 2.2×10-5. The dependence of the QE on the electric field was measured and compared with a theoretical analysis including the Schottky effect.

The dependence of quantum efficiency on a high electric field was investigated using a tungsten needle photocathode irradiated by a 3rd-harmonics Nd:YAG laser whose photon energies were lower than the work function of tungsten. The tungsten needles were fabricated by an electrolytic etching technique. The tip radii could be controlled between 0.1 and 1 µm by changing the etching conditions. The obtained quantum efficiency of the needle tip is found to be proportional to the >10th power of the electric over 500 MV/m, and it reached up to 3% at about 800 MV/m. This observed field-enhancement of quantum efficiency is qualitatively explained with a field-emission process including the Schottky effect and photo-excitation.

The current-voltage characteristics of Ti/n-GaAs Schottky diodes measured over a temperature range of 78-299 K have been interpreted on the basis of thermionic emission across an inhomogeneous Schottky contact. The experiment shows that the apparent barrier height (ap) increases from 0.437 eV at 78 K to 0.698 eV at room temperature. The plot of ap versus 1/T does not exhibit a simple linear relationship over the whole temperature range, indicating that the barrier height distribution is more complicated than the frequently observed single Gaussian distribution. A new multi-Gaussian distribution model is developed. Our experimental results can be explained by a double Gaussian distribution of the barrier heights. The weight, the mean barrier height, and the standard deviation of the two Gaussian functions are 0.00001 and 0.99999, 0.721 and 0.696, 0.069 and 0.012 eV, respectively.

The forward current-voltage (I-V) characteristics of polycrystalline CoSi2/n-Si(100) Schottky contacts have been measured in a wide temperature range. At low temperatures (≤200K), a plateau-like section is observed in the I-V characteristics around 10-4Acm-2. The current in the small bias region significantly exceeds that expected by the model based on thermionic emission (TE) and a Gaussian distribution of Schottky barrier height (SBH). Such a double threshold behaviour can be explained by the barrier height inhomogeneity, i.e. at low temperatures the current through some patches with low SBH dominates at small bias region. With increasing bias voltage, the Ohmic effect becomes important and the current through the whole junction area exceeds the patch current, thus resulting in a plateau-like section in the I-V curves at moderate bias. For the polycrystalline CoSi2/Si contacts studied in this paper, the apparent ideality factor of the patch current is much larger than that calculated from the TE model taking the pinch-off effect into account. This suggests that the current flowing through these patches is of the tunnelling type, rather than the thermionic emission type. The experimental I-V characteristics can be fitted reasonably well in the whole temperature region using the model based on tunnelling and pinch-off.

Transport properties of graphene semiconductor Schottky junctions strongly depend on interfacial inhomogeneities due to the inherent formation of ripples and ridges. Here, chemical vapor deposited graphene is transferred onto multilayer MoS2 to fabricate Schottky junctions. These junctions exhibit rectifying current–voltage behavior with the zero bias Schottky barrier height increases and ideality factor decreases with increasing temperature between 210 and 300 K. Such behavior is attributed to the inhomogeneous interface that arises from graphene ripples and ridges, as revealed by atomic force and scanning tunneling microscopy imaging. Assuming a Gaussian distribution of the barrier height, a mean value of 0.96 ± 0.14 eV is obtained. These findings indicate a direct correlation between temperature dependent Schottky barrier height and spatial inhomogeneity in graphene/2D semiconductor Schottky junctions.

Microdischarges are an important type of plasma discharge that possess several unique characteristics, such as the presence of a stable glow discharge, high plasma density and intense excimer radiation, leading to several potential applications. The intense and controllable gas heating within the extremely small dimensions of microdischarges has been exploited in microthruster technologies by incorporating a micro-nozzle to generate the thrust. This kind of microthruster has a significantly improved specific impulse performance compared to conventional cold gas thrusters, and can meet the requirements arising from the emerging development and application of micro-spacecraft. In this paper, we performed a self-consistent 2D particle-in-cell simulation, with a Monte Carlo collision model, of a microdischarge operating in a prototype micro-plasma thruster with a hollow cylinder geometry and a divergent micro-nozzle. The model takes into account the thermionic electron emission including the Schottky effect, the secondary electron emission due to cathode bombardment by the plasma ions, several different collision processes, and a non-uniform argon background gas density in the cathode-anode gap. Results in the high-pressure (several hundreds of Torr), high-current (mA) operating regime showing the behavior of the plasma density, potential distribution, and energy flux towards the hollow cathode and anode are presented and discussed. In addition, the results of simulations showing the effect of different argon gas pressures, cathode material work function and discharge voltage on the operation of the microdischarge thruster are presented. Our calculated properties are compared with experimental data under similar conditions and qualitative and quantitative agreements are reached.

A novel method is proposed to determine effective barrier heights in homogeneous nonideal Schottky contact from I-V measurements. This method takes into account the different mechanisms of current flow through the metal-semiconductor interface. The total current has been expressed as the sum of two independent terms which are: (1) the thermionic current where the ideality factor value is equal to one and (2) the contribution of different transport mechanisms. The second term responds to a general expression of the thermionic emission theory where the barrier height and the ideality factor are voltage dependent. The effective barrier height is found by means of subtraction of the transport mechanism terms from the total current. The method was applied to a group of I-V experimental curves which were reported by M Barus and D Donoval 1993 Solid State Electron. 36 969.

Molecular structure, morphological characteristics and electrical conduction in syndiotactic polypropylene (sPP) are studied. sPP is found to be highly resistant to thermal oxidation. The crystallinity measured by the x-ray diffraction meter is very low and shows no significant change with annealing, compared with that in isotactic polypropylene (iPP). The development of spherulites occurs at a lower temperature than in iPP and the size of spherulites is much smaller than that in iPP, even though the number of spherulites increases drastically. The influence of the morphological change on electrical conduction in sPP is negligibly small, in contrast to iPP, where the breakdown strength decreases markedly due to spherulite growth upon heat treatment. In electrical conduction in the high field region above 107 V m-1 , the predominant conduction mechanism is interpreted to be the Schottky effect in the low temperature region below 70 °C. But in the high temperature region above 70 °C, the conduction is considered to be ionic conduction, with the jump distance estimated to be 3.3 nm smaller than that in iPP. These results are discussed in terms of the molecular structure differences between sPP and iPP.

Current–voltage characteristics of Al lateral Schottky diodes fabricated on homoepitaxial diamond (100) films were analyzed. The currents in reverse-bias voltages smaller and larger than 3 V were well explained by the image-force lowering mechanism modified by localized-field enhancement and the thermionic-field emission mechanism, respectively. An electric field breakdown was observed when the maximum electric field was in the range 1.08–1.46 MV/cm. The breakdown electric field became large with increasing the Schottky barrier height. These features suggest that the electric field breakdown occurred at the electrode fringe due to the field enhancement.

I - V forward current characteristics of Si Schottky diodes are measured in the 80 - 291 K temperature range. The apparent decrease of the electron barrier height at the metal - semiconductor (M - S) interface, , and the increase of the ideality factor, n, with the temperature decrease (with and n calculated by the thermionic theory of Schottky diode current) are connected with the appearance of an insulating part of the space charge layer of the Schottky diode near to the M - S interface when the diode temperature is decreased. Electron tunnelling from occupied deep levels to unoccupied ones in this insulating part of the space charge layer is considered as an electron transport mechanism in these M - S Schottky diodes at temperatures near to 77 K. Electron tunnelling via deep levels at eV below the Si conduction band and variable-range hopping when the deep levels have constant energy distribution in the Si energy gap are considered separately. After the forward pulse voltage, current transients are observed in these Si Schottky diodes. The amplitude of these current transients increases and their time-constant remains unchanged when the Schottky diode temperature decreases. These current transients are also connected with electron tunnelling in the insulating part of the diode space charge layer near to the M - S interface.

The effect of the presence of ions near the metal surface on the electron field emission by an arc cathode is studied theoretically. An important electron emission process is shown to be mediated by the neutralization of the ions: one electron from the metal is captured by each ion to form an excited neutral atom which is then ionized by the macroscopic field. The neutralization - ionization rates are obtained for ions in front of a metal surface for different fields and then used to evaluate the electron emission. This ion-assisted process contributes significantly to the electron emission, well below the top of the Schottky barrier. The contribution of this resonant process to the cathode heating is also evaluated.