Space Charge Limit Research Articles

Optoelectronic properties and carrier transport mechanisms in amorphous SiCN

Amorphous silicon carbon nitride (a-SiCN) films were prepared on boron-doped silicon and quartz substrates by plasma-enhanced chemical vapor deposition. The films were characterized by using X-ray diffraction spectroscopy, atomic force microscopy, Fourier transform infrared spectroscopy, as well the results of the measurements of optical spectra, photoluminescence (PL) and current-voltage characteristics. The films demonstrate high optical transparency and the energy gap ~ 3.4 eV. The PL spectra consist of three bands at 395, 500 and 526 nm that are caused by the electronic recombination in the Si-C-N-H-O network. Forward and reverse current-voltage characteristics were measured in the temperature range of 196–353 K. The forward current is explained by unipolar space-charge-limited (SCL) current mechanism with an exponential distribution of trap states in a local energy band. The reverse current is limited by the depletion region in the p-Si substrate. Basic electrical parameters of a-SiCN layers are determined from the SCL current measurements.

Simulation of the Space-Charge-Limited Current Density for Time-Variant Pulsed Injection

Space-charge-limited (SCL) current density for time-invariant injection (under long-pulse condition) via the diode cathode is the maximum transportable density, while it can be leveraged higher when the injection pulselength becomes shorter than the transmit time for electrons (i.e., under short-pulse condition). However, both limits mentioned above apply for the time-invariant injection condition and the role of time-varying current density for injection remains elusive. In this paper, we numerically investigate the SCL electron flow with time-variant injection. Using particle-in-cell simulation, four different time-variant profiles for electron injection are enforced, and the maximum current densities are determined resulting from the space charge effect for various pulse lengths. We speculate that the time-variant density of injection via the diode cathode will contribute to transport enhancement in the short-pulse condition.

Rapid-cycling synchrotron for multi-megawatt proton facility at Fermilab

The Fermilab accelerator complex delivers intense high-energy proton beams to a variety of fixed-target scientific programs, including a flagship long-baseline neutrino program. With the advent of the Deep Underground Neutrino Experiment (DUNE) and Long Baseline Neutrino Facility (LBNF) program there is strong motivation for a 2.4 MW beam power upgrade of the Fermilab proton facility. We show the Fermilab proton facility can achieve 2.4 MW with a new rapid-cycling synchrotron (RCS) to replace the Fermilab Booster and we provide a comprehensive technical analysis of the RCS-based facility design. Past design efforts and operational experience at the Fermilab Booster, J-PARC RCS, and Oak Ridge SNS are leveraged to provide strong empirical precedent for the design. We provide a parametric study of slip-stacking accumulation, RCS extraction energy, space-charge limits, beampipe aperture, eddy current heating, injection foil heating, and lattice optics. The 2.4 MW benchmark for the long baseline neutrino program is achieved independently of a previously proposed multi-GeV linac program, but we assess the impact the linac upgrade would have on RCS performance.

Extraction and Analysis of Gate Leakage Current Mechanism in Silicon Carbide (SiC) MIS Capacitors

Wide bandgap and resulting low intrinsic carrier concentration enable longer charge retention time and high-temperature operation of Silicon carbide (SiC) flash memory devices. However, charge leakage through gate oxide reduces memory retention time and reliability of gate dielectric. Knowledge of conduction mechanisms of gate leakage current is essential for the optimization of process conditions to reduce leakage current and determination of suitable operating voltage of flash memory devices. In this work, conduction mechanisms contributing to gate leakage current in SiC MIS capacitors with different gate dielectric are extracted for the entire range of the gate electric field. Space charge limited (SCL) conduction mechanisms either Ohm’s law or trap filled limit process or combination of both are observed to be dominant at the low electric field. Hopping conduction found to be dominating the leakage current when the slope of leakage current changes from the lower value to a higher value. The sequence of mechanisms contributing to gate leakage current from low electric field to high electric field is found to be same in SiO2 annealed in O2 and N2, SiON and Al2O3/SiON stack. The trap filled limit process, trap-assisted tunneling and FN tunneling are dominating the gate leakage current at low, mid and high electric field respectively in all the investigated MIS capacitors.

Relativistic Charged-Particle Beam Space-Charge Limited Current in Finite Length Coaxial Drift Tube

In the strong axial (guide) magnetic field and uniform beam density approximations, we calculate scalar potential distribution induced by axisymmetric annular relativistic charged-particle beam in a finite-length coaxial drift tube. This scalar potential distribution allows us to find an approximate radial position at which the extremum of the scalar potential is attained in the coaxial drift tube inside the annular relativistic charged-particle beam. By knowing the approximate extremal radial position, we obtain the analytical estimate for the space-charge limited (SCL) current of axisymmetric annular (finite thickness) relativistic charged-particle beam propagating in strong axial magnetic field in a finite length coaxial drift tube and establish a simple correspondence of this result to that in a long coaxial drift tube. A comparison of numerical nonlinear calculations and the analytical estimate for the SCL current in the finite-length coaxial drift tube is also given.

Graphene nanoflakes and carbon nanotubes on porous silicon layers by spin coating, for possible applications in optoelectronics

Structures of c-Si/PS/GNF/Au and c-Si/PS/CNT/Au have been developed through a simple and inexpensive fabrication method. Porous silicon (PS) layers with nanometer pore sizes have been created by the electrochemical anodization method, which is simple, low-cost, and efficient. Then, graphene nanoflakes (GNF) and carbon nanotubes (CNT) have been deposited by the spin coating method on the porous silicon. PS/GNF and PS/CNT structures showed a considerable reduction in the luminescence of porous silicon which could be attributed by an interaction between porous silicon and carbon nanotubes or graphene nanoflakes. Moreover, SEM images revealed the distribution of these carbon materials on the porous silicon, even different agglomerations could be appreciated both CNT and GNF. Electrical characterization showed rectifying and ohmic behavior with some anomalies that can be related to these carbon structures. Furthermore, it was possible to fit a transport mechanism that resulted to be Space charge Limit Current (SCLC), due to different simulations carried out in this study.

Charge exchange injection into accelerators and storage rings

The development of charge exchange injection technology at the Budker Institute of Nuclear Physics, Siberian Branch of the Russian Academy of Sciences, mastering charge exchange injection at research centers worldwide, and the experimental observation, explanation, and damping of electron–proton instability (electron cloud effect) are overviewed. Techniques for preparing a circulating proton beam whose intensity is an order of magnitude higher than the space-charge limit are considered. Problems with stripping-foil stability in the process of injection of intense beams and laser ionization of accelerated atoms are also discussed.

Electroforming and resistive reversible switching effect in polycrystalline fullerene C60 films

It has been found that the conductivity of a polycrystalline fullerene C60 thin film increases by several orders of magnitude after lengthy exposure to an electric field of ∼ 106 V/cm. This effect is called electroforming. After electroforming, the current-voltage characteristic shows high current stability and reproducibility of results. An increase in capacitance and a decrease in resistance by 4 orders is observed in a narrow voltage range of 0.1-1 V, which is the effect of reversible resistive switching. In fields with a strength of 105-106 V/cm, the I-V characteristic of molded films is determined by the current with a limited space charge. Trap concentrations are defined as Nt-1 ≈ 2.3· 1021 m−3 and Nt-2 ≈ 5.8·1020 m−3 and their depth Et ≈ 0.17 eV for polycrystalline C60 films with thicknesses of 250 and 500 nm, respectively.

Carrier transport in methylammonium lead iodide perovskite single crystals studied by dark current, steady state and transient photocurrent measurements

Electronic transport properties of methylammonium lead iodide perovskite single crystals prepared by inverse temperature crystallisation have been investigated using dark current, steady state and transient photocurrent measurements. Above 250 K the dark conductivity at electric fields below 100 V/cm is thermally activated, with single activation energy 0.7 eV and magnitude 4×10−8 S/cm at 300 K. At higher electric fields, conductivity increases as a power law suggesting space charge limitation. Steady state photoconductivity exhibits a roughly linear dependence on photon flux below 1015 cm−2s−1 and square root dependence above this, consistent with trap-limited and band-to-band recombination mechanisms respectively. Photocurrent overshoot and undershoot on a timescale of 10-100 s are observed on application and removal of a steady light source. Transient photocurrent measurements with temperature reveal a peak in the localised density of states 0.26 eV into the band gap, with an attempt to escape frequency of 1011 s−1. Deeper states are distributed exponentially with characteristic energy 54 meV, decreasing towards mid-gap.

Core Structure Engineering in Hole-Transport Materials to Achieve Highly Efficient Perovskite Solar Cells.

In this work, the thiadiazolopyridine (PT) unit was introduced as the core structure, with N3 ,N3 ,N6 ,N6 -tetrakis(4-methoxyphenyl)-9-phenyl-9 H-carbazole-3,6-diamine as the peripheral group, to obtain a new compound, JY8, for use as a hole-transport material (HTM) in planar perovskite solar cells (PSCs). Compared with the previously reported JY5 with benzothiadiazole as the core structure, the PT unit with stronger electron-withdrawing ability enhanced the intermolecular dipole-dipole interaction. Moreover, the introduction of the PT unit made the central part in JY8 more planar than its analogue JY5, which is conducive to charge transport. Field-emission (FE)-SEM images suggested a smooth and condense morphology of the JY8 film, which could improve the contact between the perovskite layer and the metal electrode. Space-charge limitation of current results, steady-state, and time-resolved photoluminescence decay curves indicated that JY8 as HTM facilitated hole extraction and hole transport. Consequently, planar PSCs fabricated with JY8 as the HTM exhibited a decent efficiency of 19.14 % with a high fill factor of 81 %.

Simultaneous Enhancement of Three Parameters of P3HT‐Based Organic Solar Cells with One Oxygen Atom

AbstractPoly(3‐hexylthiophene) (P3HT)‐based organic solar cells (OSCs) have attracted much attention due to their advantages of low‐cost production and matured roll‐to‐roll manufacture. However, the efficiency of P3HT‐based OSCs lag much behind the non‐P3HT ones due to their negligible absorption of long wavelengths of light over 650 nm, high‐lying highest occupied molecular orbitals (HOMO), and difficulty of controlling morphology. In this study, the alkyl chains of the nonfullerene acceptors are replaced with alkoxy chains to achieve synergistic enhancement of all three parameters ( short circuit current density (JSC), open circuit voltage (VOC), and fill factor (FF)) and thus significant increase of power conversion efficiency for P3HT‐based OSCs. As a result, the OSCs exhibit a maxima efficiency of 6.6%. The P3HT‐based systems are systematically studied with optical spectroscopy, photoluminescence, cyclic voltametry, space charge limit current, grazing incident wide‐angle X‐ray scattering, transient absorption spectroscopy, transmission electron microscope, and atomic force microscopy to probe the mechanism, which reveal that introducing alkoxy chains simultaneously increases the energy levels of the HOMO and the lowest unoccupied molecular orbitals, enhances the light absorption, improves the rigidity of the backbone and charge transport mobility, and tunes the molecular orientation and film morphology, thus improving the photovoltaic performance. This contribution provides an important guidance in the design of novel nonfullerene acceptors for high‐performance P3HT‐based OSCs.

Field Effect in PbSnTe:In Films with Low Conductivity in the Mode of Injection from Contacts and Space-Charge Limitation of the Current

At T = 4.2 K, a strong change (up to 4 times) in the space-charge-limited current (SCLC) is observed for the first time at samples based on semi-insulating PbSnTe:In films grown by molecular-beam epitaxy on BaF2(111) substrates. The obtained results agree with experiments on the effect of treatment of the surface of PbSnTe:In films on the space-charge-limited current with a variation in the current to 103 times or more. At a qualitative level, the model is considered which assumes the substantial contribution of localized surface states to space charge formed in the mode of space-charge-limited current due to the injection of charge carriers from contacts.

Superlinearity, saturation, and the PMT-Tailoring and calibration methodology for prompt radiation detectors.

This work illustrates predominant measureable nonlinearities in photomultiplier tubes (PMTs) and introduces a controllable one called "Superlinearity," signifying both a positive nonlinear response and the ability to extend linear operation by counteracting gain saturation mechanisms - charge depletion, space-charge field limitation, and secondary emission surface effects. Recognizing superlinearity and its effect on the temporal step response leads to a true definition of linearity, free of a small-signal linear assumption. Furthermore, given the prevalent use of glass microchannel-plate (MCP) PMTs in favor of a faster impulse response in spite of a small charge limit, we are motivated to examine their nonlinear amplitude response and deploy tailored gain bias string methods to fully harness the maximum linear gain as is usually done for transmissive metal mesh and reflective metal dynode PMTs. Our characterization methodology applies standard NIST-traceable calibrated laboratory equipment with absolute input-referenced techniques, examining step responses over many orders of magnitude in controlled illumination. By doing so, we quantitatively reveal the superlinearity strength independent of charge depletion, yielding true linear responsivity and effectively doubling the small-signal linear limit; this is very relevant to PMT modeling and charge deconvolution efforts. With further development, the tailoring strategies we introduce could be applied to MCP detectors, extracting all useful capillary charge with a significant improvement in large linear signal quality.

A review on deterministic and stochastic models for electrical treeing initiation and propagation in solid insulation systems from the perspective of prediction assessment techniques

Electrical treeing is a challenging prospect related to degradation mechanism in dielectrics that eventually leads to its breakdown. Treeing propagation depends on dielectric and electrode configuration thus leading to types such as branch, filamentary, bush etc. Though researchers have attempted a gamut of studies related to treeing, it is evident that appropriate choice of prognostic model is essential for prediction of ageing of insulation that emulate features such as dendrite length, channel width, space charge etc. Although studies related to treeing modeling have taken up depictions such as Diffusion Limited Aggregation, Field Limiting Space Charge etc. to obtain estimates of electric potential based on boundary value equations (Laplace and Poisson) utilizing tools such as Fractal Analysis, Cellular Automata etc., challenges such as space charge, inhomogeneity factor etc. continue to confront researchers. This research gives a detailed insight into a wide spectrum of studies related to treeing and ageing prediction assessment of insulation based on physical, electrical, numerical and analytical models developed thus far. This study also summarizes succinctly, the merits and limitations in each method and avenues for improvement. Finally, this research gives clues into important future opportunities related to methodologies for development pertaining to treeing prediction and ageing assessment.

Simulation of a Distributed Cathode in a Linear-Format Crossed-Field Amplifier

The effects of a distributed cathode in a linear-format crossed-field amplifier are simulated in VSim and analyzed. A linear-format, injected-beam, 150-MHz, low-power (~100 W), moderate-gain (7 dB), meander-line crossed-field amplifier (CFA) designed and tested at Northeastern University is used as the basis for the simulation model. The simulation results were validated by the experimental results from that CFA. The injected-beam model was then replaced with a distributed cathode requiring electron injection with an energy below the sole electrode. A new method, which uses a divergence-free region, was developed to simulate this distributed cathode. This model was validated and studied using three different current injection profiles. The distributed cathode offered no improvements to gain or the signal-to-noise ratio (SNR). Increasing the cathode area lowers the noise but is counteracted by the corresponding decrease in gain. The limits to the gain in this particular device are due to the current space charge limit for the higher power operation and the SNR for the lower power operation.

Exposed the mechanism of lead chloride dopant for high efficiency planar-structure perovskite solar cells

Although the PbCl2 has been exploited to improve the performance of perovskite solar cells, the mechanism of enhancement by the introducing PbCl2 is not fully understudied. In present work, we used the PbCl2 as a dopant to improve the perovskite film quality, we found the crystalline and grain size of perovskite film both increased with 10% molar ratio of PbCl2. The in situ time-resolved grazing incidence wide-angle X-ray scattering measurements revealed that more pre-nucleated suspended in the solution when employed the PbCl2 dopant, leading to high crystalline for perovskite film. An incorporation of the space charge limit current, photoluminescence, time-resolved photoluminescence measurements, we uncover the function of PbCl2 in reduced trap density, decreased charge recombination and enhanced carrier lifetime. As a result, the efficiency of planar-structure perovskite solar cells based on PbCl2 enhanced to as high as 19.79% with reduced hysteresis, increased by ∼23% compared to the device without PbCl2 (16.05%). We believe that this work would provide a fundamental prospective for mixed perovskite and promote its application in perovskite photovoltaics.

Electron collection and thermionic emission from a spherical dust grain in the space-charge limited regime

The collection and emission of electrons from a spherical body in the Space-Charge Limited (SCL) regime are investigated. When a Virtual Cathode (VC) in the potential profile around the body is present, the barrier in the effective potential energy of electrons is assumed to be located near the position of the minimum of the VC potential, for both collected and emitted electrons. This assumption is confirmed to be reasonable in the case of a double Yukawa potential profile and allows the SCL cross-section for electron collection and the emitted electron’s trapped-passing boundary to be written in a simple way. An expression for the collection current for Maxwellian electrons is derived and is shown to recover the classical Orbital Motion Limited (OML) theory when the VC vanishes. Using the same assumptions, an expression for the thermionic emission current in the SCL regime is also obtained and comparisons with the OML+ theory are made. Finally, an expression for the dust electric charge in the SCL regime is derived and shown to give drastically different results when compared to the commonly used formula (obtained from a Yukawa potential profile). Consequences in the framework of dust in tokamak plasmas are discussed.

A detailed study on the structures of steady-state collisionless kinetic sheath near a dielectric wall with secondary electron emission. II. Inverse and space-charge limited sheaths

The inverse and space-charge limited (SCL) sheaths between a Maxwellian low-temperature plasma source and a dielectric surface that emits secondary electrons are carefully investigated by a 1D3V, steady-state, kinetic sheath model with fully self-consistent wall secondary electron emission conditions, within a broad range of plasma electron temperature Te. The multistep electron emission (MEE) phenomenon caused by the near-wall potential increment is specially considered, and it is found that the total emitted electron velocity distribution function perpendicular to the dielectric wall approximately obeys two-temperature Maxwellian and half-Maxwellian distribution. Under cold plasma ion assumption, results show that when Te>Tec (the first threshold) the sheath goes into entirely inverse sheath regime: (1) the ion energy E at the sheath edge equals 0, i.e., no presheath is necessary to accelerate ion; (2) the wall total electron emission coefficient Γ=1, i.e., no charge is lost at the wall; (3) the wall potential is just about 0∼0.03(Te/e) for all the cases; (4) the sheath depth is thinner than one Debye length. When Te≥TeSCL (the second threshold), another solution of SCL sheath is also found. It clearly indicates that the two sheaths are possible near plasma-facing dielectric wall with strong electron emission, but they do not always emerge in pairs. Further comparative calculations predict that the MEE phenomenon can significantly decrease the wall potential Φw, virtual cathode potential Φvc, and near-wall potential increment ΔΦ (i.e., Φw-Φvc) for SCL sheath, but slightly increase Φw for the inverse sheath. In addition, the differences between the two sheaths are discussed.

A detailed study on the structures of steady-state collisionless kinetic sheath near a dielectric wall with secondary electron emission. I. Classic sheath and its structure transition

It is well known that plasma sheath presents a classic sheath structure when the dielectric-wall total electron emission coefficient Γ ≤ Γc (<1). However, the structural transition of a classic sheath near a dielectric wall when Γ→Γc is controversial about transiting to a space-charge limited (SCL) sheath or inverse sheath. In this study, the classic sheath between a Maxwellian low-temperature plasma source and a dielectric surface that emits secondary electrons is carefully investigated using a 1D3V, steady-state, kinetic sheath model within a broad range of plasma electron temperatures Te. Using the Monte Carlo method to simulate secondary electron emission (SEE) events that are based on the self-consistent primary electron velocity distribution function at the wall and a detailed SEE model, it is found that the total emitted electron velocity distribution function (EEVDF) perpendicular to the dielectric wall approximately satisfies a three-temperature Maxwellian distribution. Due to the relatively high average energy of this total EEVDF, for cases of Te with cold plasma ion assumption: (1) the critical SCL sheath does not exist; (2) Γc reaches unit; (3) the sheath disappears when Te = Tec (i.e., Γ = 1); and (4) as Te increases, the classic sheath will transit to an inverse sheath structure. Further comparative calculations predict that the magnitude of emitted electrons' average energy may lead to different experimental results between thermionic emitting surfaces which have a “cold” half-Maxwellian EEVDF and SEE surfaces. However, when the finite plasma ion temperature is considered, at the transition point, the cold plasma ion assumption is expected to be invalid, and thus a fully kinetic sheath model should be built to reveal the potential new transition regime.

Structural investigations and mobility enhanced of Vanadyl 2,3-naphthalocyanine (VONc) nanostructured films under thermal effect

Powder and films of the promised Vanadyl 2,3-naphthalocyanine (VONc) materials were studied. Structural properties of these both VONc powder and films were investigated. The structural properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR). XRD shows that VONc powder has polycrystalline structure, while VONc films have nanostructured nature. SEM result shows agglomerated grains under annealing effect. The observed vibrational wavenumbers in FTIR spectra were analyzed and assigned to different normal modes of the molecule. The films of VONc were electrically studied before and after annealing at different annealing temperatures of 373, 473 and 523 K. The electrical conductivity and current–voltage dependence were studied for VONc films with these suggested annealing temperatures. Both charge carrier concentration and their mobility besides the different conduction processes have been illustrated. The experimental and calculated results have shown that the present films have two regions on the current–voltage relation. In the Ohmic district, the conduction has no traps of electrons in the forbidden gap of present films. In the range of higher voltage, the current is expected a limited space charge controlled by a single trap level. Our results obtained that the holes mobility µ0 and the density of total trap at Et were enhanced under the thermal effect.