Positive Space Charge Research Articles

Charge Transport and Space-Charge Formation in Cd1−xZnxTe1−ySey Radiation Detectors

The electron- and hole-transport properties in cadmium zinc telluride selenide (CZTS) crystals are studied using a laser-induced transient-current technique with pulsed and dc bias. The internal electric field profile and velocity of surface recombination are determined by Monte Carlo simulations of electron and hole transient currents combined with a numerical solution of the drift-diffusion equation coupled with Poisson's equation. Electron and hole drift mobilities of ${\ensuremath{\mu}}_{e}$ = 830 ${\mathrm{cm}}^{2}$/Vs and ${\ensuremath{\mu}}_{h}$ = 40 ${\mathrm{cm}}^{2}$/Vs, respectively, are determined. We also develop a simple technique for evaluating surface recombination directly from measured current waveforms without the need for numerical simulation. The good quality of the prepared detector at pulsed bias, with electron- and hole-mobility-lifetime products of $(\ensuremath{\mu}\ensuremath{\tau}{)}_{e}$ = 1.9 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{2}$/V and $(\ensuremath{\mu}\ensuremath{\tau}{)}_{h}$ = 1.4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{2}$/V, respectively, are observed. The formation of a positive space charge, originating from hole injection combined with a recombination level, is found. We observe a significant position dependence of the lifetime of electrons and holes in dc bias due to hole injection. The experiment is successfully fitted by a simple model dominated by a single deep recombination level with an energy of ${E}_{t}={E}_{C}\ensuremath{-}0.73\phantom{\rule{0.2em}{0ex}}\mathrm{eV}$; concentration of 7.3 \ifmmode\times\else\texttimes\fi{} ${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}3}$; and electron- and hole-capture cross sections of 3.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{2}$ and 6.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{2}$, respectively.

Effect of La2Li0.5Co0.5O4 in semiconductive nanocomposites on suppression of space charge injection to the insulating medium for high-voltage direct current cables

Space charge accumulation in the insulating layers of high-voltage direct current cables is a key factor in the degradation of such cables and limits their operational safety. It is necessary to weaken charge injection to the insulation. The semiconductive shielding layer sandwiched between the conductive core and the insulating layer plays a vital role in preventing carrier injecting into the insulation. In this study, a new method is proposed to suppress charge injection. A certain amount of La2Li0.5Co0.5O4 nanopowder was doped in the semiconductive screen to enhance the internal conductive network and block charge injection in the insulation. The influence of La2Li0.5Co0.5O4 in the nanocomposite on the positive temperature coefficient effect and space charge injection process were explored. The results revealed that the peak resistivity of the specimen containing 5 wt% La2Li0.5Co0.5O4 decreased by 50.5% compared to the composites without La2Li0.5Co0.5O4, and charge quantity in the insulating layer was reduced by 41.2%.

Physicochemical and electrochemical characterization of Nafion-type membranes with embedded silica nanoparticles: Effect of functionalization

Introduction of nanoparticles in membranes allows a significant enhancement of their performance in energy production, water treatment and other applications. However, the effect of nanoparticles’ surface functionalization and the mechanism of their impact on membrane properties remain poorly studied. In this paper, we examine a Nafion-based membrane and its modifications, each containing 3 wt% SiO2. The effect of functionalization by propyl, 3-aminopropyl and 3,3,3-trifluoropropyl is investigated. The water uptake, contact angle, conductivity, diffusion permeability to NaCl, current-voltage curves (CVC), chronopotentiograms (ChP), and the difference between the pH of the desalination compartment output and input solutions (characterizing the water splitting rate) are reported. It is found that the doping of the membranes with nanoparticles leads to increasing their conductivity in all cases except 3-aminopropyl, which imparts a positive charge to the nanoparticles; the diffusion permeability decreases and permselectivity increases in all cases. The latter is explained by transformation of the mesoporous membrane structure to the microporous one. The impact of nanoparticles on the membrane conductivity, CVC and ChP is mainly caused by an additional (positive) space charge introduced into the pore solution and at the membrane surface by the electric double layer surrounding the nanoparticles. The greater the surface charge density of the nanoparticles and the smaller their size, the stronger the impact. Accordingly, the highest conductivity, current density at a low fixed voltage and chronopotentiometric transition time are shown by the sample doped with SiO2 and 3,3,3-trifluoropropyl. The interplay between electroconvection and water splitting phenomena is discussed.

Особенности анализа вод, почв и донных отложений в масс-спектрометрии с индуктивно-связанной плазмой

The paper discusses the possibilities and limitations of the method of mass spectrometry with inductively coupled plasma on the example of elemental analysis of natural and drinking waters, soils and grounds. It is shown that the combination of this method with the simpler atomic emission method makes it possible to expand the range of determined elements, simplify the mass-spectral analysis and increase its reliability. It is shown that the use of the ICP-MS method in the analysis of various objects makes it possible to determine the majority of elements with extremely low detection limits. The reason for the manifestation of matrix effects is the positive space charge formed between the interface and the extractor, the composition of which is determined by the composition of singly charged argon ions. The increase in the concentration of ions in this region is the appearance of a matrix element, which facilitates the scattering of ions from this region. It was found that the heavier the ions of the matrix element, the more the space charge density increases and the scattering occurs. A serious limitation of the method is associated with interferences due to the presence of a certain amount of two and three-charged ions in the plasma. These ions, which have approximately the same mass as the isotopes of the element being determined, are formed as a result of various plasma-chemical reactions and interfere with the determination.

Charge carrier properties of single-crystal CVD diamond up to 473 K

The drift behavior of charge carriers, generated by α-particles of a reference 241Am-source, in electronic grade, single crystal chemical vapor deposition (scCVD) diamond was investigated by the transient current technique (TCT) from room temperature up to ≈473K. Furthermore, the α-spectroscopic behavior was analyzed in terms of charge collection and spectroscopic resolution for the same temperature range. All conducted measurements revealed complete charge collection up to the maximum temperature. The electron–hole-pair creation energies were derived from the TCT as well as from the spectroscopic measurements. The herein presented results imply that high temperature α-spectroscopy with diamond-based semiconductor solid state detectors, using presently available scCVD sensor substrates, is feasible at least up to 473K. Only at the highest applied temperature, the conducted TCT measurements showed distorted signal traces, indicating a uniform positive space charge built-up.

Dynamics of the spatial separation of electrons and mobile oxygen vacancies in oxide heterostructures

In the search for an oxide-based 2D electron system with a large concentration of highly mobile electrons, a promising strategy is to introduce electrons through donor doping while spatially separating electrons and donors to prevent scattering. In $\mathrm{SrTi}{\mathrm{O}}_{3}$, this can be achieved by tailoring the oxygen vacancy profile through reduction, e.g., by creating an interface with an oxygen scavenging layer. Through reduction, oxygen atoms are removed close to the interface, leaving behind oxygen vacancies in the $\mathrm{SrTi}{\mathrm{O}}_{3}$ lattice and mobile electrons in the $\mathrm{SrTi}{\mathrm{O}}_{3}$ conduction band. The commonly assumed picture is that the oxygen vacancies then remain confined close to the interface while the electrons leak a few nanometers into the bulk, resulting in an electron-defect separation and a highly mobile, oxide-based 2D electron system. So far it has remained unclear how the confinement and electron-defect separation develop over time. Here, we present transient finite element simulations that consider three driving forces acting on the oxygen vacancy distribution: diffusion due to the concentration gradient, drift due to the intrinsic electric field, and an oxygen vacancy trapping energy that holds oxygen vacancies at the interface. Our simulations show that at room temperature, three distinct regions are formed in $\mathrm{SrTi}{\mathrm{O}}_{3}$ within days: (1) Oxygen vacancies are partially held at the interface due to the oxygen vacancy trapping energy. (2) The accompanying positive space charge causes an oxygen vacancy depletion layer with large electron concentration and high mobility just below the interface. This electron-defect separation, indeed, leads to a highly conductive region. (3) While we are able to describe measured conductivity data with an oxygen vacancy trapping energy of $\ensuremath{-}0.2$ eV, this value does not prevent oxygen vacancy diffusion into the bulk: A diffusion front progresses into the bulk and leads to significant conductivity arising over the first micrometer within a couple of months. An enhanced oxygen vacancy trapping energy of $\ensuremath{-}0.5$ eV or below would suppress this loss of confinement, leading to a static and pronounced electron-defect separation. Consequently, our results highlight the importance of oxygen vacancy redistribution and suggest the trapping energy of oxygen vacancies at the interface as an important design parameter for oxygen-vacancy-based 2D electron systems.

Dual frequency DBD: influence of the amplitude and the frequency of applied voltages on glow, Townsend and radiofrequency DBDs

To study dual frequency Townsend glow and radio frequency DBDs in an Ar/NH3 Penning mixture, different voltages are applied on each electrode of a plane/plane DBD. The one which has the higher amplitude and frequency, V H, determines the discharge regime. The other one, which has a lower amplitude and frequency, V L, is more similar to a bias voltage. V H frequency ranges from 50 kHz to 13.56 MHz and V L frequency from 1 kHz to 2 MHz. The amplitude of V L is always kept lower than the breakdown voltage. The discharge behavior is characterized by space and phase resolved optical emission spectroscopy and Fourier transform of the plasma induced light intensity. When the frequencies of two voltages are close to each other, V L higher than 150 V increases the discharge light intensity. In glow and Townsend discharges, this increase is attributed to the enhancement of the plasma density, due to the ions drift from the plasma bulk to the cathode, which produces a higher secondary electron emission. For the radiofrequency discharge, the enhancement of the light intensity is attributed to an enhancement of the positive space charge due to the higher voltage amplitude.When the frequencies of two voltages are very different, e.g. for V L between 1 and 100 kHz and a 5.5 MHz α-RF discharge, the behavior largely depends on V L amplitude. Above some low frequency (LF), the discharge tends to extinguish when V L amplitude is at its maximum. This is explained by a diminution of the plasma density resulting from the ion drift to the cathode due to V L. When V L is very high, it enhances the discharge intensity. This discharge amplification is associated with a transition from α to γ RF mode, which only occurs when the two voltages have the same polarity. Whatever the RF discharge mode, γ or α, the discharge is diffuse. To determine the interest of LF-RF dual frequency DBD for the PECVD, SiOx coatings, made with a single and a dual LF-RF frequency DBDs, are compared. It is concluded that the ɣ dual frequency removes OH functions from the material lattice and densifies the layer.

Variation of Velocity of Ions in a Magnetized Plasma Sheath for Different Magnetic Field

The kinetic trajectory simulation method has been used to study ion velocity profile in a plasma sheath for varying magnetic field at fixed obliqueness. As the electrons have higher velocity compared to that of ions the wall is charged up negatively with respect to the core plasma. The negative potential then attracts the ions and repels electrons forming a thin positive space charge region in front of the wall. This positive space charge region, known as the ‘sheath’ separates the negatively charged wall from the quasineutral ‘presheath’ plasma. The ions moving towards the wall have to satisfy the Bohm criterion to ensure the stability of the overall plasma. The mean value as well as oscillation frequency of velocity of ions change as the magnetic field is varied from 1.5 to 10.5 mT. The maximum amplitude of normal component of velocity is almost independent of the magnetic field but the maximum amplitude of other components of velocity change and shows oscillating nature as the magnetic field changes.

Ionization Processes in Arc Discharge of Low Pressure

In this work, a model of the cathode spot of a vacuum arc in the framework of the drift-diffusion representation has been considered. The use of the drift-diffusion approximation eliminates the need to calculate the particle energy distribution functions and take into account various energy reactions that are not accompanied by a change in the particle charge. It has been shown that the field strength of the plasma column is significantly reduced. The column is surrounded by a space charge of slow positive ions, providing a high field strength in front of it. At the stage of the formed plasma boiler, the ionization of metal vapor occurs mainly in a thin layer in front of the ion cloud, and ion recombination prevails in the column.

Electric potential barriers in the magnetic nozzle.

Magnetic nozzles are convergent-divergent applied magnetic fields which are commonly used in electric propulsion, manufacturing, and material processing industries. This paper studies the previously overlooked physics in confining the thermalized ions injected from a near-uniform inlet in the magnetic nozzle. Through fully kinetic planar-3V particle-in-cell (PIC) modeling and simulation, an electric potential barrier is found on the periphery of the nozzle throat, which serves to confine the thermalized ions by the electric force. With the initial thermal energy as driving force and insufficient magnetic confinement, the ions overshoot the most divergent magnetic line, which results in the accumulation of positive space charges around the throat. The accumulated charges would create an ion-confining potential barrier with limited extent. Apart from the finite-electron Larmor radius (FELR) effect, two more factors are put forward to account for the limited extent of the potential barrier: the depletion of ion thermal energy and the short-circuiting effect. The influences of inlet temperature ratio of ions to electrons and magnetic inductive strength B_{0} are quantitively investigated using the PIC code. The results indicate that the potential barrier serves as a medium to transfer the gas dynamic thrust to the magnetic nozzle while providing constrain to the ions, like the solid wall in a de Laval nozzle. In high-B_{0} regime, the finite-ion Larmor radius (FILR) effect becomes dominant rather than the FELR effect in the plasma confinement of magnetic nozzles.

A Tutorial on Theoretical and Computational Techniques for Gas Breakdown in Microscale Gaps

Paschen’s law (PL), derived based on the Townsend avalanche (TA) condition, is commonly used to predict gas breakdown. For microscale gaps near atmospheric pressure, TA is insufficient to drive breakdown and ion-enhanced field emission (FE) dominates. Accurately predicting breakdown voltages for these gaps is critical for numerous applications, including environmental remediation, medicine, combustion, and propulsion. This tutorial summarizes theoretical and computational approaches for predicting this behavior and demonstrating the transition between the TA and FE mechanisms. It focuses on the derivation of closed-form solutions from a theory that accounts for the generation of additional positive space charge at the cathode due to electrons generated by the strong FE-induced electric fields. Appropriate simplifications using a matched asymptotic analysis in terms of the total ionization in the gap agree well with simulations and experiments and show the transition from FE for small gaps to PL at larger gaps. Specifically, this theory shows that the breakdown voltage varies linearly with gap distance when FE dominates, agreeing with both the experimental and simulation results. The particle-in-cell Monte Carlo collision (PIC/MCC) simulations used to predict the ionization coefficient provide additional insight into the mechanisms involved. Future benefits of extended theoretical and computational research for examining microscale and nanoscale breakdown and electron emission, particularly assessing the impact of electrode surface structure and device design and coupling with additional emission mechanisms, will be discussed.

Enhanced Performance of Organic Field‐Effect Transistor Memory by Hole‐Barrier Modulation with an N‐Type Organic Buffer Layer between Pentacene and Polymer Electret

AbstractTheoretical and experimental work has revealed that a positively charged insulator layer formed by oxygen‐related defects near pentacene–dielectric interface in organic field‐effect‐transistors (OFETs) severely affects their performance. A novel OFET memory structure with an n‐type organic buffer layer at the pentacene/polymer interface is proposed. In this structure, electrons are induced on the surface of the n‐type organic layer near the interface due to the electrostatic field of the positively charged insulator and partly compensate the local positive charges at the interface, leading to a reduction in the height of hole barrier formed at the interface. On the other hand, the positive space charges will be induced in an extended region in n‐type organic layer due to the limited density of ionized donors, blocking the back transfer of holes to pentacene as a result of the thicker positively charged layer. This idea is implemented in the n‐type PTCDI‐C13‐buffered OFETs with poly(2‐vinyl naphthalene) (PVN) dielectric, in which greatly reduced P/E gate voltages, reliable P/E endurance of more than 10 000 cycles, and excellent retention time of more than 104 s with an ION/IOFF of more than 104 under a set of P/E pulses with low amplitudes and short pulses are achieved in air.

Model of the Negative-Bias Temperature Instability of p-MOS Transistors

A new quantitative model of the negative-bias temperature instability (NBTI) of p-MOS (metal-oxide-semiconductor) transistors is developed. The model is based on the reaction of the depassivation of surface states at the Si–SiO2 interphase boundary (IPB) and hydrogen-containing hole traps near the Si–SiO2 IPB by positively charged hydrogen ions H+, accumulated in the p+-type inversion layer of the silicon substrate. The dependences of the surface and space charges in p-MOS transistors on the NBTI time are controlled by the kinetics of H+-ion diffusion and drift from the silicon substrate to the Si–SiO2 IPB. The effect of the gate voltage on the NBTI is explained by the effect of the electric-field strength on the H+ ion segregation coefficient at the Si–SiO2 IPB. The relaxation of positive space charge introduced into the gate dielectric during NBTI is described by the tunnel discharge of oxide traps by silicon-substrate electrons.

Performance of proportional counters filled with Xe + 5% TMA under high count rate-pressure scaling

The total space charge effect includes the cumulative effect of positive ions created from many different avalanches. The presence of positive ions in the avalanche multiplication region reduces the gas amplification factor. The gas gain has been measured for a low and high count rate by means of the current method for sealed cylindrical proportional counters of radius b = 12.5 mm with an axially placed anode of radius a = 50 μm. The gas gain curves covered the whole gas gain region from the ionisation chamber to that close to continuous discharge. The gas gain was determined for 5,9 keV X-rays of varying intensity and of constant high radiation intensity. For any count rate there is always a critical voltage, which divides the operation range of the counter into two regions, i.e. the region in which no changes in the gas gain with a count rate are observed and the region in which the space charge of positive ions reduces the gas gain. The values of current over which a 5% reduction in gas gain is observed have been determined. The measurements were made for a mixture pressure range from 294 hPa to 1800 hPa. The obtained results, which are presented here, can be favourable for micromegas-TPC operating in an Xe-TMA mixture.

Модель термополевой нестабильности p-МОП-транзисторов при отрицательном смещении затвора

Abstract A new quantitative model of the negative-bias temperature instability (NBTI) of p -MOS (metal-oxide-semiconductor) transistors is developed. The model is based on the reaction of the depassivation of surface states at the Si–SiO_2 interphase boundary (IPB) and hydrogen-containing hole traps near the Si–SiO_2 IPB by positively charged hydrogen ions H^+, accumulated in the p ^+-type inversion layer of the silicon substrate. The dependences of the surface and space charges in p -MOS transistors on the NBTI time are controlled by the kinetics of H^+-ion diffusion and drift from the silicon substrate to the Si–SiO_2 IPB. The effect of the gate voltage on the NBTI is explained by the effect of the electric-field strength on the H^+ ion segregation coefficient at the Si–SiO_2 IPB. The relaxation of positive space charge introduced into the gate dielectric during NBTI is described by the tunnel discharge of oxide traps by silicon-substrate electrons.

Creation and Development of the Ion Beam Technology

A historical overview of the development of domestic ion beam technology is presented. It is shown that the wide application of the ion beam technology is facilitated by (i) the creation of a method for compensating the positive space charge inside an ion-source accelerating gap by the negative space charge of electrons held in crossed electric and magnetic fields, which makes it possible to increase the density of the extracted ion current by orders of magnitude over the Child–Langmuir limitations; (ii) the development of the original cold cathode sources forming ion beams from almost any compounds; and (iii) the creation of the reactive ion beam etching technique characterized by precise etching with inert gas ion beams and the selectivity of liquid etching. Based on these advances, a series of ion source modifications and technological equipment are developed for use in the manufacture of micro- and nanoelectronic products, optics, piezoelectric quartz technology, medicine, and other fields. The ways of improving the ion beam technology by creating high-energy neutral particle sources and technologies based on their use are shown.

Line Charge Densities and Currents of Downward Negative Leaders Estimated From VHF Images and VLF Electric Fields Observed at Close Distances

In this paper, a method for retrieving the line charge density and the current of a downward leader based on ground observations of lightning images and electric field changes was proposed and examined. The method was then applied to two downward negative leaders in a rocket-triggered lightning discharge with measurements of broadband interferometric images and electric field changes at close distance. Only the descending channel portion between about 900–400 m high of the two leaders were analyzed, where the channel was in free air rather than the trace of triggering wire. The line charge density for both leaders showed a general increasing trend with a big impulsive change around 500 m high, as the leaders moved downward, which was in good agreement with the prediction of existing downward leader models. The range of the line charge density was similar for the two leaders (−0.03 to −0.32 mC/m and −0.02 to −0.33 mC/m, respectively). The impulsive change of the line charge density was corresponding to a sharp turn of the leader channel around 500 m high, which may imply that the sharp turn of the leader channel was probably associated with a pre-existing local positive space charge pocket. The estimated leader currents for two leaders ( −20 to −315 A and −77 to −541 A, respectively) were also in good agreement with independent estimations in the literature.

Impact of positive space charge depletion layer on negatively charged and neutral centers in gold–diamond Schottky junctions

It has been widely accepted that conversion between negatively charged nitrogen-vacancy (NV−) center and neutral nitrogen-vacancy (NV0) center can be realized with the Fermi level crossing the transition level corresponding to the ground state of NV−. In this study, a suppressing effect was observed in the emission of NV− center; however, emission of NV0 center was not observed to be increasing as expected and was stable when lowering of the Fermi level was induced by a Schottky junction between gold and oxygen-terminated diamond containing NV centers. The relationship between the thickness of the depletion layer as well as that of the suppression layer, appearing when the Fermi level shifted below the ground state level of NV−, and the donor concentration was derived. Moreover, a model based on the assumption that an intermediate transition level (nonfluorescence state) existed between the ground state energy levels of NV− and NV0, was proposed to explain why suppressed NV− did not convert to NV0. The results of this study will aid in further understanding of the characteristics of NV centers. In addition, a Schottky junction can be developed to determine the charge state of unknown color centers and facilitate study of defects in diamonds.

Upgrading a Detector for Studying Fast Processes on a Beam of Synchrotron Radiation

The current stage of the upgrade of the DIMEX detector for studying fast processes on a synchrotron radiation (SR) beam is described. The current version of the DIMEX is a one-dimensional high-pressure ionization chamber with a fast electron component readout. The detector can record photon fluxes from each bunch separately without mixing photons from different bunches. The maximum flux the DIMEX can detect is limited by the space charge of positive ions in a high-pressure gas. To substantially improve this and other parameters of the detector, it was decided to switch to microstrip silicon technology. A prototype based on a silicon-microstrip sensor was developed along with a special integrated circuit (ASIC) for reading out the signals from this prototype. The first tests of the new DIMEX-Si prototype on an SR beam show that it is superior to the gaseous version in all respects. Results are presented from the first tests of the silicon-microstrip detector prototype on the SR beam of the VEPP-4M storage ring.

Modeling of fast neutral atoms flow generation in channel rays of glow discharge

A physical-kinetic approach has been used for modeling of fast neutral atoms flow generation in the channel rays of abnormal glow discharge. The fast atoms are generated due to charge exchange of ions accelerated by voltage drop on the cathode layer of positive space charge. The limits of applicability of the created model and the range of operating parameters of the fast atom source are established for the Ar (gas) – Ta (cathode) pair. The work is directed towards implementation of the beam processing of dielectric optical nanostructured metamaterials with help of fast neutral atoms.