Ionization Model Research Articles

Modeling of SPAD avalanche breakdown probability and jitter tail with field lines

A new methodology to accurately simulate the Photon Detection Efficiency and the Jitter tail of SPAD devices is presented. This method first relies on the use of the electric field lines to mimic the carriers’ trajectories. A model for impact ionization and avalanche probability is then used on the obtained lines to simulate the probability of avalanche, coupled with the optical absorption, the PDE is then extracted. Finally, an advection–diffusion model is used to simulate the drift and diffusion of carriers within the device, which leads to the timing jitter due to the transport time from the photogeneration spot to the avalanche region. The results obtained numerically are compared with an extensive series of measurements and show a good agreement on a wide variety of device designs.

Effect of doping on Al2O3/GaN MOS capacitance

This paper investigates the turning-on-voltage (VFB/VTH) of Al2O3/GaN MOS stacks with n-doped GaN, p-doped GaN and not intentionally doped (NID) GaN by exploiting capacitance measurements on large gate area test structures with systematic variation of Al2O3 thickness (tox). Measurements are compared with 1D Schrödinger-Poisson simulations including incomplete ionization model. The necessity of using a quantum description of electron density is demonstrated especially for thinner gate oxides. We found that, contrary to what is expected, p-doping below the channel barely increases the VTH and the VTH is independent of tox, even if the density of activated acceptors is demonstrated to be sufficiently high. Our results highly suggest that the negative charge induced by p-doping is compensated at the oxide level.

Reconstruction of the vibronic-state density matrix based on pump-probe state-resolved energy spectra

We propose a scheme based on perturbation theory to reconstruct a molecular density matrix (DM) using the pump-probe state-resolved energy spectra. By applying the scheme to an ${\mathrm{O}}_{2}^{+}$ system produced via strong-field ionization, we demonstrate that the reconstruction scheme can retrieve the populations and coherences of vibrational-electronic (vibronic) states while maintaining good robustness. We further extend the strong-field transient ionization model to include the rotational degree of freedom. The evolution of the vibronic-state DM can be recovered, benefiting from the population stabilities between adjacent fractional revivals. The retrieved signals quantitatively capture the distinguishing decay features of the vibronic coherences. Our strategy paves the way toward understanding elaborate vibronic-state DMs in molecular systems.

Ultrafast Laser Material Damage Simulation-A New Look at an Old Problem.

The chirped pulse amplification technique has enabled the generation of pulses of a few femtosecond duration with peak powers multi-Tera and Peta–Watt in the near infrared. Its implementation to realize even shorter pulse duration, higher energy, and higher repetition rate laser systems relies on overcoming the limitations imposed by laser damage of critical components. In particular, the laser damage of coatings in the amplifiers and in post-compression optics have become a bottleneck. The robustness of optical coatings is typically evaluated numerically through steady-state simulations of electric field enhancement in multilayer stacks. However, this approach cannot capture crucial characteristics of femtosecond laser induced damage (LID), as it only considers the geometry of the multilayer stack and the optical properties of the materials composing the stack. This approach neglects that in the interaction of an ultrashort pulse and the materials there is plasma generation and associated material modifications. Here, we present a numerical approach to estimate the LID threshold of dielectric multilayer coatings based on strong field electronic dynamics. In this dynamic scheme, the electric field propagation, photoionization, impact ionization, and electron heating are incorporated through a finite-difference time-domain algorithm. We applied our method to simulate the LID threshold of bulk fused silica, and of multilayer dielectric mirrors and gratings. The results are then compared with experimental measurements. The salient aspects of our model, such as the implementation of the Keldysh photoionization model, the impact ionization model, the electron collision model for ‘low’-temperature, dense plasma, and the LID threshold criterion for few-cycle pulses are discussed.

Soil impulse ionisation model based on dynamic changes of electric field

Soil ionisation has an important influence on impulse characteristics of grounding conductor. This paper introduced improvements to an existing soil ionisation model considering the influence of electric field on the change of soil resistivity, while soil resistivity is not directly dependent on soil electric field. The validity of improved soil ionisation model was verified by surge response of vertical conductor in uniform soil and horizontally stratified soil. Furthermore, the impulse experiment was carried out on horizontal conductor in soil with finite volumes. Based on experimental configuration, the corresponding calculation model was built, and calculated results were in good agreement with experimental results.

Temperature-Dependent Characteristics of HgCdTe Mid-Wave Infrared E-Avalanche Photodiode

This paper presents the characteristics of HgCdTe mid-wavelength infrared (MWIR) electron-initiated avalanche photodiodes (e-APDs) as a function of temperature under different biases. The devices show a low dark current density of the order of <inline-formula><tex-math notation="LaTeX">${10^{ - 7}} \text{A/cm}^2$</tex-math></inline-formula> at 80 K when the reverse bias voltage is below 4 V, with an exponential gain above 100 at &#x2212; 8 V. Low excess noise factor of around 1.2 is also demonstrated for the devices at 80 K. The dark current and gain characteristics at different temperatures are analyzed, along with the device simulation results. A varied-temperature impact ionization model for MWIR e-APD devices is adopted based on the experimental results. In addition, a significant increase of the dark current in the APD device is observed at high temperature under large reverse bias, which can be associated with the local electric field redistribution due to the accumulation of holes in the depletion region at high temperatures. The analysis presented in this work paves the way to achieve a high operating temperature (HOT) of the HgCdTe APD with further optimization.

Nanoscale Impact Ionization and Electroluminescence in a Biased Scanning-Tunneling-Microscope Junction

Electroluminescence from a p-type GaAs(110) surface was induced by tunneling electrons in a scanning tunneling microscope under both polarities of bias voltage. The optical spectra exhibit a polarity-independent luminescence peak at 1.47 eV resulting from the exciton recombination. However, the quantum yield of photon emission at negative bias voltage is two orders of magnitude weaker than that at positive bias voltage. Moreover, the luminescence at negative bias voltage shows the linear dependence of bias voltage, distinct from the rapid rise due to resonant electron injection at positive bias. Furthermore, the threshold bias voltage for electroluminescence at negative bias is nearly twice the bandgap of GaAs, not simply satisfying the energy conservation for the creation of an electron-hole pair. Through theoretical calculation, we propose an impact ionization model to nicely explain the newly observed electroluminescence at negative bias voltage. We believe that this mechanism of impact ionization could be readily applied to other nanoscale optoelectronics including 2D semiconductors and 1D nanostructures.

A New 3D Solar Wind Speed and Density Model Based on Interplanetary Scintillation

Abstract The solar wind (SW) is an outflow of the solar coronal plasma, which expands supersonically throughout the heliosphere. SW particles interact by charge exchange with interstellar neutral atoms; on the one hand, they modify the distribution of this gas in interplanetary space, and, on the other hand, they are the seed populations for heliospheric pickup ions and energetic neutral atoms (ENAs). The heliolatitudinal profiles of the SW’s speed and density evolve during the solar activity cycle. A model of the evolution of the SW’s speed and density is needed to interpret observations of ENAs, pickup ions, the heliospheric backscatter glow, etc. We derive the Warsaw Heliospheric Ionization Model 3DSW—WawHelIon 3DSW—based on interplanetary scintillation (IPS) tomography maps of the SW speed. We use the IPS tomography data from 1985 to 2020, compiled by Tokumaru et al. We derive a novel statistical method of filtering these data against outliers; we present a flexible analytic formula for the latitudinal profiles of the SW speed, based on Legendre polynomials of varying order with additional restraining conditions at the poles; fit this formula to the yearly filtered data; and calculate yearly SW density profiles using the latitudinally invariant SW energy flux observed in the ecliptic plane. Despite the application of a refined IPS data set, a more sophisticated data filtering method, and a more flexible analytic model, the present results mostly agree with those obtained previously, demonstrating the robustness of IPS studies of the SW’s structure.

Research cloud electrification model in the Wisconsin dynamic/microphysical model 2: Charge structure in an idealized thunderstorm and its dependence on ion generation rate

This paper presents a cloud electrification model, which we embedded in the Wisconsin Dynamic and Microphysical Model-2 and labels it CEMW. WISDYMM-2 makes use of two-moment cloud microphysics to produce 5 hydrometeor types (e.g., cloud-droplets, raindrops, cloud-ice, snow, and graupel) that are used by CEMW for storm electrification where storm convection is initiated by a warm air bubble that is placed over an assumed flat terrain with no surface friction. In this paper, CEMW was used to examine cloud electrification in a simulated (idealized) thundercloud and to examine the impact of various formulations of the ion generation rate by cosmic rays (G) on how the storm and individual hydrometeor charges were structured. Results showed that the CEMW generates reasonable electric charge structures, which is qualitatively similar to those published by Brothers et al. (2018) in that it consists of a number of smaller positively and negatively charged regions. This structure differs from a charge structure generally depicted by conceptual models based on conventional balloon measurements of electric field. However, simulated balloon measurements in the idealized thunder clouds further revealed that CEMW produces electrostatic charge distributions and electric field profiles that are in good agreement with those reported by real balloon measurements. How charge is structured by CEMW was tested by formulating G (the ion generation rate) in two different ways. First, we derived G assuming fair weather conditions, which is the usual way applied in cloud electrification modelling. Second, we calculated Gs using the Cosmic Ray Atmospheric Cascade: Cosmic Ray Induced Ionization model for several values of solar modulation potential and cut-off rigidity. The results show that the structure of the electric charge fields does not differ much depending on G, but the fundamental difference between G is in the amount of electric discharges.

Probing the physicochemical properties of the Leo Ring and the Leo I group

We present an absorption line study of the physical and chemical properties of the Leo HI Ring and the Leo I Group as traced by 11 quasar sightlines spread over a 600 kpc X 800 kpc region. Using HST/COS G130/G160 archival observations as constraints, we couple cloud-by-cloud, multiphase, Bayesian ionization modeling with galaxy property information to determine the plausible origin of the absorbing gas along these sightlines. We search for absorption in the range 600 km/s - 1400 km/s consistent with the kinematics of the Leo Ring/Group. We find absorption plausibly associated with the Leo Ring towards five sightlines. Along three other sightlines, we find absorption likely to be associated with individual galaxies, intragroup gas, and/or large-scale filamentary structure. The absorption along these five sightlines is stronger in metal lines than expected from individual galaxies, indicative of multiple contributions, and of the complex kinematics of the region. We also identify three sightlines within a 7-degree X 6-degree field around the Leo Ring, along which we do not find any absorption. We find that the metallicities associated with the Leo Ring are generally high, with values between solar and several times solar. The inferred high metallicities are consistent with the origin of the ring as tidal debris from a major galaxy merger.

Current Filamentation and Switching Effect in Chalcogenide Glassy Semiconductors: A Review

A review on current filamentation and switching effect in chalcogenide glassy semiconductors (CGSs), which are promising materials for the development of phase change memory devices, is presented. First, a history of the research on CGSs and their properties is considered. Next, formation of a current filament in CGSs is discussed and the scale of heat release in the material as well as the geometric shape of the filament is analyzed. Finally, various hypotheses developed for the explanation of the switching effect in CGSs are reviewed. It is shown that the most relevant model of the switching effect in CGS is the model of multi-phonon tunneling ionization of the so-called ‘negative-U centers’. This model is based on the assumption that an avalanche-like increase in current at a certain point in time is associated with mass tunneling of electrons located on atoms, occuring due to thermal vibrations of atoms.

Electrical and optical properties of cobalt oxide thin films, prepared by ion-beam sputtering

Optical and electrical properties of cobalt oxide thin films obtained by ion-beam sputtering in argon atmosphere and argon with the addition of oxygen (PAr=1.1·10-5 Torr) has been investigated in the work. Optical properties investigations showed that, for films of cobalt oxide obtained in argon atmosphere, the optical band gap is independent of the film thickness and is 3.24 eV, which is in the range of given in the literature for the CoO phase values. For cobalt oxide films obtained in mixed atmosphere of argon with the addition of oxygen, two direct optical transitions with energies of 1.45 and 2.1 eV were detected. The presence of two direct allowed optical transitions is associated with the variable valence of cobalt in the Co3O4 compound and the presence of two valence states Co2+ and Co3+. The dependences of specific electrical conductivity of the synthesized films on the magnitude of the electric field were studied. It was found that for all investigated samples, the electrical conductivity does not depend on the electric field strength up to the value of E=106 V/m. The nonlinearity of the dependence of specific electrical conductivity of the synthesized films on electric field strength at E&amp;gt;106 V/m is discussed in terms of the hopping conductivity model and the trap ionization model described by the Poole?Frenkel effect. Keywords: oxide semiconductors, optical absorption coefficient, electrical conductivity, strong electric fields.

Электрические и оптические свойства тонких пленок оксида кобальта, полученных методом ионно-лучевого распыления

Optical and electrical properties of cobalt oxide thin films obtained by ion-beam sputtering in argon atmosphere and argon with the addition of oxygen (PAr = 1.1∙10-5 Torr) has been investigated in the work. Optical properties investigations showed that, for films of cobalt oxide obtained in argon atmosphere, the optical band gap is independent of the film thickness and is 3.24 eV, which is in the range of given in the literature for the CoO phase values. For cobalt oxide films obtained in mixed atmosphere of argon with the addition of oxygen, two direct optical transitions with energies of 1.45 and 2.1 eV were detected. The presence of two direct allowed optical transitions is associated with the variable valence of cobalt in the Co3O4 compound and the presence of two valence states Co2+ and Co3+. The dependences of specific electrical conductivity of the synthesized films on the magnitude of the electric field were studied. It was found that for all investigated samples, the electrical conductivity does not depend on the electric field strength up to the value of E = 106 V/m. The nonlinearity of the dependence of specific electrical conductivity of the synthesized films on electric field strength at E &gt; 106 V/m is discussed in terms of the hopping conductivity model and the trap ionization model described by the Poole–Frenkel effect.

Performance analysis of an inert gas plasma Faraday-type MHD generator by coupling thermodynamic equilibrium ionization model with UDF functions of Fluent

Performance analysis of an inert gas plasma Faraday-type MHD generator by coupling thermodynamic equilibrium ionization model with UDF functions of Fluent

Third-order transport coefficients for electrons in N2 and CF4: effects of non-conservative collisions, concurrence with diffusion coefficients and contribution to the spatial profile of the swarm

Using a multi-term solution of the Boltzmann equation and Monte Carlo simulation technique we study behaviour of the third-order transport coefficients for electrons in model gases, including the ionisation model of Lucas and Saelee and modified Ness–Robson model of electron attachment, and in real gases, including N2 and CF4. We observe negative values in the E/n 0-profiles of the longitudinal and transverse third-order transport coefficients for electrons in CF4 (where E is the electric field and n 0 is the gas number density). While negative values of the longitudinal third-order transport coefficients are caused by the presence of rapidly increasing cross sections for vibrational excitations of CF4, the transverse third-order transport coefficient becomes negative over the E/n 0-values after the occurrence of negative differential conductivity. The discrepancy between the two-term approximation and the full multi-term solution of the Boltzmann equation is investigated for electrons in N2 and CF4. While the accuracy of the two-term approximation is sufficient to investigate the behaviour of the third-order transport coefficients in N2, it produces large errors and is not even qualitatively correct for electrons in CF4. The influence of implicit and explicit effects of electron attachment and ionisation on the third-order transport tensor is investigated. In particular, we discuss the effects of attachment heating and attachment cooling on the third-order transport coefficients for electrons in the modified Ness–Robson model, while the effects of ionisation are studied for electrons in the ionisation model of Lucas and Saelee, N2 and CF4. The concurrence between the third-order transport coefficients and the components of the diffusion tensor, and the contribution of the longitudinal component of the third-order transport tensor to the spatial profile of the swarm are also investigated. For electrons in CF4 and CH4, we found that the contribution of the component of the third-order transport tensor to the spatial profile of the swarm between approximately 50 Td and 700 Td, is almost identical to the corresponding contribution for electrons in N2. This suggests that the recent measurements of third-order transport coefficients for electrons in N2 may be extended and generalized to other gases, such as CF4 and CH4.

Application of Binary Encounter Approximation and Electron Impact on Ionization of Atoms

Semi-classical binary encounter approximation has been used for theoretical calculations of electron impact single ionization cross sections of Xe and Kr and double ionization of Fe atom at ground state. An accurate expression of cross section for energy transfer ΔE (σΔE) as given by Vriens and quantum mechanical Hartree-Fock velocity distributions for target electron have been used in the calculation. In the case of single ionization of Xe, 71% theoretical results lies within valid range of ratio factor less than 2 and 55% have valueless than 1.5. In the case of Kr, 97% of results have ratio factor less than 2 and 34% of results have ratio factor less than 1.1. Gryzinski and Kune model of charged particle impact double ionization of atoms found suitable for describing double ionization of atoms and ions. In the case of double ionization of Fe by electron impact 47% of results have ratio factor less than 2 and 18% of results have ratio factor 1.2. At impact 760 eV, 800 eV and 850 eV having ratio factor 1.01, 0.989 and 0.967. Major contribution in double ionization is 60.16% from (4s,3d) and 31% is from (4s,3p).

3D spatio-kinematic modelling of Abell 48, a planetary nebula around a Wolf–Rayet [WN] star

ABSTRACT The planetary nebula (PN) Abell 48 (PN G029.0+00.4) is around a rare Wolf–Rayet [WN5] star whose stellar history is as yet unknown. Using the integral field observations of the H αλ6563 and [N ii] λ6584 line emissions, we conducted a comprehensive spatio-kinematic analysis of this PN. A three-dimensional spatio-kinematic ionization model was developed with the kinematic modelling tool shape to replicate the observed spatially resolved velocity channels and position–velocity diagrams. According to our kinematic analysis of the H α emission, this object possesses a deformed elliptic toroidal shell with an outer radius of 23 arcsec and a thickness of 15 arcsec associated with an integrated H α emission-line expansion of ∼35 ± 5 km s−1, a maximum poloidal expansion of around 70 ± 20 km s−1 at an inclination angle of ∼30° with respect to the line of sight, and a position angle of ∼130° measured from east towards north in the equatorial coordinate system. Furthermore, [N ii] kinematic modelling reveals the presence of narrow (∼3 arcsec) exterior low-ionization structures surrounding the main elliptical shell, which could have formed as a result of shock collisions with the interstellar medium. The torus-shaped morphology of this PN could be related to its unusual hydrogen-deficient [WN] nucleus that needs to be inspected further.

Investigation of vortex arms in electron vortices by counter-rotating elliptically polarized attosecond laser pulses

We theoretically investigate the vortex patterns in photoelectron momentum distributions of He+ driven by counter-rotating elliptically polarized, time delayed attosecond laser pulses by numerically solving the two-dimensional time-dependent Schrödinger equation. It is found that the number of vortex arms is extremely sensitive to the ellipticity and wavelength of counter-rotating elliptically polarized laser pulses, which is illustrated by the attosecond perturbation ionization models. In addition, the effect of different time delays between two pulses on the interference patterns is also investigated and the corresponding physical mechanism is demonstrated. Since the wavelength, ellipticity and time delay have a significant effect on the vortex interference patterns, this may be a new method for laser field detection.

Encapsulation of Metronidazole in Biocompatible Macrocycles and Structural Characterization of Its Nano Spray-Dried Nanostructured Composite.

Due to the great potential of biocompatible cucurbit[7]uril (CB7) and 4-sulfonatocalix[4]arene (SCX4) macrocycles in drug delivery, the confinement of the pharmaceutically important metronidazole as an ionizable model drug has been systematically studied in these cavitands. Absorption and fluorescence spectroscopic measurements gave 1.9 × 105 M−1 and 1.0 × 104 M−1 as the association constants of the protonated metronidazole inclusion in CB7 and SCX4, whereas the unprotonated guests had values more than one order of magnitude lower, respectively. The preferential binding of the protonated metronidazole resulted in 1.91 pH unit pKa diminution upon encapsulation in CB7, but the complexation with SCX4 led to a pKa decrease of only 0.82 pH unit. The produced protonated metronidazole–SCX4 complex induced nanoparticle formation with protonated chitosan by supramolecular crosslinking of the polysaccharide chains. The properties of the aqueous nanoparticle solutions and the micron-sized solid composite produced therefrom by nano spray drying were unraveled. The results of the present work may find application in the rational design of tailor-made self-assembled drug carrier systems.

Characterizing Extreme Emission-line Galaxies. I. A Four-zone Ionization Model for Very High-ionization Emission*

Abstract Stellar population models produce radiation fields that ionize oxygen up to O+2, defining the limit of standard H ii region models (&lt;54.9 eV). Yet, some extreme emission-line galaxies, or EELGs, have surprisingly strong emission originating from much higher ionization potentials. We present UV HST/COS and optical LBT/MODS spectra of two nearby EELGs that have very high-ionization emission lines (e.g., He ii λλ1640,4686 C iv λλ1548,1550, [Fe v]λ4227, [Ar iv]λλ4711,4740). We define a four-zone ionization model that is augmented by a very high-ionization zone, as characterized by He+2 (&gt;54.4 eV). The four-zone model has little to no effect on the measured total nebular abundances, but does change the interpretation of other EELG properties: we measure steeper central ionization gradients; higher volume-averaged ionization parameters; and higher central T e , n e , and log U values. Traditional three-zone estimates of the ionization parameter can underestimate the average log U by up to 0.5 dex. Additionally, we find a model-independent dichotomy in the abundance patterns, where the α/H abundances are consistent but N/H, C/H, and Fe/H are relatively deficient, suggesting these EELGs are α/Fe-enriched by more than three times. However, there still is a high-energy ionizing photon production problem (HEIP3). Even for such α/Fe enrichment and very high log U s, photoionization models cannot reproduce the very high-ionization emission lines observed in EELGs.