Electron Injection Model Research Articles

Interpretation of the puzzling gamma-ray spectrum of the Geminga halo

The gamma-ray halo around Geminga is formed owing to the slow diffusion of the electrons released by the Geminga pulsar wind nebula (PWN). The latest observations from the HAWC and H.E.S.S. Collaborations exhibit complex features in the TeV gamma-ray spectrum of the Geminga halo. We first show that the new results cannot be interpreted by the commonly used simple model, where a single power-law injection spectrum and an energy index of $\ensuremath{\delta}=1/3$ for the diffusion coefficient are assumed. We then propose a two-population electron injection model based on the x-ray observations of the Geminga PWN, which consists of a population of freshly accelerated electrons escaping from the PWN through rapid outflows and a population trapped longer inside the PWN before escaping. The two-population model interprets the HAWC and H.E.S.S. data well, and the goodness of fit improves significantly compared with the single power-law injection model. It also predicts a different energy dependency of the gamma-ray profile from the single power-law injection model, which could be tested by the Large High Altitude Air Shower Observatory in the coming future. We note that a $\ensuremath{\delta}$ slightly larger than 1 is needed to fit the HAWC and H.E.S.S. data consistently. We also discuss the possible improvements by adopting the two-zone diffusion model.

Synchrotron Gamma-Ray Emission Model of the Giant Outburst of Quasar 3C 279 in 2015 June: Fast Reconnection or Stochastic Acceleration with Electromagnetic Cascade?

Abstract We test the synchrotron emission scenario for the very bright gamma-ray flare of blazar 3C 279 observed in 2015 June using time-dependent numerical simulations. A bulk Lorentz factor as high as 100 can bring the synchrotron maximum energy above the GeV energy range. We find two possible solutions for the X-ray to gamma-ray spectrum. One is a prompt electron injection model with a hard power-law index as magnetic reconnection models suggest. Too strong a magnetic field yields too bright a synchrotron X-ray flux due to secondary electron–positron pairs. Even in the prompt electron injection model, the Poynting flux luminosity is at most comparable to the gamma-ray or electron luminosity. Another model is the stochastic acceleration model, which leads to a very unique picture accompanying the electromagnetic cascade and reacceleration of the secondary electron–positron pairs. In this model, the energy budget of the magnetic field is very low compared to gamma-rays and electrons.

Electron injection model for graphene-induced efficiency enhancement in dye-sensitized solar cells

A model, correlating the reduced graphene oxide (rGO)-induced shrinking of the bandgap in rGO/TiO2 composite films to the efficiency enhancement in associated dye-sensitized solar cells, is proposed. The intercalated rGO nanosheets into TiO2 cause redistribution of the density of states (DOS) and modification of the energetics. The DOS increases with rGO proportion, [rGO], and the conduction band (CB) edge moves into the gap to regain the initial critical DOS for a mobility edge. The rate of electron injection from the dye increases proportionally to the DOS, thus enhancing the DSSC efficiency.

Time-dependent quantum Monte Carlo simulation of electron devices with two-dimensional Dirac materials: A genuine terahertz signature for graphene

An intrinsic electron injection model for linear band two-dimensional (2D) materials, like graphene, is presented and its coupling to a recently developed quantum time-dependent Monte Carlo simulator for electron devices, based on the use of stochastic Bohmian conditional wave functions, is explained. The simulator is able to capture the full (DC, AC, transient and noise) performance of 2D electron devices. In particular, we demonstrate that the injection of electrons with positive and negative kinetic energies is mandatory when investigating high frequency performance of linear band materials with Klein tunneling, while traditional models dealing with holes (defined as the lack of electrons) can lead to unphysical results. We show that the number of injected electrons is bias-dependent, implying that an extra charge is required to get self-consistent results. Interestingly, we provide a successful comparison with experimental DC data. Finally, we predict that a genuine high-frequency signature due to a roughly constant electron injection rate in 2D linear band electron devices (which is missing in 2D parabolic band ones) can be used as a band structure tester.

Electron transfer and cascade relaxation dynamics of graphene quantum dots/MoS2 monolayer mixed-dimensional van der Waals heterostructures

We realize the efficient modulation of relaxation channels in MoS 2 monolayers and interfacial electron transfer by forming mixed-dimensional VdW heterostructures with graphene quantum dots (GQDs) and find that the cascaded relaxation of hot electrons in GQDs that originate from the quantum confinement effect is the ultimate reason. With femtosecond pump–probe spectroscopy, the biphasic electron injection model and modified rate equations are employed to quantitatively solve the interfacial dynamics. Van der Waals (VdW) heterostructures have emerged as promising materials for atomically thin optoelectronic and photovoltaic applications, where the efficient charge separation after photo-excitation is significant to enhance device performances. However, modulating the interfacial charge transfer is still challenging due to the weak interlayer VdW interaction. Revealing electron transfer and relaxation processes in heterostructures lays foundation to tune the interfacial dynamics and optoelectronic properties. Here, we realize the efficient modulation of relaxation channels in MoS 2 monolayers and interfacial electron transfer by forming mixed-dimensional VdW heterostructures with graphene quantum dots (GQDs). With femtosecond pump–probe spectroscopy, the biphasic electron injection model and modified rate equations are employed to quantitatively solve interfacial transfer rates of mixed-dimensional VdW heterostructures. We find that the cascaded relaxation of hot electrons in GQDs that originates from the quantum confinement effect can intensively affect the interfacial dynamics. Our established model is instructive to optimize performances of future photon-harvesting devices based on mixed low-dimensional heterostructures.

TCAD investigation on hot-electron injection in new-generation technologies

The hot electron injection model presently available in the TCAD tools has been investigated against experiments on new test devices to the purpose of gaining an insight on its predictability in the context of the new-generation technologies. The study has been carried out on electron emission in extremely high electric fields, as expected in power LDMOS devices at the onset of avalanche breakdown, reaching for the first time injection probabilities as high as 0.01. The numerical analysis clearly showed that the new Si/SiO2 interfaces experience different features with respect to the old ones. The TCAD method based on the deterministic solution of the Boltzmann equation can accurately capture such effects.

Local environments and transport properties of heavily doped strontium barium niobates Sr0.5Ba0.5Nb2O6

Undoped as well as K-doped (40%), Y-doped (40%), Zr-doped (10%), and Mo-doped (12.5%) strontium barium niobate Sr0.5Ba0.5Nb2O6 (SBN50) materials have been investigated to explore the effect of heavy doping on the structural and functional properties (thermo-power, thermal and electrical conductivities) both in the as prepared (oxidized) and reduced states.For all materials, the EXAFS spectra at the Nb – K edge can be consistently analyzed with the same model of six shells around the Nb sites. Doping mostly gives a simple size effect on the structural parameters, but doping on the Nb sites weakens the Nb–O bond regardless of dopant size and charge. Shell sizes and Debye–Waller factors are almost unaffected by temperature and oxidation state, and the disorder is of static nature.The functional effects of heavy doping do not agree with a simple model of hole or electron injection by aliovalent substitutions on a large band gap semiconductor.With respect to the undoped samples, doping with Mo depresses the thermal conductivity by ~ 30%, Y doping enhances the electrical conductivity by an order of magnitude, while Zr doping increases the Seebeck coefficient by a factor of 2–3. Globally, the ZT efficiency factor of the K-, Y-, and Zr-doped samples is enhanced at least by one order of magnitude with respect to the undoped or Mo-doped materials.

Indirect Band Gap Emission by Hot Electron Injection in Metal/MoS₂ and Metal/WSe₂ Heterojunctions.

Transition metal dichalcogenides (TMDCs), such as MoS2 and WSe2, are free of dangling bonds and therefore make more "ideal" Schottky junctions than bulk semiconductors, which produce Fermi energy pinning and recombination centers at the interface with bulk metals, inhibiting charge transfer. Here, we observe a more than 10× enhancement in the indirect band gap photoluminescence of transition metal dichalcogenides (TMDCs) deposited on various metals (e.g., Cu, Au, Ag), while the direct band gap emission remains unchanged. We believe the main mechanism of light emission arises from photoexcited hot electrons in the metal that are injected into the conduction band of MoS2 and WSe2 and subsequently recombine radiatively with minority holes in the TMDC. Since the conduction band at the K-point is 0.5 eV higher than at the Σ-point, a lower Schottky barrier exists for the Σ-point band, making electron injection more favorable. Also, the Σ band consists of the sulfur pz orbital, which overlaps more significantly with the electron wave functions in the metal. This enhancement in the indirect emission only occurs for thick flakes of MoS2 and WSe2 (≥100 nm) and is completely absent in monolayer and few-layer (∼10 nm) flakes. Here, the flake thickness must exceed the depletion width of the Schottky junction, in order for efficient radiative recombination to occur in the TMDC. The intensity of this indirect peak decreases at low temperatures, which is consistent with the hot electron injection model.

A first-principles study of Pt–Ni bimetallic cluster adsorption on the anatase TiO 2 (1 0 1) surface: Probing electron effect of Ni in TiO 2 (1 0 1)-bimetallic cluster (Pt–Ni) on the adsorption and dissociation of methanol

Abstract A density functional theory (DFT) based method in conjunction with the projector augmented wave and pseudopotential methods have been applied to investigate the adsorption of Pt4 and Pt3Ni on the anatase TiO2 (1 0 1) surface. Two stable Pt3Ni adsorptions with considerable adsorption energies on the anatase TiO2 (1 0 1) surface were identified. Analysis of the partial density (PDOS) of states and Bader charge suggest that the electronic structure of Pt is modified by Ni due to the electron transfer from Ni to Pt atoms in the Pt3Ni clusters. The 2cO (3cO)-PtNi-5cTi conformation of the adsorbed Pt3Ni on the anatase TiO2 (1 0 1) surface provides a more feasible model for electron injection through the Pt3Ni/TiO2 interface. The reactivity of Pt3Ni/TiO2 is superior to Pt4/TiO2 and effectively manifests itself in the eased decomposition of O H bonds derived by methanol and alleviative CO adsorption.

Investigation of Local Field Enhancement and Hot Electron Injection in Carbon Nano-Tube Doped Phosphor Nano-Composite for Ultra-Bright Electroluminescence

Present work focuses on the effective doping of multi-walled carbon nanotube (CNT) in the ZnS:Cu phosphor nano-composite and thereafter improvement in the optical performance of electroluminescent (EL) device due to increased local field effects. To facilitate doping of CNTs into the phosphor and decrease the operating voltage of the EL device, CNTs were shortened by milling and incorporated effectively using a flux assisted solid-state annealing reaction. Interestingly shorter the length of CNTs used; greater was the local field enhancement, improvement in brightness and efficiencies observed for the EL devices. When the field is applied, adequate charge carriers are tunneled into the ZnS:Cu system through the tips of the CNTs by forming high energy hot spots thus enhancing the local field. The improved device characteristics are due to field enhancement and effective transfer of energy from hot spots to copper activator by impact ionization. The detailed electrical characterization of the novel EL device along with its brightness measurements are presented by considering the hot electron injection model.

Impact of strain on gate-induced floating body effect for partially depleted silicon-on-insulator p-type metal–oxide–semiconductor-field-effect-transistors

This work investigates impact of mechanical strain on gate-induced-floating-body-effect (GIFBE) for partially depleted silicon-on-insulator p-type metal–oxide–semiconductor field effect transistors (PD SOI p-MOSFETs). First part, the original mechanism of GIFBE on PD SOI p-MOSFETs is studied. The experimental results indicate that GIFBE causes a reduction in oxide electric field (Eox), resulting in an underestimate of negative-bias temperature instability (NBTI) degradation. This can be attributed to the electrons tunneling from the process-induced partial n+ poly gate and anode electron injection (AEI) model, rather than the electron valence band tunneling (EVB) widely accepted as the mechanism for n-MOSFETs. And then, the second part shows that the strained FB device has less NBTI degradation than the unstrained devices. This behavior can be attributed to the fact that more electron accumulation was induced by strain-induced band gap narrowing, reducing NBTI significantly.

An Efficient Nonlocal Hot Electron Model Accounting for Electron–Electron Scattering

This paper presents a nonlocal model for channel hot electron injection in MOSFETs and nonvolatile memories, which includes a full-band description of optical phonon scattering rates and carrier group velocity. By virtue of its efficient formalism, this model can also include carrier-carrier scattering, which has a marked impact on gate current at low gate voltages. The model is compared against full-band Monte Carlo simulations of typical nor flash devices in terms of distribution functions, bulk current, gate current, and gate current density along the channel. A very good agreement is obtained for various drain and gate voltages and channel lengths.

On the Origin of Gate-Induced Floating-Body Effect in PD SOI p-MOSFETs

This letter systematically investigates the origin of gate-induced floating-body effect (GIFBE) in partially depleted silicon-on-insulator p-type MOSFETs. The experimental results indicate that GIFBE causes a reduction in the electrical oxide field, leading to an underestimate of negative-bias temperature instability degradation. This can be partially attributed to the electrons tunneling from the process-induced partial n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> polygate. However, based on different operation conditions, we found that the dominant origin of electrons was strongly dependent on holes in the inversion layer under source/drain grounding. This suggests that the mechanism of GIFBE at higher voltages is dominated by the proposed anode electron injection model, rather than the electron valence band tunneling widely accepted as the mechanism for n-MOSFETs.

Effects of non-uniform interstellar magnetic field on synchrotron X-ray and inverse-Comptonγ-ray morphology of supernova remnants

Observations of SNRs in X-ray and gamma-ray bands promise to contribute with important information in our understanding on the nature of galactic cosmic rays. The analysis of SNRs images collected in different energy bands requires the support of theoretical modeling of synchrotron and inverse Compton (IC) emission. We develop a numerical code (REMLIGHT) to synthesize, from MHD simulations, the synchrotron radio, X-ray and IC gamma-ray emission from SNRs expanding in non-uniform interstellar medium (ISM) and/or non-uniform interstellar magnetic field (ISMF). As a first application, the code is used to investigate the effects of non-uniform ISMF on the SNR morphology in the non-thermal X-ray and gamma-ray bands. We perform 3D MHD simulations of a spherical SNR shock expanding through a magnetized ISM with a gradient of ambient magnetic field strength. The model includes an approximate treatment of upstream magnetic field amplification and the effect of shock modification due to back reaction of accelerated cosmic rays. From the simulations, we synthesize the synchrotron radio, X-ray and IC gamma-ray emission with REMLIGHT, making different assumptions about the details of acceleration and injection of relativistic electrons. A gradient of the ambient magnetic field strength induces asymmetric morphologies in radio, X-ray and gamma-ray bands independently from the model of electron injection if the gradient has a component perpendicular to the line-of-sight. The degree of asymmetry of the remnant morphology depends on the details of the electron injection and acceleration and is different in the radio, X-ray, and gamma-ray bands. The non-thermal X-ray morphology is the most sensitive to the gradient, showing the highest degree of asymmetry. The IC gamma-ray emission is weakly sensitive to the non-uniform ISMF, the degree of asymmetry of the SNR morphology being the lowest in this band.

Two-dimensional model of space charge limited electron injection into a diode with Schottky contact

We present an analytical two-dimensional (2D) model of a high current space charge limited (SCL) electron injection into a solid through a Schottky contact for a n+–i–n+ diode with constant mobility and no velocity overshoot effect. The amount of electron injection is limited by the barrier at the contact, which includes the effect of electron tunnelling through the Schottky barrier. For a given barrier height, the SCL current condition can be reached if the tunnelling current becomes dominant over the drift–diffusion current. An analytical scaling is present to show the enhancement of the SCL current due to finite contact size, which is verified by a 2D device simulator over a wide range of parameters.

Power-law-type electron injection through lithium fluoride nanolayers in phosphorescenceorganic light-emitting devices

This work reports a power-law-type electron injection model that accounts for the thicknesseffect of lithium fluoride (LiF) nanolayers used as an electron injection layer inphosphorescence organic light-emitting devices (PHOLEDs). A series of PHOLEDs werefabricated with various LiF thicknesses in order to investigate the influence of LiFnanolayer thickness on the device current. The PHOLEDs exhibited pronouncedly changedelectron currents by only 0.2 nm thickness variation. The device current as a function of LiFthickness excellently followed the power-law model proposed in this study, from which aphysical constant indicating the intrinsic nature of electron injection materials has beenextracted.

Dependence of Photocurrent and Conversion Efficiency of Titania-Based Solar Cell on the Qy Absorption and One Electron-Oxidation Potential of Pheophorbide Sensitizer

Titania-based solar cells were fabricated by the use of six pheophorbide sensitizers having bacteriochlorin, chlorin, and porphyrin macrocycles, to which the carboxyl or allylcarboxyl group is directly attached for binding and electron injection to TiO2. Because of structural similarity, these sensitizers are expected to have similar physical properties except for electronic-absorption and redox properties: Concerning the former, the state energies, molar extinction coefficients (ε), oscillator strengths ( f ), and transition-dipole moments (μ) of the Soret and Qy absorptions were determined, whereas concerning the latter, one electron-oxidation potential (Eox). Alternatively, the performance of pheophorbide-sensitized solar cells, including the incident photon-to-current conversion efficiency (IPCE), short-circuit current density (Jsc), open-circuit voltage (Voc), and solar energy-to-electricity conversion efficiency (η) was determined. It was found that the Jsc and η values increased with the increasing Qy absorption and with the decreasing one electron-oxidation potential (in other words, with the increasing electron-ejection potential). To explain the clear and strong dependence, we built possible models for the pheophorbide-to-TiO2 electron injection and tried to fit the Jsc value in terms of the Qy absorption and the Eox value by the use of empirical equations. After a number of fitting trials, two successful models of reasonable fitting emerged: One, a model of parallel electron injection, that is, electron injection upon Qy excitation and redox electron transfer in the ground state, and the other, a model of electron injection simply via the excited state, in which both the Qy absorption and the Qy-state one electron-oxidation potential can contribute. These models as well as the future strategies in revealing the real mechanism of electron injection are discussed.

A unification of interface-state generation and hole-injection for hot-carrier-injection stress in low and high-voltage NMOSFET

To characterize hot-carrier-injection degradation, a typical reaction–diffusion model has been used in low-voltage NMOSFETs with only hot-electron-injection. In this paper, an improved reaction–diffusion model of hole-injection and electron-injection induced interface-state generation, with a lower time exponent, is proposed in high-voltage NMOSFETs. The dynamics of hot-carrier-injection are considered with simulation, experiment and theory. Hole-injection induced by an additional high-electric-field outside the gate is found to be also responsible for degradation mechanism. Based on a better understanding of the mechanism of HV NMOSFETs, the modified reaction–diffusion model for hot-carrier-injection stress involves hole-injection.

Capacitance-voltage characteristics of organic light-emitting diodes varying the cathode metal: Implications for interfacial states

Capacitance-voltage $(C\text{\ensuremath{-}}V)$ characteristics of organic light-emitting diodes based on a polyphenylenevinylene have been measured by means of impedance spectroscopy. The effect of the metallic cathode (Au, Ag, Al, Mg, and Ba) was analyzed in the low-frequency region $(2\phantom{\rule{0.3em}{0ex}}\mathrm{Hz})$ of the capacitive response. The $C\text{\ensuremath{-}}V$ curves collapse into a single pattern in the low bias region, and exhibit a dependence on the cathode work function showing a crossover from positive to negative (inductive) values. The voltage corresponding to the onset of the inductive behavior shifts toward higher bias as the cathode work function increases. Simulations based on an electron-injection model result in agreement with the observed $C\text{\ensuremath{-}}V$ characteristics. From the comparison between the predicted and the experimentally observed data an estimation of the energetics of the metal-organic interface could be given. First, an interfacial state located $\ensuremath{\sim}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the metal Fermi level appears independently of the metallic cathode. Second, the existence of a dipole layer with interface slope $S=\ensuremath{-}0.43$ is deduced which enhances relatively the electron injection for large work function metals. In the case of a Ba cathode, the electron barrier determining carrier injection is due to the interface states.

An electron injection model for time-dependent simulators of nanoscale devices with electron confinement: Application to the comparison of the intrinsic noise of 3D-, 2D- and 1D-ballistic transistors

For ballistic (quasi-ballistic) devices, the noise is mainly determined by the rate of injection of electrons from the contacts. Thus, the computation of the noise can be very sensible to the boundary conditions imposed on the simulators. An algorithm for the injection of electrons in nanoscale devices with (or without) electron confinement for degenerate (or non-degenerate) conditions is presented. The injection model is conceptually similar to the boundary conditions used in the Landauer approach, but it is developed for time-dependent simulators. For a simple ballistic two-terminal device, as a test, it is shown that the injection model directly reproduces the average current and the noise obtained by the Landauer–Buttiker formalism. Finally, the injection model is applied to compute noise in nanoscale transistors (without self-consistence with the Poisson equation). The results predict an increase of the Fano factor when electron confinement is present. In addition, it is also shown that other electron injection models without correlation between the injection of consecutive electrons increase the noise of nanoscale transistors unrealistically, when degenerate conditions are present.