Electron Energy Loss Rate Research Articles

Hot-electron relaxation in ZnO films

The energy relaxation of hot electrons in ZnO films was investigated by the electric-field-dependent photoluminescence. From the high-energy tail of photoluminescence, the electron temperature of the hot electrons in ZnO was determined. Longitudinal optical phonon emission alone cannot explain the relationship between the electron temperature and the electron energy loss rate. Instead, it can be explained by a model based on a combination of both the acoustic and longitudinal optical phonon emissions.

Calculation of runaway electrons stopping power in ITER

The energy loss rate of runaway electrons (RE) was analysed for ITER plasma facing components materials (Be and W). The stopping power, the energy deposition profiles, and the material erosion are estimated by using the codes MEMOS and ENDEP. The latter has been updated by including the effect of the target’s polarizability. Our calculations show that this effect is significant for high RE energies and low Z materials such as Be. We also find that the conversion of the RE’s magnetic energy into heat can explain the temperature rise on dump plate in JET. In the case of ITER, the calculated heat deposition due to RE is almost two times the melting threshold energy of Be but well below that of W.

Klein–Nishina steps in the energy spectrum of galactic cosmic-ray electrons

The full Klein–Nishina cross-section of the inverse Compton scattering interactions of electrons implies a significant reduction of the electron energy loss rate compared with the Thomson limit when the electron energy exceeds the critical Klein–Nishina energy EK=γKmec2=0.27m2ec4/(kBT), where T denotes the temperature of the photon graybody distribution. As a consequence, the total radiative energy loss rate of single electrons exhibits sudden drops in the overall -dependence when the electron energy reaches the critical Klein–Nishina energy. The strength of the drop is proportional to the energy density of the photon radiation field. The diffuse galactic optical photon fields from stars of spectral type B and G-K lead to critical Klein–Nishina energies of 40 and 161 GeV, respectively. Associated with the drop in the loss rate are sudden increases (Klein–Nishina steps) in the equilibrium spectrum of cosmic-ray electrons. Because the radiative loss rate of electrons is the main ingredient in any transport model of high-energy cosmic-ray electrons, Klein–Nishina steps will modify any calculated electron equilibrium spectrum irrespective of the electron sources and the spatial transport mode. To delineate most clearly the consequences of the Klein–Nishina decreases in the radiative loss rate, we chose as an illustrative example the simplest realistic model for cosmic-ray electron dynamics in the galaxy, consisting of the competition of radiative losses and secondary production by inelastic hadron–hadron collisions. We demonstrate that the spectral structure in the FERMI and HESS data is well described and even the excess measured by ATIC might be explained by Klein–Nishina steps.

Energy loss in degenerate quantum surfaces due to piezoelectric interaction at low lattice temperatures

The theory of energy loss rate of the non-equilibrium electrons in a degenerate quantum surface due to piezoelectric interaction has been developed under the condition of low temperature when the well-known approximations of the traditional theory are hardly valid. Compared to the traditional results for non-degenerate surface layers the theory here reveals a complex and altogether different dependence of the loss rate upon the carrier energy and the lattice temperature. The numerical results for GaAs and ZnO show how significantly does the degeneracy level or the true phonon distribution or the inelasticity of the interaction affect the loss characteristics at low temperatures. The possible applications of the theory have been discussed.

Energy relaxation in disordered two-dimensional electron gas with dynamic deformation potential

We investigated the electron energy loss rate through the emission of acoustic phonons in disordered two-dimensional electron gas in the deformation coupling limit. The energy loss rate has been determined by considering dynamic screening of electrons as various earlier studies have shown that static screening underestimates the energy loss rate. We find a change in the temperature exponent as well as the pre-factor β from the earlier reported approximate temperature power law dependence βT 6 obtained under static and dynamic strong screening and impurity limit. The difference in magnitude between the approximate analytic and numerical results mounts at decreasing sub-Kelvin temperatures with the pre-factor being enhanced about three times at a temperature 100 mK and mean free path length 8 nm , and the power dependency decreasing from the analytically derived T 6 to numerically observed T 5.2 .

Energy loss rate of hot electrons due to confined acoustic phonon modes in a freestanding quantum well structure

The energy loss rate of hot electrons in a semiconducting freestanding quantum well structure, in the presence of zero and finite quantizing magnetic field, is studied using electron temperature model. Electron interaction with confined acoustic phonons via deformation potential coupling described by elastic continuum model is considered. Numerical results are given for GaN, InAs, and InSb with zinc-blende structure and GaAs freestanding quantum well structures for the dependence of energy loss rate on electron temperature and the magnetic field. The results are compared to those obtained for bulk description of acoustic phonon modes.

Density-dependent energy relaxation of hot electrons in InN epilayers

This work investigates the dependence of the hot-electron energy relaxation in InN epilayers on electron density. From the high-energy tail of photoluminescence, the electron temperature of the hot electrons was determined. Acoustic phonons have an important role in the energy relaxation of the hot electrons. The density-dependent electron energy loss rate in InN can be explained by a combination of longitudinal optical and acoustic phonon emissions. A slowing of energy loss rate at high electron densities was observed and attributed to piezoelectric coupling to acoustic phonons.

TeV light curve of PSR B1259-63/SS2883

The inverse Compton (IC) scattering of ultrarelativistic electrons accelerated at the pulsar wind termination shock is generally believed to be responsible for TeV gamma-ray signal recently reported from the binary system PSR B1259-63/SS2883. While this process can explain the energy spectrum of the observed TeV emission, the gamma-ray fluxes detected by the Array of Imaging Atmospheric Cherenkov Telescopes (HESS) at different epochs do not agree with the published theoretical predictions of the TeV light curve. The main objective of this paper is to show that the HESS results can be explained, under certain reasonable assumptions concerning the cooling of relativistic electrons, by IC scenarios of gamma-ray production in PSR B1259-63. In this paper we study evolution of the energy spectra of relativistic electrons under different assumptions about the acceleration and energy-loss rates of electrons, and the impact of these processes on the light curve of IC gamma-rays. We demonstrate that the observed TeV light curve can be explained (i) by adiabatic losses which dominate over the entire trajectory of the pulsar with a significant increase towards the periastron or (ii) by the 'early' (sub-TeV) cut-offs in the energy spectra of electrons due to the enhanced rate of Compton losses close to the periastron. The first four data points obtained just after periastron comprise an exception - possibly due to interaction with the Be star disc, which introduces additional physics not included in the presented model. The calculated spectral and temporal characteristics of the TeV radiation provide conclusive tests to distinguish between these two working hypotheses. The Compton deceleration of the electron-positron pulsar wind contributes to the decrease of the non-thermal power released in the accelerated electrons after the wind termination, and thus to the reduction of the IC and synchrotron components of radiation close to the periastron. Although this effect alone cannot explain the observed TeV and X-ray light curves, the Comptonization of the cold ultrarelativistic wind leads to the formation of gamma-radiation with a specific line-type energy spectrum. While the HESS data already constrain the Lorentz factor of the wind, Γ ≤ 10 6 (for the most likely orbit inclination angle i = 35°, and assuming an isotropic pulsar wind), future observations of this object with GLAST should allow a deep probe of the wind Lorentz factor in the range between 10 4 and 10 6 .

Hot-electron energy-loss rate via PO-phonon scattering in CdZnSe/ZnSe quantum wire

The hot-electron energy-loss rate conditioned by confined and interface polar-optical phonons for a Cd 0.35Zn 0.65Se quantum wire embedded in ZnSe medium is investigated theoretically. It is shown that the inclusion of the polar-optical phonon confinement effects is crucial for accurate calculation of the energy-loss rate in quantum wire. Taking into account the nonequilibrium phonon populations, the hot-electron energy-loss rate is derived by a model, which includes the lowest subband occupation and the phonon confinement effects. The contribution of intersubband transitions to electron energy-loss rate as well as the relaxation time of the electron temperature with and without hot-phonon bottleneck effect is estimated.

Effect of Hyperbolic Band Structure on the Energy Loss Rate of Hot Electrons with Non-equilibrium Phonon Distribution in the Extreme Quantum Limit at Low Temperatures in n-Hg0.8Cd0.2Te

The energy loss rate of hot electrons with the non-equilibrium phonons in narrowgap semiconductors with hyperbolic band structures has been investigated in the extreme quantum limit condition in the low temperature region. The calculation is done for n-Hg0.8Cd0.2Te sample considering electron scattering by acoustic phonons via piezoelectric coupling to be the dominant loss mechanism. The value of the energy loss rate with hyperbolic band is compared with the results of parabolic and non-parabolic band structures and at the same time all the results are also compared with the experimentally observed data. It is found that with the inclusion of hyperbolic band structure, the value of energy loss rate is found to be close to the experimental values. The dependence of energy loss rate on magnetic field and lattice temperature has been studied. Using the experimental value of the energy loss rate, the phonon life time is evaluated. The value of the phonon life time is found to be of the order of the phonon boundary relaxation time indicating that phonon boundary scattering is the dominant phonon dissipation mechanism. The dependence of the phonon life time on magnetic field, and lattice temperature has also been studied. The phonon life time is also found to decrease with increase in electron temperature.

Energy balance and plasma potential in low-density hot-filament discharges

Electron energy balance is shown to play an important role in determining the plasma potential in low-density hot-filament discharges. The confined electrons that are lost to the walls are those with energy just above the plasma potential, thus the electron energy loss rate is the product of the electron loss rate and the height of the potential barrier. The sources of the electron energy are the energy at creation plus the energy gained from equilibration with energetic, unconfined electrons. An experiment in a soup-pot plasma device demonstrates that the plasma potential has values that satisfy the energy balance equation. The ion loss rate affects the electron loss rate through the quasi-neutrality condition, thus collisions of ions play a role in determining the plasma potential by reducing the particle loss rates

Absolute cross sections of electron attachment to molecular clusters: Part I. Formation of (CO2)−N

A procedure of determining absolute cross section σ− of electron attachment to (CO2)N clusters at pair collisions in crossed beams is suggested. The cross section is measured as a function of energy (E = 0.1–50 eV) and of cluster mean size N in a beam \((\bar N = 2 - 4000 molecules)\). It is found that, even at \(\bar N > 200\) and E ≤ 3 eV, σ− is equal to, or larger than, 7 × 10−13 cm2, i.e., by more than one order of magnitude exceeds the maximal cross section of CO2 ionization by electron impact. The dependences σ−\((\bar N,E)\) have two wide continua at E ≤ 5.2 eV and E ≥ 6.9 eV, which correlate well with known functions of CO2 electron-impact-induced excitation. These continua are attributed largely to formation of (CO2)N− ions during electron thermalization and solvation in the clusters. At E → 0, the polarization capture of an incident electron by the cluster leads to a sharp increase in cross section σ−(E). From the dependences σ−\((\bar N,E)\) measured, the thermalization and sovation probabilities for electrons with E ≤ 0.8 eV and the rate of electron energy loss in the cluster are found.

Ultrafast dynamics in ZnO thin films irradiated by femtosecond lasers

The damage morphologies, threshold fluences in ZnO films were studied with femtosecond laser pulses. Time-resolved reflectivity and transmissivity have been measured by the pump-probe technique at different pump fluences and wavelengths. The results indicate that two-phase transition is the dominant damage mechanism, which is similar to that in narrow band gap semiconductors. The estimated energy loss rate of conduction electrons is 1.5 eV/ps.

Size dependence investigations of hot electron cooling dynamics in metal/adsorbates nanoparticles

The size dependence of electron–phonon coupling rate has been investigated by femtosecond transient absorption spectroscopy for gold nanoparticles (NPs) wrapped in a shell of sulfate with diameter varying from 1.7 to 9.2 nm. Broad-band spectroscopy gives an overview of the complex dynamics of nonequilibrium electrons and permits the choice of an appropriate probe wavelength for studying the electron–phonon coupling dynamics. Ultrafast experiments were performed in the weak perturbation regime (less than one photon in average per nanoparticle), which allows the direct extraction of the hot electron cooling rates in order to compare different NPs sizes under the same conditions. Spectroscopic data reveals a decrease of hot electron energy loss rates with metal/adsorbates nanosystem sizes. Electron–phonon coupling time constants obtained for 9.2 nm NPs are similar to gold bulk materials (∼1 ps) whereas an increase of hot electron cooling time up to 1.9 ps is observed for sizes of 1.7 nm. This is rationalized by the domination of surface effects over size (bulk) effects. The slow hot electron cooling is attributed to the adsorbates-induced long-lived nonthermal regime, which significantly reduces the electron–phonon coupling strength (average rate of phonon emission).

Sci‐AM1 Sat ‐ 07: Monte Carlo simulation of large electron fields at extended distances: Total skin electron treatment optimization

Development of a Total Skin Electron Therapy (TSET) procedure and acquiring all of the necessary parameters by measurement is difficult tedious and time consuming. To produce large fields for TSET, an external scatterer is usually attached to treatment head. Selection of scatterer's properties would be influenced by the requirements of angular spread, minimum energy loss and bremsstrahlung contamination. The goal of this research was to select properties of an optimal scatterer (z and thickness) in order to achieve an optimum single large electron field at extended SSD, as a first step in development of a TSET optimization process. A 6 MeV electron beam from a Philips SL‐20 unit, scattered by a control copper scatterer, was simulated using EGS4 code (12 hours CPU time for transport of particles from scatterer into a cylindrical phantom, r=50 cm, at 300 SSD for 10E5 initial electrons). For optimization process, a group of 10 materials were selected as candidate materials, using the radiation length concept. Comparison of field characteristics calculated for candidate materials and control scatterer resulted in selection of a 0.059 mm lead foil as the optimal scatterer: a uniformity index of 93%, a d80% of 1.8–2.1 cm and a 30% increase in output compared to control scatterer. This simulation showed that in large scattered electron fields, energy loss and dose rate are controlled by scatterer's atomic number and thickness respectively. For the same energy loss, however, field uniformity and bremsstrahlung contamination are not influenced noticeably by the properties of the scatterer.

Hot electron cooling rates via the emission of LO-phonons in InN

We report on the energy and momentum relaxation of hot electrons in n-type epilayer InN grown on sapphire substrate using molecular beam epitaxy (MBE). Hall and pulsed I–V measurements are carried out in the temperature range between 77 and 300 K. Drift velocity versus electric field characteristics show that, at 77 K, the drift velocity saturates just above vd ∼ 8 × 106 cm s−1 at electric fields in excess of E ∼ 12 kV cm−1. The mobility comparison method together with the power balance equation is used to obtain the electron temperature as a function of applied electric field and the electron energy loss rate as a function of electron temperature. Our results show conclusively that the effective energy relaxation time constant in InN is 200 fs. This is about six times slower than the theoretical value for the e–LO phonon scattering time of 31 fs. The effects of non-equilibrium phonon generation on slowing down of the energy relaxation and increasing the momentum relaxation processes are discussed using a theoretical model first developed for GaAs and then adapted to the III-N material systems.

Hot electron energy loss rates in GaAs/GaAlAs multiple quantum wells: effects of finite barrier height and well width

Hot electron energy loss rate has been studied in GaAs/GaAlAs multiple quantum wells in the electron temperature range 3.3– 15.0 K where electron–acoustic phonon interaction dominates. Calculations are presented for power loss by incorporating the finite barrier height and screened interaction. An excellent agreement between our calculations and the experimental data of Celik et al. [Semicond. Sci. Technol. 17 (2002) 18] and Cankurtaran et al. [Phys. Stat. Sol. (b) 229 (2002) 1191] for quantum wells with widths in the range 51– 140 A ̊ and base lattice temperature in the range 1.7– 5.9 K is obtained. The analysis of the data, apart from demonstrating the importance of finite barrier height, has also brought out the relative importance of the contribution to power loss due to deformation potential and piezoelectric scatterings. The values deduced for the deformation potential constant E d lie in the range 7.5– 12.5 eV , in contrast to the large scatter in its values, usually observed in the literature. The need for more experimental data, for well widths less than 40 A ̊ and higher than 140 A ̊ , is pointed out for a better understanding of well width dependence of electron energy loss rate.

Energy and momentum relaxation of hot electrons inGaN/AlGaN

We report the experimental studies of hot-electron energy andmomentum relaxation in the steady state in GaN/AlGaN HEMTstructures with a high two-dimensional electron density of n = 1.5×1013 cm-2. From the LO-phonon-scattering-limitedcomponent of the mobility we obtain for the LO phonon the energy ofℏω~90 meV and the momentum relaxation time of τm~4 fs. Drift velocity versus electric field characteristics obtained from the pulsed I-V measurementsshow that, at TL = 77 K, the drift velocity saturates atvd = 1.0×107 cm s-1 at electric fields inexcess of E~7.5 kV cm-1, and at TL = 300 K it saturates at vd~5×106 cm s-1, at anelectric field of around E~10 kV cm-1. Electrontemperature as a function of applied electric field is obtainedby comparing the measured electric field dependence of the mobility µE at a fixed lattice temperature, with thelattice temperature dependence of the mobility at a fixed lowelectric field. The electron energy loss rate is thendetermined from the electron temperature dependence of thepower loss using the power balance equations. The effect of hot-phonon production on the observed momentum and energyrelaxation of hot electrons is discussed within the frameworkof a theoretical model, which was originally developed forIII-V material systems and has been adapted for a two-dimensionalelectron gas in GaN, and in which phonon drift is neglected.

Interactions of excited activators in rare earth and transition metal doped phosphors and their role in low energy cathodoluminescence

We have measured the time decay of spectrally resolved, pulsed cathodoluminescence (CL) and photoluminescence (PL) in several phosphors activated by rare earth and transition metal impurities, including Y2O3:Eu, Y2SiO5:Tb, and Zn2SiO4:Mn. Activator concentrations ranged from ∼0.003% to 10%. The CL decay curves are almost always nonlinear on a log (CL)-linear(time) plot—i.e., they deviate from first-order decay kinetics. These deviations are always more pronounced at short times and larger activator concentrations and are largest at low beam energies where the decay rates are noticeably faster. PL decay is always slower than that seen for CL, but these differences disappear after most of the excited species have decayed. PL decays display higher order decay kinetics and resemble CL decay profiles when the laser excitation intensity is increased. We have also measured the dependence of steady state CL efficiency on beam energy and find that larger activator concentrations accelerate the drop in CL efficiency seen at low beam energies. These effects are largest for the activators which interact more strongly with the host lattice. While activator–activator interactions are known to limit PL and CL efficiency in most phosphors, the present data suggest that a more insidious version of this mechanism is partly responsible for poor CL efficiency at low beam energies. This “enhanced” concentration quenching is due to the interaction of nearby excited activators. These interactions can lead to nonradiative activator decay, hence lower steady state CL efficiency. We suggest that this excited state “clustering” arises from the large energy loss rate of low energy primary electrons.

Hot electron capture and power loss in 2D GaN

We report the experimental studies of hot electron energy relaxation and capture in a low electron mobility GaN/GaAlN HEMT structure grown with MBE on sapphire substrate. Pulsed I− V characteristics indicate that at elevated electric fields, current pulse decays exponentially with time. The threshold electric field ( F th) above which the current pulse exhibits the decay, increases with temperature ( E th∼200 V/cm at T=77 K, E th∼350 V/cm at T=300 K). The magnitude of the decay decreases with increasing temperature and increases with applied field. Time constant associated with this decay decreases rapidly with increasing applied field. These observations indicate the presence of hot electron capture over potential barriers. Electron temperature as a function of applied electric field is obtained by comparing the measured electric field dependence of the mobility μ E at a low lattice temperature, with the lattice temperature dependence of the mobility at a low electric field. Electron energy loss rate is then determined from the electron temperature dependence of the power loss using the power balance equations. Our results show that the experimental power loss has an exponential factor, ℏ ω/ k which is much smaller than the value expected from LO phonon emission. Furthermore, the magnitude of the experimental power loss is also much lower than the theoretical value. This observation is explained in terms of capture of hot electrons.