Electron Beam Penetration Research Articles

Analysis of electrical and thermal responses of n-doped silicon to an impinging electron beam and joule heating

A detailed numerical simulation for electron-beam heating of n-doped silicon is presented. Electron-beam penetration is modeled using electron-beam transport equation (EBTE). The EBTE is solved by using a Monte Carlo (MC) method to determine the electron deposition distributions, including electron density deposition and optical phonon generation. Electron and phonon temperatures of the film are then determined using electron–phonon hydrodynamic equations (EPHDEs) coupled with the deposition distributions obtained from the MC simulation. The combined EBTE and EPHDEs results indicate that an electron beam creates a depletion region near the surface of incidence and causes non-equilibrium between electron and phonon temperatures.

Effective Green Semiconductor Lasers with Multiple CdSe/ZnSe QD Active Region for Electron Beam Pumping

Electron beam pumped (EBP) semiconductor lasers using ZnSe-based structures as active elements look very promising for the development green lasers because of no requirements of both p-n junction and low-resistivity contacts. Another advantage of EBP lasers is that the maximum conversion efficiency of electron beam energy to light is of ~30%. The possibility of using ZnSe-based quantum-size heterostructures for EBP green lasers demonstrated during last decade. Thus, room-temperature lasing at Ee<5keV and extremely low threshold current density jth=0.4-0.5 A/cm 2 (at Ee=8-10keV) have been demonstrated in laser heterostructures with short-period superlattice (SL) waveguide and single CdSe/ZnSe quantum dot (QD) active region [1]. The structure with three ZnSe/CdSe QD sheets arranged within 0.4um-thick extended SL waveguide has demonstrated laser generation with efficiency (per one facet) as high as ~4% [2]. Here we present the latest results on fabrication and study of ZnSe-based EBP laser heterostructure with multiple CdSe/ZnSe QD sheets. Laser heterostructure has been grown by molecular beam epitaxy pseudomorphically to GaAs (001) substrate and consists of 1.6μmthick ZnMgSSe cladding, active region with ten CdSe/ZnSe QD sheets equally spaced within a 0.6um-thick ZnSe/ZnSSe SL waveguide and cap 20-nm ZnMgSSe layer. The nominal thickness of CdSe QD layers was chosen to ~2.5 monolayers. All measurements were carried out at room temperature in transverse excitation geometry in pulse mode at electron beam energy Ee=10-30 keV. The lasing has been observed at the wavelength of ~542nm (T=300K) with FWHM of the emission line of ~2-3 nm. The maximum achieved values of pulse output power ~12W per facet has been registered at Ee=23keV. Maximum quantum efficiency of ~8.5% per facet has been obtained at the same electron beam energy and electron beam current Ie~5.7mA. The main reason of increase of laser efficiency seems to be more effective utilization of pumping energy by means of adjustment of both structure geometric parameters and electron beam penetration depth into the sample. Further increase of light output power is likely can be achieved by increasing Ie keeping constant electron beam energy Ee. The question on optimum number of active CdSe QD layers will be discussed. This work was partly supported by FASI Contract and ISTC Project No. 3754.

Energy dependence of electron beam penetration, area throughput rates and electron energy utilization in the low-energy region

Industrial electron accelerators used in radiation processing facilities are rated by their maximum electron energies, which are usually classified in three energy intervals. Low energies are from 75keV to 300keV, medium energies are from 300keV to 5MeV, and high energies are above 5MeV. The incident electron energy is determined by the depth and density of the irradiated material, while the electron beam current is determined by the area throughput rate and the absorbed dose. The ITS3 Monte Carlo code has been used to calculate the energy deposition per electron versus depth in polyethylene plates with incident electron energies from 75keV to 600keV. This quantity is proportional to the absorbed dose delivered during a continuous irradiation process. The energy depositions in the thin titanium beam window and the air space between this window and the irradiated material are included in these calculations. The beam window and air space have significant effects on the depth–dose distribution within the processed material in the low-energy region. The first group of calculations assumes a window thickness of 40μm (micro-meter) and an air space of 15cm with energies extending from 200keV to 600keV. The second group assumes a window thickness of 15μm and an air space of 5.0cm with energies extending from 100keV to 300keV. The third group assumes a window thickness of 6.0μm and an air space of 2.5cm with energies extending from 75keV to 250keV. The data presented include the energy deposition at the surface of the polyethylene plate, the depth where the exit dose equals half of the entrance dose, and the total energy absorbed in the polyethylene plate as a percentage of the incident electron beam energy. Additional calculations show the effects of changing the thickness of the air space in the limited low-energy region from 75keV to 125keV.

Cathodoluminescence of laser AIIBVI heterostructures

A Cathodoluminescence (CL) method has been used for studies of ZnSe-based heterostructures with a CdSe fractional-monolayer recombination region. Electron beams with energies of 1–25 keV provide the ability to analyze CL bands associated with different layers of the heterostructure and revealed on the CL spectrum at different electron-beam penetration depth. Comparative analysis of the CL bands both from the recombination region and the upper layers has been used to characterize the transport properties of the structure. The correlation between the density and size distribution of quantum dots and full width at half maximum and spectral position of the CL bands has been studied in detail.

Runaway electron production during intense electron beam penetration in dense plasma

Relativistic electrons are efficiently generated when multiterawatt lasers focused to ultrahigh intensities ≳1019W∕cm2 illuminate the surface of dense plasma targets. A theoretical study finds that during typical picosecond pulse widths, significant amounts of Dreicer produced runaway electrons can build up due to the high axial electric field driving the neutralizing return current. An important consequence is that there will be a conversion of plasma current to runaway electron current, which is maximized at some optimum value of the beam-to-plasma density ratio Nb=nb∕ne, depending on the plasma collisionality. At collisionalities representative of solid target experiments, complete conversion to runaway electrons can only take place over a certain range of Nb values. At higher collisionalities and pulse widths, applicable to the fast ignition concept for inertial confinement fusion, it was found that conversion to runaways has a peak at ∼90% around Nb∼0.06. Significant lessening of target material heating by Joule current dissipation is also possible, since part of the beam energy loss is transferred through the electric field directly to the formation of energetic runaways. Implications for beam transport inhibition by the electric field are also discussed.

Properties and structure of vapor-deposited polyimide upon electron-beam irradiation

Vapor-deposited polyimide capsules from pyromellitic dianhydride and 4,4′-oxydianiline were irradiated with an electron beam that mimicked the β-radiation emitted by tritium, a fuel that the capsules are to contain during the inertial confinement fusion process. The mechanical properties and gas permeability of the irradiated capsules were measured to examine their radiation resistance. Upon electron-beam irradiation at an energy of 8keV and a dose of 120MGy, the capsules showed 15% and 56% decrease in tensile strength and elongation at break, respectively, without significant change in gas permeability and Young’s modulus. Analyses using x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy indicated that the chain cleavage and carbonization occurred but were confined in a thin layer at the top surface of the capsules. The shallow penetration of the low-energy electron beam used, as well as the existence of cross-linking in the vapor-deposited polyimide, may have led to the smaller magnitude of property degradation in the capsules compared to that reported for solution-cast polyimide.

Luminescence of bound excitons in epitaxial ZnO thin films grown by plasma-assisted molecular beam epitaxy

Luminescence properties of ZnO films, which have been epitaxially grown on c-sapphire (0001) substrates by plasma-assisted molecular beam epitaxy, are investigated by means of different excitation sources and their measurement conditions. With the increase of measurement temperature, photoluminescence spectra clearly present, the appearance of different bound-exciton peaks (I10 line) with an abrupt increase of emission intensity at the measurement temperature of 30-50K. Hypothetical explanations on the basis of thermalization effects, vibronic/rotational resonance states, and the involvement of the B-valence level in emission are given. In cathodoluminescence (CL), the deep level emission intensity was enlarged with the electron beam penetration depth due to the higher defect density near the interface between ZnO and the sapphire. From the CL image of the ZnO film, the dislocation density was estimated as 6×108-3×109∕cm2. The lasing phenomenon was observed at the threshold power density of 1.3MW∕cm2 at 300K.

Monte Carlo simulation of electron depth distribution and backscattering for carbon films deposited on aluminium as a function of incidence angle and primary energy

Carbon films are deposited on various substrates (polymers, polyester fabrics, polyester yarns, metal alloys) both for experimental and technological motivations (medical devices, biocompatible coatings, food package and so on). Computational studies of the penetration of electron beams in supported thin film of carbon are very useful in order to compare the simulated results with analytical techniques data (obtained by scanning electron microscopy and/or Auger electron spectroscopy) and investigate the film characteristics.In the present paper, the few keV electron depth distribution and backscattering coefficient for the special case of film of carbon deposited on aluminium are investigated, by a Monte Carlo simulation, as a function of the incidence angle and primary electron energy. The simulated results can be used as a way to evaluate the carbon film thickness by a set of measurements of the backscattering coefficient.

Static and dynamic experiments in cryo-electron microscopy: comparative observations using high-vacuum, low-voltage and low-vacuum SEM.

We carried out a unique comparative study between three modes of cryo-scanning electron imaging: high-vacuum, low-voltage and low-vacuum, using ice cream as a model system. Specimens were investigated both with and without a conductive coating (Au/Pd) and at temperatures for which ice either remains fully frozen (< -110 degrees C) or undergoes sublimation (-110 to -90 degrees C). At high magnification, high-vacuum imaging of coated specimens gave the best results for 'static' specimens (i.e. containing fully frozen ice). Low voltages, such as 1 kV, could be used for imaging uncoated specimens at high vacuum, although slight 'classical' charging artefacts remained an issue, and the reduced electron beam penetration tended to decrease the definition between different microstructural features. However, this mode was useful for observing in situ sublimation from uncoated specimens. Low-vacuum mode, involving small partial pressures of nitrogen gas, was particularly suited to in situ sublimation work: when sublimation was carried out in low vacuum in the absence of an anti-contaminator plate, sublimation rates were significantly reduced. This is attributed to a small partial pressure of sublimated water vapour remaining near the specimen surface, enhancing thermodynamic stability.

First detection of return currents in solar flares by spectropolarimetry with THEMIS

Using THEMIS French-Italien telescope with the MTR mode, the Hydrogen Hα and Hβ lines have been observed to be linearly polarized up to a few percent by impact during the impulsive phase of two solar flares associated with high-frequency radio pulses. Two privileged directions of linear polarization are present, respectively radial (in the disk center to flare direction) and tangential (perpendicular to the radial direction). This 90 degree change in the linear polarization direction is interpreted as due to the chromospheric return current generated by the penetration of a non-thermal electron beam into the chromosphere.

Long time scale electron beam penetration of blowoff plasma in multipulse x-ray radiography

In the context of multipulse x-ray radiography, the interaction of a relativistic electron beam pulse with the low density halo surrounding the blowoff plasma from a target due to previous pulses is calculated in two dimensions on time scales comparable to the pulse duration. In the halo, the density of the beam and plasma system is low enough that the interaction is collisionless. At the outer edge of the halo, the density is lower than the beam density. The electric field due to the impinging beam rapidly pushes the background electrons radially outward and the beam is not disrupted by the two stream instability because of the absence of the background electrons. As the beam propagates closer to the target where the background density is much higher than the beam density, some electrons are also pushed radially outward, but most remain. The beam propagates through about the first 5 cm of the background before a part of it is disrupted at early times by the two stream instability. At later times, the instability is extinguished when a channel deficient in background electrons forms through which the beam then propagates without further significant disruption. The beam spot size is restored and the remains comparable to the initial size.

The use of trans-vinylene formation in quantifying the spatial distribution of electron beam penetration in polyethylene. Single-sided, double-sided and shielded irradiation

The use of the quantification of trans-vinylene unsaturations in irradiated ultrahigh molecular weight polyethylene (UHMWPE) in determining the spatial distribution of electron beam penetration was investigated. UHMWPE was irradiated with a 10 MeV linear electron beam accelerator to 100 kGy at both room temperature and 125°C in air. The irradiation was carried out in shielded and unshielded single-sided and unshielded double-sided irradiation modes. Dose–depth profiles were measured by quantifying the yields of trans-vinylene unsaturations as a function of depth away from the e-beam incidence surfaces using infrared spectroscopy. The extent of beam penetration increased with increasing irradiation temperature as was manifested by the increase in the iso-dose penetration with the unshielded single-sided irradiation and increased dose overlap with the unshielded double-sided irradiation. The optimum thickness with maximum uniformity in dose–depth distribution for double-sided irradiation was 85 and 90 mm for 25°C and 125°C irradiation temperatures, respectively.

Two-dimensional simulation and modeling in scanning electron microscope imaging and metrology research.

Traditional Monte Carlo modeling of the electron beam-specimen interactions in a scanning electron microscope (SEM) produces information about electron beam penetration and output signal generation at either a single beam-landing location, or multiple landing positions. If the multiple landings lie on a line, the results can be graphed in a line scan-like format. Monte Carlo results formatted as line scans have proven useful in providing one-dimensional information about the sample (e.g., linewidth). When used this way, this process is called forward line scan modeling. In the present work, the concept of image simulation (or the first step in the inverse modeling of images) is introduced where the forward-modeled line scan data are carried one step further to construct theoretical two-dimensional (2-D) micrographs (i.e., theoretical SEM images) for comparison with similar experimentally obtained micrographs. This provides an ability to mimic and closely match theory and experiment using SEM images. Calculated and/or measured libraries of simulated images can be developed with this technique. The library concept will prove to be very useful in the determination of dimensional and other properties of simple structures, such as integrated circuit parts, where the shape of the features is preferably measured from a single top-down image or a line scan. This paper presents one approach to the generation of 2-D simulated images and presents some suggestions as to their application to critical dimension metrology.

Evaluation of Several Dosimeters for Identification of Irradiated Foods Using a 5 MeV Electron Beam.

Electron-beam irradiation facilities are commercially utilized for food irradiation. This study investigates whether dosimeters used to detect gamma-rays can also be used to accurately detect electron-beam irradiation. The irradiation souses for gamma-ray at the Takasaki Institute were used throughout this study. Four kinds of dosimeters, RadiaChromic (RC), GammaChromic (GC), Amber3042 (AM), Radix (RX), and alanine (AL) were irradiated at doses from 0.1 to 60 kGy using a 5 MeV electron-beam (EB) at the Japan Irradiation Service Co. Ltd. (Japan). AL was submitted to the National Physics Laboratory in order to evaluate the doses that AL were absorbed. At low-level dose range, all dosimeters indicated correct values. This means those dosimeters were usable to accurately detect electron beam irradiation. But at higher range they all required correction due to either temperature effects or varying sensitivity to electron-beam exposure. Dose-depth profiles were obtained using ham and cheese samples, in order to check penetration of EB by the calibrated machine.

Photo- and cathodoluminescence of ZnSSe quantum well heterostructures grown by MOVPE

Lateral- and depth-resolved cathodoluminescence as well as photoluminescence of MOVPE-grown single- and multi quantum well ZnSSe-based heterostructures in the temperature range of 14–300 K were investigated. The intensity ratios between different emission peaks and their dependencies on the electron penetration depth together with the temperature dependence of photoluminescence spectra permit concluding that the transport of non-equilibrium carriers or excitons in the structure is hindered by potential fluctuations created by microvariations of the sulphur content. The deep centres are localized mostly in the near-substrate region which is indicated by an increase of the self-activated emission at 2.2 eV with increasing electron beam penetration depth. Further improvement of layer homogeneity as well as a sufficient distance between the quantum well and the substrate together with minimization of the interlayer strain is necessary to improve the radiative properties of ZnSe-based heterostructures.

Studies of electron-beam penetration and free-carrier generation in diamond films

Experimental observations of the energy-dependent electron-beam penetration in type II-A natural diamond are reported. The experimental data are compared with results obtained from numerical Monte Carlo simulations, and the results are in very good agreement. The results also reveal that a threshold energy of about 125 keV is necessary for complete penetration for a 35 μm sample. It is found that over the 30–180 keV range, the energy dependence of the penetration depth and total path length exhibits a power-law relation. Monte Carlo simulations have also been performed to investigate the excess carrier-generation profiles within diamond for a set of incident e-beam energy distributions. The simulation results demonstrate the feasibility of tailoring the internal source function, and hence influencing the diffusion currents, the internal electric fields, and charge injection through the contacts.

X-ray microanalysis of frozen-hydrated, freeze-dried, and critical point dried leaf specimens: determination of soluble and insoluble chemical elements at Erysiphe graminis epidermal cell papilla sites in barley isolines containing Ml-o and ml-o alleles

Three specimen preparation procedures were used in conjunction with scanning electron microscopy and energy dispersive X-ray microanalysis to determine, by comparison among preparation methods, both soluble and insoluble elements at Erysiphe graminis – barley leaf epidermal cell encounter site areas where attempted fungal penetration by appressoria failed. Near isogenic lines (RISO 5678-R and RISO 5678-S) of barley differing by mutation at the Ml-o locus were used. The recessive ml-o allele conditions barley epidermal cells to respond with papilla-associated resistance to E. graminis, while the dominant Ml-o allele allows for successful penetration of the majority of E. graminis germlings. Frozen-hydrated and freeze-dried specimens maintained soluble and insoluble elements, while specimens fixed by formalin – acetic acid – alcohol and critical point dried lost soluble elements. The effects of specimen preparation on electron-beam penetration and depth of X-ray excitation were calculated and are illustrated. Mean elemental intensity values were corrected for X-ray absorption by nickel coating of specimens (used for electrical conductivity) and for X-ray detector efficiency. The elements Cl, K, Mn, Ca, and Mg were highly soluble both at recently deposited (16 h) and at matured (24 h) papilla deposition sites. Elemental Si levels were elevated and in a partially soluble state in recently deposited papilla sites (16 h), but Si became bound or insoluble in matured (24 h) papilla sites. Elemental P and S are insoluble. The physiological role of each element is briefly discussed relative to its known function in healthy and diseased plants, with emphasis on E. graminis – barley epidermal cell encounter site penetration failure. Key words: Erysiphe graminis, Hordeum vulgare, X-ray microanalysis, scanning electron microscopy.

Electron scattering in microstructure processes

The study of electron beam penetration in solids is fundamental to understanding the basic processes in a variety of applications, such as microscopy, electron probe microanalysis and microlithography. The physics of electron scattering in solids has been discussed in sect. 2, in order to obtain a useful theoretical description of the electron transport problem. Due to the complexity of the electron scattering process strong simplifications have been proposed. The single-scattering approach of Everhart and the diffusion sphere approach of Archard, described in sect. 3, have the merit of modelling, in a very simple way, two extreme cases, large-angle single scattering and diffusion: the real situation can be considered as being intermediate between the two. Presently, the most basic approach to the study the electron penetration in solids is the Monte Carlo method. MC calculations consider the behaviour of individual electrons. The trajectory of an electron through the solid is calculated step by step, assuming it is scattered through randomly determined angles, on the basis of the equations used to approximate the physical processes. The great success of Monte Carlo calculations relies upon three factors: a) its adaptability to systems having a variety of geometries, with reference to size, shape or internal structure; b) the number of different output data available from MC calculations, in the form of plots of electron trajectories, energy and angular distributions of forward and backward scattered electrons; c) the physical insight into the problem, allowed by the capability of treating the process directly in terms of its basic mechanisms. The accuracy of such calculations depends on the accuracy of the modelling of the scattering, MC results being, in any case, more accurate than analytical treatments.

Comparison of Low-and High-Voltage SEM Images of Beryllium

Beryllium has a low secondary electron (SE) coefficient that produces a low signal-to-noise ratio, and in the past, this deficiency caused many workers to routinely coat beryllium with a high-Z material such as Au/Pd and examine the specimens at high voltage (HV). In principle, beryllium should be an ideal candidate for improved imaging with low voltage (LV), because its low atomic number permits excessive electron-beam penetration during HVSEM imaging — the use of LVSEM to analyze beryllium films was demonstrated previously. Unfortunately, low SE emission can seriously compromise LVSEM images from uncoated beryllium specimens, even with the improved LVSEM imaging capability of a SEM equipped with a field-emission gun (FESEM). This work demonstrates that some beryllium structures yield improved results with LVSEM imaging in a FESEM, particularly at low magnifications, while structures that require high magnification often yield better results with HVSEM.Specimens selected for this study include craters formed by a Cs+ ion beam in specimens that had been analyzed in an ion microanalyzer by secondary-ion mass spectroscopy (SIMS), machined surfaces, and fracture surfaces. SE images were obtained for this study with a Hitachi S-800 FESEM equipped with a cryogenic vacuum pumping system.Surface structure in a sputtered SIMS crater is shown at 2.0 kV in Fig. 1a and at 20 kV in Fig. 1b. This specimen was from commercial grade beryllium sheet. Prominent microtwins are visible in several grains in Fig. 1, and because of the excessive beam penetration at HV, the microtwins are better defined at LV in Fig. 1a than they are at HV in Fig. 1b. In addition, a fine faceted microstructure was formed by the ion beam that could be observed at intermediate magnifications, and it also was better defined at LV than at HV. The dark contamination film that is visible around some of the particles in Fig. 1a also is significant; this film was not apparent at voltages of 5 kV and higher, as demonstrated in Fig. 1b. SIMS ion images revealed that this material contained particles of several different compositions including carbides; since beryllium carbide hydrolyzes when exposed to moist air, the films are suspected to have formed by the decomposition of carbides when the specimen was removed from the ion microanalyzer after the SIMS analysis. This example demonstrates the excellent sensitivity of LVSEM to detect thin films, and it also suggests that contamination and the thin oxide film that forms on beryllium may impair SE imaging.

Monte Carlo simulation of kilovolt electron transport in solids

A Monte Carlo procedure to simulate the penetration and energy loss of low-energy electron beams through solids is presented. Elastic collisions are described by using the method of partial waves for the screened Coulomb field of the nucleus. The atomic charge density is approximated by an analytical expression with parameters determined from the Dirac–Hartree–Fock–Slater self-consistent density obtained under Wigner–Seitz boundary conditions in order to account for solid-state effects; exchange effects are also accounted for by an energy-dependent local correction. Elastic differential cross sections are then easily computed by combining the WKB and Born approximations to evaluate the phase shifts. Inelastic collisions are treated on the basis of a generalized oscillator strength model which gives inelastic mean free paths and stopping powers in good agreement with experimental data. This scattering model is accurate in the energy range from a few hundred eV up to about 50 keV. The reliability of the simulation method is analyzed by comparing simulation results and experimental data from backscattering and transmission measurements.