Incident Electron Beam Research Articles

An effective method to calculate RHEED rocking curves from nanoheteroepitaxial systems

An effective method to calculate RHEED rocking curves from nanoheteroepitaxial systems

Depth-dependent EBIC microscopy of radial-junction Si micropillar arrays

Recent advances in fabrication have enabled radial-junction architectures for cost-effective and high-performance optoelectronic devices. Unlike a planar PN junction, a radial-junction geometry maximizes the optical interaction in the three-dimensional (3D) structures, while effectively extracting the generated carriers via the conformal PN junction. In this paper, we report characterizations of radial PN junctions that consist of p-type Si micropillars created by deep reactive-ion etching (DRIE) and an n-type layer formed by phosphorus gas diffusion. We use electron-beam induced current (EBIC) microscopy to access the 3D junction profile from the sidewall of the pillars. Our EBIC images reveal uniform PN junctions conformally constructed on the 3D pillar array. Based on Monte-Carlo simulations and EBIC modeling, we estimate local carrier separation/collection efficiency that reflects the quality of the PN junction. We find the EBIC efficiency of the pillar array increases with the incident electron beam energy, consistent with the EBIC behaviors observed in a high-quality planar PN junction. The magnitude of the EBIC efficiency of our pillar array is about 70% at 10 kV, slightly lower than that of the planar device (≈ 81%). We suggest that this reduction could be attributed to the unpassivated pillar surface and the unintended recombination centers in the pillar cores introduced during the DRIE processes. Our results support that the depth-dependent EBIC approach is ideally suitable for evaluating PN junctions formed on micro/nanostructured semiconductors with various geometry.

Visualization of clay minerals at the atomic scale

AbstractThis review demonstrates that high-resolution transmission electron microscopy (HRTEM) imaging of clay minerals or phyllosilicates with an incident electron beam along the major zone axes parallel to the constituting layers, in which the contrast corresponds to individual cation columns in the images obtained, is indispensable for elucidating the enigmatic structures of these minerals. Several kinds of variables for layer stacking, including polytypes, stacking disorder and the interstratification of various kinds of unit layers or interlayer materials, are common in phyllosilicates. Local and rigorous determination of such variables is possible only with HRTEM, although examination as to whether the results obtained by the HRTEM images from limited areas represent the whole specimen should be made using other techniques, such as X-ray diffraction. Analysis of these stacking features in clay minerals provides valuable insights into their origin and/or formation processes. Recent state-of-the-art techniques in electron microscopy, including incoherent imaging, superior resolutions of ~0.1 nm and low-dose imaging using new recording media, will also contribute significantly to our understanding of the true structures of clay minerals.

High precision 5 MeV Mott polarimeter

We report on the design and performance of a Mott polarimeter optimized for a nominal 5-MeV electron beam from the Continuous Electron Beam Accelerator Facility (CEBAF) injector. The rf time structure of this beam allows the use of time of flight in the scattered electron detection, making it possible to cleanly isolate those detected electrons that originate from the scattering foil, and resulting in measured scattering asymmetries which are exceptionally stable over a broad range of beam conditions, beam currents, and foil thicknesses. In two separate series of measurements from two different photocathode electron sources, we have measured the Mott scattering asymmetries produced by an approximately 86% transversely polarized electron beam incident on ten gold foils with nominal thicknesses between 50 and 1000 nm. The statistical uncertainty of the measured asymmetry from each foil is below 0.25%. Within this statistical precision, the measured asymmetry was unaffected by \ifmmode\pm\else\textpm\fi{}1-mm shifts in the beam position on the target foil, and by beam current changes and dead-time effects over a wide range of beam currents. The overall uncertainty of our beam polarization measurement, arising from the uncertainty in the value of the scattering asymmetry at zero foil thickness as determined from our fits to the measured asymmetries versus scattering foil thicknesses, the estimated systematic effects, and the (dominant) uncertainty from the calculation of the theoretical Sherman function, is 0.61%. A simulation of the polarimeter using geant4 has confirmed that double scattering in the target foil is the sole source of the dependence of the measured asymmetry on foil thickness, and gives a result for the asymmetry versus foil thickness in good agreement with both our measurements and a simple calculation. Future measurements at different beam energies and with target foils of different atomic numbers will seek to bound uncertainties from small effects such as radiative corrections to the calculation of the polarimeter analyzing power. A simultaneous high-precision measurement of the beam polarization with a different polarimeter, AESOP (Accurate Electron Spin Optical Polarimeter), under development at the University of Nebraska, clearly possible at the CEBAF accelerator, will allow a high-precision comparison of our measured asymmetries with theoretical calculations of the Mott analyzing power. Finally, the improved precision of the current Mott polarimeter along with similar improvements to other Jefferson Lab polarimeters warrants another precision comparison of all of these polarimeters when measuring a beam of the same polarization.

In situ monitoring of exopolymer-dependent Mn mineralization on bacterial surfaces.

Bacterial biomineralization is a widespread process that affects cycling of metals in the environment. Functionalized bacterial cell surfaces and exopolymers are thought to initiate mineral formation, however, direct evidences are hampered by technical challenges. Here, we present a breakthrough in the use of liquid-cell scanning transmission electron microscopy to observe mineral growth on bacteria and the exopolymers they secrete. Two Escherichia coli mutants producing distinct exopolymers are investigated. We use the incident electron beam to provoke and observe the precipitation of Mn-bearing minerals. Differences in the morphology and distribution of Mn precipitates on the two strains reflect differences in nucleation site density and accessibility. Direct observation under liquid conditions highlights the critical role of bacterial cell surface charges and exopolymer types in metal mineralization. This has strong environmental implications because biofilms structured by exopolymers are widespread in nature and constitute the main form of microbial life on Earth.

Monte Carlo commissioning of radiotherapy LINAC-Introducing an improved methodology.

Monte Carlo (MC) commissioning of medical linear accelerator (LINAC) is a time-consuming process involving a comparison between measured and simulated cross beam/lateral profiles and percentage depth doses (PDDs) for various field sizes. An agreement between these two data sets is sought by trial and error method while varying the incident electron beam parameters, such as electron beam energy or width, etc. This study aims to improve the efficiency of MC commissioning of a LINAC by assessing the feasibility of using a limited number of simulated PDDs. Using EGSnrc codes, a Varian Clinac 2100 unit has been commissioned for 6 MV photon beam, and a methodology has been proposed to identify the incident electron beam parameters in a speedier fashion. Impact of voxel size in 3-dimensions and cost functions used for comparison of the measured and simulated data have been investigated along with the role of interpolation. A voxel size of 1 × 1×0.5 cm3 has been identified as suitable for accurate and fast commissioning of the LIANC. The optimum number of simulated PDDs (required for further interpolation) has been found to be five. The present study suggests that PDDs alone at times can be insufficient for an unambiguous commissioning process and should be supported by including the lateral beam profiles in the process.

Double ionization of helium by twisted electron beam

In this communication, we report our calculations of the five-fold differential cross-section (FDCS) for (e,3e) on the He atom using both the plane wave and the twisted incident electron beam. The formalism has been developed in the first Born approximation using the plane wave as well as the twisted electron wave for the incident electron. We describe plane wave, Le Sech, and 2 Coulomb wave with Gamow factor (2CG) respectively for the scattered, bound electrons of He atom and the ejected electrons. We compare the angular profile of the FDCS for different values of orbital angular momentum (OAM) (m) of the twisted electron with that of the plane wave. We present the angular profiles of the FDCS in the θ-variable and constant θ12 mode in the coplanar geometry. The effects of the opening angle (θp) of the twisted electron on the angular profile of the FDCS are also studied.

Charge transport in nnn and npn phosphorene junctions: The use of phosphorene pn junctions as rectifiers

Abstract We investigate quantum charge transport of nnn and npn junctions based on two-dimensional black phosphorus (BP), phosphorene. We observe an oscillatory behavior for both the transmission probability and tunneling conductance of phosphorene nnn and npn junctions as a function of the chemical potential and width of the middle tunnel barrier. The oscillating amplitudes and periods can be controlled by means of a top gate voltage in the second segment. Because of the anisotropy of the electronic structure of phosphorene, the transmission probability depends strongly on the direction of the incident electron beam. We show that the perfect transmission can be selected by tuning the chemical potential and width of the middle region as well as the incidence angle of the carriers. In particular, for the n n n regime for the lightly doped case of the middle region, the transmission for any width of the middle region nearly becomes perfect. Under the certain conditions an electron is not allowed to tunnel through the junction that could be used as a single electron transistor. The resulting electronic properties show that the planar pn junctions (PNJs) structures based on phosphorene could be used as rectifiers, showing excellent tunability and novel functionalities. Our findings will open a new way to nanoelectronics based on field-effect tunneling transistors as well as the single electron transistors based on phosphorene heterostructures.

Effect of Coulomb field on laser-induced ultrafast imaging methods

By performing a joint theoretical and experimental investigation on the high-order above-threshold ionization (HATI) spectrum, the dominant role of the 3rd-return-recollision trajectories in the region near the cutoff due to the ionic Coulomb field is identified. This invalidates the key assumption adopted in the conventional laser-induced electron diffraction (LIED) approach that the 1st-returnrecollision trajectories dominate the spectrum according to strong field approximation (SFA). Our results show that the incident (return) electron beams produced by the 1st and 3rd returns possess distinct characteristics of beam energy, beam diameter and temporal evolution law due to the influence of Coulomb field, and therefore the extracted results in the LIED will be altered if the significance of the 3rd-return-recollision trajectories is properly considered in the analysis. Such Coulomb field effect should be taken into account in all kinds of laser-induced imaging schemes based on recollision.

Imaging Polarity in Two Dimensional Materials by Breaking Friedel's Law

Friedel's law guarantees an inversion-symmetric diffraction pattern for thin, light materials where a kinematic approximation or a single-scattering model holds. Typically, breaking Friedel symmetry is ascribed to multiple scattering events within thick, non-centrosymmetric crystals. However, two-dimensional (2D) materials such as a single monolayer of MoS2 can also violate Friedel's law, with unexpected contrast between conjugate Bragg peaks. We show analytically that retaining higher order terms in the power series expansion of the scattered wavefunction can describe the anomalous contrast between hkl and hkl¯peaks that occurs in 2D crystals with broken in-plane inversion symmetry. These higher-order terms describe multiple scattering paths starting from the same atom in an atomically thin material. Furthermore, 2D materials containing heavy elements, such as WS2, always act as strong phase objects, violating Friedel's law no matter how high the energy of the incident electron beam. Experimentally, this understanding can enhance diffraction-based techniques to provide rapid imaging of polarity, twin domains, in-plane rotations, or other polar textures in 2D materials.

Metrology of crystal defects through intensity variations in secondary electrons from the diffraction of primary electrons in a scanning electron microscope

Understanding defects and their roles in plastic deformation and device reliability is important for the development of a wide range of novel materials for the next generation of electronic and optoelectronic devices. We introduce the use of gaseous secondary electron detectors in a variable pressure scanning electron microscope for non-destructive imaging of extended defects using electron channelling contrast imaging. We demonstrate that all scattered electrons, including the secondary electrons, can provide diffraction contrast as long as the sample is positioned appropriately with respect to the incident electron beam. Extracting diffraction information through monitoring the modulation of the intensity of secondary electrons as a result of diffraction of the incident electron beam, opens up the possibility of performing low energy electron channelling contrast imaging to characterise low atomic weight and ultra-thin film materials. Our methodology can be adopted for large area, nanoscale structural characterisation of a wide range of crystalline materials including metals and semiconductors, and we illustrate this using the examples of aluminium nitride and gallium nitride. The capability of performing electron channelling contrast imaging, using the variable pressure mode, extends the application of this technique to insulators, which usually require conducting coatings on the sample surface for traditional scanning electron microscope based microstructural characterisation.

Monte Carlo study on the temperature rise under electron irradiation

Scanning electron microscope (SEM) is frequently-used in analysis for micro/nanoscale structural information. During SEM analysis, besides the widely-known charging effect, the temperature rise because of electron beam bombardment also has become an issue due to it can modify SEM results with the downsizing of specimen, too. In this work, a sophisticated Monte Carlo (MC) model was adopted to quantitatively study the temperature rise during SEM imaging. The influences of various parameters on temperature profile for two different systems were examined: 1) semi-infinite metal copper (Cu) bulk focused on accelerating energy, incident electron beam angle (0° to 80°), incident beam size and the number of incident electron; 2) semi-infinite insulator Silicon dioxide (SiO2) bulk (amorphous SiO2) focused on scanning duration time. It was concluded that the temperature profile presents three characteristics: a) increasing first at surface until its maximum is reached, then has a sharp reduction as a function of depth from the surface for all two systems; b) being affected seriously by accelerating energy, incident electron beam angle and scanning duration time. The mechanism behind these phenomena was explained in detail. This work provides a better understanding and elucidation for physical mechanism of electron-beam-induced deposition, of which process control, performance and reliability would thus get a greater improvement based on this study.

A Method for the Measurement of Thickness of Inorganic Solid Films by Medium Energy Electron Excited X-ray Fluorescence

In this paper, a new non-destructive method of measuring the thickness of nanometer thin films by medium energy electron excited X-ray fluorescence was introduced. Due to strong decay of the intensity of incident electron beam with energy of the order of 1 keV in solid materials, the penetration depth is very small, only in the order of 1 nm. Therefore, the escape depth of the X-ray fluorescence excited by the electron beam is also limited by the penetration depth of the incident electron beam, i.e., the X-ray fluorescence detected is emitted only from the top most layers. By simply compare the intensity of an XRF peak from the substrate with/without the deposition of thin film, the thickness of the films can be easily calculated. The method described in this paper is simple in participle, nondestructive, and applicable for films of any elements. As an example, a set of Al films deposited on Si substrate was demonstrated, the results show that this method is feasible, and the detection limit can be as low as 1 nm or less.

Phase transformation at controlled locations in nanowires by in situ electron irradiation

Solid state phase transformations have drawn great attention because they can be effectively exploited to control the microstructure and property of materials. Understanding the physics of such phase transformation processes is critical to designing materials with controlled structure and with desired properties. However, in traditional ex situ experiments, it is hard to achieve position controlled phase transformations or obtain desirable crystal phase on nanometer scale. Meanwhile the underlying mechanisms of the reaction processes are not fully understood due to the lack of direct and real-time observation. In this paper, we observe phase transformation from body-centered tetragonal PX-PbTiO3 to monoclinic TiO2(B) on the atomic scale by in situ electron irradiation during heat treatment in transmission electron microscope, at pre-defined locations on the sample. We demonstrate that by controlling the location of the incident electron beam, a porous TiO2(B) crystal structure can be formed at the desired area on the nanowire, which is difficult to achieve by traditional synthesis methods. Upon in situ heating, the Pb atoms in the crystal migrate out of the pristine nanowire through inelastic scattering under incident electrons while high temperature(> 400 °C) provides energy for the crystallization of TiO2(B) and the volatilization of a substantial number of Pb atoms, which makes the resultingTiO2(B) nanowires to be porous. In contrast, at temperatures 400 °C, the segregated Pb atoms form Pb particles and the TiOx nanowires remain in the amorphous state. This work not only provides in situ visualization of the phase transition from the PX-PbTiO3 to monoclinic TiO2(B), but also suggests a crystallography engineering strategy to obtain the desired crystal phase at controlled locations on the nanometer scale.

In Situ Transmission Electron Microscopy Measurements of Ge Nanowire Synthesis with Liquid Metal Nanodroplets in Water.

The growth of Ge nanowires in water inside a liquid transmission electron microscope (TEM) holder has been demonstrated at room temperature. Each nanowire growth event was stimulated by the incident electron beam on otherwise unsupported liquid Ga or liquid In nanodroplets. A variety of conditions were explored, including liquid metal nanodroplet surface condition, liquid metal nanodroplet size and density, formal concentration of dissolved GeO2, and electron beam intensity. The cumulative observations from a series of videos recorded during growth events suggested the following points. First, the conditions necessary for initiating nanowire growth at uncontacted liquid metal nanodroplets in a liquid TEM cell indicate the process was governed by solvated electrons generated from secondary electrons scattered by the liquid metal nanodroplets. The attained current densities were comparable to those achieved in conventional electrochemical liquid-liquid-solid (ec-LLS) growths outside of a TEM. Second, the surface condition of the liquid metal nanodroplets was quite influential on whether nanowire growth occurred and surface diffusion of Ge adatoms contributed to the rate of crystallization. Third, the Ge nanowire growth rates were limited by the feed rate of Ge to the crystal growth front rather than the rate of crystallization at the liquid metal/solid Ge interface. Estimates of an electrochemical current for the reduction of dissolved GeO2 were nominally in line with currents used for Ge nanowire growth by ec-LLS outside of the TEM. Fourth, the Ge nanowire growths in the liquid TEM cell occurred far from thermodynamic equilibrium, with supersaturation values of 104 prior to nucleation. These collective points provide insight on how to further control and improve Ge nanowire morphology and crystallographic quality by the ec-LLS method.

Benchmarking of electron beam parameters based on Monte Carlo linear accelerator simulation.

BackgroundThe main objective in this paper is to build up a simplified benchmarking procedure to determine the incident electron beam parameters independently using a Monte Carlo linear accelerator model.MethodsA 10×10 cm2 field is used to acquire the parameters influences on the dose distributions for 6 MeV megavoltage electron beams.ResultsThe calculated depth doses had no significant changes when the electron mean energy and radial intensity varied from 5.5–6.4 MeV and 0.1–0.4 cm respectively. And the dose profile simulated with an electron radial intensity among 0.0–0.19 cm had a good agreement with the measured dose profile in the main field, while obvious discrepancies happened in the penumbra region. The mean angular spread was found to be 0.3° to get good match between calculations and measurements at all interested depths.ConclusionsAccurate dose distributions are acquired by using the incident electron beam parameters benchmarked with the proposed procedure. As the percentage depth doses and dose profiles are insensitive to the electron energy and radial intensity correspondingly, they are feasible to benchmark individually.

Photonuclear reactions by relativistic electron channeling radiation

The research for newly developing branch of nuclear physics, the nuclear photonics, has been accompanied since the beginning with the studies on intense MeV photon sources. One of the possible solution is in the use of channeling radiation. The channeling radiation spectrum for sub-GeV–several GeV electrons is characterized by a sharp maximum at photon energies up to several MeV, which is enough to excite separate nuclear levels as well as (γ,n) reaction for light Be and D nuclei. This maximum may even reach the region of giant dipole resonance for heavier nuclei. At equal radiator thickness the channeling radiation flux may exceed in more than one order that of bremsstrahlung. Thus, channeling radiation can be efficiently utilized in studying photonuclear reactions as well as generating pulsed neutron beams at sub-GeV electron accelerators. The latter is illustrated by detailed calculations of the neutrons yield from the light D and Be targets irradiated by channeling radiation. The non-trivial dependence of neutrons yield on the energy of incident electron beam and on the electron beam alignment with respect to the crystal channeling planes is revealed.

The Auger spectra in argon induced by electron impact – new measurements with high resolution

The Auger spectrum of argon in the kinetic energy region (135–235 eV) has been measured using a non-monochromatic electron beam (incident energy from 313 to 2019 eV) at two ejection angles of 60° and 90° with a high resolution electrostatic analyzer. The electron impact energies of 313 and 323 eV have been used in order to disentangle the contribution of the ionization and decay of the 2s state. The systematic analysis of the measured spectra shows the dominance of the Coster-Kronig transitions from the 2s ionization in the energy region 135–159 eV and 208–235 eV, while the Auger transitions from the 2p ionization are dominant in the energy region 160–208 eV. The high resolution of the present work led to the observation of a certain number of new features in two energy regions 135–159 and 208–235 eV, respectively. Recent literature data on the cascade Auger processes and threshold formation of Ar2+ satellite states allow the assignment of some of them, while other have been left without assignments. The influence of the PCI effect on the line-shape, width and energy position of the features in the kinetic energy region 200–208 eV has been also investigated and the PCI shift versus excess energy above threshold compared with previous data and theoretical predictions.

Monte Carlo study for correcting the broadened line-scan profile in scanning electron microscopy.

Line-scan profile is always broadened due to the probe shape of the primary electron (PE) beam in scanning electron microscopy (SEM), which leads to an inaccurate dimension metrology. Currently, the effective electron beam shape (EEBS) is suggested as the broadening function to overcome this issue for theoreticalanalysis, rather than the widely used Gaussian profile. However, EEBS is almost impossible to be acquired due to it strongly depends on both the sample topography and the electron beam focusing condition, which makes it is impossible to be applied in practical analysis. Taking the case of gate linewidth measurement, an approach is proposed to find a best-fit traditional Gaussian profile, which can optimally replace the EEBS in the case of the same sample structure and experimental condition for construction of a database of the parameter in traditional Gaussian profile. This approach is based on the use of the ideal and broadened line-scan profiles which are both obtained from Monte Carlo (MC) simulation, but respectively by an ideal and a focusing incident electron beam model. The expected value of parameter can be obtained through deconvoluting (here using a maximum-entropy algorithm) the broadened line-scan profile then fitting it to the ideal profile. Experimenters can benefit from this database to obtain true line-scan profiles for accurate gate linewidth measurement. This work should prove useful for samples of other structures and be an extension of the database in the future.

Triple differential cross-section measurements for electron-impact ionization of methane from a coplanar geometry to the perpendicular plane.

A new study of electron-impact single ionization of the HOMO 1t2 state of CH4 has been conducted at incident electron energies 20 eV and 40 eV above the ionization energy of the state. Triple differential cross sections were measured from a coplanar symmetric geometry, where scattered and ionized electrons were detected at equal angles, through to the perpendicular geometry where the outgoing electrons emerged orthogonal to the incident electron beam. At the lower energy, the electrons were detected with equal energies of 10 eV, whereas at the higher energy, data were obtained for equal energies of 20 eV and for unequal energies of 5 eV and 35 eV. The results are compared to a molecular 3-body distorted wave approximation that used a full averaging procedure to allow for the random orientation of the target, an orientation averaged molecular orbital model that averages the target wavefunction over all orientations prior to the collision, and a distorted wave Born approximation that does not include postcollisional interactions.