Parallel Electron Beam Research Articles

Manifestations of Electronic Correlations in the Diffraction of Electron Pairs from Crystals

An electronic system distorted externally by a photon or a charged particle impact can integrally respond by the simultaneous emission of two electrons into the vacuum. The vacuum states of these two correlated electrons can be then determined using an angular and energy-resolved coincidence technique. The spectra of low-energy electron pairs, the subject of this study, carry the signature of their mutual correlations and their coupling to other degrees of freedom of the environment while in the high energy regime information on the initial-momentum components of the pair can be extracted from the recorded spectra [1‐ 4]. These observations have been made in diverse areas of physics, such as plasma, atomic, molecular, and condensed matter physics [3,5,6]. For pair emission from localized electronic states, such as atomic and molecular orbitals, it turns out that the spectra are dominated by the interelectronic interaction of the pair, in particular at lower energies (with respect to the initial orbital energies) [6]. Thus, an adequate theoretical description of these phenomena must go beyond an independent particle description. For delocalized electronic states, as present in metallic crystals and surfaces, it is established that delocalization does not preclude correlations. E.g., in transition metals the d electrons are delocalized, yet correlation between them is far from weak. In this work it is shown theoretically and experimentally that an electron pair can be regarded as a “two-electron quasiparticle.” The scattering of this quasi-single-particle from a crystal potential results in characteristic diffraction pattern that is, for the first time, experimentally observed. The positions of the diffraction peaks are governed by a von Laue‐ like diffraction condition for the center-ofmass wave vector of the electron pair. The relative intensities of the diffraction maxima are largely determined by the internal degree of freedom of the pair, i.e., by interelectronic correlations. The experimental setup used for the angular and energy-resolved detection of the pairs, i.e., for the projection of the two-electron initial state onto the two-electron vacuum state, is schematically depicted in Fig. 1. A more detailed description can be found in Refs. [7 ‐ 9]. The sample surface defines the x-y plane, while the z axis coincides with the surface normal. In the z-x plane two position sensitive microchannel plate electron detectors are located at a distance of 160 mm to the sample surface, such that the relative angle between the detector axes and the surface normal is given by 6a . The angular acceptance of each electron detector within the scattering plane is 613.2 ‐ . A parallel electron beam of about 1 mm diameter impinges onto the sample surface including the angle g with the surface normal. For investigating the Cu(001) sample, the angles a and g are chosen to be 40 ‐ and 0 ‐ , respectively, while in the case of Fe(110) (BCC), a was set to be 50 ‐ and g › 5 ‐ . Correlated electron pairs emitted from the sample upon excitation by a primary electron are detected in coincidence. Their energies have been measured using a time-of-flight technique. In the range of electron energies detected here, we achieve an energy resolution of DE › 0.4 0.8 eV. Standard cleaning procedure of the surface is applied before each measurement under a base pressure in the range of 10 211 mbar. The probability for the two electrons to be detected with asymptotic momenta k1 and k2 is derived from an

The existence regions of structural modifications in decagonal Al-Co-Ni

A subdivision of the stability region of decagonal Al-Co-Ni quasicrystals into eight different structural modifications is presented for the temperature against composition section along the maximum extension of the decagonal phase. The only methods identifying this subdivision are transmission electron microscopy techniques. Besides two decagonal superstructures (one of which also exists as a high-temperature modification), two variants of the basic decagonal state were observed. Also, a one-dimensionally periodic fivefold quasicrystal (together with its high-temperature superstructure modification) and a one-dimensional quasicrystal are parts of the quasicrystalline stability region. Both have a close structural relationship to the decagonal states. All samples investigated were obtained by quenching from the corresponding temperatures. The eight modifications can be differentiated by a set of attributes, especially by their characteristic diffraction patterns taken with the electron beam parallel to the unique periodic axis.

Demonstration of Solid State Electron Optical Devices: Pixelated Fresnel Phase Lenses

The ability to produce features of nanometer scale offers the possibility of phase manipulation of electron waves. The first conclusive results of phase manipulation by nanometer-scale diffraction gratings directly cut by the finely focused electron beam in a scanning transmission electron microscope (STEM) have already been demonstrated. This was achieved by using a grating with a “wedge” profile. This produced an asymmetrical diffraction pattern (in violation of Friedel's law). This violation is expected only if the grating acts mostly as a strong phase object. In this paper, a demonstration of solid state Pixelated Fresnel Phase (PFP) lenses for electrons will be presented. An array of these electron lenses can be easily formed and may be utilized, for example, as compact electron-beam forming lenses for parallel electron beam lithography.In general, an incident plane wave traveling along the optic axis of a lens experiences a phase shift. A conventional Fresnel phase lens, consisting of concentric zones with a modulo of 2π phase structure, focuses an incoming electron plane wave to its focal point.

Transmission electron microscopy of Cd3As2 and eutectoid Cd3As2-As crystals

Cadmium arsenide, Cd3As2, is a narrow band-gap semiconductor with forbidden energy gap of 0.14 eV. The electron density in as-grown n-type crystals is as high as 2 3 10 my3 [1] and electron mobilities have been reported from 0.3 to 2.0 m Vy1 sy1 at 300 K, while approaching 28 m Vy1 sy1 at 4.2 K [2, 3] making them suitable for photo detectors, thermal detectors, ultrasonic multipliers, and Hall generators [4, 5]. Hence, crystal and band structures remain of interest [6]. Its accepted crystal structure is tetragonal with a ˆ 1:267 nm, c ˆ 2:548 nm and space group I41cd [7]. This work examined the crystal structure of Cd3As2 and lattice defects using transmission electron microscopy (not reported on bulk material). Crystals were grown by sublimation under two sets of conditions, by transport within an evacuated silica ampoule in a temperature gradient with condensation occurring above the solid-solid phase transition of 578 8C [8], and in an argon gas ow with recondensation in a temperature region of 335 8C to 385 8C as described by Lovett [9]. Specimens for electron microscopy were prepared by grinding and polishing followed by argon ion etching. Intensities and d-values of diffraction peaks for Cd3As2 obtained by powder X-ray diffractometry are as reported by Steigmann and Goodyear [7], supporting a body-centred tetragonal with a unit cell of 16 cubes. In each cube, the As ions are arranged approximately cubic close-packed (f.c.c.), and the Cd ions are tetrahedrally co-ordinated. Certain of the cadmium ions are shifted from their idealized positions by approximately 0.02 nm, and others by 0.025 nm. The arsenic ions are also displaced and As±As bond lengths vary from 0.448 to 0.47 nm. These ionic displacements lead to violation of extinction conditions in this structure. A series of electron diffraction patterns of Cd3As2 were obtained using a JEM-200CX transmission electron microscope (TEM). A reciprocal lattice cell, as shown in Fig. 1, was reconstructed using these electron diffraction patterns. The lattice parameters thus derived are a  1:26(7) nm, and c  2:54(8) nm. Part of the series of diffraction patterns is shown in Fig. 2. These diffraction patterns can be indexed unambiguously based on the lattice parameters obtained above. The zone axes for each diffraction pattern are [0 0 1], [2 0 1], [1 0 0], [2 1 0], and [1 1 1], respectively. However, there are additional diffractions which are not observed in the X-ray powder diffraction spectrum. These diffractions are forbidden re ections. Their occurrence in electron diffraction patterns is because of the displacements of cadmium and arsenic ions from their exact positions in the unit cell, and because of double diffraction. For example, in the pattern of Fig. 2c, there is a column of diffraction spots between 0 0 0 and 0 4 0 and three rows of diffraction spots between 0 0 0 and 0 0 8, and all the alternative columns and rows which are parallel to these. These are forbidden re ections in this structure. The faint diffraction spot of (0 0 4) in Fig. 2c becomes a strong re ection in Fig. 2d due to double diffraction of (1 2 1)‡ (1 2 3) ˆ (0 0 4). All the faint spots in Fig. 2e are forbidden diffractions, but their occurrences are due to ionic displacements, and not because of double re ections. The appearances of forbidden re ections due to ionic displacements depend also on the specimen thickness. Fig. 2f is a diffraction pattern with the electron beam parallel to [1 1 1], but is taken from a thinner specimen and is more populated with forbidden diffractions such as (h h 0) or (0 k k). Certain patterns from samples prepared under the second set of growth conditions exhibited different symmetry. Energy dispersive X-ray analysis of these distinguishable crystalline regions showed them to be arsenic. A eutectoid mixture of Cd3As2 and As with up to ,5 at % As was observed by Carpenter et al. [10] in ®lms of evaporated Cd3As2 as prepared using a pulsed-laser technique. In the present work, the proportion of As in the (Cd3As2 ±As) eutectoid was less than ,0.1 at %. Electron diffraction patterns of these arsenic phases indicated that their structure is orthorhombic with parameters a ˆ 0:71(3) nm, b ˆ 0:73(3) nm, c ˆ 1:25(1) nm and with space group Pnnn, rather than hexagonal. Electron diffraction patterns with zone axes [1 0 0] and [1 3 0] are shown in Fig. 3. The new arsenic unit cell is possibly due to the presence in solution of low levels of cadmium below the detection level of EDS X-ray analyser (Link Systems, 860 series II). Using bright-®eld and dark-®eld imaging, structural defects were observed within the Cd3As2 crystals and within the new phase of arsenic.

Semiconductor on glass photocathodes as high-performance sources for parallel electron beam lithography

The throughput of electron beam lithography has historically been limited by electron–electron interactions that cause blurring at high currents. We present a system configuration for maskless parallel electron beam lithography using a new multiple primary source technology that, by employing widely spaced beams, significantly reduces this problem. The proposed source technology, a negative electron affinity (NEA) photocathode, allows us to generate an array of high brightness, low energy spread, independently modulated beams over a large area. In order to assess the effects of electron–electron interactions in this system, Monte Carlo simulations have been performed. The results of these calculations indicate that this configuration enjoys significant advantages over existing maskless systems. By restricting the area of emission for the individual beamlets to submicron dimensions, the blurring due to statistical electron–electron interactions can be significantly reduced for a given current at the wafer. For example, at 50 kV a total current of more than 2.5 μA can be obtained with less than 10 nm beam blurring. Preliminary experimental results suggest that high brightness emission can be maintained from a NEA photocathode in a demountable vacuum system.

Structure ofthe Al-Rh-Cu decagonal quasicrystal studied by high-resolution electron microscopy

The structure of the Al-Rh-Cu decagonal quasicrystal with periodicity of about 0.4 nm has been investigated by high-resolution electron microscopy. A high-resolution structure image taken with the incident electron beam parallel to the tenfold axis can be interpreted as an aperiodic tiling composed of three kinds of subunit. The atomic arrangements in these subunits have been proposed according to the structure of the Al-Co-Cu decagonal quasicrystal determined by single-crystal X-ray analysis. A calculated image using the present model reproduces well the contrast features of the high-resolution structure image.

Inverse compton testing of fundamental physics and the cosmic background radiation

The recent precise Cosmic Microwave Background Radiation (CMB) measurements by COBE and other facilities can provide interesting possibilities for the experimental checking of some basic physical parameters. In this context the inverse Compton scattering of highly monochromatic and parallel electron beams on laser photons is proposed for testing special relativity and related basic principles. For CEBAF and SLC parameters, for example, one can expect a precision for the isotropy of the speed of light dc(θ)/c ≈ 10−13 – 10−14 and an upper limit for the hypothetical fundamental length λf ≃ 10−22 – 10−23 cm. In particular, of special interest can be the checking of the isotropy of the speed of light with respect the direction of the CMB dipole (a kind of Michelson-Morley experiment relative to the absolute "rest" coordinate system).

Two-dimensional quasicrystal with fivefold rotational symmetry and superlattice

A two-dimensional quasicrystal with a fivefold rotational axis has been found in an Al-Co-Ni-Tb alloy by means of transmission electron microscopy. An electron diffraction pattern taken with the incident electron beam parallel to the unique periodic axis shows two-dimensional quasiperiodicity. Moreover, fivefold orientation symmetry clearly appears in the pattern, when a thick sample is used in the experiment. In addition, the S1 type of superlattice has been observed in the quasiperiodic reciprocal plane. The period of the fivefold axis is suggested to be about 0 4nm for the basic structure and 0 8nm if chemical order in the structure is taken into account. The lattice image taken with the incident beam along the fivefold axis is related to a fivefold aperiodic tiling under certain assumptions.

Enhancing a hierarchical, parallel electron beam data conversion processor toward 1 Gbit memory lithography

A new hierarchical, parallel electron beam (EB) data conversion system has been developed to be used in lithography for the fabrication of memories of 1 Gbit or more. The conversion system has been enhanced not only to meet EB writing requirements but also to cover new data processing capabilities required for critical dimension control and for optical resolution enhancement. The conversion system takes full advantage of the hierarchical, parallel technique and generates patterns for various EB lithography systems, including the EX-8 mask writing system, EX-8D wafer direct writing system, and MC-100 defect inspection system. The interface with computer aided design (CAD) systems has been improved by realizing direct input from CADENCE Opus or Edge and from a design rule checker ENMA in addition to CALMA GDSII format. A design guide has been established to guarantee efficient, problem-free hierarchical data conversion. Also, a new data compaction technique has been employed. The system is equipped with functions for increasing EB writing throughput. Moreover, functions for multipass writing and resize writing are available to improve pattern transfer fidelity in the resist development or chromium etching process. In the EB proximity effect correction, Ghost and dose modulation can be selectably used. For alternating phase shifting mask fabrication, a new CAD software to automatically assign shifter phase has been added. When a model 1 Gbit dynamic random access memory pattern with 0.15 μm minimum features designed following the design guide was scaled up for 4× reticle mask making and converted to the resize writing pattern on a workstation with four central processor units, the maximum conversion time per layer was 7 min 17 s, and an average conversion time was about 5 min. The data volume per mask amounted to 456 Mbytes. Using the compaction technique, the data volume was reduced to 1/7.7 at the maximum, and an average data compaction ratio of 4.5 was obtained.

Misfit dislocations in ZnSe grown on vicinal Si(001) substrates

High resolution electron microscopy (HREM) has been used to study misfit dislocations of ZnSe films grown on vicinal Si(001) substrates tilted 4° towards the [11̄0] axis. In images taken with the electron beam parallel to the [11̄0] direction, 60° dislocations were found to predominate whereas mostly Lomer dislocations or closely spaced 60° dislocations (separated by <2 nm) were observed in images taken in the orthogonal direction. A model is presented here to explain the formation of the asymmetric dislocation structure on the basis of mechanisms for propagation and formation of misfit dislocations.

Determination of Electron Depth-Dose Curves for Water, ICRU Tissue, and PMMA and Their Application to Radiation Protection Dosimetry

For monoenergetic electrons in the energy range between 60 keV and 10 MeV, normally incident on water, 4-element ICRU tissue and PMMA phantoms, depth-dose curves have been calculated using the Monte Carlo method. The phantoms' shape was that of a rectangular solid with a square front face of 30 cm x 30 cm and a thickness of 15 cm; it corresponds to that recommended by the ICRU for use in the procedure of calibrating radiation protection dosemeters. The depth-dose curves have been used to determine practical ranges, half-value depths, electron fluence to maximum absorbed dose conversion factors, and conversion factors between electron fluence and absorbed dose at depths d corresponding to 0.007 g.cm-2, 0.3 g.cm-2, and 1.0 g.cm-2. The latter data can be used as fluence to dose equivalent conversion factors for extended parallel electron beams.

Application of microdiffraction to crystal structure identification

In a previous paper by J.P. Morniroli and J.W. Steeds [Ultramicroscopy 45 (1992) 219] it was shown that microdiffraction patterns obtained with a nearly parallel electron beam and a small spot size can be used to identify and to characterize crystal structures. A systematic method was proposed to deduce, via the Buerger extinction symbol, the space group or a set of consistent space groups. In order to illustrate how this method can be applied experimentally, three examples concerning the σ, X and R intermetallic phases are detailed. Experimental solutions are given to overcome difficulties connected with the frequent absence of high-order Laue zone reflections on high-symmetry zone axis patterns. The possibility to identify the Buerger extinction symbol from a unique zone axis pattern is also explored.

Multiple twinning in GaAs epitaxial layers grown on Si(001) and Si(111)

Multiple twinning in GaAs epitaxial layers grown on Si by metalorganic vapor phase epitaxy has been studied by transmission electron microscopy. The primary and multiple twin reflections in the diffraction patterns were indexed in terms of matrix indices for the electron beam parallel to the 〈001〉, 〈110〉, and 〈111〉 directions in GaAs.

Solution of the Atomic Structure of the Σ=5 (310) [001] Grain Boundary in Nial by Hrem and Atomistic Simulations

AbstractThe atomic structure of the Σ = 5 (310) [001] grain boundary in NiAl has been determined by a synergistic approach combining high resolution electron microscopy (HREM) and atomistic structure calculations. A bicrystal of controlled orientation was produced by diffusion bonding and imaged with the electron beam parallel to the [001] tilt axis. The results showed that the material remains chemically ordered up to the boundary plane. Atomistic structure calculations employed N-body empirical potentials developed for the NiAl phase to examine the changes in interfacial energy due to the incorporation of various point defects at the grain boundary. A self-consistent model structure was determined which was of lowest energy and produced calculated images which matched experimental images of the boundary. Monte Carlo simulations confirm the stability of this structure at finite temperatures.

Parametric excitation of the electrostatic whistler and Z mode by auroral kilometric radiation

By using a two‐dimensional magnetized electron plasma simulation model it is found that an X mode which is presumed to be the dominant component of auroral kilometric radiation can parametrically decay into an electrostatic whistler and a Z mode. The electrostatic whistler grows to large amplitude, and the associated parallel electric field accelerates electrons to several kiloelectron volts. It is proposed that this mechanism is a possible origin of Z mode radiation, broadband electrostatic noise, and energetic parallel electron beams.

The Conditions for Detecting Confined Material in the Channels/Cavities of Zeolites by Using HREM

High resolution electron microscope images of zeolites recorded with the incident electron beam parallel to the main channels show an optical artifact. The contrast in the center of the channels is severely disturbed by dark spots caused by some Bragg reflections being insufficiently transferred by the objective lens. Since zeolites are being used as containers for other materials, the spots in electron micrographs will screen any materials located in the large zeolite channels. With experimental micrographs and image simulations it is shown that by changing the focus conditions the spot can be eliminated and hence, confined material can be imaged. Two representatives, mordenite and zeolite Y, are described in detail for the cases of one-dimensional and three-dimensional channel systems, respectively.

Low‐energy electron microscopy (LEEM) and mirror electron microscopy (MEM) of biological specimens: Preliminary results with a novel beam separating system

Low-energy electron microscopy (LEEM) and mirror electron microscopy (MEM) utilize a parallel beam of slow-moving electrons backscattered from the specimen surface to form an image. If the electrons strike the surface an LEEM image is produced and if they are turned back just before reaching the surface an MEM image results. The applications thus far have been in surface physics. In the present study, applications of LEEM and MEM in the biological sciences are discussed. The preliminary results demonstrate the feasibility of forming images of uncoated cultured cells and cellular components using electrons in the threshold region (i.e. 0-10 V). The results also constitute a successful test of a novel beam-separating system for LEEM and MEM.

High-Speed Electron Beam Data Conversion System Combining Hierarchical Operation with Parallel Processing

A new high-speed electron beam (EB) data conversion system has been developed for the variable-shaped beam EB reticle writing system EX-8. The target conversion rate from computer-aided design (CAD) data to EX-8 data is five reticles per hour for the class of 16-Mbit dynamic random access memory (DRAM) and beyond. To achieve this, both the hierarchical and parallel shape data operations and the parallel EB formatting have been adopted. Preprocesses for the hierarchical operations have been integrated, such as cell-overlap eliminations and the “doughnut process” which involves partially expanding shapes near cell boundaries. A 64-Mb DRAM design was successfully converted in the hierarchical method on an engineering workstation (EWS) with a single processor, and the number of processed shapes was reduced by four orders of magnitude and the average processing time was less than nine minutes. The processing time of the shape data operations for one layer of a 129 k gate array was reduced to 1/3 when four processes were performed in parallel on an EWS with four processors.

Effect of crystal and beam tilt on simulated high-resolution TEM images of interfaces.

The effects of crystal and beam tilt on high-resolution transmission electron microscope (HRTEM) images of planar coherent interfaces were investigated by multislice image simulations. It was found that a beam tilt of 0.5 Bragg angle (theta B) was sufficient to introduce detrimental artifacts into most images of interfaces in crystals only 1/8 xi 000 thick, while crystal tilt had a much smaller effect even for crystals 1 xi 000 thick. Effects produced in HRTEM images of interfaces by crystal and beam tilt included the introduction of additional periodicities and loss of compositional detail across a boundary, translation of a boundary from its actual position, and apparent mismatch of atomic planes across a perfectly coherent interface. These results indicate that alignment of the electron beam parallel to the optic axis is critical for reliable HRTEM imaging of interfaces in materials. Techniques for obtaining accurate alignment are also discussed.

Microdiffraction as a tool for crystal structure identification and determination

Microdiffraction patterns obtained with a focused and nearly parallel incoherent electron beam allow one to obtain the crystal system and the Bravais lattice and to reveal the presence of glide planes and screw axes of a specimen. These crystallographic features are easily and reliably obtained, in a systematic way, from a few patterns by means of tables and theoretical patterns established for each crystal system. They lead to an extinction symbol which is in agreement with a small number of possible space groups. Usually, these possible space groups belong to different point groups which may be distinguished by study of the intensity distributions in the diffraction patterns. Consequently the space group or a small sub-set of consistent space groups can be deduced. The technique is therefore very well adapted to phase identification. It can be used as a routine technique and applied to most specimens and especially to small particles, and this constitutes its main advantage. It is particularly valuable with specimens which give poor convergent beam electron diffraction (CBED) patterns. It can also be used in connection with CBED.