Detection Of Low-energy Electrons Research Articles

Nanometer-scale depth-resolved hard x-ray absorption spectroscopy based on the detection of energy-loss Auger electrons with low energies

X-ray absorption spectroscopy (XAS) provides information on the chemical state and atomic structure of a target element even without long-range periodicity. A depth-resolved surface-sensitive XAS method for K-edge using hard x rays is proposed herein. The principle of this method is based on the selective detection of low-energy electrons using an electron analyzer. The detected electrons originate from the LMM Auger electrons that cascade from the KLL Auger process caused by K-shell absorption and lose the energy corresponding to their travel distance in a solid. The analysis depth was confirmed to be in the nanometer range using a GaN substrate with a thin oxide film of the defined thickness. In addition, depth-resolved information regarding the local atomic structure, including the interatomic distance and crystallinity, was obtained via the Fourier transformation of the spectral oscillations. Because the proposed technique does not require a change in the detection angle, it is also expected to be used for particles and samples with uneven surfaces.

An electron counting algorithm improves imaging of proteins with low-acceleration-voltage cryo-electron microscope

Relative to the 300-kV accelerating field, electrons accelerated under lower voltages are potentially scattered more strongly. Lowering the accelerate voltage has been suggested to enhance the signal-to-noise ratio (SNR) of cryo-electron microscopy (cryo-EM) images of small-molecular-weight proteins (<100 kD). However, the detection efficient of current Direct Detection Devices (DDDs) and temporal coherence of cryo-EM decrease at lower voltage, leading to loss of SNR. Here, we present an electron counting algorithm to improve the detection of low-energy electrons. The counting algorithm increased the SNR of 120-kV and 200-kV cryo-EM image from a Falcon III camera by 8%, 20% at half the Nyquist frequency and 21%, 80% at Nyquist frequency, respectively, resulting in a considerable improvement in resolution of 3D reconstructions. Our results indicate that with further improved temporal coherence and a dedicated designed camera, a 120-kV cryo-electron microscope has potential to match the 300-kV microscope at imaging small proteins.

Effect of Nd and Cu Impurities on the Electro-Optical Properties of Recombinant of Graphene/CH3NH3PbCl3/Borophene for Photovoltaic and Biosensing Applications

In recent years, effective measures have been increasingly taken to improve the efficiency of perovskite solar cells. The use of two-dimensional structures such as Graphene in these structures has led to increased efficiency and improved performance of solar cells. The simultaneous use of Graphene and Borophene thin films in a perovskite solar cell has not been studied so far. In this paper, the nanostructures of Graphene/CH3NH3PbCl3/Borophene and Nd-doped_Graphene/CH3NH3PbCl3/Cu-doped_Borophene have been introduced and its structural and electro-optical properties have been investigated. Studies were performed using DFT. The results of the band structure studies show that Graphene and Borophene change the energy band gap and this band gap is adjustable by adding the amount of Nd and Cu impurities. Optical properties such as extinction coefficient, refractive index and reflectance for these structures show an improvement in light absorption compared to the reported structures, which is a significant increase for structures with Cu and Nd impurities. Also, the results of electron probation showed that the studied structures are capable of transition of low-energy electrons and electron detection. Raman shift calculations also show new frequency ranges due to the presence of Graphene and Borophene. Based on this set of calculations, the recombinant structures of Graphene/CH3NH3PbCl3/Borophene and Nd-doped_Graphene/CH3NH3PbCl3/Cu-doped_Borophene show promising prospects for improving the performance of sensors and photovoltaic solar cells.

Performance of a silicon-on-insulator direct electron detector in a low-voltage transmission electron microscope.

The performance of a direct electron detector using silicon-on-insulator (SOI) technology in a low-voltage transmission electron microscope (LVTEM) is evaluated. The modulation transfer function and detective quantum efficiency of the detector are measured under backside illumination. The SOI-type detector is demonstrated to have high sensitivity and high efficiency for the direct detection of low-energy electrons. The detector is thus considered suitable for low-dose imaging in an LVTEM.

Spectral detection of spin-polarized ultra low-energy electrons in semiconductor heterostructures

The circularly polarized cathodoluminescence (CL) technique has been used to study the free spin-polarized electron injection in semiconductor heterostructures with quantum wells (QWs). A polarized electron beam was created by the emission of optically oriented electrons from the p-GaAs(Cs,O) negative electron affinity (NEA) photocathode. The prepared beam was injected in a semiconductor QW target, which was activated by cesium and oxygen to reduce the work function. To study the spin-dependent injection, we developed a spin-detector prototype, which consists of a compact proximity focused vacuum tube with the source and target placed parallel to each other on the opposite ends of the vacuum tube (photodiode). The injection of polarized low-energy electrons into the target by varying the kinetic energy in the range of 0.5-5.0 eV and temperature in the range of 90-300 K was studied. The CL was polarized to 2 % by the injection of 20 % spin-polarized electron beam with the energy of 0.5 eV at room temperature. The asymmetry (Sherman function) of spin detection was estimated. It was shown that the dependence of the CL polarization degree on the injected electron energy is satisfactory described by the model that considers the electron spin relaxation in the heterostructure matrix and QWs. The results demonstrate that semiconductor detectors are promising for the spin-polarimetry applications based on the optical detection of free-electron spin polarization.

Fabrication and performance of AC-coupled LGADs

Detectors that can simultaneously provide fine time and spatial resolution have attracted wide-spread interest for applications in several fields such as high-energy and nuclear physics as well as in low-energy electron detection, photon science, photonics and imaging. Low-Gain Avalanche Diodes (LGADs), being fabricated on thin silicon substrates and featuring a charge gain of up to 100, exhibit excellent timing performance. Since pads much larger than the substrate thickness are necessary to achieve a spatially uniform multiplication, a fine pad pixelation is difficult. To overcome this limitation, the AC-coupled LGAD approach was introduced. In this type of device, metal electrodes are placed over an insulator at a fine pitch, and signals are capacitively induced on these electrodes. At Brookhaven National Laboratory, we have designed and fabricated prototypes of AC-coupled LGAD sensors. The performance of small test structures with different particle beams from radioactive sources are shown.

Time-of-flight spectrometer for efficient coincidence detection of low-energy electrons from surfaces with 2π solid angle, pair recognition and unlimited band path (2e2πCS)

Time-of-flight spectrometer configurations combined with position sensitive single particle counting detectors have been simulated with a Monte Carlo code to find an efficient solution for coincident low energy electron detection. Photoemission experiments on polycrystalline silver have been performed at a synchrotron radiation facility with a likewise wide-angle ARPES (angle resolved photoemission spectroscopy) setup. Low-energy electron pairs created from secondary electron emission have been detected in coincidence obtaining their full momentum vectors.

From measuring electron charge to exploring particle-wave duality: A new didactic experimental approach

This paper points out the strong didactic potential of an experiment originally intended for electron source research. Using a dual stage micro-channel plate in a low-energy electron point projection microscope, electrons are spatially detected one by one. Field electron emission affords wide-ranging intensity, enabling the electron charge to be determined by comparing counting and analog measurements. The same setup is used to illustrate the buildup of an interference pattern from separate low-energy electron detection, thereby enabling the Planck constant to be determined. Both granular and continuous signals are directly observable in this educational approach.

Analysis and detection of low-energy electrons in scanning electron microscopes using a Bessel box electron energy analyser

The interpretation of images generated by scanning electron microscopes (SEMs) requires quantifiable and well-understood contrast. Furthermore, recent interest in probing samples using low-energy electrons to extract surface information is pushing towards the quantification of relative contrast in secondary-electron (SE) images. The detection and analysis of low-energy SEs remains at the heart of the development of such instruments and techniques. However, state-of-the-art SE detectors in most SEMs, which are largely using in-lens configurations, are mainly designed to favour the collection of primary-induced SEs, and hence fail to provide electron energy information that could be related to the sample composition. Here, we report on the development of a novel SE detector which makes use of the Bessel box (BB) electron energy analyser. We demonstrate a very compact analyser design (˜a volume of 12 mm3) through simulations and by acquiring an electron spectrum from a sample of graphene on copper. A resolution of 0.3% has been numerically computed and experimentally verified for the BB analyser.

First-Surface Scintillator for Low Accelerating Voltage Scanning Electron Microscopy (SEM) Imaging.

Highly luminescent thin films of zinc tungstate (ZT) have been deposited on top of conventional scintillators (Yttrium Aluminum Perovskite, Yttrium Aluminum Garnet) for electron detection in order to replace the need for a top conducting layer, such as indium tin oxide (ITO) or aluminum, which is non-scintillating and electron absorbing. Such conventional conducting layers serve the single purpose of eliminating electrical charge build-up on the scintillator. The ZT film also eliminates charging, which has been verified by measuring the Duane-Hunt limit and electron emission versus accelerating voltage. The luminescent nature of the ZT film ensures effective detection of low energy electrons from the very top surface of the structure ZT/scintillator, which we call "first-surface scintillator". The cathodoluminescence has been measured directly with a photodetector and spectrally resolved at different accelerating voltages. All results demonstrate the extended range of operation of the first-surface scintillator, while the conventional scintillators with a top ITO layer decline below 5 kV and have practically no output below 2 kV. Scintillators of different types were integrated in a detection system for backscattered electrons (BSE). The quality of the image at high accelerating voltages is comparable with the conventional scintillator and commercial BSE detector, while the image quality at 1 kV from the first-surface scintillator is superior.

High spatial resolution detection of low-energy electrons using an event-counting method, application to point projection microscopy.

An event-counting method using a two-microchannel plate stack in a low-energy electron point projection microscope is implemented. 15 μm detector spatial resolution, i.e., the distance between first-neighbor microchannels, is demonstrated. This leads to a 7 times better microscope resolution. Compared to previous work with neutrons [Tremsin et al., Nucl. Instrum. Methods Phys. Res., Sect. A 592, 374 (2008)], the large number of detection events achieved with electrons shows that the local response of the detector is mainly governed by the angle between the hexagonal structures of the two microchannel plates. Using this method in point projection microscopy offers the prospect of working with a greater source-object distance (350 nm instead of 50 nm), advancing toward atomic resolution.

PureB single-photon avalanche diodes for low-energy electron detection down to 200 eV.

Single-photon avalanche diodes (SPADs) have been fabricated on Si using PureB (Pure Boron) chemical-vapor deposition (CVD) to create both a nanometer-thin anode junction and a robust front-entrance window. The device shows high sensitivity to low radiation levels of electrons with energies down to 200eV when measurements are performed at room temperature where the dark count rate can be as low as 10Hz. An implicit guard ring, using an n-enhancement implantation in the central region of the diode, is applied, and this gives a very uniform sensitivity across the whole front-entrance window.

Direct Detection of Low-Energy Electrons With a Novel CMOS APS Sensor

Direct detection of low-energy electrons (500-2000 eV) with a novel back-thinned CMOS active pixel sensor (APS) is reported in this paper. The sensor was installed in a JEOL 6400F scanning electron microscope to quantitatively test its linearity and spatial resolution by a focused electron beam. The obtained results show good linearity at electron beam energy values from 500 to 2000 eV. The full-width at half-maximum spatial resolution was around 2 pixels, a limit set by charge diffusion in the epilayer. These results show that this CMOS APS sensor is appropriate for low-energy electron detection providing the benefits of direct detection for many applications.

X-ray and electron response of 4H-SiC vertical interdigitated Schottky photodiodes

We report on the X-ray and electron performance of a low voltage reverse biased 4H-SiC vertical Schottky photodiode, based on the pinch-off surface effect, obtained by means of self-aligned Nickel Silicide (Ni2Si) interdigitated contacts. Although these photodiodes were original designed for UV detection their open Schottky contact is attractive for soft X-ray and low energy electron detection. The open electrode structure of the device allows the incident radiation to reach the active layer with minimal absorption. The photon counting spectroscopy response for soft X-rays is reported and compared to those predicted from a Monte Carlo model for SiC devices. We also show these devices can detect beta particles from 3H and 14C radioisotope sources with a combined energy range 1–156 keV.

Direct Detection of 100–5000 eV Electrons With Delta-Doped Silicon CMOS and Electron-Multiplying CCD Imagers

We have demonstrated a direct detection of 100-5000 eV electrons with a back-illuminated boron delta-doped hybrid silicon complementary metal-oxide-semiconductor imager operating in full depletion and a silicon electron-multiplying charge-coupled device (CCD) operating in partial depletion. The delta-doping molecular beam epitaxy increases sensitivity to low-energy electrons and improves low-energy electron detection threshold relative to conventional solid-state detectors. We compare the gain measured in these two delta-doped devices with gain measured from control delta-doped CCDs.

Boron-layer silicon photodiodes for high-efficiency low-energy electron detection

Silicon photodiodes for use as low-energy electron detectors have been fabricated using a pure-boron technology to form the p+-anode region. The diode I–V characteristics are ideal and uniform over the wafer with low dark currents in the range of 0.6pA/mm2. An extremely small thickness of the front-entrance window dead layers was achieved for a 1.8nm B-layer deposition at 700°C. All other processing layers on the photosensitive surface were removed using selective wet-etching to the B-layer, a process which is studied here with respect to residues and pitting effects that can result from the etching of Al to the B-layer. For the most optimal photodiode, a high relative electron signal gain is obtained: 60% at 500eV, and 74% at 1keV. The degradation of the dark current of B-layer photodiodes is examined for 10-min-long irradiation with 1–25keV electron energies and stable performance is observed provided that the perimeter isolation-oxide is not exposed.

A time-of-flight spectrometer for angle-resolved detection of low energy electrons in two dimensions

We have developed a time-of-flight photoelectron spectrometer that simultaneously analyzes low energy electrons photoemitted from solid surfaces in an energy- and angle-resolved manner. To achieve this, a field free drift tube with an acceptance angle of 22° is combined with two-dimensional position-sensitive detection of photoelectrons, which is realized by a microchannel plate stack and a delay-line anode for position encoding. Here, we present the design considerations and principles of operation including analysis of multiple events per light pulse. The performance of the spectrometer is demonstrated by photoemission from a Cu(111) single crystalline surface by UV femtosecond laser pulses at 6.2 eV photon energy.

Microchannel plates at high rates: The challenges for future space plasma missions

Microchannel plates (MCPs) are currently the preferred sensor element for the detection of low-energy (∼0–30keV) electrons and ions in space plasma instruments, and are the base line technology for several proposed space missions. With a number of such missions currently in operation such as Cluster, Cassini, Double Star, and Mars Express using MCPs, there is a large volume of performance data, both from ground tests as well as in flight. From these experiences, it is clear that the current state of the art limits instrument performance significantly, in particular, in two areas; at high rates and the instrument lifetime. The requirements for several future missions are a lot more demanding and the push towards miniaturized and fast instruments, possibly requiring imaging at very high rates, and missions to extreme environments, presents a formidable challenge for the detector technology required. This article discusses details of the high rate performance of some of the plasma instruments, requirements of the future missions, and some of the solutions that are under study. The technology leap provided by recent advancements in microfabrication and nanofabrication techniques presents several opportunities for exploiting them to address these requirements with certain approaches. The challenges such devices face are discussed.

CMOS Monolithic Active Pixel Sensors (MAPS): New ‘eyes’ for science

Re-invented in the early 1990s on both sides of the Atlantic, Monolithic Active Pixel Sensors (MAPS) in a CMOS technology have slowly invaded the world of consumer imaging and are now on the edge of becoming the first technology in this field, previously dominated by Charge-Coupled Devices (CCD). Thanks to the advantages brought by the use of standard CMOS technology, MAPS have great potential in many areas including function integration, leading to the concept of a camera-on-a-chip, pixel size, random access to selected region-of-interest, low power, higher speed and radiation resistance. In many ways, MAPS have introduced a new way of doing imaging. Despite their success in the consumer arena, MAPS are still to make a definitive impact in the world of scientific imaging. This paper first briefly reviews the way radiation is detected by a CMOS sensor, before analysing the main noise source and its relationship with the full well capacity and the dynamic range. This paper will also show first examples of scientific results, obtained in the detection of low-energy electrons.

High-resolution, back-side illuminated monolithic active pixel sensor for low-energy electron imaging

A new domain of application of monolithic active pixel sensors, with respect to particle tracking, has been triggered by the innovative idea of a nondestructive beam monitoring system in the extraction lines of a hadron-therapy center. The beam monitoring exploits secondary electrons emitted from a submicrometer thick Al foil, intersecting the beam. Electrons are accelerated in an electrostatic field. The detection of low-energy electrons, up to 30 keV, is required. The sensitivity to these energies is obtained by thinning the detector, originally fabricated in a standard VLSI technology, down to the thickness of the radiation sensitive layer. A thin entrance window, in the order of 100 nm, is provided. Monolithic active pixel sensors for low-energy electron imaging can be prospectively used in several domains: in bioscience for cell process study using radiotracers, e.g., /sup 3/H(18 keV/spl beta//sup -/) or fluorescence imaging exploiting the Hybrid Photodiodes principle, in safety or environmental studies for neutron imaging with converters directly deposited or in micro-beam facilities for position resolving in studies of living cell irradiations. The low-energy electron imaging capabilities for installation inside an HPD test facility and the results obtained with a /sup 3/H marked source are shown. The detector used is the 1/spl times/10/sup 6/ pixel MIMOSA V chip. The back-thinning up to the epitaxial layer was applied, resulting in a high resolution, back-side illuminated imager.