Photoemission Effect Research Articles

Near-Infrared Polarization-Sensitive Detection by All-Si Plasmonic Hot Electron Detectors.

Polarization-sensitive optoelectronic detection has been achieved by an all-Si detector in the NIR range, based on plasmon hot electron generation/internal photoemission effect. An advanced architecture with a specially designed anisotropic metasurface was developed and structurally optimized for maximizing the internal quantum efficiency (IQE). Assisted by finite difference time domain (FDTD) simulations, the well-designed device exhibits a maximum optical absorption of 80% around 1.45 μm, corresponding to an optical discrimination ratio of 120. Optoelectronic measurements show the peak responsivity and detectivity of 51.2 mA/W and 8.05 × 1010 cm Hz1/2/W, respectively, at 1.45 μm. A high polarization photocurrent ratio of 35 nm is also achieved at 1.55 μm. Moreover, the optoelectronic response can be tuned by a back-gate bias. Last but not least, we built up a model for theoretically estimating the IQE, which provides instructive guidance for further enhancing the optoelectronic performance of hot electron detectors.

Importance of Electrostatic Effects for Interpretation of X‐ray Photoemission Spectra of Self‐Assembled Monolayers

AbstractThis paper reviews the relevant work regarding electrostatic effects in X‐ray photoemission from self‐assembled monolayers (SAMs) which are application‐relevant ultrathin molecular films, coupled over a suitable anchoring group to the substrate. Whereas, in most cases, the standard concept of chemical shift is fully sufficient to describe X‐ray photoelectron spectra of these systems, consideration of electrostatic effects is frequently necessary for their proper interpretation. Due to the insulator character of the SAM matrix, decoupled electronically from the substrate, the introduction of a dipolar ´sheet´ at the SAM‐substrate interface or within this matrix creates a potential discontinuity shifting the energy levels above the ´sheet´ with respect to those below it. This shift is reflected then in the matrix‐related spectra, resulting in shifts and splitting of the characteristic photoemission peaks. Several representative examples in this context are provided and discussed in detail. These examples and other relevant literature data underline the importance of electrostatic effects in photoemission and suggest that they should be considered on the equal footing as the chemical shift ones.

Near-infrared detection based on the excitation of hot electrons in Au/Si microcone array

Although the photoresponse cut-off wavelength of Si is about 1100 nm due to the Si bandgap energy, the internal photoemission effect (IPE) of the Au/Si junction in Schottky detector can extend the absorption wavelength, which makes it a promising candidate for the Si-based infrared detector. However, due to low light absorption, low photon–electron interaction, and poor electron injection efficiency, the near-infrared light detection efficiency of the Schottky detector is still insufficient. The synergistic effect of Si nano/microstructures with a strong light trapping effect and nanoscale Au films with surface plasmon enhanced absorption may provide an effective solution for improving the detection efficiency. In this paper, a large-area periodic Si microcone array covered by an Au film has successfully been fabricated by one-time dry etching based on the mature polystyrene microspheres lithography technique and vacuum thermal deposition, and its properties for hot electron-based near infrared photodetection are investigated. Optical measurements show that the 20 nm-thick Au covered Si microcone array exhibits a low reflectance and a strong absorption (about 85%) in wide wavelength range (900–2500 nm), and the detection responsivity can reach a value as high as 17.1 and 7.0 mA W−1 at 1200 and 1310 nm under the front illumination, and 35.9 mA W−1 at 1310 nm under the back illumination respectively. Three-dimensional finite difference time domain (3D-FDTD) simulation results show that the enhanced local electric field in the Au layer distributes near the air/Au interface under the front illumination and close to the Au/Si interface under the back illumination. The back illumination favors the injection of photo-generated hot electrons in Au layer into Si, which can explain the higher responsivity under the back illumination. Our research is expected to promote the practical application of Schottky photodetectors to Si-compatible near infrared photodetectors.

Charging of space debris in the LEO and GEO regions

We present theoretical estimates of the electrical charge and electrostatic surface potentials on debris objects orbiting in the LEO and GEO regions of the Earth's ionosphere. The estimates are obtained using analytic calculations based on an improved Orbital Motion Limited model as well as through numerical Particle-In-Cell simulations using the open-source SPIS code. A variety of shapes, sizes, and material compositions of the debris objects are considered, including HAMR objects with a high area-to-mass ratio and objects composed of insulating and conducting material patches. In the GEO region, we consider the charging arising from photo-emission effects and the impact of energetic charged particle beams associated with solar flares. We discuss the nature of the debris-induced collective excitations in the plasma and their potential use for debris detection.

Two‐Dimensional Characterization of Au/Ni/Thin Heavily‐Mg‐Doped p‐/n‐GaN Structure under Applied Voltage by Scanning Internal Photoemission Microscopy

The two‐dimensional characterization of the Au/Ni/thin p+‐GaN/n−‐GaN structure, which is a part of a junction barrier Schottky structure, is demonstrated by scanning internal photoemission microscopy (SIPM) method. In the SIPM photoyield image using a blue laser, small photocurrents based on the internal photoemission effect by electron injection across the metal/semiconductor interface (from Ni to GaN) are detected in the center of the electrode. On the periphery, large photocurrents in the opposite direction are detected. These currents are caused by the hole injection from Ni into p+‐GaN layer and the electrical field in the laterally extended depletion layer at the edge. By using a violet laser, large photocurrents based on the fundamental absorption generated in the depletion layer of the n−‐GaN layer are uniformly detected over the electrode. It is conformed that SIPM can be available for quasi‐three‐dimensional characterization of this structure.

Pulsed photoemission induced plasma breakdown

This article characterises the effects of cathode photoemission leading to electrical discharges in an argon gas. We perform breakdown experiments under pulsed laser illumination of a flat cathode and observe Townsend to glow discharge transitions. The breakdown process is recorded by high-speed imaging, and time-dependent voltage and current across the electrode gap are measured for different reduced electric fields and laser intensities. We employ a 0D transient discharge model to interpret the experimental measurements. The fitted values of transferred photoelectron charge are compared with calculations from a quantum model of photoemission. The breakdown voltage is found to be lower with photoemission than without. When the applied voltage is insufficient for ion-induced secondary electron emission to sustain the plasma, laser driven photoemission can still create a breakdown where a sheath (i.e. a region near the electrode surfaces consisting of positive ions and neutrals) is formed. This photoemission induced plasma persists and decays on a much longer time scale ( s) than the laser pulse length ( ps). The effects of different applied voltages and laser energies on the breakdown voltage and current waveforms are investigated. The discharge model can accurately predict the measured breakdown voltage curves, despite the existence of discrepancy in quantitatively describing the transient discharge current and voltage waveforms.

Mono- and Bilayer Graphene/Silicon Photodetectors Based on Optical Microcavities Formed by Metallic and Double Silicon-on-Insulator Reflectors: A Theoretical Investigation.

In this work, we theoretically investigate a graphene/silicon Schottky photodetector operating at 1550 nm whose performance is enhanced by interference phenomena occurring inside an innovative Fabry-Pèrot optical microcavity. The structure consists of a hydrogenated amorphous silicon/graphene/crystalline silicon three-layer realized on the top of a double silicon-on-insulator substrate working as a high-reflectivity input mirror. The detection mechanism is based on the internal photoemission effect, and the light-matter interaction is maximized through the concept of confined mode, exploited by embedding the absorbing layer within the photonic structure. The novelty lies in the use of a thick layer of gold as an output reflector. The combination of the amorphous silicon and the metallic mirror is conceived to strongly simplify the manufacturing process by using standard microelectronic technology. Configurations based on both monolayer and bilayer graphene are investigated to optimize the structure in terms of responsivity, bandwidth, and noise-equivalent power. The theoretical results are discussed and compared with the state-of-the-art of similar devices.

Geometry-Induced Spin Filtering in Photoemission Maps from WTe_{2} Surface States.

We demonstrate that an important quantum material WTe_{2} exhibits a new type of geometry-induced spin filtering effect in photoemission, stemming from low symmetry that is responsible for its exotic transport properties. Through the laser-driven spin-polarized angle-resolved photoemission Fermi surface mapping, we showcase highly asymmetric spin textures of electrons photoemitted from the surface states of WTe_{2}. Such asymmetries are not present in the initial state spin textures, which are bound by the time-reversal and crystal lattice mirror plane symmetries. The findings are reproduced qualitatively by theoretical modeling within the one-step model photoemission formalism. The effect could be understood within the free-electron final state model as an interference due to emission from different atomic sites. The observed effect is a manifestation of time-reversal symmetry breaking of the initial state in the photoemission process, and as such it cannot be eliminated, but only its magnitude influenced, by special experimental geometries.

Magic angle HAXPES

The use of higher energy X-ray sources for photoelectron spectroscopy is receiving considerable attention due to the increased availability of laboratory-based instrumentation and an improved insight into the structures and interfacial properties of technological materials. In traditional X-ray photoelectron spectroscopy the design of the instrument often compensates for anisotropy in photoelectron emission through consideration of the angles between the X-ray source and the electron analyser. X-ray polarisation and non-dipole effects in photoemission are usually assumed to be negligible. However, for high energy XPS (HAXPES) both may be significant. Polarisation at synchrotron sources is an important consideration and non-dipole effects are generally more significant at higher photon energies. In this article we demonstrate that, for certain polarisations, ‘magic angle’ geometries exist that minimise the effects of both dipole and non-dipole contributions in photoemission. However, it is not possible to find such geometries for unpolarised X-rays; achieving a ‘magic angle’ geometry in HAXPES requires the X-rays to have a degree of linear polarisation of 1/3 or greater.

Ga2O3 metal–insulator-semiconductor solar-blind photodiodes with plasmon-enhanced responsivity and suppressed internal photoemission

Metal-semiconductor-metal (MSM) architectures are popular for achieving high-responsivity Ga2O3 solar-blind photodetectors (SBPDs), however, the hot-electron-induced internal photoemission (IPE) effect restricts their detecting performance. Herein, we demonstrate the rational design of an Al/Al2O3/Ga2O3 metal–insulator-semiconductor (MIS) SBPD that has merits of enhanced responsivity, suppressed sub-gap response and ultralow dark current based on the simulation results obtained using Lumerical software. For the cylindrical patterned detectors with Al/Al2O3/Ga2O3 MIS structures, the optimized dimensions of Al electrodes with a conformed ultra-thin (2 nm) Al2O3 layer support the surface plasmon polariton resonances at 250 nm, thus improving the photoresponsivity to 74 mA W−1. Furthermore, the sandwiched Al2O3 layer lifts the barrier for hot electrons in electrodes, which significantly suppresses the IPE-induced sub-gap photoresponse by more than 105 in magnitude with respect to the Al/Ga2O3 MSM counterpart. Optical and electrical field distributions are overlapped in cylindrically patterned MIS detectors, simultaneously improving the excitation and collection efficiencies of excess carriers and resulting in the 103-boosted rejection ratio.

Modeling and Simulation of Si Grating Photodetector Fabricated Using MACE Method for NIR Spectrum

In this research, we modeled a silicon-based photodetector for the NIR-IR spectrum using a grating structure fabricated using the metal-assisted chemical etching method. A nanostructure fabricated by using this method is free of defects such as unwanted sidewall metal depositions. The device is simulated using Lumerical finite difference time domain (FDTD) for optical characteristics and Lumerical CHARGE for electrical characteristics. First, we optimized the grating structure duty cycle parameter for maximum optical power absorption using the particle swarm optimization algorithm provided in Lumerical FDTD, and then used the optimized parameter for our simulations. From Lumerical FDTD simulations, we found that the Cr masker metal used in the fabrication process acts as a resonant cavity and a potential candidate for internal photo emission (IPE) effects. By using Lumerical CHARGE, we performed electrical simulation and by adding the IPE calculation we found that at 850 nm wavelength the Si grating photodetector device exhibited 19 mA/W responsivity and detectivity of 2.62 × 106 Jones for −1 volt operating voltage.

Physically and chemically smooth cesium-antimonide photocathodes on single crystal strontium titanate substrates

The performance of x-ray free electron lasers and ultrafast electron diffraction experiments is largely dependent on the brightness of electron sources from photoinjectors. The maximum brightness from photoinjectors at a particular accelerating gradient is limited by the mean transverse energy (MTE) of electrons emitted from photocathodes. For high quantum efficiency (QE) cathodes like alkali-antimonide thin films, which are essential to mitigate the effects of non-linear photoemission on MTE, the smallest possible MTE and, hence, the highest possible brightness are limited by the nanoscale surface roughness and chemical inhomogeneity. In this work, we show that high QE Cs3Sb films grown on lattice-matched strontium titanate (STO) substrates have a factor of 4 smoother, chemically uniform surfaces compared to those traditionally grown on disordered Si surfaces. We perform simulations to calculate roughness induced MTE based on measured topographical and surface-potential variations on the Cs3Sb films grown on STO and show that these variations are small enough to have no consequential impact on the MTE and, hence, the brightness.

Light-Induced Magnetization at the Nanoscale.

Triggering and switching magnetic moments is of key importance for applications ranging from spintronics to quantum information. A noninvasive ultrafast control at the nanoscale is, however, an open challenge. Here, we propose a novel laser-based scheme for generating atomic-scale charge current loops within femtoseconds. The associated orbital magnetic moments remain ferromagnetically aligned after the laser pulses have ceased and are localized within an area that is tunable via laser parameters and can be chosen to be well below the diffraction limit of the driving laser field. The scheme relies on tuning the phase, polarization, and intensities of two copropagating Gaussian and vortex laser pulses, allowing us to control the spatial extent, direction, and strength of the atomic-scale charge current loops induced in the irradiated sample upon photon absorption. In the experiment we used He atoms driven by an ultraviolet and infrared vortex-beam laser pulses to generate current-carrying Rydberg states and test for the generated magnetic moments via dichroic effects in photoemission. Abinitio quantum dynamic simulations and analysis confirm the proposed scenario and provide a quantitative estimate of the generated local moments.

A self-powered solar-blind photodetector based on polyaniline/α-Ga2O3 p–n heterojunction

In this work, we demonstrated the self-powered solar-blind photodetector based on a polyaniline/α-Ga2O3 hybrid heterojunction. The resultant device exhibited distinct self-power characteristics with a peak photoresponsivity (R) of 8.2 mA/W, a UVC (UV light of wavelength range at 200–280 nm)/UVA (UV light of wavelength range at 320–400 nm) rejection ratio (R220 nm/R400 nm) of 2.97 × 104, and a response decay time (τdec) of 176 μs at zero bias. With an elevated bias to 5 V, the dark current remained in an ultralow level of 0.21 pA, while the rejection ratio and τdec were improved to be 7.13 × 104 and 153 μs, respectively, together with the corresponding external quantum efficiency of 38.4% and a detectivity of 6.63 × 1013 Jones. Thanks to the dual functions of bandpass transmission in the deep-ultraviolet spectral region and the hole spreading transport of the polyaniline layer, the responsivity to the visible light is suppressed with the negligible internal photoemission effect, thus leading to the improved rejection ratio. Furthermore, weak interface interactions in such polyaniline/α-Ga2O3 organic–inorganic hybrid systems avoid the introduction of interfacial trapping centers by lattice mismatch and the stabilization of negatively charged anions (O2−) by (−NH2)-+species in polyaniline deactivate oxygen vacancies at the α-Ga2O3 surface, both of which lead to the negligible persistent photoconductivity effect. As a result, in the aid of the interfacial built-in field, the constructed hybrid heterojunction exhibited a self-powered detecting characteristic and a fast response speed. These findings verify the feasibility of delivering high performance photodetectors by implementing the inorganic/organic hybrid bipolar device design to overcome the difficulty in p-type Ga2O3.

Evolution of Secondary Electrons Emission During EUV Exposure in Photoresists

Low energy electronic processes are key to chemical reactions during exposure of photoresists in extreme ultraviolet lithography. To understand and optimize the functionality of photoresists, it is of paramount important to assess the magnitude of the electron distribution inside the material. While photoemission spectroscopy is being widely used, vacuum barrier crossing and nonideal photoemissivity alter the measured spectra in a way that prevents meaningful assessment of very low energy electrons. In this work we propose a model to account for the physics of photoemission effects and to reliably estimate the distribution from solid state matter. The model also provides a quantitative value for the occupation function and density of state. We tested the model on the photoemission spectra acquired on a prototype EUV photoresist and report here the results.

Attosecond intra-valence band dynamics and resonant-photoemission delays in W(110)

Time-resolved photoelectron spectroscopy with attosecond precision provides new insights into the photoelectric effect and gives information about the timing of photoemission from different electronic states within the electronic band structure of solids. Electron transport, scattering phenomena and electron-electron correlation effects can be observed on attosecond time scales by timing photoemission from valence band states against that from core states. However, accessing intraband effects was so far particularly challenging due to the simultaneous requirements on energy, momentum and time resolution. Here we report on an experiment utilizing intracavity generated attosecond pulse trains to meet these demands at high flux and high photon energies to measure intraband delays between sp- and d-band states in the valence band photoemission from tungsten and investigate final-state effects in resonant photoemission.

Repression of Interlayer Recombination by Graphene Generates a Sensitive Nanostructured 2D vdW Heterostructure Based Photodetector.

Great success in 2D van der Waals (vdW) heterostructures based photodetectors is obtained owing to the unique electronic and optoelectronic properties of 2D materials. Performance of photodetectors based 2D vdW heterojunctions at atomic scale is more sensitive to the nanointerface of the heterojunction than conventional bulk heterojunction. Here, a nanoengineered heterostructure for the first‐time demonstration of a nanointerface using an inserted graphene layer between black phosphorus (BP) and InSe which inhibits interlayer recombination and greatly improves photodetection performances is presented. In addition, a transition of the transport characteristics of the device is induced by graphene, from diffusion motion of minority carriers to drift motion of majority carriers. These two reasons together with an internal photoemission effect make the BP/G/InSe‐based photodetector have ultrahigh specific detectivity at room temperature. The results demonstrate that high‐performance vdW heterostructure photodetectors can be achieved through simple structural manipulation of the heterojunction interface on nanoscale.

Fano resonance enhanced multiphoton photoemission from single plasmonic nanostructure excited by femtosecond laser

High-flux photoelectron pulses with femto-nano spatiotemporal properties enhanced with plasmonic metallic nanostructures can provide possibilities for potential photoelectron-based devices. In this work, a large multiphoton photoemission enhancement in a single plasmonic Au-heptamer nanostructure, which supports the Fano resonance, is investigated using photoemission electron microscopy (PEEM). By exploiting the high spatial PEEM resolution and performing calculations, it is found that the photoemission electrons are mainly emitted from the junction area between the lower surface of the nanostructure and the indium tin oxide film on the substrate. It is also observed that photoemission is strongly affected by the Coulomb field. Specifically, the strong photoemission locates in the gap region with the Coulomb attraction rather than Coulomb repulsion, even though the latter exhibits a higher charge density. The measured photoelectron enhancement achieved by employing a Fano-resonant Au heptamer is supported by calculations, combining the photoemission effects of both near-field intensity in the lower surface and nanostructure absorption.

Al Kα XPS reference spectra of polyethylene for all instrument geometries

This paper extends a previous description of XPS survey spectra from low density polyethylene (LDPE), which was specific for a single type of geometry, to all geometries. Instrument geometries are specified by two angles. The first angle, a, is between the sample-to-monochromator vector and the sample-to-analyzer vector. The second angle, b, is the dihedral angle between the anode-monochromator-sample plane and the monochromator-sample-analyzer plane. The second angle is important because of the polarization induced by the monochromator. We show, using theory, that the XPS spectrum can be decomposed into a “magic angle” reference spectrum, I1, and an anisotropy correction spectrum, f. The geometry for LDPE at which photoemission intensity is equivalent to isotropic emission is shown to be a function of a and b with extreme values for a of 64.6° (b = 0 or 180°) and 57.5° (b = 90°). The deviation of these angles from the “magic angle” a = 54.7° is due to a combination of x-ray polarization and nondipole effects in photoemission. Intensity-calibrated data from a number of instruments with two geometries with b = 180°, one set with a = 60° and the other set with a = 45° are used to determine I1 and f, and these are fitted with simple functions to allow the reproduction of LDPE reference spectra for any instrument geometry. The spectra are taken from the Versailles Project on Advanced Materials and Standards, Technical Working Area 2: Surface Chemical Analysis study A27 and are traceable to the National Physical Laboratory, UK intensity calibration spectra for argon ion sputter-cleaned gold. The functions in this paper may be used in the calibration of XPS instruments with quartz-crystal-monochromated Al Kα x-rays by the comparison of the calculated reference spectrum to data from clean LDPE.

Revealing Hidden Orbital Pseudospin Texture with Time-Reversal Dichroism in Photoelectron Angular Distributions.

We performed angle-resolved photoemission spectroscopy (ARPES) of bulk 2H-WSe_{2} for different crystal orientations linked to each other by time-reversal symmetry. We introduce a new observable called time-reversal dichroism in photoelectron angular distributions (TRDAD), which quantifies the modulation of the photoemission intensity upon effective time-reversal operation. We demonstrate that the hidden orbital pseudospin texture leaves its imprint on TRDAD, due to multiple orbital interference effects in photoemission. Our experimental results are in quantitative agreement with both the tight-binding model and state-of-the-art fully relativistic calculations performed using the one-step model of photoemission. While spin-resolved ARPES probes the spin component of entangled spin-orbital texture in multiorbital systems, we unambiguously demonstrate that TRDAD reveals its orbital pseudospin texture counterpart.