Photoemission Microscopy Research Articles

Surface potential dependence of the topological insulator Bi[formula omitted]Se[formula omitted] studied by scanning photoemission and Kelvin probe microscopy

The atomic composition and electronic structure of topological insulators (TI) Bi2Se3 grown by the two-temperature Physical Vapor Deposition on n-type Si patterned substrates, have been followed in details during slight variations of the growth temperatures and values of thickness close to that of the topological transition (about 6 nm), by means of submicron lateral probe photoemission with enhanced surface sensitivity (less than 1 nm). Raman and atomic force microscopy together with Kelvin probe microscopy provide a coherent picture where the fine tuning of Se doping and of the growth temperatures constrain the on–set of the gapless state at the exact middle point of the gap.

Local bandgap narrowing in the forming state of threshold switching materials

Threshold switching (TS) materials, such as amorphous chalcogenide, have received significant attention for their application in storage class memory and in-memory computing. These materials contribute to efficient data processing and reduced power consumption in data centers. The initial switching process after fabricating a TS device, known as “forming,” has a profound impact on its subsequent TS behavior. However, it remains unclear how TS materials undergo changes in their atomic and electronic structures during the forming process. Consequently, the key factors that govern TS behavior remain obscure, necessitating a deeper understanding of the underlying physics behind TS phenomena. In this Letter, we investigated the forming state of the TS material AlTeN by combining scanning internal photoemission microscopy (SIPM) and ab initio calculations. Thanks to nondestructive evaluation by SIPM measurements, we observed local bandgap narrowing of AlTeN after its forming process. This is an experimental demonstration showing the presence of nuclei of the conductive filament formed in its ON state. Moreover, we conducted an ab initio calculation to reveal the origin of bandgap narrowing. We applied strong electrothermal stresses to the AlTeN model by ab initio molecular dynamics simulation with high electronic and lattice temperatures. By quenching from the electrothermal stress conditions, we reproduced an experimentally observed forming state with a narrowed bandgap. Analysis of the electronic structures of the forming state revealed that the origin of bandgap narrowing is the generation of the valence band top and conduction band bottom stemming from the increased homopolar bonds.

Visualization of spin-polarized electronic states by imaging-type spin-resolved photoemission microscopy

Visualization of spin-polarized electronic states by imaging-type spin-resolved photoemission microscopy

Photoelectrical characterization of heavily doped p-SiC Schottky contacts

The availability of photoelectrical characterizations of heavily Al-doped p-SiC Schottky contacts was clarified. We conducted a systematic study of four samples with different Al doping concentrations from 1 × 1018 to 5 × 1019 cm−3. Although the current–voltage (I–V) characteristics had lost rectification, reasonable Schottky barrier height (qϕ B) values were obtained up to 1 × 1019 cm−3 by capacitance voltage, photo response, and scanning internal photoemission microscopy (SIPM) measurements. In the two-dimensional characterization by SIPM, large photocurrent spots corresponding with low qϕ B were observed in an average density of 103 to 104 cm−2. However, except for these spots, a high uniformity of about 2 meV standard deviation was obtained for qϕ B over the entire observed electrodes. These results indicate that SIPM is able to characterize the inhomogeneity of heavily doped p-SiC contacts with very leaky I–V characteristics.

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.

Momentum space separation of quantum path interferences between photons and surface plasmon polaritons in nonlinear photoemission microscopy

Abstract Quantum path interferences occur whenever multiple equivalent and coherent transitions result in a common final state. Such interferences strongly modify the probability of a particle to be found in that final state, a key concept of quantum coherent control. When multiple nonlinear and energy-degenerate transitions occur in a system, the multitude of possible quantum path interferences is hard to disentangle experimentally. Here, we analyze quantum path interferences during the nonlinear emission of electrons from hybrid plasmonic and photonic fields using time-resolved photoemission electron microscopy. We experimentally distinguish quantum path interferences by exploiting the momentum difference between photons and plasmons and through balancing the relative contributions of their respective fields. Our work provides a fundamental understanding of the nonlinear photon–plasmon–electron interaction. Distinguishing emission processes in momentum space, as introduced here, could allow nano-optical quantum-correlations to be studied without destroying the quantum path interferences.

Using wafer scale ferroelectric domains of LiNbO3 to form permanent planar p–n junction in narrow band gap nanocrystals

p–n junctions based on nanocrystals now serve as fundamental components in optoelectronics. However, the process of designing these p–n junctions has largely relied on empirical choices, either for ligand exchange or for the selection of charge transport layers. Therefore, a systematic strategy is still lacking. In this study, we explore the utilization of ferroelectric domains as a general method for remotely inducing the formation of a p–n junction. To ensure compatibility with devices of various designs, we employ a commercially available periodically poled LiNbO3 (PPLN) substrate commonly used in nonlinear optics. We engineer a PPLN/graphene/HgTe heterostructure and demonstrate its planar photodiode behavior. Through x-ray photoemission microscopy, we confirm that the rectifying behavior stems from the influence of the ferroelectric domains, by ruling out the possibility of the formation of non-ohmic contacts at the electrode/semiconductor interfaces. This approach proves to be quite general and holds promise for the future design of high-speed nanocrystal-based photodiodes.

Resistive Switching Systems: A Spectromicroscopy Approach

Memristive effects, i.e., changes in the conductivity of a nonlinear two‐terminal device after a voltage or current pulse, are of high interest for applications in novel information technology. A crucial step in controlling memristive effects, particularly in oxide‐based systems, involves the understanding of chemical and redox‐induced changes in the material during resistive switching processes. Depending on the material, the switching may occur in an areal or filamentary fashion. In both cases, the resulting changes need to be monitored by high‐resolution techniques. In this contribution, photoemission microscopy approaches are focused and the use and limitations of these techniques for the detailed analysis of memristive elements are discussed.

Angle-resolved photoelectron spectroscopy in a low-energy electron microscope.

Spectroscopic photoemission microscopy is a well-established method to investigate the electronic structure of surfaces. In modern photoemission microscopes, the electron optics allow imaging of the image plane, momentum plane, or dispersive plane, depending on the lens setting. Furthermore, apertures allow filtering of energy-, real-, and momentum space. Here, we describe how a standard spectroscopic and low-energy electron microscope can be equipped with an additional slit at the entrance of the already present hemispherical analyzer to enable an angle- and energy-resolved photoemission mode with micrometer spatial selectivity. We apply a photogrammetric calibration to correct for image distortions of the projective system behind the analyzer and present spectra recorded on Au(111) as a benchmark. Our approach makes data acquisition in energy-momentum space more efficient, which is a necessity for laser-based pump-probe photoemission microscopy with femtosecond time resolution.

Reveal Ultrafast Electron Relaxation across Sub-bands of Tellurium by Time- and Energy-Resolved Photoemission Microscopy.

Exploring ultrafast carrier dynamics is crucial for the materials' fundamental properties and device design. In this work, we employ time- and energy-resolved photoemission electron microscopy with tunable pump wavelengths from visible to near-infrared to reveal the ultrafast carrier dynamics of the elemental semiconductor tellurium. We find that two discrete sub-bands around the Γ point of the conduction band are involved in excited-state electron ultrafast relaxation and reveal that hot electrons first go through ultrafast intra sub-band cooling on a time scale of about 0.3 ps and then transfer from the higher sub-band to the lower one on a time scale of approximately 1 ps. Additionally, theoretical calculations reveal that the lower one has flat-band characteristics, possessing a large density of states and a long electron lifetime. Our work demonstrates that TR- and ER-PEEM with broad tunable pump wavelengths are powerful techniques in revealing the details of ultrafast carrier dynamics in time and energy domains.

The single power law relationship between coercivity and width in pointed composite element magnetic barcodes

Pointed magnetic elements are introduced as an improvement upon rectangular strips currently employed in composite element magnetic barcodes. The coercivity of these elements, as measured using the magneto-optic Kerr effect, is found to strictly adhere to a single power law relationship with the element width, where the power law exponent is dependent on the length of the pointed region and takes values between −0.98 and −0.91. The steeper gradients here, along with the absence of the crossover region seen in rectangular devices, present these structures as a strict improvement in terms of potential device applications. These improvements are found to be present for all structures where the pointed region is as long as, or longer than, the magnetic element is wide. The remanent magnetization configuration, imaged using photo-emission microscopy with contrast from x-ray magnetic circular dichroism (XMCD-PEEM), is compared to the results of micromagnetic simulations. It is found to cant inward in the pointed section of the strip, aligning with the edges of the point, pinning the magnetization and giving a consistent magnetization reversal behavior for all element widths investigated.

(Invited) Characterization of Metal/GaN Schottky Contacts - Review from the Early Days

This paper reviews innovation of metal/GaN contacts from the aspects of crystal quality, process technique, and basic understanding of the electrical characteristics. Five topics; (i) thermal degradation of refractory metal contacts, (ii) correlation between dislocations and electrical characteristics, (iii) exploring ideal metal/GaN interfaces by cleaving, surface treatment, and post metallization annealing, (iv) large-barrier p-GaN Schottky contacts by reducing Mg doping level, (v) two-dimensional characterization using our original method: scanning internal photoemission microscopy are described.

A Platform for Addressing Individual Magnetite Islands Grown Epitaxially on Ru(0001) and Manipulating Their Magnetic Domains.

We have grown high-quality magnetite micrometric islands on ruthenium stripes on sapphire through a combination of magnetron sputtering (Ru film), high-temperature molecular beam epitaxy (oxide islands), and optical lithography. The samples have been characterized by atomic force microscopy, Raman spectroscopy, X-ray absorption and magnetic circular dichroism in a photoemission microscope. The magnetic domains on the magnetite islands can be modified by the application of current pulses through the Ru stripes in combination with magnetic fields. The modification of the magnetic domains is explained by the Oersted field generated by the electrical current flowing through the stripes underneath the magnetite nanostructures. The fabrication method is applicable to a wide variety of rock salt and spinel oxides.

When microscopy meets soft X-ray at TLS and TPS

Microscopy using the soft X-ray as an illumination source can integrate spectroscopy into images. This capability was implemented at Taiwan Light Source (TLS) in the early 2000s through two photoelectron-based microscopy stations whose imaging optics is either zone-plate (TLS 09A1) or electron lens (TLS 05B2). The microscopy project was later expanded into multiple stations and beamlines at Taiwan Photon Source (TPS) to take advantage of TPS’s low emittance. In this report, we summarize the activities of soft X-ray microscopy at TLS, followed by the status of microscopy stations under construction at beamline TPS 27A and 39A. The new lineup at TPS is to meet the scientific challenges ahead while carrying the existing activities forward. Both photoemission and transmission X-ray microscopy will be available at TPS.

Unruh effect and Takagi's statistics inversion in strained graphene

We present a theoretical study of how a spatially varying quasiparticle velocity in honeycomb lattices, achievable using strained graphene or in engineered cold-atom optical lattices that have a spatial dependence to the local tunneling amplitude, can yield the Rindler Hamiltonian embodying an observer accelerating in Minkowski space-time. Within this setup, a sudden switch on of the spatially varying tunneling (or strain) yields a spontaneous production of electron-hole pairs, an analog version of the Unruh effect characterized by the Unruh temperature. We discuss how this thermal behavior, along with Takagi's statistics inversion, can manifest themselves in photoemission and scanning tunneling microscopy experiments. We also calculate the average electronic conductivity and find that it grows linearly with frequency $\ensuremath{\omega}$. Finally, we find that the total system energy at zero environment temperature looks like Planck's blackbody result for photons due to the aforementioned statistics inversion, whereas for an initial thermally excited state of fermions, the total internal energy undergoes stimulated particle reduction.

Mapping of ultra-high-pressure annealed n-GaN Schottky contacts using scanning internal photoemission microscopy

We characterized the effect of the ultra-high-pressure annealing (UHPA) on GaN surfaces using Au/Ni/n-GaN Schottky contacts. After the UHPA at 1573 K, the current-voltage (I–V) characteristics became very leaky and the photocurrent signal was significantly reduced due to out-diffusion of N atoms. Despite these leaky I–V characteristics, scanning internal photoemission microscopy (SIPM) revealed inhomogeneity with a variation of about 20% and a range of about 20 μm on the contacts in the photoyield (Y) distribution images. After the UHPA at 1673 K, the out-diffusion was locally enhanced and spots with large-Y-signal appeared. After the UHPA at 1753 K, we speculated that the segregated Ga atoms were etched away by surface treatments, and the spots disappeared. We confirmed that the SIPM method was a powerful tool to investigate the inhomogeneity of severely damaged GaN Schottky contacts.

Two-Photon Photoemission Spectroscopy and Microscopy for Electronic and Plasmonic Characterizations of Molecularly Designed Organic Surfaces.

Functional surfaces decorated with organic molecules and/or nanoclusters (NCs) composed of several tens of atoms are promising for use in future photoelectronic substrates, whose functionalities are governed by molecular local electronic/plasmonic excitations at the interfaces. Here, we combine two-photon photoemission spectroscopy (2P-PES) and microscopy (2P-PEEM) to investigate the local excited-state dynamics at organic surfaces functionalized with NCs. The 2P-PES and 2P-PEEM for organic fullerene (C60) layers on graphite and Au substrates demonstrated photophysical characterization of electronic and plasmonic properties, including propagating surface plasmon polaritons (SPPs). The SPP propagation at the Au interface buried by overlayered C60 can be visualized by Agn NC deposition, which enhances plasmon-induced hot electrons, where the threshold number of Ag atoms (n ≥ 9) for the plasmonic response is revealed by the size dependence of 2P-PES for Agn NCs on C60 layers.

Mapping the Energy Landscape from a Nanocrystal-Based Field Effect Transistor under Operation Using Nanobeam Photoemission Spectroscopy.

As the field of nanocrystal-based optoelectronics matures, more advanced techniques must be developed in order to reveal the electronic structure of nanocrystals, particularly with device-relevant conditions. So far, most of the efforts have been focused on optical spectroscopy, and electrochemistry where an absolute energy reference is required. Device optimization requires probing not only the pristine material but also the material in its actual environment (i.e., surrounded by a transport layer and an electrode, in the presence of an applied electric field). Here, we explored the use of photoemission microscopy as a strategy for operando investigation of NC-based devices. We demonstrate that the method can be applied to a variety of materials and device geometries. Finally, we show that it provides direct access to the metal-semiconductor interface band bending as well as the distance over which the gate effect propagates in field-effect transistors.

Study of Phase Composition in TiFe + 4 wt.% Zr Alloys by Scanning Photoemission Microscopy

The alloy TiFe is widely used as hydrogen storage material. However, the first hydrogenation is difficult. It was found that the addition of zirconium greatly improves the kinetic of first hydrogenation, but the mechanism is not well understood. In this paper, we report the use of scanning photoemission microscopy to investigate the composition and chemical state of the various phases present in this alloy and how they change upon hydrogenation/dehydrogenation. We found the presence of different oxide phases that were not seen by conventional SEM investigation. The nature of these oxides phases seems to change upon hydrogenation/dehydrogenation cycle. This indicates that oxide phases may play a more significant role in the hydrogen absorption as what was previously believed.

Inside a nanocrystal-based photodiode using photoemission microscopy.

As nanocrystal-based devices gain maturity, a comprehensive understanding of their electronic structure is necessary for further optimization. Most spectroscopic techniques typically examine pristine materials and disregard the coupling of the active material to its actual environment, the influence of an applied electric field, and possible illumination effects. Therefore, it is critical to develop tools that can probe device in situ and operando. Here, we explore photoemission microscopy as a tool to unveil the energy landscape of a HgTe NC-based photodiode. We propose a planar diode stack to facilitate surface-sensitive photoemission measurements. We demonstrate that the method gives direct quantification of the diode's built-in voltage. Furthermore, we discuss how it is affected by particle size and illumination. We show that combining SnO2 and Ag2Te as electron and hole transport layers is better suited for extended-short-wave infrared materials than materials with larger bandgaps. We also identify the effect of photodoping over the SnO2 layer and propose a strategy to overcome it. Given its simplicity, the method appears to be of utmost interest for screening diode design strategies.