Reflection Electron Energy Loss Spectroscopy Research Articles

Epitaxial growth and band offsets of β -(ScxGa1− x)2O3 thin films grown on (100) β -Ga2O3 substrate

β-(ScxGa1−x)2O3 (x = 0–0.36) thin films were epitaxially grown on (100) β-Ga2O3 substrates by oxygen-radical-assisted pulsed-laser deposition. β-(ScxGa1−x)2O3 epilayers were coherently strained up to x = 0.30, although the presence of a structural disorder was implied when x > 0.2. The bandgap energies measured by reflection electron energy loss spectroscopy increased from 4.56 to 5.25 eV with increasing Sc content. In β-(ScxGa1−x)2O3 epilayers, a slightly negative bandgap bowing behavior with a bowing parameter of −0.4 eV was observed, resulting in a larger bandgap increase than in β-(AlxGa1−x)2O3 epilayers with identical x. X-ray photoemission spectroscopy measurement revealed that the valence-band and conduction-band offsets of β-(Sc0.17Ga0.83)2O3 epilayer with respect to β-Ga2O3 were 0.0 and 0.3 eV, respectively.

Advanced spectroscopic methods for probing in-gap defect states in amorphous SiNx for charge trap memory applications

Silicon nitride (SiNx) serves as the charge trap layer in current 3D NAND flash memory devices. The precise formation mechanism and electronic structure of localized defect trap states in SiNx remain elusive. Here, we present a refined experimental methodology to elucidate the in-gap defect states and the band gaps in amorphous SiNx thin films. Our approach integrates high-resolution reflection electron energy loss spectroscopy (REELS) and spectroscopic ellipsometry (SE) for comprehensive analysis. By systematical analysis, we aim to provide a robust method for determining in-gap electronic states in SiNx. We investigated two different SiNx films prepared by plasma-enhanced chemical vapor deposition and sputtering. Our analysis revealed several distinct in-gap states and determined band gap energies. This approach not only provide advanced spectroscopic methods to characterize the defect electronic states in SiNx, but also applicable to other large band gap semiconductors or dielectrics to predict device-level characteristics for future devices.

In situ photoemission spectroscopies to reveal surface transfer doping on hydrogenated milled nanodiamonds

Nanodiamonds (ND) exhibit exceptional chemical, electronic, thermal, and optical properties, making them valuable for applications in nanomedicine, energy, quantum technologies, advanced lubricants, and polymer composites. Surface analysis techniques such as X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) and reflection electron energy loss spectroscopy (REELS) are critical for understanding the surface properties of nanomaterials, including nanodiamonds. This study investigates hydrogenated milled nanodiamonds (H-MND) by integrating UPS and XPS measurements with REELS. Through in situ annealing within an ultra-high vacuum (UHV) chamber, we examine the impact of surface termination on surface conductivity, focusing on the role of adsorbates. Our findings reveal that a surface transfer doping mechanism, akin to that observed in bulk diamond, governs a pseudo p-type conductivity in H-MND. The conductivity dependence on ambient exposure, water, and subsequent annealing demonstrates its reversibility. The study also discusses the nature of electron acceptors and the influence of ND facets on conductivity.

Optical properties of hafnium-dioxide derived from reflection electron energy loss spectroscopy spectra

The optical properties of HfO2 have practical applications. As an important gate dielectric material with high-k dielectric, HfO2 is beneficial to reduce the leakage current of the complementary metal-oxide-semiconductor (CMOS) gate and improves the gate structure. HfO2 also plays a key role in optical coating materials field for its high transmittance in the ultraviolet to near-infrared band, and high laser damage threshold. Therefore, it is of practical significance for the accurate knowledge of optical properties of HfO2. Here we present the optical constants and dielectric function of HfO2 based on the energy loss function (ELF) in the energy loss range of 0–200 eV for nanocrystalline film. Our new data are derived from the analysis of the reflected electron energy loss spectra by using the latest version of our reverse Monte Carlo modelling. Sum rules are used to check the reliability of the new data. We have found that up to 30 eV there are three main peaks, and the first-principles calculation also confirms the three peak structures. A comparison with previous results indicates that the ELF peak positions of HfO2 with different crystal structures are similar, but the peak intensity and shape are different. The heterogenicity of the crystal structure of HfO2 leads to different measurement results for different samples.

Reduced trap state density in AlGaN/GaN HEMTs with low-temperature CVD-grown BN gate dielectric

In this Letter, low-temperature (400 °C) chemical vapor deposition-grown boron nitride (BN) was investigated as the gate dielectric for AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MISHEMTs) on a Si substrate. Comprehensive characterizations using x-ray photoelectron spectroscopy, reflection electron energy loss spectroscopy, atomic force microscope, high-resolution transmission electron microscopy, and time-of-flight secondary ion mass spectrometry were conducted to analyze the deposited BN dielectric. Compared with conventional Schottky-gate HEMTs, the MISHEMTs exhibited significantly enhanced performance with 3 orders of magnitude lower reverse gate leakage current, a lower off-state current of 1 × 10−7 mA/mm, a higher on/off current ratio of 108, and lower on-resistance of 5.40 Ω mm. The frequency-dependent conductance measurement was performed to analyze the BN/HEMT interface, unveiling a low interface trap state density (Dit) on the order of 5 × 1011–6 × 1011 cm−2 eV−1. This work shows the effectiveness of low-temperature BN dielectrics and their potential for advancing GaN MISHEMTs toward high-performance power and RF electronics applications.

Electron backscattering coefficients for Cr, Co, and Pd solids: A Monte Carlo simulation study

We have calculated electron backscattering coefficients, η(Ep), at primary electron energies Ep of 0.1–100 keV for three elemental and intermediate atomic number solids, Cr, Co and Pd, with an up-to-date Monte Carlo simulation model. A relativistic dielectric functional approach is adopted for the calculation of the electron inelastic cross section, where several different datasets of optical energy loss function (ELF) are adopted. The calculated backscattering coefficient is found to be substantially affected by the ELF, where the influence can be seen to follow the f- and ps-sum rules and the resultant energy dependence of electron inelastic mean free path. To understand the uncertainties involved in a comparison with experimental data both the theoretical uncertainty due to the elastic cross-section model and the experimental systematic error for the contaminated surfaces are investigated. A total of 192 different scattering potentials are employed for the calculation of Mott's electron elastic cross section and this theoretical uncertainty is confirmed to be small. On the other hand, the simulation of contaminated Co and Pd surfaces with several carbonaceous atomic layers can well explain the experimental data. The present results indicate that accurate backscattering coefficient data should be either measured from fully cleaned surfaces or obtained from modern Monte Carlo theoretical calculations involving reliable optical constants data. With the recent progress in the accurate measurement of optical constants by reflection electron energy loss spectroscopy technique, constructing a reliable theoretical database of electron backscattering coefficients for clean surfaces of elemental solids is highly hopeful.

Optical properties of InSb derived from reflection electron energy loss spectroscopy spectrum

The energy loss function (ELF) of the narrow bandgap semiconductor, InSb, was derived from the reflection electron energy loss spectroscopy (REELS) spectrum measured at 4 keV incident electron energy in an energy loss range of 1.6–200 eV using a high energy resolution electron spectrometer. The experimental spectrum was analyzed with the reverse Monte Carlo (RMC) method, which integrates the classical trajectory Monte Carlo simulation with the simulated annealing method for optimizing the trial ELF. Subsequently, the complex dielectric function ε(ω), refractive index and extinction coefficient were determined from the obtained ELF in the energy loss (i.e. photon energy) range of 1.6–200 eV. The validity of the obtained data was verified by the f-sum rule, ps-sum rule, inertial sum rule and dc-conductivity sum rule. We found that our calculated data of InSb fulfill the sum rules with very high accuracy; therefore, the use of these calculated optical data in material science and surface analysis is highly recommended for further applications.

Advanced Fabrication of 3D Micro/Nanostructures of Gallium Oxide with a Tuned Band Gap and Optical Properties.

Gallium oxide (Ga2O3) is a promising material for high-power semiconductor applications due to its wide band gap and high breakdown voltage. However, the current methods for fabricating Ga2O3 nanostructures have several disadvantages, including their complex manufacturing processes and high costs. In this study, we report a novel approach for synthesizing β-Ga2O3 nanostructures on gallium phosphide (GaP) using ultra-short laser pulses for in situ nanostructure generation (ULPING). We varied the process parameters to optimize the nanostructure formation, finding that the ULPING method produces high-quality β-Ga2O3 nanostructures with a simpler and more cost-effective process when compared with existing methods. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were used to characterize the samples, which indicated the presence of phosphorous. X-ray photoelectron spectroscopy (XPS) confirmed the formation of gallium oxide, along with a minor amount of phosphorus-containing compounds. Structural analysis using X-ray diffraction (XRD) revealed the formation of a monoclinic β-polymorph of Ga2O3. We also measured the band gap of the materials using reflection electron energy loss spectroscopy (REELS), and found that the band gap increased with higher nanostructure formation, reaching 6.2 eV for the optimized sample. Furthermore, we observed a change in the heterojunction alignment, which we attribute to the change in the oxidation of the samples. Our results demonstrate the potential of ULPING as a novel, simple, and cost-effective method for fabricating Ga2O3 nanostructures with tunable optical properties. The ULPING method offers a green alternative to existing fabrication methods, making it a promising technology for future research in the field of Ga2O3 nanostructure fabrication.

Lithium Dendrite Propagation in Ta-Doped Li7La3Zr2O12 (LLZTO): Comparison of Reactively Sintered Pyrochlore-to-Garnet vs LLZTO by Solid-State Reaction and Conventional Sintering.

Ta-doped Li7La3Zr2O12 (LLZTO) garnet is a promising Li-ion-conducting ceramic electrolyte for solid-state batteries. However, it is still challenging to use LLZTO in Li metal batteries operating at high current densities because of the tendency for Li metal to nucleate and propagate along the grain boundaries. In this study, we carry out a detailed investigation to elucidate the effect of microstructure and grain size on the electrochemical properties and short circuit behavior in LLZTO. Pellets were prepared using reactive sintering from pyrochlore precursors (a method called pyrochlore-to-garnet, P2G) and compared with LLZTO synthesized using solid-state reaction (SSR) followed by conventional pressureless sintering. Both preparation methods were controlled to keep the phase and elemental composition, ionic and electronic conductivity, relative density, and area-specific resistance of the pellets constant. Reflection electron energy loss spectroscopy and X-ray photoelectron spectroscopy confirm that both types of LLZTO have similar band gaps and chemical states. Microstructure analysis shows that the P2G method results in LLZTO with an average grain size of around 3 μm, which is much smaller than the grain sizes (as large as 20 μm) seen in SSR LLZTO. Galvanostatic Li stripping/plating and linear sweep voltammetry measurements show that P2G LLZTO can withstand higher critical current densities (up to 0.4 mA/cm2 in bidirectional cycling and >1 mA/cm2 for unidirectional) than those seen in SSR LLZTO. Post-mortem examination reveals much less Li deposition along the grain boundaries of P2G LLZTO, particularly in the bulk of the pellet, compared to SSR LLZTO after cycling. The improved cycling behavior in P2G LLZTO despite the higher grain boundary area could be from more homogeneous current density at the interfaces and different grain boundary properties arising from the liquid-phase, reactive sintering method. These results suggest that the effect of grain size on Li dendrite propagation in LLZO may be highly dependent on the synthesis and sintering method employed.

Probing the Electronic Structure of Prussian Blue and Analog Films by Photoemission and Electron Energy Loss Spectroscopies.

X-ray photoelectron spectroscopy (XPS) and reflection electron energy loss spectroscopy (REELS) were employed to characterize the electronic properties of Prussian blue (PB) and its analogs when electrodeposited over metal-decorated carbon nanotubes (CNTs). Through an investigation of the influence of carbon nanotubes (CNTs) and preparation conditions on the electronic structure, valuable insights were obtained regarding their effects on electrochemical properties. XPS analysis enabled the probing of the chemical composition and oxidation states of the film materials, unveiling synthesis-driven variations in their electronic properties. REELS provided information on energy loss and electronic transitions, enabling further characterization of the changes in the electronic structure induced by different preparation methods. Such findings emphasize the importance of surface characterization to understand how the unique electronic properties of such materials can be harnessed to enhance their performance and functionality.

Determination of the energy loss function of tungsten from reflection electron energy loss spectroscopy spectra

We incorporate experimental reflection electron energy loss spectroscopy (REELS) spectrum data with theoretical analysis to precisely determine the energy loss function (ELF) of tungsten (W) in the energy loss range of 0.1–110 eV at the incident electron energies of 1 keV, 2 keV and 3 keV. Employing the reverse Monte Carlo (RMC) method, we have obtained an averaged ELF whose relative errors of the perfect-screening sum rule and oscillator-strength sum rule were ∼ 0.70 % and 0.68 %, respectively. This ELF was then used to derive the optical constants and complex dielectric function. Furthermore, we have successfully differentiated between bulk and surface contributions to the REELS spectra throughout the entire considered energy loss range.

Effects of Oxygen Partial Pressure on the Electronic Properties and Chemical State of MgO/SiO<sub>2</sub>/Si(100) Thin Films

Magnesium Oxide (MgO) thin films were deposited on SiO2/Si substrate by electron beam evaporation. The properties of MgO thin film with and without oxygen partial pressure have been investigated by X-ray photoelectron spectroscopy (XPS), Reflection Electron Energy Loss Spectroscopy (REELS), and Ultra-Violet Photoemission Spectroscopy (UPS). The XPS was used to investigate the chemical state of the film. REELS spectra revealed that MgO thin films deposited under oxygen partial pressure had band gaps of 6.07 eV. Meanwhile, the band gap for MgO thin films grown without oxygen partial pressure was 7.17 eV. The UPS results showed that the work functions of MgO thin film with and without oxygen partial pressure are 4.75 and 4.84 eV, respectively. In the MgO thin film with oxygen partial pressure, the intensity for the valence band peak at 12.16 eV decreased, but the work function remained relatively the same. Our results demonstrated that the oxygen partial pressure played a crucial role in improving the electronic properties of MgO thin films.

Mechanistic studies of Yb2O3/HAT-CN connection electrode in tandem semiconductor devices

The optically transparent connecting electrode is much desired in fabrication of tandem optoelectronic devices. Yet, optically transparent materials, such as oxides, are electrically insulating. In this work, we show that low work function oxides Yb2O3 combing with high work function 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) molecule can be used as effective connecting electrodes to make high performance tandem organic light emitting diodes with negligible voltage loss. For instance, in a tandem device with two emission zones, yielding a brightness of 100 cd/m2, the voltage required is 5.3 V, which is approximately twice that of a single emission zone device. To gain insights into the band alignment of this electrode, we conducted the measurements, including ultraviolet photoelectron spectroscopy to analyze the electronic structures of occupied valence and gap states and reflection electron energy loss spectroscopy to study the unoccupied states. To understand the charge transport and injection behavior of this electrode, we conducted variable temperature charge transport measurements. Our findings reveal the presence of localized gap states within the Yb2O3/HAT-CN structure. These gap states effectively form a conduction pathway for facilitating the transport of charge carriers. At higher temperatures (≥200 K), charge transport is primarily limited by the Efros–Shklovskii type of hopping conduction through the localized states in the Yb2O3. Conversely, at lower temperatures (<200 K), the electrical current is limited by the properties of HAT-CN. These discoveries suggest that localized gap states at the oxides/organic heterojunctions can be effectively utilized in the fabrication of tandem semiconductor devices.

Optimizing energy storage performance of ALD YSZ thin film devices via yttrium concentration variations

In the last decade, research has focused on fabricating new materials to accelerate the development of energy storage devices. In this work, metal-electrolyte-metal (Au-YSZ-Ru) structures were fabricated on silicon substrates using atomic layer deposition, sputtering, and thermal evaporation techniques. The effect of the yttrium concentration in YSZ films on their chemical, structural, optical, and electrical properties were studied. XPS analysis showed yttrium concentrations of 14.6, 10.3, 6.8, and 5.3 at.% for films with ALD cycle ratios Zr:Y of 2:1, 4:1, 6:1, and 8:1. Deconvolution of the oxygen XPS signal demonstrate the yttrium to oxygen vacancies ratio. The band gap was calculated through UV–vis indicating an increase in values with yttrium concentration. In addition, the trend was confirmed by reflection electron energy loss spectroscopy (REELS). The structure was investigated by performing X-ray diffraction (XRD) and infrared attenuated total reflectance spectroscopy (IR-ATR) measurements. Both results indicate a better crystallization of the cubic phase for the samples with lower concentrations, as the intensity of (111) diffraction peaks and IR band increased for lower yttrium concentrations. Impedance spectroscopy revealed an activation energy of 1.69, 1.32, and 1.28 eV for the 10.3, 6.8, and 5.3 at.% film concentration, respectively. According to cyclic voltammetry and charge-discharge tests, a lower yttrium concentration, 5.3 at.%, promotes electrode REDOX reactions improving the energy-storage properties.

Capabilities of a New Compact SEM / STEM Electron Detector for Energy Resolved Scanning Imaging, Reflection Electron Energy Loss Spectroscopy (REELS) and Elastic Peak Electron Spectroscopy (EPES).

Capabilities of a New Compact SEM / STEM Electron Detector for Energy Resolved Scanning Imaging, Reflection Electron Energy Loss Spectroscopy (REELS) and Elastic Peak Electron Spectroscopy (EPES).

Determination of electron inelastic mean free path and stopping power of hafnium dioxide

We present inelastic mean free path (IMFP) and stopping power data of the hafnium dioxide applying the relativistic dielectric response theory. The energy loss function (ELF) derived from reflection electron energy loss spectroscopy spectrum with the reverse Monte Carlo method was used for the first time to obtain the IMFP and stopping power of HfO2. The probability of the energy loss is determined by the dielectric response function εq,ω as a function of the frequency ω and the wavenumber q of the electromagnetic disturbance. Two algorithms, namely the full Penn algorithm (FPA) and the super-extended Mermin algorithm (SMA), were employed to expand the optical energy loss function, Im-1/ε0,ω, into the q,ω-plane. The results indicate that the IMFP and the stopping power obtained by using both algorithms are consistent at high electron energies, but show differences at low electron energies (less than ∼ 70 eV). This discrepancy arises from the consideration of the finite plasmon lifetimes effect in the SMA model, while it is neglected in the FPA model. Additionally, we observed that the band gap has a significant influence on the IMFP and the stopping power at low electron energies. Typically, the inclusion of the band gap leads to an increase in the IMFP, because transition channels with energies larger than E-Eg-Ev are prohibited.

Energy loss function of samarium

We present a combined experimental and theoretical work to obtain the energy loss function (ELF) or the excitation spectrum of samarium in the energy loss range between 3 and 200 eV. At low loss energies, the plasmon excitation is clearly identified and the surface and bulk contributions are distinguished. For the precise analysis the frequency-dependent energy loss function and the related optical constants (n and k) of samarium were extracted from the measured reflection electron energy loss spectroscopy (REELS) spectra by the reverse Monte Carlo method. The ps- and f-sum rules with final ELF fulfils the nominal values with 0.2% and 2.5% accuracy, respectively. It was found that a bulk mode locates at 14.2 eV with the peak width ~6 eV and the corresponding broaden surface plasmon mode locates at energies of 5-11 eV.

Demonstration of Bixbyite-Structured δ-Ga2O3 Thin Films Using β-Fe2O3 Buffer Layers by Mist Chemical Vapor Deposition

Gallium oxide (Ga2O3) possesses five polymorphs: α, β, γ, κ (ε), and δ. Although the first four polymorphs have been well-studied, there are few reports on δ-Ga2O3. Here, we demonstrate the epitaxial growth of metastable δ-Ga2O3 thin films by mist chemical vapor deposition using β-Fe2O3 buffer layers. X-ray diffraction (XRD) 2θ–ω scan pattern revealed that (004) κ-Ga2O3 grew on (111) yttria-stabilized zirconia (YSZ) without a buffer layer or with a bcc-In2O3 buffer layer, whereas (222) δ-Ga2O3 grew on (222) β-Fe2O3. The β-Fe2O3 buffer layer led to the epitaxial growth of the δ-Ga2O3 thin film. The lattice mismatch between the equivalent crystal structures of β-Fe2O3 and δ-Ga2O3 triggered this growth. XRD analysis shows that δ-Ga2O3 grew epitaxially on the β-Fe2O3 buffer layer/YSZ substrate in both the out-of-plane and in-plane orientations, and the lattice constant inferred from the diffraction peaks was estimated to be 9.255 Å. Reciprocal space mapping results indicated that the δ-Ga2O3 grown on β-Fe2O3 was fully relaxed. Selected area electron diffraction images confirmed that the δ-Ga2O3 exhibited a cubic bixbyite structure. The optical band gap of δ-Ga2O3 was 4.3 or 4.9 eV, as calculated from reflection electron energy loss spectroscopy. We successfully grew a δ-Ga2O3 epitaxial thin film for the first time.

Synthesis of Optoelectronic Nanostructures on Silicon and Gold-Coated Silicon via High-Intensity Laser Pulses at Varied Pulse Durations

This work defines the generation of nanostructures on silicon and gold-coated silicon substrates by tuning the pulse duration of our proposed method: ultra-short laser pulses for in situ nanostructure generation (ULPING) under ambient conditions. The method is a single-step novel method which is efficient in synthesizing nanostructures on the substrates. We observed a higher nanofiber generation at a shorter pulse duration using Scanning Electron Microscopy (SEM) imaging. Silicon oxide formation was confirmed by Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) analysis and a band gap of 8.19 eV was achieved for the Si + Au sample, which was determined by the Reflection Electron Energy Loss Spectroscopy (REELS) spectra. A high valence band offset of 4.93 eV was measured for the silicon-based samples for the Si/SiO2 interface. The addition of gold nanoparticles decreased the band gap and we observed a blue shift in optical conductivity for samples with nanofibers using optical spectroscopy.

Low Leakage Current Metal–Insulator–Metal Device Based on a Beryllium Oxide Insulator Created by a Two-Step Spin-Coating Method as a Novel Type of Modified Pechini Synthesis

Beryllium oxide (BeO) is considered to be an attractive alternative material for use in future industries in areas such as semiconductors, spacecraft, aircraft, and rocket technologies due to its high bandgap energy, useful melting point, good thermal conductivity, and dielectric constants. In this context, our approach is a novel method to produce BeO thin films based on a two-step spin-coating innovation of the conventional powder synthesis method. The surface morphology and the crystal structure of BeO thin films were observed to be dependent on the citric acid/beryllium sulfate ratio and the sintering temperature, respectively. To characterize the BeO films, X-ray photoelectron spectroscopy was conducted for an elemental analysis. Furthermore, the bandgap of the BeO thin films was determined by reflection electron energy loss spectroscopy. Finally, the leakage current of a planar metal–insulator–metal device consisting of Au/Ti/BeO thin film/Ti/Au electrodes was determined to be below the nA range over the linear voltage sweeping range of −20 V to +20 V. These results can assist researchers in the areas of morphology control strategies, phase transfer theories, and applications that utilize BeO thin film manufactured by a solution process.