Si L-edge Research Articles

Background subtraction in electron Compton spectroscopy

Compton scattering in electron energy loss spectroscopy (EELS) is used to quantify the momentum distribution of occupied electronic states in a solid. The Compton signal is a broad feature with a width of several hundred eV. Furthermore, the weak intensity results in a low peak-to-background ratio. Removing the background under the Compton profile is therefore particularly challenging, especially if there is an overlap with EELS core loss edges. Here an empirical background subtraction routine is proposed that uses input data from a bright-field EELS spectrum that does not have a Compton signal. The routine allows for multiple elastic-inelastic scattering within the EELS collection angles. Background subtraction is demonstrated on a Compton profile in silicon that overlaps with the Si L-edge. Systematic errors in the method are also discussed.

Broadband normal-incidence mirrors for a range of 111–138 Å based on an a-periodic Mo/Be multilayer structure

Broadband Mo/Be multilayer structures were designed for maximum uniform normal-incidence reflectivity in a broad range of 111–138 Å, which lies near and beyond the L2,3 absorption edge of Si. A comparison was made of the capabilities of two classes of aperiodic structures and of so-called “stack” structures, which are composed of several periodic structures with different periods stacked one over the other. Six-stack Mo/Be 80-layer structures were synthesized on concave (R = 1 m) superpolished fused silica substrates. Their absolute reflectivity was measured at 13% – 14% in the 111–138 Å optimization range using a laboratory reflectometer with a laser-plasma radiation source. The normal-incidence reflection spectra of the mirrors were recorded in the configuration of a transmission-grating spectrograph using the slowly varying quasicontinuum of a laser-driven tungsten plasma. Comparing the reflectivity data with the reflection spectra recorded with a CCD permitted estimating a decrease in the detector responsivity beyond the Si L-edge. The broadband normal-incidence multilayer mirrors facilitate the development of a high-resolution imaging spectrograph covering a usable range about the Si L-edge to characterize, for instance, the L-edge fine structures and chemical states. These mirrors will also find use in imaging solar instruments with a high spectral resolution operating aboard a spacecraft and in laboratory instruments for plasma diagnostics.

A method of using Si L-edge for O/Si and N/Si quantitative ratio analysis by electron energy loss spectroscopy (EELS)

In EELS core loss excitation, inverse jump ratio is defined as a ratio between intensity of a pre-core-loss-edge and a signal of the core loss excitation after background subtraction. A linear relationship between inverse jump ratio of Si L-edge and O/Si and N/Si intensity ratio is found for electron energy loss spectroscopy (EELS) analysis. This relationship is used to analyze O/Si and N/Si atomic ratio to quantify the elemental distribution in SiON films and the result is compared with XPS analysis on the same SiON films. The method is applied to blanket wafer elemental analysis and can be used for flash memory structure (3D NAND) to obtain atomic ratios of N/Si and O/Si for tunnel oxide at sub-nanometer scale.

Photoelectron based soft x-ray detector for removing high order x rays.

Soft x-ray absorption spectroscopy (XAS) in the low energy region below 200 eV is important to investigate chemical and biological phenomena under an atmospheric condition since it covers K-edges of Li and B and L-edges of Si, P, S, and Cl. Monochromatic soft x rays usually include not only first order x rays but also high order x rays due to the high order diffraction of a plane grating monochromator. It is difficult to measure XAS in the low energy region under an atmospheric helium condition since the transmitted soft x rays mostly consist of the high order x rays due to the low transmission of the first order x rays. In this study, we have developed a photoelectron based soft x-ray (PBSX) detector, where the Au 4f photoelectrons emitted by the first order x rays are separated from those by the high order x rays using a difference in kinetic energies of photoelectrons. By using the PBSX detector, we have successfully obtained Si L-edge XAS spectra of the SiC and polymer/SiC films that mainly include the first order x rays by removing the major contributions of the second order x rays at the C K-edge and the fifth order x rays at the O K-edge. In the future, several physical, chemical, and biological phenomena in solution will be investigated by XAS in the low energy region with the PBSX detector.

Soft X-ray absorption spectroscopy in the low-energy region explored using an argon gas window.

The soft X-ray region below 200 eV is important for investigating chemical and biological phenomena since it covers K-edges of Li and B and L-edges of Si, P, Sand Cl. Helium gas is generally used as the soft X-ray transmission window for soft X-ray absorption spectroscopy (XAS) under atmospheric conditions. However, the helium gas window cannot be applied to XAS in the low-energy region since transmitted soft X-rays mostly consist of high-order X-rays due to the low transmission of first-order X-rays. In this study, the argon gas window isproposed as a new soft X-ray transmission window in the low-energy region. High-order X-rays are removed by the absorption of the Ar L-edge (240 eV), and first-order X-rays become the major contribution of transmitted soft X-rays in the low-energy region. Under atmospheric argon conditions, the double-excitation Rydberg series of helium gas (60 eV), Si L-edge XAS of an Si3N4 membrane (100 eV) and S L-edge XAS of dimethyl sulfoxide gas (170 eV) are successfully measured, indicating that the argon gas window is effective for soft X-ray transmission in the low-energy region from 60 eV to 240 eV.

A versatile diamond anvil cell for X-ray inelastic, diffraction and imaging studies at synchrotron facilities.

We present a new diamond anvil cell design, hereafter called mBX110, that combines both the advantages of a membrane and screws to generate high pressure. It enables studies at large-scale facilities for many synchrotron X-ray techniques and has the possibility to remotely control the pressure with the membrane as well as the use of the screws in the laboratory. It is fully compatible with various gas-loading systems as well as high/low temperature environments in the lab or at large scale facilities. The mBX110 possesses an opening angle of 85° suitable for single-crystal diffraction or Brillouin spectroscopy and a large side opening of 110° which can be used for X-ray inelastic techniques, such as X-ray Raman scattering spectroscopy, but also for X-ray emission, X-ray fluorescence, or X-ray absorption. An even larger opening of 150° can be manufactured enabling X-ray imaging tomography. We report data obtained with the mBX110 on different beamlines with single-crystal diffraction of stishovite up to 55 GPa, X-ray powder diffraction of rutile-GeO2 and tungsten to 25 GPa and 280 GPa, respectively, X-Ray Raman spectra of the Si L-edge in silica to 95 GPa, and Fe Kβ X-ray emission spectra on a basalt glass to 17 GPa.

Angular and Spectral Bandwidth of Extreme UV Multilayers Near Spacer Material Absorption Edges.

High resolution imaging systems for EUV range are based on multilayer optics. Current generation of EUV lithography uses broadband Sn LPP sources, which requires broadband mirrors to fully utilize the source power. On the other hand, there always remains a possibility to use FEL or synchrotron as EUV source. FEL can produce very bright narrowband EUV light of a tunable wavelength, and the spectral bandwidth of the mirror is no longer a restriction. In this paper we look at the consequences of switching to different wavelengths if FEL source is used. For instance, it is known that the reflectance of Mo/Si multilayers increases when approaching Si L-edge, and the spectral bandwidth drops. But the behavior of an angular bandwidth (and its relation with the spectral bandwidth) is usually left out. It is also sometimes assumed that these bandwidths are correlated. For a large aperture EUV optical system with diffraction-limited resolution angular acceptance of a mirror is also a very important parameter. We show that the angular bandwidth of several multilayer systems (Mo/Si, Mo/Be, Ru/Si, Ru/B, La/B) actually increases close to spacer absorption edges, opposite to what occurs with the spectral bandwidth. We study this effect and show that it is caused by an interplay of changing optical constants of respective materials used in these multilayer combinations. We also provide an experimental check of the angular bandwidth of Mo/Si multilayers at 13.5 and 12.6 nm, which confirms our calculations.

Study of lamellar multilayer grating near B K-edge and Si L-edge

We present an efficient design of lamellar multilayer grating (LMG) for the extreme ultraviolet/soft X-ray region near both the Si L-edge and B K-edge. The designs of LMGs have been performed by exploring different materials combinations, which are capable of providing high resolution and peak reflectivity. The performance analysis of the designed LMGs is made analytically using the coupled wave theory for single-order regime. That provides an estimation of diffraction efficiency and resolution. The effect of structural imperfections such as tapering of the lamellar profile and interfacial width on the optical properties are also analysed to describe non-ideal LMG structures.

Large-scale hollow nanoparticle identification by X-ray absorption spectroscopy

Large-scale SiO2 hollow nanoparticles were synthesized by a sol-gel method. The composition, morphology, and chemical bonding information of SiO2 hollow nanoparticles were studied by X-ray absorption near edge structure (XANES) and scanning transmission X-ray microscopy (STXM). XANES at the Si L-edge and K-edge reveal the characteristics of hollow nanoparticles, which are essentially amorphous SiO2 with a slightly longer Si–O bond than SiO2 nanoparticles on average and deficiencies in oxygen. Individual SiO2 hollow spheres were also examined using STXM, which provides spectromicroscopic information, as well as the absolute thickness, of the sample.

Excitation and propagation of X-ray fluorescence through thin devices with hollowed ordered structures: comparison of experimental and theoretical spectra.

The lack of models describing the propagation of X-rays in waveguides and the interference mechanism between incident and reflected radiation waves hamper the understanding and the control of wave propagation phenomena occurring in many real systems. Here, experimental spectra collected at the exit of microchannel plates (MCPs) under the total X-ray reflection condition are presented. The results are discussed in the framework of a theoretical model in which the wave propagation is enhanced by the presence of a transition layer at the surface. The angular distributions of the propagating radiation at the exit of these MCPs with microchannels of ∼3 µm diameter will also be presented and discussed. These spectra show contributions associated with the reflection of the primary monochromatic beam and with the fluorescence radiation originating from the excitation of atoms composing the surface of the microchannel. The soft X-ray fluorescence spectra collected at the exit of microcapillaries were analyzed in the framework of a wave approximation while diffraction contributions observed at the exit of these hollow X-ray waveguides have been calculated using the Fraunhofer diffraction model for waves in the far-field domain. Data collected at the Si L-edge show that in glassy MCPs the fluorescence radiation can be detected only when the energy of the primary monochromatic radiation is above the absorption edge for grazing angles higher than half of the critical angle of the total reflection phenomenon. Experimental data and simulations of the propagating radiation represent a clear experimental confirmation of the channeling phenomenon of the excited fluorescence radiation inside a medium and point out that a high transmission can be obtained in waveguide optics for parameters relevant to X-ray imaging.

Current–voltage and spectral-response characteristics of surface-activated-bonding-based InGaP/GaAs/Si hybrid triple-junction cells

We measured the current–voltage (I–V) and spectral-response characteristics of InGaP/GaAs/Si hybrid triple-junction cells that were fabricated by using surface-activated bonding methods. We found by spectral response measurements that the current generated in the Si-based bottom cell was lower than those in the top and middle cells under the conditions of an air mass of 1.5G and one sun. Furthermore we observed a discrepancy between the short-circuit current, which was obtained by subtracting the estimated contribution of Si ledges surrounding InGaP/GaAs mesas to the I–V characteristics, and the results of spectral response measurements. One possible model for explaining the discrepancy was discussed on the basis of the electrical coupling scheme between subcells. The intrinsic conversion efficiency of a 5 × 5 mm2 triple-junction cell was crudely estimated to be ∼26% by compensating for the shadow loss as well as subtracting the contribution of Si ledges.

X-ray radiation channeling in micro-channel plates: Spectroscopy with a synchrotron radiation beam

We present here the angular distribution of the radiation propagated inside MultiChannel Plates with micro-channels of ∼3μm diameter. The spectra collected at the exit of the channels present a complex distribution with contributions that can be assigned to the fluorescence radiation, originated from the excitation of the micro-channel walls. For radiation above the absorption edge, when the monochromatic energy in the region of the Si L-edge hits the micro-channel walls with a grazing angle θ⩾5°, or at the O K-edge when θ⩾2° a fluorescence radiation is detected. Additional information associated to the fine structures of the XANES spectra detected at the exit of MCPs are also presented and discussed.

Insights into the physical chemistry of materials from advances in HAADF-STEM.

The observation that, "New tools lead to new science"[P. S. Weiss, ACS Nano., 2012, 6(3), 1877-1879], is perhaps nowhere more evident than in scanning transmission electron microscopy (STEM). Advances in STEM have endowed this technique with several powerful and complimentary capabilities. For example, the application of high-angle annular dark-field imaging has made possible real-space imaging at sub-angstrom resolution with Z-contrast (Z = atomic number). Further advances have wrought: simultaneous real-space imaging and elemental identification by using electron energy loss spectroscopy (EELS); 3-dimensional (3D) mapping by depth sectioning; monitoring of surface diffusion by time-sequencing of images; reduced electron energy imaging for probing graphenes; etc. In this paper we review how these advances, often coupled with first-principles theory, have led to interesting and important new insights into the physical chemistry of materials. We then review in detail a few specific applications that highlight some of these STEM capabilities.

Electronic properties of Co2FeSi investigated by X-ray magnetic linear dichroism

We present experimental XMLD spectra measured on epitaxial (001)-oriented thin Co2FeSi films, which are rich in features and depend sensitively on the degree of atomic order and interdiffusion from capping layers. Al- and Cr-capped films with different degrees of atomic order were prepared by DC magnetron sputtering by varying the deposition temperatures. The local structural properties of the film samples were additionally investigated by nuclear magnetic resonance (NMR) measurements. The XMLD spectra of the different samples show clear and uniform trends at the L3,2 edges. The Al-capped samples show similar behavior as previous measured XMLD spectra of Co2FeSi0.6Al0.4. Thus, we assume that during deposition Al atoms are being implanted into the subsurface of Co2FeSi. Such an interdiffusion is not observed for the corresponding Cr-capped films, which makes Cr the material of choice for capping Co2FeSi films. We report stronger XMLD intensities at the L3,2 Co and Fe egdes for films with a higher saturation magnetization. Additionally, we compare the spectra with ab initio predictions and obtain a reasonably good agreement. Furthermore, we were able to detect an XMCD signal at the Si L-edge, indicating the presence of a magnetic moment at the Si atoms.

X-ray spectroscopy of fluorescence radiation channeling in μ-capillary holed glass plates

Capillary optics is a fundamental X-ray technology capable of generating a high flux density with sub μm spot size. The physical basis of capillary optics refers to the mechanism of the total external reflection of X-rays although experimental data about the transport of the radiation inside such kind of structures are really limited.We present here the data of reflection and transmission soft X-ray synchrotron radiation experiments performed with different types of microchannel plates. In particular, we discuss the presence of fine structures in the XANES spectra at the energy of the Si L-edges detected at the exit of micro-capillary structures under the condition of the total X-ray reflection.

Strong-field-induced attosecond dynamics in SiO2

Striking field-induced changes in the absorption near the Si L-edge of SiO2 exposed to a near-infrared laser field of several V/A delivered by a few-cycle pulse are observed with sub-100 attosecond extreme ultraviolet pulses by means of attosecond transient absorption.

Reaction Mechanism of “SiO”-Carbon Composite-Negative Electrode for High-Capacity Lithium-Ion Batteries

The reaction mechanism of a “SiO”-carbon composite-negative electrode for high-capacity lithium-ion batteries is examined by Si K-edge X-ray absorption near edge structure (XANES), 7Li and 29Si nuclear magnetic resonance (NMR), and scanning transmission electron microscopy (STEM) with Si L-edge electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDX). According to the analytical results, “SiO” consisting of nano-size Si particles dispersed in amorphous SiO2 is converted to lithium-silicon alloys and lithium silicon oxides during the first charge in lithium-ion batteries, and the nano-size silicon particles surrounded by lithium silicon oxides are rechargeable for subsequent cycles. Molecular dynamics (MD) simulation of “SiO” is also carried out and shown that silicon particles grow during the first cycle and are surrounded by a good lithium-ion conductor of lithium silicon oxides. From these results, the reaction mechanism of “SiO” is discussed in terms of high-capacity negative electrodes for lithium-ion batteries.

Optical Response Near the Soft X-Ray Absorption Edges and Structural Studies of Low Optical Contrast System Using Soft X-Ray Resonant Reflectivity

Fine structure features of energy-dependent atomic scattering factor near the atomic absorption edge, are used for structural analysis of low-Z containing thin film structures. The scattering contrast undergoes large and abrupt change as the incident photon energy approaches the natural frequency of the atom and is sensitive to variation in atomic composition and atomic density. Soft X-ray resonant reflectivity is utilized for determination of composition at the buried interfaces with subnanometer sensitivity. This is demonstrated through characterization of Mo/Si multilayers near Si L-edge. We also demonstrate the possibility of probing variation of atomic density in thin films, through the characterization of Fe/B4C structure, near B K-edge. Sensitivity of soft X-ray resonant reflectivity to native oxide is demonstrated through characterization of BN films near B K-edge.

XANES and XPS investigations of (TiO2)x(SiO2)1−x: the contribution of final-state relaxation to shifts in absorption and binding energies

The (TiO2)x(SiO2)1−x system (0 ≤ x ≤ 0.33) was synthesized by the sol–gel method and investigated by X-ray absorption near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS). The use of both hard (Ti K-edge) and soft (Ti L-edge) X-rays provides a useful way to monitor changes in the bulk and surface, respectively, of these amorphous materials. The average CN of both bulk-Ti and surface-Ti increases with greater x in the chemical formula, due to the larger ionic radius of Ti. Comparison of Ti K- and L-edge spectra of annealed samples revealed that Ti atoms at the surface have a higher average CN than in the bulk, likely due to the presence of surface hydroxide and water groups that can coordinate to the Ti centres. The O K-edge, Ti L-edge, and Si L-edge XANES absorption energies showed little to no change with Ti content, while the O 1s, Ti 2p, and Si 2p XPS BEs were found to decrease with increasing Ti content due to nearest-neighbour and next-nearest-neighbour effects, which lead to increased final-state relaxation. The degree of final-state relaxation is more significant than previously believed for these amorphous powders.

Electronic structure of ZrCuSiAs and ZrCuSiP by X-ray photoelectron and absorption spectroscopy

The electronic structures of quaternary pnictides ZrCuSi Pn ( Pn=P, As) were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES). Shifts in the core-line XPS and the XANES spectra indicate that the Zr and Cu atoms are cationic, whereas the Si and Pn atoms are anionic, consistent with expectations from simple bonding models. The Cu 2 p XPS and Cu L-edge XANES spectra support the presence of Cu 1+. The small magnitudes of the energy shifts in the XPS spectra suggest significant covalent character in the Zr–Si, Zr– Pn, and Cu– Pn bonds. On progressing from ZrCuSiP to ZrCuSiAs, the Si atoms remain largely unaffected, as indicated by the absence of shifts in the Si 2 p 3/2 binding energy and the Si L-edge absorption energy, while the charge transfer from metal to Pn atoms becomes less pronounced, as indicated by shifts in the Cu K-edge and Zr K, L-edge absorption energies. The transition from two-dimensional character in LaNiAsO to three-dimensional character in ZrCuSiAs proceeds through the development of Si–Si bonds within the [ZrSi] layer and Zr–As bonds between the [ZrSi] and [CuAs] layers.