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For the reliable fabrication of the current and next generation of nanostructures it is essential to be able to determine their material composition and dimensional parameters. Using the grazing incidence X-ray fluoresence technique, which is taking advantage of the X-ray standing wave field effect, nanostructures can be investigated with a high sensitivity with respect to the structural and elemental composition. This is demonstrated using lamellar gratings made of Si$_3$N$_4$. Rigorous field simulations obtained from a Maxwell solver based on the finite element method allow to determine the spatial distribution of elemental species and the geometrical shape with sub-nm resolution. The increasing complexity of nanostructures and demanded sensitivity for small changes quickly turn the curse of dimensionality for numerical simulation into a problem which can no longer be solved rationally even with massive parallelisation. New optimization schemes, e.g. machine learning, are required to satisfy the metrological requirements. We present reconstruction results obtained with a Bayesian optimization approach to reduce the computational effort., Event: SPIE Optical Metrology, 2019, Munich, Germany. arXiv admin note: text overlap with arXiv:1801.04157

In this work, we present a grazing incidence X-ray diffraction study of the surface of a 0.24Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) [011] poled rhombohedral single crystal. The near surface microstructure (the top several tens to hundreds of unit cells) was measured in situ under an applied electric field. The strains calculated from the change in lattice parameters have been compared to the macroscopic strain measured with a strain gauge affixed to the sample surface. The depth dependence of the electrostrain at the crystal surface was investigated as a function of temperature. The analysis revealed hidden sweet spots featuring unusually high strains that were observed as a function of depth, temperature and orientation of the lattice planes.

Herein, we report a systematic study on the electronic structure of surface and bulk TiO2 doped with C, N or S. The results were attained with a semi-empirical method consisting of a combination between resonant X-ray emission spectroscopy (RXES) measurements and localized density of states (LDOS) simulations. Experimental TiO2 RXES data was used to fit FEFF code empirical parameters, and subsequently frozen for the theoretical analysis of LDOS of TiO2 doped materials. The results show significant electronic structure differences between bulk and surface doping, as well as in the nearest Ti atom electronic structure if it is located at the surface or sub-surface, with potential consequences to the photo-catalytic process. The methodology can be adapted to study other dopants, morphologies, structures and surface terminations and as well as other inorganic semiconductors.

We review the high energy resolution off-resonant spectroscopy (HEROS) technique. HEROS probes the unoccupied electronic states of matter in a single-shot manner thanks to the combination of off-re ...

We present experimental and theoretical X-ray emission spectroscopy (XES) data of the Fe Kβ line for Iron(II)sulfide (FeS) and Iron(II)disulfide (FeS(2)). In comparison to X-ray absorption spectroscopy (XAS), XES offers different discrimination capabilities for chemical speciation, depending on the valence states of the compounds probed and, more importantly in view of a a broader, laboratory-based use, a larger flexibility with respect to the excitation source used. The experimental Fe Kβ XES data was measured using polychromatic X-ray radiation and a compact full-cylinder von Hamos spectrometer while the calculations were realized using the OCEAN code. The von Hamos spectrometer used is characterized by an energy window of up to 700 eV and a spectral resolving power of E/ΔE = 800. The large energy window at a single position of the spectrometer components is made profit of to circumvent the instrumental sensitivity of wavelength-dispersive spectrometers to sample positioning. This results in a robust energy scale which is used to compare experimental data with ab initio valence-to-core calculations, which are carried out using the ocean package. To validate the reliability of the ocean package for the two sample systems, near edge X-ray absorption fine structure measurements of the Fe K absorption edge are compared to theory using the same input parameters as in the case of the X-ray emission calculations. Based on the example of iron sulfide compounds, the combination of XES experiments and ocean calculations allows unravelling the electronic structure of different transition metal sulfides and qualifying XES investigations for the speciation of different compounds.

In this work, Pt-Ti core-shell nanoparticles (NP) of 2 nm to 3 nm in size and 30000 u \pm 1500 u as specified single particle mass, deposited on flat silicon substrates by means of a mass-selected cluster beam source, were used for the investigation of the modification of the X-Ray Standing Wave (XSW) field intensity with increasing NP surface coverage. The focus of the investigation is on the determination of the range of validity of the undisturbed flat surface approach of the XSW intensity in dependence of the actual coverage rate of the surface. Therefore, the nanoparticles were characterized using reference-free grazing incidence X-ray fluorescence analysis (GIXRF) employing radiometrically calibrated instrumentation. In addition, near-edge X-ray absorption fine structure (NEXAFS) measurements were performed to investigate the binding state of titanium in the core-shell nanoparticles which was found to be amorphous TiO2. The combination of GIXRF measurements and of the calculated XSW field intensities allow for a quantification of the core-shell nanoparticle surface coverage. For six different samples, the peak surface coverage could be determined to vary from 7 % to 130 % of a complete monolayer-equivalent coverage. A result of the current investigation is that core-shell nanoparticles modify the intensity distribution of the XSW field with increasing surface coverage. This experimental result is in line with calculated XSW field intensity distributions at different surface coverages using an effective density approach., Comment: 19 pages, 8 figures and 3 tables

Stochastic processes are highly relevant in research fields as different as neuroscience, economy, ecology, chemistry, and fundamental physics. However, due to their intrinsic unpredictability, stochastic mechanisms are very challenging for any kind of investigations and practical applications. Here we report the deliberate use of stochastic X-ray pulses in two-dimensional spectroscopy to the simultaneous mapping of unoccupied and occupied electronic states of atoms in a regime where the opacity and transparency properties of matter are subject to the incident intensity and photon energy. A readily transferable matrix formalism is presented to extract the electronic states from a dataset measured with the monitored input from a stochastic excitation source. The presented formalism enables investigations of the response of the electronic structure to irradiation with intense X-ray pulses while the time structure of the incident pulses is preserved., Free electron X-ray laser pulses, generated by self-amplified spontaneous emission, are stochastic in nature. Here the authors present a reconstruction method for 2D spectroscopy while preserving the intrinsic properties of the incident pulses and apply it to a study towards X-ray intensity induced effects.

LiMn 2O 4 is a promising candidate for a cathode material in lithium-ion batteries because of its ability to intercalate lithium ions reversibly through its three-dimensional manganese oxide network. One of the promising techniques for depositing LiMn 2O 4 thin-film cathodes is atomic layer deposition (ALD). Because of its unparalleled film thickness control and film conformality, ALD helps to fulfill the industry demands for smaller devices, nanostructured electrodes, and all-solid-state batteries. In this work, the intercalation mechanism of Li + ions into an ALD-grown β-MnO 2 thin film was studied. Samples were prepared by pulsing LiO tBu and H 2O for different cycle numbers onto about 100 nm thick MnO 2 films at 225 °C and characterized with X-ray absorption spectroscopy, X-ray diffraction, X-ray reflectivity, time-of-flight elastic recoil detection analysis, and residual stress measurements. It is proposed that for tBu/H 2O, the Li + ions penetrate only to the surface region of the β-MnO 2 film, and the samples form a mixture of β-MnO 2 and a lithium-deficient nonstoichiometric spinel phase Li xMn 2O 4 (0 < x < 0.5). When the lithium concentration exceeds x ≈ 0.5 in Li xMn 2O 4 (corresponding to 100 cycles of LiO tBu/H 2O), the crystalline phase of manganese oxide changes from the tetragonal pyrolusite to the cubic spinel, which enables the Li + ions to migrate throughout the whole film. Annealing in N 2 at 600 °C after the lithium incorporation seemed to convert the films completely to the pure cubic spinel LiMn 2O 4.

The characterization of nanostructured surfaces with sensitivity in the sub-nm range is of high importance for the development of current and next-generation integrated electronic circuits. Modern transistor architectures for, e.g., FinFETs are realized by lithographic fabrication of complex, well-ordered nanostructures. Recently, a novel characterization technique based on X-ray fluorescence measurements in grazing incidence geometry was proposed for such applications. This technique uses the X-ray standing wave field, arising from an interference between incident and the reflected radiation, as a nanoscale sensor for the dimensional and compositional parameters of the nanostructure. The element sensitivity of the X-ray fluorescence technique allows for a reconstruction of the spatial element distribution using a finite element method. Due to a high computational time, intelligent optimization methods employing machine learning algorithms are essential for timely provision of results. Here, a sampling of the probability distributions by Bayesian optimization is not only fast, but it also provides an initial estimate of the parameter uncertainties and sensitivities. The high sensitivity of the method requires a precise knowledge of the material parameters in the modeling of the dimensional shape provided that some physical properties of the material are known or determined beforehand. The unknown optical constants were extracted from an unstructured but otherwise identical layer system by means of soft X-ray reflectometry. The spatial distribution profiles of the different elements contained in the grating structure were compared to scanning electron and atomic force microscopy and the influence of carbon surface contamination on the modeling results were discussed. This novel approach enables the element sensitive and destruction-free characterization of nanostructures made of silicon nitride and silicon oxide with sub-nm resolution.

The geometry of a Si$_3$N$_4$ lamellar grating was investigated experimentally with reference-free grazing-incidence X-ray fluorescence analysis. While simple layered systems are usually treated with the matrix formalism to determine the X-ray standing wave field, this approach fails for laterally structured surfaces. Maxwell solvers based on finite elements are often used to model electrical field strengths for any 2D or 3D structures in the optical spectral range. We show that this approach can also be applied in the field of X-rays. The electrical field distribution obtained with the Maxwell solver can subsequently be used to calculate the fluorescence intensities in full analogy to the X-ray standing wave field obtained by the matrix formalism. Only the effective 1D integration for the layer system has to be replaced by a 2D integration of the finite elements, taking into account the local excitation conditions. We will show that this approach is capable of reconstructing the geometric line shape of a structured surface with high elemental sensitivity. This combination of GIXRF and finite-element simulations paves the way for a versatile characterization of nanoscale-structured surfaces.

ECS journal of solid state science and technology 7(7), Q3108-Q3113 (2018). doi:10.1149/2.0171807jss special issue: "JSS Focus Issue on Semiconductor-Based Sensors for Application to Vapors, Chemicals, Biological Species, and Medical Diagnosis", Published by ECS, Pennington, NJ

The potential of valence to core Al X-ray emission spectroscopy to determine aluminum distribution in ferrierite zeolites was investigated. The recorded emission spectra of four samples prepared with different structure directing agents exhibit slight variations in the position of the main emission peak and the intensity of its low energy shoulder. Theoretical calculations indicate that an increased intensity of the Kβx shoulder in the Al emission spectra can be linked to a predominant occupation of the T3 site by a single aluminum atom. This study thus suggests that valence to core X-ray emission spectroscopy can be applied to help determine the occupation of aluminum at crystallographic T-sites in zeolites.

FeNi Hydroxides (FeNi-HD) have been considered as promising substitutes to noble metal electrocatalysts for oxygen evolution reaction (OER). In this work, we design and realize FeNi-HD nanotube arrays (FeNi-HDNAs) on Ni foam via an in-situ reaction and Kirkendall effect. The obtained catalysts possess higher specific surface area, more catalytic active sites and better chemical stability for OER. Electron migrations from the Fe 3d orbitals to Ni sites in the FeNi-HDNAs lead to more unoccupied Fe 3d states and a higher oxidation state. As expected, FeNi-HDNAs exhibit lower overpotential as well as lower Tafel slope and better durability than the Fe- or Ni-HD peers. DFT calculations elucidate that FeNi hydroxides lower the energy barrier of rate-determining step in OER. Moreover, a high current density of 10 mA cm−2 is obtained at a low potential of 1.49 V using FeNi-HDNAs as the bifunctional electrocatalyst for overall water splitting in basic solution.

X-ray techniques have evolved over decades to become highly refined tools for a broad range of investigations. Importantly, these approaches rely on X-ray measurements that depend linearly on the number of incident X-ray photons. The advent of X-ray free electron lasers (XFELs) is opening the ability to reach extremely high photon numbers within ultrashort X-ray pulse durations and is leading to a paradigm shift in our ability to explore nonlinear X-ray signals. However, the enormous increase in X-ray peak power is a double-edged sword with new and exciting methods being developed but at the same time well-established techniques proving unreliable. Consequently, accurate knowledge about the threshold for nonlinear X-ray signals is essential. Herein we report an X-ray spectroscopic study that reveals important details on the thresholds for nonlinear X-ray interactions. By varying both the incident X-ray intensity and photon energy, we establish the regimes at which the simplest nonlinear process, two-photon X-ray absorption (TPA), can be observed. From these measurements we can extract the probability of this process as a function of photon energy and confirm both the nature and sub-femtosecond lifetime of the virtual intermediate electronic state., Scientific Reports, 6, ISSN:2045-2322

We report on the surface-sensitive grazing emission X-ray fluorescence technique combined with synchrotron radiation excitation and high-resolution detection to realize depth-profile measurements of Al-implanted Si wafers. The principles of grazing emission measurements as well as the benefits offered by synchrotron sources and wavelength-dispersive detection setups are presented. It is shown that the depth distribution of implanted ions can be extracted from the dependence of the X-ray fluorescence intensity on the grazing emission angle with nanometer-scale precision provided that an analytical function describing the shape of the depth distribution is assumed beforehand. If no a priori assumption is made, except a bell shaped form for the dopant distribution, the profile derived from the measured angular distribution is found to reproduce quite satisfactorily the depth distribution of the implanted ions. Copyright © 2012 John Wiley & Sons, Ltd.

Depth-profiling measurements by means of synchrotron radiation based grazing XRF techniques, i.e., grazing emission X-ray fluorescence (GEXRF) and grazing incidence X-ray fluorescence (GIXRF), present a promising approach for the non-destructive, sub-nanometer scale precision characterization of ultra shallow ion-implantations. The nanometer resolution is of importance with respect to actual semiconductor applications where the down-scaling of the device dimensions requires the doping of shallower depth ranges. The depth distributions of implanted ions can be deduced from the intensity dependence of the detected X-ray fluorescence (XRF) signal from the dopant atoms on either the grazing emission angle of the emitted X-rays (GEXRF), or the grazing incidence angle of the incident X-rays (GIXRF). The investigated sample depth depends on the grazing angle and can be varied from a few to several hundred nanometers. The GEXRF setup was equipped with a focusing polycapillary half-lens to allow for laterally resolved studies. The dopant depth distribution of the investigated low-energy (energy range from 1 keV up to 8 keV) P, In and Sb ion-implantations in Si or Ge wafers were reconstructed from the GEXRF data by using two different approaches, one with and one without a priori knowledge about the bell-shaped dopant depth distribution function. The results were compared to simulations and the trends predicted by theory were found to be well reproduced. The experimental GEXRF findings were moreover verified for selected samples by GIXRF.

In this work two synchrotron radiation-based depth-sensitive X-ray fluorescence techniques, grazing incidence X-ray fluorescence (GIXRF) and grazing emission X-ray fluorescence (GEXRF), are compared and their potential for non-destructive depth-profiling applications is investigated. The depth-profiling capabilities of the two methods are illustrated for five aluminum-implanted silicon wafers all having the same implantation dose of 1016 atoms per cm2 but with different implantation energies ranging from 1 keV up to 50 keV. The work was motivated by the ongoing downscaling effort of the microelectronics industry and the resulting need for more sensitive methods for the impurity and dopant depth-profile control. The principles of GIXRF and GEXRF, both based on the refraction of X-rays at the sample surface to enhance the surface-to-bulk ratio of the detected fluorescence signal, are explained. The complementary experimental setups employed at the Physikalisch-Technische Bundesanstalt (PTB) for GIXRF and the University of Fribourg for GEXRF are presented in detail. In particular, for each technique it is shown how the dopant depth profile can be derived from the angular intensity dependence of the Al Kα fluorescence line. The results are compared to theoretical predictions and, for two samples, crosschecked with values obtained from secondary ion mass spectroscopy (SIMS) measurements. A good agreement between the different approaches is found proving that the GIXRF and GEXRF methods can be efficiently employed to extract the dopant depth distribution of ion-implanted samples with good accuracy and over a wide range of implantation energies.

We report on the design and performance of a wavelength-dispersive type spectrometer based on the von Hamos geometry. The spectrometer is equipped with a segmented-type crystal for x-ray diffraction and provides an energy resolution in the order of 0.25 eV and 1 eV over an energy range of 8000 eV–9600 eV. The use of a segmented crystal results in a simple and straightforward crystal preparation that allows to preserve the spectrometer resolution and spectrometer efficiency. Application of the spectrometer for time-resolved resonant inelastic x-ray scattering and single-shot x-ray emission spectroscopy is demonstrated.

We present single-shot measurements of the longitudinal photon source position of the SPring-8 Angstrom Compact Free Electron Laser x-ray free electron laser by means of x-ray grating interferometry. The measurements were performed in order to study the behavior of the source under normal operation conditions and as a dependence on the active undulator length. The retrieved experimental results show that x-ray grating interferometry is a powerful in situ monitoring tool for investigating and tuning an x-ray free electron laser.

Various 3-dimensional nano-scaled periodic structures with different configurations and periods deposited on the surface of silicon and silica substrates were investigated by means of the grazing incidence and grazing emission X-ray fluorescence techniques. Apart from the characteristics which are typical for particle- and layer-like samples, the measured angular intensity profiles show additional periodicity-related features. The latter could be explained by a novel theoretical approach based on simple geometrical optics (GO) considerations. The new GO-based calculations were found to yield results in good agreement with experiment, also in cases where other theoretical approaches are not valid, e.g., periodic particle distributions with an increased surface coverage. (C) 2014 Elsevier B.V. All rights reserved.

The increasing importance of well-controlled ordered nanostructures on surfaces represents a challenge for existing metrology techniques. To develop such nanostructures and monitor complex processing constraints fabrication, both a dimensional reconstruction of nanostructures and a characterization (ideally a quantitative characterization) of their composition is required. In this work, we present a soft x-ray fluorescence-based methodology that allows both of these requirements to be addressed at the same time. By applying the grazing-incidence x-ray fluorescence technique and thus utilizing the x-ray standing wave field effect, nanostructures can be investigated with a high sensitivity with respect to their dimensional and compositional characteristics. By varying the incident angles of the exciting radiation, element-sensitive fluorescence radiation is emitted from different regions inside the nanoobjects. By applying an adequate modeling scheme, these datasets can be used to determine the nanostructure characteristics. We demonstrate these capabilities by performing an element-sensitive reconstruction of a lamellar grating made of Si3N4, where GIXRF data for the O-Kα and N-Kα fluorescence emission allows a thin oxide layer to be reconstructed on the surface of the grating structure. In addition, we employ the technique also to three dimensional nanostructures and derive both dimensional and compositional parameters in a quantitative manner.