Inelastic Background Research Articles

Intensity-energy response function of Al/Cr-K[formula omitted] x-ray photoemission instruments: An inter-laboratory study

The question of the intensity-energy response of photoemission spectrometers has been approached through a round-robin involving 13 instruments working with low (Al/Mg-Kα) and high (Cr-Kα) energy photon sources. An algorithm based on the analysis of inelastic background previously proposed [S. Guilet et al., J. Electron Spectrosc. Relat. Phenom. 285 (2022) 147225)] was intensively tested against calibrated Al/Mg-Kα instruments and calculated relative sensitivity factors (RSFs) over the 15 core level peaks of coinage metals (Ag, Au, Cu). In both cases, the linear correlation within ±10 %, over two orders of magnitude in intensity and for kinetic energies ranging from ∼400 to 1400eV, showed the consistency of the approach. All the contributions to RSFs (e.g. non-dipolar terms in the photo-ionization cross section, elastic scattering effects, surface excitations, beam polarization by the monochromator) were critically reviewed and taken into account using the state-of-the-art modellings and databases. Strong variations were evidenced among instruments, regarding not only the response functions, but also the theoretical RSFs due to different measurement geometries. For the Cr-Kα hard x-ray instruments, the same analysis was performed with a set of different materials (Al, Si, Ge, Fe, Co, Ni, Cu, Zn, Nb, Mo, Ag, W, Au). A ±10 % satisfactory agreement against theoretical RSFs over 69 core levels spanning a large kinetic energy range (300–4000 eV) could be achieved with a common response function. Despite limitations that are reviewed, this work opens interesting perspectives for a systematic calibration of photoemission instruments.

HAXPES: Inelastic background for characterization of nanostructured materials

Analysis of the spectrum of inelastically scattered electrons in XPS has been used for decades to determine the structure of materials on the nanoscale. Using the higher photon energies available in HAXPES, the method was recently shown to determine structures rather accurately with a more than 100 nm probing depth. In this paper, these HAXPES applications are briefly reviewed. Only two parameters are required for the analysis, namely, the inelastic mean free path and the cross section for inelastic electron scattering. We also discuss in detail how these parameters are best selected.

The Impact of Energy Filtering on Fluctuation Electron Microscopy

Abstract Fluctuation electron microscopy (FEM) analyzes intensity fluctuations within diffraction patterns in order to draw conclusions regarding the structure of amorphous materials by calculating the normalized variance V(k,R). Ideally, such experiments only evaluate elastically diffracted electrons. However, an undesired inelastic background intensity is always present and degrades the FEM data. Energy filtered FEM experiments were performed on amorphous germanium created by self-ion implantation. FEM data were acquired in a transmission electron microscope at 60 and 300 kV with different electron doses as well as varying energy filter slit widths at two sample thicknesses. Generally, the measurements reveal that energy filtering greatly improves FEM data at both beam energies and sample thicknesses by removing a certain amount of the inelastic background intensity in the diffraction patterns. The narrower the energy filter, the larger the normalized variance. This brings energy filtered FEM data closer to the normalized variance determined by simulations under idealized conditions. Furthermore, preliminary results indicate that the medium range order length scale extracted from the pair-persistence analysis used in FEM is strongly affected by energy filtering.

Treating Knock-On Displacements in Fluctuation Electron Microscopy Experiments.

This work investigates how knock-on displacements influence fluctuation electron microscopy (FEM) experiments. FEM experiments were conducted on amorphous silicon, formed by self-ion implantation, in a transmission electron microscope at 300 kV and 60 kV at various electron doses, two different binnings and with two different cameras, a CCD and a CMOS one. Furthermore, energy filtering has been utilized in one case. Energy filtering greatly enhances the FEM data by removing the inelastic background intensity, leading to an improved speckle contrast. The CMOS camera yields a slightly larger normalized variance than the CCD at an identical electron dose and appears more prone to noise at low electron counts. Beam-induced atomic displacements affect the 300 kV FEM data, leading to a continuous suppression of the normalized variance with increasing electron dose. Such displacements are considerably reduced for 60 kV experiments since the primary electron's maximum energy transfer to an atom is less than the displacement threshold energy of amorphous silicon. The results show that the variance suppression due to knock-on displacements can be controlled in two ways: Either by minimizing the electron dose to the sample or by conducting the experiment at a lower acceleration voltage.

Miniaturized 7-in-1 fiber-optic Raman probe.

This Letter reports a miniature 7-in-1 fiber-optic Raman probe that eliminates the inelastic background Raman signal from a long-fused silica fiber. Its foremost purpose is to enhance a method for investigating extraordinarily tiny substances and effectively capturing Raman inelastic backscattered signals using optical fibers. We successfully used our home-built fiber taper device to combine seven multimode fibers into a single fiber taper with a probe diameter of approximately 35 µm. By experimentally comparing the traditional bare fiber-based Raman spectroscopy system with the miniaturized tapered fiber-optic Raman sensor using liquid solutions, the novel probe's capability is demonstrated. We observed that the miniaturized probe effectively removed the Raman background signal originating from the optical fiber and confirmed expected outcomes for a series of common Raman spectra.

Assessing advanced methods in XPS and HAXPES for determining the thicknesses of high-k oxide materials: From ultra-thin layers to deeply buried interfaces

We assess advanced XPS-based methods in determining depth distribution in alumina and hafnia ALD samples of increasing thicknesses and buried layer depths. First, dose quantification of Al in ultrathin alumina is achieved through non-destructive wavelength dispersive X-ray fluorescence (WDXRF), which is confirmed by comparison to destructive liquid-phase deposition inductively coupled plasma mass spectrometry (LPD-ICPMS) (uncertainty 10 %). Second, we compare WDXRF to parallel ARXPS (pARXPS) in precise dose and thickness quantification in ultrathin alumina, and present the ability of pARXPS to accurately track linear growth during the deposition. Thickness determination of an intermediate ALD stack of silicon dioxide, hafnia, and alumina layers of 1–2 nm by pARXPS is evaluated against quantitative, reference-free grazing incidence X-ray fluorescence (GIXRF) (uncertainty 8–9 %). An alternative technique employing inelastic background analysis (IBA) is introduced as a fast and accurate method and subsequently compared with pARXPS and GIXRF thickness determinations. Finally, in thicker multilayer systems, hard X-ray photoelectron spectroscopy (HAXPES) combined with IBA is proposed for an in-depth analysis for thicknesses up to 28 nm, with generated interface locations and layer thicknesses well-aligned to nominal values. This study is a novel application of lab-based Cr-Kα HAXPES spectra for inelastic background analysis in thick metal-oxide multilayer materials.

Reference survey spectra of elemental solid measured with Cr Kα photons as a tool for Quases analysis (3): Transition metals period 6 elements (Hf, Ta, W, Re, Ir, Pt, Au)

Several pure bulk materials were analyzed using laboratory-based hard x-ray photoelectron spectroscopy. The spectra are surveys measured using monochromatic Cr Kα radiation at 5414.8 eV after removal of surface contamination or oxidation. These aim to be references for inelastic background analysis using the Tougaard method.

Reference survey spectra of elemental solid measured with Cr Kα photons as a tool for Quases analysis (1): Transition metals period 4 elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn)

Several pure bulk materials were analyzed using laboratory-based hard x-ray photoelectron spectroscopy. The spectra are surveys measured using monochromatic Cr Kα radiation at 5414.8 eV after removal of surface contamination or oxidation. These aim to be references for inelastic background analysis using the Tougaard method.

Reference survey spectra of elemental solid measured with Cr Kα photons as a tool for Quases analysis (2): Transition metals period 5 elements (Y, Zr, Nb, Mo, Ru, Rh, Pd, and Ag)

Several pure bulk materials were analyzed using laboratory-based hard x-ray photoelectron spectroscopy. The spectra are surveys measured using monochromatic Cr Kα radiation at 5414.8 eV after removal of surface contamination or oxidation. These aim to be references for inelastic background analysis using the Tougaard method.

Reference survey spectra of elemental solid measured with Cr Kα photons as a tool for Quases analysis (4): Group III and IV elements (B, Al, In, C, Si, Ge, Sn, Pb)

Several pure bulk materials were analyzed using laboratory-based hard x-ray photoelectron spectroscopy. The spectra are surveys measured using monochromatic Cr Kα radiation at 5414.8 eV after removal of surface contamination or oxidation. These aim to be references for inelastic background analysis using the Tougaard method.

Reference survey spectra of elemental solid measured with Cr Kα photons as a tool for Quases analysis (5): Various elements (Mg, Sb, Bi, Se, Gd, Dy, Yb)

Several pure bulk materials were analyzed using laboratory-based hard x-ray photoelectron spectroscopy. The spectra are surveys measured using monochromatic Cr Kα radiation at 5414.8 eV after removal of surface contamination or oxidation. These aim to be references for inelastic background analysis using the Tougaard method.

Comparison and complementarity of QUASES-Tougaard and SESSA software

With the recent introduction of numerous commercial lab-scale high-energy X-ray photoemission spectrometers, new possibilities arise for photoemission spectra simulation. The most commonly available software are SESSA and QUASES-Tougaard. In this paper, we present and compare these software for the analysis of the inelastic background of X-ray photoelectron spectra. The QUASES-Tougaard software is a dedicated software for inelastic background analysis already well known for allowing to retrieve the depth distribution of complex stacks and deeply buried layers with a typical 5% accuracy. SESSA is a well-known software to facilitate quantitative interpretation of XPS spectra and to improve the accuracy of quantitative analysis but it is less commonly used for inelastic background analysis. This paper compares their capability to retrieve the depth distribution of elements and presents their main differences and complementarities. The two approaches are discussed using three well different systems from the microelectronic field with different levels of complexity such as the recurrence of an element in several layers or the combination of elements presenting very different cross-sections.

New directions in the analysis of buried interfaces for device technology by hard X-ray photoemission.

Photoelectron spectroscopy is a characterization technique which plays a key role in device technology, a field requiring, very often, a reliable and reproducible analysis of buried, critical interfaces. The recent advent of laboratory hard X-ray spectrometers opens new perspectives toward routine studies of technologically-relevant samples for the qualification of processes and materials. In this review, the status of hard X-ray photoelectron spectroscopy (HAXPES) implemented with chromium Kα excitation (5.414 keV) and applied to technological research in nanoelectronics is presented. After an account of the role of synchrotron HAXPES and the specific effects to care about at the practical level, different aspects are developed, first for illustrating the benefits of the technique through specific application cases in the field of resistive memories and power transistors. Then, we provide a status update on quantification in HAXPES, both from core-level intensities and inelastic background analysis. Finally, we present preliminary results in a novel analytical field, operando HAXPES, where a prototypical device is operated in situ during the laboratory HAXPES experiment, opening up the possibility of unravelling the mechanisms occurring at buried interfaces and governing device operation.

Running axial mass of the nucleon as a phenomenological tool for calculating quasielastic neutrino\u2013nucleus cross sections

We suggest an empirical rule-of-thumb for calculating the cross sections of charged-current quasielastic (CCQE) and CCQE-like interactions of neutrinos and antineutrinos with nuclei. The approach is based on the standard relativistic Fermi-gas model and on the notion of neutrino energy dependent axial-vector mass of the nucleon, governed by a couple of adjustable parameters, one of which is the conventional charged-current axial-vector mass. The inelastic background contributions and final-state interactions are therewith simulated using GENIE 3 neutrino event generator. An extensive comparison of our calculations with earlier and current accelerator CCQE and CCQE-like data for different nuclear targets shows good or at least qualitative overall agreement over a wide energy range. We also discuss some problematical issues common to several competing contemporary models of the CCQE (anti)neutrino–nucleus scattering and to the current neutrino interaction generators.

Using the inelastic background in hard x-ray photoelectron spectroscopy for a depth-resolved analysis of the CdS/Cu(In,Ga)Se2 interface

The inelastic background of hard x-ray photoelectron spectroscopy data is analyzed to paint a depth-resolved picture of the CdS/Cu(In,Ga)Se2 (CdS/CIGSe) layer structure. The CdS/CIGSe interface is the central component in next-generation chalcopyrite thin-film photovoltaic devices. By analyzing both, the (unscattered) core-level peaks and the inelastic background, and by varying the excitation photon energy from 2.1 up to 14 keV, we can derive photoemission information over a broad range of electron kinetic energies and, hence, sampling depths. With this complementary information, the CdS film thickness of a CdS/CIGSe interface can be accurately determined as a function of the CdS deposition time. For the thinner CdS films, the film thickness can be shown to vary laterally. Furthermore, small amounts of Se and process-related Rb can be detected in a thin (∼2 nm) surface layer of all investigated CdS films.

Improved depth information from routine analysis of the inelastic background of XPS and HAXPES spectra using optimized two‐ and three‐parameter cross‐sections

Determination of the depth distribution of complex nanostructures by X‐ray photoelectron spectroscopy (XPS) inelastic background analysis may be complicated if the sample materials have widely different inelastic scattering cross‐sections. It was recently demonstrated that this may be solved by using a mixture of cross‐sections. This permits retrieval of depth distributions of complex stacks and deeply buried layers with a typical 5% accuracy. This requires however that the cross‐sections of the individual sample materials are known which is often not the case and this can complicate practical use for routine analysis. In this paper, we explore to what extent a suitable two‐ or three‐parameter cross‐section can be defined independent of prior knowledge of the cross‐sections involved but simply defined by fitting the cross‐section parameters to the spectrum being analyzed. This paper presents a theoretical study following our recent paper that explored how to make the best choice of inelastic mean free path and inelastic scattering cross‐section for the inelastic background analysis with the Quases‐Tougaard software. It was previously shown that a rough analysis of the inelastic background could give a good idea of the depth distribution. Here, we demonstrate with model spectra from buried layers created with Quases‐Tougaard Generate software that a rather accurate analysis can be performed for very different cases with an average ~5% error. This analysis is easy to apply as it only needs the two‐ or three‐parameter cross‐sections generated with the Quases‐Tougaard software. This study is aimed to improve routine analysis of the inelastic background of XPS and hard X‐ray photoelectron spectroscopy (HAXPES) spectra.

Separation of the inelastic and elastic scattering in time-of-flight mode on the pinhole small-angle neutron scattering diffractometer K-WS-2.

To study and control the incoherent inelastic background in small-angle neutron scattering, which makes a significant contribution to the detected scattering from hydrocarbon systems, the KWS-2 small-angle neutron scattering diffractometer operated by the Jülich Centre for Neutron Science (JCNS) at Heinz-Maier Leibnitz Zentrum (MLZ), Garching, Germany, was equipped with a secondary single-disc chopper that is placed in front of the sample stage. This makes it possible to record in time-of-flight mode the scattered neutrons in the high-Q regime of the instrument (i.e. short incoming wavelengths and detection distances) and to discard the inelastic component from the measured data. Examples of measurements on different materials routinely used as standard samples, sample containers and solvents in the experiments at KWS-2 are presented. When only the elastic region of the spectrum is used in the data-reduction procedure, a decrease of up to two times in the incoherent background of the experimentally measured scattering cross section may be obtained. The proof of principle is demonstrated on a solution of bovine serum albumin in D2O.

Inelastic background modelling applied to hard X-ray photoelectron spectroscopy of deeply buried layers: A comparison of synchrotron and lab-based (9.25 keV) measurements

Hard X-ray Photoelectron Spectroscopy (HAXPES) provides minimally destructive depth profiling into the bulk, extending the photoelectron sampling depth. Detection of deeply buried layers beyond the elastic limit is enabled through inelastic background analysis. To test the robustness of this technique, we present results on a thin (18 nm) layer of metal–organic complex buried up to 200 nm beneath organic material. Overlayers with thicknesses 25–140 nm were measured using photon energies ranging 6–10 keV at the I09 end station at Diamond Light Source, and a new fixed energy Ga Kα (9.25 keV) laboratory-based HAXPES spectrometer was also used to measure samples with overlayers up to 200 nm thick. The sampling depth was varied: at Diamond Light Source by changing the photon energy, and in the lab system by performing angle-resolved measurements. For all the different overlayers and sampling depths, inelastic background modelling consistently provided thicknesses which agreed, within reasonable error, with the ellipsometric thickness. Relative sensitivity factors were calculated, and these factors consistently provided reasonable agreement with the expected nominal stoichiometry, suggesting the calculation method can be extended to any element. These results demonstrate the potential for the characterisation of deeply buried layers using synchrotron and laboratory-based HAXPES.

Determining nonuniformities of core‐shell nanoparticle coatings by analysis of the inelastic background of X‐ray photoelectron spectroscopy survey spectra

Most real core‐shell nanoparticle (CSNP) samples deviate from an ideal core‐shell structure potentially having significant impact on the particle properties. An ideal structure displays a spherical core fully encapsulated by a shell of homogeneous thickness, and all particles in the sample exhibit the same shell thickness. Therefore, analytical techniques are required that can identify and characterize such deviations. This study demonstrates that by analysis of the inelastic background in X‐ray photoelectron spectroscopy (XPS) survey spectra, the following types of deviations can be identified and quantified: the nonuniformity of the shell thickness within a nanoparticle sample and the incomplete encapsulation of the cores by the shell material. Furthermore, CSNP shell thicknesses and relative coverages can be obtained. These results allow for a quick and straightforward comparison between several batches of a specific CSNP, different coating approaches, and so forth. The presented XPS methodology requires a submonolayer distribution of CSNPs on a substrate. Poly(tetrafluoroethylene)‐poly(methyl methacrylate) and poly(tetrafluoroethylene)‐polystyrene polymer CSNPs serve as model systems to demonstrate the applicability of the approach.

Practical guides for x-ray photoelectron spectroscopy: Quantitative XPS

X-ray photoelectron spectroscopy (XPS) is widely used to identify chemical species at a surface through the observation of peak positions and peak shapes. It is less widely recognized that intensities in XPS spectra can also be used to obtain information on the chemical composition of the surface of the sample and the depth distribution of chemical species. Transforming XPS data into meaningful information on the concentration and distribution of chemical species is the topic of this article. In principle, the process is straightforward, but there are a number of pitfalls that must be avoided to ensure that the information is representative and as accurate as possible. This paper sets out the things that should be considered to obtain reliable, meaningful, and useful information from quantitative XPS. This includes the necessity for reference data, instrument performance checks, and a consistent and methodical method for the separation of inelastic background from peaks. The paper contains relevant and simple equations along with guidance on their use, validity, and assumptions.