New challenges associated with hard X‐ray photoelectron spectroscopy (report on the 2023 ASTM E42‐ASSD AVS workshop)

Addressing the oxidation state of functional materials such as transition metal oxides is a current critical challenge and requires new methodologies to characterize their electronic structure within surface to bulk resolution at the nanometer scale. One approach to this issue is the combination of co‐localized soft and hard X‐ray photoelectron spectroscopies for a non‐destructive depth profiling. In this work, we demonstrate the capability to characterize the oxidation state of cobalt in LiCoO 2 thin films, a model electrode material in the Li‐ion battery field, within the first 15 nm. The capability of the methodology to address the surface evolution of the cobalt oxidation state is tested through a proof of concept surface modification introduced by Ar‐ion sputtering. To address the cobalt local electronic structure at different depths from the extreme surface, we exploited Auger‐free Hard X‐ray photoelectron spectroscopy (HAXPES) spectra to fit the Co 2p core‐level features in the X‐ray photoelectron spectroscopy (XPS) spectra. This approach may pave the way for a better understanding of the surface electronic structure changes in transition metal oxides driven by their applications in a broad range of technologies.

Introduction Lithium-ion battery life is expected to increase, and the need for degradation analyses has grown [1]. Among the degradation mechanism, the change in the electrode surface state has significant influence on lithium-ion battery performance. In this study, a long-term cycle test was conducted using commercial 18650-type lithium ion cells for a current rate of 1 C at 25 °C. The electrode surface state was investigated by X-ray photoelectron spectroscopy (XPS), hard X-ray photoelectron spectroscopy (HAXPES), and transmission electron microscopy (TEM). Experimental The cycle test was performed using a 18650-type commercial lithium-ion cell equipped with a Li(Ni1/3Mn1/3Co1/3)O2 cathode and a graphite anode. After cycle test completion, the cell was analyzed at a state-of-charge of 50% (= 3.694 V) by electrochemical impedance spectroscopy at 25 °C. The degraded single electrode property was measured by preparing a half cell against lithium metal. The electrode surface state was characterized by XPS and HAXPES. HAXPES measurements were performed at the BL46XU beamline of SPring-8. The cathode surface state was also investigated by TEM. The electrolytic solution was analyzed by 1H- and 19F-NMR. Results and discussion During the cycle test, the discharge capacity decreased gradually and then drastically above 1500 cycles. The capacity retention measured at 1 C reached below 10% after 1700 cycles. Nyquist plots before and after cycle test showed an increase in electrolyte and charge transfer resistances (Fig. 1). To understand the degradation mechanism, the degraded cell was disassembled after the cell was discharged at C/20, and their electrodes and electrolytic solution were removed under argon atmosphere. Half cells were prepared, and measured electrodes were characterized. After the first cycle, the degraded cathode exhibited a coulombic efficiency of 186%, indicating that it was partially charged when the cell was opened. Conversely, the degraded anode displayed a coulombic efficiency of about 100%, showing it was fully discharged. This mismatch between cathode and anode state-of-charge may explain the decrease in cell capacity. In addition, charge/discharge curves in the second cycle showed a capacity loss of about 20% at the cathode compared to initial condition. XPS and HAXPES spectra of Mn 2p, Co 2p, and Ni 2p were measured to investigate the cathode surface state. XPS and HAXPES detection depths are approximately 6 and 30 nm, respectively. No spectral changes were observed for Mn 2p 1/2 and Co 2p 1/2. Figure 2 shows the XPS and HAXPES spectra of Ni 2p 1/2. Ni 2p 1/2 XPS peak shifted toward higher energies (around 1.9 eV), indicative of the higher valence state of Ni at the cathode. Ni 2p 1/2 HAXPES peak only slightly shifted (around 0.7 eV) as a potential result of the oxidation of Ni2+, consistent with the partially charged state of the cathode. The difference between XPS and HAXPES suggests that the cycle test changed the cathode surface state. The structural properties of the cathode surface were evaluated by TEM. The lattice fringes of the layered rock-salt Li(Ni1/3Mn1/3Co1/3)O2 structure disappeared from the active material surface, and the electron diffraction pattern of the surface was unclear. These results indicate that amorphization occurred at the cathode surface (about 3 nm), augmenting the charge transfer resistance. Moreover, electrolyte analysis revealed that LiPF6was decomposed, explaining the increase in electrolyte resistance. In summary, the long-term cycle test revealed that the capacity loss stemmed from the shift of cathode and anode reaction regions and cathode degradation. Furthermore, the increase in impedance spectrum resulted from cathode surface amorphization and LiPF6decomposition. Acknowledgment Synchrotron radiation experiments were performed at the BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Institute (JASRI) (Proposal No. 2013A1234, 2014A1558, 2014B1015, and 2014B1594) [1] T. Matsuda et al. 226th ECS meeting A5-343 Figure 1

A method for the rapid determination of theoretical relative sensitivity factors (RSFs) for hard X‐ray photoelectron spectroscopy (HAXPES) instruments of any type and photon energy has been developed. We develop empirical functions to describe discrete theoretically calculated values for photoemission cross sections and asymmetry parameters across the photon energy range from 1.5 to 10 keV for all elements from lithium to californium. The formulae describing these parameters, in conjunction with similar practical estimates for inelastic mean free paths, allow the calculation of a full set of theoretical sensitivity factors for a given X‐ray photon energy, X‐ray polarisation and instrument geometry. We show that the anticipated errors on these RSFs are less than the typical errors generated by extracting X‐ray photoelectron spectroscopy (XPS) intensities from the spectra and thus enable adequate quantification for any XPS/HAXPES experiment up to 10 keV. A spreadsheet implementation of this method is provided in the supporting information, along with example RSFs for existing commercial instruments.

Investigation of the corrosion behavior of AlCoCrFeNi high-entropy alloy in 0.5 M sulfuric acid solution using hard and soft X-ray photoelectron spectroscopy

Hard X-ray photoelectron spectroscopy (HAXPES) (⩽15 keV) at SpLine, the Spanish CRG beamline at the ESRF

X-ray Photoelectron Spectroscopy (XPS) is a widely used surface analysis technique with many well established industrial and research applications. The surface sensitivity (top 5-10 nm) of XPS and its ability to provide short-range chemical bonding information make the technique extremely popular in materials characterization and failure analysis laboratories. While its surface sensitivity is an important attribute, in some cases, the depth of analysis of XPS is not sufficient to analyze buried interfaces without first sputter etching the sample surface. However, sputter etching can often lead to alterations of the true surface chemistry. An alternative to sputter etching the sample is Hard X-ray Photoelectron Spectroscopy (HAXPES), available at some synchrotron facilities. HAXPES utilizes X-rays typically defined as having energies greater than 5 keV. By increasing the photon energy of the X-ray source, the mean free path of photoelectrons is increased, resulting in an increased information depth obtained from the sample. Depending on the energy used, these hard X-rays can provide depths of analysis three or more times than that of soft x-rays used on conventional XPS systems. HAXPES measurements are, therefore, more sensitive to the bulk, and contributions from the surface are minimized.1,2 This presentation will describe a laboratory-based instrument, the PHI Quantes, equipped with two scanning microprobe monochromated X-ray sources, Al Kα (1486.6 eV) and Cr Kα (5414.9 eV), thus enabling both traditional XPS and HAXPES experiments in the same instrument. Combining both soft and hard X-ray analyses, we can gain an even better understanding of composition with depth and information at buried interfaces. References Kobayashi, K. Hard X-ray photoemission spectroscopy, Nucl. Instr. Meth. Phys. Res. A 2009, 601, 32-47.Fadley, C.S. Hard X-ray Photoemission: An Overview and Future Perspective. In Hard X-ray Photoelectron Spectroscopy (HAXPS); Woicik, J. C., Ed; Springer: Switzerland 2016.

Resistive random access memory (ReRAM) has been proposed as a new application for oxide materials and advanced to the commercial manufacturing stage. An oxide sandwiched between two metal electrodes shows reversible electric field–induced resistance switching behaviors. These are many resistivity changing mechanisms for the oxide based ReRAM structures such as the insulator–metal transition in perovskite oxides and conductive-bridging (CBRAM). In our research, we focus on the CBRAM with nano-electrolyte reaction, whose advantage is the low forming voltage. The CBRAM with nano-electrolyte reaction comprises the generation and rupture of a metal filament using a metal such as Ag and Cu acting as a fast mobile ion in oxides. Hafnium oxide (HfO2), which is used as a high-k gate insulator for advanced complementary metal-oxide-semiconductor (CMOS) technologies, has shown resistance switching phenomena and been increased interest in the use of HfO2 and related oxides as potential ReRAM materials.1) To put the oxide based ReRAM on practical applications, understanding on controls of metal/oxide interface is essentially important. Here, we employed hard x-ray photoelectron spectroscopy (HX-PES) under bias operation. HX-PES is a powerful tool for investigating the electronic structure and chemical state of the surface/interface of stacking structures for nanoelectronics devices without any degradation because it has a longer photoelectron mean free path than conventional x-ray photoelectron spectroscopy using Al Kα radiation (hν= 1486.6 eV). The detection depth of HX-PES with an energy of 6 keV is approximately three times deeper than that of conventional XPS, so the photoelectron from a metal/oxide interface, which works as an electrical device, can be detected by HX-PES. With this method, bias-induced compositional changes around the metal/oxide interface during device operation have been directly observed. HX-PES was performed at the SPring-8 BL15XU undulator beamline. The incident X-ray energy and the total energy resolution were 5.95 keV and 240 meV, respectively. We have demonstrated resistance switching using HfO2 film with a Cu top electrode for nonvolatile memory applications, and revealed the Cu diffusion into the HfO2 layer during the conductive filament formation process. Resistive switching was clearly observed in the Cu/HfO2/Pt structure by performing current-voltage measurements. The current step from a high resistive state to a low resistive state was of the order of 103-104, which provided a sufficient on/off ratio for use as a switching device. The filament formation process was investigated by employing HX-PES under bias operation. The application of a bias to the structure reduced the Cu2O state at the interface and the intensity ratio of Cu 2p3/2/Hf 3d5/2, providing evidence of Cu2O reduction and Cu diffusion into the HfO2 layer. These results also provide evidence that the resistance switching of the Cu/HfO2/Pt structure originates in a solid electrolyte reaction containing Cu ions. HX-PES also revealed the top or bottom electrode dependences of interface reduction or oxidization, and ion migration behaviors.2-5) In the presentation, we will show the details of the correlation between the switching mechanism and the interface reaction in the electrode/high-k dielectrics based ReRAM structure.6) We are grateful to HiSOR, Hiroshima Univ. and JAEA/SPring-8 for the development of HX-PES at BL15XU of SPring-8. The HX-PES measurements were performed under the approval of the NIMS Beamline Station (Proposal Nos. 2009A 4600, 2010B 4600, 2011A 4611, 2011B 4613, and 2012A 4613).

Core-level Photoemission Spectroscopy (PES) has played a very important role in our understanding of the electronic structure of correlated transition metal and rare-earth compounds. The appearance of strong satellite structures accompanying the main PES spectra in correlated systems is well known, and systematic variations in the position and intensities of these satellites provide us with important clues to their electronic structures. In spite of these successes, the surface sensitivity of PES has often led to controversies regarding surface versus bulk electronic structure, and hence, hard X-ray photoelectron spectroscopy (HAXPES) is very important and promising. HAXPES is a bulk sensitive probe of the electronic structure due to its ability to overcome surface sensitivity of conventional PES. Unlike soft X-ray PES, 2p core-level HAXPES have shown additional well-screened features with significant intensity at the low binding energy side of the main peak. These features were explained well by the configuration-interaction model including a screening channel derived from coherent states near Fermi energy. Here, we review these advances and examine the application of HAXPES to studies of the strongly correlated electron systems, especially for 3d transition metal compounds. The details of the well-screened features are also discussed.

We had, in a previous study, found that flat cold-rolled Ni3Al foil is spontaneously activated during the initial stage of catalytic methanol decomposition, and further, this is accompanied by the gradual formation of fine Ni particles. In this study, we investigate the evolution of the foil surface structure at the beginning of the spontaneous activation by using hard X-ray (hν = 5.95 keV) photoelectron spectroscopy. The core level spectra of Ni 2p, Ni 3p, Al 2p, O 1s, Al 1s, and C 1s have been analyzed in detail. Ni in the Ni3Al foil remained in the metallic state during the reaction, and neither Ni oxide nor Ni hydroxide was formed. In contrast, Al was found to react with the gaseous products of methanol decomposition to form two compounds in succession. At the beginning of the reaction, Al was oxidized to form an Al2O3 layer on the surface. The outer surface of the Al2O3 layer then hydroxylated to Al(OH)3, thereby forming an Al(OH)3/Al2O3 two-layer structure. Thus, it was found that an Al(OH)3/Al2O3 t...

Locally resolved investigation of wedged Cu(In,Ga)Se 2 films prepared by physical vapor deposition using hard X-ray photoelectron and X-ray fluorescence spectroscopy

Hard X-ray photoelectron and X-ray absorption spectroscopy characterization of oxidized surfaces of iron sulfides

Electronic structure of single crystal UPd 3, UGe 2, and USb 2 from hard X-ray and angle-resolved photoelectron spectroscopy

Combined chemical analyses of both the surface and bulk of industrial catalysts is a significant challenge, because all microanalysis methods reveal either the surface or the bulk properties but not both. We demonstrate the combined use of hard and soft X‐ray photoelectron spectroscopy (XPS) as a powerful, practical, and nondestructive tool to quantitatively analyze the chemical composition at the surfaces (~1 nm) and subsurfaces/bulk (~10 nm) for catalysts. The surface‐bulk differentiation is achieved via an exchangeable anode system, where the Al (Kα, 1486.6 eV) and Cr (Kα, 5414.7 eV) for the XPS and hard X‐ray photoelectron spectroscopy (HAXPES) analyses, respectively, are interchanged without affecting the X‐ray beam position on the sample. As an archetypical catalyst, we study the perovskite‐type material La 0.30 Sr 0.55 Ti 0.95 Ni 0.05 O 3‐δ (LSTNO), which has differing chemical compositions at the surface and subsurface after reduction and oxidation reactions. We look at the relative changes in surface composition, which minimizes the error stemming from the differing relative sensitivity factors in the oxidized and reduced states. The HAXPES‐XPS analysis indirectly confirms the well‐known exsolution and formation of Ni nanoparticles on the surface upon reduction though following changes of Ni concentration at the surface. However, the XPS‐HAXPES analysis demonstrates an increase in not only the Ni but also the Sr, which corroborates the reorganization within the perovskite lattice upon reduction. The XPS‐measured intensities decrease for all the accessible peaks (La 3d, Sr 3d, lattice O 1s, and Ti 2p), which is attributed to the photon diffusion by the surface Ni nanoparticles.

1. Introduction GeSn alloy is an attractive material as a high hole mobility channel substituting Si and as a stressor for strained channels. Furthermore, modulation of band structure from indirect to direct gap by the increasing Sn composition up to 6-10 % is expected to improve the performance of optical devices [1, 2]. Then, a high-quality GeSn thin film with higher Sn concentration is a promising for higher performance and integrated devices on the Si platform. However, it has been difficult to increase Sn concentration, because the solubility limit of Sn within a GeSn film is considered to be approximately 1 atomic % [3]. Hence, a growth technique which enables the suppression of Sn segregation during GeSn growth and a characterization to analyze the surface segregation occurred with a thickness of several nm, are essential. For these purpose, we have developed a metal-organic chemical-vapor-deposition (MOCVD) method with new source gases that is safe, uniform, and industrially applicable [4]. In addition, we also investigated a hard X-ray photoelectron spectroscopy (HAXPES) and X-ray diffraction (XRD) on synchrotron technique to analyze thin GeSn films. In this presentation, we introduce high Sn concentration GeSn thin film exceeding 6% prepared by combining the new MOCVD growth and synchrotron analysis methods. 2. Experimental Thin GeSn films with the thickness of typically 30-100 nm were grown on (001) Ge substrates at low temperature (320-360 degrees) by the MOCVD method using Ge (t-C4H9GeH3) and specially prepared Sn ((C2H5)4Sn) source gases. The Sb was doped from the atmosphere with residual triisopropyl-antimony [(i-C3H7)3Sb]. The target compositions of Sn were 2%, 3%, 6%, respectively. To clarify the crystallinity of GeSn films, HAXPES and XRD were carried out at SPring-8. Since the inelastic mean free path (IMFP) is several times deeper than that for conventional X-ray photoelectron spectroscopy (XPS), HAXPES is expected to be useful to identify simultaneously the variation of chemical state at a surface part and the underlying bulk part of a GeSn film [5]. The Sn and Sb concentrations were calculated by the combination of Rutherford back-scattering spectroscopy (RBS) and secondary ion mass spectrometry (SIMS) measurements. Cross-sectional TEM was also carried out for the evaluation of the crystal quality. 3. Results and discussion Initially, we observed the splitting of the Sn3d5/2 spectrum into two peaks for 3% Sn composition GeSn film by the HAXPES measurement, whereas the spectrum of a low Sn composition (2%) GeSn film remained single (Fig.1). The binding energy of the newly split peak (M2) was lower than that of the Sn3d5/2 peak (M1) and the peak position of M1 approximately coincided with that of the 2% GeSn film. To clarify the newly split Sn3d5/2 peak (M2), total reflection mode HAXPES (TR-HAXPES) measurement was also carried out for the 3% GeSn film. As a result, only a single Sn3d5/2 spectrum was observed by the measurement. Since the position of the observed peak by the TR-HAXPES, closely coincided with the split peak (M2), the M2 was identified as a peak derived from the Sn segregation formed at the film surface. Hence, it is concluded that depth profile characterization of the Sn chemical state within a GeSn film possible by combining the HAXPES and TR-HAXPES measurements. In order to further increase the Sn concentration, we employed Sb as a surfactant [6]. As a result, neither extreme change of XRD spectrum due to strain relaxation or asymmetric structure of HAXPES by the Sn segregation was observed for the newly grown GeSn film of 6% Sn. A decrease in the broad peak intensity on the high binding energy side due to surface oxidation was also confirmed. Since the surface is more stabilized by Sb with lower interfacial free energy, it is expected that suppression of Sn segregation and uniform Sn concentration in depth direction are simultaneously achieved. The concentration distribution of Ge and Sb were confirmed by the RBS and SIMS, and it was revealed that the Sn concentration was uniform in the depth direction, and the composition of Sn and Sb was 6.6 at.% and 0.5 at.%, respectively. Hence, by combining newly developed MOCVD technique and synchrotron technique, it was confirmed that uniform GeSn thin layer with higher Sn composition was realized. Reference [1] S.Zaima, JJAP 52 (2013) 030001, [2] R.Cjhen et al., APL 99, (2011) 181125, [3] C. Thurmond et al., J.Chem.Phys. 25,(1956)799, [4] K.Suda et al., ECS J.SSST, 4 (2015) 152. [5] K.Usuda et al., MRS spring meeting, (2016), EP11.6.10. [6] X. Yang et al., IEEE Photon.Tech.Lett.12,(2000) 128. Figure 1

1. Introduction As a packing density in the large-scale integration (LSI) becomes higher, the devices exhibit three-dimensional complicated structure. The deep trench with high aspect ratio (AR), i.e. opening/depth, is one of the components to realize the structure [1]. The atomic layer deposition (ALD) is one of techniques suitable for trench structure, because which can conformally deposit a thin film along the high AR trench structure [2,3]. The ALD film theoretically consist of the same composition with the same thickness along the trench no matter how the AR is high. In this study, we evaluated the ALD deposited SiN films along the deep trench with high AR of 3 and 7.5. 2. Experiment 10 nm-thick SiN films were deposited using alternate supply of SiH2Cl2 and NH3 precursor with plasma on the plane and trench structure with 40 nm opening and 120 or 300 nm deep, AR of 3 and 7.5 at 350, 550, 630oC, respectively. The SiN film were conformally deposited along the trench as shown in Fig. 1. The fabricated samples were evaluated using, conventional X-ray photoelectron spectroscopy (XPS) and Hard X-ray photoelectron spectroscopy (HAXPES). For the XPS and HAXPES evaluation, we carried out angle-resolved spectroscopy. As the HAXPES equipment, a laboratory-based HAXPES (Lab. HAXPES) equipment product of Scienta Omicron Inc. was used [4]. The X-ray source energy was 9,251.74 eV from liquid GaKα. The photoemission angle (Take-off angle: TOA) was varied from 90 degree to 30, with the photoelectron energy resolution of approximately 0.5 eV. 3. Results and Discussion Figure 1 illustrates the trench sample schematics with the explanation of the angle-resolved measurements. One can understand by reducing the TOA from 90 degrees, the photoelectron from the lower part of the trench side walls cannot reach to the photoelectron energy analyzer, therefore the measurement becomes top layer sensitive. Figure 2 compares the Si 1s spectra from the samples at 350oC with (blue) and without (red) trenches. The film on the flat surface, without trenches, showed mostly composed of Si-N chemical bonds, while the spectrum from the trench sample showed more Si-O bonds in the film. The films on or in the trench structure seem to be different from the flat surface, more oxide than nitride, although the ALD was performed to deposit SiN film. Figure 3 shows the angle-resolved HAXPES results for the trench sample at 350oC. From Fig. 3, it can be recognized the Si-O component in the film decreased by reducing the TOA from 90 to 60, implying there are more oxide in the lower part of trench than the upper part. Plasma activated N precursors might lose their energy (activity) during the proceeding narrow high AR trench, resulting in the non-stoichiometric film remained close to the bottom part of trench, which might be oxidized after the film deposition, although the film thickness seems to be the same for all over the trench structures. The stoichiometry uniformity was achieved by elevating the deposition temperature up to 550oC. 4. Conclusion In conclusions, we have evaluated the ALD SiN film conformally formed physically in the high AR trench and found there were possible non-uniformity in the chemical structure along the deep trench. Acknowledgements We appreciate the SPring-8 BENTEN database for the Si 1s spectra assignment. Reference [1] T. Franza et al., ACS Appl. Mater. Interfaces 9, 1858 (2017).[2] T. Antonio et al., Materials Matters 13, 55 (2018).[3] X. Meng et al., Materials 9, 1007 (2016).[4] A. Regoutz et al., Rev. Sci. Instrum. 89, 073105 (2018). Figure 1