Hard X-ray Photoelectron Spectroscopy Research Articles

Electrically induced change in HfO2/1-monolayer TiO2/SiO2 metal-oxide-semiconductor stacks: capacitance–voltage and hard X-ray photoelectron spectroscopy studies

Metal-oxide-semiconductor capacitors with HfO2/1-monolayer TiO2/SiO2 stacks were examined to explore the origin of the interface dipole modulation. The capacitance–voltage (C–V) measurements exhibited that the polarity of the interface dipole layer changes depending on the gate bias. The hard X-ray photoelectron spectroscopy measurements demonstrated that an applied gate voltage induces small changes in the Ti–O chemical bonding and potential profile around the HfO2/SiO2 interface.

Combined Pulsed RF GD-OES and HAXPES for Quantified Depth Profiling through Coatings

Chemical characterization at buried interfaces is a real challenge, as the physico-chemical processes operating at the interface govern the properties of many systems and devices. We have developed a methodology based on the combined use of pulsed RF GD-OES (pulsed Radio Frequency Glow Discharge Optical Emission Spectrometry) and XPS (X-ray Photoelectron Spectroscopy) to facilitate the access to deeply buried locations (taking advantage of the high profiling rate of the GD-OES) and perform an accurate chemical diagnosis using XPS directly inside the GD crater. The reliability of the chemical information is, however, influenced by a perturbed layer present at the surface of the crater, hindering traditional XPS examination due to a relatively short sampling depth. Sampling below the perturbed layer may, however, can be achieved using a higher energy excitation source with an increased sampling depth, and is enabled here by a new laboratory-based HAXPES (Hard X-ray PhotoElectron Spectroscopy) (Ga-Kα, 9.25 keV). This new approach combining HAXPES with pulsed RF GD-OES requires benchmarking and is here demonstrated and evaluated on InP. The perturbed depth is estimated and the consistency of the chemical information measured is demonstrated, offering a new route for advanced chemical depth profiling through coatings and heterostructures.

Unified treatment of recoil and Doppler broadening in molecular high-energy photoemission

Doppler and recoil effects are an integral part of the photoemission process at the high kinetic energies reached in hard x-ray photo-electron spectroscopy (HAXPES) and have a major effect on the observed lineshape, resulting in broadening, energy losses and discrete excitations. These effects can be modeled with a high degree of detail for small systems like diatomic molecules, for larger systems such treatment is often superfluous as the fine spectral features are not observable. We present a united description of the Doppler and recoil effects for arbitrary polyatomic systems and offer an approximate description of the recoil- and Doppler-modified photoemission spectral lineshape as a practical tool in the analysis of HAXPES spectra of core-level photoemission. The approach is tested on the examples of carbon dioxide and pentane molecules. The C and O 1s photoelectron spectra of CO2 in gas phase were also measured at 2.3 and 7.0 keV photon energy at Synchrotron SOLEIL and the spectra were analyzed using the model description. The limitations and applicability of the approach to adsorbates, interfaces and solids is briefly discussed.

Antiferromagnetic Correlations in Strongly Valence Fluctuating CeIrSn.

CeIrSn with a quasikagome Ce lattice in the hexagonal basal plane is a strongly valence fluctuating compound, as we confirm by hard x-ray photoelectron spectroscopy and inelastic neutron scattering, with a high Kondo temperature of T_{K}∼480 K. We report a negative in-plane thermal expansion α/T below 2K, which passes through a broad minimum near 0.75K. Volume and a-axis magnetostriction for B∥a are markedly negative at low fields and change sign before a sharp metamagnetic anomaly at 6T. These behaviors are unexpected for Ce-based intermediate valence systems, which should feature positive expansivity. Rather they point towards antiferromagnetic correlations at very low temperatures. This is supported by muon spin relaxation measurements down to 0.1K, which provide microscopic evidence for a broad distribution of internal magnetic fields. Comparison with isostructural CeRhSn suggests that these antiferromagnetic correlations emerging at T≪T_{K} result from geometrical frustration.

Thermal and oxidation stability of TixW1−x diffusion barriers investigated by soft and hard x-ray photoelectron spectroscopy

The binary alloy of titanium-tungsten (TiW) is an established diffusion barrier in high-power semiconductor devices, owing to its ability to suppress the diffusion of copper from the metallization scheme into the surrounding silicon substructure. However, little is known about the response of TiW to high-temperature events or its behavior when exposed to air. Here, a combined soft and hard x-ray photoelectron spectroscopy (XPS) characterization approach is used to study the influence of post-deposition annealing and titanium concentration on the oxidation behavior of a 300 nm-thick TiW film. The combination of both XPS techniques allows for the assessment of the chemical state and elemental composition across the surface and bulk of the TiW layer. The findings show that in response to high-temperature annealing, titanium segregates out of the mixed metal system and upwardly migrates, accumulating at the TiW/air interface. Titanium shows remarkably rapid diffusion under relatively short annealing timescales, and the extent of titanium surface enrichment is increased through longer annealing periods or by increasing the bulk titanium concentration. Surface titanium enrichment enhances the extent of oxidation both at the surface and in the bulk of the alloy due to the strong gettering ability of titanium. Quantification of the soft x-ray photoelectron spectra highlights the formation of three tungsten oxidation environments, attributed to WO2, WO3, and a WO3 oxide coordinated with a titanium environment. This combinatorial characterization approach provides valuable insights into the thermal and oxidation stability of TiW alloys from two depth perspectives, aiding the development of future device technologies.

Direct observation of spin-resolved valence band electronic states from a buried magnetic layer with hard X-ray photoemission

ABSTRACT We report spin-resolved hard X-ray photoelectron spectroscopy (spin-HAXPES) for a buried Fe thin film in the valence band region. For the spin-HAXPES experiments, we developed an ultracompact built-in Mott-type spin-filter in a sample carrier, which enabled us to use the merit of two-dimensional (2D) multi-channel detector in a recent photoelectron analyser without modifying an apparatus for HAXPES. The effective Sherman function and the single-channel figure of merit (FOM) of the spin-filter were assessed to be −0.07 and 2.0 × 10−4, respectively. By utilizing the 2D detector of the photoelectron analyser, the effective FOM increased by a factor of ~4 × 104 compared to the case when only 1 channel of the 2D detector was used. We have applied spin-HAXPES to MgO(2 nm)/Fe(50 nm)/MgO(001) structures. The spin-HAXPES experiments revealed the majority and minority spin electronic states and the spin polarisation of the buried Fe thin film. Due to the large photoionization cross-section of the 4s orbital of Fe in HAXPES, the spin-resolved spectra mainly reflected the Fe 3d and 4s states. The observed spin-HAXPES and spin polarisation spectral shapes agreed well with the calculated spin-resolved cross-section weighted densities of states and spin polarisation spectra. In contrast, a small discrepancy in the energy scale was recognised due to the electron correlation effects. These results suggest that the electron correlation effects are important in the electronic structure of bulk Fe, and spin-HAXPES is useful for detecting genuine spin-resolved valence band electronic structures of buried magnetic materials.

Total reflection hard x-ray photoelectron spectroscopy: Applications to strongly correlated electron systems

We demonstrate that total reflection hard x-ray photoelectron spectroscopy (TR-HAXPES) is a versatile method for elucidating a difference between surface and bulk electronic states of strongly correlated electron systems, complementing conventional bulk sensitive hard x-ray photoelectron spectroscopy (HAXPES). To demonstrate the experimental feasibility of the method, we investigated ${\mathrm{La}}_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{MnO}}_{3}$ and the electron-doped high-${T}_{\mathrm{C}}$ superconductor ${\mathrm{La}}_{1.9}{\mathrm{Ce}}_{0.1}{\mathrm{CuO}}_{4}$. From the incident angle dependence of the spectral line shapes, we found that the surface-sensitive TR-HAXPES measurement equivalent to soft x-ray photoelectron spectroscopy is possible in the total reflection condition. The results strongly suggest that this method allows us to measure both surface and bulk electronic states without making any changes to experimental setup such as the energy resolution, x-ray energy, and the beamline.

Investigation of the effect of adding a moderate amount of hydrogen on the properties of tin oxide films deposited by DC magnetron sputtering

We investigated the effect of adding small amounts of hydrogen during DC magnetron sputtering of SnO x films followed by annealing in a nitrogen atmosphere at temperatures below the instability of SnO. Hard X-ray photoelectron spectroscopy analysis showed a difference in the initial Sn content in the as-deposited samples, which suggests an increased number of V Sn vacancies. This led to the formation of V Sn -H complexes, resulting in a doubling of the concentration of holes from 5.89 × 1017 cm−3 to 1.38 × 1018 cm−3 and a slight increase in the mobility from 1.17 cm2/(Vs) to 1.45 cm2/(Vs) after annealing. Thermal desorption spectroscopy revealed increased desorption of hydroxyl groups after the addition of hydrogen, indicating that hydrogen presence in the film as OH bonds may be the reason for the undesired increase in SnO2 content and raising of the crystallization point. The average transmittance of the hydrogen-added sample was 10% greater than that of the hydrogen-free sample.

Evolution of the local structure surrounding nitrogen atoms upon the amorphous to crystalline phase transition in nitrogen-doped Cr2Ge2Te6 phase-change material

Phase-change memory based upon resistive switching due to phase transitions between the amorphous and crystalline phases of chalcogenide-based phase-change materials (PCMs) has been widely studied for the next generation non-volatile memory (NVM). Unlike traditional PCMs such as Ge2Sb2Te5, nitrogen doped Cr2Ge2Te6 (NCrGT) was reported to not exhibit a bulk resistivity change upon the amorphous to crystalline phase transition, instead the contact resistivity contrast in the presence of metal electrodes was found to play an essential role. In the current study, a distinct difference in the local structure around N atoms in amorphous and crystalline NCrGT films was found using X-ray absorption near-edge spectroscopy (XANES) and hard X-ray photoelectron spectroscopy (HAXPES). CrN and GeN bonds and N2 molecular vibrational modes were found to be present in an amorphous NCrGT film, while N atoms were found to be present at Te substitutional positions in the crystalline phase. A previous study revealed that Cr nanoclusters are present in the undoped amorphous phase, whose concentration was found to decrease as crystallization progressed leading to an increase in the resistivity. In the present study, it was determined that N doping inhibits Cr nanocluster formation in the amorphous NCrGT phase and leads to a constant Cr nanocluster volume fraction even after crystallization, ensuring negligible resistance contrast between the amorphous and crystalline phases. These results will prove useful in understanding the role of dopant elements in the promising PCM CrGT.

Mapping nitrogen heteroatoms in carbon fibres using atom probe tomography and photoelectron spectroscopy

Carbon fibres show great potential as multifunctional negative electrode for novel structural battery composites – a rechargeable electrochemical cell with structural function. The electrochemical performance of carbon materials can be enhanced with nitrogen heteroatoms, which conveniently are inherent in polyacrylonitrile (PAN)-based carbon fibres. However, it is not fully understood how the electrochemical performance is governed by microstructure and composition of the carbon fibres, particularly the distribution and chemical states of nitrogen heteroatoms. Here we reveal the atom-by-atom three-dimensional spatial distribution and the chemical states of nitrogen in three PAN-carbon fibre types (M60J, T800 and IMS65), using atom probe tomography (APT) and synchrotron hard X-ray photoelectron spectroscopy (HAXPES), and correlate the results to electrochemical performance. The findings pave the way for future tailoring of carbon fibre microstructure for multifunctional applications.

Chemical Interaction at the MoO3/CH3NH3PbI3-xClx Interface.

The limited long-term stability of metal halide perovskite-based solar cells is a bottleneck in their drive toward widespread commercial adaptation. The organic hole-transport materials (HTMs) have been implicated in the degradation, and metal oxide layers are proposed as alternatives. One of the most prominent metal oxide HTM in organic photovoltaics is MoO3. However, the use of MoO3 as HTM in metal halide perovskite-based devices causes a severe solar cell deterioration. Thus, the formation of the MoO3/CH3NH3PbI3-xClx (MAPbI3-xClx) heterojunction is systematically studied by synchrotron-based hard X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. Upon MoO3 deposition, significant chemical interaction is induced at the MoO3/MAPbI3-xClx interface: substoichiometric molybdenum oxide is present, and the perovskite decomposes in the proximity of the interface, leading to accumulation of PbI2 on the MoO3 cover layer. Furthermore, we find evidence for the formation of new compounds such as PbMoO4, PbN2O2, and PbO as a result of the MAPbI3-xClx decomposition and suggest chemical reaction pathways to describe the underlying mechanism. These findings suggest that the (direct) MoO3/MAPbI3-xClx interface may be inherently unstable. It provides an explanation for the low power conversion efficiencies of metal halide perovskite solar cells that use MoO3 as a hole-transport material and in which there is a direct contact between MoO3 and perovskite.

Impact of oxygen on band structure at the Ni/GaN interface revealed by hard X-ray photoelectron spectroscopy

To investigate the impact of oxygen on the band structure at the Ni/p-type GaN interface, the crystal structure and nanoscale impurity distributions were evaluated using transmission electron microscopy and three-dimensional atom probe (3DAP) analysis, respectively. These measurements revealed that the oxygen region existed approximately 5 nm from the GaN surface and that the oxygen concentration was equal to or higher than the Mg acceptor concentration. The band bending and photoelectron spectrum were then simulated using the Mg and oxygen concentration profiles obtained by 3DAP to consider the impact of the interfacial oxygen donors on the photoelectron spectrum measured using hard X-ray photoelectron spectroscopy (HAXPES). The precise band bending was then determined by fitting the simulated spectrum onto the experimental measurements. This showed that the oxygen donors at the interface modulated the band structure and decreased the energy barrier by at least 0.1 eV, which demonstrates the importance of considering the existence of oxygen at the interface. It is, therefore, essential to use techniques like 3DAP and HAXPES to evaluate both the nanoscale impurity distributions and the resulting band structure to fabricate higher-performance devices.

Determination of Hafnium Zirconium Oxide Interfacial Band Alignments Using Internal Photoemission Spectroscopy and X-ray Photoelectron Spectroscopy.

Doped ferroelectric HfO2 is highly promising for integration into complementary metal-oxide semiconductor (CMOS) technology for devices such as ferroelectric nonvolatile memory and low-power field-effect transistors (FETs). We report the direct measurement of the energy barriers between various metal electrodes (Pt, Au, Ta, TaN, Ti/Pt, Ni, Al) and hafnium zirconium oxide (Hf0.58Zr0.42O2, HZO) using internal photoemission (IPE) spectroscopy. Results are compared with valence band offsets determined using the three-sample X-ray photoelectron spectroscopy (XPS) as well as the two-sample hard X-ray photoelectron spectroscopy (HAXPES) techniques. Both XPS and IPE indicate roughly the same dependence of the HZO barrier on metal work function with a slope of 0.8 ± 0.5. XPS and HAXPES-derived barrier heights are on average about 1.1 eV smaller than barrier heights determined by IPE, suggesting the presence of negative charge in the HZO.

Chemical Stability of IrO2 Top Electrodes in Ferroelectric Hf0.5Zr0.5O2‐Based Metal–Insulator–Metal Structures: The Impact of Annealing Gas

The IrO2 top electrode chemistry of ferroelectric IrO2/Hf0.5Zr0.5O2 (HZO)/IrO2 metal–insulator–metal (MIM) structures dependent on rapid thermal annealing in different gas atmospheres (nitrogen, oxygen, and forming gas [FG]) is investigated. Using hard X‐ray photoelectron spectroscopy (HAXPES), the strongly modified chemical states of the IrO2 layer dependent on the choice of annealing gas atmosphere are observed. For O2 and N2 anneals, the IrO2 electrode remains either unaffected or is just slightly chemically attacked at the top. In contrast, FG annealing causes a complete reduction of the IrO2 top electrode into metallic Ir. Surprisingly, oxygen is detected—unbound to Ir—which is incorporated in the metallic Ir layer. This mobile oxygen is thought to affect the electrical behavior of the IrO2/HZO/IrO2 device. In addition, it may serve as a test sample for future studies of the root causes of the role of oxygen‐vacancy interactions at the interface, which can influence the performance instabilities in HZO‐based MIM structures, such as wake‐up, imprint, and fatigue.

A spectrum deconvolution method based on grey relational analysis

The extensive usage of X-ray spectroscopies in studying complex material systems is not only intended to reveal underlying mechanisms that govern physical phenomena, but also used in applied studies focused on an insight-driven performance improvement of a wide range of devices. However, the traditional analysis methods for X-ray spectroscopic data are rather time-consuming and sensitive to errors in data pre-processing (e.g., normalization or background subtraction). In this study, a method based on grey relational analysis, a multi-variable statistical method, is proposed to analyze and extract information from X-ray spectroscopic data. As a showcase, the valence bands of microcrystalline silicon suboxides probed by hard X-ray photoelectron spectroscopy (HAXPES) were investigated. The results obtained by the proposed method agree well with conventionally derived composition information (e.g., curve fit of Si 2p core level of the silicon suboxides). Furthermore, the uncertainty of chemical compositions derived by the proposed method is smaller than that of traditional analysis methods (e.g., the least square fit), when artificial linear functions are introduced to simulate the errors in data pre-processing. This suggests that the proposed method is capable of providing more reliable and accurate results, especially for data containing significant noise contributions or that is subject to inconsistent data pre-processing. Since the proposed method is less experience-driven and error-prone, it offers a novel approach for automate data analysis, which is of great interest for various applications, such as studying combinatorial material “libraries”.

Cycling mechanism of Li2MnO3: Li–CO2 batteries and commonality on oxygen redox in cathode materials

Summary Li2MnO3 has been considered to be a representative Li-rich compound with active debates on oxygen activities. Here, by evaluating the Mn and O states in the bulk and on the surface of Li2MnO3, we clarify that Mn(III/IV) redox dominates the reversible bulk redox in Li2MnO3, while the initial charge plateau is from surface reactions with oxygen release and carbonate decomposition. No lattice oxygen redox is involved at any electrochemical stage. The carbonate formation and decomposition indicate the catalytic property of the Li2MnO3 surface, which inspires Li-CO2/air batteries with Li2MnO3 acting as a superior electrocatalyst. The absence of lattice oxygen redox in Li2MnO3 questions the origin of the oxygen redox in Li-rich compounds, which is found to be of the same nature as that in conventional materials based on spectroscopic comparisons. These findings provide guidelines on understanding and controlling oxygen activities toward high-energy cathodes and suggest opportunities on using alkali-rich materials for catalytic reactions.

Bulk electronic structure of high-order quaternary approximants

The authors use hard x-ray photoelectron spectroscopy to show the existence of a pseudogap at the Fermi level in high-order quaternary approximants, Al-Pd-Mo-Fe and Al-Pd-Cr-Fe.

X-SPEC: a 70 eV to 15 keV undulator beamline for X-ray and electron spectroscopies.

X-SPEC is a high-flux spectroscopy beamline at the KIT (Karlsruhe Institute of Technology) Synchrotron for electron and X-ray spectroscopy featuring a wide photon energy range. The beamline is equipped with a permanent magnet undulator with two magnetic structures of different period lengths, a focusing variable-line-space plane-grating monochromator, a double-crystal monochromator and three Kirkpatrick-Baez mirror pairs. By selectively moving these elements in or out of the beam, X-SPEC is capable of covering an energy range from 70 eV up to 15 keV. The flux of the beamline is maximized by optimizing the magnetic design of the undulator, minimizing the number of optical elements and optimizing their parameters. The beam can be focused into two experimental stations while maintaining the same spot position throughout the entire energy range. The first experimental station is optimized for measuring solid samples under ultra-high-vacuum conditions, while the second experimental station allows in situ and operando studies under ambient conditions. Measurement techniques include X-ray absorption spectroscopy (XAS), extended X-ray absorption fine structure (EXAFS), photoelectron spectroscopy (PES) and hard X-ray PES (HAXPES), as well as X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS).

In situ investigation of the bismuth vanadate/potassium phosphate interface reveals morphological and composition dependent light-induced surface reactions

Bismuth vanadate (BiVO4) is an established n-type oxide semiconductor for photoelectrochemical oxygen evolution. Direct charge carrier recombination at the solid/liquid interface is a major cause of efficiency loss in BiVO4-based devices. Intrinsic and extrinsic surface states (SSs) can act as electron and hole traps that enhance the recombination rate and lower the faradaic efficiency. In this study, we investigate the BiVO4/aqueous KPi interface using two types of samples. The samples were prepared at two different deposition and annealing temperatures (450 °C and 500 °C) leading to different morphologies and stoichiometries for the two samples. Both samples exhibit SSs in the dark that are passivated under illumination. In situ ambient pressure hard x-ray photoelectron spectroscopy experiments performed under front illumination conditions reveal the formation of a bismuth phosphate (BiPO4) surface layer for the sample annealed at 450 °C, whereas the sample annealed at 500 °C exhibits band flattening without the formation of BiPO4. These results imply that the light-induced formation of BiPO4 may not be responsible for SS passivation. Our study also suggests that slight differences in the synthesis parameters lead to significant changes in the surface stoichiometry and morphology, with drastic effects on the physical-chemical properties of the BiVO4/electrolyte interface. These differences may have important consequences for device characteristics such as long-term stability.

Effects of water and hydrogen introduction during In–Ga–Zn–O sputtering on the performance of low-temperature processed thin-film transistors

In–Ga–Zn–O (IGZO) films were deposited by sputtering in Ar + O2 + H2 and Ar + H2O atmosphere to investigate the effects of H2 and H2O introduction on physical and electrical properties of IGZO films and thin-film transistors (TFTs). A substantial reduction and oxidation of the IGZO films were confirmed by introducing H2 and H2O, respectively. We found that electron traps were formed for the Ar + H2O-sputtered IGZO TFTs, resulting in a degradation of field-effect mobility (μ FE), subthreshold swing (S.S.), and hysteresis (V H). In contrast, such degradations were not observed in the Ar + O2 + H2-sputtered IGZO TFTs and it exhibited μ FE of 12.5 cm2 V−1 s−1, S.S. of 0.15 V dec−1, and V H of 0.5 V. Hard X-ray photoelectron spectroscopy analysis revealed that excess oxygen incorporated during Ar + H2O sputtering is a possible cause of the TFT degradation. Thus, the reduction process, namely, Ar + O2 + H2 sputtering, is a promising approach for low-temperature-processed (∼150 °C) oxide TFTs for future flexible device applications.