Hard X-ray Photoelectron Spectroscopy Research Articles

Experimental and theoretical studies on atomic structures of the interface states at SiO2/4H-SiC(0001) interface

We investigated the atomic structures of the interface states (gap states) at the SiO2/4H-SiC(0001) interface using hard x-ray photoelectron spectroscopy (HAXPES), operando hard x-ray photoelectron spectroscopy, extended x-ray absorption fine structure, and first principles molecular dynamics (FPMD) calculations. For the interface states, two types were observed: continuous interface states in the whole gap and interface states with discrete energy levels near the conduction band minimum (CBM). From HAXPES, we found that carbon clusters and carbon vacancies were formed at the SiO2/4H-SiC(0001) interface. FPMD calculations on the SiO2/4H-SiC(0001) interface showed that the interface states in the whole gap were attributed to the various atomic geometries of the CßSi3 species and the carbon clusters with various sizes and surrounding atoms. For the interface states with a discrete energy level near the CBM, we could not find their atomic structure using our current calculations. We calculated the carbon vacancies prepared on the side of an SiC substrate at the SiO2/4H-SiC(0001) interface, indicating the formation of a discrete energy level in the midgap. It is likely that carbon vacancies formed at the step of the interface may modulate the midgap energy level to energy below the CBM. Therefore, we propose that the interface states with discrete energy levels near the CBM could be attributed to the carbon vacancies formed on the steps at the SiO2/4H-SiC(0001) interface.

Effects of carbon impurity in ALD-Al2O3 film on HAXPES spectrum and electrical properties of Al2O3/AlGaN/GaN MIS structure

In this study, the effects of carbon impurity in the atomic layer deposited-Al2O3 film on the hard X-ray photoelectron spectroscopy (HAXPES) spectra and the electrical properties of metal–insulator–semiconductor (MIS) structure were measured. The carbon concentration in the Al2O3 film was adjusted by varying the deposition conditions (precursor: trimethylaluminum, Al(CH3)3, or dimethylaluminum hydride, Al(CH3)2H, oxidant, and deposition temperature) in the atomic layer deposition process. The HAXPES measurements revealed the correlation between full width at half maximum (FWHM) of Al 1 s and O 1 s spectra and the carbon concentration in the Al2O3 film. Furthermore, the negative charges in the Al2O3 film could change the FWHM, attributed to the carbon impurity. The correlation between the carbon concentration and the electrical characteristics of the Al2O3/AlGaN/GaN MIS structure was analyzed. The interface state density and the effective charge density were dependent on the carbon concentration in the Al2O3 film.

Effects of Lithium Bis(oxalate)borate Electrolyte Additive on the Formation of a Solid Electrolyte Interphase on Amorphous Carbon Electrodes by Operando Time-Slicing Neutron Reflectometry.

Comprehensive analyses were performed using neutron reflectivity and hard X-ray photoelectron spectroscopy to understand the structure and composition of the solid electrolyte interphase (SEI) layer during charge-discharge processes and because of the addition of lithium bis(oxalate)borate (LiBOB) to improve the battery performance. The chemical composition of the SEI was assessed using these methods, and the amount of Li+ intercalated in the anode during the electrochemical reaction was evaluated. The results demonstrated that Li2C2O4 was produced initially but later decomposed to Li2CO3 on the first charge cycle. Presumably, the SEI layer formed by the decomposition of LiBOB was a single dense layer and chemically stable during the further charge-discharge processes owing to the difference in the reaction process. Therefore, the reduced Li+ transfer resistance and charging capacity accounted for the substantial improvement contributed by adding LiBOB. Moreover, the charges used for the intercalation of Li+ and SEI formation during the two-cycle processes were analyzed. The addition of LiBOB increased the discharge capacity of the anode and provided an additional charge used for SEI formation, presumably for decomposing Li2C2O4, which could reflect the durability of the Li-ion batteries. The electrode, electrolyte, and charge-discharge reactions affect the SEI properties and consequently the electrochemical reactions. Therefore, additional investigations under different charge-discharge conditions would reveal important characteristics such as the charge and discharge efficiency, output performance, and safety.

Multiple elastic scattering of electrons in condensed matter (new version announcement)

Multiple elastic scattering of electrons in condensed matter (new version announcement)

Depth-Dependent Understanding of Cathode Electrolyte Interphase (CEI) on the Layered Li-Ion Cathodes Operated at Extreme High Temperature

The high-temperature operation of Li-ion batteries is highly dependent on the stability of the cathode electrolyte interphase (CEI) formed during lithiation–delithiation reactions. However, knowledge on the nature of the CEI is limited and its stability under extreme temperatures is not well understood. Therefore, herein, we investigate a proof-of-concept study on stabilizing CEI on model LiNi0.33Mn0.33Co0.33O2 (NMC333) at an extreme operation condition of 100 °C using the thermally stable pyrrolidinium-based ionic liquid electrolyte. The electrochemical lithiation–delithiation reactions at 100 °C and the CEI evolution upon different cycling conditions are investigated. Further, the depth-dependent CEI chemistry was investigated using energy-tunable synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). The results reveal that the high-temperature operation accelerated the CEI formation compared to room temperature, and the surface of the interphase layer is rich in boron-based inorganic moieties than the deeper surface. Further, bulk-sensitive X-ray absorption spectroscopy (XAS) was used to investigate the transition-metal redox contributors during high-temperature electrochemical reactions; similar to room temperature, the Ni2+/4+ redox couple is the only charge-compensating redox couple during high-temperature operation. Finally, the physical nature of the conformal CEI on the cathode particles was visualized with high-resolution transmission electron microscopy, which confirms that the significant degradation of cathode particles without conformal CEI is due to the transformation of a layer-to-spinel formation at extreme temperature. In this study, understanding this high-temperature interfacial chemistry of NMC cathodes through advanced spectroscopy and microscopy will shed light on transforming the ambient-temperature Li-ion chemistry into high-temperature applications.

Evaluation of the thermal stability of TiW/Cu heterojunctions using a combined SXPS and HAXPES approach

Power semiconductor device architectures require the inclusion of a diffusion barrier to suppress or at best prevent the interdiffusion between the copper metallization interconnects and the surrounding silicon substructure. The binary pseudo-alloy of titanium–tungsten (TiW), with >70 at. % W, is a well-established copper diffusion barrier but is prone to degradation via the out-diffusion of titanium when exposed to high temperatures (≥400 °C). Here, the thermal stability of physical vapor deposited TiW/Cu bilayer thin films in Si/SiO2(50 nm)/TiW(300 nm)/Cu(25 nm) stacks were characterized in response to annealing at 400 °C for 0.5 h and 5 h, using a combination of soft and hard x-ray photoelectron spectroscopy and transmission electron microscopy. Results show that annealing promoted the segregation of titanium out of the TiW and interdiffusion into the copper metallization. Titanium was shown to be driven toward the free copper surface, accumulating there and forming a titanium oxide overlayer upon exposure to air. Annealing for longer timescales promoted a greater out-diffusion of titanium and a thicker oxide layer to grow on the copper surface. However, interface measurements suggest that the diffusion is not significant enough to compromise the barrier integrity, and the TiW/Cu interface remains stable even after 5 h of annealing.

Hard X‐Ray Photoemission Study of Heusler‐Type Fe2−xRexVAl Thermoelectric Compounds

The electronic structure of Fe2−xRexVAl compounds is investigated using bulk‐sensitive hard X‐ray photoelectron spectroscopy (HAXPES) measurements and electronic band calculations to elucidate the origin of the improvement in their thermoelectric properties. The HAXPES results show that, with the substitution of Re atoms for the Fe site, a new Re 5d state appears near the Fermi level, which leads to a change in the electronic structure with a gradual narrowing of the pseudo‐gap. The decrease in the pseudo‐gap width with Re substitution is found to be related to the increase in the p‐type Seebeck coefficient and the shift in its peak temperature to the lower temperature side.

Operando analysis of electronic band structure in an all-solid-state thin-film battery

Material characterization that informs research and development of batteries is generally based on well-established ex situ and in situ experimental methods that do not consider the band structure. This is because experimental extraction of structural information for liquid-electrolyte batteries is extremely challenging. However, this hole in the available experimental data negatively affects the development of new battery systems. Herein, we determined the entire band structure of a model thin-film solid-state battery with respect to an absolute potential using operando hard X-ray photoelectron spectroscopy by treating the battery as a semiconductor device. We confirmed drastic changes in the band structure during charging, such as interfacial band bending, and determined the electrolyte potential window and overpotential location at high voltage. This enabled us to identify possible interfacial side reactions, for example, the formation of the decomposition layer and the space charge layer. Notably, this information can only be obtained by evaluating the battery band structure during operation. The obtained insights deepen our understanding of battery reactions and provide a novel protocol for battery design.

Operando hard X-ray photoelectron spectroscopy study of buried interface chemistry of Au/InO1.16C0.04/Al2O3/p[formula omitted]-Si stacks

Operando hard X-ray photoelectron spectroscopy (operando-HAXPES) with synchrotron radiation source was used under an applied bias voltage to investigate prototypical Au/InO1.16C0.04/Al2O3/p+-Si structure. First, HAXPES analysis reveals a successful incorporation of carbon in the In2O3 matrix (InO1.16C0.04) which is intentionally used to stabilized intrinsic oxygen vacancies and limit their harmful migrations. Results show that the InO1.16C0.04 layer tends to remove the oxygen from the Al2O3 layer and create an oxygen depletion region into the Al2O3 film subsequent to the InO1.16C0.04 deposition step. Under bias, we observe a combination of band bending from Al2O3 and redox reaction in the InO1.16C0.04. From depth dependent HAXPES measurements, we conclude that the redox process mainly takes place near the Au/InO1.16C0.04 top interface. Thanks to the HAXPES extended probing depth, we highlight that the in-operando analysis may be an effective tool for a dynamic observation of TFT buried interfaces. We hope that our operando-HAXPES measurements will contribute to develop a full understanding of interfacial structure, energy bands and TFT operation mechanisms.

Real-Time Monitoring the Growth of Epitaxial CoxFe3-xO4 Ultrathin Films on Nb-Doped SrTiO3(001) via Reactive Molecular Beam Epitaxy by Means of Operando HAXPES.

In this work, we present a comprehensive study on real-time monitoring the growth of epitaxial CoxFe3−xO4 thin films grown on SrTiO3(001) substrates via reactive molecular beam epitaxy. The growth process was monitored during evaporation by means of time resolved operando hard X-ray photoelectron spectroscopy (HAXPES). We prepared ultrathin ferrite films using different oxygen partial pressures, showing pure metallic, light oxidic, and cobalt ferrite-like growth. Additional X-ray diffraction measurements confirm HAXPES results.

Standing wave hard X-ray photoemission study of the structure of the interfaces in Ta/Co2FeAl/MgO multilayer

Interfaces of the magnetic layer in the heterostructure Ta/Co2FeAl/MgO have been studied using hard X-ray photoelectron spectroscopy (HAXPES). Depth-selective information with sub-nanometer resolution is obtained using X-ray standing waves. It is found that the two magnetic species namely, Co and Fe behave very differently at the interfaces; 54% of Fe in the magnetic layer gets either oxidised or alloyed with Ta at the interfaces, as against 48% of Co. Oxidation state of both Co and Fe decreases, as one moves away from the MgO interface. On the average about 0.36 nm of Co2FeAl gets oxidised at MgO interface and 0.66 nm gets alloyed with Ta at the bottom interface. Thermal annealing at 200℃ results in further oxidation of Fe, accompanied by a partial reduction of Co. It is suggested that this opposite behaviour of Fe and Co at the MgO interface may be the cause of the thermal annealing induced shift of the centroid of Fe distribution towards MgO layer, as observed in some STEM studies in such heterostructures.

The electrical conductivity of cubic (In1−x Ga x )2O3 films (x ≤ 0.18): native bulk point defects, Sn-doping, and the surface electron accumulation layer

The alloying of the group-III transparent semiconducting sesquioxides In2O3 and Ga2O3 can lead to a modulation of the properties of the parent compounds, e.g. the shallow- and deep-donor character of the oxygen vacancy or the presence and absence of a surface electron accumulation layer, respectively. In this work, we investigate the effect of alloying on the electron transport properties of unintentionally-doped single-crystalline and textured bixbyite (In1−x Ga x )2O3 thin films annealed in oxygen and vacuum with Ga contents up to x = 0.18. Hall effect measurements demonstrate a surprising increase in electron density due to native defects with added Ga, possibly induced by Ga-related unit-cell distortions. Based on the measured electron mobility we assign this increase to oxygen vacancies rather than Ga-interstitials. The large electron density of >7 × 1019 cm−3 in vacuum-annealed, Ga-containing films would place the 2+/0 transition level of the O-vacancy in our films at least 340 meV above the conduction band minimum. A combined investigation based on hard and soft X-ray photoelectron spectroscopy measurements demonstrates the existence of the surface electron accumulation layer for all alloy films irrespective of annealing and, hence, no depletion up to x = 0.18. Comparison of Hall and Seebeck measurements confirms negligible contribution of the SEAL to the electron transport in the few-100 nm thick, unintentionally-doped films. Finally, we additionally demonstrate a single-crystalline (In0.92Ga0.08)2O3:Sn film, as a possible transparent conductive oxide with a wider band gap than that of (Sn-doped) In2O3, with transport properties similar to those of ITO.

Molecular beam epitaxy of single-crystalline bixbyite (In1−xGax)2O3 films ( x≤0.18 ): Structural properties and consequences of compositional inhomogeneity

In this work, we show the heteroepitaxial growth of single-crystalline bixbyite (In$_{1-x}$Ga$_x$)$_2$O$_3$ films on (111)-oriented yttria-stabilized zirconia substrates using plasma-assisted molecular beam epitaxy under various growth conditions. A pure In$_2$O$_3$ buffer layer between the substrate and (In$_{1-x}$Ga$_x$)$_2$O$_3$ alloy is shown to result in smoother film surfaces and significantly improved crystallinity. Symmetric out-of-plane 2$\theta - \omega$ x-ray diffraction scans show a single (111) crystal orientation and transmission electron microscopy confirms the single-crystallinity up to $x = 0.18$ and only slight film quality deterioration with increasing Ga content. Partially relaxed layers are demonstrated via reciprocal space mapping with lattice parameters fitting well to Vegard's law. However, the Ga cations are not evenly distributed within the films containing nominally $x > 0.11$: inclusions with high Ga density up to $x = 0.50$ are observed within a "matrix" with $x \approx 0.08$. The cubic bixbyite phase is preserved, in both the "matrix" and the inclusions. Moreover, for $x \geq 0.11$, both the Raman phonon lines as well as the optical absorption onset remain nearly constant. Hard x-ray photoelectron spectroscopy measurements also indicate a widening of the band gap and exhibit similar saturation of the Ga 2p core level position for high Ga contents. This saturation behavior of the spectroscopic properties further supports the limited Ga incorporation into the "matrix" of the film.

Development of Laboratory-based Hard X-ray Photoelectron Spectroscopy

Development of Laboratory-based Hard X-ray Photoelectron Spectroscopy

XAFS and HAXPES analyses of the oxidation state of a copper surface buried under a phenolic resin nanofilm

The oxidation state of a buried copper surface under a phenolic resin insulating layer was investigated by nondestructive analysis methods using conversion electron yield X-ray absorption fine structure (CEY-XAFS) and hard X-ray photoelectron spectroscopy (HAXPES). The formation of Cu2O and CuO during heat treatment at 180 °C in air and the formation of Cu(OH)2 and CuCO3 during long-term storage under atmospheric conditions at room temperature proceeded on the buried copper surface to a depth of tens of nanometers. The phenolic resin nanofilm suppressed the thermal oxidation of the underlying Cu to Cu2O and CuO by 20% but did not suppress the formation of native oxides Cu(OH)2 and CuCO3 under atmospheric conditions because of the high water absorption and high permeability of the phenolic resin nanofilm. This study demonstrated that CEY-XAFS and HAXPES technique are the powerful tools for investigation of oxidation states of the copper surface buried under the phenolic resin insulating layer.

Investigation of the Acceleration and Suppression of the Light-Induced Degradation of a Lead Halide Perovskite Solar Cell Using Hard X-ray Photoelectron Spectroscopy

Investigation of the Acceleration and Suppression of the Light-Induced Degradation of a Lead Halide Perovskite Solar Cell Using Hard X-ray Photoelectron Spectroscopy

Noble-Metal High-Entropy-Alloy Nanoparticles: Atomic-Level Insight into the Electronic Structure.

The compositional space of high-entropy-alloy nanoparticles (HEA NPs) significantly expands the diversity of the materials library. Every atom in HEA NPs has a different elemental coordination environment, which requires knowledge of the local electronic structure at an atomic level. However, such structure has not been disclosed experimentally or theoretically. We synthesized HEA NPs composed of all eight noble-metal-group elements (NM-HEA) for the first time. Their electronic structure was revealed by hard X-ray photoelectron spectroscopy and density function theory calculations with NP models. The NM-HEA NPs have a lower degeneracy in energy level compared with the monometallic NPs, which is a common feature of HEA NPs. The local density of states (LDOS) of every surface atom was first revealed. Some atoms of the same constituent element in HEA NPs have different LDOS profiles, whereas atoms of other elements have similar LDOS profiles. In other words, one atom in HEA loses its elemental identity and it may be possible to create an ideal LDOS by adjusting the neighboring atoms. The tendency of the electronic structure change was shown by supervised learning. The NM-HEA NPs showed 10.8-times higher intrinsic activity for hydrogen evolution reaction than commercial Pt/C, which is one of the best catalysts.

Design and demonstration of EID MOS-HEMTs on Si substrate with normally depleted AlGaN/GaN epitaxial layer

An extrinsic electron induced by a dielectric (EID) AlGaN/GaN MOS high-electron-mobility transistor (HEMT) on Si substrate was designed and investigated. The EID structure with SiO2 deposition and subsequent high-temperature annealing, which induces two-dimensional electron gases (2DEGs) on fully depleted AlGaN/GaN hetero-epitaxial layers with thin AlGaN barrier layer, was applied to access and drift regions in the HEMT. The fabricated HEMT exhibited enhancement-mode operation with a specific on-resistance of 7.6 mΩ cm2 and a breakdown voltage of over 1 kV. In addition, electron state analysis using hard X-ray photoelectron spectroscopy revealed that changes in the chemical states of Al and energy level lowering at the SiO2/AlGaN interface affect the induction of 2DEG in the EID structure. The proposed HEMTs should become a strong candidate for highly reliable high-power switching devices due to the damage-less fabrication without dry etching or fluorine plasma exposure processes on the semiconductor layers.

Hard x-ray photoelectron spectroscopy of Al2O3 with Cr Kα excitation

A thick alumina sample was analyzed by hard x-ray photoelectron spectroscopy by means of a lab-scale spectrometer equipped with a Cr Kα (5414.8 eV) excitation source. The reported spectra include a survey scan as well as Al 1s, Al 2s, Al 2p, O 1s, and C 1s core-level spectra.

Modification and Characterization of Interfacial Bonding for Thermal Management of Ruthenium Interconnects in Next-Generation Very-Large-Scale Integration Circuits.

Ruthenium may replace copper interconnects in next-generation very-large-scale integration (VLSI) circuits. However, interfacial bonding between Ru interconnect wires and surrounding dielectrics must be optimized to reduce thermal boundary resistance (TBR) for thermal management. In this study, various adhesion layers are employed to modify bonding at the Ru/SiO2 interface. The TBRs of film stacks are measured using the frequency-domain thermoreflectance technique. TiN and TaN with high nitrogen contents significantly reduce the TBR of the Ru/SiO2 interface compared to common Ti and Ta adhesion layers. The adhesion layer thickness, on the other hand, has only minor effect on TBR when the thickness is within 2-10 nm. Hard X-ray photoelectron spectroscopy of deeply buried layers and interfaces quantitatively reveals that the decrease in TBR is attributed to the enhanced bonding of interfaces adjacent to the TaN adhesion layer, probably due to the electron transfer between the atoms at two sides of the interface. Simulations by a three-dimensional electrothermal finite element method demonstrate that decreasing the TBR leads to a significantly smaller temperature increase in the Ru interconnects. Our findings highlight the importance of TBR in the thermal management of VLSI circuits and pave the way for Ru interconnects to replace the current Cu-based ones.