Nondestructive investigation of interface states in high‐k oxide films on Ge substrate using X‐ray absorption spectroscopy

The unoccupied electronic structures of 5 nm thick high permittivity (k) oxides (HfO₂, ZrO₂, and Al₂O₃) and SiO₂ films on Ge substrates were examined using O K‐edge X‐ray absorption spectroscopy. Comparative studies with those on Si substrates showed contrasts in the conduction bands, which should be due to the formation of interface states. In the Al₂O₃ and SiO₂ films, GeO₂ layers are formed at the interface and they suppress in part the formation of detrimental germanate phases. In contrast, in the HfO₂ and ZrO₂ films, no signature of the Ge‐oxide phase is observed but some germanate phases are expected to prevail, suggesting a degradation of the gate oxide characteristics. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reduction of charge trapping in HfO2 film on a Ge substrate by trimethylaluminum pretreatment

Trimethylaluminum pretreatment prior to HfO₂ deposition is introduced for native oxide reduction. It is identified that the trimethylaluminum pretreatment could effectively reduce native oxide, which is transformed to an aluminum oxide interfacial layer. Formation of the thin aluminum oxide layer suppresses Ge diffusion into HfO₂, reducing hysteresis in the ca‐ pacitance–voltage curve. Moreover, the device reliability of the trimethylaluminum pretreated sample is improved in a constant current stress test. This work indicates that trimethylaluminum pretreatment is an effective in‐situ method for the gate dielectric stack formation to reduce charge trapping in the HfO₂ film on a Ge substrate. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High‐resolution spectroscopy on an X‐ray absorption beamline

A bent‐crystal spectrometer based on the Rowland circle geometry has been installed and tested on the BM30b/FAME beamline at the European Synchrotron Radiation Facility to improve its performances. The energy resolution of the spectrometer allows different kinds of measurements to be performed, including X‐ray absorption spectroscopy, resonant inelastic X‐ray scattering and X‐ray Raman scattering experiments. The simplicity of the experimental device makes it easily implemented on a classical X‐ray absorption beamline. This improvement in the fluorescence detection is of particular importance when the probed element is embedded in a complex and/or heavy matrix, for example in environmental sciences.     

Development of picosecond time‐resolved X‐ray absorption spectroscopy by high‐repetition‐rate laser pump/X‐ray probe at Beijing Synchrotron Radiation Facility

A new setup and commissioning of transient X‐ray absorption spectroscopy are described, based on the high‐repetition‐rate laser pump/X‐ray probe method, at the 1W2B wiggler beamline at the Beijing Synchrotron Radiation Facility. A high‐repetition‐rate and high‐power laser is incorporated into the setup with in‐house‐built avalanche photodiodes as detectors. A simple acquisition scheme was applied to obtain laser‐on and laser‐off signals simultaneously. The capability of picosecond transient X‐ray absorption spectroscopy measurement was demonstrated for a photo‐induced spin‐crossover iron complex in 6 mM solution with 155 kHz repetition rate.     

Impact of substrate temperature and film thickness on the interfacial evolution during atomic layer deposition of HfO2 on InP

The effects of both the deposition temperature and the HfO₂ film thickness on the interfacial layer (IL) evolution were studied when tetrakis(ethylmethylamino)hafnium and H₂O based atomic layer deposition (ALD) was performed on InP substrates. While the self‐cleaning effect resulted in an IL‐free structure after formation of ～2 nm thick HfO₂ at 200 °C and 250 °C, substantial IL growth occurred at 300 °C, probably due to simultaneous InP oxidation. Following further growth to ～8 nm at 300 °C, the IL was almost removed and, in particular, a significant In incorporation into the HfO₂ film was observed, which was attributed to IL decomposition and subsequent out‐diffusion of the constituent elements. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Significantly reduced leakage currents in organic thin film transistors with Mn‐doped Bi2Ti2O7 high‐k gate dielectrics

We report the fabrication of organic thin‐film transistors (OTFTs) with high‐k gate dielectrics of Mn‐doped Bi₂Ti₂O₇ (BTO) films. 3% Mn‐doped BTO films deposited on polymer substrates by pulsed laser deposition at room temperature exhibit low leakage currents of 2.1 × 10^–8 A/cm² at an applied electric field of 0.3 MV/cm, while undoped BTO films show much higher leakage currents of 4.3 × 10^–4 A/cm². Mn doping effectively reduces the number of oxygen vacancies in the films and improves the electrical properties. Low operation voltage and significantly reduced leakage currents are demonstrated in pentacene‐based OTFTs with the Mn‐doped BTO gate dielectrics. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Silicon surface passivation by atomic‐layer‐deposited Al2O3 facilitated in situ by the combination of H2O and O3 as reactants

We investigate the effect of O₃ and H₂O oxidant pre‐pulse prior to Al₂O₃ atomic layer deposition for Si surface passivation. Interfacial oxide SiO_x formed by the O₃ pre‐pulse is more beneficial than that by H₂O to a high level of surface passivation. The passivation of thinner H₂O–Al₂O₃ films is more improved by this O₃ pre‐pulse. O₃ pre‐pulse for 10 nm H₂O–Al₂O₃ reduces saturation current density in boron emitter to 18 fA cm^–2 by a factor of 1.7. Capacitance–voltage measurements reveal this interfacial oxide plays a role of decreasing interface trap density without detrimental effect to negative charge density of Al₂O₃. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Assessment of sulfur and iron speciation in a soil aggregate by combined S and Fe micro‐XANES: microspatial patterns and relationships

To test whether synchrotron‐based spectromicroscopy can be used to identify spatial patterns of sulfur (S) and iron (Fe) speciation as well as relationships between the speciation of S and Fe in soil colloids or aggregates at the micrometre and sub‐micrometre level, an anoxically prepared dissected soil aggregate (size ～1 mm³) was analyzed by µ‐XANES at the K‐edges of S (2472 eV) and Fe (7112 eV). The experiment included (i) elemental mapping at the S K‐edge (S, Si, Al) and the Fe K‐edge (Fe, Si), (ii) acquisition of 300 µm × 300 µm images of the region of interest with X‐ray energies of 2474 eV (addressing reduced organic and inorganic S), 2483 eV (total S), 7121 eV (divalent Fe) and 7200 eV (total Fe), as well as (iii) acquisition of S and Fe µ‐XANES spectra at two different positions, where image analysis suggested the dominance of reduced and oxidized S and Fe, respectively. Image analysis revealed a heterogeneous distribution of total Si, S and Fe as well as of different S and Fe species in the aggregate. Microregions which were either enriched in reduced or in oxidized S and Fe could be identified. A microregion with a large contribution of oxidized S (sulfate, sulfonate) to total S contained exclusively Fe(III) oxyhydroxides (probably ferrihydrite) as S‐bearing phase, whereas another microregion with a large contribution of reduced organic S (thiol, organic disulfide) to total S contained a small amount of Fe(II)‐bearing silicate in addition to the dominating Fe(III) oxyhydroxides. Our results show that combined S and Fe µ‐XANES is a powerful tool for studying microscale spatial patterns of S and Fe speciation as well as microscale relationships between the speciation of S and Fe in soil aggregates.     

Soft X‐ray absorption spectroscopy and resonant inelastic X‐ray scattering spectroscopy below 100 eV: probing first‐row transition‐metal M‐edges in chemical complexes

X‐ray absorption and scattering spectroscopies involving the 3d transition‐metal K‐ and L‐edges have a long history in studying inorganic and bioinorganic molecules. However, there have been very few studies using the M‐edges, which are below 100 eV. Synchrotron‐based X‐ray sources can have higher energy resolution at M‐edges. M‐edge X‐ray absorption spectroscopy (XAS) and resonant inelastic X‐ray scattering (RIXS) could therefore provide complementary information to K‐ and L‐edge spectroscopies. In this study, M_2,3‐edge XAS on several Co, Ni and Cu complexes are measured and their spectral information, such as chemical shifts and covalency effects, are analyzed and discussed. In addition, M_2,3‐edge RIXS on NiO, NiF₂ and two other covalent complexes have been performed and different d–d transition patterns have been observed. Although still preliminary, this work on 3d metal complexes demonstrates the potential to use M‐edge XAS and RIXS on more complicated 3d metal complexes in the future. The potential for using high‐sensitivity and high‐resolution superconducting tunnel junction X‐ray detectors below 100 eV is also illustrated and discussed.     

X‐ray spectroscopy for chemistry in the 2‐4 keV energy regime at the XMaS beamline: ionic liquids,Rh and Pd catalysts in gas and liquid environments,and Cl contamination in γ‐Al2O3

The 2–4 keV energy range provides a rich window into many facets of materials science and chemistry. Within this window, P, S, Cl, K and Ca K‐edges may be found along with the L‐edges of industrially important elements from Y through to Sn. Yet, compared with those that cater for energies above ca. 4–5 keV, there are relatively few resources available for X‐ray spectroscopy below these energies. In addition, in situ or operando studies become to varying degrees more challenging than at higher X‐ray energies due to restrictions imposed by the lower energies of the X‐rays upon the design and construction of appropriate sample environments. The XMaS beamline at the ESRF has recently made efforts to extend its operational energy range to include this softer end of the X‐ray spectrum. In this report the resulting performance of this resource for X‐ray spectroscopy is detailed with specific attention drawn to: understanding electrostatic and charge transfer effects at the S K‐edge in ionic liquids; quantification of dilution limits at the Cl K‐ and Rh L₃‐edges and structural equilibria in solution; in vacuum deposition and reduction of [Rh^I(CO)₂Cl]₂ to γ‐Al₂O₃; contamination of γ‐Al₂O₃ by Cl and its potential role in determining the chemical character of supported Rh catalysts; and the development of chlorinated Pd catalysts in `green' solvent systems. Sample environments thus far developed are also presented, characterized and their overall performance evaluated.     

Spectroscopic and structural investigations of α‐, β‐, and γ‐AlH3 phases

With its reputation as a high‐energy density fuel, aluminum hydride (AlH₃) has received renewed attention as a material that is particularly suitable, not only for hydrogen storage but also for rocket propulsion. While the various phases of AlH₃ have been investigated theoretically, there is a shortage of experimental studies corroborating the theoretical findings. In response to this, we present here an investigation of these compounds based primarily on two research areas in which there is the greatest scarcity of information in the literature, namely Raman and infrared (IR) absorption analysis. To the authors' knowledge, this is the first report of experimental far‐IR absorption results on these compounds. Two different samples prepared by broadly similar ethereal reactions of AlCl₃ with LiAlH₄ were analyzed. Both Raman and IR absorption measurements indicate that one sample is purely γ‐AlH₃ and that the other is a mixture of α‐, β‐, and γ‐AlH₃ phases. X‐ray diffraction confirms the spectroscopic findings, most notably for the β‐AlH₃ phase, for which optical spectroscopic data are reported here for the first time. Copyright © 2010 John Wiley & Sons, Ltd.     

A miniature X‐ray emission spectrometer (miniXES) for high‐pressure studies in a diamond anvil cell

Core–shell X‐ray emission spectroscopy (XES) is a valuable complement to X‐ray absorption spectroscopy (XAS) techniques. However, XES in the hard X‐ray regime is much less frequently employed than XAS, often as a consequence of the relative scarcity of XES instrumentation having energy resolutions comparable with the relevant core‐hole lifetimes. To address this, a family of inexpensive and easily operated short‐working‐distance X‐ray emission spectrometers has been developed. The use of computer‐aided design and rapid prototype machining of plastics allows customization for various emission lines having energies from ～3 keV to ～10 keV. The specific instrument described here, based on a coarsely diced approximant of the Johansson optic, is intended to study volume collapse in Pr metal and compounds by observing the pressure dependence of the Pr Lα emission spectrum. The collection solid angle is ～50 msr, roughly equivalent to that of six traditional spherically bent crystal analyzers. The miniature X‐ray emission spectrometer (miniXES) methodology will help encourage the adoption and broad application of high‐resolution XES capabilities at hard X‐ray synchrotron facilities.     

Application of glancing‐emergent‐angle fluorescence for polarized XAFS studies of single crystals

X‐ray absorption fine‐structure (XAFS) data were obtained for the V K‐edge for a series of anisotropic single crystals of (Cr_xV_1–x)₂O₃. The data and the results were compared for the as‐prepared bulk single crystals (measured in fluorescence in two different orientations) and those ground to powder (measured in transmission). For the bulk single crystals, the glancing‐emergent‐angle (GEA) method was used to minimize fluorescence distortion. The reliability of the GEA technique was tested by comparing the polarization‐weighted single‐crystal XAFS data with the experimental powder data. These data were found to be in excellent agreement throughout the entire energy range. Thus, it was possible to reliably measure individual V–V contributions parallel and perpendicular to the c axis of the single crystals, i.e. those unavailable by powder data XAFS analysis. These experiments demonstrate that GEA is a premiere method for non‐destructive high‐photon‐count in situ studies of local structure in bulk single crystals.     

Structural variation in a synchrotron‐induced contamination layer (a‐C:H) deposited on a toroidal Au mirror surface

A carbon layer deposited on an optical component is the result of complex interactions between the optical surface, adsorbed hydrocarbons, photons and secondary electrons (photoelectrons generated on the surface of optical elements). In the present study a synchrotron‐induced contamination layer on a 340 mm × 60 mm Au‐coated toroidal mirror has been characterized. The contamination layer showed a strong variation in structural properties from the centre of the mirror to the edge region (along the long dimension of the mirror) due to the Gaussian distribution of the incident photon beam intensity/power on the mirror surface. Raman scattering measurements were carried out at 12 equidistant (25 mm) locations along the length of the mirror. The surface contamination layer that formed on the Au surface was observed to be hydrogenated amorphous carbon film in nature. The effects of the synchrotron beam intensity/power distribution on the structural properties of the contamination layer are discussed. The I(D)/I(G) ratio, cluster size and disordering were found to increase whereas the sp²:sp³ ratio, G peak position and H content decreased with photon dose. The structural parameters of the contamination layer in the central region were estimated (thickness ? 400 Å, roughness ? 60 Å, density ? 72% of bulk graphitic carbon density) by soft X‐ray reflectivity measurements. The amorphous nature of the layer in the central region was observed by grazing‐incidence X‐ray diffraction.     

Blueprint XAS: a Matlab‐based toolbox for the fitting and analysis of XAS spectra

Blueprint XAS is a new Matlab‐based program developed to fit and analyse X‐ray absorption spectroscopy (XAS) data, most specifically in the near‐edge region of the spectrum. The program is based on a methodology that introduces a novel background model into the complete fit model and that is capable of generating any number of independent fits with minimal introduction of user bias [Delgado‐Jaime & Kennepohl (2010), J. Synchrotron Rad. 17 , 119–128]. The functions and settings on the five panels of its graphical user interface are designed to suit the needs of near‐edge XAS data analyzers. A batch function allows for the setting of multiple jobs to be run with Matlab in the background. A unique statistics panel allows the user to analyse a family of independent fits, to evaluate fit models and to draw statistically supported conclusions. The version introduced here (v0.2) is currently a toolbox for Matlab. Future stand‐alone versions of the program will also incorporate several other new features to create a full package of tools for XAS data processing.     

Synchrotron‐radiation‐induced oxidation of selenite to selenate in coal‐derived fly ash

Systematic changes observed in consecutive XANES spectra of selenium in samples of fly ash from a power plant in Alberta, Canada, burning subbituminous coal have been interpreted as arising from synchrotron‐radiation‐induced oxidation of a selenite species to selenate. It was estimated that about 15–20% of the selenite was oxidized to selenate during the 2 h exposure of the fly ash to the synchrotron beam. In contrast, the XANES spectra of arsenic (and other trace elements not documented herein) showed no variation. It is believed that this is the first time that radiation‐induced changes have been observed for an element in coal‐derived fly ash.     

Solid‐phase epitaxy of InOx Ny alloys via thermal oxidation of InN films on yttria‐stabilized zirconia

The characteristics of InO_x N_y alloy films prepared via thermal oxidation of InN epitaxial films with In‐ or N‐polarities grown on nearly lattice‐matched, yttria‐stabilized zirconia (YSZ) substrates are investigated. The InN films were oxidized to InO_x N_y with a gradual change in O/N composition by annealing in air. Structural analysis revealed that the temperature for phase transition from wurtzite structure depends on the polarity of InN, and N‐polar InO_x N_y films can retain their wurtzite structure even at higher temperatures compared with the case of In‐polar films. Furthermore, changes in the valence band structure and optical characteristics of the InO_x N_y alloys take place via thermal oxidation. These results indicate that InO_x N_y grown via thermal oxidation of N‐polar InN on YSZ can be considered as an alloy semiconductor for optoelectronic devices. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A facile heating cell for in situ transmittance and fluorescence X‐ray absorption spectroscopy investigations

A facile heating cell has been designed for in situ transmittance and fluorescence X‐ray absorption spectroscopy (XAS) measurements up to 1273 K under vacuum or an inert atmosphere. These high temperatures are achieved using a tantalum heating element by ohmic heating. Because of the small specific heat capacity, the temperature can be changed in a matter of minutes from room temperature to high temperature. Furthermore, a commercial power controller was adapted to provide stable temperature control. The construction of the heat shielding system provides a novel approach to reducing the beam's path length and the cell's size. The cell is inexpensive and easy to build. Its performance was evaluated by in situ XAS measurements of the temperature‐dependent structure of ceria nanocrystals. Some preliminary results for the structural mechanism in ceria nanocrystal redox applications are given.     

Characteristics of a tapered undulator for the X‐ray absorption fine‐structure technique at PLS‐II

An in‐vacuum undulator (IVU) with a tapered configuration was installed in the 8C nanoprobe/XAFS beamlime (BL8C) of the Pohang Light Source in Korea for hard X‐ray nanoprobe and X‐ray absorption fine‐structure (XAFS) experiments. It has been operated in planar mode for the nanoprobe experiments, while gap‐scan and tapered modes have been used alternatively for XAFS experiments. To examine the features of the BL8C IVU for XAFS experiments, spectral distributions were obtained theoretically and experimentally as functions of the gap and gap taper. Beam profiles at a cross section of the X‐ray beam were acquired using a slit to visualize the intensity distributions which depend on the gap, degree of tapering and harmonic energies. To demonstrate the effect of tapering around the lower limit of the third‐harmonic energy, V K‐edge XAFS spectra were obtained in each mode. Owing to the large X‐ray intensity variation around this energy, XAFS spectra of the planar and gap‐scan modes show considerable spectral distortions in comparison with the tapered mode. This indicates that the tapered mode, owing to the smooth X‐ray intensity profile at the expense of the highest and most stable intensity, can be an alternative for XAFS experiments where the gap‐scan mode gives a considerable intensity variation; it is also suitable for quick‐XAFS scanning.     

Measurement of the X‐ray mass attenuation coefficients of silver in the 5–20 keV range

The X‐ray mass attenuation coefficients of silver were measured in the energy range 5–20 keV with an accuracy of 0.01–0.2% on a relative scale down to 5.3 keV, and of 0.09–1.22% on an absolute scale to 5.0 keV. This analysis confirms that with careful choice of foil thickness and careful correction for systematics, especially including harmonic contents at lower energies, the X‐ray attenuation of high‐Z elements can be measured with high accuracy even at low X‐ray energies (<6 keV). This is the first high‐accuracy measurement of X‐ray mass attenuation coefficients of silver in the low energy range, indicating the possibility of obtaining high‐accuracy X‐ray absorption fine structure down to the L₁ edge (3.8 keV) of silver. Comparison of results reported here with an earlier data set optimized for higher energies confirms accuracy to within one standard error of each data set collected and analysed using the principles of the X‐ray extended‐range technique (XERT). Comparison with theory shows a slow divergence towards lower energies in this region away from absorption edges. The methodology developed can be used for the XAFS analysis of compounds and solutions to investigate structural features, bonding and coordination chemistry.