X-ray Photoemission Measurements Research Articles

Surface scattering-dependent electronic transport in ultrathin scandium nitride films

With the constant miniaturization of device technologies, it has become essential to understand and engineer the electronic properties of semiconductors in nanoscale dimensions. Scandium nitride (ScN), an emerging rock salt indirect bandgap semiconductor, has attracted significant interest for its interesting thermoelectric, plasmonic, neuromorphic computing, and Schottky barrier device applications. However, an in-depth understanding of the electronic transport, carrier scattering mechanism, and optical properties in ultrathin ScN films is still missing. Here, we show surface-scattering dominant electronic transport in epitaxial ScN films at nanoscale thicknesses. At the ultrathin dimensions, surface scattering increases significantly due to the large surface-to-volume ratio and growth-induced texturing. As a result, mobility decreases, and resistivity increases drastically with decreasing film thickness. Temperature-dependent electronic transport shows that the mobility of the ultrathin films decreases with increasing temperature due to the ionized-impurity and dislocation scattering. Electronic transport properties are further rationalized with x-ray diffraction and pole-figure analysis that shows that while the ultrathin films maintain their predominant 002 texture, their quality degrades with decreasing thickness. However, no significant changes are observed in the electronic structure of the films, as evidenced by x-ray photoelectron spectroscopy, photoemission measurements, and first-principles density functional theory calculations. Our results elucidate the impact of surface scattering on the ultrathin ScN films and would lead to miniaturized devices with improved efficiencies.

Evidence of symmetry breaking in a Gd2 di-nuclear molecular polymer.

A chiral 3D coordination compound, [Gd2(L)2(ox)2(H2O)2], arranged around a dinuclear Gd unit has been characterized by X-ray photoemission and X-ray absorption measurements in the context of density functional theory studies. Core level photoemission of the Gd 5p multiplet splittings indicates that spin orbit coupling dominates over j-J coupling evident in the 5p core level spectra of Gd metal. Indications of spin-orbit coupling are consistent with the absence of inversion symmetry due to the ligand field. Density functional theory predicts antiferromagnet alignment of the Gd2 dimers and a band gap of the compound consistent with optical absorption.

Electronic structure of rare-earth mononitrides: quasiatomic excitations and semiconducting bands

The electronic structure of the rare-earth mononitrides LnN (where Ln = rare-earth), which are promising materials for future spintronics applications, is difficult to resolve experimentally due to a strong influence of defects on their transport and optical properties. At the same time, LnN are challenging for theory, since wide semiconducting 2p and 5d bands need to be described simultaneously with strongly correlated 4f states. Here, we calculate the many-body spectral functions and optical gaps of a series of LnN (with Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er) by a density-functional + dynamical mean-field theory (DFT + DMFT) approach treating the correlated Ln 4f shells within the quasi-atomic Hubbard-I approximation. The on-site Coulomb interaction in the 4f shell is evaluated by a constrained DFT + Hubbard-I approach. Furthermore, to improve the treatment of semiconducting bands in DFT + DMFT, we employ the modified Becke–Johnson semilocal exchange potential. Focusing on the paramagnetic high-temperature phase, we find that all investigated LnN are pd semiconductors with gap values ranging from 1.02 to 2.14 eV along the series. The pd band gap is direct for light Ln = La…Sm and becomes indirect for heavy rare-earths. Despite a pronounced evolution of the Ln 4f states along the series, empty 4f states are invariably found above the bottom of the 5d conduction band. The calculated spectra agree well with those available from x-ray photoemission, x-ray emission and x-ray absorption measurements.

Light-enhanced gating effect at the interface of oxide heterostructure

In semiconducting materials, electrostatic gating and light illumination are widely used stimuli to tune the electronic properties of the system. Here, we show a significant enhancement of photoresponse at the conducting interface of LaVO3–SrTiO3 under the simultaneous application of light and negative gate bias voltage, in comparison to their individual application. On the other hand, the illuminated LaVO3-SrTiO3 interface does not show any enhancement, when a positive gate bias voltage is applied. Our x-ray diffractometer, Raman spectroscopy and photoemission measurements show that unlike the LaAlO3–SrTiO3 interface, migration of oxygen vacancies may not be the prime mechanism for the enhanced photoresponse. Rather, we suggest that the photoresponse of our system is intrinsic and this intrinsic mechanism is a complex interplay between band filling, electric field at the interface and modification of conducting channel width.

The performance of a perovskite-silicon tandem photovoltaic device coupled with the infrared-enhanced response titanium subnitride film

Two kinds of carrier’s passivating contact capping stacks have been fostered for an asymmetric silicon based solar cell, in which ITO/a-SiOx(In) is for hole selection and possesses graded distribution of In, Si, O elements as well as intrinsic- and p-type ultrathin SiOx(In) layer trait, while the SiOx-TiNy layer on rear of n-Si absorber is for electron selection and behaves as a degenerated material with higher electron concentration (1023-1024 /cm3). By using Hall effect, X-ray photoemission and minority carrier lifetime measurements and simulation analyses with AFORS-HET software of TiNy films and ITO/a-SiOx(In)/n-Si/SiOx-TiNy device, respectively, it is found that the performance of the device is strongly correlated to the N deficiency of TiNy film, in addition the simulation results demonstrate that the reduced density of interface state (Dit(E)) and the enhanced near-infrared response (EQE) lead to an increase of open-circuit voltage from 578.6 to 744.1 mV, and power conversion efficiency (PCE) has increased by 30.65%, compared with the prototype of semiconductor-quasi-insulator-semiconductor device (SQIS). Moreover, a monolithic tandem device by n-i-p type perovskite cell tunnel-connected to the modified SQIS cell has achieved a PCE of 31.39%, which is predicted through an available estimation under the maximum values of the Jsc, Voc and pFF.

Effects of the C interstitial doping on the magnetic properties of LTP MnBi

Experimental and theoretical investigations on the structural and magnetic properties of the MnBi and MnBiC hard magnetic phases are presented. X-ray diffraction patterns showed that the highest concentration of MnBi low-temperature phase (LTP) was obtained for annealing at 400 °C for 12 h. X-ray photoemission measurements (XPS) of the core levels (Mn 2p and 3d; Bi 4d and 5p, respectively) show chemical shifts by C addition, providing evidence that the C atoms enter the MnBi structure in the vicinity of both type of metallic atoms. C atoms were assumed to occupy the 2d interstitial crystal sites of the hexagonal NiAs structure type, as suggested in earlier studies. The theoretical calculations show that C addition enhances slightly the magnetic moment of the samples compared to pristine LTP MnBi, in agreement with our magnetic measurements. Also, following both theoretical and experimental investigations, we obtained increased magnetocrystalline anisotropy energy (MAE) by adding C as interstitial dopant.

Nonuniform Debye Temperatures in Quasi-One-Dimensional Transition-Metal Trichalcogenides

The Debye–Waller factor plots for S 2p core-level, based on temperature-dependent X-ray photoemission measurements on quasi-one-dimensional (1D) chains of TiS3, suggest effective Debye temperatures...

Bistable Fermi level pinning and surface photovoltage in GaN

Band bending and Fermi level pinning at GaN(0001) surfaces have proved controversial despite their fundamental importance. By combining Kelvin probe, surface photovoltage and X-ray photoemission measurements, we clarify how the Fermi level pinning affects the band bending for n- and p-type GaN(0001). The presence of two different mid-gap pinning levels is confirmed on native oxide-covered samples, and they lie at remarkably similar energies to those previously found for atomically clean GaN(0001). The population of these bistable pinning levels and their influence on the band bending both depend on the bulk doping and level of illumination by above-gap photons.

Origin of quasilocal plasmons in Nb-substituted EuTiO3

Doped perovskite oxides are promising ``material building blocks'' for achieving tunable and intrinsically low-loss plasmonic features in the visible- to near-infrared range, which are unlikely to obtain from conventional plasmonic materials. Here, using a combination of spectroscopic ellipsometry, soft x-ray absorption, x-ray photoemission, and ultraviolet photoemission spectroscopic measurements we observe an unconventional tunable low-loss quasilocal plasmon in the IR energy (1 eV) range in the Nb-substituted perovskite oxide $\mathrm{EuTi}{\mathrm{O}}_{3}$ ($\mathrm{EuT}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{N}{\mathrm{b}}_{x}{\mathrm{O}}_{3}$). This low-loss plasmon oscillation is formed by many-body interactions, an interplay of quasifree ${d}^{1}$ electrons and hybridizations of Eu 4f-Ti $3d$ and O 2p-Ti $3d$. We find that the Nb substitution at Ti site modifies the $\mathrm{Ti}{\mathrm{O}}_{6}$ octahedral tilting and, consequently, alters the electronic structure. Our study reveals the importance of charge, orbital, and lattice to generate plasmons in $\mathrm{EuT}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{N}{\mathrm{b}}_{x}{\mathrm{O}}_{3}$.

Radiation Damage on Liquid Electrolyte during Spatially Resolved Soft X-ray Photoemission Measurements

Journal Article Radiation Damage on Liquid Electrolyte during Spatially Resolved Soft X-ray Photoemission Measurements Get access Christopher Arble, Christopher Arble Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States Corresponding author: christopher.arble@nist.gov and andrei.kolmakov@nist.gov Search for other works by this author on: Oxford Academic Google Scholar Hongxuan Guo, Hongxuan Guo SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education, Southeast University, Nanjing 210096, P. R. China Search for other works by this author on: Oxford Academic Google Scholar Brian Hoskins, Brian Hoskins Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States Search for other works by this author on: Oxford Academic Google Scholar Matteo Amati, Matteo Amati Elettra Sincrotrone Trieste SCpA, SS14, Km163-5, I-34149 Trieste, Italy Search for other works by this author on: Oxford Academic Google Scholar Patrick Zeller, Patrick Zeller Elettra Sincrotrone Trieste SCpA, SS14, Km163-5, I-34149 Trieste, Italy Search for other works by this author on: Oxford Academic Google Scholar Luca Gregoratti, Luca Gregoratti Elettra Sincrotrone Trieste SCpA, SS14, Km163-5, I-34149 Trieste, Italy Search for other works by this author on: Oxford Academic Google Scholar Andrei Kolmakov Andrei Kolmakov Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States Corresponding author: christopher.arble@nist.gov and andrei.kolmakov@nist.gov Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S2, 1 August 2019, Pages 730–731, https://doi.org/10.1017/S1431927619004380 Published: 01 August 2019

Correlation between structure, electronic properties, and magnetism in CoxGd1−x thin amorphous films

Correlation between structure, electronic properties, and magnetism in ${\mathrm{Co}}_{x}{\mathrm{Gd}}_{1\ensuremath{-}x}$ thin amorphous films was investigated. The thickness averaged properties of covered thin films are consistent with those of in-depth homogeneous amorphous alloys. In spite of that, x-ray magnetic circular dichroism (XMCD) and photoemission measurements on as-grown films have shown signatures consistent with a lateral gradient composition and a Gd surfactant effect. A tendency of Gd atoms to migrate to the surface followed by an oxidation down to 8 \AA{} has been evidenced. Our results further demonstrate the extreme sensitivity of the magnetic properties of ferrimagnetic alloys to their structure and local concentration.

Influence of chemical composition on the X-ray photoemission, thermopower, specific heat, and magnetic properties of CeNi2(Si1-yGey)2

We report our studies of the intermediate compositions between CeNi2Si2 and CeNi2Ge2, i.e., the alloys CeNi2(Si1-yGey)2 by means of the thermopower, electrical resistivity, specific heat, magnetic susceptibility, and X-ray photoemission measurements. CeNi2Si2 is a fluctuating valence system and CeNi2Ge2 is known to show the heavy fermion behaviour. The change of the temperature dependence of the resistivity towards the typical metallic behaviour occurs below y ~ 0.25. The transition between CeNi2Si2 and CeNi2Ge2 is discussed in the frames of competition between the crystal electric field and Kondo interactions. It is found that valence stabilisation occurs for Ge content y > 0.25. The hybridization energy Δ determined from the XPS Ce 3d spectrum reflects well the behaviour of the parameter Eex obtained from the analysis of the magnetic susceptibility by the interconfiguration fluctuation model. It has been also shown that thermopower data can be successfully described employing the single ion model for 0.6 < y < 1.0 and two-band model including the crystal electric field splitting for y ≤ 0.25.

Potential barrier formed at domain boundaries in twinned tetragonal BaTiO3 single crystals

An n-type barium titanate (BaTiO3) single crystal with domain structures has been investigated using electronic transport and X-ray photoemission measurements. Although cubic BaTiO3 showed transport behavior with only small departures from isotropy above its Curie temperature (Tc), twinned tetragonal BaTiO3 showed high anisotropy in electrical resistance. Indeed, the resistance in twinned BaTiO3 was high for the current flowing across the domain boundaries (DBs) and low for that parallel to the DBs. A photoemission study performed under an external bias indicated that a potential drop occurs at every DB. These results indicate that DBs in tetragonal BaTiO3 act as resistances causing anisotropic transport in twinned BaTiO3.

Synthesis and characterization of low work function alkali oxide thin films for unconventional thermionic energy converters

In this work, we present the synthesis and the characterization of low work function thin films for Micro Thermionic Converters (MTC). The objective is producing a device operating at relatively low temperature (&amp;lt;1000 K). We aim at improving the MTC efficiency by reducing the work function of the electrodes and increasing the emitted current density by alkali metal oxides electrodes coating. In particular, in this work, we analyse and compare the performances of two alkali metal oxides: potassium and caesium oxides. Our choice to exploit those materials relies on their low work function and their abundance. For both materials, we present the results on the synthesis of the oxides under high vacuum and controlled temperature. The oxide thin films were characterized by X-ray photoelectron spectroscopy, photoemission, and thermionic emission measurements. By exploiting the latter technique, a quantitative evaluation of the current density, emitted by the heated oxides, is obtained as a function of temperature. Our results demonstrate that it is possible to decrease the silicon work function by almost 3 eV, enabling significant thermionic currents despite relatively low temperatures (below 850 K).

Resonant photoemission spectroscopic studies of Eu2O3 thin film

In the present paper, we have investigated the structural and electronic properties of Eu2O3 thin film deposited by pulsed laser deposition technique on Si (100) substrate. The film was characterized by X-ray diffraction and photoemission measurements. X-ray photoemission studies suggest that Eu has 2+ (∼8%) and 3+ (∼92%) valence states in the film. Valence band spectrum of the film shows four features (2 eV, 4.8 eV, 7.7 eV, and 10.4 eV) with the prominent one at 7.7 eV due to the Eu3+ (4f 6) state. Resonance photoemission spectroscopy measurements have been performed in the Eu 4d → 4f photo absorption region, and thus, obtained constant initial state intensity plots suggest that the feature close to the Fermi level (∼2 eV) results from the hybridization of the O 2p and Eu2+ (4f7) states, while other features appearing at higher binding energies are due to the hybridization of the O 2p and Eu3+ (4f6) states.

Effect of spin-orbit and on-site Coulomb interactions on the electronic structure and lattice dynamics of uranium monocarbide

Uranium monocarbide, a potential fuel material for the generation IV reactors, is investigated within density functional theory. Its electronic, magnetic, elastic, and phonon properties are analyzed and discussed in terms of spin-orbit interaction and localized versus itinerant behavior of the $5f$ electrons. The localization of the $5f$ states is tuned by varying the local Coulomb repulsion interaction parameter. We demonstrate that the theoretical electronic structure, elastic constants, phonon dispersions, and their densities of states can reproduce accurately the results of x-ray photoemission and bremsstrahlung isochromat measurements as well as inelastic neutron scattering experiments only when the $5f$ states experience the spin-orbit interaction and simultaneously remain partially localized. The partial localization of the $5f$ electrons could be represented by a moderate value of the on-site Coulomb interaction parameter of about 2 eV. The results of the present studies indicate that both strong electron correlations and spin-orbit effects are crucial for realistic theoretical description of the ground-state properties of uranium carbide.

Depth-Resolved Composition and Electronic Structure of Buried Layers and Interfaces in a LaNiO3/SrTiO3 Superlattice from Soft- and Hard- X-ray Standing-Wave Angle-Resolved Photoemission

LaNiO3 (LNO) is an intriguing member of the rare-earth nickelates in exhibiting a metal-insulator transition for a critical film thickness of about 4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such thin films also show a transition to a metallic state in superlattices with SrTiO3 (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to better understand this transition, we have studied a strained LNO/STO superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate using soft x-ray standing-wave-excited angle-resolved photoemission (SWARPES), together with soft- and hard- x-ray photoemission measurements of core levels and densities-of-states valence spectra. The experimental results are compared with state-of-the-art density functional theory (DFT) calculations of band structures and densities of states. Using core-level rocking curves and x-ray optical modeling to assess the position of the standing wave, SWARPES measurements are carried out for various incidence angles and used to determine interface-specific changes in momentum-resolved electronic structure. We further show that the momentum-resolved behavior of the Ni 3d eg and t2g states near the Fermi level, as well as those at the bottom of the valence bands, is very similar to recently published SWARPES results for a related La0.7Sr0.3MnO3/SrTiO3 superlattice that was studied using the same technique (Gray et al., Europhysics Letters 104, 17004 (2013)), which further validates this experimental approach and our conclusions. Our conclusions are also supported in several ways by comparison to DFT calculations for the parent materials and the superlattice, including layer-resolved density-of-states results.

Ultrananocrystalline Diamond/Amorphous Carbon Composite Films Prepared by Laser Ablation of Graphite in Nitrogen and Hydrogen Atmosphere

Nitrogenated ultrananocrystalline diamond/hydrogenated amorphous carbon composite films were prepared in hydrogen and nitrogen mixed-gas atmospheres by pulsed laser deposition using graphite targets. The electrical conductivity in n-type conduction remarkably increase at room temperature with an increase in the nitrogen content. In the nitrogen content range from 7.9 to 10.4 at.%, the electrical conductivity is dramatically decreased and this accompanied by the disappearance of diamond grains in the films. Grain boundaries owing to the existence of diamond grains embedded in UNCD/a-C:H films, which is structural specific to UNCD/a-C:H, should play a significant role in the large electrical conductivity enhancement by nitrogen doping. The X-ray photoemission and near-edge X-ray fine-absorption spectroscopic measurements could not detect an evident difference in the spectra that explain the sudden irregular change in the electrical conductivity

Hydroxyl radical and thermal annealing on amorphous InGaZnO4 films for DNA immobilizations

The effect of hydroxyl radicals (OH) and thermal annealing on an amorphous InGaZnO4 (aIGZO) film surface was investigated for manipulation of DNA immobilization. X-ray photoemission and fluorescence measurements were conducted to reveal the status of surface OH coverage and DNA immobilization, respectively. Systematic examinations concerning OH termination on the film surface suggested that the surface coverage of OH leveling DNA immobilization was related to the local surface potential. Furthermore, OH affinity on the aIGZO film surface was sensitive to thermal annealing. A remarkable change in surface OH coverage was observed for the film surface annealed at high temperature. This behavior was framed by a structural change from amorphous to crystalline state, which regulated DNA immobilization. These results indicate that the OH affinity on aIGZO films is dependent on structural properties such as defects. This study suggests that an amorphous structure is critical for obtaining a high OH surface coverage governing DNA immobilization, and is hence more suitable for biosensing.

Current-induced formation of stable M2-phase vanadium dioxide

Studies are reported on the transformation of M1-phase vanadium dioxide into the stable M2 structure for a micro-fabricated device. The device is composed of a VO2 channel of length 50 μm, width 40 μm, and thickness 120 nm, and electrical contacts made of titanium and gold. Transformation is achieved in the channel region between electrical contacts by heating the entire structure to 40 °C and simultaneously driving current. The channel is observed to partially transform along its length. Micro-Raman measurements reveal the two regions to be the M1 and M2-phases of VO2. X-ray photoemission measurements show the presence of gold in both regions, but with higher content in the M2 material. The M1 portion of the channel is found to undergo a phase transition to the conductive R phase at ~ 63 °C based on resistance and micro-Raman measurements as temperature is increased. The M2-phase is found to remain stable to temperature cycling between 25 and 120 °C. We attribute both the reduced M1 → R phase transition temperature and the stable formation of M2-phase VO2 to the presence of gold.