[Show abstract][Hide abstract] ABSTRACT: Combined hard x-ray photoelectron spectroscopy (HAXPES) and electrical characterisation measurements have been shown to provide complementary information on the electrical performance of Si and GaAs based metal─oxide─semiconductor (MOS) structures. The results obtained indicate that surface potential changes at the semiconductor/dielectric interface due to the presence of different work function metals can be detected from HAXPES measurements. Changes in the semiconductor band bending at zero gate voltage and the flat band voltage values derived from C─V measurements are in agreement with the semiconductor core level shifts measured from the HAXPES spectra. These results highlight the potential application of this measurement approach in the evaluation of the efficacy of surface passivation treatments: HAXPES—hard x-ray photoelectron spectroscopy; C─V—capacitance voltage; Dit—interface state density; BE—binding energy, at reducing defect states densities in MOS structures.
[Show abstract][Hide abstract] ABSTRACT: We discuss the calculation of nondestructive compositional depth profiles from regularization of variable kinetic energy hard X-ray photoelectron spectroscopy (VKE-XPS) data, adapting techniques developed for angle-resolved XPS. Simulated TiO2/Si film structures are analyzed to demonstrate the applicability of regularization techniques to the VKE-XPS data and to determine the optimum choice of regularization function and the number of data points. We find that using a maximum entropy-like method, when the initial model/prior thickness is similar to the simulated film thickness, provides the best results for cases where prior knowledge of the sample exists. For the simple structures analyzed, we find that only five kinetic energy spectra are necessary to provide a good fit to the data, although in general, the number of spectra will depend on the sample structure and noisiness of the data. The maximum entropy-like algorithm is then applied to two physical films of TiO2 deposited on Si. Results suggest interfacial intermixing. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.
Surface and Interface Analysis 04/2014; · 1.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a detailed study of the microstructure and electronic structure of β-NaxV2O5 (x ≈ 0.33) polycrystalline films, combining film growth, X-ray spectroscopies, and first-principles calculations. High-quality crystalline and stoichiometric V2O5 and β-Na0.33V2O5 films were grown by a sol–gel process, spin-coating, and rapid thermal annealing. The V2O5 film, which exhibits a rough surface, is preferentially oriented in the (001) direction perpendicular to the surface, whereas the b-axis of β-Na0.33V2O5 is oriented in the substrate plane. The β-Na0.33V2O5 film consists of a nested layered structure composed of single-crystalline rods of a few hundred nanometers in diameter and a few micrometers in length. Photoemission and X-ray absorption measurements of β-Na0.33V2O5 confirm the Na incorporation and the presence of mixed V5+ and V4+ species and weakly occupied V 3d states. At the V L-edge, X-ray absorption and resonant inelastic X-ray measurements suggest a larger crystal field for β-Na0.33V2O5 compared with isoelectronic β-Sr0.17V2O5. We observe the lowest local crystal-field dd* transition at an energy of −1.6 ± 0.1 eV for β-Na0.33V2O5, which is substantially larger than β-Sr0.17V2O5; this large difference is interpreted as arising from the stronger distortions to the VO6 octahedra in β-Na0.33V2O5.
The Journal of Physical Chemistry C 01/2014; · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The scaling behavior of Gd- and Al-doped HfO2 films as gate dielectrics in metal-oxide-semiconductor (MOS) capacitors was studied. For equivalent oxide thicknesses (EOTs) in the range of 10 Å, crystallized Gd:HfO2 showed higher leakage current densities than crystallized Al:HfO2, with undoped HfO2 in between. Ultimately, the scalability of Al:HfO2 was limited by the ability to crystallize the films at a given thermal budget. As a result, for post-deposition annealing at 800 °C, the EOT of Al:HfO2 based MOS capacitors was limited to ∼8 Å. However, for such an EOT, leakage current densities were reduced by about 100× with respect to HfO2. This demonstrates the high potential of Al:HfO2 for low-standby-power MOS devices.
[Show abstract][Hide abstract] ABSTRACT: The electronic structure of β-Pb0.33V2O5 nanowires has been studied with x-ray photoelectron spectroscopy techniques. The recent synthesis of defect-free β-Pb0.33V2O5 nanowires resulted in the discovery of an abrupt voltage-induced metal insulator transition. First principle calculations predicted an additional V-O-Pb hybridized “in-gap” state unique to this vanadium bronze playing a significant role in facilitating the transition. We confirm the existence, energetic position, and orbital character of the “in-gap” state. Moreover, we reveal that this state is a hybridized Pb 6s–O 2p antibonding lone pair state resulting from the asymmetric coordination of the Pb2+ ions.
[Show abstract][Hide abstract] ABSTRACT: The theory of elasticity accurately describes the deformations of macroscopic bodies under the action of applied stress . In this review, we examine the internal mechanisms of elasticity for strained-layer semiconductor heterostructures. In particular, we present extended x-ray-absorption fine structure (EXAFS) and x-ray diffraction (XRD) measurements to show how the bond lengths and bond angles in semiconductor thin-alloy films change with strain when they are grown coherently on substrates with different lattice constants. The structural distortions measured by experiment are compared to valence-force field (VFF) calculations and other theoretical models. Atomic switching and interfacial strain at buried interfaces are also discussed.
[Show abstract][Hide abstract] ABSTRACT: The local structure of tetragonal BiFeO3-PbTiO3 solid solutions featuring anomalous lattice distortions has been determined using simultaneous fitting of neutron total scattering and extended X-ray absorption fine structure data. On the local scale, the large tetragonal distortion, promoted by the displacements of the A-cations (Bi and Pb), is accommodated primarily by the [FeO6] octahedra, even though both Fe and Ti acquire (5+1)-fold coordination. Bi cations exhibit considerably larger displacements than Pb. The combination of the A-cation displacements and the ability of M-cations to adopt 5-fold coordination is suggested as key for stabilizing the large tetragonality in BiMO3-PbTiO3 systems.
[Show abstract][Hide abstract] ABSTRACT: The origin of the deep subgap states in amorphous indium gallium zinc oxide (a-IGZO), whether intrinsic to the amorphous structure or not, has serious implications for the development of p-type transparent amorphous oxide semiconductors. We report that the deep subgap feature in a-IGZO originates from local variations in the oxygen coordination and not from oxygen vacancies. This is shown by the positive correlation between oxygen composition and subgap intensity as observed with X-ray photoelectron spectroscopy. We also demonstrate that the subgap feature is not intrinsic to the amorphous phase because the deep subgap feature can be removed by low-temperature annealing in a reducing environment. Atomistic calculations of a-IGZO reveal that the subgap state originates from certain oxygen environments associated with the disorder. Specifically, the subgap states originate from oxygen environments with a lower coordination number and/or a larger metal-oxygen separation.
[Show abstract][Hide abstract] ABSTRACT: Local and average structures of ceramic Ba1-xSrxTiO3 solid solutions have been reconciled using Reverse Monte Carlo (RMC) refinements of atomic positions that employ simultaneous fitting of neutron total scattering data, x-ray absorption fine structure, and patterns of diffuse scattering in electron diffraction. These refinements enable explicit reconstruction of three-dimensional atomic configurations without any effective parameters. The results reveal cube- and parallelepiped-shaped probability density distributions for Ti atoms in the cubic and tetragonal phases of BaTiO3, respectively. These distributions are consistent with the split Ti sites (eight for the cubic and four for the tetragonal polymorphs) separated by ≈0.2 Å. The characters of Ti distributions are retained in the solid solutions, but the magnitude of Ti off-centering decreases as x increases. The Ti displacements remain correlated along the octahedral chains, at least up to x = 0.5, as manifested in the sheets of diffuse scattering in electron diffraction patterns; the correlation parameters and lengths have been quantified using RMC analyses. The ion-size difference between Ba and Sr is accommodated through the approximately isotropic relaxation of the oxygen atoms, which shift toward Sr; a similar relaxation is observed for the Ti atoms. Local Ti off-centering diminishes as the number of Sr atoms in the coordination groups [TiOxBa8-nSrn] increases.
[Show abstract][Hide abstract] ABSTRACT: We have combined hard X-ray photoelectron spectroscopy with angular dependent O K-edge and V L-edge X-ray absorption spectroscopy to study the electronic structure of metallic and insulating endpoint phases in 4.1 nm thick (14 units cells along the c-axis of VO2 films on TiO2(001) substrates, each displaying an abrupt MIT centered at ~300 K with width < 20 K and a resistance change of Δ R/R > 10(3). The dimensions, quality of the films, and stoichiometry were confirmed by a combination of scanning transmission electron microscopy with electron energy loss spectroscopy), X-ray spectroscopy, and resistivity measurements. The measured endpoint phases agree with their bulk counterparts. This clearly shows that, apart from the strain induced change in transition temperature, the underlying mechanism of the MIT for technologically-relevant dimensions must be the same as the bulk for this orientation.
[Show abstract][Hide abstract] ABSTRACT: Hard x-ray photoelectron spectroscopy (HAXPES) has been used to study metal-oxide-semiconductor (MOS) structures fabricated with both high (Ni) and low (Al) work-function metals on 8-nm thick Al2O3 dielectric layers, deposited on sulfur passivated n- and p-doped GaAs substrates. A binding energy difference of 0.6 eV was measured between the GaAs core levels of the n- and p-doped substrates in the absence of gate metals, indicating different Fermi level positions in the band gap. Subsequent photoemission measurements made on the MOS structures with the different work-function metals displayed very limited change in the GaAs core level binding energies, indicating that the movement of the Fermi level at the Al2O3/GaAs interface is restricted. Using a combination of HAXPES measurements and theoretical calculations, the Fermi level positions in the band gap have been determined to be in the range of 0.4–0.75 eV and 0.8–1.11 eV above the valence band maximum for p- and n-type GaAs, respectively. Analysis of capacitance voltage (C-V) measurements on identically prepared samples yield very similar Fermi level positions at zero applied gate bias. The C-V analysis also indicates a higher interface defect density (Dit) in the upper half of the GaAs bandgap.
[Show abstract][Hide abstract] ABSTRACT: We report direct evidence of conduction band filling in 3% La-doped BaSnO3 using hard x-ray photoelectron spectroscopy. Direct comparisons with hybrid density functional theory calculations support a 3.2 eV indirect band gap. The use of hybrid DFT is verified by excellent agreement between our photoelectron spectra and O K-edge x-ray emission and absorption spectra. Our experimental and computational results demonstrate that the conduction band is primarily of Sn 5s orbital character with little O 2p contribution, which is a prerequisite for designing a perovskite-based transparent conducting oxide.
[Show abstract][Hide abstract] ABSTRACT: The origin of the almost unique combination of optical transparency and the ability to bipolar dope tin monoxide is explained using a combination of soft and hard X-ray photoemission spectroscopy, O K-edge X-ray emission and absorption spectroscopy, and density functional theory calculations incorporating van der Waals corrections. We reveal that the origin of the high hole mobility, bipolar ability, and transparency is a result of (i) significant Sn 5s character at the valence band maximum (due to O 2p–Sn 5s antibonding character associated with the lone pair distortion), (ii) the combination of a small indirect band gap of 0.7 eV (Γ–M) and a much larger direct band gap of 2.6–2.7 eV, and (iii) the location of both band edges with respect to the vacuum level. This work supports Sn2+-based oxides as a paradigm for next-generation transparent semiconducting oxides.
Chemistry of Materials. 07/2013; 25(15):3114–3123.
[Show abstract][Hide abstract] ABSTRACT: While the lattice volume in the solid-solution Ba1−xCaxTiO3 decreases with increasing x, the Curie temperature remains unaffected, in contrast to Ba1−xSrxTiO3. We have determined the origin of this phenomenon by comparing the local structures in (Ba,Ca)TiO3 and (Ba,Sr)TiO3. Reverse Monte Carlo refinements of instantaneous atomic positions using simultaneous fitting of multiple types of experimental data (neutron total scattering, X-ray absorption fine structure, patterns of diffuse scattering in electron diffraction) reveal both ferroelectric Ca displacements and their amplification of the Ti off-centering, which mitigate the lattice-volume effects. The activity of Ca is triggered by the anomalously strained Ca-O bonds.
[Show abstract][Hide abstract] ABSTRACT: X-ray photoelectron spectroscopy is an established technique for quantitative chemical analysis requiring accurate peak intensity analysis. We present evidence of focus∕alignment dependence of relative peak intensities for peaks over a broad kinetic energy range with a hemispherical electron analyzer operated in a position imaging mode. A decrease of over 50% in the Ag 2p3∕2 to Ag 3d ratio is observed in a Ag specimen. No focus∕alignment dependence is observed when using an angular imaging mode, necessitating the use of angular mode for quantitative chemical analysis.
The Review of scientific instruments 03/2013; 84(3):036106. · 1.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To continue the scaling of memory and logic devices, new materials must
be employed to replace the traditional Si/SiO2. However,
detailed understanding of the chemical and electronic structures of the
new materials and interfaces must be achieved for employment. X-ray
photoelectron spectroscopy (XPS) is an excellent tool for studying such
materials due to its unique ability to probe both the chemical and
electronic structure of materials. However, XPS analysis is inherently
limited by the short inelastic mean free paths (IMFPs) of the
photoelectrons, limiting the probe depth to the near surface region. To
overcome this limitation, XPS using hard X-rays (HAXPES) can be used,
increasing the probe depth to technology relavent thicknesses. We
present recent HAXPES results of materials and interfaces for