Hard X-ray Photoemission Spectroscopy Research Articles

Hard X‐ray photoemission study of the covalent‐chain antiferromagnets TlFeS2 and TlFeSe2

AbstractBulk electronic structures of covalent‐chain antiferromagnets TlFeS2 and TlFeSe2 have been investigated by hard X‐ray photoemission spectroscopy (HAXPES) and band calculations based on the full‐potential linearized augmented plane wave method. Competition between localized and delocalized characters of Fe 3d electrons has been suggested by the slight discrepancies between valence‐band HAXPES spectra and calculated density of states, as well as Fe 2p spectra with a broad charge‐transfer satellite. It is assumed that this competition closely related to the crystallographic onedimensionality of these materials. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Direct observation of the band gap shrinkage in amorphous In2O3–ZnO thin films

We investigated the dependence of valence- and core-level photoemission spectra of amorphous In2O3–ZnO (a-IZO) films on carrier density by using hard x-ray photoemission spectroscopy (hν=8000 eV). The valence band edge distinctly shifts toward high binding energy with the increase in carrier density from 0.80 to 3.96 × 1020 cm−3, and an abrupt jump for the shift of the valence band edge from high to low binding energy occurs at a carrier density of 4.76×1020 cm−3. After considering the effect of nonparabolic bandstructure, the shifts are still less than the width of the occupied conduction band, providing direct evidence for the band gap shrinkage. Our calculation results indicate that the contribution of the band gap shrinkage increases as the carrier density increases, which accords with the observations in doped conducting crystal materials, such as Sn doped In2O3. Moreover, it is found that the conduction electrons of a-IZO films are strongly perturbed by the ionization of core levels, which leads to obvious plasmon satellites in core photoemission spectra lines.

Development of high lateral and wide angle resolved hard X-ray photoemission spectroscopy at BL47XU in SPring-8

In this study, we have realized a high lateral resolution and wide-angle-resolved hard X-ray photoelectron spectroscopy (HAXPES) facility at BL47XU in SPring-8. The system uses Kirkpatrick–Baez focusing mirrors to achieve a beam size of 1.0μm (horizontal)×0.98μm (vertical) at the photon energy of 7.94keV and a wide-acceptance-angle objective lens installed in front of the electron energy analyzer. The objective lens system, which we had been developed originally and has achieved a total acceptance angle of ±34° with a resolution better than that of an acceptance angle of 1.5°. The performance of this system was evaluated through core spectra measurements of a typical multi-layered sample of Ir (8nm)/HfO2 (2.2nm)/thickness-graded SiO2 (0–10nm)/Si(001).

Photoemission spectroscopy study of the lanthanum lutetium oxide/silicon interface

Rare earth oxides are promising candidates for future integration into nano-electronics. A key property of these oxides is their ability to form silicates in order to replace the interfacial layer in Si-based complementary metal-oxide field effect transistors. In this work a detailed study of lanthanum lutetium oxide based gate stacks is presented. Special attention is given to the silicate formation at temperatures typical for CMOS processing. The experimental analysis is based on hard x-ray photoemission spectroscopy complemented by standard laboratory experiments as Rutherford backscattering spectrometry and high-resolution transmission electron microscopy. Homogenously distributed La silicate and Lu silicate at the Si interface are proven to form already during gate oxide deposition. During the thermal treatment Si atoms diffuse through the oxide layer towards the TiN metal gate. This mechanism is identified to be promoted via Lu-O bonds, whereby the diffusion of La was found to be less important.

B-Site Deficiencies in A-site-Ordered Perovskite LaCu3Pt3.75O12

An A-site-ordered perovskite LaCu3Pt(3.75)O12 was synthesized by replacing Ca(2+) with La(3+) in a cubic quadruple AA'3B4O12-type perovskite CaCu3Pt4O12 under high-pressure and high-temperature of 15 GPa and 1100 °C. In LaCu3Pt(3.75)O12, 1/16 of B-site cations are vacant to achieve charge balance. The B-site deficiencies were evidenced by crystal structure refinement using synchrotron X-ray powder diffraction, hard X-ray photoemission spectroscopy, and soft X-ray absorption spectroscopy, leading to the ionic model La(3+)Cu(2+)3Pt(4+)(3.75)O(2-)12. Magnetic susceptibility data for this compound indicated a spin-glass-like behavior below T(g) = 3.7 K, which is attributed to disturbance of the antiferromagnetic superexchange interaction by the B-site deficiencies.

Control of Schottky Barrier Height at Al/p-Ge Junctions by Ultrathin Layer Insertion

We have fabricated Al/p-Ge(100) junctions with an ultrathin GeO2 interfacial control layer and studied the effect of post-metallization annealing on the electrical properties from the current-voltage characteristics. Then, the changes in the chemical bonding features with annealing were directly evaluated by using hard x-ray photo-emission spectroscopy. Results of these analyses showed that a GeO2 reduction in the region near the Al/GeO2 interface occurs after annealing at temperatures over 300{degree sign}C and that the Fermi level pinning (FLP) effect increases. This suggests that inserting a thermally and chemically stable thin layer suppresses the FLP effect at the Metal/Ge interface caused by thermal annealing. The release of this FLP effect in Al/p-Ge junctions has also been demonstrated by using ultrathin N2 sputtered hafnium nitride layer instead of GeO2. And we found that the formation of a nitrogen rich region near the metal/Ge interface was an effective means of controlling the Schottky barrier height.

Angle-Resolved PES Studies on Transition Layers at SiO2/SiC Interfaces

We investigated on the chemical bonding states of thermally oxidized SiO2/SiC in detail by the angle-resolved X-ray photoemission spectroscopy (AR-XPS). Analyses of Si 2p, C 1s and O1s spectra show that the signal arising from sub-oxide is detected, and that the signals related carbon in SiO2 and C-C in SiC are detected. We also characterized SiO2/4H-SiC by Hard X-ray photoemission spectroscopy (HAX-PES) in order to check the depth profile of carbon in oxide in detail.

Ultrathin magnetic oxide EuO films on Si(001) using SiOx passivation—Controlled by hard x-ray photoemission spectroscopy

We present the chemical and structural optimization of ultrathin magnetic oxide EuO films on silicon. By applying a controlled in situ passivation of the Si(001) surface with SiOx in the monolayer regime, metallic silicide contaminations at the interface can be effectively reduced down to a sub-monolayer coverage, as was carefully quantified by interface-sensitive hard x-ray photoemission spectroscopy. Heteroepitaxial growth of EuO on Si(001) is sustained for this ultrathin SiOx-passivation, and bulk-near magnetic properties are observed for the 4 nm-thin EuO films. Our successful combination of chemically and structurally optimized EuO/Si(001) heterostructures by ultrathin in situ SiOx passivation makes this system promising for an application as alternative spin functional tunnel contacts in spin-FETs.

Room temperature redox reaction by oxide ion migration at carbon/Gd-doped CeO2 heterointerface probed by an in situ hard x-ray photoemission and soft x-ray absorption spectroscopies

In situ hard x-ray photoemission spectroscopy (HX-PES) and soft x-ray absorption spectroscopy (SX-XAS) have been employed to investigate a local redox reaction at the carbon/Gd-doped CeO2 (GDC) thin film heterointerface under applied dc bias. In HX-PES, Ce3d and O1s core levels show a parallel chemical shift as large as 3.2 eV, corresponding to the redox window where ionic conductivity is predominant. The window width is equal to the energy gap between donor and acceptor levels of the GDC electrolyte. The Ce M-edge SX-XAS spectra also show a considerable increase of Ce3+ satellite peak intensity, corresponding to electrochemical reduction by oxide ion migration. In addition to the reversible redox reaction, two distinct phenomena by the electrochemical transport of oxide ions are observed as an irreversible reduction of the entire oxide film by O2 evolution from the GDC film to the gas phase, as well as a vigorous precipitation of oxygen gas at the bottom electrode to lift off the GDC film. These in situ spectroscopic observations describe well the electrochemical polarization behavior of a metal/GDC/metal capacitor-like two-electrode cell at room temperature.

Determination of the surface band bending in InxGa1−xN films by hard x-ray photoemission spectroscopy

Core-level and valence band spectra of InxGa1−xN films were measured using hard x-ray photoemission spectroscopy (HX-PES). Fine structure, caused by the coupling of the localized Ga 3d and In 4d with N 2s states, was experimentally observed in the films. Because of the large detection depth of HX-PES (∼20 nm), the spectra contain both surface and bulk information due to the surface band bending. The InxGa1−xN films (x = 0–0.21) exhibited upward surface band bending, and the valence band maximum was shifted to lower binding energy when the mole fraction of InN was increased. On the other hand, downward surface band bending was confirmed for an InN film with low carrier density despite its n-type conduction. Although the Fermi level (EF) near the surface of the InN film was detected inside the conduction band as reported previously, it can be concluded that EF in the bulk of the film must be located in the band gap below the conduction band minimum.

Electronic band alignment and electron transport in Cr/BaTiO3/Pt ferroelectric tunnel junctions

Electroresistance in ferroelectric tunnel junctions is controlled by changes in the electrostatic potential profile across the junction upon polarization reversal of the ultrathin ferroelectric barrier layer. Here, hard X-ray photoemission spectroscopy is used to reconstruct the electric potential barrier profile in as-grown Cr/BaTiO3(001)/Pt(001) heterostructures. Transport properties of Cr/BaTiO3/Pt junctions with a sub-μm Cr top electrode are interpreted in terms of tunneling electroresistance with resistance changes of a factor of ∼30 upon polarization reversal. By fitting the I-V characteristics with the model employing an experimentally determined electric potential barrier we derive the step height changes at the BaTiO3/Pt (Cr/BaTiO3) interface +0.42(−0.03) eV following downward to upward polarization reversal.

Interfacial and bulk electronic properties of complex oxides and buried interfaces probed by HAXPES

Designing, understanding and controlling the properties of engineered and functional materials, based on oxides and buried interfaces, is one of the most flourishing research fields and one of the major challenges faced by contemporary solid state science and technology. Often, a reliable spectroscopic analysis of such systems is hindered by surface effects, as structural distortion, stoichiometry changes, strong reactivity to external agent and major atomic and/or electronic reconstruction to name but a few. Hard X-Ray PhotoEmission Spectroscopy (HAXPES) is a powerful technique to overcome such limitations, allowing to monitor truly bulk sensitive properties. We report selected HAXPES results for manganese-based oxides, both in films and crystal forms, and for buried metal–organic interfaces, with the aim of highlighting some of the important features such technique brings in the analysis of electronic properties of the solids.

Hidden relationship between the electrical conductivity and the Mn 2p core-level photoemission spectra in La1−xSrxMnO3

Core-level electronic structure of La1−xSrxMnO3 has been studied by x-ray photoemission spectroscopy (XPS). We first report, by the conventional XPS, the well-screened shoulder structure in Mn 2p3∕2 peak, which had been observed only by hard x-ray photoemission spectroscopy so far. Multiple-peak analysis revealed that the Mn4+ spectral weight was not proportional to the nominal hole concentration x, indicating that a simple Mn3+/Mn4+ intensity ratio analysis may result in a wrong quantitative elemental analysis. Considerable weight of the shoulder at x = 0.0 and the fact that the shoulder weight was even slightly going down from x = 0.2 to 0.4 were not compatible with the idea that this weight simply represents the metallic behavior. Further analysis found that the whole Mn 2p3∕2 peak can be decomposed into four portions, the Mn4+, the (nominal) Mn3+, the shoulder, and the other spectral weight located almost at the Mn3+ location. We concluded that this weight represents the well-screened final state at Mn4+ sites, whereas the shoulder is known as that of the Mn3+ states. We found that the sum of these two spectral weight has an empirical relationship to the conductivity evolution with x.

Effective attenuation length for lanthanum lutetium oxide between 7 and 13 keV

To obtain quantitative depth information from hard X-ray photoemission spectroscopy, the effective attenuation length (EAL) is required. In this paper, the EAL was determined for LaLuO3 for electron kinetic energies between 7 and 13 keV. As a result, the EAL is in the range of 100–150 Å for the investigated photon energies. In addition, higher binding energy orbitals of La and Lu were measured and are discussed. LaLuO3 is a promising high-k dielectric for future nano-scaled MOS devices.

Ab initioand photoemission study of correlation effects in SrRuO3thin films

We present the experimental and theoretical study of correlation effects in epitaxial SrRuO${}_{3}$ thin films. Experimentally, we have performed resonant ultraviolet photoemission spectroscopy (UPS) and angle-resolved hard x-ray photoemission spectroscopy (HAXPES) measurements. For resonant UPS, the two methods, Fano-profile fitting of constant initial state spectra and the energy distribution curves equidistant difference spectra, were used to extract Ru 4$d$ partial spectral weight (PSW) in the valence band. We find Ru 4$d$ PSW possessing a clearly pronounced coherent peak at the Fermi level together with angle-resolved HAXPES spectra demonstrating no difference in surface and bulk electronic structure. From comparison of experimental data with theoretical calculations done at density functional theory level, we conclude that SrRuO${}_{3}$ is a weakly correlated material and electronic structure of it can be consistently described employing first-principles approaches.

Reactive pulsed laser deposition of Cu2ZnSnS4 thin films in H2S

Cu2ZnSnS4 (CZTS) thin films have been grown by reactive pulsed laser deposition in H2S atmosphere, combining the alternate ablation from the metallic (Cu) and alloyed (ZnxSn) targets at room temperature. The morphological, structural and optical properties of as grown CZTS thin films with varying compositions as well as upon annealing in N2 atmosphere are investigated by Rutherford backscattering spectrometry, X-ray diffraction, Raman spectroscopy and optical spectrophotometry. The chemical bonding in the “bulk” of the CZTS films is elucidated via hard X-ray photoemission spectroscopy measurements. The formation of the good quality stoichiometric polycrystalline CZTS films is demonstrated upon optimization of the growth parameters.

Nondestructive characterization of a TiN metal gate: Chemical and structural properties by means of standing-wave hard x-ray photoemission spectroscopy

Standing-wave (SW) hard x-ray photoemission (HXPS, HAXPES) is applied to a thick (100 Å) film of a metal gate TiN grown on top of a Si/MoSi2 multilayer mirror. The mirror is used to produce a standing wave of 30 Å period that is scanned through the sample by varying the x-ray incidence angle over its 1st-order Bragg condition, thus generating rocking curves of various core-level intensities. The thickness and chemical state of the top, oxidized surface of TiN, as well as the buried interface between TiN and the native oxide on top of the mirror are determined by SW-HXPS. The information provided by SW-HXPS is compared to that obtained by XPS Ar+ depth profile. The SW-HXPS method not only does not require destroying the sample but also provides more quantitative results and a more detailed profile of the interfaces than XPS Ar+ depth profile. Various applications of SW-HXPS to nanoscale multilayer semiconductor systems are thus suggested.

Large spin-orbit splitting and weakly anisotropic superconductivity revealed with single-crystalline noncentrosymmetric CaIrSi3

We report normal and superconducting properties of the Rashba-type noncentrosymmetric compound CaIrSi${}_{3}$ using single crystalline samples with nearly 100$%$ superconducting volume fraction. The electronic density of states revealed by the hard x-ray photoemission spectroscopy can be well explained by the relativistic first-principles band calculation. This indicates that strong spin-orbit interaction indeed affects the electronic states of this compound. The obtained $H$-$T$ phase diagram exhibits only approximately 10$%$ anisotropy, indicating that the superconducting properties are almost three dimensional. Nevertheless, strongly anisotropic vortex pinning is observed.

Observation of boron diffusion in an annealed Ta/CoFeB/MgO magnetic tunnel junction with standing-wave hard x-ray photoemission

The CoFeB/MgO system shows promise as a magnetic tunnel junction with perpendicular magnetization and low critical current densities for spin-torque driven magnetization switching. The distribution of B after annealing is believed to be critical to performance. We have studied the distribution of B in a Ta/Co0.2Fe0.6B0.2/MgO sample annealed at 300 °C for 1 h with standing-wave hard x-ray photoemission spectroscopy (SW-HXPS). Comparing experimental rocking curve data to x-ray optical calculations indicates diffusion of 19.5% of the B uniformly into the MgO and of 23.5% into a thin TaB interface layer. SW-HXPS is effective for probing depth distributions in such spintronic structures.

Hard x-ray photoemission spectroscopy of Bi2S3 thin films

The electronic structure of polycrystalline Bi2S3 thin films deposited by thermal evaporation under high vacuum conditions was investigated with respect to their potential use as absorber materials in p-i-n solar cells by means of hard x-ray photoemission spectroscopy at the PETRA III synchrotron. A clear influence of the post-deposition treatment on the electronic structure could be observed, resulting in a lowering of the Fermi level as well as in a change of the electronic states in the valence band. Furthermore, chemical shifts of Bi2S3 were determined in the bulk-sensitive hard x-ray regime as ΔEB,Bi=1.35 eV and ΔEB,S=−2.80 eV.