Hard X-ray Photoemission Spectroscopy Research Articles

Fe 3 − x Zn x O 4 thin film as tunable high Curie temperature ferromagnetic semiconductor

Epitaxial ferri(ferro)magnetic Fe3−xZnxO4 thin films (x=0–0.9) were prepared using a pulsed-laser deposition technique. The electrical conductivity and magnetic properties of Fe3−xZnxO4 thin film were systematically modulated for the entire range of Zn substitution. Anomalous Hall coefficient measurements revealed the presence of spin-polarized carriers at room temperature. Valence band spectra obtained by hard x-ray photoemission spectroscopy revealed that the density of states near the Fermi level was reduced with an increasing Zn concentration of x. These results indicate that this system will serve as a tunable ferromagnetic semiconductor with a strong electron correlation.

Photoemission study on Sm-based filled skutterudites

Sm-based filled skutterudites SmFe 4 P 12 and SmOs 4 Sb 12 have been investigated by soft and hard X-ray photoemission spectroscopy (PES). Bulk-sensitive Sm 3d–4f resonant PES and surface-sensitive valence-band PES spectra suggest in the bulk that SmOs 4 Sb 12 has much more strongly mixed Sm valence state than SmFe 4 P 12 . A significant Sm divalent peak in the vicinity of the Fermi level ( E F ) has been observed in SmOs 4 Sb 12 , indicating that the Sm 4f states contribute to the electronic states at E F .

Electronic structure and magnetism of [formula omitted] thin films

We have studied the electronic and magnetic properties of epitaxially grown CaMn 1 - x Ru x O 3 ( x = 1.0 , 0.5) thin films by X-ray magnetic circular dichroism (XMCD) and hard X-ray photoemission spectroscopy (HXPES). The XMCD studies indicated that the spin moments of Mn and Ru are aligned in the opposite directions. The HXPES spectra indicated the Ru 4d states at around the Fermi edge ( E F ) and the Mn 3d states below E F . From these results, we propose that the localized Mn 3d t 2 g states and the itinerant Ru 4d t 2 g band are antiferromagnetically coupled and give rise to ferromagnetism, in analogy with the mechanism proposed for the double perovskite oxides such as Sr 2 FeMoO 6 .

Nondestructive diagnostics of high-κ dielectrics for advanced electronic devices

The authors present novel results on the interface between silicon and the high-κ oxides Al2O3 and HfO2 grown by atomic layer deposition. The determination of the thickness of the interfacial layer between oxide and Si(100) is crucial to the evaluation of the performances of devices based on high-κ dielectrics. They find through hard x-ray photoemission spectroscopy (HaXPES) that no interfacial layer forms between Al2O3 and Si(100) whereas almost one monolayer forms between HfO2 and Si(100). HaXPES does not involve any destructive procedure nor any sample preparation. High-energy photoemission could therefore be widely employed for the characterization of real devices.

Hard X-ray photoemission spectroscopy of pyrochlore molybdenum oxides R 2Mo 2O 7 (R=Sm, Tb)

We have carried out hard X-ray photoemission spectroscopy (HXPES) for pyrochlore molybdenum oxides Sm 2Mo 2O 7 and Tb 2Mo 2O 7 single crystals with excitation energy of 5.95 keV, in order to investigate electronic structures and correlations in the bulk. The Mo 3d core spectra show complex features, in contrast to an ordinary doublet spectral feature derived from the spin–orbit interaction, indicating significant electron correlation in the valence band. While an intense peak structure at ∼0.5 eV is observed in the valence band of Sm 2Mo 2O 7, the peak is broadened and shifted way in Tb 2Mo 2O 7, which suggests the difference of the electron correlation energy depending on the rare-earth site.

Hard X-ray photoemission spectroscopy for intrinsic electronic structure of strongly correlated electron systems

In order to clarify the intrinsic bulk electronic structure of strongly-correlated electron systems, we have realized hard X-ray photoemission spectroscopy with high energy-resolution at BL29XUL in SPring-8. The total energy resolution of 75 meV was achieved at the excitation energy of 5.95 keV. Large probing depth of about 100 Å enables us to probe intrinsic bulk electronic structure free from surface condition. The importance and capability of this technique are demonstrated by the drastic difference in the Cu 2p core-level spectra of Nd 1.85 Ce 0.15 CuO 4 crystals obtained using soft and hard X-rays.

Results and perspectives in hard X-ray photoemission spectroscopy (HAXPES) from solids

We report on the results obtained during the commissioning phase of the volume photoemission (VOLPE) project, now operational at ESRF. Test experiments were performed on noble metals and highly correlated systems, analyzing photoelectrons with kinetic energies up to 8keV. They confirmed that hard X-ray photoemission spectroscopy (HAXPES) can achieve bulk sensitivity with high energy resolution (e.g. 70meV at 6keV kinetic energy). At these kinetic energies, the surface contribution to the measured spectra is less than 5%.

High resolution hard X-ray photoemission using synchrotron radiation as an essential tool for characterization of thin solid films

Recently, we have shown that hard X-ray photoemission spectroscopy using undulator X-rays at SPring-8 is quite feasible with both high resolution and high throughput. Here we report an application of hard X-ray photoemission spectroscopy to the characterization of electronic and chemical states of thin solid films, for which conventional PES is not applicable. As a typical example, we focus on the problem of the scatter in the reported band-gap values for InN. We show that oxygen incorporation into the InN film strongly modifies the valence and plays a crucial role in the band gap problem. The present results demonstrate the powerful applicability of high resolution photoemission spectroscopy with hard X-rays from a synchrotron source.

Electronic Structures of Heavily Boron-doped Superconducting Diamond Films

ABSTRACTRecent photoemission studies on heavily boron-doped superconducting diamond films, reporting the electronic structure evolution as a function of boron concentrations, are reviewed. From soft X-ray angle-resolved photoemission spectroscopy, which directly measures electronic band dispersions, depopulation of electrons (or formation of hole pockets) at the top of the valence band were clearly observed. This indicates that the holes at the top of the valence bands are responsible for the metallic properties and hence superconductivity at lower temperatures. Hard X-ray photoemission spectroscopy observed shift of the main C 1s core level and intensity evolution of a lower binding energy additional structure, suggesting chemical potential shift, carrier doping efficiency by boron doping, and possibility of boron-related cluster formations.

Evidence for Suppressed Screening on the Surface of High TemperatureLa2−xSrxCuO4andNd2−xCexCuO4Superconductors

Hard x-ray photoemission spectroscopy (PES) of Cu core electronic states, with a probing depth of approximately 60 A, is used to show that the Zhang-Rice singlet feature is present in La2CuO4 but is absent in Nd2CuO4. Hole and electron doping in La(2-x)SrxCuO4 (LSCO) and Nd(2-x)CexCuO4 (NCCO) result in new well-screened features which are missing in soft x-ray PES. Impurity Anderson model calculations establish screening from doped states as its origin, which is strongly suppressed within 15 A of the surface. Complemented with x-ray absorption spectroscopy, the small chemical-potential shift in core levels (approximately 0.2 eV) are shown to be consistent with modifications of valence and conduction band states spanning the band gap (approximately 1 eV) upon hole and electron doping in LSCO and NCCO.

Temperature-induced valence transition in EuNi 2(Si 0.20Ge 0.80) 2 studied by hard X-ray photoemission spectroscopy

The temperature-induced mixed valence transition in EuNi 2(Si 0.20Ge 0.80) 2 has been investigated by hard X-ray (5940 eV) photoemission spectroscopy (HX-PES) for fractured surfaces, with a probing depth larger than 5 nm. The Eu 3d core-level states are studied below and above the critical valence transition temperature, T v = 80 K. The HX-PES spectra at 40 and 120 K show the mixed valence transition, with clear changes in the divalent and trivalent Eu 3d chemically shifted features. The Eu 3d HX-PES spectra indicate a mean valence of 2.70 ± 0.03 at 40 K which changes to 2.40 ± 0.03 at 120 K, in good accordance with the results of bulk Eu III-edge X-ray absorption spectroscopy measurements.

Interface reaction of poly-Si/high- k insulator systems studied by hard X-ray photoemission spectroscopy

The interface reaction between poly-Si and high- k dielectrics (including HfSiON and HfAlO) have been investigated using hard X-ray ( hν = 5.95 keV) photoemission spectroscopy (HX-PES). HX-PES method has enabled us to probe intrinsic interfacial states without surface conditioning, due to the large escape depth of high energy photoelectrons. From the results of take-off angle dependent data (O 1s and Si 1s core level photoemission spectra), we have found the difference of interfacial reaction concerning poly-Si and high- k dielectrics for their fabrication process. The oxidation proceeded near to surfaces by the difference in the existence of doped boron atom and in the annealing temperature for fabrication process.

A novel probe of intrinsic electronic structure: hard X-ray photoemission spectroscopy

We have realized hard X-ray (HX) photoemission spectroscopy (PES) with high throughput and high-energy resolution for core level and valence band studies using high-energy and high-brilliance synchrotron radiation at SPring-8. This is a brand new method because large escape depth of high-energy photoelectrons enables us to probe intrinsic bulk states free from surface condition. By use of a newly developed electron energy analyzer and well-focused X-rays, high-energy resolution of 75 meV ( E/Δ E = 79,000) was realized for 5.95 keV photoelectrons.

硬X線光電子分光による界面・バルク電子状態の研究

Recently, we have realized hard X-ray photoemission spectroscopy (HX-PES) using high-brilliance synchrotron radiation at SPring-8. High energy (6 and 8 keV) photon excitation enables us to probe photoelectrons with larger escape depth compared to conventional PES. This allows us to conduct a study of intrinsic electronic structure of bulk and embedded interface of materials. A specially designed apparatus including X-ray optics achieves both high energy resolution and high throughput. We report the characteristics and performance of HX-PES, and also the application to study of the electronic structure of new functional materials and strongly correlated electron materials.

Valence Transition ofYbInCu4Observed in Hard X-Ray Photoemission Spectra

Yb 3d and valence-band photoemission spectra of the first-order valence-transition compound YbInCu4 have been measured with hard x ray at an excitation energy of 5.95 keV. Abrupt changes are clearly observed in both spectra around the transition temperature T(V)=42 K, in comparison with ultraviolet and soft x-ray photoemission (VUV-PES and SX-PES) spectra. From the Yb 3d spectra, the Yb valence has been estimated to be approximately 2.90 from 220 down to 50 K and approximately 2.74 at 30-10 K. We propose that Yb 3d hard x-ray photoemission spectroscopy is a very powerful method to estimate the valence of Yb with high accuracy. On the other hand, the Yb2+ 4f-derived peaks in the valence-band spectra exhibit a remarkable enhancement below T(V). The shape of the valence-band spectra is different from those of the VUV-PES and SX-PES spectra above T(V), reflecting the In 5s and 5p contributions.

Hard X-ray Photoemission Spectroscopy of Temperature-Induced Valence Transition in EuNi2(Si0.20Ge0.80)2

We investigate the temperature-induced mixed valence transition in EuNi 2 (Si 0.20 Ge 0.80 ) 2 using Hard X-ray (5940 eV) photoemission spectroscopy (HX-PES), with a probing depth larger than 5 nm....