High-resolution Soft X-ray Photoemission Research Articles

Valence fluctuation in Yb3Si5 probed by synchrotron X-ray photoemission spectroscopy

We report the direct evidence of valence fluctuation in the thermoelectric material Yb3Si5 by using synchrotron X-ray photoemission spectroscopy. The Yb 3d core level photoemission spectrum of a Yb3Si5 single crystal shows clear separation of the Yb2+ peak and Yb3+ multiplet peaks. From the spectral weight of each component, the mean valence of Yb ions is estimated to be +2.67 at 20 K. Furthermore, a Kondo resonance peak just below the Fermi energy is also observed by high-resolution soft X-ray photoemission spectroscopy, suggesting the valence fluctuating state in Yb3Si5.

High-energy photoemission spectroscopy for investigating bulk electronic structures of strongly correlated systems

Progress of high-energy photoemission spectroscopy for investigating the bulk electronic structures of strongly correlated electron systems is reviewed. High-resolution soft X-ray photoemission has opened the door for revealing the bulk strongly correlated spectral functions overcoming the surface contributions. More bulk-sensitive hard X-ray photoemission spectroscopy (HAXPES) enables us to study the electronic structure with negligible surface contribution. The recent development of the polarization-dependent HAXPES is also described in this short review.

High-temperature thermal stability study of 1 nm Al2O3 deposited on InAs surfaces investigated by synchrotron radiation based photoemission spectroscopy

High-resolution soft x-ray photoemission spectroscopy (SXPS) has been used to study the high-temperature thermal stability of ultra-thin atomic layer deposited (ALD) Al2O3 layers (∼1 nm) on sulfur passivated and native oxide covered InAs surfaces. While the arsenic oxides were removed from both interfaces following a 600 °C anneal, a residual indium oxide signal remained. No significant differences were observed between the sulfur passivated and native oxide surfaces other than the thickness of the interfacial oxide layer while the Al2O3 stoichiometry remained unaffected by the anneals. The energy band offsets were determined for the Al2O3 on the sulfur passivated InAs surface using both valence band edge and shallow core-level photoemission measurements.

High temperature thermal stability of the HfO2/Ge (100) interface as a function of surface preparation studied by synchrotron radiation core level photoemission

High resolution soft x-ray photoemission spectroscopy (SXPS) have been used to study the high temperature thermal stability of ultra-thin atomic layer deposited (ALD) HfO2 layers (∼1nm) on sulphur passivated and hydrofluoric acid (HF) treated germanium surfaces. The interfacial oxides which are detected for both surface preparations following HfO2 deposition can be effectively removed by annealing upto 700°C without any evidence of chemical interaction at the HfO2/Ge interface. The estimated valence and conduction band offsets for the HfO2/Ge abrupt interface indicated that effective barriers exist to inhibit carrier injection.

W O 3 nanofibers for gas sensing applications

W O 3 nanofibers (NFs), with diameter of about 150–200nm, were grown using the electrospinning technique starting from WCl6 as tungsten source. The NFs were deposited onto a patterned substrate, for gas sensing measurements, and on a silicon substrate, for chemical and structural characterizations. The chemical composition of the NFs was determined by the x-ray photoemission spectroscopy, while high spatial resolution soft x-ray photoemission spectroscopy showed a chemical image of the NF. The gas sensing properties of these NFs were checked towards NO2 and CO, showing a good response for NO2 while no effect was observed for CO.

Soft x-ray spectroscopy and microspectroscopy of correlated materials: photoemission andmagnetic circular dichroism

Spectroscopy and microspectroscopy of strongly correlated systems, ferromagneticmaterials and magnetic microstructures by means of photoemission and magnetic circulardichroism using soft x-rays are presented. Bulk-sensitive high-resolution soft x-rayphotoemission has revealed the real bulk electronic states of rare-earth compounds withvarious degrees of electronic correlation. Soft x-ray magnetic circular dichroism (XMCD)has enabled element-specific measurements of the system dependence of the orbitalcontribution to the magnetic moment, which has been shown to be enhanced when themagnetic electrons are more localized. XMCD microspectroscopy has been carried out formagnetic microstructures, which, by fully utilizing the spectroscopic aspect, has yielded notonly the magnetic domain structures but also the distribution of the spin and orbitalmagnetic moment.

Soft X-ray photoemission study of the Heusler-type Fe 2VAl 1− zGe z alloys

The valence-band electronic structures of the Heusler-type alloys Fe 2VAl 1− z Ge z ( z = 0–0.10) have been investigated with high-resolution soft X-ray photoemission spectroscopy. The overall photoelectron spectral feature of Fe 2VAl 1− z Ge z with the Ge content z up to 0.10 is not so much changed, except for the main 3d band in the region from the Fermi level E F to the binding energy E B ∼ 1 eV. For z = 0.05, the leading edge of the main band is shifted by ∼0.07 eV, but the shift is much smaller than expected within a rigid band model for a calculated band structure. For z = 0.10, a band at E B ∼ 0.5 eV becomes prominent and the intensity of the Fermi edge is increased. The change in the valence band spectrum near E F suggests the rigid-band-like shift of the 3d bands and the appearance of new 3d states in the pseudogap, which may qualitatively explain the z-dependence of the thermoelectric properties. As z increases, the V 2p spectra show a shift towards the high binding energy and the evolution of a shoulder structure on the low binding energy side of the main line. The shoulder structure is attributed to the charge transfer to the V site induced by the Ge substitution as well as the surface effect. The change in the valence-band electronic structure is discussed in terms of the interaction between the substituted Ge and its surrounding constituent atoms.

Observation of bulk electronic states of Kondo semiconductor YbB 12 by high-resolution soft X-ray photoemission spectroscopy

Bulk valence band photoemission spectra of the Kondo semiconductor YbB 12 were measured with use of soft X-ray ( h ν = 700 eV) at 20 and 200 K and the Kondo resonance behavior was confirmed. Sharp bulk Yb 2+ 4f peaks are observed in the vicinity of Fermi level on fractured single crystals. From the spectra is estimated the Yb mean valence as 2.86 ± 0.01 at 20 K and 2.91 ± 0.01 at 200 K. The whole peaks of the Yb 3+ 4f multiplets are found to shift by ∼ 30 meV toward higher binding energies with decreasing the temperature from 200 to 20 K. On the other hand, the shift of the peak energy position of the Yb 2+ 4f spectra toward lower binding energies with decreasing the temperature is found to be less than 10 meV. The temperature dependence of the Yb 4f photoemission spectra is well explained by the non-crossing approximation (NCA) calculation based on the single impurity Anderson model (SIAM).

High resolution photoemission spectroscopy: Evidence for strong chemical interaction between Mg and 3,4,9,10-perylene-tetracarboxylic dianhydride

The interface formation between Mg and 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) was investigated by high resolution soft x-ray photoemission spectroscopy. The interface chemistry was obtained after fitting the core level spectra as a function of Mg thickness. At the initial stage of deposition, a strong chemical interaction between Mg and the single bonded oxygen atoms of PTCDA is observed leading to the formation of MgO and a modified organic molecule. Based on the experimental evidence, the molecular structure of the modified molecule is proposed. Moreover, the changes observed in the measured C1s core level spectra are supported by density functional theory calculations.

Influence of the molecular structure on the interface formation between magnesium and organic semiconductors

The interface formation between three different perylene derivatives and Mg were investigated by high-resolution soft X-ray photoemission spectroscopy using synchrotron radiation at BESSY. The chemical and electronic properties of these interfaces were obtained after fitting the C 1s, O 1s, N 1s and Mg 2p core-level emission spectra as a function of Mg thickness. A strong chemical interaction between Mg and the molecular end groups is observed leading to the formation of new chemical components and/or charge redistribution due to the presence of the metal.

A High-Resolution Photoemission Study of Nanoscale Aluminum Oxide Films on NiAl(110)

Using high-resolution soft X-ray photoemission, Al 2p, we have been able to quantify the relative populations of tetrahedrally (Al(tet)) and octahedrally (Al(oct)) coordinated Al(3+) in three distinct phases of nanoscale aluminum oxide films on NiAl(110). We have hence determined the bulk alumina phases that the nanoscale films most resemble. Adsorption of oxygen at room temperature produces a layer which predominately (90%) contains Al(tet) and is analogous to the amorphous bulk phase of alumina. Annealing this layer results in an Al enrichment of the oxide layer, through the diffusion of metal from the substrate, and an increase in the relative amount of Al(oct), producing a gamma-alumina-like layer with a relative Al(oct)/Al(tet) occupancy of 28 +/- 3%/72 +/- 3%. Oxygen adsorption at 823 K also produces a gamma-like phase, with a relative Al(oct)/Al(tet) occupancy of 27 +/- 3%/73 +/- 3%, although this layer is thicker than that formed at room temperature. Both oxidation methods produce gamma-alumina layers that display poor translational order. However, these poorly ordered layers have a relative Al(oct)/Al(tet) occupation similar to that of well-ordered oxide films produced using different oxidation conditions in previous studies. Both gamma layers undergo partial decomposition upon annealing to 1273 K, producing an alpha-alumina-like oxide, which contains only Al(oct), and is highly deficient in Al. There are significant oxide-free areas within the alpha-alumina oxide layer, which is characteristic of crystallite formation. Repeated cycles of oxidation and annealing to 1273 K do not produce a homogeneous film, but they do make the alpha-like oxide more Al rich.

Yb 4f states of YbXCu4 (X = Cd, Mg, Zn and Sn) investigated by high-resolution soft X-ray photoemission spectroscopy

Yb 4f electronic structure of YbXCu 4 (X = Cd, Mg, Zn and Sn) has been investigated by means of high-resolution soft X-ray photoemission spectroscopy (SX-PES) with the excitation energy of hν = 800 eV, in comparison with the results of the valence transition compound YbInCu 4. The SX-PES spectra of YbCdCu 4 exhibit the temperature dependence similar to YbInCu 4, while those of the other YbXCu 4 compounds almost no temperature dependence. The X-dependence of the Yb 4f states suggests that the low In 5p density of states is important for the valence transition of YbInCu 4.

High-resolution soft X-ray photoemission study of a Kondo semiconductor and related compounds

We have performed the bulk-sensitive high-resolution soft X-ray photoemission study of a Kondo semiconductor CeRhAs and related compounds CeNiSn and CePdSn. The comparison of the spectra of polycrystalline CePdSn on the fractured and scraped surfaces show that the fracturing of the samples is much better than the scraping in order to obtain intrinsic photoemission spectra. The Ce 4d core-level spectra show clear differences in the electronic states among the materials.

High resolution soft X-ray photoemission of Kondo insulator YbB 12

We have measured photoemission spectra of a single crystal YbB 12, which is known to be a Kondo insulator, with high energy resolution at 700 eV in the temperature range between 200 and 20 K to study the Kondo resonance behavior in the bulk. The sharp bulk Yb 2+ 4f 13 peaks observed on fractured YbB 12, which is only weakly influenced by the surface electronic structures, facilitate high accuracy evaluation of the temperature dependences of the Kondo peak energy and the bulk valence.

Soft X-ray photoemission study of CeRhAs and related compounds: observation of a bulk pseudo-gap

We report bulk 4f and non-4f spectral functions near the Fermi level ( E F ) of a Kondo semiconductor CeRhAs and its related compound CePdSn by virtue of high-resolution soft X-ray photoemission. The non-4f spectrum of CeRhAs measured at 20 K shows a shoulder at the binding energy of ∼ 0.18 eV and almost negligible spectral weight at E F . Although a peak similar to a tail of the Kondo resonance is observed in the 4f spectrum of CeRhAs, the intensity at E F is suppressed compared with those of metallic Ce compounds. These facts are attributed to signatures of the formation of a pseudo-gap in CeRhAs.

A high resolution photoemission study of surface core-level shifts in clean and oxygen-covered Ir(2 1 0) surfaces

High resolution soft X-ray photoemission electron spectroscopy (SXPS), using synchrotron radiation, is employed to investigate 4f core-level features of four differently-prepared Ir(2 1 0) surfaces: clean planar, oxygen-covered planar, oxygen-induced faceted, and clean faceted surfaces. Surface and bulk peak identifications are supported by measurements at different photon energies (thus probing different electron escape depths) and variable emission angles. Iridium 4f 7/2 photoemission spectra are fitted with Doniach–Sunjic lineshapes. The surface components are identified with core levels positioned at lower binding energies than the bulk components, in contrast to previous reports of binding energy inversion on Ir(1 0 0) (1×1) and (5×1) surfaces. For clean planar Ir(2 1 0) three surface Ir 4f 7/2 features are observed with core-level shifts of −765, −529, and −281 meV, with respect to the bulk; these are associated with the first, second and third layers of atoms, respectively, for atomically rough Ir(2 1 0). Adsorption of oxygen onto the planar Ir(2 1 0) surface is found to cause a suppression and shift of the surface features to higher binding energies. Annealing at T≥600 K in oxygen produces a faceted surface as verified by low energy electron diffraction (LEED). A comparison of planar and faceted oxygen-covered surfaces reveals minor differences in the normal emission SXPS spectra, while grazing emission spectra exhibit differences. The SXPS spectrum of the clean, faceted Ir(2 1 0) exhibits small differences in comparison to the clean planar case, with surface features having binding energy shifts of −710, −450, and −230 meV.

High-resolution soft X-ray photoemission spectroscopy of spinel-type compound CuIr2S4

Abstract The electronic structure of spinel-type chalcogenide CuIr 2 S 4 , which exhibits a metal–insulator transition, has been studied by photoemission spectroscopy (PES). The valence-band PES spectra indicate a gap formation below the transition temperature T MI . The line shape of the Ir 4f core-level PES spectra shows a dramatic change across T MI , which may be associated with the variation of the Ir 5d electron states.

Tuning Schottky barrier heights by organic modification of metal-semiconductor contacts

The initial stage of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) adsorption on Se passivated n-type GaAs(1 0 0)-(2×1) surface was investigated using high-resolution soft X-ray photoemission spectroscopy (SXPS). The thickness of the film was precisely controlled using previous results of core level intensity evolution and additional valence band spectra. A very small amount of PTCDA (⪡1 ML) is sufficient to induce a reduction of inhomogeneous band bending at the surface as judged from the sharpening of the core level spectra. This is interpreted in terms of preferential sticking of the organic molecules to surface defects. Thin films of PTCDA were then used as an interlayer for the electronic modification of Ag/n-GaAs(1 0 0) Schottky contacts. The electronic properties were investigated recording in situ current–voltage ( IV) and capacitance–voltage ( CV) characteristics. For H-plasma-treated substrates the effective barrier height decreases from 0.81 to 0.64 eV as a function of the PTCDA layer thickness ( d PTCDA ). In the case of the sulphur-passivated GaAs the effective barrier height first increases and then decreases, the overall range being 0.54–0.73 eV. The substrate treatment leads to a different alignment between the band edges of the GaAs and the molecular orbitals of the PTCDA, making it possible to determine the energy position of the lowest unoccupied molecular orbital (LUMO) transport level. The latter is also derived from ultraviolet photoemission spectroscopy (UPS) measurements and the investigation of the electronic structure formed upon deposition of PTCDA on differently treated n-GaAs(1 0 0) surfaces. Interface dipoles are found to form according to the electron affinities (EA) of the substrates and PTCDA films at the interfaces and, consequently, the vacuum level alignment rule does not hold. The results demonstrate that the energy offset between the conduction band minimum of n-doped inorganic semiconductors and the LUMO of organic molecular films at the interfaces can be obtained using UPS by systematically varying the EA of substrates with a known band gap.

HIGH SPATIAL RESOLUTION SOFT X-RAY PHOTOEMISSION STUDY OF WO3 THIN FILMS

WO 3 thin film surface chemical composition has been studied by means of high resolution soft X-ray photoemission spectroscopy. Valence band and W 4f core levels have been analyzed on different sample positions and high lateral resolution images have been acquired. The valence band spectra have shown for the first time a marked increase of the W 5d density of state at the Fermi level, indicating the presence of metallic tungsten on the surface. This has been confirmed by the W 4f signal, which presents both metallic and oxidized phases. For the first time, spectromicroscopy on core states has clearly evidenced the presence of nonstoichiometric areas and a spatial localization of W 4+ states on the surface.

Soft X-ray photoemission spectroscopy study on the interaction between CuFPc molecules and Si(1 1 1)7×7 surface

High resolution soft X-ray photoemission measurements have been performed onto ultra thin films of hexadecafluoro copper phthalocyanine deposited, at room temperature and in ultra high vacuum conditions, onto clean Si(111)7×7 substrate. The Si 2p spectra show the missing of the adatoms signal and the appearance of a broad feature mainly due to F–Si bonds. The Si 2p spectrum allows us to exclude the loss of copper atom from the molecule during the deposition. C 1s spectrum evidences the presence of Si–C interaction. These results confirm previously published X-ray photoemission spectroscopy fluorine spectra, which clearly present the effects of the interaction of F atoms of the molecule with the silicon substrate and a strong modification of the C 1s spectrum.