High-resolution Soft X-ray Photoemission Spectroscopy 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-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.

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).

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.

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 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.

Organic probe for inhomogeneous band bending

The initial stage of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) adsorption on Se-passivated n-type GaAs(100)-(2×1) surface was investigated using high-resolution soft x-ray photoemission spectroscopy. 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. The results presented in this letter indicate that organic molecules may also serve as very suitable probes for inorganic semiconductor surface evaluation through the photoemission studies.

Soft X-ray photoemission studies of [formula omitted

High-resolution soft X-ray photoemission spectroscopy has been applied to study the clean and H 2S-covered InP(100) surface. The clean surface is In-rich and forms a (4 × 2) reconstruction. The In 4d core levels and the P 2p levels show surface shifts which in general terms are consistent with existing models [1]. The P 2p level shows only a small surface shift, suggesting that most atoms are in a bulk-like configuration. The In 4d peak shows three surface and subsurface peaks and one bulk peak whose energies are extracted by fitting. When hydrogen sulphide is adsorbed, and the surface then annealed, a single sulphur peak is observed. The small shift of the P 2p core level is not changed significantly in contrast with the In 4d surface peaks, which change their energy and relative intensity, indicating that the sulphur adsorbs predominantly on the metal sites.

Coadsorption and reaction of sulfur and silver on a Ru(001) surface

We have investigated the interaction of Ag and S coadsorbed on a Ru(001) surface using high-resolution soft X-ray photoemission spectroscopy of the S 2p core level and valence region. The interaction of adsorbed sulfur and the first layer of Ag adatoms is weak. At ΘS + ΘAg < 0.5 ML, Ag and S are coadsorbed in segregated domains. The Ag layer is covered by chemisorbed S only after a S-saturation of the Ru surface (ΘS > 0.5 ML). At Ag coverages higher than one monolayer, the valence band and the S 2p spectra show formation of Silver sulfide at room temperature.

Effect of annealing on the band bending and the overlayer morphology at Sb/III–V (110) interfaces

The formation of the interface between Sb and GaAs(110) is studied for room temperature (RT) deposition and subsequent annealing using high-resolution soft X-ray photoemission spectroscopy (SXPS). The Sb 4d emission is found to consist of three spin-orbit doublets shifted in binding energy. Two components can be assigned to two inequivalent adsorption sites in the epitaxial Sb monolayer (ML). For coverages beyond 1 ML a third component develops which stems from the Sb in excess of the monolayer. Upon annealing above 500 K the Sb on top of the epitaxial monolayer desorbs completely leaving a well ordered monolayer behind. The changes in overlayer morphology are correlated with drastic changes in the Schottky barrier. Deposition up to 0.5 ML of Sb at RT causes a strong increase of the band bending on GaAs(110) or InP(110) substrates. In this coverage regime Schottky barriers of 0.5 eV on p-doped GaAs, 0.75 eV on n-doped GaAs and 0.75 eV on p-doped InP develop. The annealing reduces the barriers to 0.2 eV on p- and n-doped GaAs and 0.5 eV on p-doped InP. On p- and n-doped GaAs the Schottky barrier behaviour is correlated with dislocations and defects in the Sb monolayer, which on p-type GaAs were previously shown by STM spectroscopy to create electronic gap states. Upon annealing the dislocations are removed or reduced in density giving rise to a unpinning of the Fermi level. For Sb on InP(110), however, the results indicate that even after annealing donor type gap states exist with a sufficient density to pin the surface Fermi level.

Au interfaces with epitaxially grown CdTe(111): Chemistry and barrier heights

We have used high-resolution soft x-ray photoemission spectroscopy (SXPS) to study chemistry, atomic distributions, and Fermi level (EF) movement caused by Au deposition on molecular beam epitaxy (MBE) -grown CdTe(111)-B surfaces. Using etch-and-anneal cycles, we produce clean surfaces with characteristic valence-band and core-level photoemission spectra. Au atoms disrupt substrate bonds releasing both Cd and Te atoms into the overlayer. Anions segregate preferentially to the free surface, while cations are more interspersed in the Au matrix. These first SXPS studies of CdTe(111) surfaces indicate that the atom-induced disruption is comparable or less than that for cleaved (110) substrates. After correcting for photovoltaic effects, we obtain an equilibrium Fermi-level position 0.55 eV above the valence-band maximum (Ev), intermediate between stabilization positions for Au on (i) cleaved, bulk-grown CdTe(110) (Ev+0.8 to 1.1 eV), and (ii) cleaved, bulk-grown CdTe(110) with a Yb interlayer (Ev+0.45 eV). Despite clear evidence for interfacial diffusion, EF for MBE-grown CdTe(111) lies closer to the position expected on the basis of work function difference than does EF for bulk-grown CdTe(110). These results indicate that the structural and electronic quality of the CdTe crystal plays a major role in the Schottky barrier formation.