Angle-resolved Valence-band Photoemission Research Articles

Optimized Ni1-x Al x O hole transport layer for silicon solar cells.

NiO alloyed with aluminum, Ni1−xAlxO, is analyzed in terms of its stoichiometry, electronic and transport properties, as well as interfacial band alignment with Si to evaluate its potential use as a hole transport layer (HTL) in p–i–n type solar cells. The analysis is based on component material and slab structural simulations, as well as simulated and measured angle-resolved valence-band photoemission spectroscopy (PES) data, in order to reveal the best suitable stoichiometry. It is concluded that the ionization energy from the highest occupied states tends to increase with Al content as the simulated work function grows from 4.1 eV for pure NiO to 4.7 eV for heavily alloyed Al0.50Ni0.50O. The electronic structure as a function of the interface design between crystalline silicon and the transport layer is used to assess the band lineup and its correlation with the discontinuity of the affinities. The affinity rule is tested by evaluating the workfunctions of the component layers and justified best for a particular Ni-enriched interface design. Technology Computer-Aided Design (TCAD) device simulations show, that the band offset between oxide and crystalline silicon remains within the range of values to sustain a staggering alignment – a condition suitable for effective charge separation, similar to a situation in a tunneling diode. The self-energy of the hole carriers is estimated by contrasting simulated and measured photoemission data, which in the case of non-annealed Al-rich samples is shown to be an order of magnitude higher due to the disorder effects. The work functions derived from the measured PES data for the epitaxially grown oxide films with nearly identical alloy stoichiometry correlate well with the simulated values. The findings suggest that the optimal HTL is formed by starting with a pure Ni layer, followed by a graded doping AlxNi1−xO, with x high at contact/oxide interface and low at the oxide/semiconductor.

Surface, final state, and spin effects in the valence-band photoemission spectra ofLaCoO3(001)

The electronic structure of a ${\mathrm{LaCoO}}_{3}$(001) perovskite is analyzed using simulated and measured angle-resolved valence-band photoemission spectroscopy data with emphasis on the effects of the surface truncation, the final state effects, and the spin degeneracy removal. Supercell simulations reveal instability towards the formation of nonuniform spin polarization in the Co-containing layers under tensile stress. Lattice expansion reduces antiferrodistortive octahedron tilting and tips the ratio between the crystal field parameter and the exchange energy in favor of local magnetization. While the magnitude of the local moment scales with lateral strain, the Curie temperature saturates for straight Co-O-Co chains below 75 K. The comparison of simulated photoemission spectra with experiments suggests Co-${\mathrm{O}}_{2}$ to be the most stable surface stoichiometry, as proposed from transport and catalytic studies.

Atomic and Electronic Structure of V–Rh(110) Near-Surface Alloy

Vanadia-promoted Rh catalysts and Rh–V alloys have shown enhanced reactivity in several heterogeneous catalytic processes. However, little is known about the role of electronic and geometrical effects of the alloy phase on the chemical reactivity. In this article, we present experimental study on geometric and electronic structure of near-surface ordered alloys prepared by depositing V on Rh(110) surface and by subsequent annealing. STM, LEED, and XPD measurements were combined to determine the atomic structure. The electronic influence of the subsurface vanadium on the Rh(110) surface was elucidated by means of core-level and angle-resolved valence band photoemission spectroscopy. The influence of the alloying on the interaction with gas molecules was probed by CO adsorption. It was found that annealing at 973 K results in a (2 × 1) surface reconstruction of Rh surface atoms along [1̅10] rows induced by subsurface vanadium. Annealing at higher temperatures induces diffusion of vanadium into deeper layers and formation of a (1 × 2) missing-row reconstruction, which is metastable at the bare Rh surface. Photoemission spectroscopy revealed a rather small shift of the Rh valence band centroid upon alloying, which indicates only a small influence of the electronic effect on the reactivity. On the other hand, the (1 × 2) missing-row reconstruction brings about new adsorption sites and low-coordinated Rh atoms which are expected to significantly contribute to the enhanced reactivity of the Rh–V alloy.

Angle-resolved photoemission of chain-like molecules: the electronic band structure of sexithiophene and sexiphenyl

Here we report the electronic π-band structure of sexithiophene obtained from 6T(010) oriented films. The angle-resolved valence band photoemission results taken parallel and perpendicular to the molecular axis are compared to those of sexiphenyl and interpreted in terms of intra- and inter-molecular band dispersion. We show that the strong photoemission intensity variations with emission angle parallel to the molecular axis are well reproduced by the Fourier transforms of the molecular orbitals of the isolated molecules. These results imply that ARUPS can yield quite detailed information about molecular geometry, both in terms of molecular orientation and internal structure.

Adsorption-induced gap states ofh−BNon metal surfaces

The formation of hexagonal boron nitride (h-BN) monolayers on Ni(111), Rh(111), and Pt(111) has been studied by a combination of x-ray emission, angle-resolved valence band photoemission, and x-ray absorption in search for interface-induced gap states of h-BN. A significant density of both occupied and unoccupied gap states with N 2p and B 2p characters is observed for h-BN/Ni(111), somewhat less for h-BN/Rh(111) and still less for h-BN/Pt(111). X-ray emission shows that the h-BN monolayer is chemisorbed strongly on Ni(111) and very weakly on Pt(111). We associate the gap states of h-BN adsorbed on the transition metal surfaces with the orbital mixing and electron sharing at the interface because their density increases with the growing strength of chemisorption.

Matrix element effects in angle-resolved valence band photoemission with polarized light from the Ni(111) surface

Photoemission intensities from angle-resolved photoelectron spectroscopy with polarized synchrotron radiation were measured and calculated for a magnetization averaged Ni(111) single-crystal surface. Concentrating on the large variations of intensity in dependence on the experimental geometry and parameters, we discuss the exchange-split $sp$ bands crossing ${E}_{F}$ and identify strong matrix-element effects that are not dependent on selection rules involving parity. Comparison of experimental intensities with ab initio photoemission calculations provides a firm basis for disentangling symmetry from intrinsic effects due to the initial state. The calculated spectra provide a direct connection between ground-state properties (given by the spectral function) and the experimental intensities.

Photoemission study on electronic structure of TiSe 2

We have examined core-level photoemission spectra of 1T-TiSe 2 and 1T-TaS 2 to study chemical shifts associated with unusual superlattice formation. While large temperature-dependent core-level splitting was observed for TaS 2, temperature-dependent chemical shift was not observed for TiSe 2. On the other hand, angle-resolved valence-band photoemission spectra of TiSe 2 showed appearance of a new branch (band-folding) at the L points in the ordered phase at 50 K. These results suggest smaller lattice distortion in TiSe 2 than that in TaS 2.

Gas-phase-dependent properties ofSnO2(110), (100), and (101) single-crystal surfaces: Structure, composition, and electronic properties

The dependence of the surface structure, composition, and electronic properties of three low index $\mathrm{Sn}{\mathrm{O}}_{2}$ surfaces on the annealing temperature in vacuum has been investigated experimentally by low energy ${\mathrm{He}}^{+}$ ion scattering spectroscopy (LEIS), low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and angle resolved valence band photoemission (ARUPS) using synchrotron radiation. Transitions from stoichiometric to reduced surface phases have been observed at $440--520\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, $610--660\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and $560--660\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for the $\mathrm{Sn}{\mathrm{O}}_{2}$ (110), (100), and (101) surfaces, respectively. Density functional theory has been employed to assess the oxidation state and stability of different surface structures and compositions at various oxygen chemical potentials. The reduction of the $\mathrm{Sn}{\mathrm{O}}_{2}$ surfaces is facilitated by the dual valency of Sn, and for all three surfaces a transition from Sn(IV) to Sn(II) is observed. For the (100) and (101) surfaces, theory supports the experimental observations that the phase transitions are accomplished by removal of bridging oxygen atoms from a stoichiometric $\mathrm{Sn}{\mathrm{O}}_{2}$ surface, leaving a SnO surface layer with a $1\ifmmode\times\else\texttimes\fi{}1$ periodicity. For the (110) surface the lowest energy surface under reducing conditions was predicted for a model with a SnO surface layer with all bridging oxygen and every second row of in-plane oxygen atoms removed. Ab initio atomistic thermodynamic calculations predict the phase transition conditions for the (101) surface, but there are significant differences with the experimentally observed transition temperatures for the (110) and (100) surfaces. This discrepancy between experiment and thermodynamic equilibrium calculations is likely because of a dominant role of kinetic processes in the experiment. The reduction of surface Sn atoms from a Sn(IV) to a Sn(II) valence state results in filling of the $\mathrm{Sn}\text{\ensuremath{-}}5s$ states and, consequently, the formation of Sn derived surface states for all three investigated surfaces. The dispersion of the surface states for the reduced (101) surface was determined and found to be in good agreement with the DFT results. For the (110) surface, the $4\ifmmode\times\else\texttimes\fi{}1$ reconstruction that forms after sputter and annealing cycles was also investigated. For this surface, states that span almost the entire band gap were observed. Resonant photoemission spectroscopy identified all the surface states on the reduced $\mathrm{Sn}{\mathrm{O}}_{2}$ surfaces as Sn derived.

Spin-polarized surface state of MnSb(0 0 0 1)

Knowledge of the spin-dependent electronic structure at surfaces and interfaces plays an increasingly important role when assessing possible use of novel magnetic materials for spintronic applications. It is shown that spin- and angle-resolved photoelectron spectroscopy together with ab initio electronic structure methods provides a full characterization of the surface electronic structure of ferromagnetic MnSb(0 0 0 1). Two different surface reconstructions have been compared in spin- and angle-resolved valence-band photoemission. For annealing at elevated temperatures, the (1 × 1)-structure transforms into 2 × 2 and a majority-spin peak appears at −1.7 eV inside a majority-spin bulk band gap at the surface Brillouin zone centre. Its sensitivity to oxygen supports an interpretation as magnetic compound surface state. Local spin density calculations predict at the same energy (−1.75 eV) a prominent d surface state of majority spin for (1 × 1)-Mn terminated MnSb(0 0 0 1) but no such feature for (1 × 1)-Sb termination. The calculation shows that neither the bulk nor the surface is half-metallic, in agreement with the expectation for the hexagonal NiAs structure.

Angle-resolved valence-band photoemission study of the quenched superconductors: Y 1− xPr xBa 2Cu 3O 7− δ

The electronic structures of the quenched superconductivity system Y 1− x Pr x Ba 2Cu 3O 7− δ (YPrBCO; x=0, 0.4, 0.8) have been investigated by angle-resolved photoemission spectroscopy (ARPES). With increasing x, the ARPES spectra along Γ ̄ → S ̄ exhibit a binding energy shift away from E F and a pronounced decrease in the density of electronic states near E F, suggesting an enhanced size of the Fermi wave vector k F from S for x=0.4, as compared to x=0. The ARPES results have been compared with the LSDA+ U electronic structure calculations incorporating the spin–orbit interaction for PrBa 2Cu 3O 7.

Electronic and crystalline structure at the surface of epitaxial films of DyNi grown on W(110)

The electronic and crystalline structures of thin epitaxial DyNi films grown in situ by codeposition of both constituents onto a W(110) substrate followed by annealing at $500\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ were studied by angle-resolved valence-band photoemission and low-energy electron diffraction (LEED). The obtained experimental data are in good agreement with results of local-density-approximation calculations in bulk geometry. Further investigations included supercell calculations. According to the latter investigations the observed $(3\ifmmode\times\else\texttimes\fi{}1)$ overstructure of the LEED pattern and a Dy $5d$-derived surface state near the Fermi energy are related to a reconstruction of the (001) surface of DyNi resulting from missing Ni rows in the topmost atomic layer. The d-like surface state is a well-known phenomenon for metallic Dy(0001). Similar to the situation in rare-earth metals, it may influence the surface magnetic properties of thin DyNi films.

Structure and magnetism of Fe thin films grown on Rh(001) studied by photoelectron spectroscopy

Structural and magnetic properties of Fe thin films grown epitaxially on Rh(001) are studied by spin- and angle-resolved valence-band photoemission, Fe ${2p}_{3/2}$ photoelectron diffraction, and x-ray magnetic circular dichroism experiments. The valence-band structure shows characteristic features of a fcc high-spin state at 4 monolayers (ML) and those of ferromagnetic bulk bcc Fe at 8 ML. The structure of the Fe film reveals fct(001) compressed along the direction perpendicular to the surface in the low-coverage region and gradually changes to distorted bcc(110) as the film thickness increases. The magnetic moments increase as a function of the film thickness and reach the same value as in bulk bcc Fe above 6 ML. The Fe films show in-plane ferromagnetism above 2 ML, and the thickness dependence of the magnetic properties is discussed in connection with the characteristic growth mode.

Hydrogen-induced3×1phase of the Si-rich3C−SiC(001)surface

A single-domain $3\ifmmode\times\else\texttimes\fi{}1$ phase induced by hydrogen adsorption on a Si-rich $3C\ensuremath{-}\mathrm{SiC}(001)3\ifmmode\times\else\texttimes\fi{}2$ surface is investigated by photoemission using synchrotron radiation. Three surface components of the Si $2p$ core level are identified for the $3\ifmmode\times\else\texttimes\fi{}1\ensuremath{-}\mathrm{H}$ phase, which resemble those of the $3\ifmmode\times\else\texttimes\fi{}2$ surface. A H-Si bonding state is observed by angle-resolved valence-band photoemission. These results are consistent with the recent assignments of the Si $2p$ surface components and the valence band spectra of the $3\ifmmode\times\else\texttimes\fi{}2$ surface, based on the $3\ifmmode\times\else\texttimes\fi{}2$ structure model with $\frac{2}{3}$ ML Si addimers. A straightforward $3\ifmmode\times\else\texttimes\fi{}1\ensuremath{-}\mathrm{H}$ structure model is introduced featuring Si dimer-bond breaking and dangling-bond saturation.

Electronic structure of surface compounds formed under thermal annealing of the La/graphite interface

Surface compounds grown by step-by-step thermal annealing of the La/graphite interface in a temperature range from 600 K to 1400 K were studied by angle-resolved valence-band photoemission (PE), resonant and C 1s core-level PE as well as by C(KVV) Auger electron spectroscopy and low-energy electron diffraction. It was found that annealing of the system at temperatures between 600 K and 1000 K leads to formation of an ordered La carbide phase with non-dispersive valence-band electronic structure. High-temperature annealing up to 1100–1400 K results in formation of a thin layer of intercalation-like compound on top of the carbide phase. Valence-band energy distribution curves taken for this system reveal (i) graphite-like dispersion of electronic states shifted toward higher binding energies compared with their position in pristine graphite and (ii) a series of carbide-derived features seen through the thin film of intercalation-like compound. C 1s PE spectra measured for the high-temperature phase have two-component lineshape in accordance with the layered structure of the system.

Magnetic linear dichroism in valence-band photoemission: Experimental and theoretical study of Fe(110)

Magnetic linear dichroism (MLD) in angle-resolved valence-band photoemission of Fe(110) has been investigated by means of symmetry considerations, analytical theory, numerical calculations, and photoemission measurements. The dichroic photoemission spectra and their dependence on photon energy clearly show that the microscopic origin of MLD is an interference effect induced by hybridization of initial states of different spatial symmetry at band gaps arising from spin-orbit coupling. The dependence of MLD on the light incidence angle evidences that it is substantially influenced by the optical response of the metal, the approximation of which according to classical Fresnel theory is adequate to reproduce our experimental data.

Geometrical and electronic structure of the MBE-prepared GaAs(113)A surface

The GaAs (113)A surface was prepared by molecular beam epitaxy and investigated in situ by means of low-energy electron diffraction, surface core level spectroscopy and angle-resolved valence band photoemission. One stable, (8 × 1) reconstructed surface was prepared. Surface core level shifts were observed for As 3d (530 meV) and Ga (−460 meV, 360meV) which support a recently proposed model [1]. Two surface resonances were observed at −1.1 and −3.7 eV below the valence band maximum.

Initial stages of growth of Mn on Ag(100) studied by X-ray photoelectron diffraction and valence band photoemission

The growth mode of Mn on Ag(100) in the monolayer range was studied using X-ray photoelectron diffraction (XPD), angle-resolved valence band photoemission and low energy electron diffraction. Our results show that MnAg intermixing takes place. The thickness of the intermixed films depends critically on the growth temperature. At room temperature the XPD data clearly indicate that Mn is located in the first and second topmost atomic layers. The growth at higher temperatures, i.e. for 1 ML deposited at 460 K, or 1 ML deposited at RT and then annealed at 460 K, results in enhanced interdiffusion and Ag segregation.

Temperature dependence of the spin- and angle-resolved valence band photoemission spectra of Ni(110)

Abstract We have measured spin- and angle-resolved valence band photoemission (SARPES) spectra of ferromagnetic Ni(110) near X- and K-points in the Brillouin zone at various temperatures between T Tc =0.5 and 1.05. The spectral features were analyzed based on the assumption that they could be reduced into two components which show collapsing and noncollapsing behaviors. The results show that the observed wavevector dependence of the SARPES spectra is consistent with the one expected by the local band theory.

Temperature dependence of the spin- and angle-resolved valence band photoemission spectra of ferromagnetic Ni(110)

Abstract We have measured spin- and angle-resolved valence band photoemission spectra of Ni(110) at temperatures T / T C = 0.5−1.05. The temperature dependence of the spectral profiles can be reduced to two components, one that follows the local exchange field, and one that follows the magnetic field averaged over many local exchange fields, which is consistent with the local band theory.

Magnetic linear dichroism in angle-resolved valence band photoemission from Fe(001)

We report the observation of magnetic linear dichroism in angle resolved valence band photoemission from Fe(001), manifesting itself in a change of the spectrum upon reversal of the magnetization direction.