Angle-resolved Valence Band Spectra Research Articles

Bulk electronic structure of lanthanum hexaboride ( LaB6 ) by hard x-ray angle-resolved photoelectron spectroscopy

In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter physics, and for their applications in advanced technological fields. Among these compounds, LaB$_6$ has a special place, being a traditional d-band metal without additional f- bands. In this paper we investigate the bulk electronic structure of LaB$_6$ using hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. By comparing La 3d core level spectra to cluster model calculations, we identify well-screened peak residing at a lower binding energy compared to the main poorly-screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulk-like band structure of LaB$_6$ determined by hard x-ray angle-resolved photoemission spectroscopy (HARPES). We interpret HARPES experimental results by the free-electron final-state calculations and by the more precise one-step photoemission theory including matrix element and phonon excitation effects. In addition, we consider the nature and the magnitude of phonon excitations in HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that one step theory of photoemission and HARPES experiments provide, at present, the only approach capable of probing true bulk-like electronic band structure of rare-earth hexaborides and strongly correlated materials.

Small valence band offset of h-BN/Al0.7Ga0.3N heterojunction measured by X-ray photoelectron spectroscopy

Because of its large bandgap of ∼6.0 eV and suitability for high p-type doping, hexagonal boron nitride (h-BN) has become a candidate material that can serve as a p-layer by forming a heterostructure with AlGaN materials with a high Al fraction in deep-ultraviolet optoelectronic devices. The band offsets at the heterojunction are crucial to the device design because they determine the hole and electron transport properties across the heterojunction. In this study, we give the band alignment between h-BN and Al0.7Ga0.3N using the valence and conduction band offsets. The valence band offset of the h-BN/Al0.7Ga0.3N heterojunction is determined via X-ray photoelectron spectroscopy (XPS) to be as small as −0.01 ± 0.09 eV. The small valence band discontinuity that occurs at the h-BN/Al0.7Ga0.3N interface is further confirmed using angle-resolved valence band spectra from the XPS measurements. By combining the bandgap values of Al0.7Ga0.3N and h-BN which were estimated using absorption spectra measurements, the conduction band offset is found to be approximately 0.89 ± 0.09 eV. These results indicate that h-BN is an excellent material for hole injection into Al0.7Ga0.3N. Meanwhile, the electrons can be effectively blocked away from h-BN. These results will be helpful in the design of group-III-nitride-based optoelectronic devices, particularly deep-ultraviolet light-emitting diodes and lasers.

Surface enhanced covalency and Madelung potentials in Nb doped SrTiO 3 (100), (110) and (111) single crystals

The influence of surface enhanced covalency on the Madelung potential is experimentally investigated using angle-resolved photoemission for (100), (110) and (111) SrTiO 3 surfaces after annealing in UHV at 630 °C. Deconvolution of the core level spectra (O 1 s, Sr 3 d and Ti 2 p) distinguishes bulk and surface components, which are interpreted in terms of surface enhanced covalency (SEC). By comparing the experimentally measured binding energies with theoretical calculations developed in the framework of the Localized-Hole Point-Ion Model, we quantitatively determine the effective electron occupancy at bulk and surface Sr and Ti sites. Our results confirm the essentially ionic character of Sr–O bond and the partially covalent character of Ti–O bond in bulk STO. The cation Ti and Sr electron occupation is greater for all the three surfaces than in the bulk. Surface covalency shifts the Madelung potential at the surface by Δ E M. Δ E M is a minimum for the (111) surface, and increases through (100), attaining a maximum for (110). The angle-resolved valence band spectra and the work function values also confirm this trend. The results are consistent with d – d charge fluctuations dominating at the surface, whereas metal-ligand charge transfers are more energetically favourable in the bulk.

Final-state band structure-induced modulations of the photoemission linewidth in angle-resolved valence band spectra: a case study on Bi(1 1 1)

The peak width in angle-resolved photoemission spectra from 3D valence bands in Bi(1 1 1) has been analyzed as a function of final-state energy. Over the photon energy range of 9–100 eV valence band transitions display pronounced oscillations with minima at the Γ and T points. The maxima in-between these points are additionally modulated through transitions into multiple final-state bands. Model calculations show that the 3D linewidth is a function of the initial- and final-state widths as well as the respective band velocities v ⊥ = ℏ − 1 ∂ E / ∂ k ⊥ . Extending the model calculations past the commonly used linear approximation by using experimentally determined dispersion relations E ( k ) of initial- and final-state bands significantly improves the linewidth changes in regions near the symmetry points.

Lack of electron–phonon coupling along two-dimensional bands in Bi 1− xSb x single crystal alloys

From the temperature dependent broadening of high-resolution angle resolved valence band spectra of Bi 0.92Sb 0.08, we deduce the electron–phonon (e–p) interaction in several bands close to the Fermi level. Our data indicate that the e–p coupling constant λ is highly anisotropic in k ∥ . For a three-dimensional band close to Γ ¯ we find λ = 0.54 ± 0.03 while the e–p coupling with electrons from a nearby two-dimensional band is only λ = 0.05 ± 0.01. Another two-dimensional highly dispersive and very narrow band with a Fermi momentum of only k F = 0.0084 Å −1 close to M ¯ in the direction perpendicular to the bisectrix-line is nearly temperature independent. The e–p coupling constant for holes at the bottom of this band at E i = −24 meV is only λ =0.007 ± 0.005 and does not seem to further increase for holes closer to the Fermi level.

Interaction between As and InP(110) studied by photoemission

Arsenic deposition on InP(110) was studied by means of core-level and valence-band photoemission. By systematic spectral decompositions, it was found that the adsorption at room temperature is nonreactive. At elevated temperatures the As $3d$ spectra show drastic transformations, indicative of a P-As exchange reaction. The data can at this point be interpreted in terms of a recently proposed structure involving an As-covered InAs layer. Upon As redeposition the core-level spectra are clearly different from those observed prior to annealing, which means that the exchange reaction is irreversible. Angle-resolved valence-band spectra from the reacted surface are similar to those from clean InP(110), but important differences are found, facilitating the identification of surface states.

Spin-Resolved Photoemission Spectroscopy

The spin-resolved photoemission spectroscopy is utilized to study the spin dependent electronic structures of solids and solid surfaces. The spinand angle-resolved valence band spectra of ferromagnetic Ni are investigated by comparing with an atomic model and a one-electron band calculation. The electron correlation effects which appear in the spin-resolved photoemission spectra are discussed. The spin dependent electronic structures of S atoms adsorbed on Fe(100) surface are also investigated. PACS numbers: 75.25.+z, 73.20.-r, 79.60.-i

The electronic structure of In- and As-terminated InAs(001) surfaces

The InAs(001) 2 × 4 and 4 × 2 surfaces have been investigated by angle-resolved photoemission. The X 3 and X 5 points were found to be located 6.0 and 2.7 eV below the valence band maximum, respectively, and the dispersion of bulk bands along the Г-X direction in the bulk Brillouin zone were well described by a theoretical calculation. From angle-resolved valence band spectra measured along the high symmetry directions [110] and [11̄0], three surface induced states were identified on both the InAs(001)4 × 2 and the InAs(001)2 × 4 surface.

High-resolution core-level studies of silicon surfaces

We present high-resolution core-level spectroscopy results for the clean Si(100) surface and for a few cases of ordered overlayers. For the two varieties of the clean surface, i.e. the 2 × 1 room temperature phase and the c(4 × 2) low temperature phase, both core-level results and angle-resolved valence band spectra are discussed. Three different adsorbates (H, K and As) have been used to obtain different types of symmetric dimer structures on the surface. Similarities and differences between these systems with respect to the surface and sub-surface contributions to the Si2p spectra are discussed.

Geometry and electronic structure of selenium-treated InP(110)

We present theoretical studies of the atomic and electronic structure of the Se-treated InP(110) surface. The calculations are based on density functional theory within the pseudopotential approach and the local density approximation. Geometry optimization has been performed on both an epitaxial growth of Se and an ordered Se-reacted surface. Electronic band structures for both geometrical models are presented in order to explain the recently observed angle-resolved valence band spectra by photoemission method. We also discuss the similarities and contrast the differences in the atomic and electronic structures between the Se-treated and the Bi-covered surfaces.

Electronic structure of epitaxial α-Sn 1-xGe x alloys

The electronic structure and interface properties of α-Sn 1−xGe x alloy films grown by molecular beam epitaxy (MBE) on GaSb, CdTe and Ge(100) were studied by angle resolved synchrotron radiation photoemission spectroscopy (ARPES). Deposition at temperatures below ~150 C results In polycrystalline or amorphous films. At higher substrate temperatures GaSb and CdTe partially react with the overlayer forming either GeSb or SnTe in the Interface region. Homogeneous alloy films with good crystalline quality were grown on Ge(100) at ~400 C up to a critical thickness of 200–300Å. Photoemission core level analysis indicates a strong tendency to form an ordered compound with a composition close to x~0.5. Angle resolved valence band spectra show the shift of the Γ 8 point from ~0.6 eV in Ge(100) to ~0.16 eV below E F in the α-Sn 0.48 Ge 0.52 alloy. The experimental information of a constant alloy Fermi level locates the top of the Γ 8 valence band ~0.16 eV below E p. Assuming a linear band model, the direct band gap of Eg-0.2 eV for x~0.5 will then locate the bottom of the Γ 7 conduction band -0.04 eV above E F.