A first-principles study of the electronic structure of the sulvanite compounds

We have investigated by means of first-principles total energy calculations the electronic structure of the sulvanite compounds: Cu₃VS₄, Cu₃NbS₄ and Cu₃TaS₄; the later is a possible candidate as a p-type transparent conductor with potential applications in solar cells and electrochromic devices. The calculated electronic structure shows that these compounds are indirect band gap semiconductors, with the valence band maximum located at the R-point and the conduction band minimum located at the X-point. The character of the valence band maximum is dominated by Cu d-states and the character of the conduction band minimum is due to the d-states of the group five elements. From the calculated charge density and electron localisation function we can conclude that the sulvanite compounds are polar covalent semiconductors.     

Energy band structure and refractive properties of LiRbSO4 crystals

The energy band structure of mechanically free and compressed LiRbSO₄ single crystals is investigated. It is established that the top of the valence band is located at the D point of the Brillouin zone [k = (0.5, 0.5, 0)], the bottom of the conduction band lies at the Γ point, and the minimum direct band gap E _g is equal to 5.20 eV. The bottom of the conduction band is predominantly formed by the Li s, Li p, Rb s, and Rb p states hybridized with the S p and O p antibonding states. The pressure coefficients corresponding to the energies of the valence and conduction band states and the band gap E _g are determined, and the pressure dependences of the refractive indices n _i are analyzed.     

Electronic structure of the CuBS2 crystal

The band structure and spectra of the total and projected densities of states of a new crystal of the chalcopyrite family, namely, CuBS₂, have been calculated in terms of the density functional theory. It has been found that the crystal is a pseudo-direct-band-gap semiconductor, and the best theoretical estimate of the optical band gap is 3.44 eV. The upper valence band of the CuBS₂ crystal basically consists of the contributions from the p states of S atoms and the d states of Cu atoms. The crystal splitting is 0.2 eV. The bottom of the conduction band is basically formed by the sp states of boron and sulfur atoms with an admixture of the s states of copper atoms.     

Photoemission studies of single crystals of titanium carbide

The electron distribution in the valence band from single crystals of titanium carbide has been studied by photoelectron spectroscopy with photon energies h?ω = 16.8, 21.2, 40.8 and 1486.6 eV. The most conspicious feature of the electron distribution curves for TiC is a hybridization between the titanium 3d and carbon 2p states at ca. 3–4-eV binding energy, and a single carbon 2s band at ca. 10 eV. By taking into account the strong symmetry and energy dependence of the photoionization crosssections, as well as the surface sensitivity, we have identified strong emission from a carbon 2p band at ? 2.9-eV energy. Our results are compared with several recent energy band structure calculations and other experimental data. Results from pure titanium, which have been used for reference purposes, are also presented.The valence band from single crystals of titanium carbide have been studied by means of photoelectron spectroscopy, with photon energies ranging from 16.8 to 1486.6 eV.By taking into account effects such as the symmetry and energy dependence of the photoionization cross-sections and surface sensitivity, we have found the valence band of titanium carbide to consist of two peaks. The upper part of the valence band at 3–4 eV below the Fermi level consists of a hybridization between Ti 3d and C 2p states. The C 2p states observed in our spectra were mainly excited from a band about 2.9 eV below the Fermi level. The APW^5–9, MAPW¹⁰ and EPM¹¹ band structure calculations predict a flat band of p-character between the symmetry points X′₄ and K₃, most likely responsible for the majority of C 2p excitations observed. The C 2s states, on the other hand, form a single band centered around ?10.4 eV.The results obtained are consistent with several recent energy band structure calculations^{5–11, 13} that predict a combined bonding of covalent, ionic and metallic nature.     

Valence band structure of single crystal titanium carbide,studied by soft X-ray and ultraviolet photoelectron spectroscopy

Valence states of single crystal titatium carbide (TiC_x, X?0.88) have been studied with photon energies ranging from far ultraviolet (u.v.) to soft X-ray. The valence band consists of two peaks located at 3 and 10 eV below the Fermi level. This is in good agreement with recent APW band structure calculations that predict a strong hybridization of the Ti 3d and C 2p bands and a C 2s band at lower energy.     

Electronic band structure of In2S3 and CdIn2S4 semiconductor spinels from the data of x-ray spectroscopy and theoretical calculations

The electronic band structure of the chalcogenide spinels In₂S₃ and CdIn₂S₄ has been studied using the FEFF8 program. It is shown that the valence band top is formed by the S p states mixed with the In s and In p states for In₂S₃ or with the Cd s, Cd p, In s, and In p states for CdIn₂S₄. Compared to In₂S₃, the presence of Cd atoms in the nearest environment of S atoms in CdIn₂S₄ does not considerably affect the electronic band structure. In CdIn₂S₄ the Cd 4d states, as well as the In 4d states, form a narrow localized band shifted deep into the valence band. The theoretical results are in good agreement with the experimental x-ray photoelectron and x-ray spectra.     

Photoemission via bloch states and evanescent band gap states for Cu(110)

Normal emission photoelectron spectra from Cu(110) using polarized synchrotron radiation (hv < 35 eV) can be explained with a direct transition model using realistic final state bands. Prominent surface photoemission via evanescent final states is observed in the large X′₅-X₁ conduction band gap. Accurate valence band eigenvalues at K and X have been determined.     

Fundamental reflection spectrum and the electronic band structure of chlorine-doped CdTe

The effect of chlorine impurity on the fundamental reflection spectrum and the electronic band structure of cadmium telluride crystals has been studied. At the impurity concentration N _Cl>5.0×10¹⁹ cm^?3, a peak appears in the reflectance spectra. This peak is due to electron transitions at the X point of the Brillouin zone from the upper split valence band to Cl levels lying 0.05 eV above the Γ minimum of the conduction band. The other features in the reflectance spectra and band structure are explained as being due to the effect of spin-orbit splitting at the X point and to indirect electronic transitions from the Cl levels to the Γ minimum.     

Electronic structure of SiO2(111) thin film

The electronic structure of (111) surface of β-crystobalite is investigat ed using the empirical tight binding method. Our calculations identify surface states in the conduction band, band gap and valence band. The surface state formed from silicon-s and p_z orbitals, which is believed to account for the structure in the O K excitation spectra, lies in the band gap. It is seen that oxygen adsorption on the surface removes surface states and gives rise to a sharp peak at about — 3.8 eV below the valence band edge.     

The valence and conduction band structure in Cd1−xMnxF2 crystal by means of UPS

The density of states distribution in the valence and conduction band was determined for Cd_1?xMn_xF₂ crystals (x = 0; 0.02 and 0.1) on the basis of the EDCs obtained for hv = 21.2 eV and hv = 40.8 eV. The smooth changes of the shape of EDCs were obtained for different alloy compositions for hv = 40.8 eV while for the hv = 21.2 eV EDCs drastic changes occur due to the structure of the final density of states. The peaks of density of states obtained for CdF₂ valence band in X₅^′, L_2,1 and X₅, points and conduction band in X₁ and L₁ points of the B.Z. changes with the increase of x in Cd_1?xMn_xF₂ alloy.     

Separation of the partial s- and p-densities of valence states of ZnO from UPS-measurements

UPS-spectra of the cleaved (0001) Zn and (000$\text{1}$) O surfaces of ZnO are taken at hv = 16.8, 21.2, 26.9, and 40.8 eV. Two maxima in the spectra at constant final energy are ascribed to high densities of conduction band states. Using the hv-dependence of the valence band emission, the partial s- and p-densities of states are separated. They yield similar excitation probabilities for Zn-4s-, 3d-, and O-2p-electrons.     

An electronic structure study of europium chalcogenides by soft x-ray spectroscopy

The M_V, M_IV and M_III X-ray emission and absorption spectra of EuO and EuS have been studied. The relative positions and the widths of 4f states and valence or conduction distributions have been deduced from these spectra. These data are compared with those obtained by u.v. photoemission and the results of band structure calculations.     

Electronic structure of PbI2 from photoelectron spectra and the exciton problem

The valence band density of states for PbI₂ is determined from X-ray and u.v. induced photoelectron spectra. It is shown that the band derived from Pb 6s states is at 8 eV binding energy and not at the top of the valence bands as suggested by band structure and charge density calculations. A rigid shift in the predominantly iodine 5p derived bands to lower binding energy brings the band structure calculations into essential agreement with experiment. Pb 5d core level binding energies determined here are used to derive core level exciton energies of 0.7 eV from published reflectivity spectra.     

XPS and UPS studies of pyrite compounds (CoX2 → CuX2, X: S; Se): Investigation of the insulator-metal transition in the NiS2−xSexsystem

We present in this paper a photoemission study (XPS and UPS) of several transition metal disulphides and diselenides (CoX₂ → CuX₂) with the pyrite structure. Our results show that all these compounds have narrow conduction bands and that the spectra for the disulphides and the diselenides are very similar. Nevertheless some changes occur in the electronic structure when going in the same series from CoX₂ → CuX₂, which are qualitatively well understood from simple theoretical considerations. Our main purpose is to discuss the influence of correlations between electrons in these narrow band systems and particularly in the case of the half-filled band (Mott-Hubbard insulator NiS₂). Considering the energy shift of the core levels deduced by XPS experiments we show that the effects of these correlations become smaller when going from CoX₂ to CuX₂, whereas they seem to remain unchanged in NiS_2?xSe_x, where an insulatormetal transition takes place. We present an attempt to quantitatively analyse our UPS results on the conduction bands. According to our results the particular behaviour corresponding to the Mott-Hubbard insulating state of NiS₂ and NiS_2?xSe_x (x < 0.5) mixed systems can be revealed by photoemission studies.     

Electron energy loss spectroscopy study of the initial stage of lanthanum oxidation

The electron energy loss spectra (EELS) of a pure metallic lanthanum surface and variations in these spectra at the initial stages of surface oxidation were studied. The measurements were performed at primary-electron beam energies E _p from 200 to 1000 eV. A very pronounced peak at a loss energy of about 7.5 eV arises due to transitions from the La4d electronic states of the valence band into the empty La4f electronic states of the conduction band at 5.0–5.5 eV above the Fermi level. Marked changes are observed in the EELS during the oxidation of lanthanum: the peak at an energy of 7.5 eV disappears, and the peak at 13.5 eV corresponding to bulk collective energy loss in lanthanum oxide becomes more pronounced. The results obtained are discussed in terms of the electronic structure of lanthanum and lanthanum oxide.     

Photoelectron spectra of thin films of the high-T c superconductor YBa2Cu3O7−δ

The results of calculations of the energy band structure of thin films are used to obtain theoretical photoelectron spectra of the valence band of the compound YBa₂Cu₃O_7−δ for δ=0 and δ=1 at various photon energies; the spectra are averaged over the photoelectron emission angles. The principal structural features of the spectra are determined by the d states of copper atoms, while the variation of the shape of the spectra with increasing photon energy is attributed to a relative decrease in the contribution from the p states of oxygen atoms. The density of d states around the Fermi level for YBa₂Cu₃O₆ films is observed to increase relative to YBa₂Cu₃O₇ films. Fiz. Tverd. Tela (St. Petersburg) 39, 437–440 (March 1997)     

Electronic structures of MRhO2, MRh2O4, RhMO4 and Rh2MO6 on the basis of X-ray spectroscopy and ESCA data

X-ray O Kα, Rh Mγ and a series of M Lα emission spectra, ESCA spectra of the valence and inner levels, and O K and Rh M_III quantum-yield spectra for X-ray photoemission of the rhodium double oxides MRhO₂ (M = Li, Na, K), MRh₂ O₄ (M = Be, Mg, Ca, Sr, Ba, Co, Ni, Cu, Zn, Cd, Pb), RhMO₄ (M = V, Nb, Ta) and Rh₂MO₆ (M = Mo, W) have been measured and the dependence of electronic structure on the metal M analysed. For all compounds the inner part of the valence band corresponds to O 2p_σ + O 2p_π + Rh 4d states, while the outer part corresponds to Rh 4d. The valence band is separated from the conduction band by a narrow gap of width less than 1 eV. The first empty band, near the bottom of the conduction band, is formed by Rh 4d states, followed by a band due to vacant O 2p states.     

The electronic structures of Mg,Al and Si in octahedral coordination with oxygen from SCF Xα MO calculations

Molecular orbital calculations performed using the SCF Xα Scattered Wave Cluster method are presented for the octahedral oxyanions MgO₆^?10, AlO₆^?9 and SiO₆^?8. The AlO₆^?9 results are used to assign and interpret the X-ray photoelectron spectra (XPS), X-ray emission (XES) and u.v. spectra of Al₂O₃. Agreement between calculation and experiment is good for valence band and fair for conduction band orbitais. The SCF Xα results for MgO₆^?10 are also in good agreement with observed valence band energies in MgO, but in this case the lowest energy features in the u.v. spectrum are not assignable in terms of either the calculations or the X-ray spectral results. The substantial increase in covalency expected between the Mg and Si oxides is evidenced in the calculations by an increase in valence region width from 2.6 to 5.3 eV and an increase in valence-conduction band separation from 5.2 to 10.0 eV. The calculated trends are in reasonable agreement with u.v. spectral data and with absolute valence orbital binding energies derived from X-ray spectra. A comparison of the SiO₆^?8 calculation with the analogous tetrahedral SiO₄^?4 calculation shows the valence band in the octahedral oxyanion to be much simpler in structure and somewhat narrower than that in the tetrahedral oxyanion. Using the orbital structure calculated for the valence bands of tetrahedral and octahedral oxides, a method is presented for calculating atomization energies directly from X-ray spectral data for SiO₂, Al₂O₃ and MgO. Results are in good agreement with experiment but the method involves an empirical parameter which is not presently understood in detail. Studies of trends in p-type bonding orbital binding energies derived from experimental data provide a qualitative explanation for the preferred coordination numbers in the Mg, Al and Si oxides.     

Shallow impurity states and the free exciton binding energy in gallium phosphide

A new set of donor and acceptor ionization energies in GaP is deduced from photoexcitation spectra. Energy spectrum of donor states confirms an existence of the “camel's back” with a conduction band minima displacement ≈ 0.08(2π/a) from the X, and the corresponding energy shift ≈ 3meV. The free exciton binding energy in GaP is correctly determined: E_ex = 21 ± 2 meV.     

Electroreflectance observation of localized and itinerant electron states in NiO

We have observed electroreflectance spectra in NiO at 6 eV which we believe represent transitions from the oxygen 2_p derived valence band to the nickel 4s derived conduction band. We also observe the spectrum seen earlier by McNatt near 4 eV but interpret it differently in terms of transitions from the localized 3d⁸ state to the 4s band. These interpretations are consistent with the recent model of Adler and Feinleib.