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.     

Photoemission and electronic structure of crystalline Co3B and amorphous Co - P - B

We present an XPS and UPS study of crystalline Co, Co₃B and Co₇₈P₁₄B₈ glassy metal. For Co₃B the electronic distribution curves (EDC) of the valence band and the previous specific heat and magnetic results are interpreted in a qualitative model where : i) the s-p cobalt-boron bonding states lie in the low part of the band, the upper levels being mainly cobalt d states and ii) a large density of states of the majority spin band is present at the Fermi level. For amorphous Co₇₈P₁₄B₈ the EDC shows that E_F is located in a high density of states region, it is suggested that the phosphorus p states ae centred at 7.5 eV. The photoemission and the magnetic results can be also interpreted in the framework of the previous model.     

Valence band contributions to photoluminescence excitation spectra of tightly bound holes in zincblende semiconductors

Photoluminescence excitation (PLE) spectra of deep acceptor states in ZnSe, for example the Cu-related luminescence band at ≈1.95 eV, contain a prominent excitation band at ≈3.25 eV. This band lies above the structure marking the lowest direct E_O band gap E_g by the spin-orbit splitting energy Δ of the valence bands at Γ. The higher energy feature is either absent or greatly de-emphasised in the PLE spectra of shallow acceptor states in ZnSe and of the oxygen iso-electronic trap in ZnTe, where the electron rather than the hole is tightly bound. However, a significant PLE component at E_g + Δ is observed for deep acceptor-like states in ZnTe, where Δ is ≈0.95 eV. Efficient PLE at E + Δ for luminescence from deep acceptor-like states is shown to be consistent with the extended wave-vector contributions to the bound state wave-functions of holes of binding energies ≈Δ.     

Photoemission of adsorbed CO and H2O on Pt: Valence band studies

In this paper, using monochromatic He and Ne discharge lines, we report the ultraviolet photoemission results for CO and H₂O adsorbed on a Pt crystal. For chemisorbed CO, we use the energy dependence of the photoionization cross section to deduce that the 5σ?1π splitting is approximately 1 eV and that the 5σ is located at about ?8.1 eV while the 1π is at ?9.1 eV. The distribution of metal electrons changes in two ways upon CO chemisorption: a d character peak is found at ?4 to ?5 eV and emission at E_f is strongly depleted. For the weakly adsorbed water molecule, we find preferential attenuation of Pt states near E_f that has heretofore been observed for strongly chemisorbed systems.     

Relation between interface states and temperature behavior of the barrier height of silver contacts on clean cleaved n-type silicon

The properties of silver-silicon interfaces formed by cleaving n-type silicon in ultra high vacuum (UHV) in a stream of evaporating silver atoms were studied. The barrier heights of these contacts were measured at different temperatures by using C-V techniques. All measurements were performed in UHV. The dependence of the barrier height upon temperature did not follow the temperature dependence of the Si band gap as it is usually found. The measured temperature behavior depended on the roughness of the Si surface. The temperature behavior can be explained by assuming a specific band structure of the interface states. For Ag contacts on atomically smooth n-type Si, the interface states were found to be arranged in two bands, one band 4 × 10^?3 eV wide with acceptor type states 0.18 eV below the intrinsic level E_i and a density of 10¹⁷ states/cm² eV, and the other 1 eV wide with donor type states with its upper edge 0.28 eV below E_i, and a density of 4 × 10¹⁴ states/cm²eV.     

Gap states in hydrogenated amorphous silicon: A comparison of photoemission and photoconductivity results

We report photoemission results from which we directly determined the density of states g(E) in the gap of a-Si:H between the top of the valence band E_v and the Fermi level. At 0.4 eV above E_v, g(E) was found to be ≈1×10²⁰ cm^-3 eV^-1 in the undoped film; P-doping increased g(E) in this region whereas annealing reduced it. The photoconductivity-derived optical absorption spectrum matched the shape of the photoemission spectrum, and thus supports the explanation that the photoconductivity shoulder at photon energies in the region of 1.3 eV is due to transitions from localized states above the valence band to the conduction band.     

Reciprocal and nonreciprocal magnetic linear birefringence in the γ-Dy<Subscript>2</Subscript>S<Subscript>3</Subscript> sesquisulfide

A study has been made of the spectral dependence of the Cotton-Mouton effect (CME) quadratic in magnetic field, nonreciprocal birefringence (NB) linear in magnetic field, and the Faraday effect (FE) in the cubic magnetic semiconductor γ-Dy₂S₃. Unlike the FE, the CME and the NB in this crystal are anisotropic, with the pattern of the anisotropy being dependent on the photon energy. The dependence of the CME and NB dispersion on the direction of the magnetic field B indicates contribution from a variety of electronic transitions and mechanisms to these phenomena. It is shown that the resonant contributions to the CME and NB in the transparency region originate from electronic transitions near E?3.4 eV (beyond the band edge E _g=2.8 eV), which are likely transitions from the localized ground state of the Dy³⁺ ion to states derived from mixing of the band and 4f ^N?1 5d states of the dysprosium ion. The character of the CME anisotropy in the transparency region and near the local electronic transition ⁶ H _15/2 → ⁶ F _3/2 connecting states of the unfilled 4f shell of the Dy³⁺ ion suggests the presence of a strong axial component of the crystal field acting on the rare earth ion.     

Study of acceptor states in CdTe by donor-acceptor pair excitation luminescence

Excitation of donor-acceptor pair luminescence has been studied in CdTe doped with lithium or chlorine. The excitation spectrum of the lithium acceptor is determined and fitted with the effective mass theory of Baldereschi and Lipari. Revised values of the valence band parameters are deduced: μ = 0.8, δ = 0.054, Ry = 24 meV. The analysis of the 1.45 eV luminescence band in compensated Cl-doped crystals shows the existence of donor-acceptor pair transitions. Three acceptor centers are identified: E_A = 89, 111 and 119 meV, and the contribution of a deep donor (E_D > 40 meV) is demonstrated. Besides intracentre type excitation transitions of the 1.45 eV band have been observed in non-compensated chlorine-doped crystals. Thus several recombination channels and distinct acceptor states contribute to the composite 1.45 eV luminescence band.     

Disordering of the electronic structure of YBaCuO amorphous films upon incorporation of crystalline clusters formed in laser-induced plasma into their composition

The effect of crystalline clusters formed in a laser-induced plasma on the optical properties of YBa₂Cu₃O_{6 + x} amorphous films prepared by pulsed laser deposition has been investigated. It has been demonstrated that an increase in the number of clusters leads to a gradual disappearance of interference fringes inherent in optically homogeneous media. Simultaneously, the incorporation of metallic and insulating clusters into the amorphous medium results in a decrease in the optical band gap E ₀ of the YBaCuO amorphous matrix from 1.28 to 1.06 eV and a considerable decrease in the probability of interband optical transitions with charge transfer O 2p → Cu 3d due to the loosening of the structure and generation of local stresses. It has been revealed that there is an additional band gap E ₁, which decreases from 0.25–0.30 eV to zero values with a decrease in the optical band gap E ₀. The additional gap has been interpreted as an energy gap between localized states that belong to the valence and conduction bands. A decrease in the density of electronic states in the narrow 3d band leads to the overlap of tails of the density of states, so that the band gap E ₁ becomes negative.     

U-perturbation treatment of heavy fermion systems

Heavy fermion systems are described by the periodic Anderson Model (PAM), i.e. a lattice of localized, highly correlatedf-electron states hybridized with the delocalized states of a conduction band. We treat the PAM within the second orderU perturbation theory around the non-magnetic Hartree-Fock solution (U on site Coulomb correlation between thef-electrons). This treatment has the advantage that Fermi liquid relations (Luttinger theorem) are automatically fulfilled. Thef-electron selfenergy and spectral function are calculated for different temperatures, and, for the symmetric PAM, we obtain single-particle peaks near toE _f andE _f +U and in addition many-particle (Kondo) resonance peaks near to the chemical potential (E _f baref-electron energy). The resonance peaks are strongly temperature dependent and vanish on a characteristic temperature scaleT _K. For the symmetric PAM and a constant on-site hybridization the Fermi energy falls into a hybridization gap. A second, smaller characteristic temperature scaleT _coh (coherence temperature), on which the hybridization gap vanishes, is observed within this approach. For the non-symmetric PAM (i.e.E _f andE _f +U not symmetric around the chemical potential) we obtain a similar behaviour, but the single-particle peaks are no longer at the correct positionsE _f andE _f +U. The proper behaviour for the symmetric PAM but less satisfactory behaviour for the non-symmetric PAM can be understood from the fact that only for the symmetric PAM the exactly solvable limit of a vanishing hybridization is reproduced within this approach.     

Surface photoemission in the 4d band from polycrystalline silver surfaces

Surface induced local d-band states in the upper 4d band between ～ 4 and ～ 5.2 eV below E_Fermi have been identified for polycrystalline silver films in photoemission experiments using synchroton radiation. A thin over-coat (10 å) by an Al film leads to a depression of these surface induced local states whereas a change from s- to p-polarized excitation leads to an enhancement. Deposition of additional silver (～ 3 Å) at 120 K induces additional emission 4.2 eV below E_F with a FWHM of only ～ 0.4 eV.     

Electronic structure of PdO studied by photoemission (XPS,UPS)

In this paper we present the results of photoemission studies (XPS and UPS) performed on a polycrystalline surface of PdO. The electron density of states (EDOS) deduced both from XPS and UPS (He_I and He_II) are very similar. The valence band of PdO, which differs significantly from the Pd one, can be built up by four structures located at 0.5 eV, 2.2eV, 4.5 eV and 6.5 eV below E_F. The various electronic contributions (p or d) in the band are considered and, in order to explain our spectra, we discuss several hypothesis taking into account the possible existence of satellite lines or crystal field effects. Our XPS and UPS spectra show that the energy bands of PdO are narrow (～ 2–3 eV), moreover the energy shift of the core levels (|ΔE^F_B| = 2 eV) is important : these results suggest that the correlations between the d electrons may be important in PdO.     

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.     

Adsorbate band dispersions for C on Ru(0001)

We have studied carbon-induced two-dimensional energy bands on Ru(0001) using angleresolved photoelectron spectroscopy and have compared them with ab initio calculations. We find a nearly parabolic band (bottom at E_F ?9.8 eV at k_∥ = 0, effective mass ～ 1.5 m_e) which we assign to the C 2p_z valence states of a graphitic carbon overlayer. Compared to graphite, these states are bound more tightly by 2.3 eV.     

Surface photovoltage spectroscopy and surface piezoelectric effect in GaAs

“Real” (111) surfaces of n-type GaAs were investigated employing surface photovoltage spectroscopy and the surface piezoelectric effect. Surface states at the energy position E_c ? E_t ? 0.72 eV were found on both the Ga and the As surfaces. Both types of surfaces exhibited a barrier of about 0.55 V. No variations in the surface barrier or the energy position of the surface states were observed in various ambients at atmospheric pressure (dry air, wet air, ammonia and ozone). However, the capture cross-section of the surface states for electrons, as determined from the surface piezoelectric effect transients (of the order of 10^?13 cm²), was found to be sensitive to the ambient. It decreased in wet air and increased in ozone. This effect was more pronounced on the As than on the Ga surfaces. Additional surface states were found to be present in the energy region of 0.9 to 1.0 eV, below the bottom of the conduction band. However, their exact energy positions could not be determined due to interference caused by the carrier trapping of the surface states at E_c ? E_t ? 0.72 eV.     

Variations of the local density of electron states and short-range order in amorphous hydrated silicon films

Ultrasoft x-ray spectroscopy methods have been used to observed a change in the energy distribution of the silicon valence states after annealing a-Si:H films at 500 °C. This change appears as three distinct maxima in the density of states 3.5, 7.2, and 10.2 eV above the top of the valence band, which indicates ordering of the a-Si:H structural network. The energy distance between the latter two maxima (E?E _v=7.2 and 10.2 eV) supports electron-diffraction data indicating a decrease in the silicon-silicon interatomic distance by 0.2 Å in comparison with the crystal. The presence of a third maximum (E?E _v=3.5 eV) is connected with the change in the hybridization of the s-p-functions of silicon with decrease of the coordination number.     

Some comments on the electronic properties of the surface space charge layer of copper phthalocyanine thin films

A simple analysis, using a theory of the surface space charge layer of semiconductors, of the published values of the work function φ and surface ionization energy Φ_s of copper phthalocyanine (CuPc) thin films was performed. Using a well known position of the Fermi level E_F within the band gap E_g the values of its absolute band bending eV_s and surface electron affinity X_s were determined. A small negative value of the absolute band bending eV_s = −0.17 ∓ 0.15 eV has been interpreted by the existence of the filled electronic surface states localized in the band gap below the Fermi level E_F. Such states were predicted theoretically for thin films and the crystalline surface of CuPc, and attributed to surface lattice defects of a high concentration.     

Deep level defects in gamma-ray irradiated ge doped with pb or sn

Abstract

Deep level transient spectroscopy has been used to investigate the electrical properties of deep defect states in γ-ray irradiated Ge doped with the isoelectronic elements Pb or Sn. Three deep levels are observed in the irradiated Pb-doped Ge (E_v +0.28 eV, E_c ?0.33 eV, E_c ?0.39 eV) and two deep levels observed in the Sn-doped Ge (E_v + 0.19 eV, E_c ?0.15 eV). For the same γ-ray irradiation doses, Ge crystals grown from graphite crucibles and doped with Pb or Sn shows about two-thirds the total density of deep level defects observed in undoped Ge grown from synthetic quartz crucibles. All of the defect states observed were removed by a 1 hour, 250°C thermal anneal, and all but the E_c ?0.39 eV state in the Pb-doped material were neutralized by exposure to a low pressure atomic hydrogen plasma.     

Electronic structure and phase transitions of MnAs

On the basis of a two center tight binding approach the recursion method of Haydock et al. was applied for MnAs to calculate non-polarized and spin polarized local densities of states, magnetic moments and band energies for two different B8₁ crystal structures and two B31 structures. The itineracy of the Mn 3d-states was implied by allowing for hybridization of p-states centred at the As-sites with d-states centred at the Mn-sites. Self-consistency procedures were applied to determine the relative position of the paramagnetic p- and d-band centers (E_p - E_d = - 1 eV), and to calculate the magnetic moments (3.27 μ_B for the low temperature B8₁ phase and 3.14 μ_B for the corresponding B31 phase). Because of the similarities of the electronic structures of both phases it is argued that for the first-order B8₁ [rlarr2] B31 phase transition the usually applied model of a high spin-low spin transition of the Mn³⁺ ion is basically wrong. Instead of such an ionic model it is suggested to adopt an itinerant d-electron concept within that both phases are closely related but the B8₁ phase is favoured above a certain critical magnetic moment as supported by a very recent study of Kato et al. For the high temperature B8₁ phase occurring after the second-order B31 → B8₁ phase transition a magnetic moment of 3.37 μ_B was obtained.     

On the structure of wave fields in the region of linear interaction between ordinary and extraordinary waves in two-dimensionally inhomogeneous magnetoactive plasmas

The spectra of the low-temperature photodissociation (photoionization) of Landau-Pekar polarons are calculated using the theory of quantum-coherent states and a new method of variation with respect to the parameters of phonon vacuum deformation. It is shown that the final polaron states upon photodissociation may have different numbers of phonons produced in a single dissociation event and different momenta of charge carriers. The spectrum of optical absorption related to the photodissociation of polarons exhibits a superposition of bands corresponding to various numbers of phonons formed as a result of dissociation of a single polaron. Due to a large width of the energy region corresponding to the final states of charge carriers, the halfwidth of each band is on the order of the energy of polaron coupling and is much greater than the phonon energy. For this reason, the individual phonon bands exhibit strong overlap. The very broad and, probably, structureless band formed as a result of the superposition of all these components begins at an energy equal to the sum of the polaron coupling energy (E _p) and the phonon energy. This band has a maximum at a frequency of about 5.6E _p/? and a halfwidth on the order of 5.6E _p/? at a unit effective mass (m* = m _e) of band electrons. For an effective charge carrier mass within m* = (1–3)m _e, the energy of the polaron band maximum can be estimated as 5E _p with an error of about 10%, and the halfwidth falls within 3.4E _p < ?Ω_1/2 < 5.6E _p. The multiphonon character of this band is related to a decay of the phonon condensate after the escape of charge carrier from a polaron. Such polarons are likely to be observed in the spectra of complex metal oxides, including high-temperature superconductors. Examples of such polaron bands in the reported absorption and photoconductivity spectra of nonstoichiometric cuprates, manganites, nickelates, and titanates are presented. A theory of the formation of Landau-Pekar polarons with the participation of branches of the polarization oscillations of the medium is developed. It is shown that, under certain conditions, such a multiphonon-dressed polaron can possess a coupling energy on the order of 0.2–0.3 eV, so that the maximum of the corresponding absorption band may occur at 1–1.5 eV.