Comparison of site-specific valence band densities of states determined from Auger spectra and XPS-determined valence band spectra in GeS (001) and GeSe (001)

Auger lineshapes of the Ge M₁M_4,5V and M₃M_4,5V and Se _M1M_4,5V transitions in GeS (001) and GeSe (001) are measured and compared to XPS valence band spectra. Distortions in both types of spectra due to inelastic scattering, analyzer and source broadening, and core level lifetime broadening are removed by deconvolution techniques. The valence band consists of three main peaks at ?2 eV, ?8 eV, and ?13 eV. There is excellent agreement of peak positions in AES and XPS spectra. The Auger lineshapes can be interpreted in terms of site-specific densities of states. They indicate that the states at ～?8 eV and at ～?13 eV are associated with the cation and anion sites respectively. The bonding p-like states at the top of the valence band have both cation and anion character. The Auger lineshapes indicate that the states closest to the valence band maximum are preferentially associated with Ge.     

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.     

Virtual bound states in AuTi alloys

We have measured the optical absorption of dilute AuTi alloys for photon energies from 0.1 to 4.5 eV. The extra absorption we observe below the Au interband threshold can be accounted for if the Ti impurities form virtual bound states with half-width at half height δ ～ 0.6 ± 0.2 eV centered E_d ～ 0.5 ± 0.2 eV above the Fermi level and containing 2.7 ± 0.3 electrons.     

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.     

Potential parent compound of superconductor: Sr2CuM2As2O2 (M = Mn, Fe)

The electronic structure of Sr₂CuMn₂As₂O₂ and Sr₂CuFe₂As₂O₂ are studied by the first-principle calculations. These compounds have a body-centered-tetragonal crystal structure that consists of the CuO₂ layers similar to those in the high-Tc cuprate superconductor, and intermetallic MAs (M = Mn, or Fe) layers similar to the FeAs layers in high-Tc pnictides. Such special structure makes them as interesting candidates for new type of superconductor since they have two types of superconducting layers. However, our calculations indicate that the states in the range from −2.0 eV to +2.0 eV are dominated by Mn-3d or Fe-3d states, while the states of Cu-3d are far away from the Fermi level (in the range from −3.0 eV to −1.0 eV). Such results are significantly different with the Cu-based superconductor, like La₂CuO₄, where the states around Fermi level are dominated by Cu-3d states. Besides, we find that the mean-field magnetic ground state is the checkerboard antiferromagnetic in Cu sublattice and the stripe antiferromagnetic in Fe (or Mn) sublattice.     

Electronic structure of Si(111) surfaces

We report on new angle-resolved photoemission studies of Si(111) 2 × 1 and 7 × 7 surfaces. The emission from the 2 × 1 surface shows much structure. For normal emission the energy positions are insensitive to the photon energy in the range 19–27 eV. The emission has been interpreted as a probe of the surface density of states, SDOS, including both surface states, resonances and bulk-like states. The SDOS was also calculated as a function of parallel momentum k_∥ for a model of the Si(111) 2 × 1 surface obtained from energy minimization considerations. We identify emission from the dangling bond band, which has a positive dispersion of 0.6 eV, and also emission from surface resonances which have some character of the compressed and stretched back bonds. There are also other predicted surface resonances that correspond to experimental peaks which have not been identified in previous work. Except for the dangling bond band, the surface resonances are limited in k_∥ space, so that it is not possible to follow these resonance bands over all angles. Maximum intensity for the normal emission from the dangling bond is obtained at 23 eV, while the emission from the lowest s-like states monotonically increases towards 30 eV photon energy. When annealing the cleaved 2 × 1 surface to the 7 × 7 reconstructed surface, the spectra broaden significantly. The intensity of the dangling bond decreases and we see a very small metallic edge.     

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.     

Photoemission studies of clean and oxygen-covered Pt 6(111) × (100)

Employing the enhanced sensitivity obtained by using synchrotron radiation near the Cooper minimum for the 5d valence electrons, we have located the oxygen 2p and 2s levels for oxygen chemisorbed on a Pt 6(111) × (100) crystal. We find the oxygen 2p level located ?6 eV with a FWHM of 3 eV and the 2s at ?21.6 eV. A factor of four difference in saturation coverage is measured between temperatures of 300 and 120 K, but the position and width of the 2p level is independent of temperature. We observe also the 1b₁ orbital of weakly adsorbed H₂O molecules, which has pure O 2p parentage; from the intensity of this orbital, we are able to suggest why it is difficult to observe the oxygen 2p signal at low photon energies. In addition, we note a strong preferential attenuation in the Pt states near E_f for the adsorbed H₂O in spite of the weak nature of the bond.     

Study of Ag/Si(111) submonolayer interface: I. Electronic structure by angle-resolved UPS

Angle-resolved ultraviolet photoelectron spectra have been measured for well defined Ag/Si(111) submonolayer interfaces of (1) $\text{Si(111)(}\text{3}\text{×}\text{3}\text{)R30°-Ag}$, (2) “Si(111)(6 × 1)-Ag”, and (3) Ag/Si(111) as deposited at room temperature. Non-dispersive and very narrow (FWHM ～ 0.4–0.5 eV) Ag 4d derived peaks are found at 5.6 and 6.5 eV below the Fermi level for surface (1) and at 5.3 and 6.0 eV for surface (2). Dispersions of sp “binding” states in the energy range between E_F and Ag 4d states have been precisely determined for surface (1). Electronic structures similar to those of the Ag(111) surface, including the surface state near E_F, have been observed for surface (3).     

Effects of carbon doping on chemical states of amorphous Ge2Sb2Te5, measured with synchrotron radiation

We fabricated and analyzed the chemical states of carbon-doped (5.2–13.2 at.%) Ge₂Sb₂Te₅ thin films on Si substrates using high-resolution, X-ray photoelectron spectroscopy with synchrotron radiation. Thin films were completely amorphous and their phase-change temperature was 150 °C higher than for un-doped GST. As the carbon doping concentration increased, new chemical states of Ge 3d with 29.9 eV and C 1s with 283.7 eV core-levels were observed. The doped carbon was bonded only with Ge in GST and doping was saturated at 8.7 at.%.     

Electronic structure of uranium monochalcogenides and uranium monopnictides

Single crystals of the uranium chalcogenides US, USe, UTe and the pnictides UAs and USb have been cleaved under high and ultrahigh vacuum conditions and the near normal incidence reflectivity has been measured in situ from 0.03 to 12 eV. In the case of US the spectral range of the measurement was further extended down to 0.0017 eV. The various optical functions (?₁, ?₂, σ, E_loss, n_eff) have been deduced by a Kramers-Kronig analysis. Typical features of all spectra are weak intraband contributions and the extension of strong interband transitions to very low photon energies. Highly damped plasmons are found at 4.25 eV for US, 3.5 eV for USe nad 2.2 eV for UTe. For the pnictides UAs and USb the corresponding values are 2 eV and 1.6 eV, respectively.The apparent contradiction between the presence of a large number of conduction electrons and only very weak intraband contributions is resolved within the spectral exclusion model of Keller. In this self-consistent cellular multiple scattering calculation the coupling of the narrow 5f states and the much wider 6d states produces a dip in the density of d states near E_F where the f electrons form a resonance state. Quantitative agreement is obtained between theory and experiment for US. For all investigated compounds empirical energy level schemes are derived.The reflectivity measurements down to 1.7 meV for US reveal the existence of a further excitation at 39 meV which is assigned to a transverse optical phonon mode. This unique observation of an optical phonon in a direct reflectivity measurement of a metal supports the picture of a small density of conduction electrons at E_F. Further arguments in favor of the spectral exclusion model are presented and the question of the localization of thr 5f electrons in various uranium compounds is discussed.     

Excitation of lowest electronic states of thymine by slow electrons

Excitation of lowest electronic states of the thymine molecules in the gas phase is studied by elec- tron energy loss spectroscopy. In addition to dipole-allowed transitions to singlet states, transitions to the lowest triplet states were observed. The low-energy features of the spectrum at 3.66 and 4.61 eV are identified with the excitation of the first triplet states 1₃ A′ (π → π*) and 1₃ A″ (n → π*). The higher-lying features at 4.96, 5.75, 6.17, and 7.35 eV are assigned mainly to the excitation of the π → π* transitions to the singlet states of the molecule. The excitation dynamics of the lowest states is studied. It is found that the first triplet state 1³ A′(π → π*) is most efficiently excited at a residual energy close to zero, while the singlet 2₁ A′(π → π*) state is excited with almost identical efficiency at different residual energies.     

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.     

Investigation of electronic states of infinite-layer SrFeO2 epitaxial thin films by X-ray photoemission and absorption spectroscopies

We investigated the electronic states of a single-crystal SrFeO₂ epitaxial thin film in the valence-band and conduction-band regions using synchrotron-radiation X-ray photoemission and absorption spectroscopies. Fe 2p–3d resonant photoemission measurements revealed that the Fe 3d states have higher densities of states at binding energies of 3–5 eV and 5–8.5 eV in the valence-band region. The O K-edge X-ray absorption spectrum exhibited three peaks in the Fe 3d-derived conduction band hybridized with O 2p states; these can be assigned to Fe 3d_xy, 3d_xz + 3d_yz, and 3d_x²_–y². In addition, the indirect bandgap value of the SrFeO₂ film was determined to be 1.3 eV by transmission and absorption spectroscopies.     

Chemical states study of Si in SiOx films grown by PECVD

Thin films of SiO_x have been grown by low temperature (350°C) plasma enhanced chemical vapor deposition (PECVD), and the local chemical bond of Si and O has been investigated by infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Comparisons between the IR spectra of Si-rich oxides (SiO_x, x < 2) and the corresponding spectra of stoichiometric SiO₂ and low oxygen content a-Si:H:O alloys demonstrates a continuous variation in the character of the Si-O stretching, bending and rocking vibrations as a function of the oxygen concentration. In particular, the frequency of the dominant stretching vibration displays a linear frequency shift with the relative oxygen concentration. A similar behavior prevails in the XPS spectra where the position of the Si2p core states shifts monotonically with increasing oxygen concentration from a value of about 98.8 eV in a-Si to 103.1 eV in a-SiO₂. The IR and XPS then reflect the average chemical bonding in these films. In contrast, the AES spectra of suboxides display two strong features in the SiL₂₃VV spectrum. We show that one of these features at approximately 78 to 80 eV is associated with ejection of electrons from Si-O valence bonding states, and a second at about 90 eV is associated with ejection of electrons from Si-Si bonding states. Therefore, the AES spectra reflects the details of the local bonding, i.e., the relative numbers of Si-O and Si-Si bonds, while the IR and XPS spectra reflect the average chemical bonding. In this context, the combination of the three techniques confirms that the suboxide fims produced by this low temperature PECVD process are homogeneous, in contrast to being two phases with regions of a-Si and a-SiO₂.     

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.     

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 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.     

Autoionization cross section of potassium atom excited by electron impact

The autoionization cross section of potassium atoms excited by electron impact is measured in the energy range from the first autoionization threshold at 18.72 eV to 202 eV. The data are obtained by deter-mining the total intensity of electron spectra resulting from the decay of the 3p ⁵ n ₁ l ₁ n ₂ l ₂ autoionizing states. The cross section has two maxima, 1.8 × 10^?16 and 2.2 × 10^?16 cm², at 21 and 32 eV, respectively. The excitation dynamics of autoionization states suggests that the first maximum is associated with the resonance character of the near-threshold excitation. The second maximum, as well as the behavior of the cross section at energies above 50 eV, reflects the dynamics of electron excitation of quartet and doublet autoionizing states. The measured autoionization cross section is compared with known data for the total single ionization cross section of potassium atom by electron impact. The relative contribution of the autoionization cross section to the total single ionization cross section is found to reach 30% at 32 eV.