Valence Band Level Research Articles

Optimization and characterization of III–V surface cleaning

Achieving clean surfaces is a major and challenging requirement for the study of surfaces and surface reactions. Nondestructive cleaning is a crucial step in semiconductor manufacturing, growth of materials, and processing. We use photoemission spectroscopy (PES) to systematically study the core and valence band electronic structure of various chemical treatments of InP(100), GaAs(100), and GaN(0001). These surfaces undergo wet chemical cleaning of H2SO2/H2O2/H2O followed by thermal heating. In order to achieve the necessary surface sensitivity and spectral resolution, synchrotron radiation in the energy range of 60–1000 eV is used for PES. In tuning the sulfuric acid based chemistry, we achieve oxygen free GaAs and InP surfaces, as shown in our valence band and core level PES analysis. Furthermore, core level PES shows oxygen coverage of the GaN surface is reduced to less than 0.1 monolayer (ML). The carbon coverage is also reduced dramatically for the III–V surfaces, <10% ML for InP and GaAs and approximately 1% ML for GaN. The chemical reactions and species at different cleaning stages are determined and cleaning mechanisms are proposed. Our study shows that material similarities do not imply exact correlation to the chemical cleaning properties among III–V materials.

Electronic structure of YbTX compounds

The electronic structure of ternary YbTX compounds (T=Au, Pd, Rh, Pt; X=Sn, Bi) was studied by X-ray photoemission spectroscopy (XPS). XPS valence bands were compared with calculations using the tight-binding linear muffin-tin orbital method. The results showed that the valence bands in these compounds were formed mainly by the 4f orbitals of ytterbium and the 4d (5d) orbitals of the transition elements. Analysis of the Yb 4f valence band and Yb 4d core level states clearly indicated the presence of divalent Yb ions in YbAuSn and YbAuBi, and trivalent ions in YbRhSn and YbPtSn. In turn, the Yb ions in YbPdBi exhibited a mixed valence character.

Evolution of the band structure of β-In2S3−3xO3x buffer layer with its oxygen content

The evolution of the band structure of β-In2S3−3xO3x (BISO) thin films grown by physical vapor deposition, with composition x, is investigated using x-ray photoelectron spectroscopy. It is shown that the energy difference between the valence-band level and the Fermi level remains nearly constant as the optical band gap of the films increases. As a consequence, the difference between the conduction band level and the Fermi level increases as much as the optical band gap of the films. The calculation of the electronic affinity χ of the BISO thin films shows that it decreases linearly from 4.65 to 3.85 eV when x varies from 0 to 0.14. This will facilitate fabrication of efficient Cu(InGa)Se2-based solar cells having different absorber layer band gap.

Growth, alloy formation and reaction with oxygen of Ag and Au submonolayers on W(1 1 0) studied by valence band and core level photoemission

We have studied epitaxial submonolayers of Ag and Au deposited one after another onto W(1 1 0) at room temperature and subsequently annealed at 600 K. Photoemission spectroscopy of valence bands and the Ag3d 5/2 core level has been used to monitor two-dimensional alloy formation. The extent of alloying depends on the order of deposition, composition and annealing. We have also studied the reaction of alloy surfaces to exposure of molecular oxygen at 300 and 600 K.

The influence of doping on the electronic structure of R 1− xCa xBa 2Cu 3O 6+ d (R=Y, Eu) and (La 0.7Ca 0.3) 1− xMn 1+ xO 3

The valence band and core level states of R 1− x Ca x Ba 2Cu 3O 6+ d (R = Y, Eu) and (La 0.7Ca 0.3) 1− x Mn 1+ x O 3 systems were studied by means of angle integrated photoemission. The measurements have been performed on deoxygenated R 1− x Ca x Ba 2Cu 3O 6.1 samples which were insulators or superconductors depending on the calcium content as well as on oxygenated high T c RBa 2Cu 3O 6.9 superconductors. The goal was to study the effect of doping on the valence band and core level states and observe the change in the electronic structure. The studies of the valence band show an increase in the number of states located at 2.5 eV below the Fermi level. The parameters of the electronic structure are estimated (by cluster model calculations) to be: Δ∼1 to 2 eV, T∼2.5–3.5 eV and U dd∼6.5 eV. The valence band spectra of (La 0.7Ca 0.3) 1− x Mn 1+ x O 3 ( x=0, 0.1) are very similar showing two peaks at about 3.5 and 5.7 eV for x=0 and at 6.0 and 7.7 eV for x=0.1 below the upper edge of the valence band. These indicate partly hybridized Mn 3d–O 2p states. From the Mn 2p spectrum and its comparison with the relevant Auger spectrum values of the charge transfer energy Δ, the hybridization energy between Mn 3d and O 2p states T and the on site Coulomb correlation energies U dd in Mn 3d and U pp in O 2p states were estimated to be about 5, 4, 4 and 6.5 eV, respectively. These results indicate the mixed character of the ground state.

Splitting of the XPS in Ferroelectric SbSI Crystals

The paper presents the X-ray photoelectron spectra (XPS) of the valence band and core levels of the ferroelectric SbSI single crystal. Results revealed the new phenomenon--splitting of the core levels at the phase transition temperature T cl . The XPS are measured with monochromatized Al K f radiation in the energy range 0-1400 eV and the temperature range 130-330 K. Experimentally obtained energies are compared with the results of theoretical ab initio calculations. This is the first observation of the splitting of the XPS in a ferroelectric crystal. The mechanism of the XPS splitting in SbSI crystals is discussed.

Sodium Tripolyphosphate (Na5P3O10) by XPS

We report the XPS spectra of sodium tripolyphosphate. XPS spectra were collected with a VSW HA150 x-ray photoelectron spectrometer using monochromatic Al Kα x-radiation. Monochromatic radiation provides a distinct clarity to the rich peak structure in the valence band of sodium tripolyphosphate due to the absence of interfering x-ray satellites from the intense O 2s region. The features seen in the valence band are unique to the (P3O10)5− ion and therefore provide a means for distinguishing this compound from chemically similar compounds such as other phosphates. The valence band, survey, Na 1s, O 1s, C 1s, P 2s, P 2p, and Na 2s levels are reported.

Sodium Pyrophosphate Decahydrate (Na4P2O7⋅10H2O) by XPS

We report the XPS spectra of sodium pyrophosphate. XPS spectra were collected with a VSW HA150 x-ray photoelectron spectrometer using monochromatic Al Kα x-radiation. Monochromatic radiation provides a distinct clarity to the rich peak structure in the valence band of sodium pyrophosphate due to the absence of interfering x-ray satellites from the intense O 2s region. The features seen in the valence band are unique to the (P2O7)4− ion and therefore provide a means for distinguishing this compound from chemically similar compounds such as other phosphates. The valence band, survey, Na 1s, O 1s, C 1s, P 2s, P 2p, and Na 2s levels are reported.

Sodium Dihydrogenpyrophosphate Hexahydrate (Na2H2P2O7⋅6H2O) by XPS

We report the XPS spectra of sodium dihydrogenpyrophosphate. XPS spectra were collected with a VSW HA150 x-ray photoelectron spectrometer using monochromatic Al Kα x-radiation. Monochromatic radiation provides a distinct clarity to the rich peak structure in the valence band of sodium dihydrogenpyrophosphate due to the absence of interfering x-ray satellites from the intense O 2s region. The features seen in the valence band are unique to the (H2P2O7)2− ion which shows small but significant differences from the (P2O7)4− ion. The valence band, survey, Na 1s, O 1s, C 1s, P 2s, P 2p, and Na 2s levels are reported.

Sodium Polyphosphate (Na4P4O12) by XPS

We report the XPS spectra of sodium polyphosphate. XPS spectra were collected with a VSW HA150 x-ray photoelectron spectrometer using monochromatic Al Kα x-radiation. Monochromatic radiation provides a distinct clarity to the rich peak structure in the valence band of sodium polyphosphate due to the absence of interfering x-ray satellites from the intense O 2s region. The features seen in the valence band are unique to the (P4O12)4− ion and therefore provide a means for distinguishing this compound from chemically similar compounds such as other phosphates. The valence band, survey, Na 1s, O 1s, C 1s, P 2s, P 2p, and Na 2s levels are reported.

Sodium Phosphate Dodecahydrate (Na3PO4⋅12H2O) by XPS

We report the XPS spectra of sodium phosphate. XPS spectra were collected with a VSW HA150 x-ray photoelectron spectrometer using monochromatic Al Kα x-radiation. Monochromatic radiation provides a distinct clarity to the rich peak structure in the valence band of sodium phosphate due to the absence of interfering x-ray satellites from the intense O 2s region. The features seen in the valence band are unique to the phosphate ion. The valence band, survey, Na 1s, O 1s, C 1s, P 2s, P 2p, and Na 2s levels are reported.

Observation of acceptor level of p-type SrTiO 3 by high-resolution soft-X-ray absorption spectroscopy

The electronic structure of Sc-doped SrTiO 3 (SrTi 1− x Sc x O 3) has been investigated by soft-X-ray absorption spectroscopy (XAS) in the O 1s core region. Hole states at the top of the valence band and acceptor-induced levels just above the Fermi level are observed below the O 1s threshold. The XAS features and their temperature dependence are in good agreement with electrical conductivity studies on the same samples. These results conclude that SrTi 1− x Sc x O 3 is a p-type semiconductor with deep acceptor levels lying in the middle of the band gap.

X-Ray Photoelecton Spectra and Electronic Structure of Bi2S3 Crystals

The paper presents the X-ray photoelectron spectra (XPS) of the valence band and core levels of the semiconductive Bi 2 S 3 single crystal, which shows anomalies in various physical properties and weak phase transitions without change of symmetry. The XPS were measured with monochromatized Al K a radiation in the energy range 0-1400 eV and the temperature range 160-460 K. The valence band is located 0.55-6.5 eV below the Fermi level and shows non-linear dependence on temperature. Experimental energies of the valence band and core levels are compared with the results of theoretical ab initio calculations of the molecular model of the Bi 2 S 3 crystal. The chemical shifts in the Bι 2 S 3 crystal for the Bi and S states are obtained. Results revealed the origin of a weak polarity of the crystals at room temperature, which results in anomalies in the physical properties, and shifts the Fermi level and all the electronic spectrum.

CO adsorption on Pt xPd [formula omitted] single crystal alloy surfaces: a core level and valence band photoemission study

We present a photoelectron spectroscopy study of three single crystal Pt x Pd 1− x (1 1 1) alloy surfaces ( x=0.1, 0.5, 0.9) using synchrotron radiation. The surface and chemical sensitivity of core level photoemission allows us to quantify an oscillatory depth concentration profile and the palladium segregation at the surface of each alloy. The experimental results are compared to theory based on regular solution and tight binding Ising model formalisms, a good agreement is found. In the second part of the paper the effect of CO adsorption on the alloy surfaces is investigated. The variety of adsorption site geometry and chemistry as a function of surface segregation and bulk alloy concentration is revealed in both valence band and core level spectra.

X-ray Photoelectron Spectra and Electronic Structure of Bi2S3 Crystals

The paper presents the X-ray photoelectron spectra (XPS) of the valence band and core levels of the semiconductive Bi2S3 single crystal, which shows anomalies in various physical properties and weak phase transitions without change of symmetry. The XPS were measured with monochromatized Al Kα radiation in the energy range 0–1400 eV and the temperature range 160–460 K. The valence band is located 0.55–6.5 eV below the Fermi level and shows non-linear dependence on temperature. Experimental energies of the valence band and core levels are compared with the results of theoretical ab initio calculations of the molecular model of the Bi2S3 crystal. The chemical shifts in the Bi2S3 crystal for the Bi and S states are obtained. Results revealed the origin of a weak polarity of the crystals at room temperature, which results in anomalies in the physical properties, and shifts the Fermi level and all the electronic spectrum.

NdNi 4B and DyNi 4B compounds studied by X-ray photoemission spectroscopy

The hexagonal NdNi 4B and DyNi 4B compounds were studied by X-ray photoemission spectroscopy. Both valence band and core level spectra were analyzed. A comparison of the valence bands showed that the Dy(4f) levels were well localized, whereas Nd(4f) levels overlapped strongly with the Ni(3d) peak below the Fermi level. The 3d 5/2 and 3d 3/2 bands of NdNi 4B revealed additional satellites. The values of the 3d spin–orbit splitting E LS were equal to 22.8 eV for R=Nd and 38 eV for R=Dy. The coupling between the f-orbitals and the conduction states, Δ≈23 meV, was estimated for NdNi 4B basing on the Gunnarsson–Schönhammer model.

X-ray Photoelectron Spectroscopy of Sb2S3 Crystals

The paper presents the X-ray photoelectron spectra (XPS) of the valence band and core levels of semiconductor ferroelectric Sb2S3 single crystals, which show weak phase transitions and anomalies of various physical properties. The XPS were measured with monochromatized Al K α radiation in the energy range 0-1450 eV and the temperature range 160-450 K. The valence band is located 0.8-7.5 eV below the Fermi level. Experimental results of the valence band and core levels are compared with the results of theoretical ab initio calculations of the molecular model of Sb2S3 crystal. The chemical shifts in Sb2S3 crystal for the Sb and S states are obtained. Results revealed that the small structural rearrangements at the phase transition T c1 = 300 K shift the Fermi level and all electronic spectrum. Also, temperature dependence of a spontaneous polarisation shifts the electronic spectra of the valence band and core levels. Specific temperature-dependent excitations in Sb 3d core levels are also revealed.

X-ray photoelectron spectroscopy and magnetism of Gd 3Ni 8Al

X-ray photoelectron spectroscopy (XPS), magnetization and magnetic susceptibility of Gd 3Ni 8Al are reported. Both valence band and core level spectra were analyzed. Ni atoms have no magnetic moment because of charge transfer of Gd conduction electrons to the 3d band. This effect is mainly related with the 6s electrons of Gd. The filling of the Ni 3d band is revealed by the density of states at the Fermi level, the chemical shifts of Gd and Ni core levels and the value of the effective magnetic moment in the paramagnetic state. The compound Gd 3Ni 8Al exhibits a collinear ferromagnetic arrangement of Gd moments below the Curie temperature T C=61 K. A negative 3d band polarization of 0.15 μ B/Ni is induced by the exchange interactions with the Gd spins.

Direct evidence ofp-typeSrTiO3by high-resolution x-ray absorption spectroscopy

We study x-ray absorption spectra below the O $1s$ threshold in Sc-doped stoichiometric strontium titanate $({\mathrm{SrTiO}}_{3}).$ We find hole states at the top of the valence band and acceptor-induced levels just above the Fermi level. The x-ray absorption spectra features and their temperature dependence are in good agreement with electrical-conductivity studies on the same samples. It is concluded that Sc-doped ${\mathrm{SrTiO}}_{3}$ is a p-type semiconductor with deep acceptor levels lying in the middle of the band gap.

SiC(1 0 0) ordered film growth by C 60 decomposition on Si(1 0 0) surfaces

Fullerene (C 60) was deposited on Si(1 0 0) 2×1 double domain reconstructed substrate. The interface has been treated with annealing procedure in order to fragment the C 60 precursor and to induce covalent Si–C bond formation and to obtain carbidization. In this way SiC(1 0 0) films of thousands of Å have been grown. Depending on the growth procedure different surface structures, 1×1 or 2×1, have been obtained. The different stages of growth has been checked by in situ low energy electron diffraction (LEED), and Auger spectroscopy. Ex situ we verified surface order by means of LEED technique, observing either a 1×1 or a 2×1 double domain reconstruction. We characterized electronic properties collecting valence band and core level spectra employing synchrotron radiation source. Valence band spectra showed evident electronic surface states similar to those revealed on SiC(1 0 0) surfaces grown by different techniques.