2D degenerate electron gas at Ba/<Emphasis Type="Italic">n</Emphasis>-AlGaN and Ba/<Emphasis Type="Italic">n</Emphasis>-GaN interfaces

Ba/n-GaN(0001) and Ba/n-AlGaN(0001) interfaces were investigated for the first time by means of ultraviolet photoelectron spectroscopy. The spectra of the photoemission from a valence band along with the spectra of the core levels of Ga 3d, Al 2p, and Ba 4d were studied. The formation of a 2D degenerate electron gas (an accumulated layer on the n-GaN and n-AlGaN surfaces during adsorption of Ba atoms) was revealed.     

Electronic structure of a Ba/<Emphasis Type="Italic">n</Emphasis>-AlGaN(0001) interface and the formation of a degenerate 2D electron gas

The electronic structure of ultrathin Ba/n-AlGaN(0001) interfaces has been investigated in situ in an ultra-high vacuum by the ultraviolet photoelectron spectroscopy method. The photoemission spectra from the n-AlGaN valence band and the spectra of the core levels Ga 3d, Al 2p, and Ba 4d have been studied under synchrotron excitation with photon energies of 60–400 eV. The modification of the spectra in the process of the formation of the Ba/n-AlGaN interface in the mode of the Ba submonolayer coverages has been revealed. It has been found that a decrease in the intensity in some spectral regions of the valence band is attributed to the interaction of the surface states of the AlGaN substrate with the Ba adatoms. It has been revealed that the interface formation results in the appearance of a new photoemission peak of the quasimetallic character at the Fermi level in the AlGaN bandgap. It has been established that the peak is due to the formation of the degenerate electron gas in the accumulation nanolayer induced by adsorption near the n-AlGaN surface.     

Modification of the electronic structure and formation of an accumulation layer in ultrathin Ba/n-GaN and Ba/n-AlGaN interfaces

The electronic structure of the n-GaN(0001) and Al _x Ga_{1 ? x} N(0001) (x = 0.16, 0.42) surfaces and the Ba/n-GaN and Ba/AlGaN interfaces is subjected to in situ photoemission investigations in the submonolayer Ba coverage range. The photoemission spectra of the valence band and the spectra of the surface states and the core 3d level of Ga, the 2p level of Al, and the 4d and 5p levels of Ba are studied during synchrotron excitation in the photon energy range 50–400 eV. A spectrum of the surface states in Al _x Ga_{1 ? x} N (x = 0.16, 0.42) is found. The electronic structure of the surface and the near-surface region is found to undergo substantial changes during the formation of the Ba/n-GaN and Ba/AlGaN interfaces. The effect of narrowing the photoemission spectrum in the valence band region from 10 to 2 eV is detected, and surface eigenstates are suppressed. The Ba adsorption is found to induce the appearance of a new photoemission peak in the bandgap at the Fermi level in the Ba/n-GaN and Ba/n-Al_0.16Ga_0.84N interfaces. The nature of this peak is found to be related to the creation of an accumulation layer due to a change in the near-surface potential and enriching band bending. The energy parameters of the potential well of the accumulation layer are shown to be controlled by the Ba coverage.     

Bond cutting in Cs‐doped tris(8‐hydroxyquinoline) aluminium

A series of Cs 4d and Al 2p spectra associated with valence‐band and cut‐off spectra have been used to characterize the interaction between caesium and tris(8‐hydroxyquinoline) aluminium (Alq₃) molecules in a Cs‐doped Alq₃ layer. The Cs 4d and Al 2p spectra were tuned to be very surface sensitive by selecting a photon energy of 120 eV at the National Synchrotron Radiation Research Center, Taiwan. A critical Cs concentration exists, above which a new Al 2p signal appears next to the Al 2p peak of Alq₃ in the lower binding‐energy side. The Al 2p signal was analyzed and assigned as being contributed from a mixture of Alq₂, Alq and Al. Experimental data supported the observation that bond cutting of Alq₃ by the doped Cs atoms occurred at high Cs doping concentration.     

Electronic structure of CsAu

X-ray photoemission spectra of CsAu prepared in vacuum are in good agreement with the ionic character expected for this material. The Cs 5p doublet lies well below flat Au 5d bands. The Au 6s valence band is located somewhat closer to the 5d states than recent band structure calculations indicate. The charge transfer to gold is in the range 0.6 to 0.8 electrons.     

UV-photoemission study of barium,europium and ytterbium

Measurements are reported of the photoemission spectra and absolute yield of thin films of Eu, Ba and Yb for the photon energy range 2 to 21 eV. Transitions from the occupied 4f-states in Eu and Yb have been observed and the binding energies deduced. The excitation probability of the 4f electrons is found to be very low and an explanation based on their atomic-like nature is given. Transitions involving valence band states are compared with predictions based on published energy band schemes and the density of states for the unoccupied 5d band in Yb is deduced. The effect of chemisorbed oxygen on Ba and Eu surfaces is reported.     

Surface states and accumulation nanolayer induced by Ba and Cs adsorption on the n-GaN(0 0 0 1) surface

The Ba and Cs adsorption on the n-GaN(0 0 0 1) surface has been studied in situ by the threshold photoemission spectroscopy using s- and p-polarized light excitation. Two surface bands induced by Ba (Cs) adsorption are revealed in surface photoemission spectra below the Fermi level. The surface-Fermi level position is found to be changed from significantly below the conduction band minimum (CBM) at clean n-GaN surface to high above the CBM at Ba, Cs/n-GaN interfaces, with the transition from depletion to electron accumulation occurring at low coverages. Photoemission from the accumulation nanolayer is found to excite by visible light in the transparency region of GaN. Appearance of an oscillation structure in threshold photoemission spectra of the Ba, Cs/n-GaN interfaces with existing the accumulation layer is found to originate from Fabry–Perot interference in the transparency region of GaN.     

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.     

Nuclear model calculations on the production of <Superscript>125,123</Superscript>Xe and <Superscript>133,131,129,128</Superscript>Ba radioisotopes

In this study, production rates of ^125,123Xe and ^{133,131,129,128}Ba medical isotopes produced by ¹²⁷I(p, 3n)¹²⁵Xe, ¹²⁷I(p, 5n)¹²³Xe, ¹³³Cs(p, n)^133mg Ba, ¹³³Cs(p, 3n)^131mg Ba, ¹³³Cs(p, 5n)¹²⁹Ba, and ¹³³Cs(p, 6n)¹²⁸Ba reactions have been investigated up to 100 MeV incident proton energy. The preequilibrium calculations involve the hybrid model, the geometry-dependent hybrid model and the cascade exciton model. The calculated results are compared with the experimental data taken from the literature.     

Resonant photoemission and absorption spectroscopy of the Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiSe<Subscript>2</Subscript> compound

Single crystals of the Cu _x TiSe₂ compound with x = 0.05, 0.09, and 0.33 have been grown. Resonance photoelectron Cu 3p-3d and 2d-3d spectra of the valence bands, the spectra of the core levels, and the L absorption spectra for titanium and copper have been obtained. It is shown that the degree of oxidation of titanium atoms is +4 and the state of copper atoms is close to the state of free copper ions. It is found that the spectra of the valence bands obtained under the Cu 3p and 2p resonance conditions radically differ. For the spectra in the Cu 2p excitation regime, several bands corresponding to different decay channels of the excited state are observed. According to calculations of the density of states, the 3d states of copper are filled incompletely; the occupancy of the 3d band of copper is 9.5 electrons per atom.     

Analysis of the polarization dependence of the N K edge X-ray absorption fine structure in InN

The polarization dependence of the x-ray absorption near-edge structure (XANES) of InN beyond the N K edge is calculated. The XANES calculations are performed for different values of the angle θ between the XY plane of crystalline indium nitride and the incident x-radiation (θ = 15°, 30°, 45°, 60°, 75°, and 90°). It is shown that, in the case of N K XANES for InN, a strong polarization dependence of the specific features of the spectrum is observed. The calculated spectra are compared with previously measured experimental spectra. The partial densities of the electronic states of InN are calculated near the top of the valence band and near the bottom of the conduction band.     

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.     

Free-to-bound radiative recombination in highly conducting InN epitaxial layers

We present a theoretical simulation of near-band-edge emission spectra of highly conducting n-InN assuming the model of ‘free-to-bound’ radiative recombination (FBRR) of degenerate electrons from the conduction band with nonequilibrium holes located in the valence band tails. We also study experimental photoluminescence (PL) spectra of highly conducting InN epitaxial layers grown by MBE and MOVPE with electron concentrations in the range (7.7×10¹⁷–6×10¹⁸) cm⁻³ and find that the energy positions and shape of the spectra depend on the impurity concentration. By modeling the experimental PL spectra of the InN layers we show that spectra can be nicely interpreted in the framework of the FBRR model with specific peculiarities for different doping levels. Analyzing simultaneously the shape and energy position of the InN emission spectra we determine the fundamental bandgap energy of InN to vary between E_g=692 meV for effective mass m_n0=0.042m₀ and E_g=710 meV for m_n0=0.1m₀.     

Study of the electronic structure of the quasicrystalline Ti—Zr—Ni system

Photoelectron spectra and optical absorption spectra for clean surfaces of quasicrystalline samples of the Ti—Zr—Ni system are measured. The resonant photoelectron spectra are also measured in the range of photon energies corresponding to Ni 2p and Zr 3d absorption thresholds. The change in the intensities of the spectrum of the valence band near the Zr 3d threshold is insignificant. The emission of Auger electrons increases near the Ni 2p threshold in a resonance way. Our studies make it possible to reveal certain specific features of the electronic structure of quasicrystals, which can be used to construct models of actual electronic structures of quasi-crystalline materials.     

Electronic structure of ZnS:Co and ZnSe:Co crystals

The x-ray photoelectron spectra of the valence bands and inner Co 2p levels of solid solutions of the wide-gap semiconductors ZnS, ZnS:Co, ZnSe, and ZnSe:Co are investigated. The x-ray photoelectron Co 2p spectrum of the ZnS:Co crystal indicates a 3d configuration of the ground state of cobalt and strong covalence of the cobalt-sulfur bond, but somewhat weaker than for CoS. It is established that the edge of the valence band recedes by 0.7±0.1 eV in transition from ZnSe to ZnS. Fiz. Tverd. Tela (St. Petersburg) 39, 1971–1974 (November 1997)     

On the valence band photoemission of highT c superconductors

The valence band photoemission spectrum of highT _c -superconductors is discussed based on the half-filled single band Hubbard Hamiltonian with the strong Coulomb interaction. We discuss how to analyze these valence band and deep core level excitation spectra, concerning particularly with which orbital, Cud or Op state, a hole will occupy in the CuO₂ plane.     

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.     

Electronic structure and X-ray spectra of transition metal sulfides NiS,CuS, and ZnS

A numerical electronic band structure calculations for sulfides NiS, CuS, and ZnS are carried out. Using the results a detailed analysis of a valence states is performed; obtained partial densities of states are compared with X-ray SL _2,3 and $ SK_{\beta _{1,3} } $ SK_{\beta _{1,3} } -emission spectra. We showed that spectrum lineshape depends on hybridization strength between various Me(3d)-orbitals and 3p-states of sulfur. The hybridization strength and the symmetry of hybrid Me(3d)-orbitals are defined by crystal lattice structure. Finally a well splitted in energy bonding and antibonding states Me(3d)-S(3p) appear while weakly hybridized Me(3d)-states mainly contribute to spectra intensity in the energy between them. A good agreement between the theoretical and the experimental spectra of valence band for considered sulfides is obtained.     

Electronic states near fermi level in superconductors La2 − x Sr x CuO4 by means of x-ray absorption spectra near Cu-K and La-L edges

The electronic states of La_{2? x} Sr _x CuO₄ with 0.00 ≤ x ≤ 0.20 are studied by means of X-ray absorption spectroscopy (XANES, EXAFS) near the K-edge of Cu²⁺ ion and the L-edges of La³⁺ ion. It is found that characteristic white lines occurring near L _II and L _III edges of La³⁺ ion show a slight energy shift depending on substituted Sr²⁺ ions, x and temperature. The white lines suggest that unoccupied high-density 5dπ and 5dδ bands of La³⁺ ion just above a Fermi level transform to a hybridized single band of 5dπδ at 78?K in the superconductors with x = 0.10, 0.16 and 0.20. On the other hand, the XANES spectra near the Cu-K edge including a pre-edge region do not depend on x and temperature in the region of 0.00 ≤ x ≤ 0.20. It is considered that there is no reconstruction of electronic states at the Fermi level in a Mott–Hubbard band gap between an O 2p valence band and a Cu 3d conduction band. The electronic states at the Fermi level are probably consisted of the unoccupied 5dπδ band and an empty charge-transfer 3d?⁹ L band at low temperature, bands of which occur in a band gap between a filled O 2p valence band and an unoccupied O 2p conduction band. The insulator–superconductor–metal transitions in La_{2? x} Sr _x CuO₄ are related to the 5dπδ and 3d?⁹ L bands and holes, which site at a top region of the O 2p valence band near the Fermi level produced by a substitution of La³⁺ with Sr²⁺ ions.