Charge accumulation layer in Cs,Ba/<Emphasis Type="Italic">n</Emphasis>-GaN(0001) ultrathin interfaces: Electronic and photoemission properties

Experimental studies and theoretical calculations of the photoemission from Cs/n-GaN(0001) and Ba/n-GaN(0001) ultrathin interfaces were carried out. The electronic properties of the interfaces were studied in situ using threshold photoemission spectroscopy under vacuum at a residual pressure of P ～ 5 × 10^?11 Torr. A new effect was revealed, namely, photoemission with a high quantum yield under excitation with light in the transparency region of GaN. It was shown that adsorption of Cs or Ba on n-GaN brings about the formation of a quasi-two-dimensional electron channel, i.e., a charge accumulation layer directly near the surface. The dependences of the photoemission spectra and work function on the thickness of Cs and Ba coatings were investigated. It was established that adsorption of Cs and Ba leads to a sharp decrease in the work function by ～1.45 and ～1.95 eV, respectively. The photoemission spectra were calculated, and parameters of the accumulation layer, such as the energy position of the layer below the Fermi level for different Cs and Ba coverages, were determined. It was demonstrated that the energy parameters of the accumulation layer on the n-GaN(0001) surface can be controlled by properly varying the Cs or Ba coverage. The layer thickness was found to reach a maximum for a cesium coverage of ～0.5 monolayer.     

Emission of ballistic photoelectrons from <Emphasis Type="Italic">p</Emphasis>-GaN(Cs,O) with the effective negative electron affinity

This paper reports on a study of the emission of ballistic photoelectrons from p-GaN(Cs,O) with an effective negative electron affinity. At photon energies less than the GaN band gap width, where emission of electrons originates from photoexcitation of surface and near-surface states, an increment in the energy of ballistic electrons is equal to that of exciting photons, which is substantiated by the dispersionless character of the initial states. At photon energies exceeding the band gap width, the excess energy of light is partitioned among the kinetic energies of ballistic photoelectrons and holes in accordance with their effective masses. This relation was used to determine the effective hole mass along the c axis of the GaN lattice of the wurtzite structure, which turned out to be m* _h‖ = (0.60 ± 0.15)m ₀.     

Atomic Rearrangements and Photoemission Processes at a <Emphasis Type="Italic">p</Emphasis>-GaN(Cs)–Vacuum Interface

Spontaneous changes in photoemission properties of a р-GaN(Cs)–vacuum interface with effective negative electronic affinity induced by rearrangements of its atomic structure have been studied for the first time. The optimum Сs coating that ensures both the maximum photoelectron escape probability and its stability has been found. A thermodynamic model has been proposed to escape the relation of the photoemission properties of the р-GaN(Cs)–vacuum interface to its free energy and entropy.     

Electronic structure and spectra of CuO

We report the electronic structure of monoclinic CuO as obtained from first principles calculations utilizing density functional theory plus effective Coulomb interaction (DFT + U) method. In contrast to standard DFT calculations taking into account electronic correlations in DFT + U gave antiferromagnetic insulator with energy gap and magnetic moment values in good agreement with experimental data. The electronic states around the Fermi level are formed by partially filled Cu 3d _x²?y² orbitals with significant admixture of O 2p states. Theoretical spectra are calculated using DFT + U electronic structure method and their comparison with experimental photoemission and optical spectra show very good agreement.     

Studying the occupied and unoccupied electronic structure of LaCoO<Subscript>3</Subscript> by using DFT+embedded DMFT method with the calculated value of <Emphasis Type="Italic">U</Emphasis>

In this work, we present a systematic study of the occupied and unoccupied electronic states of LaCoO₃ compound using DFT, DFT+U and DFT+embedded DMFT methods. The value of U used here is evaluated by using constrained DFT method and found to be ~6.9 eV. It is found that DFT result has limitations with energy positions of PDOS peaks due to its inability of creating a hard gap although the DOS distribution appears to be fine with experimental attributes. The calculated value of U is not an appropriate value for carrying out DFT+U calculations as it has created an insulating gap of ~1.8 eV with limitations in redistribution of DOS which is inconsistent with experimental spectral behavior for the occupied states mainly. However, this value of U is found to be an appropriate one for DFT+embedded DMFT method which creates a gap of ~1.1 eV. The calculated PDOS of Co 3d, La 5d, La 4f and O 2p states are giving a remarkably good explanation for the occupied and unoccupied states of the experimental spectra in the energy range ~–9.0 eV to ~12.0 eV.     

Substituent Dependent Optical Properties of <Emphasis Type="Italic">p</Emphasis>-phenyl Substituted ethenyl-<Emphasis Type="Italic">E</Emphasis>-thiophenes

Various electron donor and acceptor substituted (NO₂, CN, Cl, H, OCH₃, NH₂) p-phenyl ethenyl-E- thiophenes (1–6) were synthesized and substituent dependent optical properties (dipole moment, transition dipole moment, oscillator strength, optical band gap, hyperpolarizability) were studied using Solvatochromism and Density functional theory. It is shown that thiophene acts as a weak electron donor in presence of an electron withdrawing p-phenyl substituent (NO₂, CN, Cl), whereas thiophene acts as a weak electron acceptor in presence of an electron donating p-phenyl substituent (OCH₃, NH₂). In comparison to ethenyl thiophene 4, the HOMO-LUMO energy band gap is decreased upon increasing the electron donating or electron withdrawing capacity of p-phenyl substituent. From the excited state dipole moment calculation, it is shown that the excited state is highly dipolar for nitro and amino compounds 1 and 6, whereas compounds 2–5 show a non-polar excited state. As compared to the ethenyl thiophene 4, the first hyperpolarizability (β) increases upon substitution either with a strong electron withdrawing or strong electron donating p-phenyl substituent. A large β value is found for p-nitro phenyl ethenyl-E-thiophene and p-amino phenyl ethenyl-E- thiophene. Overall, these studies provide useful information in understanding the optical properties of phenyl and heterocyclic based ethenyl systems.     

Energy band structure and X-ray spectra of phenakite Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

The electronic structure of crystalline phenakite Be₂SiO₄ is investigated using x-ray emission spectroscopy (XES) (Be K _α XES, Si L _{2, 3} XES, O K _α XES) and x-ray absorption spectroscopy (XAS) (Be 1s XAS, Si 2p XAS, O 1s XAS). The energy band structure is calculated by the ab initio full-potential linearized augmented-plane-wave (FLAPW) method. The total and partial densities of states and the dispersion curves for the Be₂SiO₄ compound are presented. It is shown that the top of the valence band and the bottom of the conduction band of the Be₂SiO₄ compound are predominantly formed by the oxygen 2p states. According to the results obtained, the electron transition with the lowest energy supposedly can occur at the center of the Brillouin zone. The effective masses of electrons (0.5m _e ) and holes (3.0m _e ) for the Be₂SiO₄) compound are estimated.     

Low-temperature method of cleaning <Emphasis Type="Italic">p</Emphasis>-GaN(0001) surfaces for photoemitters with effective negative electron affinity

The changes in the chemical composition, atomic structure, and electronic properties of the p-GaN(0001) surface upon chemical treatment in an HCl-isopropanol solution and vacuum annealing are investigated by x-ray photoelectron spectroscopy, high-resolution electron energy-loss spectroscopy, and low-energy electron diffraction. It is demonstrated that a considerable part of the surface gallium oxide is removed upon chemical treatment of the GaN surface. Subsequent annealing of the surface under vacuum at temperatures of 400–450°C leads to a decrease in the residual carbon and oxygen contamination to 3–5% of the monolayer. The preparation of a clean p-GaN(0001) surface with a (1×1) structure identical to that of the bulk unit cells is confirmed by the low-energy electron diffraction data. The cesium adsorption on the clean p-GaN surface results in a decrease in the work function by ～2.5 eV and the appearance of an effective negative electron affinity on the surface. The quantum efficiency of the GaN photocathode at a wavelength of 250 nm is equal to 26%.     

Optical properties and mechanisms of current flow in Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> films prepared by spray pyrolysis

Thin films Cu₂ZnSnS₄ (up to 0.9 μm thick) with p-type conductivity and band gap E_g = 1.54 eV have been prepared by the spray pyrolysis of 0.1 M aqueous solutions of the salts CuCl₂ · 2H₂O, ZnCl₂ · 2H₂O, SnCl₄ · 5H₂O, and (NH₂)₂CS at a temperature T_S = 290°C. The electrophysical properties of the films have been analyzed using the model for polycrystalline materials with electrically active grain boundaries. The energy and geometric parameters of the grain boundaries have been determined as follows: the height of the barriers is E_b ≈ 0.045–0.048 eV, and the thickness of the depletion region is δ ≈ 3.25 nm. The effective concentrations of charge carriers p₀ = 3.16 × 10¹⁸ cm^–3 and their mobilities in crystallites μ_p = 85 cm²/(V s) have been found using the technique for determining the kinetic parameters from the absorption spectra of thin films at a photon energy hν ≈ E_g. The density of states at grain boundaries N_t = 9.57 × 10¹¹ cm^–2 has been estimated.     

Charge accumulation layer on the <Emphasis Type="Italic">n</Emphasis>-GaN (0001) surface with ultrathin Ba coatings

The adsorption of Ba on the n-type GaN(0001) surface is studied. It is found that submonolayer Ba coatings induce cardinal changes in the electronic properties of the surface with the formation of a charge accumulation layer in the region of the near-surface band bending. The excitation of the Ba/n-GaN system by light from the region of GaN transparency results in photoemission. The lowest value of the work function corresponds to ～1.90 eV at a Ba coverage of ～0.4 ML. Two surface bands induced by Ba adsorption are found in the surface photoemission spectra.     

Energy levels of ions of silver and rhodium isoelectronic sequences with <Emphasis Type="Italic">Z</Emphasis> < 86

The method of extrapolation of the parameter of a model potential for states of one electron (4f, 5s, 5p, 5d, 5f) above the core 1s ²2s ²2p ⁶3s ²3p ⁶3d ¹⁰4s ²4p ⁶4d ¹⁰ and one vacancy (4d ⁹) in the same core is applied to calculate the energy levels in the silver and rhodium isoelectronic sequences with the maximum nuclear charge Z = 86. The energy levels of Ag-and Rh-like ions were used for the calculation of the energies of resonance transitions to the ground state ¹ S ₀ in Pd-like ions. Good agreement between the theoretical and calculated energies of the resonance transitions in Pd-like ions indicates the reliability of the results obtained.     

Concerning positive-parity collective states in <Superscript>18</Superscript>O

I discuss collective positive-parity states in ¹⁸O in light of two recent experiments. Single-particle widths have been calculated and used to extract spectroscopic factors. Special attention is given to 4p -2h and 6p -4h cluster bands.     

On the 2<Emphasis Type="Italic">s</Emphasis>-like relaxed excited state of the <Emphasis Type="Italic">F</Emphasis> center in alkali halide crystals

The energy and spectroscopic characteristics of the first excited 2s-like electron state of the F center are presented according to the calculations performed in the framework of the variational model proposed by Gourary and Adrian. The relaxation of the crystal lattice in the vicinity of the excited F center is described. The problem of the spatial propagation of the F center is discussed. The results obtained for the 2p and 2s states of the F center are compared with each other and with the relevant experimental data.     

Intrinsic and Zn-, Ce-, Tb-, Er-, Sm-, and Eu-Activated photoluminescence of pseudoamorphous GaN and InGaN thin films

Thin films of pseudoamorphous GaN (a-nc-GaN), as well as of its alloys with indium, In_xGa_1−x N (x=0.04, 0.16), were prepared by magnetron sputtering of a metallic target in the plasma of a reactive nitrogen and argon mixture. The a-nc-GaN films were codoped by the Zn acceptor impurity and a set of rare-earth metal (REM) dopants, namely, Ce, Tb, Er, Sm, and Eu. Photoluminescence (PL) spectra excited by a nitrogen laser with wavelength λ=337 nm at room temperature and 77 K were measured for all compositions and a set of impurities. It was shown that the high-energy PL edge of the pseudoamorphous (a-nc) GaN matrix lies at the same energy as that of the crystalline (epitaxial) c-GaN. As in c-GaN, the Zn acceptor impurity stimulates blue luminescence; however, the PL spectrum is substantially more diffuse, with practically no temperature quenching of the PL present. Indium doping in an amount of 16 at. % results in strong PL with a diffuse peak at 2.1–2.2 eV; the PL of the alloy exhibits temperature quenching as high as a factor of three to four in the interval 77–300 K. The decay time of the PL response increases up to 50 μs. RE impurities enter the amorphous GaN host as trivalent ions and produce narrow-band (except Ce) high-intensity spectra, thus indicating both a high solubility of RE impurities in a-nc-GaN and the generation of an effective crystal field (by the GaN anion sublattice) whose local symmetry makes the intracenter f-f transitions partly allowed. __________ Translated from Fizika Tverdogo Tela, Vol. 45, No. 3, 2003, pp. 395–402. Original Russian Text Copyright ? 2003 by Andreev.     

Accumulation nanolayer and surface states of ultrathin Cs,Ba/<Emphasis Type="Italic">n</Emphasis>-GaN interfaces

It is found that Cs and Ba ultrathin coatings induce formation of a charge accumulation layer (2D electron channel) on the n-GaN(0001) surface. Photoemission of conduction electrons from the accumulation layer under excitation by light in the range of GaN transparency is revealed. It is established that the depth of the potential well of the accumulation layer can be deliberately controlled.     

X-ray photoelectron spectroscopy of tin oxide nanolayers

X-ray photoelectron spectra of 30- and 100-nm nanolayers, recorded in the energy range 0–35 eV, show a strong dependence of both the distribution of the density of Sn 5s, p+ O₂ p valence states and the change in the intensity ratio for the Sn 4d and O 2s subvalence states on the annealing temperature and nanolayer thickness. In the nanolayers fabricated at an annealing temperature of 450°C, an unusually strong band of O 2s states of unbound oxygen is observed, which is retained for nanolayers doped with palladium and disappears for nanolayers doped with gold and silver.     

Electron-stimulated desorption of cesium atoms from cesium layers deposited on a germanium-coated tungsten surface

The yield and energy distribution of Cs atoms from cesium layers adsorbed on germanium-coated tungsten were measured, using the time-of-flight technique with a surface-ionization-based detector, as a function of the energy of bombarding electrons, germanium film thickness, the amount of adsorbed cesium, and substrate temperature. The threshold for the appearance of Cs atoms is ～30 eV, which correlates well with the germanium 3d-level ionization energy. As the electron energy increases, the Cs atom yield passes through a broad maximum at ～120 eV. For germanium film thicknesses from 0.5 to 2 monolayers, resonance Cs yield peaks were observed at electron energies of 50 and 80 eV, which can be related to the tungsten 5p and 5s core-level ionization energies. As the cesium coverage increases, the Cs atom yield passes through a flat maximum at monolayer coverage. The energy distribution of Cs atoms follows a bell-shaped curve. With increasing cesium coverage, this curve shifts to higher energies for thin germanium films and to lower energies for thick films. The Cs energy distribution measured at a substrate temperature T = 160 K exhibits two bell-shaped peaks, namely, a narrow peak with a maximum at ～0.35 eV, associated with tungsten core-level excitation, and a broad peak with a maximum at ～0.5 eV, deriving from the excitation of the germanium 3d core level. The results obtained can be described within a model of Auger-stimulated desorption.     

Mittlere Energien und Driftgeschwindigkeiten von Elektronen in Stickstoff,Argon und Xenon,ermittelt aus Bildverstärkeraufnahmen von Elektronenlawinen

Local and temporal development of electron avalanches in a pulsed discharge gap (d=3,00 cm) are investigated in N₂, Ar, Xe and mixtures of N₂ and CH₄ by simultaneously applying high gain image intensifier- and photomultiplier techniques. Electron drift velocities are obtained from time-of-flight and way-of-flight measurements in these gases. The mean energy of agitation of the electrons is derived both from electron mobility and avalanche image trace profile (diffusion broadening). The results obtained (for 20°C), being in fair agreement with one another, read N₂: (4·6...5·0) eV forE/p=50...200 V/cm Torr; Ar: (9·0...9·5) eV forE/p=24... 45 V/cm Torr; Xe: (4·8...5·0) eV forE/p= 40... 90 V/cm Torr; CH₄(10% N₂): 6·3 eV forE/p= 89 V/cm Torr. The mean energy of agitation does not change very much withE/p in the ranges investigated. Some results concerning the radiation properties of these gases are included such as lifetime of the excited states, quenching pressure etc.     

Charge transfer and thermopower in TlGdS<Subscript>2</Subscript>

The temperature dependences of the dc conductivity and thermoelectric coefficient of TlGdS₂ in the temperature interval of 77–373 K have been studied for the first time. It has been found that, at low temperatures (114–250 K), the compound has conductivity of the p-type and charge transfer in its energy gap follows the hopping mechanism. The main parameters of localized electronic states in the energy gap have been determined.