Electron impact ionization of (1s2s2p)4 P j autoionization states of the lithium atom

Electron spectroscopy using intersecting electron and atomic beams was used for the first time to measure the electron excitation function of the (1s2s2p)⁴ P _1/2,3/2 autoionization states of the lithium atom in the range of collision energies between the level excitation threshold and 90 eV with an energy resolution better than 0.4 eV and a magic observation angle θ=54.7°. An analysis of the intense near-threshold structure observed revealed for the first time that autodetachment states of the negative lithium ion having excitation thresholds of 58.2±0.1 and 59.5±0.1 eV exist in the autoionization energy range. Pis’ma Zh. Tekh. Fiz. 25, 62–67 (July 12, 1999)     

Electron energy loss spectra of NTCDA organic films

The electron energy loss (EEL) spectra of thin NTCDA films on the ZnO(0001) surface were measured. In the low-energy range (<60 eV), the EEL spectra (displaying the characteristic loss peaks at E _loss=4.0, 5.6, 12.5, and 14.5 eV) reflect the structure of transitions between the valence and conduction bands. As the primary electron energy increases, the dominating energy losses are due to the excitation of plasma oscillations at ?ω₁=6.5 eV (π plasmon) and ?ω₂=25 eV (π?σ plasmon).     

Superluminescence in an AlGaAsSb/InGaAsSb/AlGaAsSb double heterostructure

Intense electroluminescence is observed for the first time in a AlGaAsSb/In_0.9Ga_0.1As_0.89Sb_0.21/AlGaAsSb double heterostructure in the 3–4 μm wavelength range at T=77 K. The structure was grown on a GaSb substrate by liquid-phase epitaxy. The photon energy at the maximum of the narrow emission band with a half-width of 9–10 eV is hv=387 meV which is 60 meV greater than the band gap of the narrow-gap InGaAsSb solid solution (E _g =326 meV). This behavior is attributed to the population inversion characteristics of the active region when an external bias is applied Pis’ma Zh. Tekh. Fiz. 23, 68–74 (May 12, 1997)     

Synthesis,structural and optical characterization of Ni-doped ZnO nanoparticles

Nanocrystalline Zn_1−x Ni _x O (x = 0.00, 0.02, 0.04, 0.06, 0.08) powders were synthesized by a simple sol–gel autocombustion method using metal nitrates of zinc, nickel and glycine. Structural and optical properties of the Ni-doped ZnO samples annealed at 800 °C are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis using X-rays (EDAX), UV–visible spectroscopy and photoluminescence (PL). X-ray diffraction analysis reveals that the Ni-doped ZnO crystallizes in a hexagonal wurtzite structure and secondary phase (NiO) was observed with the sensitivity of XRD measurement with the increasing nickel concentration (x ≥ 0.04). The lattice constants of Ni-doped ZnO nanoparticles increase slightly when Ni²⁺ is doped into ZnO lattice. The optical absorption band edge of the nickel doped samples was observed above 387 nm (3.20 eV) along with well-defined absorbance peaks at around 439 (2.82 eV), 615(2.01 eV) and 655 nm (1.89 eV). PL measurements of Ni-doped samples illustrated the strong UV emission band at ~3.02 eV, weak blue emission bands at 2.82 and 2.75 eV, and a strong green emission band at 2.26 eV. The observed red shift in the band gap from UV–visible analysis and near band edge UV emission with Ni doping may be considered to be related to the incorporation of Ni ions into the Zn site of the ZnO lattice.     

Deep surface states on the interface between SiC and its native thermal oxide

Deep surface states are discovered on the interface between 6H-SiC and its native thermal oxide by analyzing the C-V characteristics of metal-oxide-semiconductor structures measured at a high temperature (600 K). The maximum of the density of states distributed according to energy (D _tm=2×10¹² cm⁻²· eV⁻¹) is at an energy about 1.2 eV below the bottom of the conduction band of SiC. It is postulated that the states discovered are similar in nature to the P _b centers observed in the SiO₂/Si system. Pis’ma Zh. Tekh. Fiz. 23, 55–60 (October 26, 1997)     

Electronic and Phonon Contributions to the Pairing in the Cuprates

Thermal Difference Reflectance (TDR) Spectroscopy has been used to determinethe superconducting gap parameter for several of the superconducting cupratesover a wide range of energies, extending from the infrared (0.3 eV)to the ultraviolet (5.3 eV). A contribution to the pairing is found in eachcase from the phonons, and from an electronic excitation with energy thatranges from 1.6 eV and 2.3 eV for the different compounds attributed to thed ⁹–d ¹⁰ L charge-transfer excitation between Cu and O. In every case thereflectance ratio between the superconducting and normal state, Rs/Rnplotted as a function of photon energy can be well described using theEliashberg theory. The theory also predicted a characteristic shape for thelow energy part of such spectra due to the phonons. We report theobservation of this feature in measurements on films of Tl₂Ba₂CaCu₂O₈. Wediscuss the significance of the success of the Eliashberg theory inexplaining these results and successfully predicting new effects in thelight of the correlations that had been thought to invalidate such atheoretical approach.     

Coherent elastic energy calculation of ω particles in β matrix for Zr–Nb alloys

The misfit and coherent elastic energy caused by ω particles in β matrix is quantitatively calculated in this study. First, the coherent strain matrixes for four ω variants are established including the misfit parameters based on Khachaturyan’s theory. Then, the misfit and coherent elastic energy in athermal β → ω transition, and isothermal β → ω transition are calculated, respectively. The calculation results indicate that the coherent elastic energy gets maximum value when x _Nb = 0.08 (Nb content) and gets minimum value when x _Nb = 0.1518 in quenching Zr–Nb alloys, which are in fair agreement with experimental results. For isothermal β → ω transition, the misfit and coherent elastic energy depend on composition and aging temperature. The misfit caused by isothermal ω phase is much larger than the one caused by athermal ω phase. This results in larger coherent elastic energy in isothermal β → ω transition. In addition, the misfit is found as an approximate linear function of temperature and composition for Zr–Nb alloys, and the coherent elastic energy is revealed as an increasing function of |v _F –v _S | for the two kinds of transition.     

Formation and electron properties of the interface between organic (TPD) and inorganic (ZnO) materials

The initial stage of formation of an organic coating of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) on a crystalline (ZnO) surface in the course of thermal deposition in high vacuum has been studied using the low-energy total current spectroscopy technique. A change in the work function and the density of unoccupied electron states in an energy interval of 0–20 eV above the vacuum level was traced as the organic coating thickness increased up to 8 nm. The energy positions of the bands of unoccupied electron states in the TPD film have been determined, including π* band (7–9 eV above the Fermi level), σ₁* band (10–12 eV), σ₂* band (14–16 and 18–20 eV).     

Structural and electrical measurements of CdZnSe composite

TheI—Vcharacterization and the electrical resistivity of selenium rich Se₈₅Cd_15-xZn_x (x = 0, 3, 7, 11 and 15) system at room temperature have been studied. Samples were obtained using melt cooling technique. So prepared samples were then characterized in terms of their crystal structure and lattice parameter using X-ray diffraction method. The materials were found to be poly crystalline in nature, having zinc blend structure over the whole range of zinc concentration. The measurements ofI—V bdcharacteristics have been carried out at different temperatures from room to 140°C. The electrical resistivity of the samples with composition at room temperature has been found to vary between maximum 2.7 x 10⁸ Ωm and minimum 7.3 x 10⁵ Ωm and shows a maximum at 3 at. wt.% of Zn. The carrier activation energy of the samples with composition has also been determined and found to vary from 0.026 eV to 0.111 eV.     

Photovoltaic properties and photoconductivity in multilayer Ge/Si heterostructures with Ge nanoislands

Interband optical transitions in multilayer heterostructures with SiGe nanoislands were investigated using photocurrent spectroscopy and photo-emf. The n-p heterostructures containing Ge nanoislands in the area of the potential barrier were prepared by molecular-beam epitaxy at the temperature about 500 °C. It was shown that electron transitions from the ground state of the valence band in a nanoislands to the conduction band of Si surrounding made the main contribution into the vertical photo-emf in the range 0.75–1.05 eV, which is below the interband absorption edge of Si. The lateral photoconductivity observed in the range 0.63–0.8 eV at 77 K can be attributed to indirect interband transitions from the ground state of a nanoisland to L-state of the conduction band of a nanoisland. Analysis of Raman scattering spectra revealed that the Ge composition x in a nanoisland is about 0.87, while elastic deformation value amounts to ε _xx  = −0.016. The calculated energies of interband transitions from the ground state of a nanoisland to the conduction band of Si surrounding (0.63 eV) and to L-state of the conduction band of a nanoisland (0.81 eV) fit the experimental data with a rather good accuracy.     

Nature of chemical bonding in ThF<Subscript>4</Subscript>

The fine structure of the X-ray photoelectron spectrum of ThF₄ in the range of valence electrons (binding energy from 0 to 35 eV) is examined. The analysis takes into account the results of theoretical calculation of the electronic structure of the ThF₈⁴⁻ cluster (point group C ₂) as a model of the nearest surrounding of the Th atom in ThF₄, performed by the relativistic discrete variation method. It is demonstrated theoretically and confirmed experimentally that formation of a chemical bond gives rise to filled states of Th5f electrons (∼0.5 Th5f electron) in the energy range of electrons of outer valence molecular orbitals (valence band). The Th6p electrons noticeably participate in formation not only of inner valence but also of outer valence (∼0.4 Th6p electron) molecular orbitals. The composition and order of inner valence molecular orbitals in the energy range from 13 to 35 eV is determined, and the density of states of valence electrons in the range from 0 to 35 eV in ThF₄ is calculated. The results obtained allowed the fine structure of the high-resolution O _4,5(Th)-emission spectrum of thorium in ThF₄ in the photon energy range from ∼60 to ∼85 eV, associated with the formation of outer and inner valence molecular orbitals, to be interpreted for the first time.     

An electron microscope study of the amorphous magnetic material 2605 CO (Fe67Co18B14Si1)

Electrical resistivity and transmission electron microscope studies have been carried out on the primary crystallization process occurring in two batches of the amorphous alloy 2605 CO. The transformation is characterized by an activation energy of ∼ 2.3eV and a pre-exponential constant, InK ₀ ∼ 37min⁻¹. The microscope investigations show that the final dendrite size and dendrite volume density (cm⁻³) are both temperature dependent — moreover the former is also batch dependent. On the other hand the fractional change in resistivity during the anneal seems to be a strong function of final dendrite size and is independent of batch. Observation on partially crystallized samples lends credence to the theory of a rapidly diminishing nucleation rate such that the activation energy of the whole transformation is largely determined by the activation energy associated with the growth of dendrites.     

Electronic conductivity of Sr3-3xLa2x█x(VO4)2 solid solutions

The electronic conductivity of Sr_3-3xLa_2x█_x(VO₄)₂ solid solutions was measured at oxygen pressures from 10^-13 to 10⁵ Pa and temperatures from 1070 to 1270 K, and their band gap was determined as a function of composition. The activation energy (≏2.85 eV) and enthalpies of electron generation (≏4.2 eV) and migration (≏0.75 eV) were determined. A correlation between the band gap and electronic conductivity of the solid solutions was revealed     

Temperature dependence of transport losses in multicored high-temperature superconducting composites

Measurements were made of the ac transport losses at temperatures T≥77K in silver-sheathed (Bi, Pb)₂Sr₂Ca₂Cu₃O_x composites with N=19, 61, and 127 filaments. It was observed that an increase in temperature causes an increase in the transport losses which depend on the amplitude of the working transport current. It is shown that the increase in the transport losses results from a reduction in the critical current of the composites. Pis’ma Zh. Tekh. Fiz. 25, 83–87 (June 12, 1999)     

ZnGeP2 heterocontact with layered III–VI semiconductors

The photoelectric properties of heterojunctions fabricated by clamping wafers of ternary p-ZnGeP₂ and InSe and GaSe layered semiconductors onto an optical contact have been studied for the first time and results are reported. The photosensitivity in these structures is greatest when they are illuminated from the side with the ZnGeP₂ wafer, and reaches 150 V/W at T=300 K. In InSe/ZnGeP₂ heterostructures, a window effect is observed in the range 1.2–2 eV, whereas for GaSe/ZnGeP₂ structures, the photovoltaic effect has a maximum near 2 eV because of the proximity of the band gaps in the contacting semiconductors. Pis’ma Zh. Tekh. Fiz. 23, 1–5 (June 12, 1997)     

Coupled Phonon-Ripplon Modes in the Electron Crystal at Different Driving Fields

Coupled phonon-ripplon oscillations in a two-dimensional electron crystal over liquid helium with surface electron density n _s =1.2×10⁹ cm⁻² at the temperature T=83 mK are studied in a Corbino experimental cell. The measurements are performed in the frequency interval where resonances of the coupled oscillations are observed and at different driving voltages V _∥ <10 mV at which nonlinear features in the crystal conductivity start to appear. A special attention is paid to a relatively narrow frequency interval ω/2π≃3–5 MHz. In this interval the jumps in the oscillation spectrum are observed. From the data obtained, the mobility and electron effective mass are calculated as functions of driving electric field. The electron effective mass in the crystal and dissipative losses are found to increase with the driving field increase. A possible reason for that can be an anharmonicity of the electron-ripplon interaction, which become noticeable if the electron crystal velocity along the surface is high enough.     

Characteristics of stimulated emission from an optically pumped GaN/AlGaN double heterostructure

The luminescence properties of a GaN/Al_0.1Ga_0.9N double heterostructure grown by vapor-phase deposition from organometallic compounds are studied. When luminescence is observed from the end, the radiation intensity shows a sharply defined threshold dependence on the pump density. The threshold excitation density at T=77 K was ∼40 kW/cm² and the wavelength of the stimulated emission was λ=357 nm. The long-wavelength shift of the emission line at high pump densities may be attributed to renormalization of the band gap caused by many-particle interactions in the electron-hole plasma. Pis’ma Zh. Tekh. Fiz. 23, 53–59 (August 12, 1997)     

Adsorption of sulfur on heated (10\overline 1 0) Re

High-resolution Auger electron spectroscopy is used to study the adsorption of sulfur on a Re surface and to determine the regions of thermal stability of the resulting adsorption states. The formation of surface sulfide is observed and its stoichiometry, Re₂S, and the absolute sulfur concentration on the rhenium surface N _S =(6.0±1)×10¹⁴ atom/cm² are determined. It is shown that the surface sulfide is destroyed by thermal desorption of sulfur from the rhenium surface and the desorption activation energy is estimated as E _d =(3.3±0.2) eV for a coverage close to the surface sulfide and E _d =(4.9±0.2) eV in the limit θ→0. Pis’ma Zh. Tekh. Fiz. 25, 57–64 (July 26, 1999)     

Energy barriers and trapping centers in silicon metal-insulator-semiconductor structures with samarium and ytterbium oxide insulators

The method of internal photoemission of carriers into the insulator has been used to determine the energy barriers for the electrons at Al-Sm₂O₃ (2.89–2.91 eV), Ni-Sm₂O₃ (3.29–3.33 eV), Si-Sm₂O₃ (2.70–2.72 eV), Al-Yb₂O₃ (2.90–2.92 eV), Ni-Yb₂O₃ (3.30–3.32 eV), and Si-Yb₂O₃ (3.18–3.21 eV) interfaces in silicon metal-insulator-semiconductor (MIS) structures with samarium and ytterbium oxide insulators. The parameters of deep electron traps in samarium and ytterbium oxide were investigated. The energy position of the electron trapping centers, the magnitude and “centroid” of the trapped charge, and the ratio of the charge trapped in the insulator to that transmitted by the structure were determined. It is shown that the centroid of the trapped charge is positioned almost at the center of the insulating layer for both structures studied. It was established that in these MIS structures the rare-earth oxide-Si interface is abrupt and contains no extended damaged layer. Pis’ma Zh. Tekh. Fiz. 24, 24–29 (March 26, 1998)     

Effect of thickness on the structural and optical properties of GaN films grown on Si(111)

GaN films with different thicknesses were grown on Si(111) substrates by Plasma—Assisted Molecular Beam Epitaxy (PA-MBE). The optical properties of the films were investigated using spectrophotometric measurements of the reflectance in the wavelength range 200–3,300 nm. With increasing film thickness, the refractive index (n) increased slightly, while the optical energy gap (E_g) changed with no specific trend. The structural properties of the grown films were studied at (002) reflections using two types of rocking curve measurements; normal rocking curve (ω-scan) and triple axis rocking curve (ω/2θ-scan). The Full Width at Half Maximum (FWHM) of rocking curve decreased with increasing film thickness. Hall effect measurements showed that all the samples were n-type with carrier concentrations decreasing from 8.025 × 10¹⁸ to 5.65 × 10¹⁷ cm⁻³, and mobility increasing from 14 to 110 cm² V⁻¹ s⁻¹ as increasing the film thickness from 590 to 1,420 nm, respectively. Photoluminescence (PL) spectra for the grown GaN films with different thicknesses were measured at room temperature. PL spectra for all the samples exhibited band edge (BE) emissions at peak energies of 3.24 eV, with peak intensities increased with increasing the film thickness.