Study of As-Sb-Se chalcogenide glasses by NQR and EPR spectroscopy methods

In this paper experimental results obtained by both ⁷⁵As NQR and EPR spectroscopy are presented for the three-component system As-Sb-Se. The ⁷⁵As NQR spectra of glasses of structures (As₂Se₃)_0.78 (Sb₂Se₃)_0.22, (As₂Se₃)_0.75 (Sb₂Se₃)_0.25, (As₂Se₃)_0.5 (Sb₂Se₃)_0.5 have broad lines with two Sb-NQR lines (corresponding to the Sb₂Se₃ units) and two ⁷⁵As-NQR lines (corresponding to the As₂Se₃ units). Differences in the EPR spectra of the different glasses arise because of the different amounts of arsenic and antimony in their structure.     

Anomalous X-ray scattering studies on superionic glassy (As2Se3)-(AgX) systems (X = halides)

Anomalous X-ray scattering experiments for glassy room-temperature superionic conductors (As₂Se₃)_0.4 (AgI)_0.6 and (As₂Se₃)_0.4(AgBr)_0.6 were performed close to the As, Se, Ag, and Br K edges using a third-generation synchrotron radiation facility, ESRF. The differential structure factors, Δ_iS(Q), were obtained from detailed analyses, indicating that Δ_AsS(Q) and Δ_SeS(Q) of both the glassy superionic semiconductors are similar to those of glassy As₂ Se₃ except the prepeak in Δ_SeS(Q). The Δ_AgS(Q) spectrum of (As₂Se₃)_0.4 (AgI)_0.6 looks molten salt-like. However, the Δ_Ag S(Q) of (As₂Se₃)_0.4(AgBr)_0.6 glass have quite different features from that of (As₂Se₃)_0.4 (AgI)_0.6 glass in the low Q range, and the Δ_BrS(Q) has even a pre-shoulder around 13 nm^? 1 unlike molten salts. In the differential pair distribution functions Δ_ig(r) obtained from the Fourier transforms of Δ_iS(Q), the first peaks of Δ_Asg(r) and Δ_Seg(r) show no correlation with those of Δ_Agg(r) and Δ_Brg(r), and vice versa. From these results, it can be concluded that a pseudo-binary mixture of the As₂Se₃ network matrix and AgX-related ionic conduction pathways is a good structural model for these superionic glasses. Differences between the AgBr- and AgI mixtures were found in the second-neighbor structures around the Ag atoms, which may reflect those in the crystal structures of the AgX salts.     

ESR in X-irradiated As2O3 glass

X-irradiation of glassy As₂O₃ at 77K or 300K produces an unusually large density (～5×10¹⁸ cm^-3) of paramagnetic centers which are stable at 300K. The average spin-Hamiltonian parameters (g = 1.998, g_⊥ = 1.984, A₆ = 243G, A_⊥ = 114G) indicate that these centers are analogous to those previously observed in As₂Se₃ and As₂S₃ glasses and that they consist of unpaired electrons localized on a non-bonding 4p orbital of an As atom. Unlike the results obtained for As₂Se₃ and As₂S₃, the concommitant holes in As₂O₃ are trapped on Fe²⁺ impurity sites which become Fe³⁺ and not on non-bonding oxygen p orbitals. The radiation induced ESR is also accompanied by a stable optical absorption tail which lies within the band gap and increases exponentially with energy. This absorption can be partially bleached with the application of sub-band-gap light.     

The effect of arsenic vapour species on electrical and optical properties of GaAs grown by molecular beam epitaxy

Hall effect, DLTS and low-temperature photoluminescence measurements were used to study the effect of dimeric (As₂) vs tetrameric (As₄) vapour species on the electrical and optical properties of nominally undoped and of Ge-doped GaAs layers grown by molecular beam epitaxy (MBE). The arsenic molecular beam was generated from separate As₂ and As₄ sources, respectively, and from a single source providing an adjustable As₂/As₄ flux ratio. The occurence of the previously described defect related bound exciton lines in the luminescence spectra at 1.504–1.511 eV was found to be directly correlated with the presence of three deep states (M1, M3, M4) which are characteristic of MBE grown GaAs. The intensity of the extra luminescence lines and simultaneously the concentration of the deep electron traps can be reduced substantially simply by decreasing the As₄/As₂ flux ratio. The incorporation of defect related centers as well as of amphoteric dopants like Ge strongly depends on the surface chemistry involved. Therefore, a considerably lower autocompensation ratio in Ge-dopedn-GaAs is obtained with As₂ molecular beam species which provide a higher steady-state arsenic surface population.     

Hydrostatic pressure study of the structural phase transitions and superconductivity in single crystals of (Ba1−xKx)Fe2As2 (x=0 and 0.45) and CaFe2As2

We studied the effect of hydrostatic pressure (P) on the structural phase transitions and superconductivity in the ternary and pseudo-ternary iron arsenides CaFe₂As₂, BaFe₂As₂, and (Ba_0.55K_0.45)Fe₂As₂, by means of measurements of electrical resistivity (ρ) in the 1.8-300 K temperature (T) range, pressures up to 20 kbar, and magnetic fields up to 9 T. CaFe₂As₂ and BaFe₂As₂ (lightly doped with Sn) display structural phase transitions near 170 and 85 K, respectively, and do not exhibit superconductivity in ambient pressure, while K-doped (Ba_0.55K_0.45)Fe₂As₂ is superconducting for T<30 K. The effect of pressure on BaFe₂As₂ is to shift the onset of the crystallographic transformation down in temperature at the rate of ~−1.04 K/kbar, while shifting the whole ρ(T) curves downward, whereas its effect on superconducting (Ba_0.55K_0.45)Fe₂As₂ is to shift the onset of superconductivity to lower temperatures at the rate of ~−0.21 K/kbar. The effect of pressure on CaFe₂As₂ is first to suppress the crystallographic transformation and induce superconductivity with onset near 12 K very rapidly, i.e., for P<5 kbar. However, higher pressures bring about another phase transformation characterized by reduced-resistivity, and the suppression of superconductivity, confining superconductivity to a narrow pressure dome centered near 5 kbar. Upper critical field (H_c2) data in (Ba_0.55K_0.45)Fe₂As₂ and CaFe₂As₂ are discussed.     

Low frequency Raman spectra of chalcogenide glasses

Low frequency (< 30 cm^-1) Raman scattering lineshapes for the chalcogenide glasses As₂S₃ and As₂Se₃ have been interpreted in terms of a model which explicitly includes acoustic (∼ ω2 density-of-states) and optic (rigid-layer like) vibrational modes.     

Coordination dependent vibrational properties of amorphous semiconductors alloys

The i.r., Raman and depolarization spectra of the amorphous pseudo-binary alloy systems (GeS₂)_1-χ(As₂S₃)_χ and (GeSe₂)_1-χ(As₂Se₃)_χ are reported here. In contrast to the one-mode behavior that is anticipated on the basis of the mass ratio of the alloy atoms (Ge/As), two bond-stretching modes are observed in both the i.r. and Raman spectra. This new type of phonon behavior is interpreted in terms of differences in the local coordination via a chemical bonding model.     

Temperature and pressure dependences of the thermal conductivity of As2(Se1-xTex)3 solid solutions

The effect of temperature and pressure on the thermal conductivity of solid solutions based on the As₂(Se_{1 - x}Te_x)₃ system was investigated in glassy and polycrystalline samples at 273–450 K and hydrostatic pressures of up to 0.35 GPa. The compound As₂Se₃ was studied in a temperature range of 300–760 K. Analysis showed that the short-cange order structure in As₂Se₃ remains unchanged upon the glass—liquid transition right up to 760 K.     

Studies on electrodeposited As2S3 thin films by double exposure holographic interferometry technique

Arsenic trisulphide (As₂S₃) thin films have been deposited onto stainless steel and fluorine doped tin oxide (FTO) coated glass substrates by electrodeposition technique using arsenic trioxide (As₂O₃) and sodium thiosulphate (Na₂S₂O₃) as precursors and ethylene diamine tetracetic acid (EDTA) as a complexing agent. Double exposure holographic interferometry (DEHI) technique was used to determine the thickness and stress of As₂S₃ thin films. It was observed that the thickness of the thin film increases whereas film stress to the substrate decreases with an increase in the deposition time. X-ray diffraction and water contact angle measurements showed polycrystalline and hydrophilic surface respectively. The bandgap energy increases from 1.82 to 2.45 eV with decrease in the film thickness from 2.2148 to 0.9492 μm.     

Glass formation in the As2Se3-As2Te3-Sb2Te3 system

Chalcogenide glasses from the As₂Se₃-As₂Te₃-Sb₂Te₃ system were synthesized for the first time. The glass-forming region was determined by X-ray diffraction and electron microscopic analyses.The basic physicochemical parameters such as density (d), microhardness (HV) and temperatures of phase transformations (glass transition T_g, crystallization T_cr and melting T_m) were measured. Compactness and some thermomechanical characteristics such as volume (V_h) and formation energy (E_h) of micro-voids in the glassy network as well as the elasticity module (E) were calculated. The glass-forming ability was evaluated according to Hruby's criteria (K_G). The correlation between composition and properties of the (As₂Se₃)_x(As₂Te₃)_y(Sb₂Te₃)_z glasses was established and comprehensively discussed.     

Far infrared absorption spectra of amorphous (As2S3)?(Sb2S3) semiconductors and the spatial charge fluctuation

Far infrared absorption spectra of amorphous (As₂S₃)_1–x(Sb₂S₃)_x system (x=0.00.6) were measured in the wavenumber region 725 cm^–1 with Lamellar grating far infrared Fourier Transform Spectrometer, and the spatial fluctuation of charges and its correlation range were estimated with charge fluctuation model. The charge fluctuation of (As₂S₃)_1–x(Sb₂S₃)_x system increases with Sb₂S₃ concentration x. On the other hand, the correlation length of the charge fluctuation decreases from 7 A to 4.5 A with an increase in x. As a result, it can be found that the medium range order of As₂S₃ changes drastically with an incease in Sb₂S₃ concentration x.     

Band structure of (Sr<Subscript>3</Subscript>Sc<Subscript>2</Subscript>O<Subscript>5</Subscript>)Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> as a possible basis phase of new FeAs superconductors

The results of the ab initio FLAPW-GGA computations of the band structure of the recently synthesized layered tetragonal (space group I4/mmm) arsenide (Sr₃Sc₂O₅)Fe₂As₂ as a possible basis phase of a new group of FeAs superconductors are presented. For (Sr₃Sc₂O₅)Fe₂As₂, the energy bands, electron state density distributions, Fermi surface topology, low-temperature electron specific heat, molar Pauli paramagnetic susceptibility, and effective atomic charges have been determined. These results are discussed compared to similar data for the layered tetragonal crystals LaFeAsO, SrFeAsF, SrFe₂As₂, and LiFeAs that are the basis phases of the recently discovered high-temperature (T _C ～ 26–56 K) 《1111》, 《122》, and 《111》 FeAs superconductors.     

Infrared properties of AsχTe1−χ glasses

Infrared reflection and transmission measurements in As_χTe_1?χ glasses (45?×?55) show well-defined Gaussian vibrational absorption peaks. Comparisons with spectra observed in crystalline and glassy As₂S₃ and As₂Se₃ indicate that the local order in As_χTe_1?χ glasses is not like that found in crystalline As₂Te₃, but rather it consists primarily of AsTe₃ pyramids which are probably linked together in a fashion similar to that found in As₂S₃ and As₂Se₃.     

Photoinduced transformations in (As1–xBix)2S3 glass observed by Raman spectroscopy

Raman spectra of (As_1–xBi_x)₂S₃ glass samples with x ≤ 0.2 measured at the excitation with above-bandgap (532 nm) laser light at a relatively low power density (P_exc = 4 kW/cm²) clearly confirm the amorphous character, thereby markedly extending the known compositional interval of existence of the (As_1–xBi_x)₂S₃ glass previously known (x ≤ 0.06). Spectra measured at an increased P_exc (40 kW/cm²) reveal a photostructural transformation in the illuminated area of the glass leading to an additional contribution of Bi–S bonds as well as to an increasing number of cage-type As₄S₄ units with homopolar As–As bonds. A number of new features in a broad range up to about 1,000 cm⁻¹, which emerge in the Raman spectra of the (As_1–xBi_x)₂S₃ glasses with high (x ≥ 0.14) Bi content and increase in intensity with the exposure time, are related to a photochemical transformation, namely, oxidation of arsenic and sulphur on the (As_1–xBi_x)₂S₃ glass surface with formation of units containing arsenate AsO₄³⁻ and sulphate SO₄²⁻ ions. These processes are irreversible and occur only in the presence of a sufficient amount of bismuth.     

A computational study of the electron detachment energies of Al2As2 − and Al3As3 −

Ab initio and density functional theory calculations are reported for the low-lying electronic states of Al₂As₂ ^?, Al₂As₂, Al₃As₃ ^?, and Al₃As₃. The ²B_2g ground electronic state of Al₂As₂ ^? has a rhombic structure with the Al atoms occupying the shorter diagonal. In contrast, the As atoms occupy the shorter diagonal of the ground state rhombic structure of Al₂As₂. Electron detachment energies computed for Al₂As₂ ^? are presented and discussed. The adiabatic electron affinity of Al₂As₂ ^? is calculated to be 2.1 eV at the CCSD(T) level, using B3LYP and MP2 optimized geometries. The ground states of both Al₃As₃ ^? (²A₁′) and Al₃As₃ (¹A₁′) have planar hexagonal D_3h geometry. Electron detachment energies computed for the anion are reported. At the CCSD(T)//B3LYP level, the electron affinity of Al₃As₃ is calculated to be 2.47 eV.     

Effect of nonlinear refraction and two-photon absorption on the optical limiting in amorphous chalcogenide films

This paper reports on the results of investigations into the nonlinear optical characteristics of chalcogenide films (As₂S₃, As₂₀S₈₀, 2As₂S₃/As₂Se₃, 3As₂S₃/As₂Se₃). The nonlinear refractive indices and two-photon absorption coefficients for these films are measured using the Z-scan technique at wavelengths of a picosecond Nd: YAG laser (λ=1064 and 532 nm). The optical limiting due to Kerr-type nonlinearities is analyzed.     

Kinetics of photo‐darkening and self‐bleaching in amorphous As2S3 and As2Se3 thin films

The kinetics of photo‐darkening of amorphous As₂S₃ and a‐As₂Se₃ thin films follows a single exponential, but the magnitude and the rate of the process is higher in case of As₂S₃. The kinetics of self‐bleaching (dark relaxation) in advance photo‐darkened state follows a stretched exponential (SRE) with different stretching parameter for a‐As₂S₃ and a‐As₂Se₃. Within the J. C. Phillips approach we suppose that photo‐darkening in amorphous As₂S₃ films is, to some extent, accompanied by changes in short‐range order interactions, while photo‐darkening of amorphous As₂Se₃ is accompanied rather by changes in Coulomb interactions. The self‐bleaching process reduced the magnitude of photo‐darkening up to 45% and 60% for amorphous As₂S₃ and As₂Se₃ films, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical limiting in fullerenes, colloidal metal solutions, and semiconductors in the field of pico-and nanosecond pulses of an Nd:YAG laser

Optical limiting of pico-and nanosecond pulsed radiation of a Nd:YAG laser (λ=1064 and 532 nm) is studied in solutions of fullerenes (C₆₀ and C₇₀), semiconductor crystals and films (GaAs, As₂S₃, As₂₀S₈₀, 2As₂S₃/As₂Se₃, and 3As₂S₃/As₂Se₃), and colloidal solutions of metals (Au, Ag, Pt, and Cu). The effect of the particle aggregation in silver solutions on the optical limiting is considered. Optical limiting mechanisms, specifically, reverse saturable absorption, two-photon absorption, and self-action effects, are discussed. Nonlinear absorption coefficients of the studied materials are measured.     

Thermally and photo-induced changes in the valence states of vapour-deposited As2S3 films: A comparative photoemission study of As2S3 and As4S4

During thermal annealing or light irradiation, the changes in the valence states of vapour-deposited As₂S₃ and As₄S₄ films were observed by UPS. The experimental results reveal that an as-deposited As₂S₃ film contains considerable numbers of As₄S₄ molecular units, which polymerize or cross link to form a As₂S₃ glassy network on annealing or irradiation.     

Photo-synthesis of As3S2 crystal from AsAs2S2 mixture

Existence of As₃S₂ compound which is stable in a certain temperature range higher than room temperature has been confirmed in AsAs₂S₂ system. Photo-synthesis effect of As₃S₂ crystal from As and As₂S₂ mixture, namely light induced conversion from a crystalline state to another crystalline state, has also been investigated.