Experimental characterization of amorphous As–Se–Sb alloys

Thin films were thermally evaporated from ingot pieces of the As₃₀Se_70−xSb_x (with 2.5 ⩽ x ⩽ 17.5 at.%) glasses under vacuum of ∼10⁻⁵ Torr. Increasing Sb content was found to affect the thermal and optical properties of these films. Non-direct electronic transition was found to be responsible for the photon absorption inside the investigated films. The chemical bond approach has been applied successfully to interpret the decrease of the glass optical gap with increasing Sb content. Decreasing the thermal stability of the As₃₀Se_70−xSb_x specimen by increasing Sb content is responsible for occurring the amorphous–crystalline process at lower temperatures. Binary As₂Se₃ and Sb₂Se₃ phases are the main components of the stoichiometric As₃₀Se₆₀Sb₁₀ composition.     

Effect of the substitution of S for Se on the structure and non-linear optical properties of the glasses in the system Ge0.18Ga0.05Sb0.07S0.70−xSex

Glasses in the system Ge–Ga–Sb–S/Se have been elaborated with different S/Se ratios in order to increase the non-linear optical properties of these glasses. We report results of a systematic study examining the relationship of the physical properties to the structure of the glasses in the system Ge_0.18Ga_0.05Sb_0.07S_0.70−xSe_x with x = 0, 0.02, 0.05, 0.10, 0.20, 0.30 and 0.40 where the replacement of S by Se has been made. The non-linear refractive index has been measured using the Z-scan technique, with picosecond pulses emitted by a 10 Hz Q-switched mode-locked Nd-Yag laser at 1064 nm under conditions suitable to characterize ultrafast non-linearities. The decrease of the glass transition temperature, the increase of the non-linear refractive index and of the density with the progressive replacement of S by Se have been correlated along with the red shift of the absorption band gap, to associated structural reorganization. A corresponding progressive decrease of corner-sharing GeS_4/2 and the formation of mostly two edge-sharing Ge₂S₄S_2/2, S₃Ge–S–GeS₃ as well as mixed GeS_4−xSe_x units have been identified by Raman spectroscopy.     

Nanocrystallites in Bi–As–S system

Bi₂S₃–As₂S₃ composite formation was performed by two methods: by the direct insertion of Bi₂S₃ nanocrystals into a molten As₂S₃ glass which was further solidified and by the crystallization of a rapidly quenched (As₂S₃)_1−x (Bi₂S₃)_x glasses with x = 0.005, 0.01, 0.02 and 0.04 at different conditions. Fine tuning of the annealing of the quenched glass as well as the mixing of nanocrystals in to the molten glass resulted glass-crystalline composites with different amounts and distribution of 20–50 nm large Bi₂S₃ nanocrystals as well as larger, up to few micrometer long, needle-like crystals. Structural and optical investigations support the presence of the Bi₂S₃ crystalline phase in all composites. Optical absorption and the photoconductivity of bulk composite samples follow the structural changes of the structure in the amorphous and amorphous-crystalline phase. In addition, the 290 cm⁻¹ characteristic band in Raman spectra may be used for tracing the formation of the nanocomposites.     

Physical aging effects in selenide glasses accelerated by highly energetic γ-irradiation

Effect of ⁶⁰Co γ-irradiation on As–Se glasses stored for 20 years and subjected to saturated natural physical aging is studied using differential scanning calorimetry. It is shown that γ-irradiation activates further physical aging, which leads to an increase in glass transition temperature and endothermic peak area near glass transition region for As_xSe_100−x samples with x < 30. The observed changes are associated with additional structural relaxation of radiation-modified glass network.     

A combined 77Se NMR and Raman spectroscopic study of the structure of GexSe1-x glasses: Towards a self consistent structural model

The short-range structures of stoichiometric and Se-deficient binary Ge_xSe_100 ?x glasses with 42 ≥ x ≥ 33.33 have been investigated using a combination of Raman and ⁷⁷Se Car–Purcell–Meiboom–Gill (CPMG) spikelet nuclear magnetic resonance (NMR) spectroscopy. When taken together, these spectroscopic results allow for self-consistent assignment of average ⁷⁷Se NMR isotropic chemical shifts to Se atoms in various coordination environments in Ge_xSe_100 ?x glasses. Analysis of the compositional variation of the relative concentrations of these Se environments indicates considerable violation of chemical order in the nearest-neighbor coordination environments of the constituent atoms in the stoichiometric Ge_33.33Se_66.67 glass. On the other hand, the presence of a random distribution of Ge―Ge bonds can be inferred in the Se-deficient glasses. Simulations of the previously published ⁷⁷Se NMR line shapes of Se-excess glasses on the basis of the revised structural assignments of ⁷⁷Se NMR chemical shifts obtained in this study conclusively indicate that the structure of these glasses is intermediate between a randomly connected and a fully clustered network of GeSe₄ tetrahedra and Se chains.     

Non-linear properties of chalcogenide glasses and fibers

High purity chalcogenide glasses were prepared in the series As₂S_(3?x)Se_x where x = 0 to 3. The measured third order non-linearities increase with the value of x, and are up to about 1000 times larger than silica for As₂Se₃ glass. We show that the anharmonic oscillator model, using the normalized photon energy, gives an excellent fit to the data over three orders of magnitude. Single mode optical fibers based on As₂S₃ and As₂Se₃ glasses have been fabricated using the double crucible technique and the Stimulated Brillouin Scattering (SBS) investigated. The threshold intensity for the SBS process was measured and used to estimate the Brillouin gain coefficient. Preliminary results indicate record high values for the figure of merit and theoretical gain, compared to silica, which bodes well for slow-light based applications in chalcogenide fibers.     

Glassy state and structure of Sn–Sb–Se chalcogenide alloy

The effect of Sn addition on the glass transition and structure of c-Sb₂₀Se₈₀ chalcogenide alloy have been studied by X-ray diffraction and differential scanning calorimetric studies. The increase in the glass forming region and the glass transition temperature with the addition of Sn is discussed by considering the formation of [SnSe₄] tetrahedra, another type of network former, which inhibits the crystallization. The differential scanning calorimetric studies on Sn_xSb₂₀Se_80−x (8 ⩽ x ⩽ 18) glassy samples reveal a single glass transition temperature for all values of x while a single crystallization peak was obtained only for 10 ⩽ x < 12. The X-ray diffraction studies reveal that the glass crystallizes to Sb₂Se₃ and SnSe₂ phases upon annealing. The glass formation and composition dependence of glass transition temperature in the Sn–Sb–Se chalcogenide alloy could be understood by considering the topological phase transitions and a chemically ordered network model.     

Optical properties and structure of amorphous films Agx(As0.33S0.67−ySey)100−x

As₃₃S_67−ySe_y, where y = 0, 16.75, 33.5, 50.25 and 67, amorphous thin films were prepared by a vacuum thermal evaporation technique. The films with known silver concentrations and good optical quality were prepared by thermal vacuum evaporation of a silver film on the top of As₃₃S_100−ySe_y films with sequential step-by-step optically- and thermally-induced diffusion and dissolution (OIDD) of silver. The range of silver content was x = 0–25 at.%. The kinetics of OIDD of silver were measured optically by monitoring the change of thickness of the undoped part of the chalcogenide during broadband illumination. Compositions of the reaction products have been determined by scanning electron microscope with energy-dispersive X-ray microanalyser EDS. Optical properties $(T\text{,}n\text{,}{E}_{\text{g}}^{\text{opt}})$ of thin films were measured and/or calculated by the Swanepoel method [R. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) 1214]. The refractive index increase with increasing silver and selenium concentration has been shown. The difference of the refractive index (Δn) between undoped and silver doped films was ∼0.4 and between As₃₃S₆₇ and As₃₃Se₆₇ was films ∼0.42. Non-linear indices of refraction were estimated according to Tichy’s formula [H. Ticha, L. Tichy, J. Optoel, Adv. Mat. 4 (2002) 381]. The values of non-linear refractive index grew with increasing silver and selenium content. The difference of optical bandgap, $\mathrm{\Delta }{E}_{\text{g}}^{\text{opt}}$, between undoped As₃₃S₆₇ and fully doped films with Ag and Se was ∼1 eV. Raman spectroscopy showed a decrease in S–S or Se–Se bonds with increasing silver content.     

Refractive-index dispersion of glassy semiconductors in the pseudo-binary As2Se3–SbSI system

The bulk chalcogenide glasses in the pseudo-binary system (As₂Se₃)_100?x(SbSI)_x (with x = 20, 30, 50, 70 and 80 at.%) were prepared from high-purity elemental components by melt-quenching technique. It is a system with the variable ratio of classical amorphous compound As₂Se₃ and the molecule of antimony-sulphoiodide, SbSI, which in the monocrystal form is characterized as a ferroelectric. The refractive-index behaviour (magnitude and spectral dispersion) of investigated glasses was determined by the prism method and analyzed. To calculate and discuss the parameters of dispersion in the band gap region three different approaches were used (Cauchy, Sellmeier and Wemple–DiDomenico single-oscillator model). The comparison of the results shows good agreement between all applied models. It was found that the value of the refractive-index shows normal dispersion behaviour and increases with the increase of Sb (or SbSI) content in the investigated system, whereas the optical gap of these glasses E_g slightly decreases.     

Ab initio calculation of vibrational frequencies of GexSe1−x glass

We have used the density functional theory to make the models of Ge_xSe_1?x glass for which the energy is a minimum. The clusters, Ge₂Se₂, Ge₂Se₃, Ge₃Se, Ge₃Se₂, Ge₄Se, GeSe₃, GeSe₄, chain mode zig-zag Ge₄Se₃, corner sharing GeSe₄, and edge sharing Ge₂Se₆, have been made successfully and their vibrational spectra have been calculated from the first principles. We are able to optimize the bond distances as well as the bond angles. The calculated values of the frequencies of vibrations of the various clusters have been compared with those obtained from the experimental Raman spectra of actual glasses, Ge_xSe_1?x(0 < x < 0.3). The local concentration, x within 0.25 nm is nonuniform in the amorphous material. When the same cluster occurs in two stable configurations, low frequency vibrations of frequency, ν < 100 cm^?1, are found. The corner sharing GeSe₄ has low frequency modes at 54 cm^?1 and 93 cm^?1 whereas these modes disappear in the pyramidal configuration. The low frequency modes are therefore associated with the breaking of C₄ symmetry of the pyramidal configuration. The computed vibrational frequencies of clusters Ge₃, Ge₄Se₃, Ge₂Se₃, GeSe₃ and Ge₃Se₂ are actually present in the Raman spectra of the glass, Ge_xSe_1?x(0 < x < 0.3).     

Topology and chemical order in AsxGexSe1 − 2x glasses: A high-resolution X-ray photoelectron spectroscopy study

The evolution of topology and chemical order along the As_xGe_xSe_1 ? 2x composition-line within the As–Ge–Se glass-forming region is studied by high-resolution X-ray photoelectron spectroscopy. It is shown that cation–cation bond formation becomes a dominant process for the compositions with x > 0.09. The results explain the peculiarities observed around this composition recently in the temperature-modulated differential scanning calorimetry data. Substitution of two selenium atoms within constituent structural units (pyramids and/or tetrahedra) by corresponding cations explains a second peculiarity point at x > 0.16 compositions observed recently with the above technique. The present observations show segregation of As and then Ge at high concentrations of cations in the system (x > 0.20).     

Structure of SbxGe40-xSe60 glasses around 2.67 average coordination number

The evolution of the structure of Sb_xGe_40-xSe₆₀ (x = 8, 12, 15, 18 and 20) chalcogenide glasses is determined by high resolution X-ray photoelectron spectroscopy as Sb is added and the average coordination number (Z) is varied from 2.60 to 2.72. For glasses with Z < 2.67 the structure consists of deformed tetrahedra and pyramids, in which at least one Se atom is substituted by Ge or Sb atoms. Increase in the concentration of cations beyond Z ≥ 2.67 leads to a strengthening of the above regularity and formation of the structure by shared pyramids and tetrahedra with two or more Se atoms substituted by cations. At the same time Se–Se dimers are present in all the investigated compositions while they are not expected in these Se-deficient glasses. Their formation dictates that there must be metal-rich regions somewhere in the structure.     

Emission properties and local structure of Tm3+ in Ge–Ga–S–Br glass

Spectroscopic properties of Tm³⁺ in (1 − x) (Ge_0.25Ga_0.10S_0.65)–xBr (or CsBr) glasses (x = 0.0 and 0.1) were investigated. Emission properties of Tm³⁺ in 0.9(Ge_0.25Ga_0.10S_0.65)–0.1Br glass were similar to those in Ge_0.25Ga_0.10S_0.65 glass, while there was significant improvement when doped into 0.9(Ge_0.25Ga_0.10S_0.65)–0.1CsBr glass. The lifetime of the Tm³⁺:³H₄ level increased from 0.23 to 1.22 msec with 10 mol% CsBr addition. The presence of Cs⁺ facilitated the formation of [GaS_3/2Br]⁻ units by donating an electron to the Ga tetrahedron, resulting in the homogeneous distribution of Br. In this way, Tm³⁺ ions have their local environment made of Br only. When Br ions were added instead of CsBr, [GaS_(4−x)/2Br]⁻ units with x > 1 were formed and Tm³⁺ ions were surrounded by both S and Br, producing a high phonon environment.     

Micro-structural studies of GeSe2–Ga2Se3–MX (MX = CsI and PbI2) glasses using Raman spectra

Microstructure of the chalcohalide glasses: GeSe₂–Ga₂Se₃–CsI and GeSe₂–Ga₂Se₃–PbI₂ ternary system were investigated by Raman spectra, lifetime of Dy³⁺ infrared emission and glass transition temperature (T_g). The evolution of the Raman spectra shows that the fundamental structural groups of these studied glasses consist of [Ge(Ga)Se₄] tetrahedral and some complex structure units [Ge(Ga)I_xSe_4?x](x = 1–4). The x value varied when the different iodide was added in Ge–Ga–Se matrix. For GeSe₂–Ga₂Se₃–CsI glasses, the [Ge(Ga)I_xSe_4?x](x = 1–4) mixed-anion tetrahedral and [Ga₂I₇]^? units occurred. For GeSe₂–Ga₂Se₃–PbI₂ glasses, the [Ge(Ga)I₂Se₂], [Ge(Ga)I₃Se] units can be formed. The changes of Dy³⁺ infrared emission lifetime and T_g support the results. Additionally, [PbI_n] structural units will be formed in GeSe₂–Ga₂Se₃–PbI₂ glasses due to high form-ability of these units when the PbI₂ content is high.     

Optical phonons and Raman scattering in ternary II–VI spheroidal nanocrystals embedded in a glass matrix

Theoretical and experimental studies of the spatial phonon confinement in ternary CdS_xSe_1−x nanocrystals embedded in a glass matrix formed by the composites (40)SiO₂₋(30)Na₂CO₃–(29)B₂O₃–(1)Al₂O₃ (mol%) + [(2)CdO + (2)S + (2)Se] (wt%) were carried out. From the analysis of the surface phonon modes, the theoretical procedure has allowed the determination of the geometrical characteristics of the nanocrystals. The calculated frequencies were compared with the experimental values obtained from the Raman spectra of CdS_xSe_1−x nanocrystals grown under different thermal treatments. A good correlation between the experimental and calculated CdS-like and CdSe-like surface optical modes was observed. The Raman selection rules and their connection with the nature of the surface optical phonons is discussed in order to use Raman spectroscopy as a probe to determine the composition x and the geometrical shape of the semiconductor nanocrystals.     

Structural study of GexSb40−xS60 (x = 10, 20 and 30) glasses

The atomic structures of Ge–Sb–S ternary glasses have been investigated by using the neutron diffraction method. The structure factor, S(q), for Ge_xSb_40−xS₆₀ (x = 10, 20 and 30) has a first sharp diffraction peak (FSDP) at around 1.1 Å⁻¹. The oscillation in S(q) persisted up over 25 Å⁻¹ indicates that the chemical short-range order is presented in these glasses. The aspect of S(q) for Ge₁₀Sb₃₀S₆₀ is different from those for Ge₂₀Sb₂₀S₆₀ and Ge₃₀Sb₁₀S₆₀. These results show that the atomic structure changes drastically between Ge₁₀Sb₃₀S₆₀ and Ge₂₀Sb₂₀S₆₀. The short-range order of tetrahedral GeS₄ and pyramidal SbS₃ units seems to exist in the Ge_xSb_40−xS₆₀ (x = 10, 20 and 30) glasses.     

Molecular dynamics simulation of ternary glasses Li2S–P2S5–LiI

The structure of superionic glasses in ternary systems x(0.4Li₂S–0.6P₂S₅)–(1 − x)LiI and x(0.5Li₂S–0.5P₂S₅)–(1 − x)LiI (x = 0.9, 0.75) has been studied by molecular dynamics (MD) simulations. The configurations obtained by MD were analyzed by graph theory. Phosphorus is surrounded by sulfur and iodine atoms. Li is surrounded by sulfurs alone and LiI clusters are not present as speculated by earlier spectroscopic reports. The equilibrium configuration is made up of (Li, S) and (P, S, I) rich regions which creates wide channels for Li⁺ movement. Reported variations of glass transition temperature (T_g) and ionic conductivity (σ) with LiI addition are explained based on the simulation results.     

Te-rich Ge–As–Se–Te bulk glasses and films for future IR-integrated optics

Ge₁₅As₁₅Se_70−xTe_x materials with x = 56, 60 and 63, of potential use in IR-integrated optics, were prepared by the classical melt-quenching method. A macroscopic phase separation was observed with a crystalline phase on the top and an amorphous one at the bottom. The glasses from the bottom were transparent from 1.9 to 16 μm without any purification of the elemental precursors Ge, As, Te and Se. The higher the Te/Se content in the glasses the lower their glass transition temperature and thermal stability. Films 7–12 μm thick of the above stated compositions were deposited by thermal evaporation. The higher the tellurium content, the larger the optical band gap shift of the films in the infra-red and the higher the refractive index.     

Conductivity percolation transition of Agx(Ge0.25Se0.75)100−x glasses

The ionic conductivity of several chalcogenide glasses increases abruptly with mobile ion addition from values typical of insulating materials (10⁻¹⁶–10⁻¹⁴ Ω⁻¹ cm⁻¹) to values of fast ionic conductors (10⁻⁷–10⁻¹ Ω⁻¹ cm⁻¹). This change is produced in a limited concentration range pointing to a percolation process. In a previous work [M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259] the transition from semiconductor to fast ionic conductor of Ag_x(Ge_0.25Se_0.75)_100−x glasses was detected at x¹ ≅ 10 at.% in the form of a steep change in the conductivity. Ag_x(Ge_0.25Se_0.75)_100−x glasses with x ⩽ 25 at.%, prepared by a melt quenching method, are investigated by impedance spectroscopy in the frequency range 5 Hz–2 MHz at different temperatures, T, from room temperature to 363 K and by DC measurements at room temperature. The conductivity of the glasses, σ, was obtained as a function of silver concentration and temperature. For x ⩾ 10 at.% our results are in agreement with those reported by Kawasaki et al. [M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259]. The percolation transition was observed in the range 7 ⩽ x ⩽ 8. The temperature dependence of the ionic conductivity follows an Arrhenius type equation σ = (σ_o/T) · exp(−E_σ/kT). The activation energy of the ionic conductivity, E_σ, and the pre-exponential term, σ_o, are calculated. The results are discussed in connection with other chalcogenide and chalcohalide systems and linked with the glass structures.     

Single oscillator energy and dispersion energy of uniform thin chalcogenide films from Cu–As–S–Se system

This paper presents some of the results obtained by using the modified envelope method, which takes substrate absorption into account. Samples investigated in this paper are the series of amorphous thin chalcogenide uniform films from system Cu_x[As₂(S_0.5Se_0.5)₃]_100−x. Thin films were deposited under vacuum on glass substrates by thermal evaporation technique, from previously synthesized bulk samples. The dispersion of the refractive index is discussed in terms of the single oscillator model proposed by Wemple and DiDomenico. By using this model, i.e. by plotting (n² − 1)⁻¹ against (ℏω)² and fitting a straight line, oscillator parameters, E₀ – the single oscillator energy and E_d – the dispersion energy, were directly determined.