Emission and local structure of rare-earth ions in chalcogenide glasses

Local structures of rare-earth ions in Ge–Ga–S–CsBr glasses were investigated to understand the structural origin on the emission property enhancement. The frequency of the phonon vibration controlling the multiphonon relaxation was changed to 245 cm⁻¹ due to the formation of Ga–Br bonds with CsBr addition to sulfide glasses. Formation of this new chemical bond was also confirmed from the phonon side band spectra of Eu³⁺ ions. Analyses of the EXASF spectra proved that Tm³⁺ ions were surrounded by approximately seven S ions in Ge_0.25Ga_0.10S_0.65 glass but were coordinated by ∼6 Br ions in the glass with 10 mol% of CsBr.     

Structure of ultraphosphate glasses with small rare-earth ions by X-ray diffraction

Rare-earth ultraphosphate glasses with nominal R₂O₃ fractions of 10 and 15 mol% and small ionic radius (large atomic number) R³⁺ ions (R = Tb, Tm, Lu) are measured by X-ray diffraction at a synchrotron with photons of 119 keV (maximum scattering vector 260 nm^− 1). The total correlation functions T(r) show well-resolved R-O and O-O first-neighbor peaks. In contrast to all ultraphosphate RP₅O₁₄ crystals and the ultraphosphate glasses of larger R³⁺ ions, where RO₈ polyhedra (mean R-O coordination numbers of ~ 8 for the glasses) are observed, the R³⁺ ions in glasses with R = Lu, Tm, Tb have mean R-O coordination numbers of ~ 7.5. The R-O first-neighbor peaks extracted from the T(r) functions are compared with those obtained from atomic coordinates of related RP₅O₁₄ and RP₃O₉ crystals. The R-O distances of the ultraphosphate glasses studied are found to fall between those of the two crystals but with tails to the side of longer bonds.     

Time-resolved photoluminescence in chalcogenide glasses

The time resolved spectroscopy of the photoluminescence with pulse-dye-laser excitation supplied the direct evidence of the strong electron-phonon interaction and the distributed localized states at band edges and the important information of mobility edge in chalcogenide glasses. The excitation energy dependence of the time resolved photoluminescence indicated that a-As₂S₃ and a-GeSe₂ at 4.2 K had the mobility gaps of 2.6 eV and 2.25 eV, respectively.     

Infrared conductivity of chalcogenide glasses

The general features of the conductivity spectra of chalcogenide glasses in the frequency range 10⁹ to 10¹³ Hz are briefly reviewed.     

Optical properties of chalcogenide glasses

Over the past two decades chalcogenide glasses have been researched in order to assess their suitability as passive bulk optical component materials for 3–5μm and 8–12μm infrared applications. This research has led to a greater understanding of the physical properties of these materials, and the present paper concentrates on the optical properties and applications of these bulk chalcogenide glasses. The various factors influencing the intrinsic and extrinsic optical loss mechanisms in materials are discussed, numerical data on the basic optical properties of chalcogenide glasses is presented and applications are discussed.     

High-speed transient currents in chalcogenide glasses

High-speed transient currents are often observed in chalcogenige glasses. We explain these effects by invoking the presence of Valence Alternation Pairs (VAPs). After an electric field is applied across the chalcogenide glass, carriers can tunnel directly from the electrodes onto the appropriately charged defect centers. This leads to the appearance of potential barriers near the contacts, resulting in a rapid decay of the current. Such a mechanism cannot occur in materials without a large concentration of negatively correlated defects.     

Dielectric relaxation study of chalcogenide glasses

The paper reports dielectric measurements carried out for a variety of threshold and memory alloys of glassy AsGeTe and SeGeTe at different temperatures (83 to 373 K) and various frequencies (0.2, 0.5, 1.0, 2.6 and 5.0 MHz). It is found that the glassy system of chalcogenides exists in the form of molecular dipoles which remain frozen at low temperatures and, as the temperature is increased, the molecules attain freedom of rotation at temperatures which are sometimes as low as 253 K. All the materials displayed dielectric dispersion in the radio frequency range. Gevers' formula has been used to calculate the dielectric loss (?′') and loss-angle (tan δ) from the measured values of the real part of dielectric constant (?′). The curves: log ?′ versus temperature, ?′ versus log ω, ?″ versus log ω, tan δ versus log ω and tan δ versus temperature, gave a direct evidence of the existence of a Debye-type relaxation having a wide distribution of relaxation times.Cole-Cole diagrams have been used to determine the distribution parameter (α) and the molecular relaxation time (τ). The temperature dependence of α and τ for all the alloys is consistent with the “molecular relaxation mechanism”. The paper also reports accurate values of the static and optical dielectric constants for all the alloys.Eyring's relaxation rate equations have been used to determine the free energy of activation (ΔF), and enthalpy of activation (ΔH) for all the alloys. These results indicate the existence of a stronger intermolecular interaction for SeGeTe alloys. mott's concept of “dangling bonds” has also been used to explain the existence of a stronger intermolecular interaction, and hence a greater density of defect states in case of SeGeTe as compared with AsGeTe alloys.It has been finally concluded that the dielectric behaviour of chalcogenide glasses, in general, can be successfully explained by using the theory of molecular relaxation.     

Electrical characterizations of silver chalcogenide glasses

We present a study of the electrical properties of silver chalcogenide glasses ‘40AgI’–30Ag₂S–30GeS₂, 45AgI–27.5Ag₂S–27.5GeS₂ and 50AgI–25Ag₂S–25GeS₂ in the 77–400 K temperature and the 20 Hz to 1 MHz frequency ranges. In our temperature range, a large variation of the real permittivity is observed, in relation with an electrodes polarization effect. As the amount of silver iodide increases in the Ag₂S–GeS₂ matrix, the glass transition temperature and the activation energies decrease, the electrical conductivity increases and reaches 4 Ω⁻¹ m⁻¹ at room temperature for the glass with 50% AgI. The study of the conductivity shows a behavior due to a high ionic conductivity, thermally activated with E_dc = 0.21 eV, E₁ = 0.075 eV (40AgI–30Ag₂S–30GeS₂, 45AgI–27.5Ag₂S–27.5GeS₂), E_dc = 0.17 eV, E₁ = 0.055 eV for 50AgI–25Ag₂S–25GeS₂. For these glasses, we have seen three conductivity regimes. The first two terms are thermally activated. The third term cannot be actually clearly identified because either it is thermally activated with a very low activation energy and frequency dependent, or it is almost non-thermally activated and frequency dependent.     

The structure of some chalcogenide glasses

A study using replica electron microscopy, scanning electron microscopy, electron diffraction, X-ray diffraction, sputter etching and differential thermal analysis of the structural properties of glasses having a range of compositions within the AsTeGeSi quaternary system has shown that phase separation generally occurs in the bulk material. The second phase is dispersed as inclusions in the non-crystalline matrix and is apparently non-crystalline away from the boundary of the glass forming region but is crystalline near to the high Te boundary. The crystalline second phase is probably Te. This quaternary system, AsTeGeSi is often used as source material for the fabrication of devices studied for their monostable electrical switching behaviour and the phase separation observed in these materials needs to be considered in the interpretation of the switching properties and for failure analysis of such devices.     

Low frequency Raman scattering in chalcogenide glasses

Low frequency Raman spectra of chalcogenide glasses are analyzed in terms of matrix element effects and modes of a layered structure. The spectra of GeSe₂ at low temperature shows no peaks which can be assigned to layer modes. The reduced spectra indicates that the density of states exhibits nearly ω² dependence for ω < 60 cm^?1, and the coupling constant approaches ω² dependence at frequencies less than 20 cm^?1.     

Photoluminescence dynamics in chalcogenide glasses and crystals

Time-resolved photoluminescence studies reveal distinct differences between the recombination processes in a chalcogenide glass and in its crystalline counterpart. Here the three luminescence bands of a-As₂S₃ are interpreted in terms of the recombination of an excition, a self-trapped exciton and a pair of electron- and hole-like small polarons. The two luminescence bands observed in the crystal are attributed to the recombination of two types of excitons composed respectively of a hole bound to a self-trapped electron, and a hole which is induced to self-trap in the presence of a self-trapped electron.     

Self refractive non-linearities in chalcogenide based glasses

We report third order non-linear absorption and refraction measurements at 1.20 and 1.52 μm on selected gallium-Lanthanum sulfide-based glasses (Ga:La:S) showing negligible non-linear absorption and a refractive non-linearity close to one hundred times that of SiO₂. Their potential use in telecommunication as base materials for all-optical switching practical devices is evaluated resulting in large figures of merit. The addition of a glass modifier to the Ga:La:S matrix has improved thermal and optical properties, resulting in ease of fibre drawing. The non-linear optical response of this new variant of the Ga:La:S family is studied.     

Multiphonon energy gap law in rare-earth doped chalcogenide glass

The parameters for multiphonon relaxation of rare earth (RE) ions in the sulfide glass Ge₂₅As_8.33Ga_1.67S₆₅ have been re-evaluated using the temperature dependence of the fluorescence lifetime to separate out true multiphonon decay from other non-radiative processes. It is found that for energy gaps to the next lowest level greater than 2500 cm⁻¹, other non-radiative processes become dominant over multiphonon decay. The most likely process for this additional non-radiative decay is energy transfer to vibrational impurities such as OH and SH. The newly derived parameters lead to an electron–phonon coupling parameter ε=0.058, which is more in line with other glass types than the previously accepted value of ε=0.36. These new multiphonon relaxation parameters and the existence of additional non-radiative decay mechanisms has implications for the modeling of chalcogenide-based active devices.     

Temperature dependence of dielectric losses in chalcogenide glasses

The frequency (2 × 10⁴?2 × 10⁷ Hz) and temperature (250–600 K) dependence of the imaginary part of the permittivity, ?′', of As₂Se₃ and Tl₂SnSe_8.61 is explained within a theoretical model. The dielectric premittivities of these compounds are correctly explained by means of a model of a hopping process over a potential barrier between localized sites.We have found that the glassy system of chalcogenide can exist in the form of dipoles. The theory shows, in agreement with the experimental results, that ?′' = Aω^m, where m is a function of temperature.     

Polynary silicon arsenic chalcogenide glasses with high softening temperatures

The introduction of Ag in SiAsTe glasses permits the incorporation of Se, otherwise volatile and/or degradable as a constituent in Si-containing chalcogenide glasses. SiAsAgTeSe glasses exhibit much higher softening ranges and glass transition temperatures than encountered in known chalgogenide systems. A glass Si₃₅As₁₅Ag₁₀Te₂₀Se₂₀ had the viscosity log ν = 13 at about 500°C, as compared to 370°C for the base glass Si₃₅As₂₅Te₄₀, the viscosity of log ν = 9.8 at about 560°C, as compared to 442°C for the base glass. Phase separation occurs in the system SiAsAgTeSe and becomes manifest in two glass transitions indicated by changes in the slopes of the expansion curves and breaks in the softening point-composition relations. The existence and behavior SiAsAgTeSe glasses suggests the possible development of higher T_g i.r. transparencies and higher T_g semiconductor glasses than described so far.     

Field effect in small filaments of chalcogenide glasses

A field effect has been measured in a semiconducting chalcogenide glass, of composition Te₂AsSi. The sample was in the form of a fine filament, encapsulated within a rod of insulating glass, with the bias field applied radially. The effect is small, and the change in conductivity increases more slowly with field at high fields than at low fields. The results are explained in terms of a model with a uniform density of bulk states near the Fermi energy and a sharp surface level just below the Fermi energy. There is a depletion layer at the surface at zero bias, hence the surface barrier with no external field varied with each sample.     

Scattering studies of photostructural changes in chalcogenide glasses

Structural changes which occur in chalcogenide glasses upon absorption of bandgap light have been investigated using neutron diffraction. The first part of this paper describes studies by small-angle neutron scattering of the irreversible photostructural changes which occur in obliquely evaporated amorphous chalcogenide films. The second part deals with the reversible photostructural effect observed in chalcogenide glasses, investigated by conventional neutron diffraction and EXAFS experiments. The various models which have been proposed for the photostructural effect are discussed in the light of these experimental results.