Thermal diffusivity measurements on As-Te-Ga glasses by photo-thermal deflection technique: Composition dependence and topological thresholds

The thermal diffusivity (α) of As₂₀Te_80−xGa_x glasses (7.5 ? x ? 18.5) has been measured using photo-thermal deflection (PTD) technique. It is found that the thermal diffusivity is comparatively lower for As₂₀Te_80−xGa_x glasses, which is consistent with the memory type of electrical switching exhibited by these samples. Further, the thermal diffusivity of As₂₀Te_80−xGa_x glasses is found to increase with the incorporation of gallium initially (for x ? 9), which is consistent with the metallicity of the additive. This increase in α results in a maximum at the composition x = 9; beyond x = 9, a decrease is seen in α leading to a minimum at the composition x = 15. The observed composition dependence of thermal diffusivity of As₂₀Te_80−xGa_x glasses has been found to be similar to that of Al₂₀As_xTe_100−x glasses, based on which it is proposed that As₂₀Te_80−xGa_x glasses exhibit an extended stiffness transition with compositions x = 9 and x = 15 being its onset and completion, respectively. Also, the composition x = 17.5 at which a second maximum is seen in the thermal diffusivity has been identified to be the chemical threshold (CT) of the As₂₀Te_80−xGa_x glassy system, as at CT, the glass is configurationally closest to the crystalline state and the scattering of the diffusing thermal waves is minimal for the chemically ordered phase.     

Composition dependence of photoconductivity of Al20AsxTe80−x glasses

The time dependent photocurrent of Al₂₀As_xTe_80−x glasses has been studied at low temperatures. It is found that the photocurrent of all the Al₂₀As_xTe_80−x samples studied does not decrease appreciably during illumination, which is consistent with the behavior of other narrow band gap amorphous chalcogenides. Further, the photosensitivity is found to be maximized for the composition x=25, which can be associated with rigidity percolation.     

Network topological thresholds in gallium doped As–Te glasses – Electrical and thermal investigations

Electrical switching and differential scanning calorimetric studies are undertaken on bulk As₂₀Te_80−xGa_x glasses, to elucidate the network topological thresholds. It is found that these glasses exhibit a single glass transition (T_g) and two crystallization reactions (T_c1 & T_c2) upon heating. It is also found that there is only a marginal change in T_g with the addition of up to about 10% of Ga; around this composition an increase is seen in T_g which culminates in a local maximum around x = 15. The decrease exhibited in T_g beyond this composition, leads to a local minimum at x = 17.5. Further, the As₂₀Te_80−xGa_x glasses are found to exhibit memory type electrical switching. The switching voltages (V_T) increase with the increase in gallium content and a local maximum is seen in V_T around x = 15. V_T is found to decrease with x thereafter, exhibiting a local minimum around x = 17.5. The composition dependence of T_c1 is found to be very similar to that of V_T of As₂₀Te_80−xGa_x glasses. Based on the present results, it is proposed that the composition x = 15 and x = 17.5 correspond to the rigidity percolation and chemical thresholds, respectively, of As₂₀Te_80−xGa_x glasses.     

Atomic structure and chemical order in binary Ge–Te and As–Te glasses: A Te K-edge X-ray absorption fine structure spectroscopic study

The nearest-neighbor coordination environments of Te atoms in Ge_xTe_100?x glasses with x = 15 and 20 and in As_xTe_100?x glasses with 40 ? x ? 65 have been studied with Te K-edge EXAFS spectroscopy. The average coordination number of Te atoms in all glasses is found to be ～2.0 and no violation of the 8-N rule is observed. The compositional makeup of the first coordination shell of Te atoms indicates that chemical order is largely preserved in both glass-forming binary systems. Sudden changes in the Te coordination environment and violation of chemical order are observed at the stoichiometric As₄₀Te₆₀ glass implying formation of a constrained network. The compositional dependence of the physical properties in both systems can be correlated to short-range chemical order.     

The presence of a thermally reversing window in Al-Te-Si glasses revealed by alternating differential scanning calorimetry and electrical switching studies

Alternating differential scanning calorimetric (ADSC) studies and electrical switching experiments have been undertaken on Al₁₅Te_85−xSi_x (2 ? x ? 12) system of glasses. These glasses are found to exhibit two crystallization reactions (T_c1 and T_c2), for compositions with x < 8. Above x = 8, a single-stage crystallization is seen. Further, a trough is seen in the composition dependence of non-reversing enthalpy (ΔH^NR), based on which it is proposed that there is a thermally reversing window in Al₁₅Te_85−xSi_x glasses, in the composition range 4 ? x ? 8. Electrical switching studies indicate that Al₁₅Te_85−xSi_x glasses exhibit a threshold type electrical switching at ON state currents less than 2 mA. Further, the switching voltages are found to increase with the increase in silicon content. It is interesting to note that the start (x = 4) and the end (x = 8) of the thermally reversing window are exemplified by a kink and a saturation in the composition dependence of switching voltages, respectively.     

Mechanical damping of silver-containing SiAsChalcogenide glasses

Mechanical damping measurements were used to study the structural conditions responsible for the increase in thermal stability that has been observed when Ag and Se are substituted simultaneously in Si₃₅As_25?xAg_xTe_40?ySe_y glasses. Glasses containing Ag exhibited a mechanical damping peak whose magnitude was approximately proportional to the Ag content and which was absent in the more completely cross-linked Si₃₅As₂₅Te₄₀ base glass. This peak shifted to lower temperatures and split into two overlapping peaks with increasing Se content. The splitting of the peak has been tentatively attributed to phase separation which has been postulated to occur at higher Se contents.     

A sharp thermally reversing window in As45Te55−xIx chalcohalide glasses revealed by alternating differential scanning calorimetry and photo thermal deflection studies

The composition dependence of different thermal parameters such as glass transition temperature, non-reversing enthalpy, thermal diffusivity etc., of bulk As₄₅Te_55−xI_x chalcohalide glasses (3 ? x ? 10), has been evaluated using the temperature modulated Alternating Differential Scanning Calorimetry (ADSC) and Photo Thermal Deflection (PTD) studies. It is found that there is not much variation in the glass transition temperature of As₄₅Te_55−xI_x glasses, even though there is a wide variation in the average coordination number . This observation has been understood on the basis that the variation in glass transition temperature of network glasses is dictated by the variation in average bond energy rather than . Further, it is found that both the non-reversing enthalpy (ΔH_nr) and the thermal diffusivity (α) exhibit a sharp minimum at a composition x = 6. A broad hump is also seen in glass transition and crystallization temperatures in the composition range 5 ? x ? 7. The results obtained clearly indicate a sharp thermally reversing window in As₄₅Te_55−xI_x chalcohalide glasses around the composition x = 6.     

High field electrical switching behavior of Ge10Se90−xTlx glasses

Electrical switching studies on bulk Ge₁₀Se_90−xTl_x (15 ? x ? 34) glasses have been undertaken to examine the type of switching, composition and thickness dependence of switching voltages. Unlike Ge-Se-Tl thin films which exhibit memory switching, the bulk Ge₁₀Se_90−xTl_x glasses are found to exhibit threshold type switching with fluctuations seen in their current-voltage (I-V) characteristics. Further, it is observed that the switching voltages (V_T) of Ge₁₀Se_90−xTl_x glasses decrease with the increase in the Tl concentration. An effort has been made to understand the observed composition dependence on the basis of nature of bonding of Tl atoms and a decrease in the chemical disorder with composition. In addition, the network connectivity and metallicity factors also contribute for the observed decrease in the switching voltages of Ge₁₀Se_90−xTl_x glasses with Tl addition. It is also interesting to note that the composition dependence of switching voltages of Ge₁₀Se_90−xTl_x glasses exhibit a small cusp around the composition x = 22, which is understood on the basis of a thermally reversing window in this system in the composition range 22 ? x ? 30. The thickness dependence of switching voltages has been found to provide an insight about the type of switching mechanism involved in these samples.     

Crystallization and local order of bulk AsxTe1−x glasses

The crystallization and the local order of the bulk As_xTe_1?x (0.2 ? × ? 0.8) glasses have been investigated by DSC, X-ray and Mössbauer methods. During the crystallization a metastable phase has been observed between 0.35 ? × ? 0.5 and it was identified as fcc AsTe. The local surrounding of these As_xTe_1?x glasses characterized by the ¹²⁵Te Mössbauer measurements were compared with those of the stable monoclinic As₂Te₃, hexagonal Te and metastable fcc AsTe.     

Evaluation of basic physical parameters of quaternary Ge–Sb-(S,Te) chalcogenide glasses

New quaternary chalcogenide Ge_xSb_40?xS₅₀Te₁₀ (x = 10, 20 and 27 at.%) and Ge_xSb_40?xS₅₅Te₅ (x = 20 and 27 at.%) glasses have been synthesized and the compositions have been characterized applying prompt gamma-ray activation analyses, neutron diffraction, and material density measurements. Using the experimental data, the basic physical parameters, such as average atomic volume, packing density, compactness, average coordination number, number of constrains, average heat of atomization and cohesive energy, of the synthesized glasses are evaluated and the results are discussed in a function of glass composition.     

Signatures of an extended rigidity percolation in the photo-degradation behavior and the composition dependence of photo-response of Ge-Te-In glasses

Time dependent photocurrent measurements have been undertaken on bulk Ge₁₅Te_85−xIn_x (1 ? x ? 11) series of glasses. It is found that samples with x < 3 do not exhibit any photo-degradation whereas a decrease in photo-conductivity under illumination is observed in samples with x ? 3. Further, the photosensitivity of Ge₁₅Te_85−xIn_x glasses is found to reveal specific signatures at compositions x = 3 and 7. The observed composition dependent photo-degradation behavior and photo-response of these glasses have been understood on the basis of an extended rigidity percolation and its influence on network related properties.     

Electron spin resonance of Mn2+ IN As–Se and As–Te glasses

The electron spin resonance of Mn has been studied in As_xSe_100?x with 0 ? x ? 70 and As_xTe_100?x with 40 ? x ? 70. All samples, except those with x < 20 in As_xSe_100?x, exhibit six hyperfine lines centered at g = 4.3. A g = 2.0 line is observed in As–Se with largely scattered linewidth by samples, but not in As–Te unless oxygen contamination is included in the samples. The g = 4.3 line in As–Se is closely related to a formation of As₂Se₃-type layer structure and interpreted as being caused by Mn situated at the interlayer position and surrounded by four Se atoms in an arrangement of rhombic symmetry. In As–Te, a similar model by four Te atoms is valid in composition near As₂Te₃, but the surrounding Te is replaced by As as As content increases. The g = 2.0 line is concluded to come from phase-separated antiferromagnetic particles of Mn–O and MnSe. The linewidth is scattere by differences in the relative amounts of the two kinds of particles and in particle size.     

Infrared studies of Se-based polynary chalcogenide glasses (II): YxZxSe100−2x (Y = Ge,As; Z = As,Te)

The infrared (IR) absorption spectra for Y_xZ_xSe_100?2x glasses (Y = Ge, As;Z = As, Te), x = 2.5 and 5.0 are measured in the wavenumber region 700-60 cm^?1 at room temperature. These IR spectra are explained by comparing with the IR spectra already reported for the binary glasses such as Ge–Se, As–Se and Se–Te. In Ge_xAs_xSe_100-2x glasses (x ? 5.0), the main spectral features as well explained by both the spectra of Ge_xSe_100?x and As_xSe_100?x glasses. Main structural units in these glasses are considered to be GeSe₄ tetrahedra and AsSe₃ pyramids, and Se₈ rings and Se_n chains which are the units in pure glassy Se. In Ge_xTe_xSe_100?2x glasses (x ? 5.0) and IR band which cannot be explained by either the spectra of Ge_xSe_100?x or Se_100?xTe_x glasses appears at 210 cm^?1. This band is considered to be due to Ge–Te bonds. The IR spectra of As_xTe_x Se_100?2x glasses (x ? 5.0) are well explained by both the spectra of As_xSe_100?x and Se_100?xTe_x glasses. It is concluded that As and Te atoms combine with Se atoms in the forms of AsE₃ pyramids and Se₅Te₃ mixed rings, respectively.     

Optical properties of thin films of system (As2Se3)80−x(As2Te3)x(SnTe)20 prepared by PLD

Thin amorphous films from system (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ were prepared by pulsed laser deposition (PLD) from their bulk glasses and their optical properties were studied by spectral ellipsometry. Spectral dependencies of refractive index, absorption and extinction coefficient and optical gap (1.41–1.66 eV for (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ with x = 20 resp. x = 0) were calculated from optical tansmittance, from ellipsometric data by Tauc method. High values of refractive index n₀ (2.49–2.60) and of non-linear χ⁽³⁾ coefficient of index of refraction (4.9–7.5 × 10⁻¹² esu for the glass (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ with x = 0 resp. x = 20) made studied thin films of system (As₂Se₃)_80−x(As₂Te₃)_x(SnTe)₂₀ promising candidates for application in optics and optoelectronics.     

Electrical switching studies on Ge-Te-Tl chalcogenide glasses: Effect of thallium on the composition dependence of switching voltages

Bulk Ge₁₇Te_83−xTl_x glasses (0 ≤ x ≤ 13), have been found to exhibit memory type electrical switching. The switching voltages (also known as threshold voltage — V_th) of Ge₁₇Te_83−xTl_x glasses are found to decrease with increasing thallium content. The rate of decrease of V_th is greater at lower concentrations and V_th falls at a slower rate for higher thallium concentrations (x ≥ 6).The addition of thallium to the Ge-Te network fragments the covalent network and introduces ionic nature to it; the reduction in network connectivity leads to the decrease in switching voltages with thallium content. The decrease in the glass transition temperatures of Ge₁₇Te_83−xTl_x glasses with increasing thallium concentration supports the idea of decrease in network connectivity with Tl addition. The more metallic nature of Tl also contributes to the observed reduction in the switching voltages of Ge₁₇Te_83−xTl_x glasses with Tl content.Further, there is an interesting correlation seen between the threshold voltage V_th and the average bond energy, as a function of Tl content. In addition, the switching voltages of Ge₁₇Te_83−xTl_x glasses have been found to decrease with sample thickness almost linearly. The set-reset studies indicate that the Ge₁₇Te₈₁Tl₂ sample can be switched for more than 10 cycles, whereas other glasses could not be reset beyond two switching cycles.     

Infrared studies of Se-based polynary chalcogenide glasses (III): YxZxSxSe100−3x (Y = Ge,As; Z = As,Te)

The infrared (IR) absorption spectra for Y_xZ_xS_xSe_100?3x glasses (Y = Ge, As; Z = As, te), with x = 2.5 and 5.0, are measured in the wavenumber region 700-60 cm^?1 at room temperature. These IR spectra are qualitatively explained by comparing with the IR spectra of the binary and ternary glasses. In Ge_xAs_xS_xSe_100?3x glasses (x ? 5.0), the main spectral features are explained by both spectra of the two ternary glasses Ge_xS_xSe_100?2x and As_xS_xSe_100?2x. In Ge_xS_xTe_xSe_100?3x glasses (x ? 5.0), the main spectral features are well explained by both spectra of the two ternary glasses Ge_xS_xSe_100?2x and Ge_xTe_xSe_100?2x. On the other hand, main spectral features in As_xS_xTe_xSe_100?3x glasses (x ? 5.0) are well explained by both spectra of the ternary glasses As_xS_xSe_100?2x and the binary glasses Se_100?xTe_x. In these glasses with low concentrations (x ? 5.0) some chemical bonds are confirmed and some structural units estimated.     

Effect of antimony addition on the thermal and electrical-switching behavior of bulk Se-Te glasses

The thermal properties and electrical-switching behavior of semiconducting chalcogenide Sb_xSe_55−xTe₄₅ (2 ? x ? 9) glasses have been investigated by alternating differential scanning calorimetry and electrical-switching experiments, respectively. The addition of Sb is found to enhance the glass forming tendency and stability as revealed by the decrease in non-reversing enthalpy ΔH_nr, and an increase in the glass-transition width ΔT_g. Further, the glass-transition temperature of Sb_xSe_55−xTe₄₅ glasses, which is a measure of network connectivity, exhibits a subtle increase, suggesting a meager network growth with the addition of Sb. The crystallization temperature is also observed to increase with Sb content. The Sb_xSe_55−xTe₄₅ glasses (2 ? x ? 9) are found to exhibit memory type of electrical switching, which can be attributed to the polymeric nature of network and high devitrifying ability. The metallicity factor has been found to dominate over the network connectivity and rigidity in the compositional dependence of switching voltage, which shows a profound decrease with the addition of Sb.     

Studies on the AsSbSe glass system

Results are reported of measurements of the glass transition temperature (T_g), conductivity and density (d) for glasses of the AsSbSe system, with compositions (As, Sb)₄₀Se₆₀ and As_xSb₁₅Se_85?x. The results are compared with glasses of similar compositions of the As_xSe_100?x about compositions in the Ge_xSb₁₅Se_85?x and As_xSe_100?x glasses, in the case of As_xSb₁₅Se_85?x glasses, the As-rich compositions exhibit higher values of T_g and d compared to the stoichiometric composition As₂₅Sb₁₅Se₆₀. These results are discussed in the light of a chemically ordered structural arrangement in these glasses.     

Electrical transport in n-type semiconducting Ge120BixSe70−xTe10 glasses

Resistivity and thermoelectric power were measured as a function of temperature and composition for Ge₂₀Bi_xSe_70?xTe₁₀ glasses (x = 0–11). The results were compared with the case of of Ge₂₀Bi_xSe_80?x glasses to see on the electrical properties the influence of the substitution of Te for a part of Se. The glasses show n-type conduction for x ? 9, which was not affected by the substitution of Te. The resistivity was about three orders of magnitude lower for the glasses with x < 10, and remained almost the same for x ? 10, compared with the glasses not containing Te. From the composition dependence of the calculated concentration of covalent bonds in the glasses, it was proposed that the appearance of n-type conduction was closely related to the formation of a sufficient number of BiSe bonds and the disappearance of the bonds between two chalcogen atoms such as TeSe or SeSe bonds, and that the remarkably low resistivity in the present glasses with x < 10 was likely to be attributed to the formation of TeSe bonds.     

Role of additive (Zn) incorporation on the glass/crystal thermodynamics and stability of Se80 − xTe20Znx (x = 2, 4, 6, 8 and 10) glassy alloys

Different thermodynamic parameters (quantities) of Se_80 − xTe₂₀Zn_x (x = 2, 4, 6, 8 and 10) glasses have been obtained from the phase transformation studies using Differential Scanning Calorimetry (DSC) under non-isothermal condition at five different heating rates (10-50 K/min). Specific heat measurements have been made to see the effect of Zn additive in Se-Te-Zn glasses which have further been utilized to evaluate various thermodynamic quantities such as entropy difference (ΔS), enthalpy difference (ΔH) and Gibbs free energy difference (ΔG) between the undercooled melt and the corresponding equilibrium solid phases as a function of temperature. The data obtained from different thermodynamic quantities have been used to determine the stability of these glasses. It has been found that the stability of the samples increases with the increase of Zinc (Zn) content in the investigated series of the glassy alloys.