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.     

Electrical switching in As-Te-Ag system-composition and temperature dependence

The current-voltage characteristics related to switching phenomena in silver doped arsenic telluride glasses, As₂₀Te_80−xAg_x and As₄₀Te_60−xAg_x, have been investigated over a wide composition range (4?x?14). The samples are found to show threshold switching behavior with the number of switching cycles withstood by the samples depending on the ON state current. The switching voltages are found to decrease with increase in silver content and a sharp minimum is seen at the composition x=12 for the As₂₀Te_80−xAg_x glasses and x=11 for the As₄₀Te_60−xAg_x glasses. An effort has been made to understand the observed composition dependence on the basis of increase in the conductance of the samples with silver addition and local structural effects.     

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.     

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.     

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.     

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.     

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.     

Thermal conductivity and specific heat of bulk amorphous chalcogenides Ge20Te80−xSex (x = 0,1,2,8)

Bulk amorphous chalcogenide samples of Ge₂₀Te_80−xSe_x (x = 0, 1, 2, 8) have been prepared using a melting-quench method, and characterized by the differential scanning calorimetry, X-ray powder diffraction, high-resolution transmission electron microscopy, specific heat and thermal conductivity measurements. The low temperature specific heat measurements identified some localized low-frequency oscillation modes (Einstein modes) in conjunction with a Debye-like behavior. It was found that with increasing Se concentration the characteristic Debye temperature did not change whereas the Einstein temperature slightly decreased. The lattice thermal conductivity of all Ge₂₀Te_80−xSe_x samples exhibited typical amorphous heat conduction behavior, which has been discussed in connection with the phonon mean free path and in the context of a phenomenological model of heat conduction for highly disordered system.     

Non-isothermal crystallization kinetics study on new amorphous Ga20Sb5S75 and Ga20Sb40S40 chalcogenide glasses

Bulk glasses of the system Ga₂₀Sb_xS_80−x (x = 5 and 40) were prepared for the first time by the known melt quenching technique. Non-isothermal differential scanning calorimetric (DSC) measurements of as-quenched Ga₂₀Sb_xS_80−x (x = 5 and 40) chalcogenide glasses reveal that the characteristic temperatures e.g. the glass transition temperature (T_g), the temperature corresponding to the maximum crystallization rate (T_p) recorded in the temperature range 400-650 K for x = 5 and 480-660 K for x = 40 are strongly dependent on heating rate and Sb content. Upon heating, these glasses show a single glass transition temperature (T_g) and double crystallization temperatures (T_p1 and T_p2) for x = 5 which overlapped and appear as a single crystallization peak (T_p) for x = 40. The activation energies of crystallization E_c were evaluated by three different methods. The crystallization data were examined in terms of recent analysis developed for non-isothermal conditions. The crystalline phases resulting from (DSC) have been identified using X-ray diffraction.     

Nano- and micro-indentation studies on lithium borate-barium bismuth niobate glasses

The structure and mechanical properties of multifunctional lithium tetra-borate based glasses and glass-ceramics of the system (100 − x) Li₂B₄O₇₋x(BaO-Bi₂O₃-Nb₂O₅) with x = 10, 20 and 30 in molar ratio, have been characterized. Nano- and micro-indentation techniques were employed to evaluate the elastic modulus, hardness and toughness of the as-cast and annealed glasses. These were complemented with detailed structural investigations using X-ray diffraction, microscopy (optical, scanning electron and high resolution transmission electron microscopies) and nuclear magnetic resonance spectroscopy. These investigations reveal a smooth variation of the mechanical properties with composition except for the composition corresponding to x = 20. This deviation has been attributed to subtle changes in the glass-structure due to amorphous phase-separation and heat-treatment-assisted nano-crystallization.     

Ge and Ga K-edge EXAFS analyses on the structure of Ge-Ga-S-CsBr glasses

The local structure of Ge and Ga ions in (1 − x)(Ge_0.25Ga_0.10S_0.65)-xCsBr glasses (x = 0.00, 0.05, 0.10 and 0.12) were investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. CsBr formed [GaS_3/2Br]⁻ structural units in glass while Ge ions remained in GeS_4/2 tetrahedra, unaffected by CsBr addition. Rare-earth ions can be surrounded by Br ions only when CsBr/Ga ratio in glass became larger than unity.     

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.     

Spectroscopic properties of Er-doped xGeO2-(80 − x)TeO2-10ZnO-10BaO glasses

Erbium-doped glasses with composition xGeO₂-(80 − x)TeO₂-10ZnO-10BaO were prepared by melt-quenching technique. The phonon sideband spectra and the optical absorption band edges for the host matrix were confirmed by means of the spectral measurements. Standard Judd-Ofelt calculations have been completed to these glasses. The dependence of up-conversion and infrared emission under 980 nm excitation on the glass composition was studied. The quantum efficiencies for the ⁴I_13/2 → ⁴I_15/2 transition of trivalent erbium in the glasses were estimated.     

Electrical and optical properties of the AsTeIn and GeSeIn chalcogenide systems

The influence of indium on the optical and properties of As_2−xTe_3−xxIn_2x, As_20−xTe_80−xIn_2x and Ge_20−xSe_80−xIn_2x is described. In Te-containing glasses the Fermi level is shifted by 0.05 eV and in Se-containing glasses by 0.2 eV towards the valence band.     

Metamorphic growth of tensile strained GaInP on GaAs substrate

Optical and structural properties of tensile strained graded Ga_xIn_1−xP buffers grown on GaAs substrate have been studied by photoluminescence, X-ray diffraction, atomic force microscopy, and scanning electron microscopy measurements. The Ga composition in the graded buffer layers was varied from x=0.51 (lattice matched to GaAs) to x=0.66 (1% lattice mismatch to GaAs). The optimal growth temperature for the graded buffer layer was found to be about 80–100 °C lower than that for the lattice matched GaInP growth. The photoluminescence intensity and surface smoothness of the Ga_0.66In_0.34P layer grown on top of the graded buffer were strongly enhanced by temperature optimization. The relaxation of tensile GaInP was found to be highly anisotropic. A 1.5 μm thick graded buffer led to a 92% average relaxation and a room temperature photoluminescence peak wavelength of 596 nm.     

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.     

Low-temperature/high-temperature AlN superlattice buffer layers for high-quality AlxGa1−xN on Si (1 1 1)

We present MOVPE-grown, high-quality Al_xGa_1−x N layers with Al content up to x=0.65 on Si (1 1 1) substrates. Crack-free layers with smooth surface and low defect density are obtained with optimized AlN-based seeding and buffer layers. High-temperature AlN seeding layers and (low temperature (LT)/high temperature (HT)) AlN-based superlattices (SLs) as buffer layers are efficient in reducing the dislocation density and in-plane residual strain. The crystalline quality of Al_xGa_1−xN was characterized by high-resolution X-ray diffraction (XRD). With optimized AlN-based seeding and SL buffer layers, best ω-FWHMs of the (0 0 0 2) reflection of 540 and 1400 arcsec for the (1 0 1¯ 0) reflection were achieved for a ∼1-μm-thick Al_0.1Ga_0.9N layer and 1010 and 1560 arcsec for the (0 0 0 2) and (1 0 1¯ 0) reflection of a ∼500-nm-thick Al_0.65Ga_0.35N layer. AFM and FE-SEM measurements were used to study the surface morphology and TEM cross-section measurements to determine the dislocation behaviour. With a high crystalline quality and good optical properties, Al_xGa_1−x N layers can be applied to grow electronic and optoelectronic device structures on silicon substrates in further investigations.     

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.     

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.     

Post-irradiation relaxation in vitreous arsenic/antimony trisulphides

Instability caused by high-energy γ-irradiation was studied in (As₂S₃)_1−x(Sb₂S₃)_x glasses (x = 0, 0.1, 0.2 and 0.3) using positron annihilation lifetime spectroscopy, the obtained results being treated within a two-state trapping model. The observed decrease in the positron trapping rate of the glasses tested just after irradiation was explained due to renovation of destroyed covalent chemical bonds. This process was governed by monomolecular relaxation kinetics agreed well with corresponding changes in fundamental optical absorption edge.