Calorimetric study of characteristic temperatures and crystallization behavior in Ge-As-Se-Te glass system

Results of differential scanning calorimetry of high purity Ge_xAs_40−xSe₄₀Te₂₀ (x=0-40) chalcogenide glasses are reported. The glass transition temperatures and crystallization behavior were studied under non-isothermal conditions at different heating rates (2.5-35 K/min). The glass transition temperature changes from 140 °C up to 320 °C with increasing the Ge content in Ge_xAs_40−xSe₄₀Te₂₀ glass. The studied glasses with x≤35 have no exothermal peaks of crystallization, indicating their high glass-forming ability. The glass of Ge₄₀Se₄₀Te₂₀ composition has one-stage glass transition and double-stage crystallization process during phase change. The activation energy of the glass transition (E_g), the activation energy of crystallization (E_c), the Avrami exponent (n), the frequency factor (K₀) and the crystallization criteria of Ge₄₀Se₄₀Te₂₀ glass were determined.     

PAL spectroscopy of rare-earth doped Ga–Ge–Te/Se glasses

Positron annihilation lifetime (PAL) spectroscopy was applied for the first time to study free-volume void evolution in chalcogenide glasses of Ga–Ge–Te/Se cut-section exemplified by glassy Ga₁₀Ge₁₅Te₇₅ and Ga₁₀Ge₁₅Te₇₂Se₃ doped with 500 ppm of Tb³⁺ or Pr³⁺. The collected PAL spectra reconstructed within two-state trapping model reveal decaying tendency in positron trapping efficiency in these glasses under rare-earth doping. This effect results in unchanged or slightly increased defect-related lifetimes τ₂ at the cost of more strong decrease in I₂ intensities, as well as reduced positron trapping rate in defects and fraction of trapped positrons. Observed changes are ascribed to rare-earth activated elimination of intrinsic free volumes associated mainly with negatively-charged states of chalcogen atoms especially those neighboring with Ga-based polyhedrons.     

Crystallization kinetics in new Sb14As29Se52Te5 amorphous glass

Differential scanning calorimetry (DSC) under non-isothermal conditions is used to study the crystallization kinetics of Sb₁₄As₂₉Se₅₂Te₅ chalcogenide glass. In addition, two approaches are used to analyze the dependence of glass transition temperature (T_g) on the heating rate (α). One is empirical linear relationship between (T_g) and ln(α). The second approach is the use of straight line vs. 1/T_g for the evaluation of the activation energy for glass transition. The phases at which the alloy crystallizes after the thermal treatment have been identified by using X-ray diffraction. The diffractogram of the transformed material shows the presence of some crystallites of As, Te, AsSb, As₂Se₃, Sb₂Se₃ and AsSe_.5Te_.5 in the residual amorphous matrix.     

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.     

Positron trapping defects in free-volume investigation of Ge–Ga–S–CsCl glasses

Evolution of free-volume positron trapping defects caused by crystallization process in (80GeS₂–20Ga₂S₃)_100−х(СsCl)_x, 0 ≤ x ≤ 15 chalcogenide-chalcohalide glasses was studied by positron annihilation lifetime technique. It is established that CsCl additives in Ge–Ga–S glassy matrix transform defect-related component spectra, indicating that the agglomeration of free-volume voids occurs in initial and crystallized (80GeS₂–20Ga₂S₃)_100−х(СsCl)_x, 0 ≤ x ≤ 10 glasses. Void fragmentation in (80GeS₂–20Ga₂S₃)₈₅(СsCl)₁₅ glass can be associated with loosing of their inner structure. Full crystallization in each of these glasses corresponds to the formation of defect-related voids. These trends are confirmed by positron-positronium decomposition algorithm. It is shown, that CsCl additives result in white shift in the visible regions in transmission spectra. The γ-irradiation of 80GeS₂–20Ga₂S₃ base glass leads to slight long-wavelength shift of the fundamental optical absorption edge and decreasing of transmission speaks in favor of possible formation of additional defects in glasses and their darkening.     

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.     

Initial stage of physical ageing in network glasses

An atomistic view on Johari–Goldstein secondary β-relaxation processes responsible for structural relaxation far below the glass transition temperature (T_g ) in network glasses is developed for the archetypal chalcogenide glass, As₂₀Se₈₀, using positron annihilation lifetime, differential scanning calorimetry, Raman scattering and nuclear magnetic resonance techniques. Increased density fluctuations are shown to be responsible for the initial stage of physical ageing in these materials at the temperatures below T_g . They are correlated with changes in thermodynamic parameters of structural relaxation through the glass-to-supercooled liquid transition interval. General shrinkage, occurred during the next stage of physical ageing, is shown to be determined by the ability of system to release these redundant open volumes from the glass bulk through the densification process of glass network.     

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.     

Studying the crystallization behavior of the Se85S10Sb5 chalcogenide semiconducting glass by DSC and X-ray diffraction

Crystallization kinetics of the Se₈₅S₁₀Sb₅ chalcogenide glassy alloy is studied by differential scanning calorimeter (DSC) non-isothermally. The glassy state of the as-prepared sample and the crystalline phases of the heat treated sample are characterized using X-ray diffraction. The glass transition activation energy E_g is found to be 65.2±0.8 kJ/mol and the crystallization activation energies for the first and the second crystallization peaks (E_c1 and E_c2) are found to be 70±0.8 and 85.2±0.8 kJ/mol, respectively. The determined kinetic parameters have made it possible to postulate the type of crystal growth exhibited in the crystallization process. The phases at which the alloy crystallizes after the thermal process have been identified by X-ray diffraction. The diffractogram of the transformed material indicates the presence of nanocrystallites of Sb₂Se₃, Se-S and Se, with a remaining additional amorphous matrix.     

The Cu and Te coordination environments in Cu-doped Ge–Te glasses

The structure of Ge₂₀Te₈₀, Ge₁₅Cu₈Te₇₇ and Ge₁₅Cu₅Te₈₀ glasses was investigated by diffraction techniques and extended X-ray absorption fine structure measurements. Large structural models were generated by fitting experimental data by the reverse Monte Carlo simulation technique. In Ge₂₀Te₈₀ glass, both Ge and Te obey the 8−N rule, and the structure is built up of GeTe₄ tetrahedra connected via Te–Te bonding or shared Te atoms connected to two Ge atoms. The coordination number of Te is significantly higher than 2 in Ge₁₅Cu₈Te₇₇. The average coordination number of Cu is 3.41±1 in this alloy. In Ge₁₅Cu₅Te₈₀ glass, Cu binds mostly to Te, while Cu–Cu bonding is significant in Ge₁₅Cu₈Te₇₇.     

Reversible and athermal photo-vitrification of As50Se50 thin films deposited onto silicon wafer and glass substrates

Photo-vitrification of As₅₀Se₅₀ thin films deposited onto silicon wafer and glass substrates has been studied using X-ray diffraction, far-infrared, and differential infrared spectroscopies. The optical study of this photo-amorphization effect has been carried out by two different methods enabling the determination of the average thickness and refractive index of a wedge-shaped thin film. The refractive-index behaviour of the as-evaporated, crystallized, and photo-vitrified As₅₀Se₅₀ films is analyzed within the single-oscillator approach. The optical-absorption edge is described using the non-direct transition model, and the optical energy gap is calculated. In the course of the vitrification of an As₅₀Se₅₀ thin film deposited on a silicon substrate the photo-oxidation of the film has been additionally detected and arsenic trioxide micro-crystals were formed on the surface of the film. Such oxidation has not been observed with As₅₀Se₅₀ films deposited on glass substrates, which demonstrates that the photo-vitrification phenomenon depends also on the type of substrate. Finally, it is concluded from the optical study that a reversible photo-darkening effect accompanies the photo-induced vitrification phenomenon. Received: 3 August 1998 / Accepted: 13 January 1999 / Published online: 7 April 1999     

The effect of addition of gallium on the thermal stability and crystallization kinetic parameters of GaxSe100−x glass system

Differential scanning calorimetry (DSC) technique was used to study the kinetics of amorphous to crystalline transformation for Ga_xSe_100−x glass system (x=0, 2.5 and 5 at%). The kinetic parameters of Ga_xSe_100−x glass system under non-isothermal conditions are analyzed by the model-free and model-fitting models at different constant heating rates (5-50 K/min). A strong heating rate dependence of the effective activation energy of crystallization was observed. The analysis of the present data shows that the effective activation energy of crystallization is not constant but varies with the degree of crystallization and with temperature as well. The crystallization mechanisms examined using the local Avrami exponents indicate that one mechanism (volume nucleation with one-dimensional growth) is responsible for the crystallization process for heating rates 5-50 K/min for Se glass and two mechanisms (volume nucleation with two- and one-dimensional growth) are working simultaneously during the amorphous-crystalline transformation of the Ga_2.5Se_97.5 and Ga₅Se₉₅ glasses (5-50 K/min). The reaction model that may describe crystallization process of all the compositions of Ga_xSe_100−x glass system is Avrami-Erofeev model (g(α)=[−ln(1−α)]^1/n) with n=2 for Se glass. While for Ga_2.5Se_97.5 and Ga₅Se₉₅ glasses, the values of n are equal to 3 and 2 for the heating rates 5-20 and 35-50 K/min, respectively. A good agreement between the experimental and the reconstructed (α-T) curves has been achieved. The transformation from amorphous to crystalline phase in Ga_xSe_100−x glass system demonstrates complex multi-step involving several processes.     

On the absence of photodarkening in pnictide amorphous semiconductors

Parallel photodarkening experiments have been performed at 77 K on two glassy chalcogenides (As₂Se₃ and As₂Se_1.5Te_1.5 and one amorphous pnictide (α-As). Thermally reversible photodarkening effects are observed in both chalcogenides, but no photodarkening is observed in α-As even though α-As and As₂Se_1.5Te_1.5 have very similar band gaps. The negative result in α-As confirms the importance of non-bonding valence band electrons in the photodarkening process.     

Magnetic resonance study of arsenic bonding sites in ternary chalcogenide glasses

⁷⁵As NQR and high-field NMR experiments have been performed on Ge_xAs_ySe_1−x−y glasses. Evolution of As bonding structure from arsenic sites with axially symmetric distribution of the electric field gradient (EFG) to highly asymmetric As surroundings has been revealed. Arsenic atoms form pyramidal structural units in Ge₂As₂Se₇ with no evidence of significant concentration of homopolar bonds. In Ge₂As₂Se₅ most of arsenic atoms form structural units with two As-As bonds per atom and asymmetric EFG distribution. Arsenic bondings become more complicated in Ge_0.33As_0.12Se_0.55 where all arsenic sites are highly distorted. The combination of NQR and NMR data provide valuable information on arsenic bonding dynamics in these glasses.     

Model fitted and model free approaches for crystallization kinetics in Se85Te15-xBix glasses

In this research work, we have described the model-fitted and model free approaches for the study of crystallization kinetics in Se₈₅Te_15-xBi_x chalcogenide glasses. Se₈₅Te_15-xBi_x bulk alloys were synthesized by melt quenching technique. High Resolution X- Ray diffraction (HRXRD) was used to confirm the amorphous nature of prepared alloys. Non-isothermal Differential Scanning Calorimetry (DSC) measurements were done at heating rates of 5, 10, 15, 20 and 25 K/min for crystallization kinetics studies in Se₈₅Te_15-xBi_x glasses. The various characteristic temperatures, such as glass transition (T_g), on-set crystallization (T_c) temperature, peak crystallization temperature (T_p) and melting temperatures (T_m) have been obtained from various DSC thermograms. The activation energies of glass transition (ΔE_t) were calculated by using Kissinger and Moynihan approaches and found to be minimum for Se₈₅Te₁₂Bi₃ chalcogenide glass which indicates that this alloy has maximum probability to jump into a less configurational energy state and has larger stability. The model-free approaches; Kissinger–Akahira–Sunose (KAS), Flynn-Wall-Ozawa (FWO), Tang and Straink (TS) reveal that the activation energy of crystallization varies with crystallization degree and temperature both. This variation shows that amorphous to crystalline phase transformation in Se₈₅Te_15-xBi_x chalcogenide glasses is a complex process with various nucleation and growth mechanisms.     

Free-volume defects investigation of GeS2-Ga2S3-CsI chalcogenide glasses by positron annihilation spectroscopy

The transformation behavior of free-volume defect in (80GeS₂-20Ga₂S₃)_100-x (CsI)_x (x = 0, 5, 10, 15 mol%) chalcogenide glasses was studied by employing positron annihilation spectroscopic technique, which could reveal valuable information for in-depth understanding of nano-structural defects in glassy matrix. The results indicate that the structural changes caused by CsI additives can be adequately described by positron trapping modes determined with two-state model. The initial addition of CsI (x = 5 mol%) led to a void contraction, whereas, the void agglomeration occurred with the increase of CsI and the free-volume defects of the glasses were obviously reduced. The atomic density ρ is inversely proportional to the number of these defects. Meanwhile, the UV cut-off edge shifts toward short-wavelength with increasing of CsI. This study provides the valuable information of defects evolution in GeS₂-Ga₂S₃-CsI glasses.     

Synthesis of homogeneous pentenary chalcopyrite alloys with a classical two-step growth process

Homogeneous single-phase Cu(In_0.75Ga_0.25)(Se_1−yS_y)₂ chalcopyrite alloys were prepared by a novel two-step growth process. CuIn_0.75Ga_0.25 precursors were deposited by DC magnetron sputtering and the subsequent reaction processes in a reactive H₂Se/Ar/H₂S atmosphere was optimized to prevent the formation and separation of stable ternary phases. X-ray diffraction (XRD) analysis of these films revealed characteristic chalcopyrite peaks with a high degree of symmetry, indicative of homogeneous rather than compositionally graded material. The lattice parameters of the single-phase Cu(In_0.75Ga_0.25)(Se_1−yS_y)₂ pentenary alloys decreased linearly with an increase in the S/(S+Se) ratio in accordance with Vegard's law. X-ray photoelectron spectroscopy (XPS) depth profiling confirmed the in-depth compositional uniformity of the pentenary alloys, prepared under optimized selenization/sulfurization conditions.     

Effect of replacement of Se by S on structural and physical properties of Ge–Sb–As–Se–S chalcogenide glasses

In the present study, the structural and opto-mechanical properties of Ge–Sb–As–Se–S chalcogenide glasses have been investigated. For this purpose, different bulk glasses of Ge₂₀Sb₅As₁₅Se_60?xS_x (0 ≤ x≤50) were prepared by conventional melt quenching technique in quartz ampoule and different characteristics of prepared glasses such as glass transition temperature, density, hardness, transmittance, optical band gap energy and refractive index were determined. The value of hardness and glass transition temperature of prepared glasses were found to increase with increasing the sulfur content as a result of formation of GeS₄ tetrahedral units and increasing the network connectivity and average bonding energy. The optical energy gap (according to Tauc’s relation), transmittance and refractive index of prepared glasses are in direct relation with sulfur content. In this study, the highest value of transmittance (about 70%) and lowest value of refractive index (2–2.3) was achieved in Ge₂₀Sb₅As₁₅Se₄₀S₂₀ and Ge₂₀Sb₅As₁₅Se₁₀S₅₀ glasses, respectively.     

Thermal kinetics and short range order parameters of Se80X20 (X = Te,Sb) binary glasses

Bulk Se₈₀Te₂₀ and Se₈₀Sb₂₀ glasses were prepared using the melt–quench technique. Differential scanning calorimetry (DSC) curves measured at different heating rates (5 K/min≤α≤50 K/min) and X-ray diffraction (XRD) are used to characterize the as-quenched specimens. Based on the obtained results, the activation energy of glass transition and the activation energy of crystallization (E _g, E _c) of the Se₈₀Te₂₀ glass are (137.5, 105.1 kJ/mol) higher than the corresponding values of the Se₈₀Sb₂₀ glass (106.8, 71.2 kJ/mol). An integer n value (n=2) of the Se₈₀Te₂₀ glass indicates that only one crystallization mechanism is occurring while a non-integer exponent (n=1.79) in the Se₈₀Sb₂₀ glass means that two mechanisms are working simultaneously during the amorphous–crystalline transformations. The total structure factor, S(K), indicates the presence of the short-range order (SRO) and the absence of the medium-range order (MRO) inside the as-quenched alloys. In an opposite way to the activation energies, the values of the first peak position and the total coordination number (r ₁, η ₁), obtained from a Gaussian fit of the radial distribution function, of the Se₈₀Te₂₀ glass are (2.42 nm, 1.99 atom) lower than the corresponding values (2.55 nm, 2.36 atom) of the Se₈₀Sb₂₀ specimens.     

Observation of thermally induced stress in amorphous semiconductors by i.r. crossed nicols

In thin slices of amorphous semiconductor Se₅₀Te₂₀As₂₀Ge₁₀, stress caused by switching was studied by crossed nicols method. The purely thermal effect due to the localized current flowing along the switching path is thought to be responsible for the stress by comparison with the experiment using focussed sun beam.