Glassy Se Research Articles

Observation of compensation effect for isothermal crystallization in glassy Se80−x Ge20Inx and Se78−x Ge22Bix alloys

Many activated phenomena, including solid state diffusion in crystals and polymers, dielectric relaxation, conduction and thermally stimulated processes in polymers, and electronic conduction in amorphous semiconductors obey the compensation law or Meyer-Neldel rule. In the present paper, we discuss the results of investigation into the compensation effect observed under isothermal crystallization in glassy Se70Te30−xSbx alloys. It is found that the relation between the pre-exponential factor Ko and activation energy of crystallization Ec follows the Meyer-Neldel rule for the alloys studied.

Investigation of the Meyer-Neldel rule for AC conduction in glassy Se100 − x Te x alloys

In many amorphous and liquid semiconductors and other class of materials, the Meyer-Neldel rule is observed in the dc conductivity, where the preexponential factor (σ0) is found to increase exponentially with the activation energy (ΔE). In the present paper, we report on the observation of the Meyer-Neldel rule in case of ac conductivity at high temperatures (300–350 K) in bulk samples of glassy Se100 − xTex (x = 10, 20, 30) alloys. In this temperature range, the approximate variation in ac conductivity with temperature is found to be exponential and the activation energy is found to vary with frequency. The observation of the Meyer-Neldel rule in the present study is explained in terms of the compensation effect in the relaxation time.

Collective dynamics of [formula omitted] glass investigated by inelastic X-ray scattering

Dynamic structure factors S(Q,ω) of As2S3 glass, which is one of the most stable chalcogenide glass-formers, were measured at room temperature in the Q range from 2.0 to ∼10nm−1 using high-resolution inelastic X-ray scattering. The obtained spectra clearly demonstrate the existence of longitudinal propagating modes. A positive dispersion is not clearly found in the low Q region unlike other glassy materials, such as glassy Se, SiO2, and a fragile metallic glass, which indicates a highly homogeneous nature of this glass in the microscopic sense of nm range.

Correlation between glass forming tendency and rate of crystallization in glassy Se100–xTex and Se100–xInx alloys

AbstractThe rate of crystallization in glassy Se100–x Tex (5 ≤ x ≤ 30) and Se100–x Inx (5 ≤ x ≤ 30) alloys has been determined by using the Arrhenius dependence of the rate constant of crystallization. For this, the pre‐exponential factor K0 of the rate constant K (T) in these alloys has been determined using isothermal and non‐isothermal methods. The glass forming tendency is determined using the Sun–Rawson–Minaev criterion. A correlation is observed between the glass forming tendency and the rate of crystallization in these alloys. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarization-dependent photoinduced mechanical deformations in covalent chalcogenide glasses

The optomechanical effect and photoinduced anisotropic deformation induced by illumination of linearly polarized light have been comparatively studied for glassy As2S3 and Se films. The two materials show qualitatively the same behaviors in the mechanical effect and the deformation, which suggests that these photoinduced phenomena are inherent to the covalent chalcogenide glass. But, the two phenomena show different dependences upon intensity, spectrum and exposure time of excitation light, which imply different underlying mechanisms.

A.C. conduction in glassy Se70Te30−xSbxalloys: observation of Meyer-Neldel rule

Temperature and frequency dependence of a.c. conductivity have been studied in glassy Se 70 Te 30− x Sb x (2 ≤ x ≤ 10). An agreement between experimental and theoretical results suggests that the a.c. conductivity behaviour of Se-Te-Sb system can be successfully explained by correlated barrier hopping (CBH) model. The observation of Meyer-Neldel rule in case of a.c. conductivity at high temperatures is also reported for glassy Se 70 Te 30− x Sb x (2 ≤ x ≤ 10) alloys.

Effect of Ag additive on the photoelectric and dielectric properties of Se–Te alloys

The present paper reports photoelectric and dielectric studies in glassy Se 80− x Te 20Ag x ( x=5, 10, 15) alloys. The results show anomalous behavior of transient photoconductivity in the present glassy system. However, temperature and frequency dependence of dielectric constant is normal as observed in other chalcogenide glasses. The results observed for transient photoconductivity are explained in terms of interaction between Ag + ions and photoexcited holes. The results observed for dielectric constant are explained in terms of dipolar type relaxation due to the formation of dipoles of Ag + ions and D − defects.

Observation of compensation effect for isothermal crystallization in glassy Se80−xGe20Inx and Se78−xGe22Bix alloys

Many activated phenomena, including solid state diffusion in crystals and polymers, dielectric relaxation, conduction and thermally stimulated processes in polymers, and electronic conduction in amorphous semiconductors, obey the compensation law or Meyer–Neldel rule. In the present paper, we report the compensation effect in the isothermal crystallization of glassy Se80−xGe20Inx (0 ⩽ x ⩽ 20) and Se78−xGe22Bix (0 ⩽ x ⩽ 10) alloys. We have observed the Meyer–Neldel rule between the preexponential factor K0 and the activation energy of crystallization Ec in the present case.

Chemical bond approach to activation energy of crystallization in some Se–Ge–M ( M = Bi, In) chalcogenide glasses

The crystallization kinetics of glassy Se 78− x Ge 22Bi x (0 ≤ x ≤ 10) and Se 80− x Ge 20In x (0 ≤ x ≤ 15) alloys has been studied by an isothermal method. For this purpose, conductivity measurements are done during isothermal annealing at various temperatures between the glass transition and crystallization temperatures. Avrami's equation is used to calculate the activation energy of crystallization ( E c). The composition dependence of E c in these alloys has been discussed using chemical bond approach.

Space charge limited conduction in bulk Se 100 − xSb x chalcogenide glasses

The present paper reports the measurements on space charge limited conduction in bulk glassy Se 100 − x Sb x ( x = 2, 4, 6, 8). I– V characteristics have been measured at various fixed temperatures. These characteristics show that, at low electric fields, an ohmic behaviour is observed. However, at high electric fields ( E ∼10 4 V/cm), the current becomes superohmic. At high fields (10 4 V/cm), current could be fitted to the theory of space charge limited conduction in case of uniform distribution of localized states in the mobility gap of these materials. Using the theory of SCLC for the uniform distribution of the traps, the density of localized states near Fermi level is also calculated. Density of localized states initially increases with Sb concentration till 6 at.% of Sb and then decreases with further increase of Sb. This decrease is explained in terms of average coordination number.

Kinetic parameters of crystallization in glassy Se100–xSbx alloys

AbstractDifferential scanning calorimetry (DSC) has been performed at different heating rates under non‐isothermal conditions to study the crystallization kinetics of glassy Se100–xSbx (2 ≤ x ≤ 10) alloys. The crystallization kinetics have been studied using four different methods of analysis of the non‐isothermal DSC data and various kinetic parameters (the activation energy of crystallization, order parameter, rate constant and frequency factor) have been determined. The difference Tc – Tg (indicator of thermal stability) has been determined from the DSC thermograms for each sample. The rate constant K (indicator of the rate of amorphous to crystalline phase transformation) has been evaluated using the method of Augis and Bennett. The increasing sequence of the rate constant in the glasses studied is opposite to that of Tc – Tg. This indicates that chalcogenide glasses having a higher rate of crystallization have less thermal stability. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Applicability of Meyer–Neldel rule for isothermal crystallization in glassy Se 70Te 30− xSb x alloys

Many activated phenomena, including solid state diffusion in crystals and polymers, dielectric relaxation, conduction and thermally stimulated processes in polymers, and electronic conduction in amorphous semiconductors obey the compensation law or Meyer–Neldel rule. In the present paper, we report the compensation effect in the isothermal crystallization in glassy Se 70Te 30− x Sb x alloys. We have observed Meyer–Neldel rule between pre-exponential factor K o and activation energy of crystallization E c in the present case.

Investigation of compensation effect for isothermal crystallization in glassy Se80−xTe20Mx (M = Cd, Ge, Sb) alloys

The compensation effect (also known as the Meyer–Neldel (MN) rule) is observed in many activated phenomena, including solid-state diffusion in crystals and polymers, dielectric relaxation, conduction and thermally stimulated processes in polymers and electronic conduction in amorphous semiconductors. In the present paper, we have investigated the compensation effect for isothermal crystallization in glassy Se80−xTe20Mx (M = Cd, Ge, Sb) alloys. The extent of crystallization was estimated from the dc conductivity measurements for the present chalcogenide glasses. We have observed the MN rule relating the pre-exponential factor K0 and the activation energy of crystallization Ec for all three glassy systems.

Study of electrical properties of glassy Se100- xTexalloys

Temperature and frequency dependence of a.c. conductivity have been studied in glassy Se100- x Te x(x = 10, 20 and 30) over different range of temperatures and frequencies. An agreement between experimental and theoretical results suggests that the a.c. conductivity behaviour of selenium-tellurium system (Se100- xTex)can be successfully explained by correlated barrier hopping (CBH) model. The density of defect states has been determined using this model for all the glassy alloys. The results show that bipolaron hopping dominates over single-polaron hopping in this glassy system. This is explained in terms of lower values of the maximum barrier height for single-polaron hopping. The values of density of charged defect states increase with increase in Te concentration. This is in agreement with our previous results obtained from SCLC measurements.

Observation of phase separation in some Se–Te–Ag chalcogenide glasses

Differential scanning calorimetric (DSC) measurements have been made in glassy Se 70Te 30− x Ag x alloys. At low concentration of Ag ( x = 2, 4, 6), only one endothermic and one exothermic peaks are observed at glass transition and crystallization temperatures. However, at higher concentration of Ag ( x = 8, 10), two endothermic and two exothermic peaks have been observed in DSC scans. The double stage crystallization at higher concentration of Ag has also been reported in various other glassy systems by other workers. The activation energy of crystallization ( E c) has been determined for each stage.

Photo-induced volume changes in selenium. Tight-binding molecular dynamics study

Tight-binding molecular dynamics simulations of photo-excitations in small Se clusters (isolated Se 8 ring and helical Se chain) and glassy Se networks (containing 162 atoms) were carried out in order to analyze the photo-induced instability in amorphous selenium. In the cluster systems after taking an electron from the highest occupied molecular orbital to the lowest unoccupied molecular orbital a bond breaking occurs. In the glassy networks photo-induced volume expansion was observed and at the same time the number of coordination defects changed significantly due to illumination.

A Fourier-transform photoluminescence study of radiative recombination mechanism in chalcogenide glasses

In this paper, we report photoluminescence (PL) studies carried out using Fourier-transform method in glassy Se (a-Se) and Ge-based Ge 20Se 80 and Ge 20Se 70Te 10 glasses. It was observed that the PL spectra of a-Se as well as that of Ge-based glasses exhibit fine features. The details of these fine features are extracted by deconvoluting the experimental spectra and are explained based on phenomenological defect models. Utilizing the deconvoluted information, the radiative recombination transitions occurring at ‘valence-alternation pairs’ are distinguished from those occurring at ‘intimate valence-alternation pairs’ and are illustrated with the help of a configurational coordinate diagram representation.

Glass transition phenomena, crystallization kinetics and thermodynamic properties of ternary Se 80Te 20− xIn x ( x=2, 4, 6, 8 and 10) semiconducting glasses: theoretical and experimental aspects

This paper presents the results of kinematical studies of glass transition and crystallization in glassy Se 80Te 20− x In x ( x=2,4, 6, 8 and 10) using differential scanning calorimetry (DSC). Also the thermodynamic properties of these glasses in the transformation range of temperatures have been studied. Glass transition region has been investigated in terms of activation energy, dependence of glass transition temperature on coordination number with the varying composition and heating rates. The growth kinetics and its dimensionality have been investigated using three different models viz. Kissinger, Ozawa and Matusita equations. On the basis of the obtained experimental results on phase transformations in these glasses, thermodynamic parameters like entropy difference between metastable states in the glassy region, difference of Gibbs free energy, specific heat, entropy between the two phases and the enthalpy released during crystallization have been determined. On the basis of experimental observations and theoretical calculations of thermodynamic properties, it has been found that Se 80Te 10In 10 is the most stable glass.

Kinematical studies of the glass transition in glassy Se80 − xTe20Sbx

The present paper reports kinematic studies of the glass transition in glassy Se80-xTe20Sbx alloys using the differential scanning calorimetric technique. The glass transition temperature (Tg) is found to increase monotonically with the heating rate. Tg also increases with the increase of Sb concentration in ternary Se80-xTe20Sbx system. From the heating rate dependence of glass transition temperature, the activation energy (Δ Et) for the relaxation time controlling the structural enthalpy, is calculated. The composition dependence of Tg and Δ Et is discussed in terms of the structure of the Se-Te-Sb glassy system.

Dielectric relaxation in glassy Se80-xTe20Gex

Dielectric relaxation studies have been made in glassy Se80−xTe20Gex alloys where 0 ≤ x ≤ 20. The measurements of dielectric constant (∈′) and dielectric loss (∈″) are made in the audio frequency range (1 kHz to 10 kHz) at various temperatures from 30°C to 150°C. The results indicate that the dielectric dispersion occurs in the above frequency and temperature range. An analysis of the results shows that the dielectric losses are dipolar in nature and can be understood in terms of hopping of charge carriers over a potential barrier.