Rings, chains and planes: Variation ofT g with composition in chalcogenide glasses

We propose a microscopic, phenomenological model for the decrease in the viscosity observed at glass transition. Our model is primarily applicable to chalcogenide glasses. According to this model, the decrease in the viscosity at glass transition is mainly due to the breaking of the Van der Waals bonds in the chalcogenides. Using this model, we derive a relationship between the glass transition temperature,T _g ,and the molar volume V _m.The validity of this relation is checked using experimental data available in the literature for two binary systems (Ge-Se and As-S) and a pseudo-binary system (As ₄₀ Se _x Te _60-x .     

Effect of some metallic impurities on the density of localized states in a-Se80Te20 thin films

The present paper reports the d.c. conductivity measurements at high electric fields in vacuum-evaporated thin films of amorphous Se₈₀Te₂₀, Se₇₅Te₂₀Ge₅ and Se₇₅Te₂₀Sb₅ systems. Current-voltage (I-V) characteristics have been measured at various fixed temperatures. In all the samples, at low electric fields ohmic behavior is observed. However, at high electric fields (E∼10⁴ V/cm), non-ohmic behavior is observed. An analysis of the experimental data confirms the presence of space charge limited conduction (SCLC) in all the glassy materials studied in the present case. From the fitting of the data to the theory of SCLC, the density of defect states (DOS) near Fermi level is calculated. The role of the aforesaid impurities in a-Se₈₀Te₂₀ is found to be entirely different. In case of Sb, an increase in DOS is observed. However, a decrease is observed in case of Ge. The change in DOS on impurity incorporation is explained in terms of the change in structure of these glasses.     

Study of defect states in a-Se<Subscript>85</Subscript>Te<Subscript>15−<Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> thin films by space charge limited conduction mechanism

Current–Voltage (I–V) characteristics have been studied at various temperatures in vacuum evaporated thin films of a-Se₈₅Te_15−x Pb _x (x = 0, 2, 4, 6) alloys. These characteristics show that, at low electric fields, an ohmic behaviour is observed. However, at high electric fields (E ∼ 10⁴ V/cm), the current becomes superohmic. At high fields, in case of samples having 0 and 2 at% of Pb, the experimental data fits well with the theory of space charge limited conduction (SCLC) in case of uniform distribution of localized states in the mobility gap. Such type of behaviour is not observed at higher concentration of Pb in the present glassy system due to high conductivity. In these samples, joule heating due to large currents may prohibit the measurement of SCLC. Using the theory of SCLC for the uniform distribution of the traps, the density of localized defect states near Fermi level is calculated for these compositions. The results indicate that the density of defect states near Fermi level increases on addition of Pb to binary Se₈₅Te₁₅alloy. This is explained in terms of electronegativity of Pb as compared to host elements.     

Effect of In additive on the photosensitivity of glassy Se80Te20 alloy

Photo-electrical measurements are done on thin films of Se_80?x Te₂₀In _x (0 ≤ x ≤ 20) to see the effect of In on the photoconductive properties of binary Se₈₀Te₂₀ alloy. The photosensitivity (σ_ph/σ_d) and activation energy of photoconduction (ΔE _ph) are determined for this purpose. It has been found that σ_ph/σ_d and ΔE _ph are decreased with increasing concentration of the third element. The results are explained in terms of the increase in density of the defect states with the increase in concentration of the additive in thin films of Se_80?x Te₂₀In _x .     

Optical properties of samarium doped zinc-tellurite glasses

Glasses with the composition, (Sm₂ O ₃) _x (ZnO)_(40-x)(TeO ₂)₍₆₀₎, were prepared by conventional melt quenching method. The density, molar volume, and optical energy band gap of these glasses have been measured. The refractive index, molar refraction and polarizability of oxide ion have been calculated by using Lorentz-Lorentz relations. Optical absorption spectra of these glasses were recorded in the range 300–700 nm at room temperature. The oxide ion polarizabilities deduced from two different quantities, viz. refractive index and optical energy band gap, agree well compared with other glasses. The nonlinear variation of the above optical parameters with respect to samarium dopant has been explained.     

Electrical transport properties of thin film of a-Se87Te13 nanorods

We have studied the electrical transport properties of thin film of a-S₈₇Te₁₃ nanorods. Initially, the glassy alloy of S₈₇Te₁₃ is prepared by melt-quenching technique. The amorphous nature of this alloy is verified by using X-ray diffraction technique. The nanorods of a-S₈₇Te₁₃ are synthesised on a glass substrate under an ambient gas (Ar) atmosphere using physical vapour condensation system. The morphology and microstructure of these nanorods are studied using scanning electron microscopy and transmission electron microscopy. The temperature dependence of dc conductivity for these nanorods is also studied over a temperature range of 500–100?K. On the basis of the temperature dependence of dc conductivity, the conduction mechanism in these nanorods is investigated. The results reveal that the thermally activated process is responsible for the transport of carriers in the temperature range 500–300?K. While the conduction takes place via variable range hopping (VRH) for temperature region 300–100?K. It is therefore, suggested that three-dimensional Mott's variable range hopping (3D VRH) is the conduction mechanism responsible for the transport of charge carriers in the temperature region 300–100?K. Various Mott's parameters such as density of states, degree of disorder, hopping distance and hopping energy are estimated on the basis of best fitting to our experimental data for Mott's 3D VRH model.     

Spectroscopic investigations of Cu2+ in Li2O-Na2O-B2O3-Bi2O3 glasses

Pure and copper doped glasses with composition,x Li ₂ O-(40-x)Na ₂ O-50B ₂ O ₃-10Bi ₂ O ₃,have been prepared over the range 0 ≤ x ≤ 40. The electron paramagnetic resonance (EPR) spectra of Cu²⁺ ions of these glasses have been recorded in the X-band at room temperature. Spin Hamiltonian parameters have been calculated. The molecular bonding coefficients, α² and β², have been calculated by recording the optical absorption spectra in the wavelength range 200–1200 nm. It has been observed that the site symmetry around Cu²⁺ ions is tetragonally distorted octahedral. The density and glass transition temperature variation with alkali content shows non-linear behaviour. The IR studies show that the glassy system contains BO₃ and BO₄ units in the disordered manner.     

Non-ohmic behavior in a-Se80−xTe20Cdx thin films

The present paper reports the d.c. conductivity measurements at high electric fields in vacuum evaporated thin films of amorphous Se_80−xTe₂₀Cd_x (x = 0, 5, 10, 15) systems. Current–voltage (I–V) characteristics have been measured at various fixed temperatures. In all the samples, at low electric fields ohmic behavior is observed. However, at high electric fields (E ∼ 10⁴ V/cm), non-ohmic behavior is observed. An analysis of the experimental data confirms the presence of space charge limited conduction (SCLC) in all the glassy materials studied in the present case. From the fitting of the data to the theory of SCLC, the density of defect states (DOS) near Fermi level is calculated. It is found that the incorporation of Cd like a third element as an impurity could not change the density of defect states significantly in pure binary Se–Te glassy system.     

Further MN rule in thermally activated a.c. conduction of Se-Te-Sb chalcogenide glasses

Temperature and frequency dependence of a.c. conductivity have been studied in glassy Se₇₀Te_30−xSb_x (4 ≤ x ≤ 10) alloys. The observation of Further Meyer-Neldel rule in case of a.c. conductivity is reported. The observation of the correlation between Meyer-Neldel pre-factor σ₀₀ and Meyer-Neldel energy is explained by multiple excitations stimulated by optical phonon energy as described by Yelon and Movaghar.     

Explanation of Meyer–Neldel rule in the thermally activated a.c. conduction in some chalcogenide glasses using correlated barrier hopping model

We have investigated Meyer–Neldel rule in thermally activated a.c. conduction for a-Se₈₀Te₂₀ and a-Se₈₀Te₁₀M₁₀ (M = Cd, In, Sb) alloys by two different approaches. In the first case, the temperature dependence of a.c. conductivity is studied at different audio frequencies without changing the composition of glassy system. In the second case, the composition itself varies at a particular audio frequency. The results are explained by using well-known correlated barrier hopping model.     

Optical, electrical and thermal studies on (As2Se3)3−x(As2Te3)x glasses

Optical properties and conductivity of glassy (As₂Se₃)_3−x(As₂Te₃)_x were studied for 0 ≤ x ≤ 3. The films of the above mentioned compound were prepared by thermal evaporation with thickness of about 250 nm. The optical-absorption edge is described and calculated using the non-direct transition model and the optical band gap is found to be in the range of 0.92 to 1.84 eV. While, the width of the band gap tail exhibits opposite behaviour and is found to be in the range of 0.157 to 0.061 eV, this behaviour is believed to be associated with cohesive energy and average coordination number. The conductivity measurement on the thin films is reported in the temperature range from 280 to 190 K. The conduction that occurs in this low-temperature range is due to variable range hopping in the band tails of localized states, which is in reasonable agreement with Mott's condition of variable range hopping conduction. Some parameters such as coordination number, molar volume and theoretical glass transition temperature were calculated and discussed in the light of the topological bonding structure.     

Study of electrical properties of CIGS thin films prepared by multistage processes

In this work, the dispersion mechanisms affecting the electric transport in CuIn_1−xGa_xSe₂ (CIGS) thin films grown by a chemical reaction of the precursor species, which are evaporated sequentially in two and three-stage processes are analyzed. It was found, through conductivity and Hall coefficient measurements carried out as functions of temperature, that the electrical conductivity of the CuIn_1−xGa_xSe₂ films is affected by the transport of free carriers in extended states of the conduction band as well as for variable range hopping transport mechanisms, each one predominating in a different temperature range.     

Anisotropic behaviour of semiconducting tin monosulphoselenide single crystals

Single crystals of ternary mixed compounds of group IV-VI in the form of a series, SnS_xSe_1-x (wherex = 0, 0.25, 0.50, 0.75 and 1), have been grown using direct vapour transport technique. The grown crystals were characterized by the X-ray diffraction analysis for their structural parameter determination. All the grown crystals were found to be orthorhombic. The microstructure analysis of the grown crystals reveals their layered type growth mechanism. From the Hall effect measurements Hall mobility, Hall coefficient and carrier concentration were calculated with all crystals showingp-type nature. The d.c. electrical resistivity measurements perpendicular toc-axis (i.e. along the basal plane) in the temperature range 303–453 K were carried out for grown crystals using four-probe method. The d.c. electrical resistivity measurements parallel to c-axis (i.e. perpendicular to basal plane) in the temperature range 303–453 K were carried out for the same crystals. The electrical resistivity measurements showed an anisotropic behaviour of electrical resistivity for the grown crystals. The anisotropic behaviour and the effect of change in stoichiometric proportion of S and Se content on the electrical properties of single crystals of the series, SnS_xSe_1-x (wherex = 0, 0.25, 0.50, 0.75 and 1), is presented systematically.     

Synthesis and magnetic study of Co-Al substituted calcium hexaferrite

A series of calcium substituted polycrystalline ferrite ceramics with magnetoplumbite structures were synthesized using perfect stoichiometric mixtures of oxides with chemical composition, CaAl_xCo_xFe_12−2xO₁₉ (x = 2−5), by standard ceramic technique. The variation in the values ofH _candM _s,which depends on the additive content and the temperature, was studied by means of a vibration magnetometer. The strong variation observed in coercivity, saturation magnetization and Curie temperature with chemical composition give rise to the possibility of controlling these properties and hence applying these compounds in the millimeter— microwave range.     

Theoretical study of superconductivity in MgB2 and its alloys

Using density-functional-based methods, we have studied the fully-relaxed, fulltronic structure of the newly discovered superconductor, MgB ₂, and BeB₂, NaB₂ and AlB₂. Our results, described in terms of (i) total density of states (DOS) and (ii) the partial DOS around the Fermi energy, E_F, clearly show the importance of B p-electrons for superconductivity. For BeB₂ and NaB₂, our results indicate qualitative similarities but significant quantitative differences in their electronic structure due to differences in the number of valence electrons and the lattice constantsa andc. We have also studied Mg _1-xM_xB₂ (M = Al, Li or Zn) alloys using coherent-potential to describe disorder, Gaspari-Gyorffy approach to evaluate electron-phonon coupling, and Allen-Dynes equation to calculate the superconducting transition temperature, T_c. We find that in Mg_1-xM_xB₂ alloys (i) the way Tc changes depends on the location of the added/modified k-resolved states on the Fermi surface and (ii) the variation of Tc as a function of concentration is dictated by the Bp DOS.     

Simultaneous measurements of thermal conductivity and diffusivity of Se80Te20-xInx (x = 2, 4, 6 and 10) chalcogenide glasses at room temperature

Measurements of thermal conductivity and thermal diffusivity of twin pellets of Se₈₀Te_20-xIn_x (x = 2, 4, 6 and 10) glasses, prepared under a load of 5 tons were carried out at room temperature using transient plane source (TPS) technique. The measured values of both thermal conductivity and diffusivity were used to determine the specific heat per unit volume of the said materials in the composition range of investigation. Results indicated that both the values of thermal conductivity and thermal diffusivity increased with the addition of indium at the cost of tellurium whereas the specific heat remained almost constant. This compositional dependence behaviour of the thermal conductivity and diffusivity has been explained in terms of the iono-covalent type of bond which In makes with Se as it is incorporated in the Se-Te glass.     

Structural and Transport Properties of Sn-Doped Bi2Te3 – x Se x Single Crystals

The transport properties of Bi_{2 – y} Sn _y Te_{3 – x} Se _x solid solutions are studied. The results demonstrate that doping with Sn has a strong effect on the temperature dependences of the thermoelectric power and electrical conductivity of the crystals. This suggests that the valence band of the crystals contains Sn-related resonance states. The point defects and dislocation system in Bi₂Te₃ and Bi_{2 – y} Sn _y Te_{3 – x} Se _x solid solutions are studied by transmission electron microscopy. It is shown that the predominant defects in the crystals studied, grown by the Czochralski technique, are dislocations lying in the (0001) plane. The estimated dislocation density is 10⁸ to 10⁹ cm^–2, and the primary slip plane is (0001). Electron-microscopic examination indicates the presence of stacking faults and very small dislocation loops in both Bi₂Te₃ and Bi_{2 – y} Sn _y Te_{3 – x} Se _x single crystals. Since all of the crystals are highly degenerate semiconductors, it is reasonable to assume that structural defects have an insignificant effect on their electrical properties.     

Evaporated SexTe1-x Thin Films with Tunable Bandgaps for Short-Wave Infrared Photodetectors

Semiconducting absorbers in high-performance short-wave infrared (SWIR) photodetectors and imaging sensor arrays are dominated by single-crystalline germanium and III–V semiconductors. However, these materials require complex growth and device fabrication procedures. Here, thermally evaporated Se_xTe_1-x alloy thin films with tunable bandgaps for the fabrication of high-performance SWIR photodetectors are reported. From absorption measurements, it is shown that the bandgaps of Se_xTe_1-x films can be tuned continuously from 0.31 eV (Te) to 1.87 eV (Se). Owing to their tunable bandgaps, the peak responsivity position and photoresponse edge of Se_xTe_1-x film-based photoconductors can be tuned in the SWIR regime. By using an optical cavity substrate consisting of Au/Al₂O₃ to enhance its absorption near the bandgap edge, the Se_0.32Te_0.68 film (an optical bandgap of ≈0.8 eV)-based photoconductor exhibits a cut-off wavelength at ≈1.7 μm and gives a responsivity of 1.5 AW⁻¹ and implied detectivity of 6.5 × 10¹⁰ cm Hz^1/2 W⁻¹ at 1.55 μm at room temperature. Importantly, the nature of the thermal evaporation process enables the fabrication of Se_0.32Te_0.68-based 42 × 42 focal plane arrays with good pixel uniformity, demonstrating the potential of this unique material system used for infrared imaging sensor systems.     

Electrical transport properties of amorphous Se78−x Te22Bi x films

D.C. Conductivity measurements on the thin films of a-Se_78–x Te₂₂Bi _x system (where x = 0, 0.5, 2 and 4) are reported in the temperature range 213–390 K and the density of states (DOS) near the Fermi level is calculated using dc conductivity data. It is found that the conduction in all the samples takes place in the tails of localized states. The conduction in the high temperature region 296–390 K is due to thermally assisted tunneling of electrons in the localized states at the conduction band edge. In the low temperature region 213–296 K conduction takes place through variable range hopping in the localized states near the Fermi level.     

Pre-exponential factor in<Emphasis Type="Italic">a</Emphasis>-Se<Subscript>75</Subscript>In<Subscript>25-<Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> thin films

Temperature dependence of d.c. conductivity is studied ina- Se₇₅In_25-x Pb _x thin films wherex is varied from 0–10. From these measurements, the values of the pre-exponential factor (σ₀) and activation energy (ΔE) are calculated for each glassy alloy. An approximate linear dependence of ln σ₀ on AE is observed in this glassy system with good agreement between the expected and calculated σ₀ values using Meyer-Neldel rule. Linear dependence of ln σ₀ on ΔE in case of amorphous materials indicates that the conduction band tails a finite energy distance towards the valence band and Fermi level is controlled by fixed dominant hole levels deeper in the gap.