Composition dependence of the energy gap and thermal diffusivity in bulk As-Se glasses

We have measured the composition dependence of the optical energy gap and thermal diffusivity in bulk As _x Se_1−x (0.10⩽x⩽0.50) glasses using photoacoustic technique. The energy gap shows a threshold minimum value and thermal diffusivity has a threshold maximum value at the stoichiometric composition As₂Se₃ corresponding tox=0.40. The decrease in energy gap is explained on the basis of chemical bonding. It is argued that the threshold percolation of rigidity in the random network is responsible for the peaking of the thermal diffusivity at the stoichiometric composition.     

Photoconduction in amorphous thin films of Se90Sb10-xAgx glassy alloys

The present paper reports the steady state photoconductivity and photosensitivity response of thermally evaporated amorphous thin films of Se₉₀Sb_10-xAg_x(x = 2, 4, 6, 8, 10). Temperature dependence of dark conductivity is studied and activation energy is calculated for different samples. Temperature dependence of photoconductivity is also studied at different intensities. From temperature dependence of photoconductivity activation energy is computed at different intensities which are found to vary from 0.26 to 0.47 eV. Intensity dependence of photoconductivity has also been studied at different temperatures. These curves are plotted on logarithmic scale and found to be straight lines which show that photoconductivity follows a power law with intensity. Composition dependence of dark conductivity, activation energy of DC conduction and photosensitivity show that these parameters are highly. composition dependent and show a discontinuity at a particular composition when Ag concentration becomes 6 at. %. This is explained in terms of transition from floppy state to mechanically stabilized state at this composition.     

Characterization of new quaternary chalcogenide As–Ge–Se–Sb thin films

This paper reports the effect of replacement of selenium by antimony on the optical gap and some other physical parameters of new quaternary chalcogenide As₁₄Ge₁₄Se_{72? x} Sb _x (where x = 3, 6, 9, 12 and 15 at%) thin films. Thin films with thickness 200–220 nm of As₁₄Ge₁₄Se_{72? x} Sb _x were prepared by thermal evaporation of the bulk samples. Increasing antimony content was found to affect the average heat of atomization, the average coordination number, number of constraints and cohesive energy of the As₁₄Ge₁₄Se_{72 ?x} Sb _x alloys. Optical absorption measurements showed that the fundamental absorption edge is a function of composition. Optical absorption is due to allowed, non-direct transition and the energy gap decreases with the increasing antimony content. The chemical bond approach has been applied successfully to interpret the decrease in the optical gap with increasing antimony content.     

Low temperature elastic behaviour of As-Sb-Se and Ge-Sb-Se glasses

The ternary glasses of arsenic and germanium with antimony and selenium can be prepared in large sizes for optical purposes. The elastic behaviour of eight compositions of each glass has been studied down to 4.2 K using a 10 MHz ultrasonic pulse echo interferometer. The glasses have a normal elastic behaviour, with the velocities gradually increasing as the temperature is lowered. An anharmonic solid model of Lakkad satisfactorily explains the temperature variations. The elastic moduli of Ge _x Sb₁₀Se_90?x glasses increase linearly as the Ge content is increased up to 25 at. % and beyond this the increase is nonlinear. (AsSb)₄₀Se₆₀ glasses show a linear increase in elastic moduli with increasing Sb content. The elastic moduli of As _x Sb₁₅Se_85?x glasses exhibit a drastic change near the stoichiometric composition As₂₅Sb₁₅Se₆₀. These behaviours have been qualitatively explained on the basis of the structural changes in glasses.     

Composition dependence of photoconductivity in amorphous thin films of Se70Te30−xSbx

Summary The present paper reports the composition dependence of steady-state and transient photoconductivity in amorphous thin films of Se₇₀Te_30-xSb_x, wherex is varied from 0 to 10. The results indicate that photoconductivity is highly composition dependent in this system and a discontinuity in various electrical parameters is observed at 4 at. % of Sb. This is explained in terms of increased disorder up to 4 at. % of Sb. However, at higher concentration of Sb, an ordered structure may be established due to the formation of microcrystalline phases as observed in X-ray diffraction patterns.     

Urbach tails and the structure of chalcogenide glasses

A novel analysis of optical absorption tails of inorganic network glasses is shown to provide important information on the structure of the glass. The anomalous composition and temperature dependence of absorption tails in Ge_xSe_1?x and As_xSe_1?x systems indicate that these glasses retain locally layered structures at particular stoichiometries corresponding to GeSe₂ and As₂Se₃, and a reversible structural change is taking place well below the glass transition temperature. A phenomenological model for the absorption tail slope of glasses is proposed, analogous to the Urbach rule for crystalline materials.     

Thermal conductivity of GexSb(As)ySe100‐x‐y glasses measured by Raman scattering spectra

We systematically measured thermal conductivity of Ge_xSb(As)₁₀Se_90−x, Ge_xSb₁₅Se_85−x, and Ge_xSb(As)₂₀Se_80−x chalcogenide glasses by measuring their Stokes and anti‐Stokes Raman scattering spectra and estimating the temperature raised by laser irradiation via the ratio of Stoke and anti‐Stokes scattering cross‐section. We aimed at demonstrating the viability of Raman scattering method for thermal conductivity measurements, and understanding the role of chemical composition in determining thermal conductivity of the chalcogenide glasses. We found that, while the values of the thermal conductivity measured in the paper are in a range from ~0.078 to 1.120 Wm^‐1K^‐1 that are in agreement with those reported data in the literatures, thermal conductivity increases before it reaches a maximum at the glass with chemically stoichiometric composition, and then decreases with increasing Ge content. We ascribed the threshold behavior of the thermal conductivity to the demixing of the structural units like GeSe₂, As₂Se₃ and Sb₂Se₃ from the main glass network. The present study demonstrated that Raman scattering method is simple and easy to measure thermal conductivity of the material. Copyright © 2014 John Wiley & Sons, Ltd.     

Observation of Meyer-Neldel rule in thermally activated Poole-Frenkel photoconduction in a-Se<Subscript>70</Subscript>Te<Subscript>30−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> thin films

In the present paper, we report the compensation effect on photoconductivity of thin films of Se₇₀Te_30−x Cd _x (x = 2, 4, 6) alloys for high field conduction. Steady state photoconductivity measurements have been made in thin films for different electric fields. The photoconduction was found to be ohmic at low fields and Poole-Frenkel type at high fields. Meyer-Neldel rule (MN rule) for the pre-exponential factor (σ _ph)₀ and activation energy (ΔE _ph) of photoconduction in thin films of Se₇₀Te_30−x Cd _x is observed in the present study.     

Thermoelectric figure-of-merit in <Emphasis Type="Italic">p</Emphasis>-type bismuth- and antimony-chalcogenide-based solid solutions above room temperature

The specific features of the variation in the thermoelectric figure-of merit Z for p-type bismuth- and antimony-chalcogenide-based solid solutions p-(Bi,Sb)₂(Te,Se)₃ have been analyzed with allowance made for the data amassed in the investigation of thermoelectric and galvanomagnetic properties. It has been shown that, in the samples with optimum carrier concentrations, the increase in Z in the multicomponent p-Bi_{2 − x} Sb _x Te_{3 − y} Se _y composition (x = 1.3, y = 0.06) in the temperature range 300–370 K is mediated by the high carrier mobility and the low lattice thermal conductivity. The higher effective mass of the density of states and the larger slope of the temperature dependence of the mobility as compared to the other compositions bring about an increase in Z in the p-Bi_{2 − x} Sb _x Te₃ solid solution for x = 1.6 in the temperature range 370–550 K. The increase in the figure-of merit reached in the compositions under study stems also from the increasing contraction of constant-energy ellipsoids along the binary and bisector directions and from the change in the angle θ between the principal axes of the ellipsoids and the crystallographic axes.     

Elastic constants and mechanical stability of InxAl1 − xAsySb1 − y lattice-matched to different substrates

Based on a pseudopotential approach under the virtual crystal approximation, the elastic modulus of In_xAl_1???xAs_ySb_1???y quaternaries lattice-matched to InP, GaSb and InAs substrates has been investigated. Our findings show a reasonably good accord with experiment. The dependence of the elastic features of interest on the indium concentration x shows a monotonic behaviour when In_xAl_1???xAs_ySb_1???y is lattice-matched to InP substrate. In that case, the elastic constants have larger values and the material system of interest becomes less harder and its rigidity becomes weaker. The mechanical stability criteria is verified in terms of elastic constants and shows that In_xAl_1???xAs_ySb_1???y is mechanically stable for each x and substrate being considered here. The change in indium content x and the substrate is found to have no much effect on both the Poisson ratio and machinability. The present study showed that a proper choice of the indium composition x and substrate may provide more diverse opportunities as regards the elastic modulus of In_xAl_1???xAs_ySb_1???y.     

Low temperature photoconductivity of Pb1?xSnxTe?In? films at millimeter wavelengths

Photoconductivity of single crystal Pb_1−xSn_xTe〈In〉, (0.24≤x≤0.25), films at 4.2K in the wavelength region 4÷5 mm and magnetic fields up to 8 T was studied. Preparation technology of the films with high photoconductivity (P_FEP ≤ 10⁻¹² W×Hz^1/2) and photoresponsiveness (τ ≤ 10⁻⁶ s), as well as the measurements procedure are described in the paper. Also the mechanism of photoconductivity is discussed.     

Photoconductivity in amorphous thin films of Ge22Se78−xBix

Temperature and intensity dependence of photoconductivity is studied in amorphous thin films of Ge₂₂Se_78−x Bi_x with x = 0, 2 and 10. Transient photoconductivity measurements have also been made on the same samples. Our results show that photosensitivity decreases as Bi concentration is increased from x = 0 to x = 2. However, at high concentration of Bi(x = 10), photosensitivity again increases. Transient photoconductivity also show a different behaviour at low and high concentration of Bi. Results have been explained in terms of defect states produced due to Bi incorporation in GeSe system.     

Study of As-Sb-Se chalcogenide glasses by NQR and EPR spectroscopy methods

In this paper experimental results obtained by both ⁷⁵As NQR and EPR spectroscopy are presented for the three-component system As-Sb-Se. The ⁷⁵As NQR spectra of glasses of structures (As₂Se₃)_0.78 (Sb₂Se₃)_0.22, (As₂Se₃)_0.75 (Sb₂Se₃)_0.25, (As₂Se₃)_0.5 (Sb₂Se₃)_0.5 have broad lines with two Sb-NQR lines (corresponding to the Sb₂Se₃ units) and two ⁷⁵As-NQR lines (corresponding to the As₂Se₃ units). Differences in the EPR spectra of the different glasses arise because of the different amounts of arsenic and antimony in their structure.     

Carrier mobility in nonstoichiometric n-Bi2Te3−x Sex solid solutions

A study has been made of the thermoelectric and galvanomagnetic properties of n-Bi₂Te_3−x Se_x solid solutions (x=0.3 and 0.36) in the temperature range 80–300 K. The lowest carrier concentrations, (0.8–1)×10¹⁸ cm⁻³, were obtained by displacing the solid solution from the stoichiometric to a Te-rich composition. At such carrier concentrations, the second subband in the conduction band of n-Bi₂Te_3−x Se_x is not filled, which results in a growth of mobility because of the absence of interband scattering, and brings about an increase of thermoelectric efficiency in the 80–120-K range. Fiz. Tverd. Tela (St. Petersburg) 39, 483–487 (March 1997)     

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.     

Simple solar cells of 3.5% efficiency with antimony sulfide‐selenide thin films

Thin films of antimony sulfide‐selenide solid solutions (Sb₂S_x Se_3–x) were prepared by chemical bath deposition and thermal evaporation to constitute solar cells of a transparent conductive oxide (FTO)/CdS/Sb₂S_x Se_3–x/C–Ag. The cell parameters vary depending on the sulfide‐selenide composition in the films. The best solar cell efficiency of 3.6% was obtained with a solid solution Sb₂S_1.5Se_1.5 prepared by thermal evaporation of the precipitate for which the open circuit voltage is 0.52 V and short circuit current density, 15.7 mA/cm²under AM 1.5G (1000 W/m²) solar radiation. For all‐chemically deposited solar cells of Sb₂S_1.1Se_1.9 absorber, these values are: 2.7%, 0.44 V, and 15.8 mA/cm², and for Sb₂S_0.8Se_2.2, they are: 2.5%, 0.38 V and 18 mA/cm². (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Electrical and thermal properties of a-(Se70Te30)100−x(Se98Bi2)x (0⩽x⩽20) alloys

In electrical properties, the dc conductivity and photoconductivity measurements have been made in vacuum evaporated thin films of a-(Se₇₀Te₃₀)_100−x(Se₉₈Bi₂)_x system, in the temperature range (308–355 K). It has been observed that dc conductivity and activation energy depend on the Bi concentration. Photocurrent dependence on incident radiation has also been observed which follow the power law (I_ph∝F^γ). Transient photocurrent exhibits the non-exponential decay time. All these parameters show that the recombination within the localized states is predominant. In crystallization kinetics, the heating rate dependence of glass transition and crystallization temperatures is studied to calculate the activation energy for thermal relaxation and activation energy for crystallization. The composition dependence of the activation energy for thermal relaxation and activation energy for crystallization is discussed in terms of the structure of Se–Te–Bi glassy system.     

Effect of composition and light intensity on the electrical properties of Sn–Sb–Se glassy films

Electrical and photoelectrical measurements have been performed on Sn_xSb₂₀Se_70-x (8≤x≤16) glassy films. The dc activation energy, optical gap and photoconduction parameters show a typical variation near x=10 composition indicating the occurrence of a rigidity percolation threshold in the present system. The photosensitivity increases with the increase in Sn content up to x=14 and an abrupt decrease for x=16 composition. Negative photoconductivity region have been observed in the higher temperature side for samples with x=10 and 16. This system belongs to the type II category of photoconductors. The results are explained on the basis of a change in the density of localized states present in the mobility gap with the change in the composition. PACS 71.20.Nr; 72.20.-I; 78.66.Jg; 81.05.Gc; 73.50.Pz     

Influence of electric current and magnetic field on terahertz photoconductivity in Pb1 − x Sn x Te(In)

Photoconductivity of Pb_{1 ? x} Sn _x Te(In) solid solutions in the terahertz spectral range is defined by a new type of local electron states linked to the quasi-Fermi level. The paper deals with investigation of the influence of electric current and magnetic field on the amplitude of the terahertz photoconductivity in Pb_{1 ? x} Sn _x Te(In) alloys of different composition. It is shown that the density of local electron states responsible for the positive persistent photoconductivity decreases with increasing electric current via a sample, as well as with transition to the hole conductivity in samples with a high content of tin telluride (x > 0.26). It is found that the magnetic field dependence of the positive photoconductivity is non-monotonous and has a maximum. The maximum position in magnetic field is proportional to the terahertz radiation quantum energy. Mechanisms responsible for the effects observed are discussed.     

Kinetics of nonisothermal crystallization in Sn<Subscript>10</Subscript>Sb<Subscript>20−<Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>70</Subscript> glassy semiconductors

The crystallization kinetics of Sn₁₀Sb_20−x Bi _x Se₇₀ (x=0,2,4,6,8) chalcogenide system has been studied. Crystallization studies using differential scanning calorimetry under nonisothermal conditions with different heating rates are reported. The glass transition temperature is found to increase with the increase in heating rate as well as with the addition of bismuth. The apparent activation energy for glass transition and that for crystallization have been determined using the Kissinger equation. Thermal stability and glass forming tendency have also been studied.