Effect of germanium addition on the physical properties of Se–Te glassy semiconductors

The effect of germanium addition on the physical properties, i.e. density, molar volume, compactness, number of lone-pair electrons, average coordination number, heat of atomization, mean bond energy, cohesive energy and glass-transition temperature, of (Se₈₀Te₂₀)_{100? x} Ge _x (x = 0, 2, 4, 6) bulk glassy alloys was investigated. The density of the glassy alloys is found to decrease with increasing Ge content. The molar volume and compactness of the structure of the glass were determined from the measured density. The mean bond energy is proportional to the glass-transition temperature. The cohesive energy of the samples has been calculated using a chemical bond approach and is correlated with an increase in the optical energy gap with increase in the Ge content. The heat of atomization was also calculated and correlated with the optical energy gap. The glass-transition temperature has been estimated using different methods and is found to increase with an increase of Ge content.     

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.     

Compositional dependence of the optical properties of amorphous Se100– x Te x thin films

Thin films of amorphous Se_{100 –x} Te _x with different compositions (x = 10, 20, 30 and 40 at%) were deposited on glass substrates by thermal evaporation. Transmission spectra T(λ) of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. A straightforward analysis proposed by Swanepoel [J. Phys. E: Sci. Instrum. 17 896 (1984)], using of the maxima and minima of the interference fringes, was applied to derive the real and imaginary parts of the complex index of refraction plus film thickness. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model [Phys. Rev. B 3 1338 (1971)]. The optical band gap was determined from the spectral dependence of the absorption coefficient using the Tauc formula [in The Optical Properties of Solids, edited by F. Abeles (North Holland, Amsterdam, 1970), p. 227]. The refractive index increases and the optical band gap decreases with increasing tellurium content.     

ВЛИЯНИЕ СТАРЕНИЯ НА Э ЛЕКТРИЧЕСКИЕ СВОЙСТ ВА ПОЛУПРОВОДНИКОВОЙ С ИСТЕМЫ Bi2Te3-Bi2Se3

, Bi₂Te₃-Bi₂Se₃. , , , . .

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Influence of ageing on change in electrical properties of semiconducting systems of Bi₂Te₃-Bi₂Se₃

The paper describes the effect of ageing observed on a semiconducting system Bi₂Te₃ — Bi₂Se₃. It is shown that the change in electrical conductivity and thermoelectric force, which takes place during ageing, is caused by the change in concentration of the free electrons. The influence of this process on the efficiency of equipment employing the Peltier effect is analyzed.

     

Se，Te纳米线系统的结构稳定性研究

采用第一性原理计算及经验方法研究了Se，Te纳米线的结构稳定性. Se与Te的晶体是由三角Se，Te原子链构成并且链间相互作用相对较弱. 小直径的纳米线(<30?)进行了第一性原理计算； 同时对于大直径的纳米线，采用了一种只考虑了链间相互作用的经验方法. 六边形截面的构型比其他各种结构都要稳定，虽然在原子链数目的限制下无法保证其为正六边形. 该结论与被广泛接受的Se，Te纳米线的六边形稳定结构相一致. 关键词： 第一性原理 纳米线 硒 碲     

Se改良的远红外Te基硫系玻璃

将成玻性良好的Se逐步掺入到远红外Ge-As-Te玻璃中,观察其物理、光学特性的渐变过程.采用传统熔融淬冷法制备Ge_(10)As_(40)Te_(50-x)Se_x(x=0,10,20,30,40,50)系列玻璃,用X射线衍射仪、Raman光谱仪、热膨胀仪测试玻璃的内部微观结构和物化性质;用傅里叶红外光谱仪、分光光度计测试玻璃的可见/近红外与红外透射光谱等频谱性质;利用经典Tauc方程估算玻璃样品的直接和间接带隙.测试结果表明:Se含量的增加能有效提高玻璃的热稳定性,最高玻璃转化温度可达到233℃;可见/近红外短波截止边发生蓝移,光学带隙增大,透过范围广且透过性良好,红外透过率最高可达到56%,红外截止边仍然保持在20μm,当Se含量配比在低于2 mol的情况下,其热稳定性改善明显但光谱变化最小.最后,讨论了一种加还原剂Mg对该类玻璃进行提纯的方案,实验表明提纯后玻璃的透过谱线均匀平滑且无明显杂质峰.     

Crystallization kinetics and Avrami index of Sb-doped Se–Te–Sn chalcogenide glasses

Bulk amorphous samples of Sb-substituted Se_78?xTe₂₀Sn₂Sb_x (0 < x < 6) have been prepared using melt quench technique. The structure of Se_78?xTe₂₀Sn₂Sb_x (x = 0, 2, 4, 6) glassy alloys has been investigated using X-ray diffraction technique. Calorimetric studies of the prepared samples have been performed under non-isothermal conditions using differential scanning calorimetry (DSC) and glass transition temperature as well as crystallization temperature has been evaluated using DSC scans. The activation energy of crystallization kinetics (E_c) has been determined using model-free approaches such as Kissinger, Ozawa, Tang and Starink methods. The Avrami index (n) and frequency factor (K_o) have been calculated by Matusita and Augis–Benett method.     

Effect of SnI2 on the thermal and optical properties of Ge–Se–Te glasses

A systematic series of (Ge₂₀Se₁₅Te₆₅)_1?x–(SnI₂)_x (x = 0, 0.05, 0.1, 0.15) chalcogenide glasses have been prepared. The amorphous nature can be confirmed by XRD and SEM. With the SnI₂ content increasing, the indirect optical band gaps are decreased from 0.662 to 0.622 eV according to Tauc laws. The introduction of SnI₂ makes the glasses much easier to prepare and more stable against crystallization, making them drawable as optical fibers. The highest ΔT (130 °C) value for (Ge₂₀Se₁₅Te₆₅)_0.9–(SnI₂)_0.1 glass composition can be obtained. A slight red-shifting of the long-wavelength cutting-off edge from 18.4 to 19.4 μm was shown and it seems that SnI₂ in these glasses offers the improvement in the far-infrared properties.     

The effect of Bi content on the thermal stability and crystallization of Se–Te chalcogenide glass

Glass formation and devitrification of intermediate alloys in the Se–Te–Bi system were studied by differential scanning calorimetry. A comparison of various simple quantitative methods to assess the level of stability of the glassy materials in the above-mentioned system is presented. All of these methods are based on characteristic temperatures, such as the glass transition temperature, T _g, the onset temperature of crystallization, T _in, the temperature corresponding to the maximum crystallization rate, T _p, or the melting temperature, T _m. In this work, a kinetic parameter, K _r(T), is added to the stability criteria. The thermal stability of several compositions of Se–Te–Bi was evaluated experimentally and correlated with the activation energies of crystallization by this kinetic criterion and compared with those evaluated by other criteria. All the results confirm that the thermal stability decreases with increasing Bi content in this glassy system. The crystallization results are analysed and the activation energy and mechanism of crystallization characterized.     

Effect of bond polarity in Sb2Te3−x Se x single crystals

Sb₂Te_3?x Se _x (x=0·00?1·25) single crystals were prepared from 5N purity elements using a modified Bridgman method. Measurements of the reflectivity spectra in the plasma resonance frequency range, Hall constantR _H(B∥c) and electrical conductivityσ _⊥C were carried out on these samples at room temperature. With increasing selenium content a shift of the reflectivity minimum towards longer wavelengths was observed as well as an increase of the Hall constant and a decrease of the electrical conductivity — the incorporation of Se atoms into the Sb₂Te₃ crystal lattice results obviously in a decrease in the concentration of free carriers. This effect is accounted for by a change in the polarity of bonds in the Sb₂Te₃ crystal lattice, due to the formation of Se _Te ^x substitutional defects.     

Se替代Te对BiCuTeO电热输运性能的影响

层状氧硫族化合物由于其本征的低晶格热导率和可观的热电性能吸引了广泛关注,其中以BiCuSeO化合物的热电性能最为优异.但是,其同晶型化合物BiCuTeO,由于带隙较小且存在大量本征Cu空位,导致载流子浓度较高,热电性能较差,从而研究较少.针对BiCuTeO存在的上述问题,本文利用Se替代部分Te,以期能够展宽带隙并减少Cu空位,提高其热电性能.采用固相反应结合快速热压烧结制备了BiCuTe_(1-x)Se_xO(x=0, 0.1, 0.2, 0.3和0.4)块体热电材料,并系统地研究了该体系的电热输运性能.研究结果表明,利用Se替代Te,可以使BiCuTeO导电层化学键强度增加、带隙增大、载流子有效质量增加以及载流子散射增强,从而导致载流子浓度和迁移率同时降低,进而电导率随着Se含量增加而剧烈降低, Seebeck系数则显著增大.由于综合电输运性能恶化,功率因子随着Se含量增加而减小,导致热电优值zT随着Se含量增加而降低.最终,Se含量为x=0.1的样品,在室温和723 K时的zT值分别达到约0.3和0.7,仍然在较宽温区内保持较高的zT值.由于Se替代Te改变了BiCuTeO的能带结构,通过载流子浓度优化,有望进一步提高其热电性能.     

Elastic moduli and theoretical estimations of physical properties of Ge3Se7–As2Te3

The Bulk (100-x)Ge₃Se₇ - (x)As₂Te₃ (0 ≤ x ≤ 30) glassy alloys were prepared using the melt quenching technique. The elastic moduli (Bulk (K), Micro-hardness (H), Young (Y)), and Poisson's ratio (P_r) of the prepared glasses have been determined using the measured values of the ultrasonic velocities and density (ρ). Values of ρ were measured, then the molar volume (V_m) was estimated theoretically. The DSC thermograms are used to determine the glass transition temperature (T_g). The thermal evaporation method was used to prepare the (100-x)Ge₃Se₇- (x)As₂Te₃ (0 ≤ x ≤ 30) thin films under a vacuum of about 10⁻⁴Pa. The absorbance (A) of the films in the spectral range from 0.45 to 0.85 μm has been measured, then the absorption coefficient (α) and energy gap (E_g) were determined. The Chemical Bond Approach (CBA) has been used successfully to estimate theoretically various physical and structural characteristics of the studied system. Theoretical values of E_g and T_g have been obtained using different methods. The results proved a remarkable agreement between the theoretical and experimental values for both the E_g and T_g. The E_g values decreased from 2.1 to 1.73 eV by increasing the As₂Te₃ content from 0 to 30 at. %. Therefore, these compounds can be used as an absorbing layer for electromagnetic radiation in photovoltaic devices and sensors.     

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.     

Optical properties of polyvinyl alcohol doped (Se80Te20)100–xAgx (0 ≤ x ≤ 4) composites

Polyvinyl alcohol (PVA) doped (Se₈₀Te₂₀)_100–xAg_x (0 ≤ x ≤ 4) thin films were prepared by the spin-coating technique on a quartz substrate. The optical parameters of PVA-doped (Se₈₀Te₂₀)_100–xAg_x (0 ≤ x ≤ 4) composites at the same chalcogen concentration (S₀ = 0.1 mg ml^?1) and PVA/(Se₈₀Te₂₀)₉₆Ag₄ composites at three different chalcogen concentrations viz. S₁ = 0.3 mg ml^?1, S₂ = 0.6 mg ml^?1 and S₃ = 1 mg ml^?1 have been studied. The semi-crystalline nature of the as-deposited thin filmsisdetermined by X-ray diffraction. The transmission and reflection spectra of PVA-doped Se–Te–Ag thin films were obtained in a 350–650 nm spectral region. The optical-band gap has been calculated from the transmission and reflection data. The refractive index has been calculated by the measured reflection data. It has been found that the optical-band gap increases, but the refractive index, extinction coefficient, and the real and imaginary parts of the dielectric constant decrease, with increase in Agcontent in PVA-doped (Se₈₀Te₂₀)_100–xAg_x (0 ≤ x ≤ 4) thin films. Such type of behavior is explained on the basis of decrease in density of the defect states. However, the optical-band gap has been found to be decreased and all other optical parameters show increase in their values with increase in concentration of (Se₈₀Te₂₀)₉₆Ag₄ glass in PVA-doped composites. The results have been explained on the basis of cluster-size formation at the time of dissolution. This study shows that the optical properties of new composites are affected by the change in silver and chalcogen concentration.     

Effect of γ-Radiation on the Conduction Phenomena of Amorphous As4Se4Te2 at Low Temperatures

The I–V characteristic of amorphous As₄Se₄Te₂ was studied at relatively low temperature ranging from – 158°C to –67°C. Three distinct regions of current-voltage range were observed and discussed on the basis of space-charge limited-current mechanism. A number of conduction parameters were calculated for fresh and γ-radiation samples. The effect of γ-radiation and thermal annealing process on the conduction behaviour of the test sample and on the conduction parameters were also discussed.     

Influence of γ-irradiation on the optical and electrical properties of Se(0.85) Te(0.15) films

Abstract

Se_(0.85) Te_(0.15) films were prepared by thermal evaporation under vacuum on glass substrate. The optical and electrical properties of as deposited and irradiated Se_(0.85) Te_(0.15) films with different γ-doses are reported.

The optical constants (absorption coefficient (α), extinction coefficient (k), refractive index (n) and dielectric constants (?, ?) of unirradiated and irradiated films were calculated. The value of allowed direct optical energy gap of Se_(0.85) Te_(0.15) films increased from 1.47 eV. to 1.72 eV. with increasing the γ-doses to 2.5 Mrad. The irradiated films have lower resistivity than those as deposited films (unirradiated). The activation energy (ΔE) increases from 0.72 eV. to 0.86 eV. with increasing γ-doses to 2.5 Mrad.     

Radiation effects on the optical properties of Se85−x Te15Sb x thin films

The optical absorption of the as-deposited and γ-irradiated (doses of 3, 10, 12, 15 Mrad) thermally evaporated Se_85?x Te₁₅Sb _x (x=0, 6, 9) films was measured. The refractive index n and the extinction coefficient k of the films were calculated using an analytical method developed by Zheng et al. The Urbach relation was used in the fundamental edge region to calculate the width of the band tail states. The behavior of the composition with high concentration of antimony (x=9) was attributed to the produced Se–Sb bonds and an excess of Te–Te bonds, which break the chain structure of the Se–Te system and generate defects in the film. The related optical parameters were also calculated and discussed.     

One-pot synthesis of oleic acid-capped cadmium chalcogenides (CdE: E = Se,Te) nano-crystals

Surface-capped CdSe and CdTe nano-crystals (NCs) have been synthesized using cadmium acetate, oleic acid and respective tri-octylphosphine chalcogenide (TOPE; E = Se/Te) in diphenyl ether (DPE). Well-dispersed CdSe particles showed two absorption bands at the region of 431–34 and 458–60 nm in optical absorption study. A band-edge emission resulted at 515 nm with an excitation energy of 400 nm, in its photoluminescence (PL) spectrum. Similarly, UV–visible absorption study of CdTe revealed an absorption band at <700 nm. The broadened X-ray diffraction (XRD) pattern showed that at higher reaction temperature cubic CdSe but hexagonal CdTe can be obtained with crystallite size of <10 nm. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that agglomerated particles are of spherical nature. The inter-planar spacing in CdTe was measured to be 0.406 nm, a characteristic of (100) lattice plane in hexagonal CdTe. X-ray photoelectron spectroscopy (XPS) showed that CdSe NCs have better air stability stable than CdTe. Presence of organic moiety around the semiconductor particles was confirmed by infra-red (IR) spectroscopy.