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1.
The evolution of the structure of SbxGe40-xSe60 (x = 8, 12, 15, 18 and 20) chalcogenide glasses is determined by high resolution X-ray photoelectron spectroscopy as Sb is added and the average coordination number (Z) is varied from 2.60 to 2.72. For glasses with Z < 2.67 the structure consists of deformed tetrahedra and pyramids, in which at least one Se atom is substituted by Ge or Sb atoms. Increase in the concentration of cations beyond Z  2.67 leads to a strengthening of the above regularity and formation of the structure by shared pyramids and tetrahedra with two or more Se atoms substituted by cations. At the same time Se–Se dimers are present in all the investigated compositions while they are not expected in these Se-deficient glasses. Their formation dictates that there must be metal-rich regions somewhere in the structure.  相似文献   

2.
We have used the density functional theory to make the models of GexSe1?x glass for which the energy is a minimum. The clusters, Ge2Se2, Ge2Se3, Ge3Se, Ge3Se2, Ge4Se, GeSe3, GeSe4, chain mode zig-zag Ge4Se3, corner sharing GeSe4, and edge sharing Ge2Se6, have been made successfully and their vibrational spectra have been calculated from the first principles. We are able to optimize the bond distances as well as the bond angles. The calculated values of the frequencies of vibrations of the various clusters have been compared with those obtained from the experimental Raman spectra of actual glasses, GexSe1?x(0 < x < 0.3). The local concentration, x within 0.25 nm is nonuniform in the amorphous material. When the same cluster occurs in two stable configurations, low frequency vibrations of frequency, ν < 100 cm?1, are found. The corner sharing GeSe4 has low frequency modes at 54 cm?1 and 93 cm?1 whereas these modes disappear in the pyramidal configuration. The low frequency modes are therefore associated with the breaking of C4 symmetry of the pyramidal configuration. The computed vibrational frequencies of clusters Ge3, Ge4Se3, Ge2Se3, GeSe3 and Ge3Se2 are actually present in the Raman spectra of the glass, GexSe1?x(0 < x < 0.3).  相似文献   

3.
《Journal of Non》2006,352(50-51):5413-5420
Glasses in the system Ge–Ga–Sb–S/Se have been elaborated with different S/Se ratios in order to increase the non-linear optical properties of these glasses. We report results of a systematic study examining the relationship of the physical properties to the structure of the glasses in the system Ge0.18Ga0.05Sb0.07S0.70−xSex with x = 0, 0.02, 0.05, 0.10, 0.20, 0.30 and 0.40 where the replacement of S by Se has been made. The non-linear refractive index has been measured using the Z-scan technique, with picosecond pulses emitted by a 10 Hz Q-switched mode-locked Nd-Yag laser at 1064 nm under conditions suitable to characterize ultrafast non-linearities. The decrease of the glass transition temperature, the increase of the non-linear refractive index and of the density with the progressive replacement of S by Se have been correlated along with the red shift of the absorption band gap, to associated structural reorganization. A corresponding progressive decrease of corner-sharing GeS4/2 and the formation of mostly two edge-sharing Ge2S4S2/2, S3Ge–S–GeS3 as well as mixed GeS4−xSex units have been identified by Raman spectroscopy.  相似文献   

4.
B.J. Madhu  H.S. Jayanna  S. Asokan 《Journal of Non》2009,355(52-54):2630-2633
Bulk Ge7Se93?xSbx (21 ? x ? 32) glasses are prepared by melt quenching method and electrical switching studies have been undertaken on these samples to elucidate the type of switching and the composition and thickness dependence of switching voltages. On the basis of the compressibility and atomic radii, it has been previously observed that Se-based glasses exhibit memory switching behavior. However, the present results indicate that Ge7Se93?xSbx glasses exhibit threshold type electrical switching with high switching voltages. Further, these samples are found to show fluctuations in the current–voltage (IV) characteristics. The observed threshold behavior of Ge7Se93?xSbx glasses has been understood on the basis of larger atomic radii and lesser compressibilities of Sb and Ge. Further, the high switching voltages and fluctuations in the IV characteristics of Ge–Se–Sb samples can be attributed to the high resistance of the samples and the difference in thermal conductivities of different structural units constituting the local structure of these glasses. The switching voltages of Ge7Se93?xSbx glasses have been found to decrease with the increase in the Sb concentration. The observed composition dependence of switching voltages has been understood on the basis of higher metallicity of the Sb additive and also in the light of the Chemically Ordered Network (CON) model. Further, the thickness dependence of switching voltages has been studied to reassert the mechanism of switching.  相似文献   

5.
《Journal of Non》2007,353(13-15):1388-1391
Ge15As15Se70−xTex materials with x = 56, 60 and 63, of potential use in IR-integrated optics, were prepared by the classical melt-quenching method. A macroscopic phase separation was observed with a crystalline phase on the top and an amorphous one at the bottom. The glasses from the bottom were transparent from 1.9 to 16 μm without any purification of the elemental precursors Ge, As, Te and Se. The higher the Te/Se content in the glasses the lower their glass transition temperature and thermal stability. Films 7–12 μm thick of the above stated compositions were deposited by thermal evaporation. The higher the tellurium content, the larger the optical band gap shift of the films in the infra-red and the higher the refractive index.  相似文献   

6.
Alternating differential scanning calorimetric (ADSC) studies have been performed to understand the thermal behavior of bulk GexSe35?xTe65 glasses (17 ? x ? 25); it is found that the glasses with x ? 20 exhibit two crystallization exotherms (Tc1 & Tc2). On the other hand, those with x ? 20.5, show a single crystallization reaction upon heating. The exothermic reaction at Tc1 has been found to correspond to the partial crystallization of the glass into hexagonal Te and the reaction at Tc2 is associated with the additional crystallization of rhombohedral GeTe phase. The glass transition temperature of GexSe35?xTe65 glasses is found to show a linear but not-steep increase, indicating a progressive, but a gradual increase in network connectivity with Ge addition. It is also found that Tc1 of GexSe35?xTe65 glasses with x ? 20, increases progressively with Ge content and eventually merges with Tc2 at x  20.5 (〈r = 2.41); this behavior has been understood on the basis of the reduction in TeTe bonds of lower energy and increase in GeTe bonds of higher energy, with increasing Ge content. Apart from the interesting composition dependent crystallization, an anomalous melting behavior is also exhibited by the GexSe35?xTe65 glasses.  相似文献   

7.
Gao Tang  Cunming Liu  Zhiyong Yang  Lan Luo  Wei Chen 《Journal of Non》2009,355(31-33):1585-1589
Microstructure of the chalcohalide glasses: GeSe2–Ga2Se3–CsI and GeSe2–Ga2Se3–PbI2 ternary system were investigated by Raman spectra, lifetime of Dy3+ infrared emission and glass transition temperature (Tg). The evolution of the Raman spectra shows that the fundamental structural groups of these studied glasses consist of [Ge(Ga)Se4] tetrahedral and some complex structure units [Ge(Ga)IxSe4?x](x = 1–4). The x value varied when the different iodide was added in Ge–Ga–Se matrix. For GeSe2–Ga2Se3–CsI glasses, the [Ge(Ga)IxSe4?x](x = 1–4) mixed-anion tetrahedral and [Ga2I7]? units occurred. For GeSe2–Ga2Se3–PbI2 glasses, the [Ge(Ga)I2Se2], [Ge(Ga)I3Se] units can be formed. The changes of Dy3+ infrared emission lifetime and Tg support the results. Additionally, [PbIn] structural units will be formed in GeSe2–Ga2Se3–PbI2 glasses due to high form-ability of these units when the PbI2 content is high.  相似文献   

8.
《Journal of Non》2006,352(30-31):3230-3235
The present paper reports the dc conductivity measurements at high electric fields in vacuum evaporated amorphous thin films of (Ge20Se80)1−xSnx glassy alloys where 0 < x < 1. Current–voltage (IV) characteristics have been measured at various fixed temperatures. In these 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 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. These results show that the effect of incorporation of Sn in the Ge–Se system is quite different at its low and high concentration. This peculiar role of third element Sn as an impurity in the pure binary Ge20Se80 glassy alloy is also discussed.  相似文献   

9.
《Journal of Non》2007,353(8-10):729-732
The purpose of this work is to study the change in the structure of the Ge–Se network upon doping with Ag. We report here a neutron diffraction study on two glasses of the system Agx(Ge0.25Se0.75)100−x with different silver contents (x = 15 and 25 at.%) and for two different temperatures (10 and 300 K). The total structure factor S(Q) for the two samples has been measured by neutron diffraction using the two-axis diffractometer dedicated to structural studies of amorphous materials, D4, at the Institut Laue Langevin. We have derived the corresponding radial distribution functions for each sample and each temperature, which gives us an insight about the composition and temperature dependence of the correlation distances and coordination numbers in the short-range. Our results are compatible with the presence of both GeSe4/2 tetrahedra and Se–Se bonds. The Ag atoms are linked to Se in a triangular environment. Numerical simulations allowing the identification of the main peaks in the total pair correlation functions have complemented the neutron diffraction measurements.  相似文献   

10.
The evolution of topology and chemical order along the AsxGexSe1 ? 2x composition-line within the As–Ge–Se glass-forming region is studied by high-resolution X-ray photoelectron spectroscopy. It is shown that cation–cation bond formation becomes a dominant process for the compositions with x > 0.09. The results explain the peculiarities observed around this composition recently in the temperature-modulated differential scanning calorimetry data. Substitution of two selenium atoms within constituent structural units (pyramids and/or tetrahedra) by corresponding cations explains a second peculiarity point at x > 0.16 compositions observed recently with the above technique. The present observations show segregation of As and then Ge at high concentrations of cations in the system (x > 0.20).  相似文献   

11.
Bulk Ge15Te85 ? xSnx and Ge17Te83 ? xSnx glasses, are found to exhibit memory type electrical switching. The switching voltages (Vt) and thermal stability of Ge15Te85 ? xSnx and Ge17Te83 ? xSnx glasses are found to decrease with Sn content. The composition dependence of Vt has been understood on the basis of the decrease in the OFF state resistance and thermal stability of these glasses with tin addition. X-ray diffraction studies reveal that no elemental Sn or Sn compounds with Te or Ge are present in thermally crystallized Ge–Te–Sn samples. This indicates that Sn atoms do not interact with the host matrix and form a phase separated network of its own, which remains in the parent glass matrix as an inclusion. Consequently, there is no enhancement of network connectivity and rigidity. The thickness dependence of switching voltages of Ge15Te85 ? xSnx and Ge17Te83 ? xSnx glasses is found to be linear, in agreement with the memory switching behavior shown by these glasses.  相似文献   

12.
《Journal of Non》2007,353(32-40):3045-3048
The atomic structures of Ge–Sb–S ternary glasses have been investigated by using the neutron diffraction method. The structure factor, S(q), for GexSb40−xS60 (x = 10, 20 and 30) has a first sharp diffraction peak (FSDP) at around 1.1 Å−1. The oscillation in S(q) persisted up over 25 Å−1 indicates that the chemical short-range order is presented in these glasses. The aspect of S(q) for Ge10Sb30S60 is different from those for Ge20Sb20S60 and Ge30Sb10S60. These results show that the atomic structure changes drastically between Ge10Sb30S60 and Ge20Sb20S60. The short-range order of tetrahedral GeS4 and pyramidal SbS3 units seems to exist in the GexSb40−xS60 (x = 10, 20 and 30) glasses.  相似文献   

13.
Chalcogenide bulk glasses Ge20Se80?xTex for x  (0, 10) have been prepared by systematic replacement of Se by Te. Selected glasses have been doped with Er and Pr, and all systems have been characterized by transmission spectroscopy, measurements of dc electrical conductivity and low-temperature photoluminescence. Absorption coefficient has been derived from measured transmittance and estimated reflectance. Both absorption and low-temperature photoluminescence spectra reveal shifts of absorption edge and/or dominant luminescence band to longer wavelength due to Te  Se substitution. Arrhenius plots of dc electrical conductivity, in the temperature range 300–450 K, are characterized by activation energies roughly equal to the half of the optical gap. Arrhenius plots for temperatures below 300 K yield much lower activation energies. The dominant low-temperature luminescence band centered at about half the band gap energy starts to quench above 200 K and a new band appears at 900 nm. The band at 900 nm, due to band to band transitions, overwhelms the spectra at room temperature. Systems doped with Er exhibit a strong luminescence due to 4I13/2  I15/2 transition of Er3+ ion at 1539 nm, and Pr doped samples exhibit a relatively weak luminescence peak at 1590 nm, which we tentatively assign to 3F3  3H4 transition of Pr3+ ion.  相似文献   

14.
《Journal of Non》2007,353(32-40):3314-3317
The ionic conductivity of several chalcogenide glasses increases abruptly with mobile ion addition from values typical of insulating materials (10−16–10−14 Ω−1 cm−1) to values of fast ionic conductors (10−7–10−1 Ω−1 cm−1). This change is produced in a limited concentration range pointing to a percolation process. In a previous work [M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259] the transition from semiconductor to fast ionic conductor of Agx(Ge0.25Se0.75)100−x glasses was detected at x1  10 at.% in the form of a steep change in the conductivity. Agx(Ge0.25Se0.75)100−x glasses with x  25 at.%, prepared by a melt quenching method, are investigated by impedance spectroscopy in the frequency range 5 Hz–2 MHz at different temperatures, T, from room temperature to 363 K and by DC measurements at room temperature. The conductivity of the glasses, σ, was obtained as a function of silver concentration and temperature. For x  10 at.% our results are in agreement with those reported by Kawasaki et al. [M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259]. The percolation transition was observed in the range 7  x  8. The temperature dependence of the ionic conductivity follows an Arrhenius type equation σ = (σo/T) · exp(−Eσ/kT). The activation energy of the ionic conductivity, Eσ, and the pre-exponential term, σo, are calculated. The results are discussed in connection with other chalcogenide and chalcohalide systems and linked with the glass structures.  相似文献   

15.
《Journal of Non》2006,352(6-7):704-708
Effect of 60Co γ-irradiation on As–Se glasses stored for 20 years and subjected to saturated natural physical aging is studied using differential scanning calorimetry. It is shown that γ-irradiation activates further physical aging, which leads to an increase in glass transition temperature and endothermic peak area near glass transition region for AsxSe100−x samples with x < 30. The observed changes are associated with additional structural relaxation of radiation-modified glass network.  相似文献   

16.
《Journal of Non》2007,353(13-15):1261-1263
Field emission-scanning electron microscopy (FE-SEM) and electric force microscopy (EFM) measurements were carried out on bulk Agx(Ge0.25Se0.75)100−x glasses with x = 5 and 15 at.%. The presence of chemical contrasts and electrical inhomogeneity throughout the samples indicated that the glasses were phase separated. Moreover it appears that while silver-rich nodules of ∼1 μm were embedded in a silver-poor connecting phase in the glass containing 5 at.% Ag, it was the opposite that occurred in the glass containing 15 at.% Ag. Such an inversion explains the large difference of seven orders of magnitude in the conductivity of the two glasses.  相似文献   

17.
Nanoindentation studies on Ge15Te85 ? xInx glasses indicate that the hardness and elastic modulus of these glasses increase with indium concentration. While a pronounced plateau is seen in the elastic modulus in the composition range 3  x  7, the hardness exhibits a change in slope at compositions x = 3 and x = 7. Also, the density exhibits a broad maximum in this composition range. The observed changes in the mechanical properties and density are clearly associated with the thermally reversing window in Ge15Te85 ? xInx glasses in the composition range 3  x  7. In addition, a local minimum is seen in density and hardness around x = 9, the chemical threshold of the system. Further, micro-Raman studies reveal that as-quenched Ge15Te85 ? xInx samples exhibit two prominent peaks, at 123 cm? 1 and 155 cm? 1. In thermally annealed samples, the peaks at 120 cm? 1 and 140 cm? 1, which are due to crystalline Te, emerge as the strongest peaks. The Raman spectra of polished samples are similar to those of annealed samples, with strong peaks at 123 cm? 1 and 141 cm? 1. The spectra of lightly polished samples outside the thermally reversing window resemble those of thermally annealed samples; however, the spectra of glasses with compositions in the thermally reversing window resemble those of as-quenched samples. This observation confirms the earlier idea that compositions in the thermally reversing window are non-aging and are more stable.  相似文献   

18.
《Journal of Non》2007,353(13-15):1243-1246
Silver-containing chalcogenide glasses are of considerable interest for applications in optical and electrical recordings and as solid electrolytes. Therefore, insight in their structural properties is important. A neutron thermodiffraction study of Agx(Ge0.25Se0.75)100−x glasses with x = 15 and 25 at.% was carried out. The amorphous samples were heated in situ from room temperature up to 350 °C, while the neutron diffraction spectra were collected every degree. For both glasses the Ag8GeSe6 phase appeared first, followed by the crystallization of GeSe2. The crystallization process for the Ag-rich glass was more complex with the appearance of a non-identified third phase. Rietveld refinement helped us in studying the evolution of the lattice constants of each phase upon heating.  相似文献   

19.
C.W. Chang  T. Matsui  M. Kondo 《Journal of Non》2008,354(19-25):2365-2368
Paramagnetic defects of undoped hydrogenated microcrystalline silicon–germanium alloys (μc-Si1?xGex:H) grown by low temperature (200 °C) plasma-enhanced chemical vapor desposition (PECVD) have been measured by electron spin resonance (ESR) and compared with those of hydrogenated amorphous silicon–germanium (a-Si1?xGex:H). The spin density of μc-Si1?xGex:H increases with Ge content and shows a broad maximum of ~1017 cm?3 at x  0.5, which reasonably accounts for the decreased photoconductivity. While the Ge dangling bond defects prevail in a-Si1?xGex:H for Ge-rich compositions, we detected no ESR signal in μc-Si1?xGex:H for x > 0.75 where an electrical change occurs from weak n- to strong p-type conduction. These results indicate that dangling bonds are charged in large densities due to the presence of the acceptor-like states in undoped μc-Si1?xGex:H.  相似文献   

20.
S. Sen  S. Joshi  B.G. Aitken  S. Khalid 《Journal of Non》2008,354(40-41):4620-4625
The nearest-neighbor coordination environments of Te atoms in GexTe100?x glasses with x = 15 and 20 and in AsxTe100?x glasses with 40 ? x ? 65 have been studied with Te K-edge EXAFS spectroscopy. The average coordination number of Te atoms in all glasses is found to be ~2.0 and no violation of the 8-N rule is observed. The compositional makeup of the first coordination shell of Te atoms indicates that chemical order is largely preserved in both glass-forming binary systems. Sudden changes in the Te coordination environment and violation of chemical order are observed at the stoichiometric As40Te60 glass implying formation of a constrained network. The compositional dependence of the physical properties in both systems can be correlated to short-range chemical order.  相似文献   

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