Photoluminescence and optical absorption in glassy AsxSe1−x

The photoluminescence and optical absorption spectra in glassy As_xSe_1−x with 0.08 ? x ? 0.40 are essentially independent of x. The PL spectra peak at approximately half the optical gap and the exponential slopes of the optical absorption edges are all approximately 75 meV. For x > 0.4, the PL peaks shift to higher energies, the widths of the PL spectra increase, and there is a strong component to the optical absorption well below the optical gap. Comparisons with ESR experiments in the As_xS_1−x system suggest the possibility of two PL peaks. The second PL peak and the optical absorption below the optical band gap for x > 0.4 are attributed to the presence of As-As bonds.     

Raman scattering in amorphous films of In1−xSex alloys

Raman scattering (RS) in amorphous films of In_1−xSe_x with 0.67±x?0.38 has been studied in backscattering geometry with the use of a microscope. Recorded RS spectra are revealing a mixed vibrational density-of-states and molecular character. The spectra spread from the Rayleigh line up to 200-250 cm⁻¹. The bands superimposed on the continua are related to zone center modes of the relevant crystal counterpart, Se-Se or In-In vibrations. The RS spectra suggest the structure of the In_1−xSe_x alloys to be the continuous random network built up of In centered tetrahedral clusters with In and Se atoms at the corners. The structure of the Se-rich alloys is similar to 4-2 networks with dominant InSe_4/2 clusters and two-fold coordination of Se bridging atoms. That of the In-rich alloys is expected to resemble 4-3 network with rather strong involvement of In atoms at corner of the In-centered tetrahedral clusters and Se atoms being linked to three In ones.     

Intermediate-range order in glassy GexSe1−x around the stiffness transition composition

X-ray scattering measurements on glassy Ge_xSe_1−x were performed in a concentration range x=0.07-0.333 in fine steps of 0.005-0.05, in order to explore the relation between the intermediate-range order (IRO) and the stiffness transition in this glassy system. The oscillations beyond the first peak around 20.5 nm⁻¹ in the structure factor, S(Q), remain almost unchanged or damp very slowly with decreasing the Ge concentration x, suggesting the preserve of the short-range order. On the other hand, the pre-peak around 10-12 nm⁻¹, indicating the existence of IRO, systematically changes with decreasing x; its Q position shifts towards the higher Q values and its area decreases. Especially near the onset of the stiffness transition, x=0.20, the peak position starts to deviate from the linear relation. The origin of the pre-peak is discussed in connection with results of a recent anomalous X-ray scattering measurement indicating two contributions of Ge-Ge and Se-Se correlations. Around the onset composition of the stiffness transition, the area of the pre-peak associated with the Ge-Ge correlation has a plateau-like gradual decrease with x followed by a rapid decrease at x<0.18, which is in good agreement with Raman data.     

Negative photoconductivity of illuminated α-Se layers

The investigation results of the negative conductivity effect of α-Se layers illuminated by white light are presented in this paper. It is shown that with illumination of the α-Se layer the value of the current can decrease several times as compared with the dark value of the current. This effect is observed only in the case when the voltage-current characteristic changes into the function dependence (J vs Vⁿ), the exponent index of which n=7.7. This effect is caused by recharge of deep states when the sample is illuminated.     

Compositional dependence of crystallization temperature of Fe80−xMxB20 glasses

The effect of the first group of periodic elements M on the crystallization temperature and heat of crystallization of Fe_80?xM_xB₂₀ alloys has been investigated, where M stands for Ti, V, Cr, Mn, Co or Ni. The addition of the alloying elements, except for large amounts of cobalt and nickel (? 40 at.%), enhances the thermal stability of the glasses, in particular, titanium and vanadium in small amounts significantly increases the crystallization temperature. Elements possessing fewer outer electrons and higher cohesive energies tend to increase the thermal stability of the glasses. The heat of crystallization of the glasses decreases effectively by an addition of titanium or nickel.     

Glass formation and crystallization behavior in Mg65Cu25Y10−xGdx (x=0, 5 and 10) alloys

The glass forming ability and crystallization behavior of Mg₆₅Cu₂₅Y_10−xGd_x (x=0, 5 and 10) alloys have been investigated. The glass forming ability (GFA) is significantly improved when Y in Mg₆₅Cu₂₅Y₁₀ is substituted with Gd. Ternary Mg₆₅Cu₂₅Gd₁₀ bulk metallic glass (BMG) with diameter of at least 8 mm was successfully fabricated by conventional Cu-mold casting method in air atmosphere. Mg₂(Y, Gd) is the first competing crystalline phase against the glass formation in the Mg₆₅Cu₂₅Y_10−xGd_x (x=0, 5) alloys, while Mg₂Cu and Cu₂Gd are the competing crystalline phases in the Mg₆₅Cu₂₅Gd₁₀ alloy. Therefore, the suppression of the formation of Mg₂(Y, Gd) during cooling from the liquid improves the GFA significantly.     

Non-isothermal crystallization kinetics study on new amorphous Ga20Sb5S75 and Ga20Sb40S40 chalcogenide glasses

Bulk glasses of the system Ga₂₀Sb_xS_80−x (x = 5 and 40) were prepared for the first time by the known melt quenching technique. Non-isothermal differential scanning calorimetric (DSC) measurements of as-quenched Ga₂₀Sb_xS_80−x (x = 5 and 40) chalcogenide glasses reveal that the characteristic temperatures e.g. the glass transition temperature (T_g), the temperature corresponding to the maximum crystallization rate (T_p) recorded in the temperature range 400-650 K for x = 5 and 480-660 K for x = 40 are strongly dependent on heating rate and Sb content. Upon heating, these glasses show a single glass transition temperature (T_g) and double crystallization temperatures (T_p1 and T_p2) for x = 5 which overlapped and appear as a single crystallization peak (T_p) for x = 40. The activation energies of crystallization E_c were evaluated by three different methods. The crystallization data were examined in terms of recent analysis developed for non-isothermal conditions. The crystalline phases resulting from (DSC) have been identified using X-ray diffraction.     

Crystallization of Al23Te77 glasses

The crystallization of Al₂₃Te₇₇ glasses has been studied by DSC techniques. Two peaks occur, showing an earlier excess Te crystallization and a later one of the remaining amorphous matrix. Isothermal determination of the kinetics is only possible for the second process, so a model for non-isothermic crystallization is developed on the basis of the Avrami theory, which is in agreement with isothermic results. The shift of the crystallization peaks with the scan rate allows knowledge of the activation energy for both processes. Those are found to be 1.9 and 2.8 eV respectively. Fitting of the experimental data with this model also indicates a diffusion mechanism for the Te crystallization and a homogeneous nucleation and growth process for the second stage. Re-scanning of the sample after the completion of the first peak shows a second T_g commonly associated with phase separation. Results are discussed in terms of the studies of structure recently reported.     

Energetics of CdSxSe1−x quantum dots in borosilicate glasses

The thermodynamics of CdSe quantum dots embedded in a glass matrix is of great interest because of the numerous applications as optical materials. In this study, the energetics and stability of CdSe quantum dots in a borosilicate glass matrix is investigated as a function of size using high-temperature oxide melt solution calorimetry. CdS_0.1Se_0.9 nanoparticles (1-40 nm) embedded in glass were analyzed by photoluminescence spectroscopy, electron microprobe, X-ray fluorescence, high-energy synchrotron X-ray diffraction, and (scanning) transmission electron microscopy using both electron energy loss and energy dispersive X-ray spectroscopy. As CdSe particles coarsen, their heat of formation becomes more exothermic. The interfacial energy of CdSe QDs embedded in a borosilicate glass, determined from the slope of enthalpy of drop solution versus calculated surface area, is 0.56 ± 0.01 J/m².     

Crystallization and local order of bulk AsxTe1−x glasses

The crystallization and the local order of the bulk As_xTe_1?x (0.2 ? × ? 0.8) glasses have been investigated by DSC, X-ray and Mössbauer methods. During the crystallization a metastable phase has been observed between 0.35 ? × ? 0.5 and it was identified as fcc AsTe. The local surrounding of these As_xTe_1?x glasses characterized by the ¹²⁵Te Mössbauer measurements were compared with those of the stable monoclinic As₂Te₃, hexagonal Te and metastable fcc AsTe.     

X-ray photoelectron spectroscopy studies of thin GexSb40−xS60 chalcogenide films

Thin Ge_xSb_40−xS₆₀ (x=5, 15, 20, 25 and 27) chalcogenide films have been investigated by X-ray photoelectron spectroscopy (XPS). X-ray photoelectron spectra show that there is a peculiarity in the relative intensity ratio of the Sb 4d photoelectron peak associated with Sb₂S₃ to the Sb 4d photoelectron peak associated Sb₂S₅ at an average co-ordination number Z of 2.65-2.67. After contamination and photo-oxidation layers were removed from the surface of the films, X-ray photoelectron spectra were measured again. It has been found that binding energies of the Ge 2p and Sb 3d_3/2 photoelectron peaks, which reflect the electronic structure at lower core energy levels, are independent of Z. However, the binding energies of the Ge 3d and Sb 4d photoelectron peaks are more sensitive to Z and have a discontinuity at Z=2.65.     

Sodium tracer diffusion in glasses of the type (CaO·Al2O3)x(2 SiO2)1−x

Tracer diffusion coefficients of the radioactive isotope Na-22 were measured in glasses of the type (CaO·Al₂O₃)_x(2 SiO₂)_1−x to study the diffusion of sodium as a function of glass composition, x, temperature and initial water content. The diffusion of Na-22 in glasses diffusion-annealed in dry air can always be well described by a single tracer diffusion coefficient, but sometimes not in samples annealed in common air. It was found that the sodium tracer diffusion coefficient decreases by about six orders of magnitude when the glass composition x changes from 0 to 0.75 at 800 °C. The temperature dependence of the diffusion of sodium seems to decrease as the silica content increases. Variations of the initial water content in some of the glasses investigated did not very significantly influence the rate of the tracer diffusion of sodium.     

Study of CaO-WO3-P2O5 glass system by dielectric properties, IR spectra and differential thermal analysis

Dielectric constant ε^′, loss tanδ and ac conductivity σ_ac of 40CaO-xWO₃-(60−x)P₂O₅ (with 0?x?15) glasses are studied over a range of frequencies and temperature. The dielectric breakdown strength of these glasses is also determined at room temperature. The values of dielectric parameters, viz., ε^′, tanδ and σ_ac of CaO-P₂O₅ glasses are found to decrease with the introduction of WO₃ up to 3 mol% and increase with further increase in the concentration of WO₃; the probable reasons for such an increase are identified and explained with the aid of IR spectra and differential thermal analysis of these glasses. The variation of tanδ with temperature at different frequencies of CaO-P₂O₅ glasses has exhibited dielectric relaxation effects with decreasing relaxation intensity with increase in the concentration of WO₃ from 0 to 3 mol%; such relaxation effects seem to have been absent in glasses containing WO₃ beyond 3 mol%. The relaxation phenomenon has been analysed by a pseudo-Cole-Cole plot method and the possible mechanism responsible for such relaxation effects has been suggested.     

Electrical transport in n-type semiconducting Ge120BixSe70−xTe10 glasses

Resistivity and thermoelectric power were measured as a function of temperature and composition for Ge₂₀Bi_xSe_70?xTe₁₀ glasses (x = 0–11). The results were compared with the case of of Ge₂₀Bi_xSe_80?x glasses to see on the electrical properties the influence of the substitution of Te for a part of Se. The glasses show n-type conduction for x ? 9, which was not affected by the substitution of Te. The resistivity was about three orders of magnitude lower for the glasses with x < 10, and remained almost the same for x ? 10, compared with the glasses not containing Te. From the composition dependence of the calculated concentration of covalent bonds in the glasses, it was proposed that the appearance of n-type conduction was closely related to the formation of a sufficient number of BiSe bonds and the disappearance of the bonds between two chalcogen atoms such as TeSe or SeSe bonds, and that the remarkably low resistivity in the present glasses with x < 10 was likely to be attributed to the formation of TeSe bonds.     

Insulator-like electrical transport in structurally ordered Al65Cu20+xRu15−x (x = 1.5, 1.0, 0.5, 0.0 and −0.5) icosahedral quasicrystals

Low-temperature resistivities, in zero-field and 8 T field, and magnetoresistance have been measured down to 1.4-300 K for stable icosahedral quasicrystals Al₆₅Cu_20+xRu_15−x (x = 1.5, 1.0, 0.5, 0.0 and −0.5). The analysis of the magnetoresistance data shows an overwhelming presence of anti weak-localization effect (τ_so ∼ 10⁻¹² s). But the sample with x = −0.5 shows anomalous magnetoresistance and the anti weak-localization effect breaks down (τ_so to be 10⁻¹⁵ s). The in-field σ-T between 5 K and 20 K, for x = 1.0, 0.5, 0.0 and −0.5 samples, and between 1.4 K and 40 K for x = 1.5 sample, follow a power-law behavior with an exponent of 0.5 and above ∼30 K the exponent ranges from 1.17 to 1.58. The observed power-laws basically characterize the presence of critical regime of the metal-insulator (MI) transition, dominated by electron-electron and electron-phonon inelastic scattering events respectively. In samples with x = 1.0, 0.5, 0.0 and −0.5 the in field σ-T has been found to follow ln σ-vs-T^1/4 below 5 K, which indicates the presence of variable range hopping. The observed transport features indicate the occurrence of proximity of metal-insulator transition in these Al-Cu-Ru quasicrystal samples.     

Occurrence of Kondo-like behavior in electrical-conductivity of Al70Pd20+xMn10−x (x = 0, 1 and 2) icosahedral quasicrystals

Zero-field and in-field (at 8 T) conductivity vs temperature (σ-T), magneto-resistance (Δρ/ρ), magnetization vs temperature (M-T) and magnetization vs field (M-H) of unannealed Al₇₀Pd₂₀Mn₁₀ and annealed Al₇₀Pd₂₀Mn₁₀, Al₇₀Pd₂₁Mn₉ and Al₇₀Pd₂₂Mn₈ quasicrystalline alloys have been studied in the temperature range of 1.4-300 K. Room temperature resistivity and the low-temperature magneto-resistance show a correlation with the corresponding magnetization. The σ-T for all the studied samples shows a pair of minima and maxima. The σ-T maxima show a correlation with the total magnetization. The analysis shows that σ-T is dominated by weak-localization effects. The minima are arising due to competing inelastic scattering times τ_i (e-ph scattering in the dirty metallic limit, τ_i ∝ T⁻²) and the Kondo-type spin-flip scattering time τ_sf whereas the maxima has been attributed to ‘Kondo-maxima’, occurring due to maxima in the spin-flip rate . The magneto-resistance of these samples shows a changeover from negative to positive where the negative component shows a correlation with the magnetization of the sample. The values of parameters derived from refinement give spin-flip scattering fields, which are found to be correlated with the total sample magnetization.     

Atomic layer deposition of Al2O3, ZrO2, Ta2O5, and Nb2O5 based nanolayered dielectrics

Ta₂O₅, Ta-Nb-O, Zr-Al-Nb-O, and Zr-Al-O mixture films or solid solutions were grown on Si(1 0 0) substrates at 300 °C by atomic layer deposition. The equivalent oxide thickness of Ta₂O₅ based capacitors was between 1 and 3 nm. In Zr-Al-O films, the high permittivity of ZrO₂ was combined with high resistivity of Al₂O₃ layers. The permittivity, surface roughness and interface charge density increased with the Zr content and the equivalent oxide thickness was between 2.0 and 2.5 nm. In the Zr-Al-Nb-O films the equivalent oxide thickness remained at 1.8-2.0 nm.     

Electrical properties of glassy AsxGe10Te90−x alloys

This paper analyses the electrical properties of glassy alloys of As_xGe₁₀Te_90?x, while reporting the conductivity and dielectric constant of As₅Ge₁₀Te₈₅ and As₁₅Ge₁₀Te₇₅ compositions in the temperature range 77–383 K and the frequency range from dc to 5 MHz. The dc conductivity has been shown to be of the form

The ratio σ₀₁/σ₀₂ is of the order of 10⁶. ΔE₁, the higher temperature activation energy, is dependent on the composition, while ΔE₂, the lower temperature activation energy, is less dependent on the composition. The dielectric constant has been found to be independent of temperature and frequency up to about 253 K. However, at higher temperatures, it becomes activated and proportional to log ω.Some common features of As_xGe₁₀Te_90?x are a kink in dc conductivity, a ω^0.8 relationship for ac conductivity, no evidence of variable-range hopping at low temperatures, field-dependent conductivity and memory switching. The data can be interpreted in terms of the dangling-bond theory of Mott and his collaborators. A high density of states of the order of 10²⁰eV^?1 cm^?3 near the Fermi level may be expected.     

Glass-forming ability and crystallization of bulk metallic glass (HfxZr1−x)52.5Cu17.9Ni14.6Al10Ti5

We have produced a series of bulk metallic glasses of composition (Hf_xZr_1−x)_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ (with x=0-1) by an arc melting/suction casting method. The density of these alloys increases by nearly 67% with increasing Hf content from 6.65 g/cm³ (x=0) to 11.09 g/cm³ (x=1). Over the same composition range the glass-forming ability decreases, as demonstrated by the size of the largest amorphous ingots that can be cast without crystallization. Although both the glass transition temperature and the melting temperature increase linearly with increasing Hf content, the reduced glass transition temperature (T_g/T_m) decreases, from 0.64 (x=0) to 0.62 (x=1), which suggests that the `confusion principle' correlating increased glass-forming ability with increased number of components, does not apply in this case due to the chemical similarity between Zr and Hf. A different crystallization behavior is observed for Zr-based and Hf-based glasses. The final crystalline phases are CuZr₂ and Zr₂Ni for Zr-based alloys, and Al₁₆Hf₆Ni₇ and CuHf₂ for Hf-based alloys.