Glass transition temperature shift under pressure for some semiconducting glasses

Glass transition temperatures (T_g) were measured as a fucntion of pressure for the semiconducting glasses As₂S₃, Te₁₅Ge₂As₃ and Cd₆Ge₃As₁₁, using changes in the transit time of an ultrasonic pulse to detect the glass transition. The measurements of T_g as a function of pressure were compared to the predictions of the free volume theory of the glass transition. For As₂S₃ and Te₁₅Ge₂As₃, T_g first increased and then levelled off with increasing pressure, as predicted by the free volume theory. The amorphous semiconductor Cd₆Ge₃As₁₁ exhibited anomalous behavior: T_g first increased with pressure, but at the relatively low pressure of 1.5 kbar it began to decrease with increasing pressure.     

Direct measurement of isothermal compressibility of amorphous semiconductors

The isothermal compressibilities of the chalcogenide glasses Te₁₅Ge₃As₂, Te₁₅Ge₂As₃ and As₂Se₃ have been measured to 5 kbar by direct measurement of the change in sample length using a differential transformer placed inside the hydrostatic pressure vessel. Calibration data was obtained from the measurement of the compressibilities of alkali halide single crystals. For Te₁₅Ge₃As₂, the isothermal compressibility was found to be 6.00 × 10^?12 ? 0.7 × 10^?22P cm²/dyne; for Te₁₅Ge₂As₃, 5.58 × 10^?12 + 0.3 × 10^?22P cm²/dyne; for As₂Se₃, 6.30 × 10^?12 ? 0.8 × 10^?22P cm²/dyne where P is the pressure in dynes/cm². Errors are near 5%.     

Cd2Ge2O6 nanowires grown by a simple hydrothermal route

Cd₂Ge₂O₆ nanowires have been synthesized by a simple hydrothermal route in the absence of any surfactants. The diameter and length of the Cd₂Ge₂O₆ nanowires with flat tips are 30‐300 nm and several dozens of micrometers, respectivley. X‐ray diffraction and high‐resolution transmission electron microscopy results show that the nanowires are composed of monoclinic Cd₂Ge₂O₆ phase. The growth condition dependence results show that the formation of the Cd₂Ge₂O₆ nanowires undergoes three morphological changes from spherical particles to nanorods, and finally to nanowires. The photoluminescence spectrum of the Cd₂Ge₂O₆ nanowires exhibits three fluorescence emission peaks centered at 422 nm, 490 nm and 528 nm showing the potential application for optical devices. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Neutron studies of an inorganic plastic glass

A neutron diffraction investigation has been performed of the structure of four chalcogenide glasses, using the D4c diffractometer on the high-flux reactor at the Institut Laue-Langevin (ILL). Vitreous Ge₃As₅₂S₄₅ is shown to have a structure based on As₄S₃ molecules, whereas that of its selenide analogue, vitreous Ge₃As₅₂Se₄₅, involves far fewer As₄Se₃ molecules. Two As₂X₃ reference glasses (X = S or Se) have purely network structures. As-X, and As-As bond lengths have been extracted, together with their associated co-ordination numbers, from peak fits to the real space correlation functions, T(r), and suggest that all four glasses are chemically ordered; i.e. that they contain the maximum possible number of As-X (and Ge-X) bonds. The molecular nature of vitreous Ge₃As₅₂S₄₅ is evident from the enhanced first diffraction peak relative to the As₂S₃ reference glass, and has been further investigated by comparing its interference function, Qi(Q), with that expected for an isolated As₄S₃ molecule. On the other hand, the second diffraction peak for vitreous Ge₃As₅₂S₄₅ is not reproduced by a random orientation molecular model, indicating that there is orientational correlation between adjacent molecules, and this is discussed with respect to the structure of the corresponding As₄S₃ crystalline phases. The neutron-weighted vibrational density of states (VDOS) for vitreous Ge₃As₅₂S₄₅, obtained with the ISIS MARI spectrometer, unambiguously confirms the existence of As₄S₃ molecules, as revealed by a peak at ~ 34 meV arising from the triangle of As atoms that form the base of the molecule. MARI data recorded at ambient temperature also reveal a strong quasi-elastic component in the scattered intensity, which has been further investigated using the IN5 quasi-elastic scattering spectrometer (ILL) and indicates the presence of rotational diffusion of the As₄S₃ molecules; i.e. that, at ambient temperature, vitreous Ge₃As₅₂S₄₅ behaves as a plastic glass. A possible structural model for vitreous Ge₃As₅₂S₄₅ comprises a two-dimensional tangled GeS₂ net with As₄S₃ molecules trapped in its folds whereas, for vitreous Ge₃As₅₂Se₄₅, a clathrate model appears more appropriate.     

Kinetics of self-bleaching in some photodarkened Ge–As–S amorphous thin films

Three amorphous Ge–As–S films with the average coordination numbers of 2.4–2.8 were prepared by thermal evaporation. True relaxation that is self-bleaching of photodarkened state has been studied. Considerable differences in kinetics of relaxation of photodarkened state were observed for Ge₁₂As₁₇S₇₁ (Sa 1) and Ge₁₅As₂₀S₆₅ (Sa 3) amorphous thin films. In both cases, the relaxation (self-bleaching) followed stretched exponential, however, flexible matrix of Sa 1 film relaxed significantly faster than the matrix of Sa 3 film. The amorphous film Ge_25.5As_29.5S₄₅ (Sa 8) was found to be insensitive to illumination. It is suggested that the network rigidity may significantly influence the magnitude of photodarkening and the rate of relaxation of photodarkened state.     

Ultrasonic wave velocities and attenuation in IVb-Vb-VIb chalcogenide glasses: 2–300 K

The velocities and attenuation of longitudinal and shear ultrasonic (12 MHz) waves have been measured for several chalcogenide glasses (Ge₁₀Si₁₂As₃₀Te₄₈, Ge₂₀As₃₀Se₅₀, Si₂₀As₃₂Te₄₈, Ge₁₀Si₁₂As₂₉Te₄₉, Ge₁₂S₁₄As₃₅Te₄₉). The bulk, shear and Young's moduli and the Poisson ratio are found to be insensitive to the glass composition. The temperature dependences of the longitudinal and shear-wave velocities are negative at higher temperatures and approach 0 K with a zero slope. The ultrasonic attenuation does not exhibit either the broad loss peak or the smaller low-temperature peak found in many other glasses: the elastic and anelastic behaviour is quite different from that of oxide glasses — no evidence for the existence of two-level systems has been obtained from the ultrasonic measurements.     

Evolution of chemical structure during silver photodiffusion into chalcogenide glass thin films

The change of chemical structure resulting after X-ray and photo-induced silver diffusion into chalcogenide glass (ChG) thin films is monitored by high resolution X-ray photoelectron spectroscopy (XPS). As₄₀S₆₀ and Ge₃₀Se₇₀ thin films, which are based on pyramids and tetrahedral structural units, are investigated as model materials. Survey, core level (As 3d, S 2p, Ge 3d, Ge 2p, Se 3d, Ag 3d_5/2, O 1s, C 1s) and valence band spectra have been recorded and analyzed. Reference point for the binding energy is established by the subsequent deposition of thin gold film on top of the measured samples. The chemical structure gradually changes during diffusion of silver in all the samples. The mechanism of change depends on the chemical composition, thickness of the diffused silver layer and conditions of irradiation. It is revealed that surface oxygen can play important role in the Ag photodiffusion process, leading to phase separation on the surface of the films. Photodiffusion of Ag into As₄₀S₆₀ film leads to the formation of a uniform ternary phase and arsenic oxides on the surface. The formation of ethane-like Ge₂(S_1/2)₆ units together with germanium oxidation are the main outcomes of X-ray induced Ag diffusion into Ge₃₀Se₇₀ film.     

Bimolecular relaxation kinetics observed in radiation-optical properties of Ge–As(Sb)–S glasses

Numerical parameters of decaying bimolecular relaxation kinetics are analysed for radiation-optical properties of chalcogenide glasses within ternary stoichiometric As₂S₃–GeS₂ and Sb₂S₃–GeS₂ as well as non-stoichiometric As₂S₃–Ge₂S₃ systems in dependence on their composition. It is shown, that the observed compositional dependences of bimolecular relaxation parameters have a monotonic character in the glasses of both stoichiometric systems, while the anomalous extremum-like behavior is observed in the vicinity of average coordination number close to 2.7 in non-stoichiometric glasses.     

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 formation and properties of chalcogenide systems XXVI: Permittivity and the structure of glasses AsxSe1−x and GexSe1−x

Correlation between the real part of the dielectric constant and the structure of glasses in the system As_xSe_1?x and Ge_xSe_1?x is reported. The mole polarization is calculated using the Sellmeyer approximation neglecting the Lorentz field. The vibrationally caused part of the permittivity which is obtained by subtraction of the mole refraction reflects ordered states in the investigated series. Besides the known crystalline compounds As₂Se₃ and As₄Se₄ the formation of the vitreous AsSe₃ and As₃Se₂ from the liquid state has to be supposed. In the system Ge_xSe_1?x the formation of the compounds GeSe₂ and Ge₂Se₃ is completed by GeSe₄ which as Ge₂Se₃ obviously only exists in the non-crystalline state. GeTe₄ has been reported as a metastable crystalline phase. The temperature dependence of ?_r of vitreous As₂Se₃ is tentatively interpreted in terms of the dipole orientation caused by conversion of the charge in valence alternation pairs.     

ESR study of structural changes in chalcogenide glasses

Structural changes in chalcogenide glasses induced by various processes such as annealing, illumination or application of a high pressure were investigated from a microscopic viewpoint by measuring changes of the ESR spectrum from Mn doped as a probe. Here, Mn incorporated in the glass network exhibits a characteristic ESR signal with g = 4.3 having a hyperfine structure. It is found that the above-mentioned processes cause a change of the lineshape of the hyperfine line and a change of the hyperfine structure constant A, both of which reflect the bonding characteristics around Mn. One of the annealing effects for bulk glasses is the increase of the A-value. The change of the A-value for well-annealed As₂Se₃ and As₄Se₅Ge₁ films occurs reversibly in sequential illumination and annealing cycles, as does the shift of the optical gap. The reversible change of the lineshape is observed for well-annealed As₂Se₃ films in the illumination and annealing cycles. Application of a high pressure to bulk glasses causes changes similar to those by illumination for annealed films. From these results it is concluded that the randomness of amorphous structure decreases by annealing and increases by illumination or application of high pressure.     

Low-temperature specific heat of amorphous and crystalline Te0.81Ge0.15As0.04

We have measured the specific heats of amorphous and crystalline specimens of Te_0.81Ge_0.15As_0.04 between 0.2 and 20 K, and of crystalline Te_0.93As_0.07 between 1 and 20 K. Amorphous Te_0.81Ge_0.15As_0.04 shows a low-temperature linear specific heat anomaly whose magnitude, 0.027 mJ/mol-K², is similar to that of other amorphous insulators. Crystalline Te_0.81Ge_0.15As_0.04 exists as a two-phase material comprised of GeTe and As-doped Te. The specific heat of this material is analyzed in terms of a weighted average of the properties of its two constituents.     

Fine embossing of chalcogenide glasses – a new fabrication route for photonic integrated circuits

For the first time embossing of ribs, from 1 to 10 μm wide and ∼10 mm long, has been carried out in chalcogenide glass layers sputtered onto semiconductor wafer substrates, with potential to act as monomode waveguides; these features have been similarly embossed in the surface of bulk chalcogenide glasses. The embossing shows very good replication of the GaAs mould patterning to 1 μm definition, with evidence also for sub-micron replication. For the embossing, thin coatings of the chalcogenide glasses were sputtered onto wafer substrates as follows: (i) a 6 μm layer of Ge₁₇As₁₈Se₆₅ (at.%) onto porous Si-on-Si wafer substrates and (ii) a 4 μm layer of Ge₁₅As₁₅Se₁₇Te₅₃ onto uncoated GaAs substrates. The Ge₁₇As₁₈Se₆₅ sputtered glass layer on porous Si-on-Si was demonstrated to slab waveguide at 1.55 μm wavelength; it was designed to achieve monomode waveguiding at 1.55 μm after embossing, for the 5 μm wide rib. The series of ribs, 1–10 μm wide, were successfully embossed in the Ge₁₇As₁₈Se₆₅ glass sputtered layer on porous Si-on-Si, but cracking of the glass layer occurred during the embossing process. Successful embossing of ribs without the glass layer cracking was achieved for the Ge₁₅As₁₅Se₁₇Te₅₃ sputtered glass layer on uncoated GaAs. Due to its relative simplicity, it is likely that hot embossing of this type of glass-based matrix offers an extremely promising route for producing high-resolution, guided-wave optical components and circuitry at low-cost, high-volume, and for a wide wavelength range.     

γ-ray irradiation induced multiple effects on Ge–As–Se chalcogenide glasses

In this work, multiple effects of γ-ray irradiation on properties of bulk Ge–As–Se chalcogenide glasses were studied. Increased density (ρ), thermal expansion coefficient (α) and decreased optical band gap (E_opt) were observed after irradiation, depending on glass compositions. Glasses with stoichiometric (GeSe₂)_100?x(As₂Se₃)_x compositions showed linear correlations between As₂Se₃ proportion x and irradiation sensitivity, which is expressed by Δρ/ρ, Δα/α and ΔE_opt/E_opt. Nonstoichiometric glasses (Ge₂Se₃)_100?x(As₂Se₃)_x exhibited irregular variations. The phenomena are discussed in terms of chemical bonds transition and structural evolution under γ-ray irradiation.     

Crystal structure of α‴-(Zn1−x Cd x )3As2 (x = 0.26)

Single crystals of the α?-phase of (Zn_{1 ? x} Cd _x )₃As₂ solid solution (x = 0.26) have been prepared and investigated by X-ray diffraction analysis. The tetragonal unit-cell parameters are found to be a = b = 8.5377(2) Å, c = 24.0666(9) Å, sp. gr. I4₁/amd, Z = 16. Zn and Cd atoms in the crystal statistically occupy three symmetrically independent positions in the mirror planes and are tetrahedrally coordinated by arsenic atoms. (Zn,Cd) tetrahedra share edges to form a three-dimensional structure framework. The α?-phase is geometrically related to the fluorite structure. The character of arrangement of tetrahedral vacancies in fluorite-like unit cells is revealed. Chains of tetrahedral vacancies form microchannels oriented parallel the a and b axes, which pierce the three-dimensional structure framework at different levels along the c axis. The structure of α″-Cd₃As₂ crystals is found to be similar to that of α?-(Zn_0.74Cd_0.26)₃As₂.     

Heat capacity of Ag6Ge10p12 from 180 to 550 K

The molar heat capacity at constant pressure of Ag₆Ge₁₀P₁₂ is measured in the temperature range from 180 to 550 K and the standard molar enthalpies and entropies relative to 298.15 K are calculated on the basis of these data. From a comparison of the temperature variation of the apparent Debye temperatures in CuGe₂P₃ and Ag₆Ge₁₀P₁₂ it is concluded that lattice anharmonicity is more pronounced in Ag₆Ge₁₀P₁₂ than in CuGe₂P₃.     

Electron diffraction RDF analysis of amorphous As2Se3,AAs2Se2Te,As2SeTe2 and As2Te3 films

The local order in amorphous films of As₂Se₃, As₂Se₂Te, As₂SeTe₂, and As₂Te₃ has been examined by scanning electron diffraction with direct recording of the intensity of the elastically scattered electrons. The radial distribution functions indicate that there is a systematic increase in mean nearest neighbor distance as the Te concentration is increased, butthe mean coordination number increases slightly around 2.4. Pair function calculation of models shows that the 3-aand 2-fold coordinations of arsenic and chalcogens are retained in these glasses and the interatomic distances are close to those predicted from the Pauling covalent atomic radii of the constituent atomic species. The short range order appears to be similar in amorphous and crystalline As₂Se₃, but different in the case of As₂Te₃ as found by previous workers on bulk materials.     

Electrical and optical properties of the AsTeIn and GeSeIn chalcogenide systems

The influence of indium on the optical and properties of As_2−xTe_3−xxIn_2x, As_20−xTe_80−xIn_2x and Ge_20−xSe_80−xIn_2x is described. In Te-containing glasses the Fermi level is shifted by 0.05 eV and in Se-containing glasses by 0.2 eV towards the valence band.     

Infrared studies of Se-based polynary chalcogenide glasses (II): YxZxSe100−2x (Y = Ge,As; Z = As,Te)

The infrared (IR) absorption spectra for Y_xZ_xSe_100?2x glasses (Y = Ge, As;Z = As, Te), x = 2.5 and 5.0 are measured in the wavenumber region 700-60 cm^?1 at room temperature. These IR spectra are explained by comparing with the IR spectra already reported for the binary glasses such as Ge–Se, As–Se and Se–Te. In Ge_xAs_xSe_100-2x glasses (x ? 5.0), the main spectral features as well explained by both the spectra of Ge_xSe_100?x and As_xSe_100?x glasses. Main structural units in these glasses are considered to be GeSe₄ tetrahedra and AsSe₃ pyramids, and Se₈ rings and Se_n chains which are the units in pure glassy Se. In Ge_xTe_xSe_100?2x glasses (x ? 5.0) and IR band which cannot be explained by either the spectra of Ge_xSe_100?x or Se_100?xTe_x glasses appears at 210 cm^?1. This band is considered to be due to Ge–Te bonds. The IR spectra of As_xTe_x Se_100?2x glasses (x ? 5.0) are well explained by both the spectra of As_xSe_100?x and Se_100?xTe_x glasses. It is concluded that As and Te atoms combine with Se atoms in the forms of AsE₃ pyramids and Se₅Te₃ mixed rings, respectively.     

Ab initio calculation of vibrational frequencies of Ge0.25S0.75−yIy glass

The vibrational frequencies of Ge₄, S₄, I₄, SI₃, Ge₂I₂, S₂I₂, Ge₂S₂, Ge₂SI, GeS₃, GeI₂S, Ge₃S and GeI₃ are computed using first principles. These frequencies are compared with those found experimentally in the Raman spectra of Ge_0.25S_0.75−xI_x glass. In this way, it is found that Ge₄, GeI₂S and GeI₃ tetrahedra occur in the glass. These tetrahedra are of different sizes so they stack to form the floppy phase of the glass.