Ac conductivity of Ag-doped bulk,glassy As2S3

Measurements of the electrical conductivity of Ag-doped bulk As₂S₃ glasses have been made as functions of temperature, pressure, frequency and Ag doping level. A Debye-like loss peak was observed near 10⁴ Hz. The frequency of the loss peak is dependent on temperature, pressure and doping level, but these dependences are different from those of the dc conductivity. The ac loss is attributed to the Maxwell-Wagner losses characteristic of inhomogeneous materials. The materials are presumed to be inhomogeneous mixtures of As₂S₃ and Ag₂S. We have also searched unsuccessfully for ac conductance in several bulk chalcogenide glasses.     

X-ray photoelectron spectroscopic studies on Se/As2S3 and Sb/As2S3 nanomultilayered film

Photoinduced diffusion in Se/As₂S₃ and Sb/As₂S₃ nanomultilayered thin films are studied by X-ray photoelectron spectroscopy (XPS). The XPS measurements show the atomic movements during photoinduced diffusion in Se/As₂S₃ and Sb/As₂S₃ nanomultilayered film. The analysis of experimental data describes the nature of light induced changes in different structural units.     

Effect of hydrostatic pressure on the elastic moduli of As2S3 and As2Se3 glasses at 195 and 296 K

Velocities of 30 MHz longitudinal and shear ultrasonic waves have been measured in As₂S₃ and As₂Se₃ glasses as a function of hydrostatic pressure up to 1.5 kbar at 195 K and 3 kbar at 296 K. The elastic stiffness moduli are found to have relatively large, positive, pressure dependences which are about the same at both temperatures for both glasses. This behavior is attributed to the weakness of bonding between layers comprised of AsS₃ and AsS₃ pyramids.Inspection of data for a variety of glasses reveals a correlation between the value of C_L/3C_T and whether the elastic moduli are increased or decreased by pressure. (C_L is the longitudinal modulus and C_T the shear modulus.)Using the pressure dependences of the elastic moduli obtained in the present work, it is found that volume change is responsible for most of the temperature dependences of the moduli. In addition elastic gammas are obtained which are consistent with thermal Grüneisen gammas at 12 K. The pressure dependence of the volume of As₂S₃ glass at 296 K is calculated using the present results in the Murnagham equation. Agreement with volumetric data of Weir is obtained.     

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.     

Vibrational properties of As2S3 glass

Vibrational densities of states and infrared and Raman spectra have been calculated for a structural model of As₂S₃ glass. The calculations are based on simple semi-empirical forms for interatomic potentials, electric dipole moment and Raman polarizability. The bands of the calculated spectra agree well with those of the observed infrared and Raman spectra of As₂S₃ glass in intensity and position, although a small concentration of the wrong SS bonds remains in the structural model and causes an additional peak in the higher frequency region. The calculated depolarization ratio of the Raman spectra is consistent with the observed one.     

Raman spectrum of vitreous As2S3

The spontaneous Stokes and anti-Stokes Raman spectrum of vitreous As₂S₃ is reported. The spectrum was recorded with both HeNe and Ar ion laser excitation lines in the transmission and reflection modes respectively. Spectra were recorded at various temperatures between 20°K and 465°K, the softening temperature of As₂S₃ glass. It is shown conclusively that the quasicontinous scattering observed at low wave number shifts (< 100 cm⁻¹) is real in agreement with the theory of Shuker and Gammon and not an arbitrary background as previously reported. An approximate density of vibrational states is deduced from the polarized Raman spectra.     

The structure of Hg-doped As2Se3 glasses

The structure of the semiconducting glassy As₂Se₃Hg_x system was investigated in a composition range x = 0.005?0.12. An explanation of the anomalous behaviour of the macroscopic density is proposed, based on the analysis of radial electron density distribution curves. A formula is given which correlates quantitatively the magnitude of coordination spheres with the experimental macroscopic density.     

The structure of liquid As2Se3 and GeSe2 by neutron diffraction

The liquid structures of As₂Se₃ and GeSe₂ have been investigated using the neutron diffraction patterns. In both cases the structure factor showed a low first peak maximum which follows a weak but apparent pre-peak at very low momentum transfer. It was also observed that the radial distribution function of both materials are characterized by the well-defined first neighbor shell because of the deep minimum on its right-hand side although in the liquid state. These results indicate that strong covalent bondings between unlike atoms in the solid state still remain when melting. Both the structure factor and the distribution curves of these alloys are, on the whole, similar to those in the amorphous phase which have already been examined. A slight difference in the coordination number, however, is found between amorphous and liquid phases of these materials.     

Thermally stimulated depolarization of glassy As2S3

Decay of dark polarization in glassy As₂S₃ is investigated by thermostimulated depolarization (TSD). An attempt is made to analyse the observed maximum at about 360 K. To get more insight into the TSD phenomena, dc conductivity measurements at corresponding temperature are also presented. It is not possible at present to identify the trapped species responsible for the non-uniform polarization, despite finding that identical traps are active in both surface and bulk trapping.     

Relief holograms in thin films of amorphous As2Se3 under high laser exposures

Hologram recording into thin films of amorphous As₂Se₃ was investigated experimentally. Apart from a hologram recorded at a relatively low exposure of several joules per square centimeter, a new hologram can be recorded at exposures of 10⁴–10⁵ J/cm². At these exposures a relief hologram is formed on the surface of the As₂Se₃ film. The holograms can be erased by heating or by illumination and new holograms can be recorded.     

Intrinsic and impurity infrared absorption in As2Se3 glass

A quantitative study of infrared absorption in the 250–4000 cm^?1 region of As₂Se₃ glasses doped with small amounts of As₂O₃ or purified by various procedures has been carried out with particular attention to absorption in the wavelength regions of the CO₂ and CO lasers. The dependence of the relative intensities of the oxide impurity bands in the 650–1340 cm^?1 region on the total amount of As₂O₃ added to the glass indicates the existence of three distinct oxide-impurity species. A number of higher-frequency impurity bands which are due to the presence of hydrogen in the glass and whose intensities are highly dependent on the glass-melting conditions have been observed and classified. Intrinsic multiphonon absorption in the 400–1100 cm^?1 region has been interpreted in terms of combination and overtone bands of the two highest-frequency fundamental vibrational modes. Absorption coefficients of As₂Se₃ glass in the 920–1090 cm^?1 CO₂ laser region are limited by intrinsic multiphonon absorption to values of around 10^?2 cm^?1. The lowest absorption coefficients measured in the 1700–2000 cm^?1 CO laser region were around 2 × 10^?3 cm^?1 and may contain contributions from hydrogen-impurity bands.     

On a chaotic potential field in vitreous As2Se3

Some experimental results of the vitreous As₂Se₃ investigation (the photoconductivity spectrum of volume examples, the temperature dependence of the thermoelectric E.M.F., the shift of the optical edge under the temperature and pressure) may be explained by the use of fluctuations of an internal potential field. A hypothesis about the nature of chaotic potential field is proposed, which is based on a negligible magnitude of the gap deformation potential D_g = D_c ? D_v, where D_c and D_v are deformation potentials of band edges. By this hypothesis the fluctuations of the substance density in glass generate the chaotic potential field of magnitude about a half of the gap E_g, while the gap magnitude fluctuates only slightly within some per cent of E_g. It is shown that the displacement of the optical edge at As₂Se₃ amorphization may be a demonstration of the deformation nature of the chaotic potential field in this material.     

Plasma etching of As2S3 films for optical waveguides

Chalcogenide glasses are good candidate materials for ultra-fast non-linear optic devices. In this work, we present the photolithographic process and the plasma etching of arsenic tri-sulphide (As₂S₃) film. The films were deposited on thermally oxidized silicon substrates by ultra-fast pulsed laser deposition. To protect As₂S₃ film from photo-resist developer, thin resist layer ∼100-200 nm was remained on the UV exposed area by controlling resist development time. After removing the protective layer in oxygen plasma, As₂S₃ waveguides were patterned in inductively coupled plasma reactive ion etching (ICP-RIE) system using CF₄-O₂ gas mixture. We investigated the etch rate and the etch selectivity to photo-resist of As₂S₃ as a function of bias power, induction power, operating pressure, and gas flow rate ratio of CF₄ and O₂. The film is mainly etched by the chemical reaction with fluorine radicals. The content of oxygen in the plasma determines the etched sidewall profiles and nearly vertical profile was obtained at high oxygen content plasma.     

Fabrication of highly homogeneous As2Se3 glass under argon flow

The present study relates to a new method for the synthesis of As₂Se₃ glass in a controlled atmosphere. The advantage of this technique is that it does not require sealing of the silica reaction container and therefore makes it likely to substitute the current industrial batch by batch synthesis which actually needs very expensive single-use sealed silica vessels.An experimental device has been developed for these purposes. It is equipped with a stirring mechanism to homogenize the molten bath. In order to avoid contamination by oxygen and moisture, the synthesis is carried out under argon flow (pressure of 1 bar). Material losses during synthesis can be reduced to less than 2% when temperature is progressively increased up to 430 °C. Bulk glass ingots are finally obtained according to a two-step annealing process. Their chemical composition is analyzed by EDS and shows a variation range of less than 0.2%. The excellent reproducibility of the given method is also confirmed by the refractive indexes, that do not differ for more than 1 · 10⁻³ from one another.Adverse absorption bands due to oxygen do not occur in the 8-12 μm spectral region when 1000 ppm of Mg is added. As no distilling operation has been carried out until now, the magnesium oxide partially keeps staying in the glass and leads to scattering losses at short wavelengths.     

As2S3 and AgI thin layers as ion sensitive membranes

The possibilities of using thin layers of As₂S₃ and AgI as ion sensitive membranes for ion selective field-effect transistors (ISFETs) are investigated. The thin films have been prepared by vacuum deposition on static and rotating substrates. The As₂S₃ layers were additionally doped with silver. The influence of the type of substrates and preparation conditions on electrochemical properties of the layers was studied. Electrochemical measurements revealed a reasonable sensitivity of chalcogenide and halide layers to silver and iodide ions, respectively. The near Nernstian behavior of sensitivity of As₂S₃ layers to Ag⁺ ions and of AgI layers to I⁻ ions is observed. The results obtained are promising for the development of ISFETs.     

Medium range order and optical properties of As2S3 glass

Low frequency Raman scattering and optical absorption edge were measured for As₂S₃ glasses quenched at temperature in the supercooling region of the glasses. It was found that both the Raman spectrum and the optical absorption edge shift to the lower energy side with the rise of the quenching temperature. The effects were interpreted in terms of the order of the arrangements of the layer-like clusters, which become more random as the quenching temperature goes higher.     

Study of recombination of photocarriers in a-As2Se3 and As2Te3 by photoconductivity and photo-absorption measurements

Time-dependent photoconductivity and photo-absorption measurements, the latter using probe beams of 0.1 eV and 1.4 eV, were analyzed on the basis of a saturated band tail model. By a best fit procedure of theory to experiment band structure and recombination parameters for a-As₂Se₃ were obtained.     

Anomalous anisotropic deformations of As2S3 flakes induced by linearly-polarized bandgap illumination

Semi-free As₂S₃ flakes undergo visible-scale anisotropic deformations when exposed to linearly-polarized bandgap illumination. We investigate the behavior and also those in amorphous Se, GeS₂, AgAsS₂, and crystalline As₂S₃. These results suggest that the deformation occurs through photo-induced birefringence, photo-induced fluidity, and optical force.     

Stimulated interdiffusion and optical recording in Sb/As2S3 nanomultilayers

The role of the compositional modulation at nano-scale dimensions (2–10 nm) in the enhancement of optical recording parameters in nanomultilayers, which contain Sb as active, optical absorbing and diffusing layers and As₂S₃ as barrier (matrix) layers was investigated. Comparison was made with single homogeneous layers made of ternary (As₂S3)_xSb_1−x glasses and co-deposited from Sb and As₂S₃. It was shown that essential increase of the recording efficiency, sensitivity of the bleaching process, broadening of its spectral range occurs due to the stimulated interdiffusion of adjacent components in Sb/As₂S₃ nanomultilayers with optimized Sb layer thickness.     

On the photodissolution kinetics of silver in glassy As2S3

The kinetics of the photodissolution of Ag into glassy As₂S₃ films and its dependence on temperature have been studied by monitoring the changes that occur both in their transmission spectra and transmission of weakly absorbed broadband light. It was shown that besides of a low induction period, the photodissolution kinetics consists of two linear steps with different activation energies, followed by a parabolic tail. The transitions between photodissolution steps was found to be not monotonous and explained in terms of Elliott’s model, which asserts a simultaneous ionic and electronic charge transport controlled by chalcogenide properties, illumination and temperature. The evidence is given that the islanding of Ag layer in the course of photoreaction, results in an inversion of maxima and minima of transmission spectra. It is suggested that the islanding of Ag layer is not a consequence of a non-uniform dissolution but arises itself at critical thickness, at which Ag forms a continuous film.