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.     

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.     

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.     

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.     

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.     

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.     

Preparation and thermal properties of novel Li2S-Sb2S3 glassy system

There is great interest in sulfide glasses because of their high lithium ion conductivity. A new Li₂S-Sb₂S₃ glassy system has been prepared by a classical quenching technique. Thermal characterization has been carried out and it has been observed that the T_g decreases with an increase in the concentration of Li₂S. However, this tendency stops around 17% Li₂S content. Results have been discussed on the basis of the modification that would occur in the base Sb₂S₃ glass network by the addition of Li₂S.     

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.     

Durability of an As2S3 chalcogenide glass: Optical properties and dissolution kinetics

The durability of a As₂S₃ chalcogenide glass composition was studied in de-ionized water at different temperatures (60-90 °C) for different periods of time, up to 120 days. The evolutions of the chemical composition and the pH of the solutions as well as the optical transmission of bulk samples, in the 2-10 μm region, were measured as a function of corrosion time. Atomic force microscopy and optical microscopy were used to investigate the roughness of corroded surfaces and the evolution of surface defects. The water corrosion of As₂S₃ glass was found to follow a congruent dissolution mechanism, a possible glass-water reaction mechanism was proposed. The optical transmission of the glass was found to be affected by the corrosion. The optical loss increased from 4 to 21% with corrosion time, this variation was attributed to the texturation of the surface by the reaction of corrosion. Moreover, the experimental results show that high temperature value enhances the corrosion reaction: an activation energy of 103 ± 2 kJ/mol was computed from experimental measurements.     

Wavelength selective materials modification of bulk As2S3 and As2Se3 by free electron laser irradiation

In this study we report first measurements of wavelength-selective infrared-induced materials modification of bulk As₂S₃ and As₂Se₃. These materials are currently being considered as candidate materials for infrared optical fiber transmission in the range of 1–10 μm. Our study is aimed at modifying oxygen, hydrogen and carbon impurities bound to chalcogenide constituent elements in the materials to reduce absorption. Tunable infrared radiation from the W.M. Keck Free Electron Laser (FEL) at Vanderbilt was used to excite specific vibrational modes, S–O–H and CH_x modes in bulk As₂S₃ and Se–H, CH_x and S–H₂ modes in bulk As₂Se₃. Changes in vibrational mode amplitudes are monitored by measuring the intensity of the Fourier transform infrared (FTIR) spectra before and after irradiation at appropriate wavelengths. By tuning wavelengths to hydrogen vibrational modes, we find evidence that hydrogen is released and/or redistributed athermally. In particular, following irradiation at specific resonant wavelengths, vibrational mode amplitudes as monitored by FTIR associated with CH_x are significantly reduced in bulk As₂S₃ and As₂Se₃ samples. In As₂S₃, the changes in CH_x modes are reversed by heat treatment at 115°C for 35 min in nitrogen atmosphere.     

The preparation and electrical switching properties of nickel sulphides in As2S3 glass

A preliminary investigation into the formation of ‘glasses’ from mixtures of arsenic and nickel sulphides is reported. Memory- and threshold-switching devices have been produced from glasses containing particles of nickel sulphide. Threshold switches show a high resistance in the off-state ($\text{> 10}{}^6\text{μ}$), and a linear on-state.     

Detailed study of the photodarkening process in amorphous As2S3

The photodarkening process in amorphous As₂S₃ was measured and analyzed in detail as a function of illumination time and incident photon flux. Three main results were obtained: (1) the degree of photodarkening approaches a saturation value with time in a simple exponential decay; (2) the saturation value increases with increasing incident photon flux; (3) independent of the incident photon flux, the number of photons required for the saturation is about 10²³/cm³.Detailed analyses based on a simple first-order rate equation show that a rate constant of the photodarkening increases superlinearly with respect to incident photon flux. This means that the reciprocity law between the illumination time and the incident photon flux does not hold in the photodarkening.Photo-modulation of excited electronic states is proposed here in order to interpret the nonlinear relationship between the rate constant and the incident photon flux.     

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.     

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.     

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.     

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.     

A comparison of the medium-range order in glassy GeS2 and Sb2S3

In amorphous (a-) GeS₂ there is only one medium-range order (MRO) dependent X-ray scattering feature, i.e., the ‘first sharp diffraction peak’ (FSDP), whereas in a-Sb₂S₃ all scattering features in the interval 1–2.4 Å⁻¹ are in fact MRO-dependent. The basic element of MRO in a-GeS₂ is a part of a chain of corner bound tetrahedra extracted from the high temperature form of c-GeS₂; in a-Sb₂S₃ it is a part of a crystalline-like (Sb₂S₃)_n band. Ordering of parts of these bands is characterized by a greater interband separation than in the crystal. By contrast with a-GeS₂, the MRO-dependent medium-angle scattering features in a-Sb₂S₃ are less sensitive to fluctuations of the interband (intercluster) ordering distance.     

Electrical and photoconductive properties of the As2---Se3---Bix system

The electrical and photoconductive properties of amorphous As₂---Se₃---Bi_x compact samples were studied. The thermal activation energy decreases slightly and the dark conductivity increases with increasing Bi concentration within the region of glass formation which is less than 5 at.%. Photoconductive spectral distribution showed a peak for all bulk samples. A band model can be used to explain these results.     

The kinetics of photodissolution of Ag in amorphous As2S3 films

A new technique, measurement of the electrical resistance change of the Ag layer, is developed to study the kinetics of photodissolution of Ag in amorphous As₂S₃. It is shown that the photodissolution rate is proportional to the light intensity absorbed in the As₂S₃ at the As₂S₃Ag interface, but photoelectrons ejected from the Ag into the As₂S₃ also contribute. The process is shown to be two-stage. Firstly a critical “radiation damage” dose must be accumulated in the As₂S₃. Secondly, the Ag atom is photon-assisted across the As₂S₃Ag interface activation barrier.     

Light and heat induced interdiffusion in Sb/As2S3 nano-multilayered film

The light and heat induced changes in the optical band gap of Sb/As₂S₃ nanomultilayered chalcogenide film has been studied. Even though the changes in optical bandgap are attributed to the light and heat induced interdiffusion, the diffusional intermixing between the layers is rather different with light and heat. The observed difference in the light and heat induced interdiffusion is due to unequal diffusion coefficients of light and heat predicted by thermal spike model.