M. Vlcek

Structure of As–Se and As–P–Se glasses studied by Raman spectroscopy

Abstract The structure and the optical properties of bulk and thin films of the amorphous As x Se 100− x ( x =0, 28.6, 33, 40, 50 and 57.1) and As x P 4− x Se 3 ( x =0, 1, 2, 3 and 4) chalcogenide systems have been studied in this work. Bulk samples were prepared from high purity elements, and thin films ( d ≈1000 nm) were deposited by the vacuum thermal evaporation. Both types of As x Se 100− x samples show long-term chemical stability. In the case of As x P 4− x Se 3 , especially the films, chemical stability of as-prepared samples generally decreases with increasing phosphorus content. Structure of bulk and film samples has been determined by Raman spectroscopy as a function of exposure to light. It was found that exposure to halogen lamp light causes structural changes in short range order mainly in phosphorus containing films such that their structure approaches the structure of bulk samples of the same composition. In addition, the medium and/or long range structures are also affected by light, which then further contribute to the photo-induced changes in the optical properties of as-evaporated films.

Raman study of photostructural changes in amorphous GexSb0.4−xS0.6

Abstract Raman spectra of amorphous GexSb0.4−xS0.6 films and bulk samples have been measured for several values of x. The observed Raman bands are due to heteropolar M–S (M=Ge, Sb) bonds in the GeS4 tetrahedra and SbS3 pyramids, as well as homopolar (defective) M–M bonds whose population increases with increasing x. Illumination of Ge-rich ternary films by band gap light induces photostructural changes, the most prominent of which being an increase of the relative population of M–M to M–S bonds. At larger intensities of illumination (by laser light), partial crystallization of Sb occurs in both films and bulk glasses and, again, this photostructural effect is larger in Ge-rich ternary samples. It is revealed that the SbS3 pyramidal population decreases after either of the above treatments, thus underlining the key role of the Sb–S bonds in the photostructural changes. Furthermore, the extent of these effects is discussed in terms of the defective bond population, the free volume and dimensionality of the glassy network.

Application of As40S60−xSex layers for high-efficiency grating production

Abstract Here we investigate image formation properties of As 40 S 60− x Se x layers and their application for gratings fabrication. The light sensitivity of layers, the dissolution kinetics of as-evaporated and exposed layers in amine-based etching solutions have been studied. The surface relief formation was investigated by means of numerical simulation and experimental investigations. The aim was to optimize treatment conditions to increase the efficiency of gratings. Transmission gratings with line periodicity 6–12 μm were prepared by exposure to a halogen-lamp through Cr masks and subsequent etching achieved diffraction efficiencies from 20% to 26%. The recording of holographic gratings was carried out by exposure with Ar or He–Ne laser. Diffraction efficiencies of reflection gratings with spatial frequencies 1800–2400 mm −1 in polarized light were 80–90%. Polymer copies were obtained using the master gratings on the base of As–S–Se layers.

Modulated-temperature differential scanning calorimetry and Raman spectroscopy studies of AsxS100-x glasses

Thermal properties of chalcogenide AsxS100−x glasses in the glass transition region have been studied by modulated-temperature differential scanning calorimetry (MTDSC). All samples in this work were given the same thermal history by heating to a temperature above the glass transition, equilibrating and then cooling at a rate of 5°C/min to a temperature of 20°C. The reversing and non-reversing heat flows through the glass transformation region during both heating and cooling schedules were measured and the values of the parameters Tg, ΔH, Cp and ΔCp, which characterize the thermal events in the glass transition region, were determined as a function of the glass composition. The structurally determined parameters Tg, ΔH, Cp and ΔCp reveal major extrema when the composition of AsxS100−x glass becomes As40S60, that is the same as the composition of the corresponding stoichiometric compound. In addition, we observe “small thresholds” in these properties at 28.5 at % As (As28.5S71.5) around the same composition as that reported in the As-Se glasses. No such thermal analysis had been done on the AsxS100−x glasses previously. It is shown that AsxS100−x glasses where x < 25 at % As are formed from two glass phases. From MTDSC measurements, it was possible to establish the probable composition of the high temperature glass phase and from Raman spectroscopy it was possible to correlate the MTDSC results with the structure of the As-S glasses.

Photoinduced changes of the structure and physical properties of amorphous chalcogenides

Abstract The recent results of the study of photoinduced effects in chalcogenide glasses and layers were reviewed and discussed. The main attention was paid to changes of structure, optical transmissivity and reflectivity and to photoinduced optical anisotropy.

In situ high-resolution X-ray photoelectron spectroscopy of light-induced changes in As–Se glasses ☆

Abstract To establish the mechanism of reversible photoinduced changes such as in the recently discovered opto-mechanical effect in chalcogenide glasses, we have measured the effect of He–Ne laser light on the electronic structure of As–Se glasses using high-resolution X-ray photoelectron spectroscopy (XPS). We present the first results during in situ irradiation of As–Se bulk glass and amorphous films in the spectrometer, as well as after ex situ exposure in air. The non-reversible light-induced changes are due to the segregation of Se on the surface, increased homogeneity for As, and a greater heterogeneity in the chemical environment of Se. In addition, there are reversible changes in the valence band (VB), indicating transfer of lone-pair states of Se to a new kind of bonding state deeper in the band, which have a smaller population in the absence of light.

Photoinduced changes in the surface morphology of As50Se50 chalcogenide glass films

Abstract The photoinduced changes in the surface morphology of As50Se50 chalcogenide glass films are observed using atomic force microscopy. The thin film samples, on single crystal silicon or a microscope glass slide as substrate, were exposed to laser light of wavelength λ=632 or 685 nm. The results show nucleation and gradual growth of micrometer size pyramids that grow with laser irradiation, ultimately covering ∼15% of exposed area in 20 min. The rate of growth of a given pyramid slows down with time until it ceases to grow on reaching a height of several 100 nm. The maximum height of these faceted pyramids is ∼1 μ m, sometimes even larger than the film thickness, which cannot be understood from the reported photoinduced expansion. A photoinduced oxidation is suspected, in which As atoms in the surface layer move to the sites of pyramids. These observations raise new questions about the impact of ambient atmosphere on photodarkening and other photoinduced effects in chalcogenide glasses.

On the structural phase transformations in GexSb40−xSe60 glasses

Abstract A correlation between the observed peculiarities in the variation of some physicochemical parameters of Ge x Sb 40− x Se 60 ( x =10, 20, 25, 27, 32, 35) glasses and their structure has been found by studying the average coordination, Z . In particular, the composition dependences of the microhardness and its derivative characteristics (micro-voids volume, their formation energy, and module of elasticity) have shown an extremal behaviour at Z =2.65–2.67. The results on the density and compactness have also indicated the occurrence of an additional feature at the Ge 27 Sb 13 Se 60 composition having Z =2.67. Furthermore, the increase of the intensity ratio between homopolar and heteropolar Raman bands with Z is accompanied by a break at Z =2.65–2.67. The observed Raman modes have been related to the corresponding bond vibrations and the structure of the glasses studied has been specified. Accepting a tendency towards strong chemical ordering for these glasses, the distribution of the possible chemical bonds has been determined. The obtained defective bond population is confirmed by the experimental Raman data.The established peculiarities at Z =2.65–2.67 have been attributed to structural rearrangements of the backbone of the glasses.

Structure and imaging properties of As40S60−xSex glasses

Abstract Compositional dependence of photo- and thermally induced structural changes in As 40 S 60− x Se x layers ( x =0,20,30,40,60) have been measured. Dispersion of the refractive index was obtained from the transmission data. Single-oscillator model parameters (oscillator energy, dispersion energy) were estimated. The evolution of the optical dielectric constant was analyzed using the Penn model. Raman measurements have shown that two-phase structures characteristic of glasses are preserved in As 40 S 60− x Se x layers and indicate different bonding structures for as-evaporated, exposed, and annealed layers. These correspond to photo- and thermally induced changes. The single-oscillator energy changes linearly with composition. The compositional dependencies of such structurally determined parameters as glass transition temperature, relaxation enthalpy, specific heat capacity and dispersion energy are explained by a small deviation from the statistical effects of substitution of Se atoms by S in the composition change.

Structure and photoinduced changes in bulk and films of As-Ge-S system

Abstract Structure and basic physico-chemical properties of the bulk samples and vacuum evaporated thin films of AsxGe40−xS60 (x=0, 10, 20, 30, 40) were studied in this work. The Raman spectra showed that the structure of bulk samples as well as the films consists of AsS3/2 and/or GeS4/2 basic units, and other units containing homopolar bonds (As–As and/or Ge–Ge and S–S). The concentration of homopolar bonds in the as-prepared thin layers is higher than that in the bulk samples of the same composition. Exposure to halogen light decreases the concentration of these homopolar bonds as the structure of films approaches that of the bulk samples. Considerable changes in optical properties of films such as optical transmission, refractive index and chemical resistance accompany this light-induced chemical homogenization. These glasses have a broad window of high IR transmissivity (≈70%) up to λ=10–11 μm. In the IR spectrum of binary Ge40S60, intensive absorption bands appear, which indicate partial hydrolysis of the sample. In contrast to bulk samples, the films are photosensitive. Exposure to halogen lamp light causes both photodarkening and photobleaching of thin films depending on the Ge content, with corresponding decrease and/or increase of refractive index in the visible and near IR regions. The addition of Ge into the films of As–S system can eliminate or alter the character of photoinduced changes of optical parameters.