Infrared laser ablation of glassy As2Se3

In this study, we report the first measurements of wavelength and pulse length dependence of ablation thresholds in glassy As₂Se₃. Ablation yields, obtained from time-of-flight spectra were compared to laser intensity curves to obtain laser ablation thresholds. The wavelength dependence of the ablation thresholds were obtained by tuning the Vanderbilt–Keck free electron laser in the range of 3.2–5 μm. Pulse dependence of the ablation thresholds were obtained with a 5 μs macropulse in the wavelength range 3.2–5 μm and with macropulses ranging from 200 to 1000 ns at the wavelength of 3.5 μm. The wavelength dependence of the ablation threshold did not indicate that impurity absorption was a major factor in the ablation process in glassy As₂Se₃. The ablation threshold decreased with wavelengths corresponding to higher photon energies. In addition, a sharp increase in ablation threshold was observed with decrease in the macropulse length. Optical images of ablation craters at intensities above the threshold reveal significant melting on the sample surface. We will discuss the wavelength and pulse length dependence of the laser ablation threshold in glassy As₂Se₃ in terms of a multi-photon process and a cumulative heating process.     

Te-rich Ge–As–Se–Te bulk glasses and films for future IR-integrated optics

Ge₁₅As₁₅Se_70−xTe_x materials with x = 56, 60 and 63, of potential use in IR-integrated optics, were prepared by the classical melt-quenching method. A macroscopic phase separation was observed with a crystalline phase on the top and an amorphous one at the bottom. The glasses from the bottom were transparent from 1.9 to 16 μm without any purification of the elemental precursors Ge, As, Te and Se. The higher the Te/Se content in the glasses the lower their glass transition temperature and thermal stability. Films 7–12 μm thick of the above stated compositions were deposited by thermal evaporation. The higher the tellurium content, the larger the optical band gap shift of the films in the infra-red and the higher the refractive index.     

Effect of Pb impurity on the localized states of Se–Ge glassy alloy

Space charge limited conduction (SCLC) is investigated in vacuum evaporated thin films of (Ge₂₀Se₈₀)_1−xPb_x (x = 0, 0.02, 0.04, 0.06). I–V characteristics have been measured at various fixed temperatures. At high fields (∼10⁴ V cm⁻¹), current could be fitted to the theory of space charge limited conduction in case of uniform distribution of localized states in the mobility gap of these materials. Using the above theory, the density of localized states near Fermi level is calculated. A reversal in density of localized states is obtained at 4 at.% of Pb.     

One- and two-phonon Raman spectra of amorphous AsSe systems

The polarization dependence of the one- and two-phonon Raman scatterings of the amorphous semiconductor As_xSe_1?x systems for 0 ? x ? 0.5 was measured. The analysis of the Raman data by the superposition of the vibrations of AsSe₃ pyramids and SeSe bonds suggests the existence of the intermediate range interaction in the As_xSe_1?x systems.Two phonon Raman spectra for 0.3 ? x are explained by the combination plus overtone of the first order Raman scatterings. For x ? 0.2, however, the overtone processes become predominant.The dependence of the Raman spectra on the As concentration suggests that the chemical orderings are predominant.     

Anomalous density variations of the molten chalcogenide system GeAsSe

An experimental method of measuring densities of melts with high vapour pressures is described. This method is based upon the absorption of high-energy gamma rays. The method enables densities to be measured even in the case of melts solidifying (or bulk samples melting) inside a sealed system, as long as certain conditions are satisfied. The results of investigations of some melts of the chalcogenide system GeAsSe are presented.The melts so far investigated show an increase in density at high temperatures which is super-imposed on the thermal expansion. The reason for these irregularities is an increase of the average coordination number caused by a change of the bonding system. The bonding system with one-sided overlapping of (sp^x)-hybrid bonding functions changes at higher temperatures to a system in which pσ-bonding functions are extended on more than one side of the atoms. This change of the bonding system explains the transition of the electrical conductivity of rhese melts at high temperatures from a semiconductor-like to a metallic-like conductivity. Because a pσ-bonding system enables the atoms to have a relatively high mobility the viscosities of these melts are very low at high temperatures and are similar to those of molten metals.     

Optical absorption property of glasses coloured with Cd (S, Se, Te)

In the glasses coloured with Cd (S, Se, Te), the dependence of the absorption coefficient () on photon energy (E) and temperature (T) has been determined. Some results that differ from the Urbach rule were obtained.     

Chemical characteristics of Dy–S bonds in Ge–As–S glass

From Dy L₃-edge extended X-ray absorption fine structure spectroscopic analysis of Dy-doped Ge–As–S glass, we verified that both the coordination number and Dy–S distance are decreased in this representative chalcogenide glass compared to those in the Dy₂S₃ crystalline counterpart. The strong covalent nature inherent in chemical bonds between the constituent atoms of Ge–As–S glass would be responsible for the enhanced covalency of the Dy–S bonds, which would then be sensitively related to the optical characteristics of the 4f ↔ 4f transitions of Dy³⁺ as well as the low rare-earth solubility exhibited by the Ge–As–S glass.     

The evolution of bond structure in Ge33As12Se55 films upon thermal annealing

The evolution of bond structure of laser deposited Ge₃₃As₁₂Se₅₅ films under various processing conditions has been investigated by X-ray photoelectron spectroscopy. It was found that a large number of Se-rich structures in the as-grown film may coalesce with As and Ge after annealing at high temperatures. In addition, both Ge and As 3d spectra show the presence of oxides. The oxygen distribution exponentially decays along the normal direction of the films regardless of different processing conditions. The critical thickness of the oxidized layer was extracted for the film annealed at various pressures and temperatures.     

Surface morphology of spin-coated As–S–Se chalcogenide thin films

Surface morphology and roughness of amorphous spin-coated As–S–Se chalcogenide thin films were determined using atomic force microscopy. Prepared films were coated from butylamine solutions with thicknesses d ∼ 100 nm and then annealed in a vacuum furnace at 45 °C and 90 °C for 1 h for their stabilization. The root mean square surface roughness analysis of surfaces of as-deposited spin-coated As–S–Se films indicated a very smooth film surface (with R_q values 0.42–0.45 ± 0.2 nm depending on composition). The nanoscale images of as-deposited films confirmed that surface of the films is created by domains with dimensions 20–40 nm, which corresponds to diameters of clusters found in solutions. The domain character of film surfaces gradually disappeared with increasing annealing temperature while the solvent was removed from the films. Middle-infrared transmission spectra recorded a decrease of intensities of vibration bands connected to N–H (at 3367 and 3292 cm⁻¹) and C–H (at 2965, 2935 and 2880 cm⁻¹) stretching vibrations. Temperature regions of solvent evaporation T = 60–90 °C and glass transformation temperatures T_g = 135–150 °C of spin-coated As–S–Se thin films were determined using a modulated differential scanning calorimetry.     

Electrical properties of glasses in the GeBiSbSe and GeBiS systems

Measurements of optical absorption, dc conductivity and thermopower were carried out on glasses of the types (GeSe_3.5)₈₈sbPin12?xBi_x. The main results are: 1) observation of n-type conduction in the Ge₂₀Bi_xS_80?x glasses containing high concentrations of Bi; 2) in the mixed doped glasses, transition from p-type for a Sb-doped glass (x = 0) to n-type on addition of Bi at nearly equal concentrations of Sb and Bi. The possible types of bonding of the Bi atoms in the structure and their influence on the electrical behaviour are examined.     

Photostructural relaxation in As–Se–S glasses: Effect of network fragility

The effect of photoinduced structural relaxation in As–S–Se glass is investigated during sub-bandgap irradiation. It is shown that the glass undergoes rapid optically induced structural relaxation upon photoexcitation of bonding electrons. Fragile systems exhibit larger relaxation as expected from their enthalpy profile. This suggests that the process is thermodynamically driven and that the kinetic impediment to relaxation at low temperature is lifted through photoinduced softening of the glass matrix. Activation energy for enthalpy relaxation measurement and an annealing study near T_g show that the photorelaxation effect is not a thermally activated process. The 〈r〉 dependence of photostructural changes is addressed and explained using the energy landscape formalism.     

Effect of γ-irradiation on non-linear I-V behaviour and thermoelectric measurements in amorphous semiconducting AsSeTe system

An amorphous semiconducting compound of the composition As₂SeTe₂ is studied for its non-linear current-voltage characteristics, conductivity and thermoelectric measurements. It has been observed that γ-irradiation of the sample gives a lower threshold voltage and greater thermoelectric power. The threshold voltage shifts to a lower voltage value for the irradiated sample. The non-linear I-V behaviour and p-type nature of unirradiated and γ-irradiated As₂SeTe₂ are discussed on the basis of charged defect states existing and also created after γ-irradiation in the material. The thermoelectric power for unirradiated and γ-irradiated As₂SeTe₂ is discussed on the basis of a quantity, related to conductivity and thermoelectric power which explains to a great extent the effect observed experimentally.     

Pressure dependence of the Boson peak in glassy As2S3 studied by Raman scattering

A detailed pressure dependence study of the low energy excitations of glassy As₂S₃ is reported over a wide pressure range, up to 10 GPa. The spectral features of Boson peak are analyzed as a function of pressure. Pressure effects on the Boson peak are manifested as an appreciable shift of its frequency to higher values, a suppression of its intensity, as well as a noticeable change of its asymmetry leading to a more symmetric shape at high pressures. The pressure-induced Boson peak frequency shift agrees very well with the predictions of the soft potential model over the whole pressure range studied. As regards the pressure dependence of the Boson peak intensity, the situation is more complicated. It is proposed that in order to reach proper conclusions the corresponding dependence of the Debye density of states must also be considered. Comparing the low energy modes of the crystalline counterpart of As₂S₃, as well as the experimental data concerning the pressure dependencies of the Boson peak frequency and intensity, a structural or glass-to-glass transition seems to occur at the pressure ∼4 GPa related to a change of local dimensionality of the glass structure. Finally, the pressure-induced shape changes of the Boson peak can be traced back to the very details of the excess (over the Debye contribution) vibrational density of states.