V. Pamukchieva

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.

Irreversible and reversible changes in band gap and volume of chalcogenide films

Abstract Irreversible and reversible changes induced by illumination and/or annealing of thin evaporatively deposited Ge–As–S films at normal incidence are studied. Attention is paid to the volume (thickness) changes and their correlation with the changes induced in the band gap of the ternary and binary chalcogenides. The revealed dependences are compared to that obtained for obliquely deposited chalcogenide films and substantial differences have been found.

X-ray photoelectron spectroscopy studies of thin GexSb40−xS60 chalcogenide films

Thin GexSb40-xS60 (x = 5, 15, 20, 25 and 27) chalcogenide films have been investigated by X-ray photoelectron spectroscopy (XPS). X-ray photoelectron spectra show that there is a peculiarity in the relative intensity ratio of the Sb 4d photoelectron peak associated with Sb2S3 to the Sb 4d photoelectron peak associated Sb2S5 at an average co-ordination number Z of 2.65-2.67. After contamination and photo-oxidation layers were removed from the surface of the films, X-ray photoelectron spectra were measured again. It has been found that binding energies of the Ge 2p and Sb 3d(3/2) photoelectron peaks, which reflect the electronic structure at lower core energy levels, are independent of Z. However, the binding energies of the Ge 3d and Sb 4d photoelectron peaks are more sensitive to Z and have a discontinuity at Z = 2.65.

Study of As2Se3 and As2Se2Te glass structure by neutron- and X-ray diffraction methods

Neutron and high-energy X-ray diffraction measurements have been performed on As40Se60 and As40Se50Te10 glasses. Both the traditional Fourier transformation technique and the reverse Monte Carlo (RMC) simulation of the experimental data have been applied to model the 3-dimensional atomic configurations. From the analysis of the partial atomic correlation functions and structure factors the first and second neighbour distances, coordination numbers and bond-angle distributions were calculated. It is established that substitution of Se by Te does not change the basic glassy network structure. For the As-Se bonding is revealed that the first neighbour distance is at 2.42 A, the average coordination numbers are CNAsSe=2.6±0.1 and CNSeAs=1.8±0.1 atoms and the three-atom-bond angle for , configurations is 95°, the same value within limit of error, as obtained from RMC calculations.

The boson peak in Raman spectra of AsxS1−x glasses

The Raman spectra of AsxS1−x glasses with x < 40 at % (Z < 2.4) have been studied in a wide temperature range (20–300 K). A well resolved boson peak is observed in the low-frequency portion of the spectrum, not withstanding the appearance of floppy modes in the glasses under study. It is shown that the boson peak is characterized by two parameters: intensity and the peak position. A comparison of the intensity variation for the boson peaks indicates that the degree of disorder increases as x decreases. This effect is caused by floppy modes in the glass network and by sulfur phase separation. Studies of reduced boson peaks in the Raman spectra of AsxS1−x glasses confirms the theoretical assumption that the shape of the peaks is independent of composition and temperature.

Concentration dependence of radiation-induced changes in the optical properties of GexAs40−xS60 films

Radiation-induced changes in the optical properties of chalcogenide glasses in the Ge-As-S system are investigated as a function of the concentration. Theoretical calculations are performed with due regard for possible constraints on the range of variation in the number of homobonds and heterobonds upon their switching in the structural network of chalcogenide glasses. The experimental data are obtained upon irradiation of GexAs40−xS60 thin films with fast electrons (6 MeV). The possible mechanism of structural transformations responsible for the specific features in the concentration dependence of the change in the band gap of chalcogenide glasses is discussed.

Single-oscillator description of spectral dependence of the refractive index of new chalcogenide Ge-Sb-(S,Te) glasses

New quaternary telluride glasses based on Ge-S system with addition of Sb and partial substitution of S by Te have been synthesized and their optical properties in dependence on composition have been studied by spectroscopic ellipsometry in 400–820 nm spectral range. The dispersion behavior of the refractive index of these glasses with composition of GexSb40−xS50Te10 and GexSb40-xS55Te5 (x= 10, 20, and 27) was examined applying the single-oscillator theory and using the Wempel-DiDomenicos model. From the data analysis the optical constants (n, k, e), optical bandgap energy Eog and film thickness, as well as the oscillator energies E0 and Ed have been determined. The observed increase of E0 and decrease of Ed with increasing the Ge content or decreasing the Te content are explained by the change of amount and strength of heteropolar chemical bonds and by the less covalent character of chemical bonding in these materials. Correlation between the oscillator energy E0 and optical band gap energy Eog is established.

Structure study of chalcogenide glasses from high Q-range neutron diffraction experiment and RMC modelling

A neutron diffraction structure study has been performed on newly synthesized Ge20As20Se50Te10 and Ge27As13Se50Te10 chalcogenide glasses. Oscillations in the structure factor, S(Q) have been measured with good sign-to-noise up to 35 A -1 . The reverse Monte Carlo simulation was used to model the 3-dimensional atomic configuration. The partial atomic correlation functions and structure factors have been revealed. Several first and second neighbour distances, and coordination numbers have been calculated. We have established that several first neighbour atomic distances are overlapping at two characteristic distances, namely the Ge-Ge, Ge-Se and As-Se are centred at 2.42 A, while the Ge-Te, As-Te and Se-Te are centred at 2.60 A.