E. Vateva

Optical properties of Ge-As-S thin films

Based on two independent methods, the spectral dependence of the refractive index n and the absorption coefficient α in the regions of weak and strong absorption is calculated. Conclusions are drawn concerning the influence of variation in the composition, technology and treatment on the films optical parameters; the relation is also shown to the films selective solubility.

New medium-range order features in GeSbS glasses☆

Abstract A detailed analysis of X-ray diffraction data for Ge x Sb 40 - x S 60 glasses indicates the existence of a topological phase transition and medium-range order related not only to the first but also to the pre-first sharp diffraction peak. The compositional dependence of the intensity, the correlation length and the ratio between the position and the width of the two peaks are compared. Differences between the two peaks are revealed and their structural implications are discussed.

The topological phase transition in some Ge-Sb-S glasses and thin films

Abstract The composition dependences of the mean gramme-atomic volume, density and compactness of bulk glasses from the (GeS1.5)y·(Sb2S3)1−y line are investigated, as well as the optical bandgap, solubility and selective solubility of the thin films prepared from the glasses. At an average coordination number Z≈ 2.67, well expressed peculiarities are observed, related to a topological transition. It is demonstrated that the metallization of the bonds during the substitution of arsenic with antimony affects but does not prevent the transition, typical of the structure of glasses and thin films.

On the structural stability of amorphous Se/CdSe multilayers: a Raman study

Abstract The effects of thermal annealing and laser beam illumination on the structure of amorphous Se/CdSe multilayers of up to 20 periods and various sublayer thicknesses (3.5, 5, 6.5 and 10 nm) have been studied by measuring the higher frequency (vibrational) Raman spectra. Three Raman bands have been observed in this spectral region at 209 cm−1 (CdSe), 237 cm−1 (Se) and 256 cm−1 (Se). After annealing, the intensity of these bands increases for all amorphous multilayers (AML) samples which, in a first approach, indicates an improvement of interface quality. In AML of thin ( 5 nm) sublayers the opposite effect is concluded for Se sublayers, and hardly any change for CdSe ones. A gradual increase of laser power density causes a likewise modification of Raman spectra, which is attributed to crystallization of Se sublayers; in similar low temperature (25 K) experiments (using even higher laser power densities) there is no indication of substantial crystallization, implying that the latter is mainly a thermal effect. From the threshold for crystallization, we observed that the structural stability of Se/CdSe AML is better, the smaller the sublayer thickness, an important result for electrophotographic applications.

Topological threshold in GexAs40−xSe60 glasses and thin films

Abstract Composition dependences of the mean atomic volume of glasses of the GexAs40−xSe60 system, as well as of the optical bandgap, solubility, and their changes on irradiation in thin films of the same system, are measured. It is shown that there is a topological threshold at an average coordination number Z≈2.67 both in glasses and in thin films of this chalcogenide system, similar to the transition observed in GexAs40−xS60 glasses and films.

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.

Amorphous Se/SeTe multilayers

Abstract In our study of Se/SeTe multilayers bubble type features, characteristic of quantum size effects, were established in the perpendicular I-V characteristics. An increase with time in the parallel currents is well expressed at high voltages which can lead to a S-type switching. It is mentioned that voltage-induced metastabilities are connected with high-voltage polarization. The depolarization currents follow the extended exponential law.

Photoinduced anisotropy in photobleached Ge–As–S films

Abstract Linear optical dichroism and birefringence induced by linearly polarized light were studied in the photobleached optically isotropic three-component GexAs40−xS60 films. The anisotropy can be erased by nonpolarized or circularly polarized light. Multiple reorientation under the action of linearly polarized light with orthogonal orientation of the electrical vector was investigated. The anisotropy relaxes gradually in the dark with a large time constant. The sign of dichroism was the same as in the binary As–Se and As–S photodarkened chalcogenide films. Therefore, the inducing polarized light always decreases the absorption of the light polarized parallel to itself. Photodarkening instead of photobleaching was observed in annealed films. Annealing resulted also in a strong decrease of photoinduced anisotropy that is different from the behaviour of binary chalcogenide films.

Small-angle X-ray diffraction studies on interface sharpness of amorphous Se/CdSe superlattices

Abstract X-ray diffraction measurements have been performed to evaluate the interface sharpness of Se/CdSe amorphous multilayers as well as their thermal stability. A value of about 1.6 nm has been measured as the effective interface thickness, di. It decreased to di ≈ 1.3 nm after the samples were annealed at an optimum temperature, Ta ≈ 360 K. A negative interdiffusion coefficient has been obtained in this case. More complicated temperature dependences of the X-ray diffraction peaks have been observed at higher annealing temperatures. Two different thermally dependent processes have been considered.

radiation-induced effects in Ge-As-S chalcogenide glasses

The radiation-optical properties of vitreous semiconductors in the Ge-As-S system are studied near the 2D-3D topological phase transition. The results obtained are interpreted within the framework of the model of coordination defect centers, which are characteristic of chalcogenide vitreous semiconductors.