H. Tichá

Covalent bond approach to the glass-transition temperature of chalcogenide glasses☆

Abstract Good correlation between the glass-transition temperature, T g , and the overall mean bond energy, 〈 E 〉, of a covalent network of a glass was found for 186 binary and ternary chalcogenide glasses. This correlation satisfies the Arrhenius relation for viscosity where the apparent activation energy of viscosity, E μ , is empirically related to the overall mean bond energy. The chemical bond arrangement is probably the main factor influencing T g in chalcogenide glasses.

Electrical properties of glasses in the GeBiSbSe and GeBiS systems

Abstract Measurements of optical absorption, dc conductivity and thermopower were carried out on glasses of the types (GeSe3.5)88sbPin12−xBix. The main results are: 1) observation of n-type conduction in the Ge20BixS80−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.

Optical properties of amorphous As-Se and Ge-As-Se thin films

Abstract Amorphous As–Se and Ge–As–Se thin films were prepared by thermal evaporation. From parameters of the Wemple–DiDomenico model, the values of the third-order non-linear susceptibility were estimated using the generalized Millers rule. Measurements of photodarkening at 78 K showed that the optical gap and the slope of the optical absorption edge are intercorrelated. The kinetics of the photoinduced shift of the gap are described using a stretched exponential law. A possible role of the concentration of Se atoms on the rate of photodarkening on one hand and of the network rigidity (measured by the mean coordination number) on the other are briefly discussed.

Remark on the glass-forming ability in GexSe1−x and AsxSe1−x systems

Abstract Some experimental results from new and old literature are summarized indicating that a maximum of glass-forming ability in GexSe1−x (I) and AsxSe1−x (II) systems lies close to an eutectic point; it is at x≅0.09 (I), and x≅0.20 (II). This one is more Se-rich than the Phillips–Thorpe threshold where the mean coordination number 〈CN〉 equals 2.4 (x=0.2 (I), and x=0.4 (II), respectively). It is speculated that the origin of the maximum of the glass-forming ability in GexSe1−x and AsxSe1−x systems could in fact result from `specific bonding arrangement, that is an optimal cross-linking of the short Se-chains crossed together by Ge or As atoms.

On the nature of bleaching of amorphous Ge30S70 films

Abstract The irreversible photobleaching of amorphous Ge30S70 films is found to depend on the air pressure during exposure. This is most probably a result of photo-oxidation and/or hydrolysis of the layers. The appearance of a Ge-O-Ge vibrational band in the infrared transmission spectra of exposed layers clearly indicates the extrinsic origin of photobleaching. Annealing of the layers is also accompanied by considerable bleaching. In this case the role of oxygen seems to be of less importance and an increase in the density of Ge-S bonds is most probably responsible for the increase of the optical gap.

Some physical properties of the glassy (GeS2)x(Sb2S3)1−x system

Some physical properties of a (GeS2)x(Sb2S3)1−x noncrystalline system are described. In contrast to the simple compositional dependences of e.g. the glass transition temperature (Tg), optical gap (Egopt) and high-frequency dielectric constants(ϵ∞), the compositional dependences of both the DC electrical activation energy (ΔEσ) and preexponential factor (σom), and also those of the reciprocal slope of the exponential absorption edge, show abrupt changes in the vicinity of 0.6 < Y < 0.7 (Y is volume fraction of GeS2). From the results of the application of the effective medium theory to the calculation of σom(Y) dependence it can be supposed that there exists an inhomogeneous transport regime.

Photoinduced changes in the short wavelength optical absorption edge of a-As2S3 and a-As2Se3 thin amorphous films

Abstract Photoinduced changes in the optical gap ( E g ) and the slope ( B ) of the short wavelength optical absorption edge (SWOAE) were studied in a-As 2 S 3 and a-As 2 Se 3 thermally evaporated thin films at 78 K. It was found that E g and B values are intercorrelated, as indicated by the existence of a focal point of ( αℏω ) 1/2 vs. ℏω dependencies, where α is an absorption coefficient and ℏω is photon energy. The correlation between E g and B values is discussed within both the model of Davis and Mott, and Taucs model.

A review of the specific role of oxygen in irreversible photo- and thermally induced changes of the optical properties of thin film amorphous chalcogenides

Abstract Recent results concerning the influence of oxygen on the photo- and thermally induced changes of optical properties of thin amorphous chalcogenides are summarized. It is shown that interaction of the surface of thin films with oxygen considerably affects the shift of the optical gap. The influence of oxygen is perhaps twofold. It behaves probably like a “catalyzer in the process of oxygen-assisted bond reconstruction” as proposed by Spence and Elliott [Phys. Rev. B 39 (1989) 5452], but it also enters directly into the film network forming strong covalent bonds with germanium and chalcogen atoms, respectively. Hence, the presence of oxygen during illumination or annealing of thin amorphous chalcogenide films most probably affects (i) the density of dangling and homopolar bonds, respectively, (ii) and actual chemical composition of the surface layer making this one oxygen rich (in normally deposited thin films) but also, chalcogen poor (in some obliquely deposited thin films). The role of oxygen in the case of illumination or annealing of amorphous chalcogenide films is of considerable importance and should not be neglected.

Photoinduced changes of optical properties of amorphous chalcogenide films at ambient air pressure

Abstract It is shown that Ge30Se70 and As38S62 amorphous films are relatively stable against photooxidation. The Ge35S65 film is easily photooxidized as indicated by an increase of absorption around 825 and 690 cm−1/due to the presence of the Ge-O bonds/. It is accompanied by irreversible bleaching. Vacuum annealed Ge35S65 film is also bleached. This bleaching, however, is accompanied only by an increase of Ge-S bonds density, thus it is an intrinsic effect indeed.

Some optical properties of Ge–S amorphous thin films

Abstract Amorphous Ge x S 1− x films ( x =0.27, 0.32, 0.36 and 0.4) were prepared by thermal evaporation. The values of the refractive index, the optical gap and the parameters of the Wemple–DiDomenico single oscillator model were determined. Using Millers generalized rule the values of the third-order non-linear susceptibility were also estimated. Thermally induced bleaching was observed for films studied. Photobleaching was observed for sulfur rich and nearly stoichiometric films ( x =0.27, 0.32) while for a germanium rich film ( x =0.4) no response to the illumination was observed. The differences in photobleaching are attributed to the role of p-lone pair states and to an overall network rigidity of the films.