P. Villares

Structural characteristics and some concepts related to switching properties in As0.45Se0.10Te0.45 glassy alloy

A structural model of the bulk Aso.4sSeo.roTeo.4s amorphous alloy has been built by the random Monte Carlo technique. The model is in good agreement with the atomic radial distribution function obtained by X-ray diffraction. The mean coordination number obtained was 2.3, thus allowing the sample to be characterized as a bistable material. Chalcogenide glasses are studied mainly because they present a switching phenomenon and a memory effect, and therefore can be used in the making of a great number of electronic devices [l]. The main interest in the analysis of the glass system As-Se-Te [2,3], amongst other reasons, is the dependence of the electrical conductivity on sample composition, as well as presenting memory effect together with the switching process. For these reasons, a structural analysis as well as an electrical study of the glassy alloy As0.45Se0.10Te0.45 has been undertaken, given that the establishment of the structural units, the distribution of chemical bonds between atoms, the radii of the coordination spheres and the mean values of the bonds, besides being of intrinsic interest, allows the process of electrical conductivity to be explained, as it is closely linked to the type and amount of chemical bonds existing in the material. The ‘4s0.45Se0.10Te0.45 composition was prepared so that the influence of the amount of Te could be observed, previous studies having been carried out with a smaller atomic fraction of Te [2,3]. Electrical resistance measurements of the composition with a greater amount of Te indicate that it reduces electrical resistance, due to the dislocation of covalent bonds as a result of the metallization of the chemical bond. Analogous behaviour has been reported in the literature [41. The number of bonds between each of the elements,

Structural models for semiconductor As0.20Se0.50Te0.30 glass alloy by X-ray diffraction

Abstract Two structural models of the bulk As 0.20 Se 0.50 Te 0.30 amorphous alloy have been built by a random Monte Carlo technique. These models are in good agreement with the atomic radial distribution function obtained by X-ray diffraction. The first model was made to fit the hypothesis of coordinated tetrahedral As, and the second to fit the hypothesis of threefold As coordination. The structural parameters of both models are in agreement with those mentioned in the literature for similar alloys.

Analytical determination of the area below the first peak in the RDF for amorphous alloys

Abstract An analytical expression has been obtained for the area below the first peak in the radial distribution function of an amorphous material, in which on one hand, the products of atomic factors, ƒ i ƒ i (∑ i x i ƒ i ) 2 , are approximated by the polynomial which is best fitted to the tabulated values and on the other, the parameters reflecting structural properties are introduced, on which hypotheses permitting the elaboration of a short-range order model of the alloy may be carried out.

A structural model of the amorphous alloy As0.40Se0.30Te0.30 by random technique

Abstract An analysis is performed of the atomic radial distribution function of the amorphous alloy As0.40Se0.30Te0.30, obtained from the quenching of the molten mixture of the elements. A spherical-shaped model of this alloy has been studied by means of the random Monte Carlo method. The model describes the experimental radial distribution function quite well.

Electrical conductivity and phenomenology of switching in the glassy alloy Ge0.09As0.20Te0.71

Abstract The essential characteristics of the switching phenomenon, excluding memory effects, have been analyzed in the bulk chalcogenide semiconductor glass Ge 0.09 As 0.20 Te 0.71 . A device has been designed and built to activate the samples correctly, and with which the contact pressure of the electrodes can be regulated. One of the results of this research is the constancy of the electrical power with which switching takes place. The functional dependence between the physical variables, delay time and applied voltage, is typical of bulk glasses. We have found a dependence of electrical conductivity on the applied field, which shows a deviation from the linear behaviour of the I - V characteristics in the blocking state. Also, the temporal dependence of the intensity has been studied, for voltages lower and higher than V th , which provides information about the evolution of the temperature of the material during electrical stimulation. The experimental results support an electrothermal model for the switching effect in the glass studied, and with the configuration of the electrodes used. The electronic contribution to the switching process is demonstrated by the dependence of electrical conductivity on the applied field.

A study on short-range order in some alloys of the Cu-As-Se glassy system by X-ray radial distribution analysis

Abstract The radial atomic distribution was studied in the amorphous alloys Cu x As 0.5− x Se 0.5 , with x = 0.05 (MI), x = 0.10 (MII) and x = 0.20 (MIII), using X-ray diffraction data of samples of these alloys obtained by quenching the molten material. The short-range order proposed was determined through the interpretation of the radial distribution function (RDF), using a theoretical expression which takes into account the variation in the atomic dispersion factors with s (scattering vector module) and approximating them to polynomic functions. Different coordination hypotheses for Cu, quoted in the literature on glassy alloys containing this element, were taken into account. The result of the study is that tetracoordinated Cu is the hypothesis which best agrees with the experimentally-obtained structural information, so the most probable short-range order for the Cu-As-Se system alloys studied is made up of tetrahedral units centred on interlaced copper and arsenic atoms, forming a network.

GeAsTe glassy system. Atomic distribution models

Abstract Atomic structure models were generated for the semiconducting glassy alloys Ge x As 0.20 Te 0.80− x ( x =0.05, 0.10), from shortrange order information given by the corresponding radial distribution functions, obtained from X-ray diffraction intensities. The Metropolis Monte Carlo random method was used in the building of the models, and the fact that Ge atoms can be four- and three-coordinated has been taken into account. The structural parameters obtained from the models agree with those given in the literature for similar alloys.

The formation process and the electrical nature of the lock-on filament in Ge0.09As0.20Te0.71 glass

Lock-on (memory) phenomena observed in the amorphous semiconductor Ge0.09As0.20Te0.71 have been investigated. The threshold voltage, Vth, and sample resistance, R, were measured during the formation of the filament. A relationship was obtained between the parameters Vth and R during the formation process. The electrical conductivity of the lock-on filament was deduced to be ≈ 1.1 × 104 Ω−1 m −1. The influence of an existing lock-on filament on the formation of a new filament is also described.

Pre-switching non-linear region and delay time in the Cu0.15As0.35Se0.50 semiconducting glass

Abstract The current-time and non-linear current-voltage characteristics in the Cu0.15As0.35Se0.50 chalcogenide semiconductor glass are analyzed. From the measurements it is deduced that the electrical conductance varies according to G0 exp ( V V 0 ) . The role played by Joule heating in the switching phenomenon is investigated. Results obtained for the delay time are in agreement with solutions given by the thermal balance equation for the impulse breakdown limiting case.

dc electrical properties of some metal-chalcogenide glassy alloys of the Cu-As-Se system

Abstract The dc electrical conductivity of metal-chalcogenide glassy alloys Cu x As 50− x Se 50 , with x = 5, 10 and 20 at%, derived from their linear current-voltage characteristics, and its variation with temperature, have been studied. The electrical conductivity, plotted versus inverse temperature, obeys an Arrhenius-type dependence, and the calculated preexponential factors seem to indicate that the electrical conduction mechanism is the so-called hopping between localized states, both in the band tails and near the Fermi level. Nevertheless, the electrical conductivity-temperature characteristics show a slight curvature, that could be explained by the influence of the variable range hopping conduction mechanism on the main conduction mechanism, or by the consideration of the small polaron model as the only electrical conduction mechanism.