Threshold switching characteristics

The paper describes experiments on thresholds switches, based on a multicomponent chalcogenide glass (Te₄₀As₃₅Ge₇Si₁₈) of about 1 μm thickness, and pyrolytic graphite electrodes. The results demonstrate that “forming processes” (of presumably electrolytic character) in the off-state affect only the contact area which is not engaged in filamentary conduction when the switch is in the on-state. The detailed on-state characteristics are shown to depend on (a) the maximum on-current reached during the switching event, and (b) the rate of voltage change, and this is why depend on the external series resistance. Forming processes in the on-state must arise from a very different mechanism, and the operational reality of the minimum holding current has been established. The resultsfavor non-thermal interpretations of threshold switching as such, albeit inevitably with certain “thermal overtones”.     

On-state behaviour and degradation effects in thin-film amorphous switching devices

The on-state behaviour of thin-film chalcogenide threshold switches was studied with special reference to the influence of electrode materials and the progressive effects of device degradation. For operations during the forming period, the on-state current-voltage characteristics remain identical for both rising and falling current. The on-state minimum voltage is found to be closely proportional to the square root of the electrode thermal conductivity and, in addition, shows some polarity dependence with dissimilar electrode combinations. This behaviour is interpreted in terms of a model in which the voltage drop across the on-state current channel is dominated by constrictions and associated hot spots near the channel/electrode boundaries. After forming and after a further number of operations depending both on electrode material and ambient temperature, one or more stable intermediate steps or “branches” develop in the I–V characteristic of the transition from off-state to on-state. The exact pattern shows some polarity dependence. Branching is identified with the progressive precipitation of tellurium-rich “islands” in the glass as the number of device operations increases and as a result of which the growth of the on-state channel proceeds via a number of “island-hopping” stages.     

Switching in Al-As-Te glass system

The switching properties previous to the memory effects of bulk chalcogeni de semiconductor glasses in the Al-As-Te system are investigated. The values of the switching voltage are related to the glass transition temperatures as well as to glass composition and a linear relation between ln V_th and Te at% is found. A constant electrical power for switching occurrence has been found. The behaviour of the switching voltage V_th with temperatures ranging from room temperature to 100°C follows an exponential law previously reported by other authors. The delay time versus applied voltage shows a typical glass bulk behaviour. The experimental results support an electrothermal model for switching in this system for the used conditions, although a low-field dependence must be introduced for a complete agreement.     

Thermal Analysis of LiGaO2 and NaGaO2

The high-temperature thermal properties of the ternary oxides LiGaO₂ and NaGaO₂ are studied by simultaneous differential thermal analysis and thermogravimetry between room temperature and about 1700 °C. For the melting temperature of LiGaO₂ a value of 1595 ± 10 °C is determined. NaGaO₂ undergoes a solid state phase transition at 1280 ± 10 °C and melts at 1395 ± 10 °C.     

Microstructure and properties of spark plasma sintered Fe–Cr–Mo–Y–B–C bulk metallic glass

Fabrication of Fe-based amorphous alloy using spark plasma sintering (SPS) process has been reported. Fully amorphous compacts with ～95% relative density were successfully sintered at temperature about 100 °C lower than glass transition temperature (T_g: 575 °C). Formation of crystalline Fe₂₃(C, B)₆ phases within near-fully dense (～99%) amorphous matrix is observed at sintering temperatures (>550 °C) close to glass transition temperature. Microstructure evolution in sintered compacts indicated that density, degree of crystallinity, and mechanical properties can be effectively controlled by optimizing SPS parameters.     

Crystallization of amorphous Ni35Zr65 and Fe40Ni40P14B6 under proton irradiation

Two amorphous alloys, Ni₃₅Zr₆₅ and Fe₄₀Ni₄₀P₁₄B₆, were irradiated using 400 keV protons at several temperatures below the crystallization temperature, T_x, to peak doses in the neighborhood of 3.5 to 4.5 dpa. Irradiation at 250°C resulted in the crystallization of both alloys, which were examined by transmission electron microscopy of samples electrolytically polished to various distances from the irradiated surface to study the effect of dose. Samples masked from the proton beam remained amorphous during irradiation. In the Ni₃₅Zr₆₅ alloy crystallization of the equilibrium phases propagated throughout the entire sample, while the in the Fe₄₀Ni₄₀P₁₄B₆ alloy crystallization was observed only in those parts of the samples lying within the proton range. Neither alloy crystallized during irradiation at 100°C. In both these alloys the amorphous phase is therefore evidently stable at irradiation temperatures below approximately 0.6 T_x. An examination of the literature on irradiation damage of binary alloys and intermetallic compounds suggests that there is a tendency for initially amorphous alloys to remain amorphous at irradiation temperatures, T_irr < 0.3 T_L, where T_L (≈T_x) is the “melting” temperature (either a eutectic, peritectic or congruent melting temperature). Also, these same alloys, even when they are initially crystalline, transform to the amorphous state during irradiation at T < 0.3 T_L. Some other crystalline alloys have also been shown to transform to the amorphous state at T_irr < 0.3 T_L even though they have never been prepared in this condition by rapid quenching techniques. The temperature 0.3 T_L appears to be a lower limit, however, since the crystalline to amorphous transformation occurs in many of these alloys at temperatures greater than 0.3 T_L. It is suggested, by analogy with results on void formation in irradiated metals, that this low temperature limit is related to the low mobility of vacancies in these materials, although the mechanism of crystallization, or conversely amorphization, is not fully understood.     

Mössbauer study of the amorphous Pd70Fe10Si20 alloy upon transition to the crystalline state

Foils of the amorphous alloy Pd₇₀Fe₁₀Si₂₀ were investigated after isothermal heating at the temperature range 350–450°C with a heating period from 15 min to 4 h. At 350°C no changes were observed in the shape of the Mössbauer spectra. At 400°C and at 450°C up to 30 min only the parameters of Mössbauer spectra consisting of two quadrupole doublets corresponding to palladium and silicon vicinities of iron atoms were changed. After further heating at 450°C crystalline phases of Pd₃Fe and Fe₃Si appeared on the Mössbauer spectra.     

Effect of indium content and rapid solidification on microhardness and micro‐creep of Sn‐Zn eutectic lead free solder alloy

The Sn‐Zn alloys have been considered as lead‐free solders. In this paper, the effect of 0.0, 0.5, 1.0, 1.5 and 2.0 wt.% Indium as ternary additions on melting temperature, structure, microhardness and micro‐creep of the Sn‐9Zn lead‐free solders were investigated. It is shown that the alloying additions of Indium to the Sn‐Zn binary system result in a suppression of the melting point to 187.9 °C. From x‐ray diffraction analysis, a new intermetallic compound phase, designated β‐In₃Sn is detected. The formation of an intermetallic compound phase causes a pronounced increase in the electrical resistivity and mechanical strength. Also, an interesting connection between dynamic Young's modulus and the axial ratio (c/a) of the unit cell of the β‐Sn was found in which Young's modulus increases with increasing the axial ratio (c/a). The ternary Sn‐9Zn‐xIn exhibits creep resistance superior to Sn‐9Zn binary alloy. The better creep resistance of the ternary alloy is attributed to solid solution effect and precipitation of In₃Sn in the Sn matrix. The addition of small amounts of In is found to refine the effective grain size and consequently, improves hardness. The 89%Sn‐9%Zn‐2%In alloy is a lead‐free solder designed for possible drop‐in replacement of Pb‐Sn solders. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Glass formation and crystallization in highly undercooled Te-Cu alloys

The large undercoolings required for glass formation have been achieved by the slow cooling (10-20°C/min) of liquid Te-Cu alloys in the form of a fine droplet emulsion. Within the region of glass formation, between 19 and 39 at.% Cu, DTA measurements indicate that the glass (T_g) and crystallization (T_c) temperatures during heating exhibit a broad maximum at the eutectic. During slow cooling of Te-rich alloy droplets, the maximum undercooling for nucleation increases from 213°C for pure Te to 264°C for Te-12.5 at.% Cu. An enhanced depression of the nucleation (T_n) temperature compared with the change of the liquidus develops in Te-rich alloys upon approaching the glass forming composition range and can be a useful feature in assessing the glass forming tendency. Thermal cycling experiments indicate that even at an undercooling of 181°C crystallization in an eutectic Te-29 at.% Cu alloy is limited by an inadequate nucleation rate in clean droplet samples. For a eutectic alloy, at undercoolings in excess of 200°C crystal nucleation does develop in the droplet samples, but complete crystallization is hindered by a rapidly rising liquid viscosity with increased undercooling.     

Structure,Crystallization and Electrochemical Corrosion Behaviour of Amorphous Cu66Ti34 Alloy

The effect of thermally induced structural changes on electrochemical corrosion behaviour of amorphous Cu₆₆Ti₃₄ alloy is studied using electrochemical potentiodynamic polarization technique in conjunction with XRD, TEM and DTA. The heat treatment has been carried out at two temperatures: 300 °C — where the process of structural relaxation is only possible and 500 °C — where crystallization of the alloy occurs. As model corrosive media 1N H₂SO₄ and 1N HNO₃ solutions are used. The short range order of the amorphous samples is studied with the total pair correlation function. Some changes of the interatomic distances and a tendency to increasing of the density of the amorphous structure after low temperature heating are established. The crystallization leads to formation of Cu₃Ti₂, β-Cu₃Ti and Cu₂Ti crystalline phases. It has been found that structural relaxation may have a beneficial effect on the susceptibility of the amorphous alloy to passivation, while crystallization lowers considerably its corrosion resistance.     

Some features relevant to switching processes in the amorphous semiconductor Si12Ge10As30Te48

The current-voltage characteristics of the amorphous semiconductor glass Si₁₂Ge₁₀As₃₀Te₄₈ have been measured under various experimenal conditions. The experimental results show that the resistance of the device in the ‘off’ state and the threshold voltage for the onset of switching action decrease with increasing the maximum current (I_p) passing through the device in the ‘on’ state, and that the threshold input power to set in the switching action is practically independent of temperature. The stability and the consistency of the device depend on the magnitude of I_p. When I_p is increased to a certain value the glass within and near the current filament between the electrodes become softened, and when it reaches a critical value the device is changed from its ‘switching-on’ state to a ‘memory’ state. All the results are in good agreement with the model that the filament formed to cause switching consists of a mixture of crystalline domains and amorphous domains with phase separations.     

Electrical properties of vanadate-glass threshold switches

The electrical properties of thick-film vanadate-glass switches, with both planar and sandwich geometries, have been investigated using a remote double-pulse technique. The effect on delay time of pulse separation, overvoltage and polarity were examined. No forming step was required for any of the devices. Photographic evidence of filament formation confirmed the thermal model of switching suggested by these experiments. Additional evidence, such as the effect of temperature on threshold voltage and electrical conductivity, established the importance of the semiconductor-metal transition in the VO₂ present in the material for the switching action. A computer simulation based on the discontinuous change in electrical conductivity, occurring at 68°C, and due to this transition, was shown to be in broad agreement with the experimental facts.     

On the decomposition behaviour of Al-Zn(4.5 at.%)-Mg(2;3 at.%) alloys during continuous cooling from the temperature range of homogeneity

The decomposition processes taking place in the Al-Zn (4.5)-Mg(2;3) alloys were studied by means of XSAS and resistivity measurements and by TEM investigations after continuous slow cooling to certain temperatures T_i and during an isothermal ageing at T_i following the continuous cooling procedure. During the cooling between 380 °C and 320 °C mostly particles of the η-phase are growing. The formation of particles homogeneously distributed in the matrix starts at 165 °C in the alloy with 2 at.% Mg and at 175 °C in the one with 3 at.% Mg. Below these temperatures new subcritical GP zones are formed and particles already present in the matrix are growing. The dominating process depends on T_i.     

Domain structure of Na1 − x LixNbO3 crystals

The domain structure of single-crystal and ceramic samples of Na_{1 ? x} Li_xNbO₃ solid solutions (at x ≤ 0.14) in the orthorhombic ferroelectric and antiferroelectric phases at room temperature is investigated by optical and electron microscopies and X-ray diffraction. The characteristic feature of the domain structure is the formation of 90° complexes consisting of laminar domains with a specific orientation relative to the lattice of the initial cubic phase. Consideration is given to the specific features in packing of these complexes and typical configurations of domains in the crystals. Observations revealed that the domain structure can involve 90°, 60°, 120°, and 180° boundaries, as well as (hhl) boundaries of the S type whose orientation depends on the cell distortion and changes with a variation in _x. The indices of these boundaries are determined. The density of 180° boundaries in the ferroelectric phase is very low compared to that of non-90° boundaries.     

On the reliability of amorphous chalcogenide switching devices

Switching devices consisting of a thin amorphous AsTeGe film sandwiched between two molybdenum electrodes were prepared by electron-beam evaporation and investigated by subsequent switching. The number of switching events from the beginning of the test until a prefixed change in switching voltage appeared, was defined as the “lifetime”. Samples which were subjected to an ageing process showed relatively stable operation, and longer lifetime than unaged samples. In addition, the lifetime was found to depend strongly on device geometry and to be limited by the formation of crystalline regions in the amorphous film. This was explained to be due to heating effects during the preswitching region, during the transition from off-state to on-state, and during the on-state, although the underlying switching model is assumed to be non-thermal.     

X-ray investigation of the crystallization of chalcogenide glasses of the type (As2Se3)1−x:(Tl2Se)x

Amorphous and crystalline states of As₂Se₃, (As₂Se₃)₃ : Tl₂Se and As₂Se₃ : Tl₂Se have been studied using X-ray diffraction techniques. Structural changes arise during the process of annealing in the temperature range between their softening and melting points are reported and their rates investigated. The crystallization temperatures were found to be 105 ± 5 °C, 135 ± 5 °C and 180 ± 5 °C respectively. The unit cell parameters are identified for each of the three resulting crystalline phases, that for As₂Se₃ : Tl₂Se being orthorhombic while the other two are monoclinic.     

The structure of the amorphous TISx system

Samples of bulk amorphous material were prepared in the range from TIS to TIS_2.5. Characteristic values of phase transitions (T_g, T_c, T_m) were measured by means of DTA. On this basis the entropies of melting and crystallization were calculated and found to be extremely low in this system (δS_m = 1.2 to 1.7 cal/K g-at). A composition- and temperature-dependent solid-to-liquid transition was observed: at room temperature samples with low sulphur content behaved like solids, while samples with high sulphur content behaved more or less like liquids. The TIS₂ material which was liquid-like at room temperature was solid at a temperature under 0°C (T_g = 6°C).On the basis of the analysis of radial electron density distribution curves on samples TIS, TIS_1.3, TIS₂ and TIS_2.5 we found, that chains of corner-bound TIS₄ tetrahedrons which develop with decreasing sulphur content in a Tl-matrix in which Tl atoms are arranged similarly as in a Tl-layer of the Tl₂S structure are responsible for the composition-dependent solidification.The origin of the temperature-dependent liquid-to-solid transformation observed for TIS₂ is in the development of TlS bonds of that type which binds a thallium layer to a sulphur one in crystalline Tl₂S.     

Two phase diagrams of Pb(Ti0.485Zr0.515)0.98(Nb0.5Bi0.5)0.02O3 solid solution in the temperature range 20 ≤ t ≤ 600°C

A high-temperature analysis of Pb(Ti_0.485Zr_0.515)_0.98(Nb_0.5Bi_0.5)_0.02O₃ solid solution has been carried out in the temperature range 20 ≤ t ≤ 600°C by X-ray powder diffraction. Two different phase diagrams are plotted: one is based on the change in the diffraction pattern of main reflections, while the other is based on the diffraction pattern of real (defect) structure. A transition to the cubic phase can be observed at t ≥ 360°C (in the former diagram) and t ≥ 450°C (in the latter diagram). The nonmonotonic (with inflection points) behavior of the temperature dependence of the cell volume in the cubic phase is explained.     

Glass formation and switching in the SbSe systems

New glass forming regions in the SbSe system have been observed using splat cooling techniques. The glass region is limited to compositions with less than 50 at % Sb. The glass whose composition corresponds to Sb₂Se₃ exhibits memory and can be switched off only by cooling in liquid nitrogen in the presence of an electric field. The off-state electrical resistance is of the order of 10⁵ times greater than that of the memory on-state.     

Experimental evidence of energy-controlled switching in amorphous semiconductors

The switching delay time and transition time, and threshold voltage for the onset of switching in the amorphous semiconductor Si₁₂Ge₁₀As₃₀Te₄₈ have been measured under various conditions using rectangular voltage pulses. The results show that both the threshold voltage and the delay time decrease, but the transition time increases with increasing temperature; and that these switching properties are strongly dependent on the width and the repetition rate of applied pulses. It is proposed that the delay time is associated with the time required for the formation of a filament to cause switching, and that the transition time is associated with the transit time of a carrier across the switching filament. All the experimental phenomena indicate clearly that the switching process is energy-controlled.