Switching phenomena and dielectric breakdown in amorphous Nb2O5 films

Repititive switching from the high-to the low-resistance states in thin amorphous Nb₂O₅ films are investigated. The switching processes are found to be associated with four conduction states. The transitions among the first three states are reversible, exhibiting a cyclic switching behavior. The resistive transitions are abrupt, taking place within a range of less than a millivolt. The switching time from the first conduction state to others is <2 ns, while the switching from the second state occurs with a voltage-dependent delay time. The values of resistivity, thermal activation energy and I–V characteristics of the four conduction states are given and show that the final or irreversible dielectric breakdown process occurs throuh the formation of highly conductive filaments. The current-voltage characteristics for the four conduction states are briefly discussed.     

The composition dependence of electrical switching behavior of Ge7Se93−xSbx glasses

Bulk Ge₇Se_93?xSb_x (21 ? x ? 32) glasses are prepared by melt quenching method and electrical switching studies have been undertaken on these samples to elucidate the type of switching and the composition and thickness dependence of switching voltages. On the basis of the compressibility and atomic radii, it has been previously observed that Se-based glasses exhibit memory switching behavior. However, the present results indicate that Ge₇Se_93?xSb_x glasses exhibit threshold type electrical switching with high switching voltages. Further, these samples are found to show fluctuations in the current–voltage (I–V) characteristics. The observed threshold behavior of Ge₇Se_93?xSb_x glasses has been understood on the basis of larger atomic radii and lesser compressibilities of Sb and Ge. Further, the high switching voltages and fluctuations in the I–V characteristics of Ge–Se–Sb samples can be attributed to the high resistance of the samples and the difference in thermal conductivities of different structural units constituting the local structure of these glasses. The switching voltages of Ge₇Se_93?xSb_x glasses have been found to decrease with the increase in the Sb concentration. The observed composition dependence of switching voltages has been understood on the basis of higher metallicity of the Sb additive and also in the light of the Chemically Ordered Network (CON) model. Further, the thickness dependence of switching voltages has been studied to reassert the mechanism of switching.     

Thermal switching due to a local inhomogeneity existing between the electrode and the amorphous semiconductor

Switching transients in sandwich-type amorphous switching devices with electrodes of large area were studied experimentally. The results were analyzed in terms of a modified thermal switching model; the current concentration in the inhomogenous region existing at the electrode-semiconductor interface causes a thermal switching. The reciprocal delay time 1/t_D is proportional to the square of an applied voltage V²_p, indicating that the switching is caused by a certain kind of thermal process. The slope on the characteristic, however, is much larger than the theoretical value, in spite of using the resistivvity obtained from the current-voltage characteristic in the regime just before switching. These results must be attributed to a localized small region with a resistivity much lower tthan the bulk resisitivity. Some examples of asymmetric switching characteristics due to a local inhomogeneity are also shown.     

High field electrical switching behavior of Ge10Se90−xTlx glasses

Electrical switching studies on bulk Ge₁₀Se_90−xTl_x (15 ? x ? 34) glasses have been undertaken to examine the type of switching, composition and thickness dependence of switching voltages. Unlike Ge-Se-Tl thin films which exhibit memory switching, the bulk Ge₁₀Se_90−xTl_x glasses are found to exhibit threshold type switching with fluctuations seen in their current-voltage (I-V) characteristics. Further, it is observed that the switching voltages (V_T) of Ge₁₀Se_90−xTl_x glasses decrease with the increase in the Tl concentration. An effort has been made to understand the observed composition dependence on the basis of nature of bonding of Tl atoms and a decrease in the chemical disorder with composition. In addition, the network connectivity and metallicity factors also contribute for the observed decrease in the switching voltages of Ge₁₀Se_90−xTl_x glasses with Tl addition. It is also interesting to note that the composition dependence of switching voltages of Ge₁₀Se_90−xTl_x glasses exhibit a small cusp around the composition x = 22, which is understood on the basis of a thermally reversing window in this system in the composition range 22 ? x ? 30. The thickness dependence of switching voltages has been found to provide an insight about the type of switching mechanism involved in these samples.     

Electrical switching in As-Te-Ag system-composition and temperature dependence

The current-voltage characteristics related to switching phenomena in silver doped arsenic telluride glasses, As₂₀Te_80−xAg_x and As₄₀Te_60−xAg_x, have been investigated over a wide composition range (4?x?14). The samples are found to show threshold switching behavior with the number of switching cycles withstood by the samples depending on the ON state current. The switching voltages are found to decrease with increase in silver content and a sharp minimum is seen at the composition x=12 for the As₂₀Te_80−xAg_x glasses and x=11 for the As₄₀Te_60−xAg_x glasses. An effort has been made to understand the observed composition dependence on the basis of increase in the conductance of the samples with silver addition and local structural effects.     

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.     

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.     

Threshold switching in niobate glass — thick-film devices

Thick-film, niobate-glass threshold switches have been made in reducing gas atmospheres at high temperatures (1100–1400°C) using SiO₂ and B₂O₃ as glass formers. Their temperature characteristics are far superior to those of similar vanadate switches previously reported [1,2], and they are stable at low ON currents (< mA) if they are protected from atmospheric oxidation. They do not require forming.The process consists in pre-firing the printed, powdered glass in air and then reducing in argon/hydrogen gas. While the reducing conditions are not critical and virtually full reduction to tetragonal NbO₂ occurs, the pre-firing conditions in air are critical if high-resistance devices are to be obtained. Many devices can be made on a single alumina chip and the results of a statistical analysis are given.The electrical characteristics — measured, chiefly, using a double-pulse technique to examine the change in delay time with pulse separation, ON current, pulse width, etc. — are similar to those previously reported for the vanadate switches [2]. These measurements, together with the results of a microscope and electron-microprobe examination, indicate that a thermal mechanism of switching with filament or channel formation is the most likely. A steady-state computer simulation based on the thermal model indicates the temperatures to be expected during switching, and these are calculated to be less than the NbO₂ semiconductor-metal transition temperature of roughly 800°C, for all the cases examined.     

The effects of electrode materials on the switching behaviour of the amorphous semiconductors Si12Ge10As30Te48

The switching behaviour of the amorphous chalcogenide alloy Si₁₂Ge₁₀As₃₀Te₄₈ has been systematically investigated using silver, indium, aluminum, and graphite for electrodes. The experimental results show that the stability in both the threshold voltage for the onset of switching action, and the holding current required to maintain the conducting state, depends strongly on electrode materials. The switching mechanisms related to the electrical and thermal properties of various electrode materials are discussed, and experimental evidence of the deteriorating effect of some electrode materials is given.     

Die Eigenschaften des leitfähigen Durchschlagkanals und sein Einfluß auf den bistabilen Schalteffekt in Cu2O

The current-voltage-characteristics and the temperature dependence of the thermopower and of the resistivity are measured on Cu₂O bistable switching elements in the lowresistance state. The behaviour of the low-resistance state is essentially determined by narrowings of a quasi-metallic copper filament. The narrowings result from the contact of copper particles inside the Cu₂O matrix. The behaviour of return switching into the highresistance state may be deduced from the structure of the filaments. The statistical fluctuations of the switching parameters are explained by the properties of the on-switching and off-switching process.     

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.     

Electrical switching and thermal studies on bulk Ge-Te-Bi glasses

Bulk, melt quenched Ge₁₈Te_82-xBi_x glasses (1 ≤ x ≤ 4) have been found to exhibit memory type electrical switching behavior, which is in agreement with the lower thermal diffusivity values of Ge-Te-Bi samples. A linear variation in switching voltages (V_th) has been found in these samples with increase in thickness which is consistent with the memory type electrical switching. Also, the switching voltages have been found to decrease with an increase in temperature which happens due to the decrease in the activation energy for crystallization at higher temperatures. Further, V_th of Ge₁₈Te_82-xBi_x glasses have been found to decrease with the increase in Bi content, indicating that in the Ge-Te-Bi system, the resistivity of the additive has a stronger role to play in the composition dependence of V_th, in comparison with the network connectivity and rigidity factors. In addition, the composition dependence of crystallization activation energy has been found to show a decrease with an increase in Bi content, which is consistent with the observed decrease in the switching voltages. X-ray diffraction studies on thermally crystallized samples reveal the presence of hexagonal Te, GeTe, Bi₂Te₃ phases, suggesting that bismuth is not taking part in network formation to a greater extent, as reflected in the variation of switching voltages with the addition of Bi. SEM studies on switched and un-switched regions of Ge-Te-Bi samples indicate that there are morphological changes in the switched region, which can be attributed to the formation of the crystalline channel between two electrodes during switching.     

Electrical switching studies on Ge-Te-Tl chalcogenide glasses: Effect of thallium on the composition dependence of switching voltages

Bulk Ge₁₇Te_83−xTl_x glasses (0 ≤ x ≤ 13), have been found to exhibit memory type electrical switching. The switching voltages (also known as threshold voltage — V_th) of Ge₁₇Te_83−xTl_x glasses are found to decrease with increasing thallium content. The rate of decrease of V_th is greater at lower concentrations and V_th falls at a slower rate for higher thallium concentrations (x ≥ 6).The addition of thallium to the Ge-Te network fragments the covalent network and introduces ionic nature to it; the reduction in network connectivity leads to the decrease in switching voltages with thallium content. The decrease in the glass transition temperatures of Ge₁₇Te_83−xTl_x glasses with increasing thallium concentration supports the idea of decrease in network connectivity with Tl addition. The more metallic nature of Tl also contributes to the observed reduction in the switching voltages of Ge₁₇Te_83−xTl_x glasses with Tl content.Further, there is an interesting correlation seen between the threshold voltage V_th and the average bond energy, as a function of Tl content. In addition, the switching voltages of Ge₁₇Te_83−xTl_x glasses have been found to decrease with sample thickness almost linearly. The set-reset studies indicate that the Ge₁₇Te₈₁Tl₂ sample can be switched for more than 10 cycles, whereas other glasses could not be reset beyond two switching cycles.     

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”.     

New insight into phase change memories

The switching mechanism in phase change memories was described on the basis of minimum switching unit: the commuton. A commuton is a minimum cluster of atoms that supports a reversible phase change from high to low electrical conduction state and back under the influence of an external signal. The switching process in a phase change chalcogenide film was modeled using two dimensional cellular automata approach. A system of 50 × 50 cells, each cell containing a commuton, was simulated. In the particular case of Ge₂Sb₂Te₅ (investigated here) this system corresponds to a 30 × 30 nm area. The formation of the percolation path as a function of phase change induced in commutons explains the switching phenomenon. The influence of the percent of defects in the material on the percolation threshold has been studied.     

Crystalline–amorphous and amorphous–amorphous transitions in phase-change materials

The transition from the crystalline state to amorphous state and back has been studied in the particular case of the GeSb₂Te₄ phase-change material by a computer simulation procedure. Modelling at the nanoscale indicates specific structural characteristics, especially the multiplicity of the amorphous phase as opposite to the uniqueness of the crystalline phase. In the particular case of the Si₁₂Ge₁₀As₃₀Te₄₈ switching glass two types of ordering have been pointed out and characterized.     

The presence of a thermally reversing window in Al-Te-Si glasses revealed by alternating differential scanning calorimetry and electrical switching studies

Alternating differential scanning calorimetric (ADSC) studies and electrical switching experiments have been undertaken on Al₁₅Te_85−xSi_x (2 ? x ? 12) system of glasses. These glasses are found to exhibit two crystallization reactions (T_c1 and T_c2), for compositions with x < 8. Above x = 8, a single-stage crystallization is seen. Further, a trough is seen in the composition dependence of non-reversing enthalpy (ΔH^NR), based on which it is proposed that there is a thermally reversing window in Al₁₅Te_85−xSi_x glasses, in the composition range 4 ? x ? 8. Electrical switching studies indicate that Al₁₅Te_85−xSi_x glasses exhibit a threshold type electrical switching at ON state currents less than 2 mA. Further, the switching voltages are found to increase with the increase in silicon content. It is interesting to note that the start (x = 4) and the end (x = 8) of the thermally reversing window are exemplified by a kink and a saturation in the composition dependence of switching voltages, respectively.     

Switching in amorphous semiconductors

This article presents experimental data on switching and other related properties of (1?x)As₂Se₃·xSb₂Se₃ materials. These data indicate that the threshold switching mechanism is electronic in thin devices (<10 μm) and thermal in thick devices. In addition, it raises some questions about the crystallization/phase separation model for memory switching.     

机械磨抛对CdZnTe薄膜阻变特性的影响

采用磁控溅射法在ITO玻璃上制备了CdZnTe薄膜,探究机械磨抛对CdZnTe薄膜阻变特性的影响。通过对XRD图谱、Raman光谱、AFM显微照片等实验结果分析阐明了机械磨抛影响CdZnTe薄膜阻变特性的物理机制。研究结果表明,磁控溅射制备的薄膜为闪锌矿结构,F43m空间群。机械磨抛提高了CdZnTe薄膜的结晶质量;CdZnTe薄膜粗糙度(Ra)由磨抛前的3.42 nm下降至磨抛后的1.73 nm;磨抛后CdZnTe薄膜透过率和162 cm^-1处的类CdTe声子峰振动峰增强;CdZnTe薄膜的阻变开关比由磨抛前的1.2增加到磨抛后的4.9。机械磨抛提高CdZnTe薄膜质量及阻变特性的原因可能是CdZnTe薄膜在磨抛过程中发生了再结晶。     

Temporal characteristics and the sign of photoacoustic effect in high-resistivity CdS crystals

The influence of light illumination on the nonelastic properties of high-resistivity light-sensitive CdS crystals under ultrasound (the so-called photoacoustic effect (PAE)) is studied. It is shown that the crystal steady state is attained with two different characteristic times τ₁ and τ₂ (τ₁ ≪ τ₂), which correspond to switching on (>₁) and switching off (τ₂) light, respectively. The PAE sign (i.e., the increase or decrease in the ultrasonic attenuation) under illumination is not a permanent characteristic of CdS crystals.