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.     

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.     

Differential anomalous X-ray scattering studies of amorphous Cd59As41 and Cd26As74

The local structure of amorphous cadmium arsenide semiconducting films has been studied by differential anomalous X-ray scattering. Intensity measurements were carried out on two samples, containing 41 and 74 at.% As, in the vicinity of the absorption K edges of both constituents using synchrotron radiation. The computational procedure, similar to that proposed by Warren for an amorphous sample with more than one kind of atom, was applied to obtain the structural parameters from the experimental data. It has been found that atoms in the amorphous CdAs films remain almost tetrahedrally coordinated and that the investigated films are chemically ordered. The structural changes going from cadmium- to arsenic-rich composition have been revealed. The differential anomalous X-ray scattering technique proved to be effective, providing the evidence for the CdCd and AsAs near neighbour correlations in Cd₅₉As₄₁ and Cd₂₆As₇₄, respectively. The simulations of the differential radial distribution functions have shown that for the amorphous film containing 41 at.% As the distorted tetrahedral structure, intermediate between the CdAs and Si III type structures, is adequate to account for the experimental data. At 74 at.% As, the atomic arrangement can be described satisfactorily by the structural model based on the tetragonal CdAs₂ structure. The structural parameters obtained from the present study and those previously derived using the extended X-ray absorption fine structure and conventional large angle X-ray scattering techniques are compared.     

Change in corrosion behavior of amorphous FePC alloys by alloying with various metallic elements

Comprehensive work to examine the effect of metallic alloying elements M on the corrosion behavior of amorphous FeM13 P7 C alloys was performed. All alloying elements except manganese are effective in increasing the corrosion resistance of amorphous alloys in various acidic and neutral solutions, regardless of anionic species, pH and oxidizability. The addition of elements more active than iron to amorphous FePC alloy decrease the corrosion rate in an active region during immersion tests, depending upon the passivation capabilities of the alloying elements themselves. When elements nobler than iron are added, the nobler is the alloying element, the higher is the corrosion resistance of amorphous FeM13 P7 C alloys.     

Electrical switching in chalcogenide liquid semiconductors

Reversible electrical switching from a higher resistance off-state to a lower resistance on-state has been observed in several chalcogenide semiconductor alloys both in the solid and liquid phases. Experiments conducted between 100 and 500°C showed that the threshold switching voltage decreases with increasing ambient temperature, and that the on-state resistance remains relatively constant up to the melting region but generally increases thereafter. The alloy Se_0.77Te_0.13S_0.10 shows a preserved on-state which can be maintained using 60 Hz ac in both the solid and liquid phases. The on-state can be reversed to the off-state by decreasing the applied field, and hence is not a conventional memory state. Below the melting region (250°C) the on-state I–V curve has a distinct knee or barrier voltage above which the current increases steeply. Above the melting range the knee becomes less clearly defined and occurs at a lower voltage. Below 345°C the above alloy has an activation energy of 0.37 eV. At this temperature an abrupt slope change or a discontuinity exists in the temperature dependence of off-state resistance, threshold field, threshold power and on-state resistance.     

High E-field microwave properties of bulk amorphous semiconductors

The effects of high microwave electric fields on bulk amorphous semiconductors were studied at room temperature. Samples of eight different glasses were mounted in the center of a reduced height, X-band waveguide in a sandwich geometry parallel to the electric field of the dominant mode. Samples were 90 mil (2.286 mm) square by 8.5 mil (0.2159 mm) thick with thin-film CrAu electrodes. The microwave conductivity remained constant at field strengths below a critical value near 10⁴V/cm. Above this value, which depends upon the microwave pulse length, switching to a low-resistance state was observed with the thermal breakdown as the switching initiation mechanism.     

Corrosion-resistant amorphous FeC alloys containing chromium and/or molybdenum

An attempt was made to improve corrosion resistance of amorphous iron-base alloys, which contain only carbon as a metalloid element, by the addition of chromium and/or molybdenum. The addition of both metallic elements is effective in decreasing the corrosion rate. Especially, the addition of certain amounts of chromium leads to spontaneous passivation even in 1 N HCl. The corrosion resistance of amorphous FeCrC alloys is intermediate between those of amorphous FeCrPC and FeCrB alloys. The all-amorphous alloys examined are characterized by immunity to pitting corrosion, even by anodic polarization in 1 N HCl.     

Crystallization of Li2OSiO2 glass studied by positron annihilation

The crystallization of 34 Li₂O65 SiO₂1 P₂O₅ glass has been studied by positron annihilation. The changes in the positron lifetime spectra have been correlated to the magnitude of the amorphous X-ray scattering. A linear relationship has been found for heat-treatment temperatures less than 700°C, above which non-linear phenomena in the lifetime spectra set in. The volume crystallinity reaches a saturation value, which varies from 70% at the heat-treatment temperature of 535°C to 80% at 700°C because of the phase separation. The dimensionality of the crystal growth in this glass is found to be n = 1.5 ± 0.1. The effect of the phase separation on the rate and dimensionality of the crystallization is discussed. The results show that positron annihilation is a sensitive and accurate method to follow crystallization in vitreous materials.     

Dark conductivity in amorphous undoped silicon films

The temperature dependence of the dark conductivity was investigated in amorphous undoped silicon films deposited by glow-discharge in a SiCl₄H₂ mixture. Different transport processes were indentified according to the investigated temperature range. The dependence of the dark conductivity was also examined as a function of some deposition parameters. The experimental results are discussed in terms of the two-phase structure of the film.     

Conducting filaments and switching phenomena in chalcogenide semiconductors

The sizes of the conducting filaments formed after switching operations in chalcogenide semiconductors have been measured as functions of input power in the on-state and specimen thickness using a scanning electron microscope. The experimental results show that the threshold switching involves double injection from both metallic contacts in the sandwich structure, and the combination of the electrothermal and the electronic processes. The conducting filament responsible for the switching consists of two permanent portions, one started from the anode and the other from the cathode, which have undergone a permanent change in material composition and structure after even one switching operation; and one temporary portion between the ends of the two permanent portions, which has not undergone any change in material composition or structure after many switching operations. The size of each portion depends strongly on the current level in the on-state, at which the filament is formed.     

Formation of nanocrystalline and amorphous phases in Cu80P20, Ni82P18 and Cu87Ni13 mechanically alloyed systems

Variations in the crystallite size and stored strain caused by mechanical milling of binary alloy systems of copper, nickel and phosphor were compared. During mechanical alloying process, an amorphous phase is partially produced in CuP and NiP alloy systems and only a nanoscale crystalline phase in CuNi alloy system. The CuP alloy system first forms a Cu₃P intermetallic compound and then the partial amorphous phase by the reaction of primary copper phase and Cu₃P intermetallic compound, while the NiP alloy system directly forms a partial amorphous phase by the reaction of primary nickel and phosphor phases. When the crystallite sizes of primary copper and nickel phases become of the order of a few nanometers in the CuP and NiP alloy systems, they are in an almost strain-free state.     

Photoinduced metal surface deposition in amorphous GeSAg films

Metallic silver photosurface deposition (PSD) and photoinduced stractural transformation in amorphous GeSAg films containing a large quantity of Ag have been studied. The photosensitivity of the films was greatly enhanced by the addition of gold, and also the morphology and distributing density of the deposits were markedly influenced. It has been found that the PSD effect can be reversed by a thermal annealing.     

Laser produced amorphous phases in NbSi and VSi thin films

The binary systems NbSi and VSi are investigated by laser induced melting and quenching of vapor deposited thin films. Glassy phases of various compositions are produced. The established compositional glass forming ranges support that the equilibrium compounds Nb₄Si is stable only at high temperature. Thermal decomposition of the amorphous NbSi films proceeds via several intermediate stages. In particular, the AuCu₃-type configuration of Nb₃Si was found to be formed. In addition, amorphous NbSi films around 20 at.% si show the formation of an undentified metastable compounds upon post-irradiation annealing. Amorphous VSi films proved to be stable up to at least 500°C. Above this temperature they decompose directly into their respective equilibrium phases.     

A model for photostructural changes in the amorphous AsS system

The photodarkening effect was studied in thin film, bulk glasses and powdered glasses of composition As₂₉S₇₁, As₄₀S₆₀ and As_42.5S_57.5. The amount of photodarkening produced in a sample depended strongly on the samples state and composition as well as temperature, incident light intensity and wavelength. Using Raman spectroscopy, vibrational bands at 231 and 491 cm^?1 (assigned to AsAs and SS bond vibrations) were found to be enhanced in photodarkened samples. We present a model in which photodarkening is due to the formation of As_n clusters (n ? 2) which are loosely coupled to the amorphous network.     

The structure of some chalcogenide glasses

A study using replica electron microscopy, scanning electron microscopy, electron diffraction, X-ray diffraction, sputter etching and differential thermal analysis of the structural properties of glasses having a range of compositions within the AsTeGeSi quaternary system has shown that phase separation generally occurs in the bulk material. The second phase is dispersed as inclusions in the non-crystalline matrix and is apparently non-crystalline away from the boundary of the glass forming region but is crystalline near to the high Te boundary. The crystalline second phase is probably Te. This quaternary system, AsTeGeSi is often used as source material for the fabrication of devices studied for their monostable electrical switching behaviour and the phase separation observed in these materials needs to be considered in the interpretation of the switching properties and for failure analysis of such devices.     

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”.     

Threefold coordinated model structure of amorphous GeS,GeSe and GeTe

The black P structure is presented as a model for the structures of amorphous GeS, GeSe and GeTe. It is shown that the short interatomic distances, low near neighbor coordinations and high covalencies of the amorphous materials, relative to the crystalline, can be rationalized with the model. When scaled to the near neighbor interatomic distances in the amorphous materials, the model yields satisfactory agreement with the observed position and area of the second neighbor X-ray radial distribution function peaks. The model predicts: (a) A first neighbor peak area for GeS which is significantly different from that predicted by the random covalent model and (b) phase separation in certain composition regions which, for the Ge-S system, should be observable by means of transmission electron microscopy. An explanation of why phase separation is not likely to be observable through transmission electron microscopy studies of amorphous GeTe and GeSe is also presented.     

Structural studies of amorphous CoP and NiP by EXAFS

The EXAFS spectra of amorphous CoP and NiP alloys have been recorded and analyzed on both edges of the metal and metalloid. The spectra cannot be directly fitted by the classical neutrons or X-rays results: these appear to be the average values of asymmetric radial distribution functions that we have to use in EXAFS analysis. Then we show that the environment of the phosphorus atoms in the amorphous state is very reminiscent of that of the Co2P or Ni3P crystalline materials.     

Threshold switching and the on-state in non-crystalline chalcogenide semiconductors: An interpretation of threshold-switching research

Experimental research on chalcogenide glasses is reviewed and an interpretation concerning the cause of threshold switching and the basis of the low-impedance on-regime is presented. Observed behavior is consolidated in terms of a single conductivity mechanism and two conducting species governing the off- and on-state electrical characteristics. Phenomenological equations and their consequences are given which are shown to be capable of successfully describing both in functional form and accurate magnitude a very wide variety of experimental data including current — voltage transient characteristics, the Haberland condition, delay time, maximum allowable interruption times, rise and decay characteristics and negative capacitance. Recent research is presented concerning two types of non-thermal radiative emission during the on-regime of a threshold switching event. Various switching models are described and critically discussed in terms of the experimental data and phenomenological equations.     

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。