The switching mechanisms in amorphous chalcogenide memory devices

Measurements of the resistance of chalcogenide memory devices switched with pulses which have a long or very short trailing edge, showed no significant difference. This contradicts the switching model as proposed by Cohen et al. Experimental data is presented and a discussion of the complexity of crystallization suggests a modification in which the inner zone of the filament is usually quenched below the crystallization temperature during the set pulse, as a result of current redistribution. Further modifications to the model for the reset event take account of an annulus around the reset filament which is at the optimum temperature for crystallization. Crystallization in that annulus results in shifting of the filament axis. The use of multiple-pulse resetting reduces the chance of crystal growth in the annulus.     

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.     

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.     

Growth, characterization and modeling of alternating-current thin-film electroluminescent devices

This article seeks to put together the most important and updated information and to present a unified, systematic and concise overview of the classification, physical principles and methods of the ACTFEL device growth, characterization and modeling. The review takes a comprehensive look at the evolution of the equivalent schemes of ACTFEL devices from the early purely interfacial model to the most recent models enabling simulation of the space charge formation. Comparison of experimental and computer simulated characteristics of the ACTFEL stack may bring a better understanding of the possibilities of charge storage in various regions of the device and help in finding how this may impact device performance. Some peculiarities of the SPICE simulation of the devices are also discussed.

Main peculiarities of the characterization techniques are described and their role in the understanding of the device operation is presented. Some important features of the conventional characterization techniques, such as VIQ, PDP, L-V, Q-V, C-V, Q_max-V_max, and Q_p-F_int measurements in identifying carrier traps, their energy position, and some other main device parameters are emphasized. An important role of the destructive methods of analysis, such as the scanning election microscopy, secondary electron contrast profiling, and SIMS measurements of ACTFEL devices as complimentary tools to the non-destructive characterization techniques is demonstrated.

The paper addresses the well-known as well as “non-traditional” theoretical and experimental techniques of ACTFEL device analysis. The understanding of the ACTFEL devices can be greatly improved by using the innovative analogy approach of the analysis, some aspects of which were originally developed in Luxell and converted to a characterization technique.     

Nonvolatile resistance switching in amorphous In–Zn–O films

In amorphous In–Zn–O thin films we found a polar reversible sharp transition between a high-resistance state (HRS) with insulating properties and a degenerated semiconducting or metallic low-resistance state (LRS). The switching with resistance ratio up to 10³ is completely similar to the colossal electrical resistance (CER) phenomenon, discovered in the highly correlated oxides with the structure of perovskite. We suggest that the polar reversible resistance switching occurs due to the electric field induced oxygen exchange between high-resistance thin interface part and low-resistance thick bulk part of In–Zn–O. The oxygen exchange changes the donor concentration (oxygen vacancies) in the transition layer which leads to the Fermi level shift. The transition between hopping and band conductivity occurs when the Fermi level crosses the mobility edge.     

Instrinsic instability and current channelling in thermally controlled,two-terminal switching devices

Electrical threshold switching characteristics are reported for a bulk, two-terminal chalcogenide device. In addition to a simple load line instability, a more abrupt switching transition is observed, which is identified as an intrinsic instability, associated with the formation of a current channel. Thermally controlled current channelling is discussed in relation to bulk and thin-film devices and emphasis is given to some consequences of channel constriction at the electrode interface in thin-film devices. A simple analysis of the constriction yields a minimum holding voltage of the same order as is observed in thin-film devices.     

Negative conductance and sequential tunneling in amorphous silicon-silicon carbide double barrier devices

Negative conductance has been observed in the electronic transport perpendicular to hydrogenated amorphous silicon carbide/amorphous silicon double-barrier structures at low temperatures. Some devices present negative resistance even at room temperature. These results are consistent with a sequential tunneling phenomenon.     

High field effects in amorphous germanium

Measurements of high field current have been made on amorphous Ge (a-Ge) films over the temperature range from 100 to 300 K. Non-ohmic conduction in a-Ge occurs at electric fields greater than 1–2 × 10⁴ V/cm. Field dependence of the conductivity has been explained in terms of the enhanced emission probability of carriers from the screened coulombic trap centers. Assuming the optical dielectric constant for a-Ge films, the screening length of the trap centers and the density of states at the Fermi level are estimated to be 12 Å and ～6.1 × 10²⁰ cm^?3 eV^?1, respectively.     

Interpretation of the switching effect in amorphous semiconductors as a recombination instability

For an explanation of the switching effect appearing on thin layers of amorphous semiconductors, a double-injection model is developed where the presence of diffusion currents in front of the electrodes is assumed in the high-resistance state. In the diffusion regions recombination takes place. The cause of the switching effect is seen in a recombination instability. Because of a saturation of the recombination current the life time of the free carriers becomes long enough for the diffusion length to extend over the full length and the high-resistance state to collapse.     

Enhanced spectral response by silicon nitride index matching layer in amorphous silicon thin-film solar cells

We employed the low temperature hydrogenated amorphous silicon nitride (a-SiN_x:H) prepared by plasma-enhanced chemical vapor deposition as a refractive index (n) matching layers in a silicon-based thin-film solar cell between glass (n = 1.5) and the transparent conducting oxide (n = 2). By varying the stoichiometry, refractive index and thickness of the a-SiN_x:H layers, we enhanced the spectral response and efficiency of the hydrogenated amorphous silicon thin-film solar cells. The refractive index of a-SiN_x:H was reduced from 2.32 to 1.78. Optimizing the a-SiN_x:H thickness to 80 nm increased the J_SC from 8.3 to 9.8 mA/cm² and the corresponding cell efficiency increased from 4.5 to 5.3%, as compared to the cell without the a-SiN_x:H index-matching layer on planar substrate. The a-SiN_x:H layers with graded refractive indices were effective for enhancing the cell performance.     

Thirty years trajectory of amorphous and nanocrystalline silicon materials and their optoelectronic devices

A review is given on research trajectory of hydrogenated amorphous and nanocrystalline silicon (a-Si:H and nc-Si) materials with their device applications ongoing since the period of 1970. A brief explanation on the motivation to start amorphous semiconductors research is given to produce a new kind of synthetic semiconductor having continuous energy gap controllability with valency electron controllability. Due to the result of some basic research on the film quality improvement of a-Si:H and nc-Si, some innovative devices had been developed since middle of 1980s in R&D phase such as a-SiC/a-Si heterojunction solar cells, a-Si/a-SiGe and also a-Si/nc-Si tandem type solar cells. Finally, the state of the art on the industrialization of the new devices is introduced and discussed.     

Torsion annealing influence on the impedance behaviour in amorphous FeSiB and CoSiB wires

An experimental study on the influence of a dc magnetic field on the real and imaginary parts of axial diagonal (ζ_zz) and off-diagonal (ζ_ϕz) components of the surface magnetoimpedance (MI) tensor in amorphous Fe_77.5Si_7.5B₁₅ and Co_72.5Si_12.5B₁₅ wires has been performed. The impedance characteristics have been analysed in the as-cast wires and after being torsion annealed. The ac drive current was 5 mArms flowing along the wires in the frequency range from 0.45 to 0.8 MHz. The MI behaviours can be ascribed to the different domain structures of positive and negative magnetostriction wires, before and after the intrinsic anisotropy distribution has been also modified by the annealing treatment.     

The relaxation behaviour of strenght and fracture toughness of amorphous metal-metalloid alloys

The effect of an annealing treatment on crack and fracture kinetics and on mechanical properties of Fe₄₀Ni₄₀P₁₀B₆ and Fe₅Co₇₀Si₁₅B₁₀ amorphous ribbons — both pure and with defined impurities — was investigated by in-situ tensile experiments in SEM. The effect of relaxation processes on tensile strength, microhardness and fracture toughness was analysed. A result of technological interest is the increase of fracture toughness after an annealing treatment in the temperature range T_ann. = (0.55 … 0.6) · T_cryst.. This effect will be enhanced by defined chemical impurities, and it can be well interpreted by a model of fracture toughness variation during a temper process. In this model the deformation behaviour cannot be described within the scope of a homogeneous solid, but it is determined by a relationship between closely localized deformation instabilities.     

Electrolyte effects on amorphous and supercooled sugar systems

The purpose of the present work was to analyze the modification of sugar thermal transitions in the presence of salts. Solid systems consisted of freeze-dried solutions of trehalose or sucrose, with or without salts (potassium and magnesium chlorides, acetates or citrates at 5:1 sugar:salt molar ratio). The freeze-dried systems were humidified at water activities 0.22 and 0.43 at 25 °C and then incubated at 70 °C. The presence of electrolytes affected the kinetics of several relaxation phenomena in sugar systems. Trehalose and sucrose crystallization was delayed in systems containing salts and this effect was dependent on water–cation interactions and on the size of the anion, being citrate the most inhibiting anion. The delaying effect of ions on sugar crystallization correlated with the degree of changes observed in the asymmetry of the sugar melting peak, and with the magnitude of enthalpy relaxations. Salts decreased peak and onset temperatures of trehalose melting crystals and increased the difference between these temperatures compared to the system without salt. Systems containing potassium citrate and MgCl₂ showed the highest Δc_p, relaxation enthalpies in the amorphous state and crystallization delay, and reflected higher anomalies in trehalose melting behavior.     

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.     

Charge collection in amorphous silicon solar cells: Cell analysis and simulation of high-efficiency pin devices

The drift length L_drift = μτE within the i layer of a-Si:H solar cells is a crucial parameter for charge collection and efficiency. It is strongly reduced not only by light-induced reduction of μτ, but also by electric field deformation ΔE by charges near the p–i and i–n interfaces. Here, a simple model is presented to estimate contributions of free carriers, charges trapped in band tails and charged dangling bonds to ΔE. It is shown that the model reproduces correctly trends observed experimentally and by ASA simulations: charged dangling bonds contribute most to ΔE of meta-stable cells. Electrons trapped in the conduction band tail near the i–n interface lead to the strongest field deformation in the initial state, while positively charged dangling bonds near the p–i interface get more important with degradation under AM1.5g spectrum. The measurable parameter V_coll is proposed as an indirect parameter to estimate the electric field, and an experimental technique is presented that could enable the distinction of defects near the p–i and the i–n interfaces.     

Crystal field effects in amorphous rare earth magnetic materials

A model proposed by Harris et al. in which crystal field affects are important, is extended to consider the magnetic properties of amorphous materials containing localized ionic moments of a single, heavy rare earth, magnetic element. The paramagnetic and ferromagnetic properties are investigated for all the heavy rare earths. It is found that the average spontaneous moment per ion in the amorphous state can be lowered below the free ion moment by more than 50%, but the relative reduction in Curie temperature is much less marked than found by Harris et al.