A.J. Snell

Analogue memory and ballistic electron effects in metal-amorphous silicon structures

Abstract We present experimental results showing that p+ amorphous silicon memory structures exhibit polarity-dependent analogue memory switching. The effect is non-volatile and we propose that it is associated with changes in a tunnelling barrier within the structure. It is also observed that conduction in the memory ON state is restricted to a narrow conducting channel through which the electrons can, under certain conditions, travel ballistically. As a conseauence, auantized resistance levels associated with ballistic electron transport are observed under certain circumstances. In the presence of a magnetic field, additional steps in the auantized resistance levels occur. A particular feature of this auantized resistance is that the effect can be observed at relatively high temperatures (up to about 190 K).

Theory of room temperature quantized resistance effects in metal-a-Si:H-metal thin film structures

Abstract A theoretical model of room temperature quantized resistance steps is described in terms of movement of a free electron Fermi surface into a quantized k -space (associated with electron confinement in real space) under the influence of applied electric field. The model is in good accordance with experimental observations such as the non-periodicity in quanta and voltage space and gives very realistic values for the parameters of electron lifetime, Fermi wavevector and geometry. Room temperature quantization is observed when the value of free electron lifetime (∼ 10 −14 s) is matched with a small size for electron confinement (∼ 7 nm or less).

Capacitance anomaly near the metal-non-metal transition in Cr-hydrogenated amorphous Si-V thin-film devices

Abstract We present experimental results that show a metal—non-metal (MNM) transition occurring in hydrogenated amorphous Si (a-Si: H) analogue memory devices as a function of temperature. The dc resistance of the devices undergoes a continuous change in the range 65–100 K from semiconductor-like behaviour to metallic behaviour, as the temperature increases. The ac conductivity, measured over the frequency range 1–3·1 × 107 Hz, shows an anomalous change as the temperature is varied over the MNM transition. Ac characteristics were modelled using multicomponent RC and RL equivalent circuits below and above the MNM transition region respectively. It is found that the capacitance increases markedly when the temperature approaches the MNM transition from the semiconductor side. Near the transition temperature this capacitance disappears, and the equivalent circuit now requires an inductive component together with a resistance which has a positive temperature coefficient of resistance equivalent to that of the ...

Quantization effects in metal/a-Si:H/metal devices

Abstract We present experimental results showing that metal/p+/metal amorphous silicon (a-Si: H) memory structures exhibit room temperature quantized electron transport associated with quantized resistance. The quantization of resistance is observed at values of R = h/2ie2, where i is an integer or a half integer.

DC and ac measurements on metal/a-Si:H/metal room temperature quantised resistance devices

Abstract Direct-current (dc) and alternating-current (ac) conductivities of room temperature electroformed Cr/p + -a-Si:H/V thin film quantised resistance devices have been measured as a function of temperature, applied field and frequency. The quantised resistance does not change over a temperature range. This invariance is in accordance with the theoretical model suggested for high temperature quantised resistance phenomena. The onset of quantised resistance jumps is associated with the formation of an inductive component within the structure indicating a temporary formation of a metallic conduction channel under the effect of an applied electric field.

Current-induced instability in Cr-p+ a-Si:H-V thin film devices

Abstract Experimental results on the constant current stressing in hydrogenated amorphous silicon (a-Si:H) Cr–p+–V thin film devices are presented. With increasing injection of charge via either increasing bias or time, the current-voltage characteristics of devices exhibit instability, as shown by a decrease in the reverse current. This is interpreted in terms of the creation of defects in the a-Si:H. The defect generation rate. as measured by the voltage shift ΔV at a constant reverse current in the J-V curve, is found to follow a square-root time dependent law. In addition, a decrease in device conductance after stressing is also observed, which is described by a mechanism of dopant equilibrium during and after stressing.

AC characteristics of Cr/p/sup +/a-Si:H/V analog switching devices

Experimental results on the ac characteristics of electro-formed Cr/p/sup +/ hydrogenated amorphous silicon (a-Si:H)/V thin film memory devices are presented. The impedance spectrum of the memory switching device has been measured over a wide frequency range from 1 Hz-32 MHz while keeping the ac voltage amplitude below 0.02 V. Simulation of the measured impedance spectrum using an equivalent circuit indicates that the capacitance associated with a conducting filament tends to increase as the memory resistance decreases. This is explained on the basis of an activated tunnelling mechanism. Charge transport is dominated by electron tunnelling via metallic particles in the filament, and hence small changes in interparticle spacing influences the tunnelling process considerately, leading to changes in both memory resistance and effective dielectric constant.

Aspects of Non-Volatility in a-Si:H Memory Devices

ABSTRACT.: a-Si:H p + -n-i devices, after a once only forming process, switch between two distinct states, both of which are memory states, and are electrically programmable with pulses in the nanosecond range with at least a 1 million cycle endurance. They are known to be non-volatile memory states which persist for long periods. This paper examines the nature of this non-volatility by looking at the effects of time, temperature, bias and radiation. It is found that these digital memory states persist with no change in state for at least four years under zero bias, and that they can withstand high temperatures both under bias and at zero bias. This and a resistance to radiation and a space environment shows that a mechanism of charge storage is unlikely and that they may have applications in hostile environments. The reason for such stability is unclear, but may be associated with the incorporation and distribution of metal in the filamentary region.

Analogue memory effects in metal/a-Si:H/Metal memory devices

In this paper we present experimental data for Metal/a-Si:H/Metal structures which demonstrate that they can be programmed into a range of non-volatile resistance states between 1 kΩ and 1 MΩ with nanosecond pulses of less than 5 V magnitude. A number of results are presented which show the importance of the top metal in the device operation.

Analogue Memory Effects in Metal/ a-Si:H /Metal Thin Film Structures

The ac conductivities of non-volatile analogue memory states are measured in electro-formed Cr/p + /V amorphous silicon structures for a broad frequency range (from 0.1 Hz to 32 MHz). The results suggest that the memory action is associated with electronic processes.