Electro-formed characteristics of evaporated Mn/SiO thin cermet films

Abstract

It has been conclusively demonstrated [1][2][3] that after electroforming, metal-insulator metal (MIM) sandwich structures undergo an increase in electrical conductivity by several orders of magnitude. A voltagecontrolled (or current-controlled) negative resistance, VCNR (or CCNR) is produced, and switching, electron emission, electro-luminescence and memory phenomena are observed. Switching [4 6] and electron emission [7,8] have also been observed to occur in planar discontinuous metallic films.Depending on composition, cermet thin films may possess three kinds of microstructure: metallic (continuous; positive temperature coefficient of resistance (TCR)); dielectric (discontinuous metal islands dispersed throughout a dielectric matrix; negative TCR); and transitional or intermediate (interconnected and randomly dispersed metal islands; negative TCR). Although a substantial amount of work has been carried out on the various properties of cermet films in the metallic and transitional microstructural regimes [9], relatively few investigations have commented on the d.c. and a.c. current-voltage characteristics of electro-formed cermet films in the dielectric regime. Thus the purpose of this letter is to present some results of studies for the Mn/SiO system with /> 80 wt % SiO.The metal-Mn/SiO-metal sandwich structures were prepared on conventionally cleaned Corning 7059 alkali-free substrates by single-boat evaporation in vacuo ~<0.5mPa at a deposition rate of 0.8 nm sec ~. In all cases the base electrode was aluminium and the counter-electrode was gold. The cermet thickness was monitored using a quartz crystal, but measured interferometrically for an accurate determination.Electroforming was investigated in vacuo ,-, 1 mPa. It was observed that the forming voltage, Vr, for all compositions studied (80 to 95 wt % SiO) lay between 1.0 and 5.0 V and the voltage, Vmax, corresponding to the peak current, Icm,x' was between 3.0 and 7.0 V. In all electro-formed samples the circulating current, Ic, was observed to be about four decades higher than the corresponding pre-formed structures. However, both Icm,x and Vf were found to decrease as the cermet thickness increased (up to the maximum value of 300 nm investigated). The slow decrease in Vr with increase in thickness probably occurs because the metallic islands increase in size and the inter-island separation decreases -both factors should aid filament formation. However, unlike SiOx in which Vf is independent of thickness up to a maximum value of 600 nm [10], electro-forming in Mn/SiO cermet films is not strictly voltage-controlled because it is also temperature dependent between -1 1 0 and 280°C. A comprehensive discussion of the parametric dependence of the electro-forming process in MIM structures on insulator thickness, environment and impurity content has been reviewed by Ray and Hogarth [11]. Fig. 1 illustrates that as the potential difference is increased above Vm,x, Ic decreases rapidly by one to two decades and then either continues to decrease before suddenly...

Waveform Analysis of the Dynamici−u Characteristics of CuSiOx Thin Film Sandwich Structures

“…This conclusion seems to apply with: (i) substrate temperatures between -60 and 100 "C, thc lowest and highest substrate temperatures investigated; (ii) 200 nm thick films; (iii) 2 vol% Cu film, 100 nm thick, at 20 "C. Relations (b) and (c) were found to apply equally well to a 6 vol% Cu film, 100 nm thick at 20 "C. Now the local filament temperature T can be calculated by interpreting (4) in terms of the Poole-Frenkel (PF) and the Schottky ( S ) conduction mechanisms. b = p/kTd'I2, k is Boltzmann's constant, d the film thickness, ps = (e3/(4n&~o)"2, and jPF = 2p,.…”