Conducting Channels Structure and Dielectric-Metal Switching Stability in Thin amorphous Films

Since the first discovery of dielectric-metal switching effect in amorphous materials (As-Te-I system)1,2 in 1962 there have been published numerous reports on switching phenomena in various mostly low-conducting amorphous films. In all known cases the conducting state was imagined to be related to spatially separated current paths of irregular shape (filaments). Then bistable switching effects were summarized in review articles (see, for example,3,4). As it turns out to be the bistable switching may occur in metal-insulator-metal systems with an insulator material of thickness 10 nm to 2 μm 3 or 5 nm to 1 μm 4. In any case thickness of 1–3 μm is a threshold value for such effects. Some early switching models require filamentary current paths3–6. The background insulator-metal state switching was described as growth mechanism, involving dielectric relaxation and ion motion3, regrowth5, two-stage process: regrowth and switching6. The inverse switching to the insulating state is rupture5,6 or thermal rupture3. Such “percolation”-type switching models have been developed for individual switching systems (MIM-metal/insulator/metal sandwich3, Al-polystyrene-Al5, Au on quartz glass6). As a rule amorphous film switching requires preliminary treatment (so called “forming”). It is a voltage larger than some threshold value. The earlier models of forming process suppose ion injection from anode to insulator7, conducting channel growth through insulating film8, local dielectric-electrode alloy and following ion injection9. Detailed review of forming models contains, for instance, in paper3. Very interesting and important result of this analysis is the forming voltage independence on insulating film thickness d and current peak I max ~d -3 (of course, it takes place for d