Further studies of electroforming effects in mim structures
Thin film sandwich structures of metal-SiO/B203- metal with the dielectric thickness in the range 400 to 4000 A have been prepared by thermal evaporation in a vacuum of less than 10-5 torr. The metal electrodes used were Cu,, Ag and Al. After the electroforming process the devices showed a voltage-controlled negative resistance (VCNR) and emission of electrons into a vacuum. Devices with Cu electrodes were found to give the most suitable characteristics. The effect of air pressure and temperature on VCNR were studied. The vacuum-electroformed samples exhibited a pressure-voltage memory effect when operated in air at atmospheric pressure and the highimpedance state induced then could be erased by reducing the air pressure and by applying a bias voltage exceeding the threshold voltage, VT. Thermal-voltage memory effects were investigated in devices carrying Cu electrodes and the time-dependent transitions of the high-impedance memory states to a normal low impedance state at low temperatures were measured. The results have been explained employing the filamentary model of Dearnaley, Morgan and Stoneham. The localised defect regions on the surfaces of the samples, produced during the electro-forming process and electrical operation of the device, were investigated by means of a scanning electron microscope. The defects are believed to be mainly due to the formation of conducting filaments across the insulator and to the consequential Joule heating and electrolytic processes occurring along the filaments. A memory switching effect was observed in devices having Ag and Cu electrodes. The samples were formed initially in the atmosphere by the application of a direct voltage in series with a large resistance. The formed sample then could switch between a low conductivity 'off' state and a high-conductivity 'on' state, which has an ohmic characteristic. The observed switching behaviour is explained in terms of a monofilamentary model based on the formation of a metallic filament across the insulator during the forming process of the sample.