Low-Frequency Negative Resistance in Thin Anodic Oxide Films

Abstract

Negative resistance and large current densities have been observed in the direct‐current—voltage characteristics of five metal‐oxide‐metal sandwiches prepared from evaporated metal films. The systems studied and their voltages for maximum current are: Al‐SiO‐Au, 3.1 V; Al‐Al₂O₃‐Au, 2.9 V; Ta‐Ta₂O₅‐Au, 2.2 V; Zr‐ZrO₂‐Au, 2.1 V; and Ti‐TiO₂‐Au, 1.7 V. For aluminum oxide, which has been most extensively studied, the voltage for maximum current is independent of film thickness for films between 150 and 1000 Å thick; the phenomenon is not field dependent. Peak‐to‐valley ratios of 30:1 and current densities of 10 A/cm² are typical. Maximum current densities at peak voltage are 25 A/cm²; minimum current densities are 0.01 A/cm². Switching time from peak current to valley current is <0.5 μsec but negative resistance is not found for 60‐cycle voltages. Establishment of the dc characteristics and dependence on temperature and atmosphere are described. Electron emission from aluminum oxide sandwiches can occur at 2.5 V. Space‐charge‐limited currents in the insulator provide a possible mechanism for the current‐voltage curves and large currents below the voltage for maximum current through the oxide films. The mechanism responsible for negative resistance is uncertain.