Impurity Conduction and Negative Resistance in Thin Oxide Films

Abstract

The conductivity of Al‐Al₂O₃‐metal diodes that show low‐frequency negative resistance in their current—voltage characteristics depends on impurities in the oxide and on the metal used as counterelectrode. For heavily doped Al₂O₃, development of diode conductivity by application of voltages occurs at ∼4 V, independent of oxide thickness. For oxide films that are not deliberately doped, the field in the insulator is more important than voltage in developing conductivity. Al‐Al₂O₃‐metal diodes have been constructed with Ag, Au, Cu, Co, Sn, In, Bi, Pb, Al, and Mg counterelectrodes. The current—voltage characteristics which develop depend on the metal and on polarity of the diode voltage during development of conductivity. With Ag as counterelectrode, most diodes were initially shorted; with Mg as counterelectrode, no diode conductivity could be developed. Other metals fall in between and give peak currents in the current—voltage characteristics in the sequence Au, Cu, Co, Pb, Sn, Bi, In, Al. There is no correlation between Al‐Al₂O₃‐metal diode conductivity and metal radius or work function.