Formation of Ohmic contacts: A breakdown mechanism in metal-insulator-metal structures

The occurrence of low currents in metal-insulator-metal (MIM) structures when high fields are present depends on the presence of a large energy barrier to charge injection at both metal-insulator interfaces. Formation of an Ohmic contact at either metal-insulator interface results in the easy injection of charge carriers into the insulator. The temperature dependence of current-voltage (I-V) characteristics of an Al-Al2O3-Au diode with 23 nm of anodically formed Al2O3, after nondestructive dielectric breakdown, is reported. An Ohmic contact forms after initial breakdown as shown by the occurrence of space-charge limited currents, I proportional to V-2, with an effective barrier height of 0.01-0.02 eV that is independent of bias. Further breakdown of the MIM sample results in the forming of voltage-controlled negative resistance (VCNR) in the I-V characteristic and the observation of electroluminescence. Elimination of the VCNR conduction regime results in I-V characteristics of a form I proportional to V-n, where n is temperature dependent and varies between 3.2 and 2.25 as the diode temperature increases from 100 to 300 K. The effective barrier height decreases with bias. All three conduction regimes depend on the presence of an Ohmic contact at both metal-insulator interfaces. The Ohmic contact results from the interaction of the metal electrodes with an impurity band in the insulator.