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.
Al–SiO–Al–SiO–Au triodes with SiO thicknesses between 150 and 500 Å have been used to measure the potential distribution in thin oxide films before, during, and after the development of voltage-controlled negative resistance (VCNR) in the current-voltage characteristics. Development of VCNR in the triode is accompanied by the establishment of a high-field region about 120 Å in thickness near the negative electrode. If triode potentials are reversed after developing conductivity, VCNR is still found in the current-voltage (I-V) characteristic of the triode but the potential distribution in the triode is only slightly changed. VCNR in the I-V characteristic is a high-field phenomenon but it does not depend on field emission of electrons from the metal electrodes. Conductivity in the bulk of the insulator is Ohmic with electron mobilities ∼10−3−10−2 cm2/V-sec. The behavior of Al–SiO–Au diodes is identical to that of triodes. Electroluminescence of Al–SiO–Au diodes, which appears when conductivity is developed, is characterized by a steep rise in intensity at 1.8 V, the voltage at which electron emission into vacuum from such diodes is first detected. Both electroluminescence and electron emission provide evidence for high-energy processes in the oxide film. A phenomenological model of conductivity and voltage-controlled negative resistance in thin oxide films is developed in which impurity conduction is the most important conduction mechanism.
Theories of dielectric breakdown in insulating films normally assume that dielectric breakdown depends on the electric field in the sample; that is, the thicker the film the higher the breakdown voltage. Contrary to theoretical expectations, voltage-dependent dielectric breakdown is observed in Al-Al 2 O 3 -Au diodes where Al 2 O 3 is made by anodizing in different electrolytes. The breakdown voltage is ϳ4.5 V, independent of Al 2 O 3 thickness and anodizing electrolyte. Voltage-controlled negative resistance ͑VCNR͒ develops in the current-voltage (I -V) characteristics of Al-Al 2 O 3 -Au diodes after voltage-dependent breakdown. Electron emission into vacuum accompanies the formation of VCNR in the I -V characteristics. Detailed studies of the development of VCNR show that the maximum current, the voltage for maximum current, and the voltage threshold for electron emission depend on the maximum voltage applied to the sample. A large current increase occurs for maximum applied voltage between 5 and 7 V. A fully developed VCNR characteristic has an ohmic contact suggesting that the development of an ohmic contact at a metal-insulator interface initiates breakdown.
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.
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