Defect conduction bands, localization, and temperature-dependent electron emission from Al-Al_2O_3-Au diodes

摘要

The temperature dependence of several phenomena connected with voltage-controlled negative resistance (VCNR) of Al-Al_2O_3-Au diodes has been measured between 200 and 300 K. These include the current-voltage (I-V) characteristics, electroluminescence (EL), and electron emission into vacuum (EM) of diodes with 12-20 nm of anodic Al_2O_3. There is an abrupt decrease in EM by 3 orders of magnitude as temperature decreases from 285 to 280 K. EM recovers to the same magnitude as at 300 K at -260 K and is nearly constant between 260 and 200 K. The lower temperature at which EM recovers depends on the anodizing electrolyte. EM is decoupled from the major conduction mechanism because the voltage for maximum current of the I-V curve and the voltage threshold for EL are nearly constant over the same temperature range. A model is proposed in which defect levels of oxygen vacancies form two defect conduction bands in amorphous Al_2O_3. The concentration of oxygen vacancies is estimated from measurements of polarization of Al-Al_2O_3-Au diodes that do not break down to form VCNR in their I-V characteristics. EM at high and low temperatures is through defect conduction bands. Suppression of EM in the intermediate temperature range is due to localization of electrons caused by the irregular potential present when defect centers in the lower defect conduction band are nearly fully occupied. EM shows a temperature dependent metal-nonmetal transition while the conduction current does not.