Surface plasmon polariton enhanced electroluminescence and electron emission from electroformed Al-Al2O3-Ag diodes

Abstract

Electroforming of Al-Al2O3-Ag diodes results in voltage-controlled negative resistance (VCNR) in the current-voltage (I-V) curves. Electroluminescence (EL) and electron emission into vacuum (EM) develop simultaneously. The temperature dependence between 200 and 300 K of VCNR, EL, and EM of Al-Al2O3-Ag diodes with anodic Al2O3 thicknesses between 12 and 41 nm has been studied. I-V curves and VCNR are slightly temperature dependent. The voltage for the onset of EL, VEL, is between ∼1.3 and ∼1.9 V for the range of Al2O3 thicknesses, with small temperature dependence. The density of defects in anodic Al2O3 is >1.5 × 1020 cm−3. Defect conduction bands that form from excited states of F- or F+-centers, oxygen vacancies in Al2O3, determine the value of the barrier height at the Al-Al2O3 interface, ϕA, and they control EM. EM is anomalous. The threshold voltage for EM, VEM, is ∼1.9 to ∼2.5 V for the range of Al2O3 thicknesses, which is less than the work function of Ag, 4.6 eV. EM at 300 K is ∼10−9 A. As temperature is lowered, EM drops to ∼10−12 A at TD ≅ 290 K and recovers to ∼10−9 A at TR ≅ 260 K. The particular values of TD and TR depend on sample preparation and Al2O3 thickness. The source of anomalous EM is electrons that tunnel through the high field region at the Al-Al2O3 interface into defect conduction bands. They gain energy and momentum by combining with surface plasmon polaritons (SPPs) that are generated at the Al2O3-Ag interface by EL photons. EL from Al-Al2O3-Ag diodes with 12 nm or 15 nm of Al2O3 is much larger than EL from diodes with thicker Al2O3 layers. The conducting channel of electroformed diodes with the thinnest Al2O3 acts as a microcavity in which the electromagnetic field due to SPPs stimulates EL from defect centers by the Purcell effect.