Electron emission and ultraviolet electroluminescence from valence-band states and defect conduction bands of electroformed Al-Al_2O_3-Ag diodes

摘要

Electroforming of metal-insulator-metal (MIM) diodes is a soft dielectric breakdown which results in the formation of a conducting filament through the diode. It is a critical step in the development of conducting states between which switching can occur in resistive switching memories. Conduction, electron emission into vacuum (EM), and electroluminescence (EL) have been studied in two groups of electroformed Al-Al2O3-Ag diodes with amorphous anodic Al2O3 thicknesses between 20 nm and 49 nm. EM and EL appear simultaneously with the abrupt current increase that characterizes electroforming of Al-Al2O3-metal diodes. There is voltage-controlled differential negative resistance in the current-voltage (I-V) characteristics after electroforming. There is a temperature-independent voltage threshold for EM, V-EM congruent to 2.2 V. Three EM regions occur. In region I, V-EM Vs less than or similar to 4 V, where V-s is the applied voltage, and there is an exponential increase of EM. There is a second exponential increase of EM in region III when V-s exceeds a second temperature-independent voltage threshold, U-EM. U-EM is similar to 6.6 V for one group of Al-Al2O3-Ag diodes; it is similar to 7.9 V for the second group. EM is nearly constant in region II for 4 V less than or similar to V-s less than or similar to U-EM. Two band-pass filters have been used to characterize EL from electroformed Al-Al2O3-Ag diodes. The long-pass (LP) filter plus photomultiplier responds to photons with energies between similar to 1.8 eV and similar to 3.0 eV. The short-pass (SP) filter measures ultra-violet (UV) radiation between similar to 3.0 eV and similar to 4.2 eV. Corresponding to region I of EM, there are exponential increases of EL for V-s greater than temperature-independent voltage thresholds: V-LP congruent to 1.5 V and V-sp congruent to 2.0 V. There is a second exponential increase of UV with the SP filter in region III for V-s greater than a temperature-independent voltage threshold, U-sp. U-sp congruent to 7.9 V for one group of electroformed Al-Al2O3-Ag diodes and U-sp congruent to 8.8 V for the second group; U-sp U-EM. Both groups exhibit EM from valence band states of amorphous Al2O3. The difference in U-EM and U-sp of the two groups of electroformed Al-Al2O3-Ag diodes is attributed to the presence or absence of a defect conduction band formed from the ground state of F-0- or F+-centers, oxygen vacancies in amorphous Al2O3. The observation of exponentially increasing EM or EL in the low conductivity state of electroformed Al-Al2O3-metal diodes is not consistent with switching mechanisms of MIM diodes that involve rupture of the conducting filament since rupture that affects diode current, if it occurs, should also cut off EM and EL. Published under license by AIP Publishing.