Čerenkov phonon radiation and phonon structure in electron emission, electroluminescence, and current–voltage characteristics of electroformed Al–Al2O3–Ag diodes

Abstract

Electroforming of metal–insulator–metal (MIM) diodes is a non-destructive dielectric breakdown that results in a conducting filament in the insulator and changes the high resistance of the as-prepared MIM diode into a low-resistance state. It is a critical step in the development of conducting states between which switching can occur in memristors or resistive switching memories. Electroforming of Al–Al2O3–Ag diodes in vacuum results in the formation of a conducting Al2O3 filament. There are Ohmic contacts at the Al–Al2O3 and Al2O3–Ag interfaces. Electronic processes develop, such as voltage-controlled negative resistance in the current–voltage (I–V) characteristics, electron emission into vacuum (EM), and electroluminescence (EL). I–V curves, EM, and EL of three electroformed Al–Al2O3–Ag diodes with anodic Al2O3 thicknesses between 36 nm and 49 nm have been measured with 20 mV resolution between voltage steps. Periodic voltage structure occurs for the three electronic processes for applied voltages between the voltage threshold for EL, ∼2 V, and the maximum applied voltage, 11 V. The voltage peak period, 60 mV to 70 mV, is the same as the range of LO phonon energies of anodic Al2O3. The proposed mechanism is Čerenkov phonon generation by ballistic electrons whose velocity exceeds the velocity of sound in Al2O3. The phonons, in turn, modify the motion and number of electrons that are emitted into vacuum, that are responsible for EL radiation, and that contribute to conduction through the diode. The occurrence of LO phonons shows that the conducting filament is Al2O3, not Ag or Al. The phenomena may provide a new method of generating terahertz radiation.