Photo and particle induced transport of excited carriers in thin film tunnel junctions

Abstract

Photo and particle induced charge carrier (electrons and holes) transport is studied in metal-insulator-metal tunnel junctions of $\mathrm{Ag}\text{\ensuremath{-}}\mathrm{Al}{\mathrm{O}}_{x}\text{\ensuremath{-}}\mathrm{Al}$ type. The electronic excitation induced by photo irradiation with $h\ensuremath{\nu}=4.67\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ is compared with that induced by the impact of argon ions with a kinetic energy of $12\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$. The common feature of the two experiments is that only charge carriers with energies above (electrons) or below (holes) the tunnel barrier are detected. The electron to hole induced current ratio is adjustable by applying a bias voltage to the sample. A similar bias dependence of the induced current was found in both experiments. While the bias dependence of the photo induced current cannot be unambiguously explained due to the possibility of simultaneous excitation of both metal films, one can discuss the bias dependence of particle induced currents clearly in terms of electrons and holes excited in the top electrode only. Within a computer simulation based on a three dimensional model, we show that the photo induced conduction in $\mathrm{Ag}\text{\ensuremath{-}}\mathrm{Al}{\mathrm{O}}_{x}\text{\ensuremath{-}}\mathrm{Al}$ junctions is mainly determined by the simultaneous excitation of electrons and holes in the top Ag film. Moreover, the model indicates that the dominant contribution to the induced current is given by the charge carriers excited near the silver-oxide interface. Finally, we show that the particle induced current can be modeled by carrier transport between two free-electron gases, excited to an elevated electron temperature and to room temperature, respectively. Hereby, the influence of the tunnel barrier parameters and the elevated electron temperature on the bias dependence is discussed.