Dynamical effects and conductance asymmetry in metal-insulator-metal systems with different electrodes

We investigated the electron tunneling process in a planar system consistin g of two semi-infinite metal electrodes of different dielectric properties separated by a narrow vacuum gap. This system can be viewed as a model of a metal-insulator-metal structure, or even as a very rough model of scanning tunneling microscopy. We used a local limit of a self-consistent dynamic c alculation to evaluate effective tunneling barriers, and discussed the infl uence of different screening properties of the metallic electrodes on these barriers as well as on the conductance. We found that the barriers are sig nificantly additionally lowered due to the difference in the charge fluctua tion frequencies of the electrodes, and that the conductance minimum in suc h systems is shifted to a nonzero value. We compared our results with some measurements of I-V characteristics and work functions and found that our c alculations contribute to the understanding of the observed conductance asy mmetry. [S0163-1829(99)06735-1].