A defect-free thin film pentacene diode: Interplay between transport and scanning tunneling microscope tip tunneling injection

A diodelike device has been fabricated by organic molecular beam deposition of pentacene on a surface-modified, single-crystalline Au substrate. Using the substrate as the first and the tip of a scanning tunneling microscope (STM) as the second electrode, transport characteristics of this organic semiconductor (OSC) device were investigated. The probed pentacene islands are single crystalline and defect-free and consist of few molecular layers only. The current-voltage characteristics of this device reveal a pronounced asymmetry. For negative polarity, the current characteristics is almost independent of the layer thickness. For positive polarity, the current onset is shifted significantly to larger voltages with increasing layer thickness. Numerical simulations for a two-dimensional model system allow us to identify the injection properties of the STM tip as reasons for this pronounced asymmetry. For negative substrate bias the creation of holes in the valence band occurs by tunneling of electrons to the tip whereas in the opposite case holes have to be transported through the OSC layer from the substrate. Thus, for low positive voltage the hole current limits the device current. Once the resulting voltage drop between layer and tip becomes larger than the barrier for electron injection, direct tunneling of electrons into the pentacene conduction band becomes possible and n conduction begins to dominate.