Interface-induced Gap States in Alkyl Monolayers

Organic molecules, especially saturated ones, are classically considered to be complete insulators: electronic carriers can pass through such molecules only by tunneling. Yet measurements of the tunneling current through a self-assembled monolayer of alkyl chains (C_nH_2n+1, n=12-18) bonded to silicon consistently show higher values than expected from the frontier levels (HOMO and LUMO) of the molecule and the Si band edge. While tunneling is generally modeled by a single-level barrier, we have found that transport of carriers across the junction is facilitated by a continuum of interface-induced states. Electron spectroscopy and theory show that these interface states extend both spatially– over a significant fraction of the molecule– and energetically, over the range between the molecular levels and the substrate band edges. Current flow thus occurs simultaneously through several channels, each with a different barrier. The single level barrier that is commonly derived from such measurements is therefore an effective value, and reflects both the energy distribution of the density of interface states as well as that of states that are available to tunnel into.