Comparative study of electrical properties of metal-monolayer-semiconductor junctions at macro and nano scales

The electrical properties of metal-monolayer-semiconductor junctions were examined at the macroscale using mercury drop and thermally evaporated gold pad electrodes, and at the nanoscale using ballistic emission electron microscopy (BEEM) and high-resolution transmission electron microscopy (TEM). Oxide-free silicon wafers were modified with n-alkyl or ω-functionalized monolayers prepared via organometallic or thermal reactions. The mercury-molecule-silicon junctions displayed a clear dependence of the barrier height on both chain length and terminal functional groups of the monolayer. Measurements using thermally deposited gold contacts (i.e., gold/monolayer/silicon) yielded identical barrier heights for all monolayers, indicating that the gold atoms penetrated into the molecular layer causing a shorting of the junctions. BEEM and TEM studies showed uniform penetration of the gold atoms into the monolayer at the nanoscale. It was evident that thiol-functionalized monolayers are able to inhibit gold penetration, preserving an intact organic monolayer at the metal-semiconductor interface.