Conduction properties of metal/organic monolayer/semiconductor heterostructures

We have fabricated and characterized rectifying devices made of metal/organic monolayer/semiconductor heterostructures. The devices consist of an organic barrier layer sandwiched between an aluminum (Al) metal contact and a p-type Si semiconductor. The barrier materials were chosen from three types of self-assembled monolayers (SAMs) with different electronic properties: (1) wide-band gap poly(diallydimethyl ammonium) chloride (PDDA), (2) narrow-band gap PDDA/NiPc (nickel phthalocyanine tetrasulfonate), and (3) donor type PDDA/PPP (poly p-quaterphenylene-disulfonic-dicarboxylic acid). From current-voltage (I-V) measurements at room temperature, we have found the turn-on voltage of the devices can be tuned by varying the structure, hence electronic properties, of the organic monolayers, and that there exists a power-law dependence of I on V, I∝V^α, with the exponent α=2.2 for PDDA, 2.7 for PDDA/NiPc, and 1.44 for PDDA/PPP as the barrier layer, respectively. Our results imply that the transport properties are controlled by both the electronic properties of the SAMs and those of the metal and semiconductor, as indicated by the power-law dependence of the I-V characteristics.