Physical Characterization and Electrical Transport in End-Group Functionalized Self-Assembled Monolayers

We have previously demonstrated that a self-assembled monolayer (SAM) of alkyltrichlorosilane molecules, covalently bonded to the native oxide of a silicon substrate, allows the fabrication of MIS (Metal-Insulator-Semiconductor) devices with excellent electrical properties [1]. Here we demonstrate that we can functionalize, with π-electron moieties, the end-groups of the molecules, once grafted to the substrate, without disturbing the low conductivity through the SAM (σ⊥), while strongly increasing the in-plane conductivity (σ//). The functionalization of the monolayer is monitored by FTIR, ellipsometry and X ray reflectivity. Conductivity, photoconductivity and capacitance measurements are carried out to study the effect of these chemical functionalizations on the electrical properties of the monolayers. We demonstrate that the low perpendicular conductivity (σ⊥≈10−15 S/cm) is not altered by the end-group functionalities while high anisotropic conductivities (σ///σ⊥ ≈ 107-108) are obtained at a monolayer level. Interfacial (Si/SAM and SAM/metal) energy barriers are investigated as a function of the end-group functionalities. We also show that conducting monomers substituted by alkyl chains (3,6- dibromocarbazole-N-octyltrimethoxysilane), directly grafted to the substrate, exhibit similar behavior.