Electrostatic properties of silane monolayers in an electrolytic environment

Self-assembled silane molecules are commonly used in a wide variety of electrolyte-insulator-silicon fieldeffect transistors. We combine capacitance-voltage measurements with equivalent-circuit modeling to study such a structure, where the main steps of the self-assembly process, namely surface solvent-cleaning, surface activation, and self-assembly, were sequentially characterized. It is found that UV radiation (254 nm) used in the cleaning and hydroxylation induces interface states between the Si and SiO₂ insulator with a concentration around 1013 cm^-2. When the UV radiation is blocked, the interface states concentration is reduced by 1 order of magnitude. Subsequent self-assembly of polar monolayers of 3-aminopropyltrimethoxysilane and 11-aminoundecyltriethoxysilane do not induce any change in the flat-band of the Si when in contact with an electrolyte, in contrast with the same monolayers measured under dry conditions. This is attributed to electrostatic screening by the electrolyte.