1-octadecene monolayers on Si(111) hydrogen-terminated surfaces: Effect of substrate doping

We have studied the electronic properties, in relation to structural properties, of monolayers of 1-octadecene attached on a hydrogen-terminated (111) silicon surface. The molecules are attached using the free-radical reaction between $CC$ and $SiH$ activated by an ultraviolet illumination. We have compared the structural and electrical properties of monolayers formed on silicon substrates of different types (⁠$n$ type and $p$ type) and different doping concentrations, from low-doped $(∼1014cm−3)$ to highly doped $(∼1019cm−3)$⁠. We show that the monolayers on $n$-, $p$-, and $p+$-silicon are densely packed and that they act as very good insulating films at a nanometer thickness with leakage currents as low as $∼10−7Acm−2$ and high-quality capacitance-voltage characteristics. The monolayers formed on $n+$-type silicon are more disordered and therefore exhibit larger leakage current densities $(>10−4Acm−2)$ when embedded in a silicon∕monolayer∕metal junction. The inferior structural and electronic properties obtained with $n+$-type silicon pinpoint the important role of surface potential and of the position of the surface Fermi level during the chemisorption of the organic monolayers.