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 S-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 (similar to10(14) cm(-3)) to highly doped (similar to10(19) cm(-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 similar to10(-7) A cm(-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(-4) A cm(-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. (C) 2004 American Institute of Physics.