Molecular Layer Deposition of Thiol−Ene Multilayers on Semiconductor Surfaces

The fabrication of organic thin films with controlled chemical structure in the vertical direction (parallel to surface normal) is important for many practical and technological applications in organic electronics, chemical-resistant films, and biocompatible materials, among others. In order to achieve composition control in the z-direction, molecular layer deposition (MLD: covalent layer-by-layer assembly) of thin, organic films on silicon, silicon oxide, and germanium surfaces was carried out, using the well-established UV-induced thiol-ene reaction. Through successive contact of an interface with dithiol and diene molecules under UV irradiation for short periods (approximately 30 min, room temperature), well-defined thin films can be obtained. Linear increases in film thickness with respect to layer number were obtained for shorter aliphatic dienes and dithiols (C < or = 8), but with longer molecules and with aromatic substrates a self-limiting situation sets in whereby both ends of the molecule react with the surface, arresting film growth. The functionalized interfaces were characterized by ellipsometry, X-ray photoelectronic spectroscopy, and atomic force microscopy.