Monolayer-Controlled Deposition of Silicon Oxide Films on Gold, Silicon, and Mica Substrates by Room-Temperature Adsorption and Oxidation of Alkylsiloxane Monolayers

Abstract

Ultrathin SiO2 films with thicknesses between 0.3 and 8 nm were grown on native silicon (Si/SiO2), muscovite mica and polycrystalline gold substrates via repeated application of a binary reaction sequence, which involved the formation of a self-assembled alkylsiloxane monolayer (step A) and UV−ozone oxidation of the hydrocarbon groups (step B). Using octadecyltrichlorosilane as a precursor, SiO2 films could be grown in a strictly linear, layer-by-layer mode on each of the three substrates with a growth rate of 3.0 ± 0.3 Å per deposition cycle, which corresponds to a monolayer of SiO2. The properties of these oxide films (composition, structure, packing density) were found to be essentially identical and independent of the substrate, as evidenced by ellipsometry, infrared reflection, and X-ray photoelectron spectroscopy. Furthermore, the quality of the oxide layers was investigated as a function of the hydrocarbon chain length of the alkylsiloxane monolayer formed in step A, using four different alkyltrichlorosilanes, R−SiCl3 (R = C18H37, C11H23, C4H9, CH3), as precursors. For each compound, a linear increase of the SiO2 film thickness with the number of applied deposition cycles was again observed, but the growth rate increased noticeably from 2.8 Å/cycle for the C18 and the C11 compound to 3.2 Å/cycle and 6.5 Å/cycle for the C4 and the C1 compound, respectively, concomitant with an increase of the surface roughness in atomic force microscopy images of the oxide films. The packing density of the Si atoms in these films remains essentially constant for longer-chain precursors (R ≥ C4), although the structure of the hydrocarbon layer changes drastically from a highly-ordered, perpendicular alignment on the surface (R = C18) to a random, isotropic arrangement (R ≤ C11). For films prepared from shorter precursors (R < C4), however, multilayer formation sets in and results in film growth rates clearly beyond the monolayer level. A minimum chain length of about four C atoms is therefore required to restrict the alkylsiloxane film formation (step A) to the monolayer level and to provide reproducible and precise control of the oxide film thickness in this deposition process.