Monolayer-controlled deposition of silicon oxide films on gold, silicon, and mica substrates by room-temperature adsorption and oxidation of alkylsiloxane monolayers

Ultrathin SiO2 films with thicknesses between 0.3 and 8 nm were grown on na tive 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 octadecyltrichlorosila ne 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 Angstrom per deposition cycle, which corresponds to a monolayer of SiO2. The properties of these oxide films (composition, structure, packing densi ty) were found to be essentially identical and independent of the substrate , as evidenced by ellipsometry, infrared reflection, and X-ray photoelectro n spectroscopy. Furthermore, the quality of the oxide layers was investigat ed as a function of the hydrocarbon chain length of the alkylsiloxane monol ayer 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 Angstrom/cycle for the C-18 and the C-11 compound to 3.2 Angstrom/cycle a nd 6.5 Angstrom/cycle for the C-4 and the C-1 compound, respectively, conco mitant 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 fi lms remains essentially constant for longer-chain precursors (R greater tha n or equal to C-4), although the structure of the hydrocarbon layer changes drastically from a highly-ordered, perpendicular alignment on the surface (R = C-18) to a random, isotropic arrangement (R less than or equal to C-11 ). For films prepared from shorter precursors (R < C-4), however, multilaye r formation sets in and results in film growth rates clearly beyond the mon olayer level. A minimum chain length of about four C atoms is therefore req uired to restrict the alkylsiloxane film formation (step A) to the monolaye r level and to provide reproducible and precise control of the oxide film t hickness in this deposition process.