Low-Temperature Photochemical Conversion of Organometallic Precursor Layers to Titanium(IV) Oxide Thin Films

Abstract

Thin layers of titanium(IV) ethoxide [Ti(OEt)4] as a metal–organic precursor were spin-coated onto silicon wafers under inert conditions and subsequently photochemically converted to thin titanium(IV) oxide (TiOx) films employing vacuum ultraviolet (VUV) radiation from a xenon excimer lamp. The photochemical conversion was performed below 35 °C and at ambient pressure in a nitrogen atmosphere with an optimized content of oxygen. Ti(OEt)4 decomposition and its kinetics were monitored and analyzed by gas chromatography and infrared spectroscopy. Precursor layers with a thickness between 270 and 1060 nm could be converted into much thinner TiOx films (40–165 nm). The decrease in thin film thickness was found to coincide with the removal of organic side chains and densification to a compact oxide network. For precursor layers with a thickness of up to 550 nm, VUV irradiation with a moderate radiant exposure (He) of 2.3 J cm–2 led to almost carbon-free amorphous layers with a composition close to stoichiometric titanium dioxide (TiO2) having a density of ∼2.95 g cm–3 determined by X-ray photoelectron spectroscopy and X-ray reflectometry, respectively. In turn, crack-free thin films exhibiting high UV–visible transparency and smooth surface topography were obtained. The highlighted example of Ti(OEt)4 shows that photochemically initiated decomposition of a metal alkoxide is a powerful approach for the generation of thin metal oxide layers at normal pressure and near ambient temperatures.