Probing the Interfacial Chemistry of Ultrathin ALD-Grown TiO2 Films: An In-Line XPS Study

Ultrathin TiO2 films received renewed attention in the field of photoelectrochemical water splitting as corrosion protection layers for unstable, small-bandgap semiconductors. Because nucleation on the substrate can differ from steady-state growth of the film itself, it is important to understand the nucleation behavior on a specific surface. In this work, we studied the nucleation mechanism of atomic layer deposition-grown TiO2 from TiCl4 and H2O on as-received silicon by means of in-line X-ray photoelectron spectroscopy. Within a region of ∼0.4 nm of the SiO2/TiO2 interface, the presence of Ti3+ states are detected. In this region, the Ti, O, and Cl species are found to be more strongly bonded. At the initial stages of film growth, prolonged TiCl4 exposure is necessary to reach a saturated surface chemistry, which is in contrast to the outcome of growth per cycle saturation curve analysis. A prolonged water exposure experiment suggests that residual chlorine impurities can be prevented by using a sufficiently long water dose. This is particularly interesting for photoelectrode systems that cannot tolerate high temperatures. When this restriction does not apply, a postdeposition anneal at 400 °C in vacuum is a well-known option to reduce the chlorine content from the surface and the bulk of up to 10 nm thick films without affecting the stoichiometry. These insights will facilitate the optimization of the electronic properties and the materials design of efficient ultrathin protection layers for photoelectrodes for photoelectrochemical water splitting applications.