Room-temperature growth of single-crystalline TiO2 thin films on nanometer-scaled substrates by dc magnetron sputtering deposition

We have investigated single crystallization of titanium oxide (TiO2) thin films deposited at room temperature (RT) on variously sized substrates with or without an additional deposition of an amorphous silicon oxide thin film by using a low-energy particle-beam deposition method. The employed substrates were composed of a number of isolated right-prism-like diamond columns on nanometer scales, which were formed using a self-assembling process of a Pt thin film during a hydrogen-plasma exposure, an oxygen-plasma etching process, and a subsequent microwave-plasma chemical-vapor-deposition (CVD) process of diamond. The relation between the size of each prism-like substrate and the crystallinity of the TiO2 thin films thus grown was studied mainly by means of transmission electron microscope observations. The obtained evidences revealed that reduction of the substrate area to ≈100×100 nm2 led to single-crystallization of the RT-deposited TiO2 films. It was found that single-crystalline rutile TiO2 films grew at RT on flat substrates with areas of ≈2×104 nm2, no matter whether the top surface material of the substrate was crystalline (the diamond facets) or not (the silicon oxide). The mechanism of the single-crystallization observed is discussed.