Low-temperature processing and control of structure and properties of TiO₂/c-sapphire epitaxial heterostructures

We have investigated the formation of the rutile and the anatase polymorphs of TiO2, with emphasis on epitaxial growth characteristics, and defect content as a function of laser and substrate variables. X-ray diffraction (XRD) studies revealed that the rutile phase is more stable at higher substrate temperatures and lower oxygen pressures; in contrast, decreasing the temperature and increasing the oxygen pressure gave rise to formation of anatase. Epitaxial rutile films with a <100> orientation were obtained at 450 °C using the pressure of 5 × 10-4Torr, and laser energy of 3.5-4.0 J/cm2. The epitaxial relationship, determined by 2θ-θ and Φ scan of XRD and confirmed by transmission electron microscopy (TEM) diffraction patterns, was shown to be rutile(100)‖sapphire(0001), rutile[001]‖sapphire[1010] and rutile[010]‖sapphire[1210]. An atomically sharp interface between the rutile epitaxial film and the sapphire substrate was observed in the scanning transmission electron microscopy (STEM) images. The films exhibited a transmittance of 75-90% over the visible region. The absorption edge was observed to shift toward longer wave lengths at higher deposition temperatures or lower pressures. X-ray photoelectron spectroscopy and photoluminescence results showed that concentration of lattice point defects, namely oxygen vacancies and titanium interstitials, increased at lower oxygen pressures. Formation of nanostructured films with a surface roughness of -1.5-13.1 nm was confirmed by atomic force microscopy investigations.