Structural impact of chromium incorporation in as-grown and flash-lamp-annealed sputter deposited titanium oxide films

Abstract

We address the impact of chromium (Cr) incorporation (<15 at.%) in the structure of titanium dioxide (TiO₂:Cr) films for as-grown and after flash-lamp-annealing (FLA) states. Samples were produced by DC magnetron sputtering on either unheated or heated (400 °C) substrates. Complementary medium- and local-order information was extracted by X-ray diffraction and absorption near-edge structure, respectively. TiO₂:Cr grown on unheated substrates are amorphous with the major contribution from Cr³⁺ and progressive formation of Cr⁶⁺ with Cr. On heated substrates, anatase phase is dominant for low Cr levels (≤7 at.%) and the structure evolves with Cr towards a disordered mixed-oxide with rutile structure. By tuning the FLA energy density, customized (single or mixed) phase formation is achieved from (initially amorphous) Cr-free TiO₂. For amorphous TiO₂:Cr with low Cr (≤7 at.%), FLA induces a short-range rutile structure but structural ordering is not observed at higher Cr levels. Nonetheless, FLA annihilates Cr⁶⁺ sites and promotes Cr⁴⁺, which is associated to the mixed-oxide rutile. FLA also improves the pristine structure of anatase TiO₂:Cr grown on heated substrates. These results provide relevant information about the atomic structure of mixed oxides and the use of FLA for the synthesis of band-gap engineered TiO₂-based materials.