Influence of Amorphous-to-Crystalline Transformation on the Negative Thermo-Optic Properties of TiO2 Films

In this study, we systematically investigate the structure–property relationship of atomic-layer-deposited titanium dioxide (TiO2) to identify governing factors that determines the negative thermo-optic property of TiO2. In addition, we demonstrate the negatively enhanced thermo-optic coefficient (TOC) values (from −1.8 to −2.3 × 10–4 /°C) from the visible to the near-infrared regime via mild-thermal budget processing. Structural characterizations based on X-ray diffraction and reflectivity and temperature-dependent refractive index measurements based on spectroscopic ellipsometry prove that the higher crystallinity of anatase TiO2 leads to greater negative TOC values, ascribed to the higher film density and lower porosity compared to amorphous states. Based on the Prod'homme model and bandgap analysis, we also report that the negative TOC of TiO2 is influenced more by the volume expansion than the polarizability.