Effects of Thermally Induced Phase Transition on the Negative Thermo-Optic Properties of Atomic-Layer-Deposited TiO₂ Films

The negative thermo-optic properties of TiO₂ have been considered promising for athermal photonic devices that can mitigate the optical-performance instability due to temperature variations. When temperature increases, its negative thermo-optic coefficient (TOC) can compensate for the unfavorable increase in refractive index exhibited by widely used optical materials such as Si, which have positive TOCs. Herein, the structure-property relationship of TiO₂ is thoroughly investigated to understand the negative thermo-optic behaviors of TiO₂. Through atomic layer deposition and mild thermal annealing, the obtained negative TOC values are as high as −2.30 × 10^-4/°C in the visible to the near-infrared regime. X-ray diffraction/reflectivity and temperature-dependent refractive index measurements identify that the higher crystallinity of anatase TiO₂ leads to greater negative TOC values due to its higher density and lower porosity, compared to those of amorphous or weakly crystalline states. The Prod’homme model and band gap analysis reveal that the effect of volume expansion is more dominant on the enhanced negative TOC of TiO₂, rather than the polarizability. Photoelectron spectroscopy measurements suggest an amorphous relaxation process during annealing that further supports the amorphous-to-crystalline transformation in TiO₂. The findings of the structure and chemical properties governing the negative TOC of TiO₂ for athermal applications may be of significant relevance to many photonic devices showing strong performance instability due to the high positive TOCs.