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

Abstract

Titanium dioxide (TiO2) has long been hailed in various optoelectronic applications, including metaphotonics and photovoltaics, on account of its high refractive index, low loss tangent, and excellent transparency at a broad range of wavelengths. In particular, due to the negative thermo-optic properties of TiO2, it has been also employed for the fabrication of temperature-insensitive optical devices (called, athermal devices) that can mitigate the optical-performance instability from temperature variations. When the temperature rises, the negative thermo-optic coefficient (TOC, dn/dT) of TiO2 can compensate the unfavorable increase in refractive index exhibited by widely used optical materials such as Si, Ge, and III-Vs, which have a positive TOC. To realize the TiO2-based athermal devices, it is important to establish strategies to engineer and enhance negative TOC of TiO2 with fundamental understandings of processing-structure-property relationships. In this study, we systematically investigate the structure-property relationship of atomic-layer-deposited TiO2 to identify governing factors that determines the negative thermo-optic property of TiO2, and we demonstrate negatively enhanced TOC values up to −2.30 × 10–4 /°C in 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 higher crystallinity of anatase TiO2 leads to greater negative TOC values, ascribed to the higher film density and lower porosity compared to amorphous or less crystalline states. Furthermore, a photoelectron spectroscopy analysis shows an amorphous relaxation process facilitates the overall TiO2 crystallization during annealing by altering the local coordination of Ti. In addition, according to the Prod’homme model and bandgap analysis, we also report that the negative TOC of TiO2 is more influenced by the volume expansion than the polarizability. The findings about 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 TOC of active materials. Figure 1