Determining thermo-optic coefficients of wide band-gap thin films via thermoelectric effect

Abstract

Index of refraction of a material (n) is significantly affected by heat change. Three thin film samples, TiO₂, AlN and ZnO are subjected to reflectivity measurements at temperatures between 16-70°C operating from 400 to 1100nm with an incident light angle of 90°. From the raw data the thermo- optic coefficients of the three samples (dn/dT) were computed and were found to be -2.9x10^-3 for TiO₂, -1.4x10^-3 for AlN and 9.0x10^-4 for ZnO. Both TiO₂ and AlN thin films have negative values for their thermo-optic coefficients, which are in good agreement to the positive values of their coefficient of thermal expansion. ZnO has a positive thermo-optic coefficient, which could be attributed by the material’s polarisability.