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Thermo-Optic Coefficients of 4H-SiC, GaN, and AlN for Ultraviolet to Infrared Regions up to 500 °C

The temperature dependence of the refractive indices of 4H-SiC, GaN, and AlN were determined in a wavelength range from the near band edge (392 nm for 4H-SiC, 367 nm for GaN, and 217 nm for AlN) to infrared (1700 nm) and a temperature range from room temperature to 500 °C. Optical interference measurements with vertical incidence along a c-axis configuration were employed to precisely evaluate ordinary refractive indices. Near the band-edge region, the temperature dependence of the refractive index mainly originates from the temperature change of the bandgap. At 450 nm, the thermo-optic coefficients of 4H-SiC, GaN, and AlN were measured to be 7.8 ×10-5, 1.6 ×10-4, and 3.6 ×10-5 K-1, respectively.