Over the last decade, we have assisted to the rapid development of industrial applications of microelectronic devices based on new semiconducting materials, such as Silicon Carbide (SiC) and Gallium Nitride (GaN). Power electronic applications, in particular, have taken huge advantages from the adoption of these wide bandgap semiconductors, both of which allow the effective fabrication of high breakdown voltage, low on-resistance, diodes and transistors1,2,3,4, capable moreover of operating at much higher temperatures compared to Silicon (Si) ones5,6. As the deployment of these materials in electronics is destined to grow in the coming years7, it can be expected that this will trigger the development of passive and active photonic devices for communication or sensing purposes, possibly monolithically integrated within the same chip8,9,10. An obvious prerequisite for the development of such devices is the precise knowledge of the optical properties of these materials, in particular their dependence on temperature variations that can be reached during the operating life.
However, although the temperature dependence of the refractive index is essential for understanding the optical potential of novel materials, such as 4H-SiC and GaN, investigations on the thermo-optic coefficient (TOC) are limited. To date, only few studies on these wide-band gap materials can be found in the literature11, based on different experimental methods, as interferometry12, z-scan13, thermal lens14 and light-induced transient thermal grating techniques15, and carried out in various wavelength ranges.
In particular, for the hexagonal (4H) polytype of SiC, although usually regarded among the best materials for high-power electronic and optoelectronic applications16, an accurate value of the TOC and its temperature dependence is still lacking at the common ଁber-optic wavelengths of λ ~ 1.55 µm.
Scajev et al.15 applied a time-resolved four-wave mixing technique for the determination of TOCs in heavily doped n-type and p-type 4H-SiC substrates at room temperature (RT). Spatial modulation of thermal properties was achieved by intraband carrier excitation using a light-interference pattern at λ = 1064 nm and subsequent carrier thermalization. Measurements were carried out in a range of temperature T=10-300 K, with a calculated value dn/dT=3.6×10−5 K−1 at T=300 K.
Watanabe et al.17 investigated the temperature dependence of the refractive indices of 4H-SiC and GaN in a wavelength range from the near band edge (λ = 392 nm for 4H-SiC, λ = 367 nm for GaN) to infrared (λ = 1700 nm) from RT to T=500°C. Optical interference measurements 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 λ = 450 nm, the TOCs of 4H-SiC and GaN were measured to be 7.8×10−5 K−1 and 1.6×10−4 K−1, respectively.
Xu et al.18 measured, by the method of minimum deviation, from T=293 K to 493 K, the temperature-dependent refractive indices of 4H- and 6H-SiC over a spectral range from λ = 404.7 nm to λ = 2325.4 nm. The TOC dispersion formula as a function of wavelength and temperature was derived from the Sellmeier equation. For 4H-SiC, at the wavelength of λ = 450 nm, at T=493 K, they calculated a dn/dT=8.18×10−5 K−1, very close to what reported in Ref.17. At the same temperature, for 6H-SiC, the TOC at the wavelength of λ = 1523 nm is 5.94×10−5 K−1, not far from the value of 5.54×10−5 K−1 reported in Ref.11.
To our knowledge, to date, there is however a lack of studies in the literature highlighting that the TOC for 4H-SiC and GaN, at λ = 1550 nm, is itself temperature dependent, and in a not negligible manner. Therefore, in this letter we report an experimental characterization of this coefficient for both wide bandgap materials, from RT up to T=480 K, about, at said fiber optic communication wavelength. The results should be helpful for the proper design of 4H-SiC or GaN-based passive and active optoelectronic devices, like waveguides, couplers, interferometers, lasers, switches, modulators, etc. An unknown thermo-optic effect may cause, in fact, incorrect functioning of devices where the refractive index dependence scales with wavelength and temperature.