SiC absorption of near-infrared laser radiation at high temperatures

We report on a theoretical and experimental investigation of the temperature-dependent optical absorption of nitrogen-doped 4H-SiC for a temperature range between room temperature and the decomposition point. The theoretical model is based on free carrier absorption including the temperature dependence of the electron mobility. With respect to laser material processing of silicon carbide, the analysis focusses on a near-infrared wavelength range. At room temperature, the calculated absorption is in excellent agreement to transmission and reflection measurements. For the experimental study of the absorption at higher temperatures induced by intense 1070-nm laser irradiation, a two-color pyrometer is employed with the thermal emission of the laser interaction zone being collected coaxial to the impinging laser. Exemplarily, the simulated temperature-dependent absorption is used to determine the heating of a 0.4-mm-thick 4H-SiC specimen during laser irradiation and compared to the experimentally determined temperature. In an initial time domain of the irradiation with an attained temperature below 1350 K, the simulated and measured temperatures are in good agreement. Above 1350 K, however, the measured temperature reveals a sharp and fast increase up to 2100 K which is not predicted by the model. This discrepancy is attributed to a strong additional absorption mechanism caused by carbonization at the surface which is confirmed by EDX analysis.