Temperature Dependence of the Absorption Coefficient in 4H- and 6H-Silicon Carbide at 355 nm Laser Pumping Wavelength

We report on the absorption coefficient at 355 nm laser wavelength for 4H- and 6H-SiC over a wide temperature range. The measurements were carried out using a depth- and time-resolved free-carrier-absorption technique. The values of the absorption coefficient for the fundamental E ⊥ ĉ and E ‖ ĉ light polarization were extracted from the instantaneous profiles of the excess carrier concentration. The absorption dependency in 4H epilayers is consistent with a model based on the indirect optical transitions and shift of the energy gap with temperature. A discrepancy in absorption observed in heavily-doped 4H-SiC substrates at elevated temperatures is associated with a doping-induced narrowing of the band gap.