A diffraction-based technique for determination of interband absorption coefficients in bulk 3C-, 4H- and 6H-SiC crystals

Abstract

Knowledge of absorption coefficient values for wavelengths above the bandgap and the injected carrier density profile is an important issue for analysis of carrier dynamics in highly excited semiconductors, e.g. for evaluation of the carrier density in photoexcited layer, density-dependent recombination rate and diffusivity. In this work we present a novel way for determining the interband absorption coefficient α for SiC crystals in a wide temperature range. The proposed method is based on recording of a transient free carrier grating in a bulk semiconductor by strongly absorbed light and measurements of probe beam diffraction efficiencies on the grating for the Bragg and symmetric anti-Bragg directions. The method was applied for 3C-, 6H-, 4H-SiC polytypes at 351 nm wavelength and revealed 3 to 10-fold increase in the interband absorption coefficients in the 80–800 K temperature range. Increase in absorption coefficients with temperature was simulated by bandgap shrinkage and increase in phonon density. A good agreement of the determined α values with a priori known room-temperature data verified validation of this technique.