Temperature- and excitation-dependent carrier diffusivity and recombination rate in 4H-SiC

Abstract

Time-resolved optical ‘pump-probe’ techniques were applied to monitor carrier dynamics in CVD-grown 4H-SiC for investigation of carrier diffusivity and recombination rate over a wide temperature (80–800 K) and excitation (1016–1019 cm−3) range. The light-induced transient grating technique provided the dependence of ambipolar diffusion coefficient on nonequilibrium carrier density at various temperatures, attributable to bandgap renormalization at low excitation levels and to plasma degeneracy at high ones. The free carrier absorption technique allowed us to monitor carrier lifetimes at various excitation levels and revealed its essential decrease from microseconds to a few nanoseconds. The latter effect was attributed to phonon-assisted Auger recombination with a temperature-independent, unscreened coefficient, C0 = (5 ± 1) × 10−31 cm6 s−1, which was Coulomb-enhanced at low excitations (up to C300 K ∼ 10−30 cm6 s−1 at 1018 cm3) but gradually decreased with increasing excitation due to screening of the carrier–polar optical phonon interaction. The measurements provided an ambipolar diffusion length of ∼30 µm which was almost temperature independent at low excitations (up to ΔN = 1017 cm−3) but decreased to 1 µm at high excitations, mainly due to the impact of nonlinear recombination.