The determination of high-density carrier plasma parameters in epitaxial layers, semi-insulating and heavily doped crystals of 4H-SiC by a picosecond four-wave mixing technique

We applied a picosecond four-wave mixing technique for measurements of carrier lifetimes and diffusion coefficients in highly excited epitaxial layers, semi-insulating and heavily doped 4H-SiC substrates. Optical carrier injection at two different wavelengths (266 and 355 nm) allowed us to vary the depth of the excited region from 1–2 µm to 50 µm, and thus determine photoelectric parameters of carrier plasma in the density range from 2 × 1016 to 1019 cm−3. Strong dependence of carrier lifetime and mobility on carrier density was found in the epitaxial layers. The origin of fast decay components, not resolved previously by photoluminescence and free-carrier absorption techniques in SiC, was attributed to nonlinear carrier recombination. Numerical modelling provided a value of bimolecular recombination coefficient equal to B = (2–4) × 10−11 cm3 s−1 and verified a surface recombination velocity S = 4 × 104 cm s−1. In heavily doped crystals, nonlinear carrier recombination reduced the carrier lifetime down to 1.1 ns, while in semi-insulating ones a lifetime of 1.5–2.5 ns was measured. Temperature dependences of four-wave mixing provided monopolar carrier mobility in heavily doped and bipolar one in semi-insulating crystals, and revealed the contribution of ionized impurity and phonon scattering mechanisms.