DYNAMICS OF THE NITROGEN-BOUND EXCITONS IN 6H SIC

We have measured the photoluminescence decay time of the P, R, and S bound excitons at the neutral nitrogen donors in 6H SiC using picosecond pulsed excitation. At 2 K the decay times are 8.0, 1.8, and 1.5 ns, respectively, which are significantly faster than previously reported values for shallow donors in other indirect-band-gap materials such as Si or GaP. Each of the observed decay times is found to be independent of the doping level in the sample, and also temperature independent at low temperatures, but decreases when the bound excitons are thermally ionized. The decay time related to different donors exhibits a strong dependence on the binding energy of the donor level. We suggest that the dominating mechanism responsible for the observed decay time is a phononless Auger process. In high-purity samples we have also measured the free-exciton decay time at low temperatures to be 12 ns.