DYNAMICS OF THE NITROGEN-BOUND EXCITONS IN 6H SIC

We have measured the photoluminescence decay time of the P, R, and S b ound excitons at the neutral nitrogen donors in 6H SiC using picosecon d 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 independen t of the doping level in the sample, and also temperature independent at low temperatures, but decreases when the bound excitons are thermal ly ionized. The decay time related to different donors exhibits a stro ng dependence on the binding energy of the donor level. We suggest tha t the dominating mechanism responsible for the observed decay time is a phononless Auger process. In high-purity samples we have also measur ed the free-exciton decay time at low temperatures to be 12 ns.