Recombination lifetime modification in bulk, semi-insulating 4H-SiC photoconductive switches

A series of high temperature annealing experiments were performed to characterize the processing parameters that alter the recombination lifetime in high purity, semi-insulating (HPSI) silicon carbide (SiC). All annealed samples were diced from a single 4H-SiC wafer with a measured resistivity of greater than 10⁹ Ω-cm. The samples were annealed for various lengths of time in a PID-controlled high temperature induction furnace at 1810 °C. A 35 GHz microwave photoconductivity decay (MPCD) system was used to measure the transient photoconductivity of the as-grown and processed samples. Through numerical processing of the temporal characteristics of the illuminating laser pulse, the photoconductivity transients were simulated with various recombination lifetimes to fit the experimental MPCD data. The results show that the as-grown material has an average recombination lifetime of 6 ns. However, samples annealed for more than 100 minutes demonstrated recombination lifetimes in excess of 100 ns. The annealing process reduces the concentration of shallow point defects (Z1/Z2) in the bulk material which serve as recombination centers in HPSI 4H-SiC, extending the carrier lifetime. Finally, the impacts of increased recombination lifetime in photoconductive switch operation and performance are presented and discussed.