Effects of annealing on carrier lifetime in 4H-SiC

We present results of a thermal anneal process that increases the minority carrier lifetime in SiC substrates to in excess of $3\mu \mathrm{s}$ , compared to the starting as-grown substrates with lifetimes typically in the $<10\mathrm{ns}$ range. Measurement of lifetimes was conducted using microwave-photoconductive decay. Electron beam induced current measurements exhibited minority carrier diffusion lengths of up to $65\mu \mathrm{m}$ , confirming the enhanced carrier lifetime of the annealed substrate material. Additionally, positron annihilation spectroscopy and deep level transient spectroscopic (DLTS) analysis of samples subjected to this anneal process indicated that a significant reduction of deep level defects, particularly $Z_1∕Z_2$ , may account for the significantly enhanced lifetimes. The enhanced lifetime is coincident with a transformation of the original as-grown crystal into a strained or disordered lattice configuration as a result of the high temperature anneal process. The operational performance of $p\text{-}i\text{-}n$ diodes employing drift layers fabricated from the annealed high-lifetime substrates confirmed conductivity modulation in the diodes consistent with ambipolar carrier lifetimes in the microsecond range.