Lifetime limiting defects in 4H-SiC epitaxial layers: The influence of substrate originated defects

The minority carrier lifetime is a decisive factor to obtain 4H-SiC bipolar devices with a low forward voltage drop at high blocking voltages. The lifetime is directly correlated with the concentration of the so-called Z(1/2) deep level and depends on the epitaxial growth process and post-epi processing like ion implantation, annealing, and thermal oxidation. The substrate has so far been attributed a subordinate role for the minority carrier lifetime. In this work, the influence of substrate quality on the minority carrier lifetime after epitaxial growth and post-epi processing is studied on substrates from different manufacturers. The investigation revealed a significant impact of the substrate contamination on the minority carrier lifetime of epitaxial layers and on the efficiency of lifetime enhancement by thermal oxidation. A deep level named SD2 was found in the samples which acts as an additional Shockley-Read-Hall recombination center. The deep level SD2 was traced back to the substrates themselves showing different levels of incorporation of recombination sites from the substrate into the epitaxial layer during the growth. A comparison of the energy levels and electron capture cross sections of the SD2 deep level with defects caused by a tungsten contamination shows a good agreement.