Impacts of recombination at the surface and in the substrate on carrier lifetimes of n-type 4H-SiC epilayers

After remarkable reduction in the Z(1/2) center in n-type 4H-SiC epilayers, the measured carrier lifetimes can be severely affected by other recombination paths. Impacts of carrier recombination at the surface as well as in the substrate are investigated in detail by using numerical simulation based on a diffusion equation. The simulation reveals that a very thick (>100 mu m) epilayer is required for accurate measurement of carrier lifetimes if the bulk lifetime in the epilayer is longer than several microsecond, due to the extremely short lifetimes in the substrate. The fast decay often observed at the initial stage of decay curves can be explained by fast recombination at the surface and in the substrate. In experiments, the carrier lifetime is improved from 0.69 to 9.5 mu s by reducing the Z(1/2) center via two-step thermal treatment (thermal oxidation and Ar annealing) for a 148-mu m-thick n-type epilayer. This lifetime must be still, to large extent, affected by the recombination at the surface and in the substrate, and the real bulk lifetime may be much longer. The carrier recombination paths and their impacts on the decay curves are discussed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3498818]