Numerical Simulations of Transient Photoconductance Decay

Transient photoconductance decay (PCD) in ingots or wafers has been numerically simulated by a finite-element method (FEM). We examined two types of light sources for carrier injection in transient PCD measurements. Large discrepancies between the widely used asymptotic approximation and the FEM simulations of wafers were seen, and an empirical fitting of the FEM results suggests that the effect of surface recombination velocity on wafer lifetime may be much smaller, although the diffusion-limited surface lifetime remains the same. A single-exponential decay representing overall quality of a multicrystalline wafer is obtained even though the wafer comprises of grains with different lifetimes. If the grain sizes are much smaller than the carrier diffusion length, and if no surface or grain boundary recombination is present, then the inverse effective lifetime is found to be the volume-weighted sum of the inverse local lifetimes.