Correct Carrier Concentration for Lifetime Measurements with Local Illumination

Spatially resolved measurements of minority carrier lifetime are a valuable tool to monitor the quality of silicon wafers and of single processing steps. Common implementations of this technique generate excess minority carriers by scanning the variable focus of a pulsed light beam over the wafer surface. The resulting carrier distribution is spatially inhomogeneous; therefore minority carriers diffuse out of the illuminated area. However, most existing evaluation models neglect carrier diffusion, and hence overestimate the photogenerated minority carrier concentration by several orders of magnitude. This contribution presents a simulation model to calculate the correct local minority carrier concentration due to carrier diffusion for variable line and spot focus geometries. Carrier density distributions from spatially resolved photoluminescence measurements confirm the results of the model.