DIRECT OBSERVATION OF A SCALING EFFECT ON EFFECTIVE MINORITY-CARRIER LIFETIMES

The effective minority carrier lifetime in semiconductors with recombination inhomogenities depends on the spatial distribution of recombination sites and not only on their absolute number. We use time-resolved microwave reflection meaurements to monitor the carrier recombination in silicon wafers with a periodic metallization pattern on one surface. This contact scheme simulates the distribution of sites of enhanced carrier recombination. The experiments reveal a sensitive dependence of the effective minority carrier lifetime on the interdistance and the size-i.e., the scaling-of the metallization pattern at fixed metallization area ratio. This so-called scaling effect occurs whenever the size and the interdistance of recombination sites are comparable to the minority carrier diffusion length. Our experiments are in good agreement with results from a new analytical three-dimensional simulation which is based on the solution of the electronic transport equations in Fourier space. Our model is applicable to the optimization of the backside ohmic contact pattern for high efficiency solar cells.