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

The effective minority carrier lifetime in semiconductors with recombi nation inhomogenities depends on the spatial distribution of recombina tion 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 surfac e. 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 th e size-i.e., the scaling-of the metallization pattern at fixed metalli zation area ratio. This so-called scaling effect occurs whenever the s ize and the interdistance of recombination sites are comparable to the minority carrier diffusion length. Our experiments are in good agreem ent with results from a new analytical three-dimensional simulation wh ich is based on the solution of the electronic transport equations in Fourier space. Our model is applicable to the optimization of the back side ohmic contact pattern for high efficiency solar cells.