Mapping of minority carrier lifetime and mobility in imperfect silicon wafers

Abstract

The microwave phase-shift (μW-PS) technique is used to determine the bulk lifetime ( τ b) of minority carriers. In this contactless technique, the phase-shift between a microwave beam (10 GHz) and a sine-modulated infrared excitation is related to τ b and to the surface recombination velocity ( S). Surfaces are passivated by means of an aqueous iodine solution or by short phosphorus diffusion (850 °C for 20 min). By varying the excitation, the modulation frequency ( S) can be deduced. The technique works at a quasi-constant excitation level, using either a large excitation beam or a focused beam (50 μm) to establish a lifetime scan map. The phosphorus-diffused samples are transformed into n +p diodes and light beam-induced current (LBIC) maps lead to a mapping of minority carrier diffusion length ( L). From the τ b and L maps, we can get a map of the minority carrier diffusion coefficient (or mobility). Comparison of the measured values of τ b in surface-passivated samples leads to an evaluation of S, which is in agreement with that obtained directly by changing the excitation frequency. The technique proposed works for Si wafers containing oxygen precipitates and for multicrystalline Si wafers. Features of extended defect recombination strength, at the defects and far from them, as well as the local variation of S, can be deduced. In conclusion, using in association the lifetime maps obtained by μW-PS and the diffusion length maps obtained by LBIC leads to an enhanced knowledge of the properties of silicon wafer minority carriers.