Minority carrier lifetime imaging of silicon wafers calibrated by quasi-steady-state photoluminescence

Abstract

Reliable process control or predictions of solar cell efficiencies from minority carrier lifetimes on silicon wafers require precise lifetime measurements. For inhomogeneous material quality this implies the necessity of adequately averaged spatially resolved lifetime measurements. Materials such as, e.g. multicrystalline upgraded metallurgical grade silicon frequently feature relatively low lifetimes, high trap densities, and several material parameters (charge carrier mobilities and net dopant concentration) that are not straightforwardly predictable or measurable. As this may substantially compromise conventional lifetime measurements, we present a solely luminescence based lifetime imaging technique, which requires virtually no a priori information about material parameters. Our approach is based on a calibration of a wafer's photoluminescence image through a precise lifetime determination of a part of this wafer via quasi-steady-state photoluminescence. Carrier mobilities, net dopant concentration, and surface morphology leave the determination of lifetime virtually unaffected, the injection dependence of lifetime is properly taken into account, and lifetimes down to the timescale of a microsecond can be reliably measured.