Excess Carrier Density Variations in Test Structures for Photoconductance-Based Contact Recombination Current Measurements

We recently proposed a novel test structure for the extraction of recombination characteristics of metal-contacted areas on silicon wafers using quasi-steady state photoconductance measurements. In this test structure, photoconductance measurements are performed on several wafer areas with different contact fractions. Each area consists of a lattice of point contacts on an otherwise passivated wafer. The requirement of constant excess carrier densities throughout the quasi-neutral wafer bulk is absolutely essential for the simple extraction of figures of merit for contact recombination. In this study, we first discuss the requirements for constant injection levels during photoconductance measurements, without reference to a particular geometry. Then, we use a simple 1-D model to elucidate how injection levels vary in the test structure plane. We use the model to investigate limiting cases for which the requirement of constant injection levels is satisfied. In addition, we investigate the breakdown of the assumption of constant injection levels. We discuss the influence of nonconstant excess carrier concentrations on photoconductance measurements in the context of our test structure. Finally, we validate our model by comparison with experiments. Our analysis is particularly useful in the context of understanding the design rules for contact size and pitch in our test structure.