Silicon defect and impurity studies using float-zone crystal growth as a tool

The float-zone (FZ) method for silicon crystal growth produces very high purities and low defect levels because no crucibles or other heated components are used, and the melt is only in contact with solid silicon. FZ growth is thus a useful tool for the study of defect and impurity effects on material properties such as minority charge carrier lifetime τ or photovoltaic efficiency η using controlled samples in which specific defects or impurities are deliberately introduced into the high-purity, low-defect baseline material, as desired. We have used this tool for defect studies on grain size, dislocations, silicon self interstitial clusters, and fast-cooling defects, as well as for impurity studies on H, N, Fe, p-type dopants, and interactions between Fe and Ga. Float zoning was conducted in vacuum or pure argon using induction heating at 2 MHz. The ASTM F28-91 photoconductive decay method was used to measure τ after growth. Arrays of isolated diagnostic solar cell devices were used to characterize defect and impurity effects on photovoltaic parameters.