Silicon Float-Zone Crystal Growth as a Tool for the Study of Defects and Impurities

Because of its ability to produce silicon crystals of exceptionally high purity and crystallographic perfection, the float-zone method lends itself to use as a tool for the controlled study of deliberately introduced defects and impurities in Si crystals and their effects on materials properties such as minority charge-carrier lifetime or photovoltaic conversion efficiency. Some examples of such studies are presented here. Defects we’ve studied include grain size, dislocations, swirl defects, and fast-cooling defects. Impurity studies have focused on H, N, Fe, and interactions between Fe and Ga. We used the bulk DC photoconductive decay lifetime characterization method and small diagnostic solar cell characterization techniques to assess material quality. The low defect and impurity concentrations obtainable by float zoning allow baseline lifetimes over 20 milliseconds and photovoltaic device efficiencies over 22%, so small effects of impurities and defects can be detected easily.