Lithium Diffusion in Silicon with Respect to Si Solar Cells

Abstract

Improved techniques for the diffusion of lithium in silicon are described. These techniques have been studied in view of their application to the fabrication of lithium-doped silicon solar cells. The basic process selected consists of lithium diffusion, in a gas atmosphere, from a vacuum-evaporated lithium layer. The improvements introduced afford protection of this layer from chemical reaction with the humidity of the laboratory atmosphere prior to diffusion. One way to obtain this protection is to combine the evaporation chamber with the diffusion furnace. Another way consists of converting the freshly evaporated lithium into Li3N by reaction with nitrogen. This delays the hydroxidation in the laboratory atmosphere without reducing the quantity of lithium available for diffusion. Experimental results obtained with both techniques are presented. They concern the doping density and profile, the electrical parameters of uniformly diffused slices, and the alloying effects observed at the surface. The diffusion constants of lithium in silicon have also been verified.