GROWTH OF SILICON THIN-LAYERS ON CAST MG-SI FROM METAL SOLUTIONS FOR SOLAR-CELLS

In pursuit of device-quality layer formation on cast, metallurgical-gr ade silicon (MG-Si) substrates for solar cells, the growth kinetics of silicon liquid phase epitaxy (LPE) from metal solutions was studied. We found an ideal solvent system, Cu-Al, for growth of Si layers with thicknesses of tens of microns on cast MG-Si substrates by LPE at temp eratures near 900 degrees C. This solvent system utilizes Al to ensure good wetting between the solution and substrate by removing silicon n ative oxides, and employs Cu to control Al doping into the layers. Iso tropic growth is achieved because of a high concentration of solute si licon in the solution and the resulting microscopically rough interfac e. As a result, macroscopically smooth Si layers have been grown on ca st MG-Si that are suitable for device fabrication. With the microscopi cally rough interface, the growth rate has been studied with a diffusi onal model involving a boundary layer that takes the melt convection i nto account. The model was found to be in good agreement with experime ntal results, indicating only a small boundary layer (similar to 0.1 c m) and a silicon diffusivity of similar to 2 x 10(-4) cm(2) s(-1) in t he liquid. The thin layer (similar to 30 mu m) grown on the MG-Si subs trate has a minority-carrier diffusion length greater than the layer t hickness.