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-grade 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 temperatures near 900 degrees C. This solvent system utilizes Al to ensure good wetting between the solution and substrate by removing silicon native oxides, and employs Cu to control Al doping into the layers. Isotropic growth is achieved because of a high concentration of solute silicon in the solution and the resulting microscopically rough interface. As a result, macroscopically smooth Si layers have been grown on cast MG-Si that are suitable for device fabrication. With the microscopically rough interface, the growth rate has been studied with a diffusional model involving a boundary layer that takes the melt convection into account. The model was found to be in good agreement with experimental results, indicating only a small boundary layer (similar to 0.1 cm) and a silicon diffusivity of similar to 2 x 10(-4) cm(2) s(-1) in the liquid. The thin layer (similar to 30 mu m) grown on the MG-Si substrate has a minority-carrier diffusion length greater than the layer thickness.