Fast growth of thin multi-crystalline silicon ribbons by the RST method

The ribbon on sacrificial template (RST) process is a ribbon direct-wafering technology with specific ability for high throughput and thin multicrystalline wafer production, in the range of 60-140m. Mechanical and electrical properties of the RST material were investigated. Ball on ring and four-point bending tests showed good fracture stress values up to 260MPa. The conversion efficiency potential for passivated emitter and rear cells (PERC) made out from the RST material, around 16%, is shown to be limited by defects reducing minority carrier lifetime. The interaction between impurities, such as C and transitions metals, with structural defects such as dislocations, results in highly recombinative areas in RST wafers. A model is proposed which shows that the carbon substrate is an important source of carbon contamination in the silicon melt during the growth of the ribbon. This high C contamination can be accompanied by transition metal contamination and can have an influence on the growth stability and on the generation of structural defects, especially if C accumulates in a boundary layer just above the growth interface. The study of the segregation of Sb indicates that the process conditions are close to the case of the diffusive regime near the solid/liquid interface, with a boundary layer thickness of about 70 mu m.