Multielemental analysis of silicon impurities by LIBS: boron case in metallurgical and photovoltaic grade silicon

The principal subject matter of this work is the development of a technique for the multi-elemental analytical characterization of different qualities of solid silicon. The physical process upon which the technique is based is the temporally resolved emission spectra emitted by a micro-plasma generated by a laser focused on the surface of a given sample. A preliminary study was made to choose the various system components. After having determined the optimal environmental parameters for the measurement, and having verified reproducibility, identification of several metallic, non-metallic, and dopant impurities was made. Particular attention was given to boron, an essential dopant for photovoltaic cells. A detection limit of 0.05 ppmm of boron was found in an atmosphere of 85%He/15%Ar and calibration curves was built in the range of 1 to 100 ppmm. LIBS was used on silicon samples issuing from different production techniques (4C and directed solidification) which permitted the segregation of different impurities along the length of a silicon ingot. The LIBS measurements have been validated by comparison with GDMS and ICP, and a correlation has been made between carrier lifetime and the presence of certain impurities. (author)