The effect of a CO growth ambient on the defect structure of EFG silicon

Edge-defined film-fed growth (EFG) Si grown with carbon monoxide (CO) added to the Ar ambient during crystal growth yields solar cells with higher efficiencies than when grown without CO. This increase in cell efficiency is not fully understood. Surface photovoltage (SPV), deep level transient spectroscopy (DLTS), and Fourier transform infrared (FTIR) spectroscopy were used to determine the minority carrier diffusion lengths, impurity distributions, and defect structures in uncontaminated, Cr contaminated, and V contaminated EFG material grown with and without CO added to the Ar ambient. The authors find, from SPV, that CO ambient material consistently has a higher average minority carrier diffusion length than non-CO ambient material, and that the CO ambient material has a surface region with lower minority carrier diffusion lengths than those found in the bulk. DLTS shows “nontraditional”, and large wide band DLTS signals at the surface of CO ambient material that are not present in non-CO ambient material. FTIR spectroscopy is used to study carbon based “SiC-like” complexes near the surface of the CO grown material. It is concluded that these complexes act as gettering sites during crystal growth which results in lower bulk impurity levels and higher solar cell efficiencies