Gettering of transition metal impurities during phosphorus emitter diffusion in multicrystalline silicon solar cell processing

We have investigated the gettering of transition metals in multicrystalline silicon wafers during a phosphorus emitter diffusion for solar cell processing. The results show that mainly regions of high initial recombination lifetime exhibit a significant lifetime enhancement upon phosphorus diffusion gettering. Nevertheless, transition metal profiles extracted by secondary ion mass spectrometry in a region of low initial lifetime reveal significant gradients in Cr, Fe, and Cu concentrations towards the surface after the emitter diffusion, without exhibiting a significant enhancement in the lifetime. In a region of higher initial lifetime, however, diminutive concentration gradients of the transition metal impurities are revealed, indicating a significantly lower initial concentration in these regions. From spatial maps of the dislocation density in the wafers, we find that lifetime enhancements mainly occur in regions of low dislocation density. Thus, it is believed that a generally higher concentration of transition metals combined with an impurity decoration of dislocations in regions of high dislocation density limit the initial lifetime and the lifetime after the phosphorus diffusion, in spite of the notable gettering of transition metal impurities towards the surface in these regions. Furthermore, after a hydrogen release from overlying silicon nitride layers, we observe that only regions of low dislocation density experience a significant lifetime enhancement. This is attributed to impurity decoration of the dislocations in the regions of both high dislocation density and high transition metal impurity concentration, reducing the ability of hydrogen to passivate dislocations in these regions. (C) 2006 American Institute of Physics.