Lifetime investigations of degradation effects in processed multicrystalline silicon wafers

The first part of this work shows that, in commercial systems used for lifetime mapping, some electronic defects present in multicrystalline silicon wafers can lead to an apparent improvement of the effective lifetime {tau}{sub eff} after in-situ light soaking. By performing quantitative analyses of the decay curves after light soaking and comparing them with the initial curves, such defects are identified as iron-boron complexes. Their recombination activity is weakly injection dependent when Fe and B are paired and strongly injection dependent after light soaking, when Fe is at interstitial sites. In the last case, the laser pulse used to probe the sample creates injection conditions which are higher than those achieved with a 100 mW cm{sup -2} bias light on the samples and therefore the {tau}{sub eff} values calculated by the lifetime scanner have to be considered with care when analysing such wafers. The possibility to dissociate FeB pairs in the lifetime scanner using high intensity laser pulses permits, however, an extremely fast control of the presence of such contaminants. The second part shows how a phosphorus diffusion step at the appropriate temperature can lead to a dramatic decrease of the iron concentration contained in the lowermost part of block-cast multicrystalline ingots. A final Fe concentration [Fe] of 2 x 10{sup 12} cm{sup -3} is found for a diffusion at 950 C, but after diffusion at 830 C, an upper limit of {proportional_to} 10{sup 11} cm{sup -3} for the residual Fe is estimated, indicating that the material responds positively to phosphorus gettering. (orig.)