Analysis of the high-temperature improvement of Cz-silicon
The carrier-induced lifetime degradation observed in boron-doped Czochralski silicon (Cz-Si) has its origin in a metastable defect limiting the bulk lifetime even in absence of other external defects. In a lifetime investigation on standard boron-doped Cz-Si we observed a permanent improvement of the stable bulk lifetime by a factor of 2 to 3 achieved by an optimized high-temperature process at 1050°C. In the present work we demonstrate that an effective reduction of the defect concentration is already possible by an optimized anneal step at 750°C for only 10 min. On the other hand we investigate the extreme sensitivity of Cz-silicon to the process scheme of a high-temperature oxidation: on a boron-doped oxygen-contaminated sample the initial lifetime of 7.7 µs was either increased up to 24.6 µs or decreased down to 0.75 µs depending on the ramping conditions. These results perfectly correspond with solar cell results: varying the ramping conditions of the high-temperature oxidation steps the same solar cell structure (RP-PERC) led to efficiencies of 19.6 % and 12.0 %, respectively. Such a severe lifetime reduction due to an unsuitable high-temperature process was observed in all oxygen-contaminated materials (including gallium-doped Cz-Si) while all oxygen-free materials showed no sensitivity to the process scheme. Our investigation of the interstitial oxygen concentration indicates that the severe lifetime reduction is probably caused by the formation of oxygen precipitates.