Lifetime-limiting defects in monocrystalline Silicon

This thesis discusses studies performed by the author at the Fraunhofer Institute for Solar Energy Systems, ISE in cooperation with the Freiburg Materials Research Center, FMF. The main achievements are: A new measurement technique was developed that allows the investigation of the distribution of interstitial oxygen atoms in silicon wafers. The method is based on photoluminescence imaging and the measurement of resistivity changes upon annealing at 450 °C. This results in a high lateral resolution on full samples with reasonable effort. It thereby opens up pathways to improve the understanding of the distribution of impurities during crystal growth. Furthermore, the method can be applied to very thin wafers without loosing precision. It is therefore feasible for typical sample thicknesses in photovoltaic research, where application of infrared absorption spectroscopy becomes problematic. An extensive literature review of the broad field of studies of the light-induced degradation caused by boron-oxygen defects is given. The review provides an overview over aspects that were subject of vivid discussions in literature. It reduces the pronounced fragmentation of the scientific discourse in the field by identifying established and controversial findings in literature. Detailed experiments to investigate the activation kinetics of boron-oxygen defects were performed on compensated n-type silicon. The results provide unambiguous confirmation of the strong dependence of the activation rates on the concentration of holes during illumination. This influence was demonstrated to apply to both, the fast and the slow activation processes. This finding indicates the involvement of two holes in both defect state transitions. The recombination activity of boron-oxygen defects was investigated independence of sample doping and injection conditions. The experiments provide strong evidence that boron-oxygen defects introduce at least two energetic levels in the silicon band gap that interact during recombination. A light-induced degradation of the charge carrier lifetime in p-type float-zone silicon was observed at elevated temperature and studied in detail. The investigations indicate that the effect arises from bulk defects involving hydrogen introduced from dielectric surface passivation layers. Several similarities to a light-induced degradation at elevated temperature in multicrystalline silicon are observed. A long-term stability test of several passivation schemes involving aluminium oxide was performed. The experiment demonstrates that the good passivation quality of such layers is stable for illumination durations above 1000 hours at elevated temperature.