Process-Induced Degradation of SiO and a-Si:H Passivation Layers for Photovoltaic Applications

The passivation characteristics of thermally grown silicon dioxide (SiO₂) and hydrogenated amorphous silicon (a-Si:H) layers are investigated, using a combination of photoluminescence and capacitance-voltage analysis techniques. Key findings are the significant passivation degradation of SiO₂ and a-Si:H layers induced by metallization through electron beam evaporation. The degradation correlates with an increase in silicon dangling bond defect density at the interface with silicon (for both SiO₂ and a-Si:H) or in the passivation layer (a-Si:H). Performing the metallization by thermal evaporation is an effective method to avoid such process-induced damage, as is forming gas annealing at 450°C, which effectively recovers the interface characteristics of SiO₂ layers. Deposition of amorphous silicon on a thermal SiO₂ layer induces bulk and interface defects in the SiO₂ layer-but in this case, a 450°C forming gas anneal is not possible due to the thermal budget limitations of a-Si:H, thereby posing problems for solar cell structures which rely on a combination of PECVD a-Si:H and thermal SiO₂ passivation layers.