Process-Induced Degradation of SiO2 and a-Si:H Passivation Layers for Photovoltaic Applications

Abstract

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 degrees 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 degrees 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.