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On the Control of the Fixed Charge Densities in Al2O3-Based Silicon Surface Passivation Schemes

Simon, Daniel K., Jordan, Paul M., Mikolajick, Thomas, Dirnstorfer, Ingo
ACS Applied Materials & Interfaces 2015 v.7 no.51 pp. 28215-28222
aluminum oxide, photovoltaic cells, silica, silicon
A controlled field-effect passivation by a well-defined density of fixed charges is crucial for modern solar cell surface passivation schemes. Al₂O₃ nanolayers grown by atomic layer deposition contain negative fixed charges. Electrical measurements on slant-etched layers reveal that these charges are located within a 1 nm distance to the interface with the Si substrate. When inserting additional interface layers, the fixed charge density can be continuously adjusted from 3.5 × 10¹² cm–² (negative polarity) to 0.0 and up to 4.0 × 10¹² cm–² (positive polarity). A HfO₂ interface layer of one or more monolayers reduces the negative fixed charges in Al₂O₃ to zero. The role of HfO₂ is described as an inert spacer controlling the distance between Al₂O₃ and the Si substrate. It is suggested that this spacer alters the nonstoichiometric initial Al₂O₃ growth regime, which is responsible for the charge formation. On the basis of this charge-free HfO₂/Al₂O₃ stack, negative or positive fixed charges can be formed by introducing additional thin Al₂O₃ or SiO₂ layers between the Si substrate and this HfO₂/Al₂O₃ capping layer. All stacks provide very good passivation of the silicon surface. The measured effective carrier lifetimes are between 1 and 30 ms. This charge control in Al₂O₃ nanolayers allows the construction of zero-fixed-charge passivation layers as well as layers with tailored fixed charge densities for future solar cell concepts and other field-effect based devices.