Ultrathin Plasma Oxide for Passivation of Phosphorus-Diffused Silicon Solar Cell Emitters

Different passivation layers on phosphorous diffused emitters have been studied. A comparison between silicon nitride (SiN _x ) single layer at different deposition temperatures, stacks of “outgassing” silicon oxide (that was grown during the moving in/out of the furnace in ambient air during the outgassing process) and an ultrathin SiO _x layer formed by plasma oxidation capped with SiN _x layers have been investigated. Optimization of plasma enhanced chemical vapor deposition (PECVD) SiNx at different deposition temperatures showed good passivation quality depending on the hydrogen content. Plasma oxidation process by a PECVD tool without utilizing a silicon source in the process gas flow is an obvious choice to form a thin SiOx as it allows the formation of a SiO _x /SiN _x stack in a sequence of deposition processes in the same inline PECVD tool. Such an ultrathin SiO _x layer was found to not only improve the surface passivation after a fast firing process but also to maintain the excellent anti-reflection coating (ARC) property of the SiN _x . Low emitter saturation current density J _oe values of 15 fA/cm ² for planar and 24 fA/cm ² for textured surfaces for an emitter with a sheet resistivity of 161 Ω/ sq has been reached after firing at 850 °C, which is comparable with the outgassing oxide. These stacks gave an improvement of the J _oe values with a factor of 6-9 for emitters with low surface concentration comparing to single SiN _x layer.