Boron Emitter Passivation With Al$_{\bf 2}$O $_{\bf 3}$ and Al$_{\bf 2}$O$_{\bf 3}$/SiN$_{\bm x}$ Stacks Using ALD Al$_{\bf 2}$O $_{\bf 3}$

Thin layers of Al ₂ O ₃ are known to feature excellent passivation properties on highly boron-doped silicon surfaces. In this paper, we present a detailed study of the passivation quality of Al ₂ O ₃ single layers and stacks of Al ₂ O ₃ and antireflection SiN _x on boron-doped emitters, where the Al ₂ O ₃ was deposited by plasma-assisted atomic layer deposition and the SiN _x by plasma-enhanced chemical vapor deposition. The passivation quality was studied for different atomic layer deposition temperatures, as a function of the Al ₂ O ₃ layer thickness, as well as on samples with planar and random pyramids textured surfaces. These investigations were performed for different boron emitter diffusions, such as shallow, industrial emitters with high surface concentrations, as well as driven-in emitters with low surface concentrations. For all these variations, we compared systematically different thermal post-deposition treatments to activate the Al ₂ O ₃ passivation, i.e., annealing processes at moderate temperatures and short high-temperature processes, as required for firing printed metal contacts. Therefore, symmetrically processed p ⁺ np ⁺ samples were fabricated, which were characterized with the photoconductance decay technique to determine emitter saturation current densities. Finally, the longtime stability of the Al ₂ O ₃ /SiN _x stacks with planar and textured surfaces was investigated with an accelerated ultraviolet (UV) exposure experiment, miming about 34 month of outdoor performance.