Development and Characterization of N₂O-Plasma Oxide Layers for High-Temperature p-Type Passivating Contacts in...

Full-area passivating contacts based on SiO/poly-Si stacks are key for the new generation of industrial silicon solar cells substituting the passivated emitter and rear cell (PERC) technology. Demonstrating a potential efficiency increase of 1 to 2% compared to PERC, the utilization of n-type wafers with an n-type contact at the back and a p-type diffused boron emitter has become the industry standard in 2024. In this work, variations of this technology are explored, considering p-type passivating contacts on p-type Si wafers formed via a rapid thermal processing (RTP) step. These contacts could be useful in conjunction with n-type contacts for realizing solar cells with passivating contacts on both sides. Here, a particular focus is set on investigating the influence of the applied thermal treatment on the interfacial silicon oxide (SiO) layer. Thin SiO layers formed via ultraviolet (UV)-O₃ exposure are compared with layers obtained through a plasma treatment with nitrous oxide (N₂O). This process is performed in the same plasma enhanced chemical vapor deposition (PECVD) chamber used to grow the Si-based passivating layer, resulting in a streamlined process flow. For both oxide types, the influence of the RTP thermal budget on passivation quality and contact resistivity is investigated. Whereas the UV-O₃ oxide shows a pronounced degradation when using high thermal budget annealing (T > 860 °C), the N₂O-plasma oxide exhibits instead an excellent passivation quality under these conditions. Simultaneously, the contact resistivity achieved with the N₂O-plasma oxide layer is comparable to that yielded by UV-O₃-grown oxides. To unravel the mechanisms behind the improved performance obtained with the N₂O-plasma oxide at high thermal budget, characterization by high-resolution (scanning) transmission electron microscopy (HR-(S)TEM), X-ray reflectometry (XRR) and X-ray photoelectron spectroscopy (XPS) is conducted on layer stacks featuring both N₂O and UV-O₃ oxides after RTP. A breakup of the UV-O₃ oxide at high thermal budget is observed, whereas the N₂O oxide is found to maintain its structural integrity along the interface. Furthermore, chemical analysis reveals that the N₂O oxide is richer in oxygen and contains a higher amount of nitrogen compared to the UV-O₃ oxide. These distinguishing characteristics can be directly linked to the enhanced stability exhibited by the N₂O oxide under higher annealing temperatures and extended dwell times.