PO_x/Al₂O₃ stacks for surface passivation of Si and InP

Passivation of semiconductor surfaces is crucial to reduce carrier recombination losses and thereby enhance the device performance of solar cells and other semiconductor devices. Thin-film stacks of phosphorus oxide (PO_x) and aluminum oxide (Al₂O₃) have recently been shown to provide excellent passivation of semiconductor surfaces, including crystalline silicon and indium phosphide, and can also be highly interesting for passivation of other semiconductor materials such as Ge and III-V semiconductors. On silicon, the excellent passivation is attributed to the combination of a high positive fixed charge and a very low interface defect density. On InP nanowires, application of the PO_x/Al₂O₃ stacks improves charge carrier lifetime threefold as compared to unpassivated nanowires. In this work, we review and summarize recent results obtained on PO_x/Al₂O₃ stacks for semiconductor surface passivation. Several topics are discussed, including the passivation performance on various semiconductor surfaces, the processing of the PO_x and Al₂O₃ layers, the role of the capping layer, and aspects related to device integration. The PO_x/Al₂O₃ stacks feature some unique properties, including an unusually high positive fixed charge density, a low interface defect density, and can be prepared over a wide deposition temperature range. These unique properties arise in part from the mixing process that occurs between the PO_x and Al₂O₃ layers, which upon post-deposition annealing leads to the formation of AlPO₄. The surface passivation provided by PO_x/Al₂O₃ stacks is highly stable and the stack can be used to conformally coat high-aspect-ratio structures such as nanowires, showing their promise for use in semiconductor devices.