High-temperature stability of c-Si surface passivation by thick PECVD Al2O3 with and without hydrogenated capping layers

We are studying the thermal stability of thick hydrogenated amorphous aluminum oxide (Al 2O 3) layers (20-50 nm) prepared by a high-throughput plasma-enhanced chemical-vapor-deposition (PECVD) technique for the electrical passivation of crystalline silicon surfaces. These passivation layers can be applied alone or covered by a capping layer like amorphous hydrogenated silicon nitride (SiN x) or amorphous hydrogenated silicon oxide (SiO x), also prepared by PECVD. After firing at 870 °C for approximately 3 s, the layers show blistering for Al 2O 3 of 30 nm or higher, independently from the capping layer. For thinner Al 2O 3, no blistering can be observed even using scanning electron microscope (SEM). Very long carrier lifetimes up to 900 s was obtained in passivated p-Si (1 cm) wafer after annealing and firing, without observing a strong influence of the layer thickness and the capping layer. All the layer stacks, including the stacks with SiN x capping layer, show high negative charge densities in the layer (1-4 × 10 12 cm -2). Additionally, low interface defect densities (10 11 cm -2 eV -1), which could be achieved with and without a hydrogenated capping layer, were measured even after firing. To explain these phenomena, hydrogen concentration depth profiles were measured by nuclear reaction analysis. These measurements have shown that, at the Al 2O 3-Si interface, hydrogen atomic concentration ranging 5-7% after annealing and 4% after firing are obtained independently from the capping hydrogen concentration. We conclude that PECVD Al 2O 3 layers of 20 nm or thicker can provide enough hydrogen to passivate the interface defects, even after a high temperature step. However, the layer thickness should be limited to 30 nm in order to avoid the blistering.