Al₂O₃/TiO₂ stack layers for effective surface passivation of crystalline silicon

For silicon surface passivation, we investigate stack layers consisting of a thin Al₂O₃ layer and a TiO₂ capping layer deposited by means of thermal atomic layer deposition (ALD). In this work, we studied the influence of different thermal post-deposition treatments and film thickness for the activation of passivating ALD Al₂O₃ single layers and Al₂O₃/TiO₂ stack layers. Our experiments show a substantial improvement of the passivation for the Al ₂O₃/TiO₂ stack layers compared to a thin single Al₂O₃ layer. For the stacks, especially with less than 10 nm Al₂O₃, a TiO₂ capping layer results in a remarkably lower surface recombination. Effective fixed charge density of Al₂O₃/TiO₂ stack layers increases after TiO₂ deposition and O₂ annealing. It is also demonstrated that the enhanced surface passivation can be mainly related to a remarkably low interface defect density of 1.1 × 10¹⁰ eV^-1 cm ^-2, whereas post-TiO₂ heat treatment in O₂ ambience is not beneficial for the passivation of silicon, which is attributed to increasing interface defect density of stack layers.