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 Al2O3/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.