Al2O3 thin films with thickness between 2 and 100 nm were synthetized at 250 degrees C by thermal atomic layer deposition on silicon substrates. Characterizations of as-deposited andannealed layers were carried out using ellipsometry, X-ray reflectivity, and X-ray photoelectron spectroscopy. A silicon-rich SiOx layer at the interface between Si and Al2O3 was introduced in the optical models to fit the experimental data. Surface passivation performances of Al2O3 layers deposited on n-type float-zone monocrystalline silicon were investigated as a function of thickness and post-deposition annealing conditions. Surface recombination velocity around 2 cm.s-1 was measured after the activation of the negative charges at the Si/Al2O3 interface under optimized annealing at 400 degrees C for 10 min. The evolution of the interface layer and of the material properties with the thermal treatment was studied. (C) 2016 Elsevier B.V. All rights reserved.