We have studied the surface passivation of p- and n-type silicon by thermal atomic layer deposited (ALD) Al2O3. The main emphasis is on different ALD reactant combinations and especially on using ozone as an oxidant. Thermal stability of Al2O3 will also be briefly addressed. Our results show that in p-type CZ-Si Al2O3 leads to much higher passivation than thermal oxidation, independent of the reactants. The best minority carrier lifetimes are measured when a combination of Al2O3 and TiO2 is used. In n-type CZ-Si similar results are obtained except the choice of reactants seems to be more crucial. However, the combination of Al2O3 and TiO2 results again in the best passivation with measured lifetimes well above 10 ms corresponding surface recombination velocities of ∼2 cm/s. Finally, we demonstrate that Al2O3 passivation is also applicable in high resistivity n-type FZ-Si and in ∼1 Ωcm p-type multicrystalline Si.