In this Letter, we report that both thermal atomic layer deposition (ALD) with H2O, and plasma ALD with an O2 plasma, can be used to deposit Al2O3 for a high level of surface passivation of crystalline silicon (c-Si). For 3.5 Ocm n-type c-Si, plasma ALD Al2O3 resulted in ultralow surface recombination velocities of Seff <0.8 cm/s. Thermal ALD Al2O3 also showed an excellent passivation level, with Seff <2.5 cm/s. In contrast to plasma ALD Al2O3, thermal ALD Al2O3 provides some surface passivation in the as-deposited state, although annealing is required to activate it to the full extent. For thermal ALD, the optimal temperature for this anneal was found to be slightly lower, 375 °C, than for plasma ALD Al2O3, 425 °C. The minimal Al2O3 thickness without compromising the passivation properties was 5 nm for plasma ALD Al2O3, whereas for thermal ALD, films >10 nm were required. Thermal stability against a high temperature firing step was demonstrated for ultrathin thermal and plasma ALD Al2O3 films of 5 nm by Seff <9.2 and <6.5 cm/s, respectively.