Differences in Si surface passivation by aluminum oxide (Al2O3) films synthesized using H2O and O-3-based thermal atomic layer deposition (ALD) and plasma ALD have been revealed. A low interface defect density of D-it = similar to 1011 eV(-1) cm(-2) was obtained after annealing, independent of the oxidant. This low D-it was found to be vital for the passivation performance. Field-effect passivation was less prominent for H2O-based ALD Al2O3 before and after annealing, whereas for as-deposited ALD films with an O-2 plasma or O-3 as the oxidants, the field-effect passivation was impaired by a very high Dit. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3501970] All rights reserved.