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Boron-diffused p+/n/p+ and undiffused silicon samples with (1 0 0) and (1 1 1) orientations passivated by aluminum oxide (Al2O3) that is synthesized by atomic layer deposition (ALD) have been investigated. Emitter saturation current densities of ~24, 29, and 33 fA/cm2 were obtained for (1 0 0) samples with symmetrical 85Ω/□ B diffusions that were passivated by plasma-assisted, H2O-based, and O3-based ALD Al2O3, respectively. Compared with undiffused samples, it was found that the additional surface doping from the diffusion reduces recombination at the Al2O3/Si interface in the case of relatively low surface boron concentrations (<; 2×1019 cm-3). The degree of surface passivation that is observed on (1 0 0) surfaces was generally better than on (1 1 1) surfaces, particularly for undiffused samples, but this difference effectively disappeared following the application of more negative charge by corona charging. From capacitance- voltage measurements, it was found that Al2O3 films on substrates with a (1 0 0) orientation display a higher negative fixed charge density Qf than films on (1 1 1) samples. On the other hand, the interface state density Dit was not strongly influenced by surface orientation of the substrate. It appears that the difference in negative charge density is at least partly responsible for the differences in the observed passivation.