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Aluminum oxide films (Al2O3) synthesized by atomic layer deposition (ALD) provide an excellent level of surface passivation of n-, p-, and heavily doped p-type crystalline silicon (c-Si). It has been shown that a negative fixed charge density in the order of 1012-1013 cm-2 can be present at the interface of Al2O3 with c-Si, inducing a high level of field-effect passivation. In this contribution we report on results obtained by two complementary, contactless techniques to investigate the field-effect passivation for the Al2O3/c-Si case: optical second-harmonic generation (SHG) and corona charging. From an extensive data set on Al2O3 thin films, we demonstrate that the combination of SHG and corona charging offers the advantage of probing the presence of fixed charges at the interface non-intrusively (SHG) and of extracting the fixed charge density quantitatively (corona). These two techniques are therefore powerful tools for investigating the mechanisms responsible for the passivation properties of thin films.