Polarity-dependent charge trapping and defect generation have been observed in SiO2 / Al2O3 gate stacks with TiN electrodes. For the substrate injection case, electron trapping in the bulk of the Al2O3 films dominates, whereas hole trapping near the Si substrate is observed for gate injection. This asymmetry in defect creation causes an asymmetry in oxide reliability. For gate injection, reliability is limited by the thin SiO2 interfacial layer, yielding low beta values, independent of the Al2O3 thickness. For substrate injection, reliability is limited by electron trap generation in the bulk of the Al2O3 film, yielding a strong thickness dependence of the beta values, as expected from the percolation model and as observed in SiO2 layers of similar thickness.