The uncontrolled formation of an interfacial layer between high-κ oxides and semiconductors is a major concern in advanced microelectronics not only for Si-based devices but also for those exploiting the higher mobility of Ge and GaAs. Using transmission electron microscopy, the authors investigate the interfacial layer formed between as-grown thin Al2O3 films, deposited using atomic layer deposition, and oxide free Si(100), Ge(100), and GaAs(100). In particular, they compare the effects of two different oxygen sources (H2O and O3) on interfacial layer formation during the growth process. They show that no interfacial layer can be distinguished unambiguously between the Al2O3 films, grown using TMA and H2O or O3, and all the semiconductor substrates.