a b s t r a c tFour atmospheric pressure ozone oxidation methods were used to produce ultra-thin layers of aluminum oxide for organic thin-film transistors. They are UV/ozone oxidation in ambient (UV-AA) and dry (UV-DA) air, UV/ozone oxidation combined with high-voltage discharge-generated ozone in dry air (UV+O 3 -DA), and discharge-generated ozone in dry air (O 3 -DA). The lack of the high-energy UV photons during the O 3 -DA oxidation led to low relative permittivity and high leakage current density of the AlO x layer that rendered this method unsuitable for transistor dielectrics. Although this oxidation method led to the incorporation of oxygen into the film, the FTIR confirmed an increased concentration of the subsurface oxygen while the XPS showed the highest portion of the unoxidized Al among all four methods. The remaining three oxidation methods produced AlO x films with thicknesses in excess of 7 nm (2-h oxidation time), relative permittivity between 6.61 and 7.25, and leakage current density of (1-7) Â 10 À7 A/cm 2 at 2 MV/cm, and were successfully implemented into organic thin-film transistors based on pentacene and DNTT. The presence of -OH groups in all oxides is below the detection limit, while some carbon impurities appear to be incorporated.