We report on the electrical properties of amorphous aluminum oxide films which are the potential gate dielectric material of organic thin-film transistors. The oxide films were deposited on indium-tin-oxide glasses using rf magnetron sputtering at room temperature under various Ar/O-2 ambient of 6:1, 10:1, and Ar only. As measured by Rutherford backscattering spectrometry, the stoichiometry (x:y) of the AlxOy oxide films approaches up to 2:3.5 when the gas ratio increases. The highest breakdown electric field (similar to3 MV/cm) was obtained from the samples deposited at the Ar only condition. According to capacitance-voltage measurements, all of our aluminum oxide films show similar to7 as their dielectric constant, k. (C) 2003 American Vacuum Society.