Aluminum oxide films were grown on Si under ultrahigh vacuum conditions for use as tunnel barriers in spin injection studies. X-ray photoelectron spectroscopy was performed to characterize the film stoichiometry. It was observed that all the aluminum was bonded to the oxygen for the films grown in 1 nm steps. Whereas the 2 nm sample grown in one 2 nm step left a partially unoxidized aluminum film. Current-voltage measurements were performed and fitted to a tunnel model. The resistance area products fall within the range needed for spin injection and nonlocal readout. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3386589]