Co/AlOx/Co with AlOx barriers of various oxidation states were fabricated and investigated using inelastic electron tunneling (JET) spectroscopy and X-ray photoelectron spectroscopy (XPS). XPS revealed that AlOx-oxidized for 8 h contained an inhomogeneous distribution of metallic Al, whereas AlOx oxidized for 24 h contained a homogeneous distribution. The inhomogeneous and homogeneous distributions of metallic Al corresponded to asymmetric and symmetric IET spectra, respectively. These junctions showed peaks at +/- 0.03 V. AlOx oxidized for 168 h contained no metallic Al, and this junction had no peaks, suggesting that peaks at +/- 0.03 V originate from metallic Al. (C) 2009 Elsevier B.V. All rights reserved.