Ballistic electron emission microscopy is used to study the formation of ultrathin tunnel barriers by the oxidization of aluminum. An ${\mathrm{O}}_2$ exposure, $\sim 30\mathrm{mTorr}\sec$, forms a uniform tunnel barrier with a barrier height ${\phi }_b$ of 1.2 eV. Greater ${\mathrm{O}}_2$ exposure does not alter ${\phi }_b$ or the ballistic transmissivity of the oxide conduction band. Tunneling spectroscopy indicates a broad energy distribution of electronic states in the oxide. With increasing ${\mathrm{O}}_2$ dose the states below 1.2 eV gradually become localized, but until this localization is complete these states can provide low-energy single-electron channels through the oxide.

• Received 20 June 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.046805