Ultrathin aluminum oxide tunnel barriers

Ballistic electron emission microscopy is used to study the formation of ultrathin tunnel barriers by the oxidization of aluminum. An O-2 exposure, similar to30 mTorr sec, forms a uniform tunnel barrier with a barrier height phi(b) of 1.2 eV. Greater 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 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.