We present X-ray photoelectron spectroscopy data which show that the chemisorbed oxygen previously observed to be on the surface of thin AlO x layers formed by room temperature thermal oxidation is bound by oxygen vacancies in the oxide. Increasing the electric field across the oxide, either by over-coating with a metallic electrode, or by electron bombardment, drives this surface chemisorbed oxygen into the vacancy sites. Due to the low bonding energies of these oxygen sites, subsequent oxygen exposures draw these

Ballistic electron emission microscopy is used to study the formation of ultrathin tunnel barriers by the oxidization of aluminum. An O2 exposure, approximately 30 mTorr sec, forms a uniform tunnel barrier with a barrier height straight phi(b) of 1.2 eV. Greater O2 exposure does not alter straight 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 O2 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.

Ballistic electron emission microscopy has been used to study thin aluminum oxide tunnel junction barriers formed both by magnetron sputter deposition and thermal evaporation. We have found that the barriers made by oxidation of evaporated Al become fully formed at a significantly thinner mean deposited thickness (∼6 Å) than junctions made by sputter deposition. The effective barrier height of the aluminum oxide has been determined to be 1.22±0.05 eV and is independent of the method of deposition, thickness, and oxidation conditions.

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