Oxygen stoichiometry and instability in aluminum oxide tunnel barrier layers

The use of aluminum oxide sAlOxd layers of nanometer thickness formed by ,300 K oxidation of aluminum thin films has long been the most successful approach to the fabrication of high-performance metal-insulator-metal tunnel junctions, originally for low-temperature superconducting Josephson junctions 1 sJJsd, and more recently for magnetic tunnel junctions 2 sMTJsd. The goals of reliably forming still thinner barriers to achieve higher critical current densities Jc sfor JJsd and lower specific resistances sfor MTJsd, of producing tunnel junctions with lower levels of 1 / f noise for sensor and quantum computing applications, and of forming higher performance gate insulators for molecular electronics studies 3 have continued to focus attention on the objective of obtaining a better understanding and improved control of the oxide barrier layer. Recently a scanning tunneling and ballistic electron emission microscopy sSTM/BEEMd study 4 of the thermal oxidation of aluminum revealed that after oxidation, a layer of oxygen remains on the surface of the oxide indefinitely in ultrahigh vacuum in the form of nanoscale clusters of chemisorbed O2 ˛ , raising questions regarding the cause of this chemisorbed oxygen and its effect on the resultant tunnel barrier when overcoated by the counter electrode. Here we report on an x-ray photoemission spectroscopy sXPSd study of the thermal oxidation of Al, which extends previous XPS studies 5,6 and shows that this chemisorbed oxygen is associated with and bound to the surface by O vacancies in the oxide. Low-energy electron bombardment can drive the chemisorbed oxygen into the oxide layer and substantially stabilize it there but subsequent exposure to oxygen ambient pulls some of this oxygen back to the surface, demonstrating the low bonding energies of a portion of the oxygen sites in the oxide. Depending on the relative work function F of the metal used, much of this chemisorbed oxygen is also driven into the oxide layer upon overcoating by a metallic counter electrode. These results yield insights into the structure and electronic properties of thermally formed AlOx layers and suggest pathways whereby thinner, more stable, and potentially lower 1 / f noise and higher electrical resistance tunnel barriers and gate insulators might be produced.