Oxygen stoichiometry and instability in aluminum oxide tunnel barrier layers

Abstract

We present x-ray photoelectron spectroscopy data that show that the chemisorbed oxygen previously observed to be on the surface of thin $\mathrm{Al}{\mathrm{O}}_{x}$ layers formed by room temperature thermal oxidation is bound by oxygen vacancies in the oxide. Increasing the electric field across the oxide, either by overcoating 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 ${\mathrm{O}}^{\ensuremath{-}}$ ions back to the surface, reforming chemisorbed $\mathrm{O}_{2}{}^{\ensuremath{-}}$. $\mathrm{Al}∕\mathrm{Al}{\mathrm{O}}_{x}∕\mathrm{Al}$ tunnel junctions incorporating electron-bombarded $\mathrm{Al}{\mathrm{O}}_{x}$ barriers show a significant reduction in the low-frequency junction resistance noise level at $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.