Evidence for interstitial hydrogen as the dominant electronic defect in nanometer alumina films

We provide experimental and first principles theoretical evidence that electron transport through alumina films on Al(111) occurs through electron tunneling mediated by interstitial (i.e., nonbonded) H defects within the 6-nm amorphous surface oxide. Experiments indicate field-assisted tunneling into a defect level near midgap in the oxide and conductance that increases with increasing H content. Using density functional theory, we find interstitial neutral H indeed produces a midgap trap. Moreover, with a negatively charged H, the full 1s² orbital is almost at the same mid-gap energy. This shows H^- electron-electron interactions are well screened in this wide gap insulator and can mediate conduction.