Experimental and first-principles theoretical evidence was provided to show that electron transport through alumina films on Al(111) occurred via electron tunneling, mediated by interstitial (i.e., non-bonded) H defects within the 6nm amorphous surface oxide. Experiments indicated a field-assisted tunneling into a defect level near to the mid-gap in the oxide and a conductance that increased with increasing H content. By using density functional theory, it was found that interstitial neutral H indeed produced a mid-gap trap. Moreover, with a negatively charged H, the full 1s2 orbital was almost at the same mid-gap energy. This showed H– electron-electron interactions were well screened in this wide gap insulator and could mediate conduction.

Evidence for Interstitial Hydrogen as the Dominant Electronic Defect in Nanometer Alumina Films. D.R.Jennison, P.A.Schultz, J.P.Sullivan: Physical Review B, 2004, 69[4], 041405 (4pp)