Hole trapping in ultrathin Al2O3 and ZrO2 insulators on silicon

Optical injection of electron-hole pairs in 3–5 nm thick layers of SiO2, Al2O3, ZrO2 and their stacks on (100)Si is found to result in positive oxide charging, suggesting trapping of holes. In thin layers of the high-permittivity metal oxides (Al2O3, ZrO2), the positive charge exhibits a remarkable stability to neutralization by electrons which is neither observed in thicker layers of the same oxides nor in thermal SiO2. Most of the positive charge is associated with diamagnetic centers (possibly, protons).