A study was made of the instability and recovery of a large negative mid-gap voltage that was introduced, into metal-thermally grown SiO2/Si capacitors with a transparent Al gate, during vacuum ultra-violet exposure to a Kr lamp (10eV). High-frequency capacitance-voltage data were presented which showed that this mid-gap voltage instability was caused by the generation of unstable fast donor states, stable and unstable slow donor states, and drifting ions. It was noted that 2 fundamentally different mechanisms contributed to the gradual recovery of the samples. One was the disappearance of the unstable centers (fast and slow donor states and mobile ions), and the other was hole emission from slow states and from mobile ions which arrived at the substrate or at the gate. The annealing of unstable fast donor centers was inhibited when the states were positively charged. It was argued that the unstable fast and slow donor states, and the drifting ions, had an identical microscopic nature. That is, all of them were associated with a H atom that was attached to an O atom in the network. The unstable fast donor states were attributed to H atoms that were trapped at, or very close to, the Si substrate. The unstable slow donor states were attributed to H that was trapped further away. Annealing of these unstable slow states was explained in terms of the gradual disappearance of atomic H from the oxide. It was shown that the stable slow states were related to near-interfacial O vacancies that were activated by first trapping a hole.

K.G.Druijf, J.M.M.De Nijs, E.Van der Drift, E.H.A.Granneman, P.Balk: Journal of Applied Physics, 1995, 78[1], 306-16