RECOVERY OF VACUUM-ULTRAVIOLET IRRADIATED METAL-OXIDE-SILICON SYSTEMS

This article is concerned with the instability and recovery of the large negative midgap voltage induced in metal-thermally grown SiO2-silicon capacitors with transparent Al gate upon vacuum ultraviolet exposure by a Kr lamp (hν=10 eV). Evidence is presented from high-frequency capacitance-voltage measurements, showing that this midgap voltage instability is caused by the generation of unstable fast donor states, stable and unstable slow donor states, and drifting ions. Two fundamentally different mechanisms contribute to the gradual recovery of the samples: the disappearance of the unstable centers (fast and slow donor states and mobile ions) and hole emission from slow states and from mobile ions arriving at the substrate or at the gate. The annealing of unstable fast donor centers is inhibited when the states are charged positively. It is argued that the unstable fast and slow donor states and the drifting ions have an identical microscopic nature: all are associated with a H atom attached to an O atom in the network. The unstable fast donor states are attributed to H atoms trapped at or very close to the Si substrate, the unstable slow donor states to H trapped somewhat further away. Annealing of these unstable slow states is explained from the gradual disappearance of the atomic H from the oxide. It is shown that the stable slow states are related to near-interfacial O vacancies, activated by first trapping a hole. © 1995 American Institute of Physics.