Random Telegraph Signal in the Quasi-Breakdown Current of MOS Capacitors

Observations of multilevel fluctuations (random telegraph signal, RTS) in the quasi breakdown (QB) current in thin (below 5nm) SiO2 MOS capacitors are reported, for the first time to our knowledge. Two or three level RTS with a relative amplitude, ¿Ig/Ig, of up to 30% has been observed. A simple model of current fluctuations is proposed, where electron capture-emission near the cathode induces local field fluctuations causing variations in the tunnelling rate across the oxide layer. The crucial element of the model is that the QB current is confined to a few small active areas, probably associated with electrical stress induced defects. The model explains (i) a step-like increase in Ig observed during constant voltage stress (by an increase in the number of active areas) and (ii) the current fluctuations (by trapping-induced electric field variations in the active area). The surface area can be determined from ¿Ig/Ig and is found to be of the order of 10^¿12cm² which is consistent with a point-like appearance of light sources accompanying the QB regime.