Effects of bulk trapping on the memory characteristics of thick-oxide MNOS variable-threshold capacitors

Charge-storage measurements have been performed at room temperature on thick-oxide MNOS structures using on automatic measuring system which allows a detailed and quite accurate investigation of the hysteresis loops (flat-band voltage versus stress voltage characteristics) of such devices. The experimental data have revealed several anomalous features, previously unreported in the literature, which are very general properties of double-layer devices. It is found that the initial hysteresis loop of a virgin device is very similar to the ‘conventional’ hysteresis loop, with flat bottoms and tops and steeply-rising sides. However, the second and subsequent hysteresis loops show dramatic changes in the flat-band voltage in the formerly flat regions. The results are interpreted in a selfconsistent manner, in terms of trapping phenomena throughout the bulk of both insulator regions. The trapped charge is found to produce strong internal fields that cause redistribution of the trapped charge in both the nitride and the oxide; it is these trapping effects which lead to the dramatic changes in the hysteresis loops after the initial pulse sequence. The existence of trapping levels in the bulk of the nitride is a necessary extension of the idealized model in which trapping levels are assumed to exist only at the oxide-nitride interface.