Carrier transport and storage in Si3N4 for metal-nitride-oxide-semiconductor memory applications

Abstract

The carrier transport and storage properties in the nitride of metal-nitride-oxide-semiconductor (MNOS) capacitors have been studied by combining repeated voltage ramp stress and constant voltage stress measurements. From the positive flat band voltage shift that results after bias application to the samples, it is clear that electrons dominate in the transport current and trapping. However, it is shown that, whereas at intermediate and low fields electrons are the single species involved in the capture-emission processes in the nitride, at higher electric fields a certain mechanism responsible for positive charge accumulation in the oxide-nitride interface region is also evident. By fitting theoretical results describing electron trapping in the nitride to the experimental flat band voltage shift dependence on bias, the density of electron traps in the nitride and the trapping efficiency have been estimated to be 3.7 × 1018 cm-3 and 10-2s cm2 respectively. We have also compared the experimental flat band voltage shift dependence on current obtained in constant current stressed MNOS capacitors with theoretical values derived from the fitted parameters. Quite good agreement has been found (except at low stress levels where leakage currents through the parallel resistance of the current source become significant), thus supporting the validity of the proposed model.