Interpretation of flatband voltage shifts in terms of charge distributions in MNOS structures

Abstract

It is theoretically shown, how profiles of spatially and energetically distributed trapped charge influence the flatband voltage shift of MNOS capacitors during discharge. A monoenergetic charge density increasing from the oxide-nitride interface towards the gate and a charge distribution over a continuum of energy levels both cause a slower drop of the flatband voltage, whereas a spatial decrease in the charge distribution yields a more rapid change of the flatband voltage. Applied to the experiments, it is shown that for structures with oxides in the range 2–3 nm or when charged with high voltages (30 V) for structures with oxides even thicker than 3 nm, an initial increase in the charge profile was obtained. In the case of tunneling oxides below 2 nm a thin layer close to the oxide-nitride interface is shown to be free of trapped charge. As an extension to the charge profiling the comparison of theory and experiment allows us to determine important discharge parameters, so it is shown that a high temperature treatment of the memory nitride after deposition yields a trap level 0.4 V closer to the bottom of the nitride conduction band.