Observation of high interface state densities at the silicon/oxide interface for low doped polysilicon/oxide/silicon capacitor structures

In this work, we present new observations noted in the capacitance–voltage behaviour of polysilicon/oxide/silicon capacitor structures. As the active doping concentration reduces in the polysilicon layer, an anomalous capacitance–voltage behaviour is measured which is not related directly to depletion into the polysilicon gate. From examination of the frequency dependence of the capacitance–voltage characteristic, in conjunction with analysis and simulation, the anomalous capacitance–voltage behaviour is explained by the presence of a high density of near-monoenergetic interface states located at the silicon/oxide surface. The density and energy level of the interface states are determined. Furthermore, the work presents a mechanism by which the polysilicon doping level can impact on the properties of the silicon/oxide interface.