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.