SPACE-CHARGE CAPACITANCE OF FERROELECTRIC MEMORY CELLS IN THE NONLINEAR REGIME

Recently ferroelectric thin-film capacitors have been modelled using t he sub-surface space charge region. The model is similar to the well k nown abrupt p-n junction theory or semiconductors. This paper describe s the sub-surface space charge using the landau free energy within a p olarized medium. The capacitance of the ferroelectric storage cell is described from these first principles. The model predicts that the res ult of the abrupt p-n junction depletion capacitance (linear medium mo del) is the limit of the ferroelectric p-n junction model (nonlinear m edium) when the polarization approaches zero. The results of the model are applied successfully to 60/40 PZT capacitors where the intrinsic heterogeneity (high election concentration in the near surface region) is always present due to high oxygen vacancy concentration. Both the linear and the nonlinear models are reasonable at high Voltages but on ly the nonlinear model is accurate at lower voltages. The results are also useful to characterize device parasitics and elucidate the effect of the microstructure on device behavior.