A model for electrical conduction in metal-ferroelectric-metal thin-film capacitors

Time-zero current-voltage characteristics and time-dependent current behavior of metal-ferroelectric-metal (Pt-PZT-Pt) capacitor structures have been studied. Under constant-voltage stressing, the current density through the 1500-Å-thick lead-zirconate-titanate (PZT) film exhibits a power-law dependence on time, with the exponent (∼0.33) independent of temperature and voltage. Electrode material dependence of current density indicates that the conventional model of trap-limited single-carrier injection over nonblocking contacts is inadequate to explain the time-zero current. A change in top electrode material from Pt to In leads to the observation of work-function-driven Schottky contacts between the metal and ferroelectric. The current-voltage characteristics fit a two-carrier injection metal-semiconductor-metal model incorporating blocking contacts, with distinct low- and high-current regimes (PZT is assumed to be p-type and trap-free in this model). Temperature-dependent I-V measurements indicate a Pt-PZT barrier height of 0.6 eV and an acceptor doping level of ∼1018 cm−3 in PZT. The implications of this model on the optimization of ferroelectric capacitors for dynamic random access memory applications are discussed.