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

Abstract

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. Electrodematerial 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 electrodematerial 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 ferroelectriccapacitors for dynamic random access memory applications are discussed.