Electrical characteristics of ferroelectric PZT thin films for DRAM applications

Ferroelectric lead zirconate titanate (PZT) films with as much as 2.5 times the storage capacity of the best reported silicon oxide/nitride/oxide (ONO) stacked dielectrics have been fabricated. A 2000-Å film with an effective SiO₂ thickness of 10 Å is demonstrated. Because of the extremely high dielectric constant (ε_r≳>1000), even larger storage capacities can be obtained by scaling the ferroelectric film thickness, whereas the thickness of ONO films is limited by direct tunneling through the film. Electrical conduction in the PZT films studied is ohmic at electric fields below 250 kV/cm and follows an exponential field dependence at higher fields, which is shown to be consistent with a simple model for electronic hopping through the film. Leakage current as low as 9×10^-8 A/cm² at 2.5 V for a 4000-Å film is obtained with the addition of La and Fe to compensate for Pb and O vacancies in the film. Further improvement in both leakage current and time-dependent dielectric breakdown characteristics are necessary to ensure reliable DRAM operation