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-angstrom PbZr0.5Ti0.5O3 film with an effective SiO2 thickness of 10 angstrom is demonstrated. Because of the extremely high dielectric constant (epsilon(r) greater than or similar to 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 here is ohmic at electric fields below 250 kV/cm and follows an exponential field dependence at higher fields. This type of behavior is shown to be consistent with a simple model for electronic hopping through the film. Leakage current as tow as 9 x 10(-8) A/cm2 at 2.5 V for a 4000 angstrom film is obtained with the addition of lanthanum and iron to compensate for lead and oxygen vacancies in the film. Further improvement in both leakage current and time-dependent dielectric breakdown characteristics are necessary to ensure reliable DRAM operation.