Characterization of lead zirconate titanate (PZT) thin films for ferroelectric memory applications.

Abstract

Recently, significant progress has been made in integrating ferroelectric materials and semiconductor technology to achieve high density, semiconductor memories. The hysteresis behavior of the polarization versus the electric field and high dielectric constant of ferroelectric materials are useful for non-volatile and dynamic random access memories (DRAMs), respectively. Lead Zirconate-Titanate (Pb(Zr,Ti)O₃), commonly called PZT, is considered to be potentially important in memory applications. PZT is the ferroelectric material studied here. In this dissertation, the measurement methods for polarization and current-voltage characteristics of ferroelectric thin films are investigated. A new method for measuring current-voltage characteristics of ferroelectric materials is developed to distinguish the leakage current from the switching current. Further, the reliability concerns for ferroelectric memories, such as fatigue, retention, and temperature effect, are discussed based on the polarization and leakage-current characteristics of sol-gel derived PZT thin films. In addition to the studies on the charge storage capability, a model of the ferroelectric thin films is presented. Considering the spontaneous polarization of the ferroelectric film, a back-to-back Schottky barrier system having asymmetric barriers is proposed as a model for the platinum-PZT-platinum capacitor. The potential and electric field distributions in PZT thin films are calculated by solving Poisson's equation numerically for different doping concentrations. In this analysis, the permittivity variation with respect to the electric field is considered. Based on these numerical results, capacitance-voltage characteristics of the PZT thin film capacitor are predicted. For a doping concentration of 1 x 10¹⁸ cm⁻³ and the maximum relative permittivity of ∼8000, the calculated C-V curve fits well with the experimental result.